Current and Voltage Transformers _23_10_2021.pptx

Current and Voltage
Transformers

Introduction
• The purpose of electric power systems is to provide energy for human use in a secure, reliable
and economic manner.
• Electric power systems are made up of facilities and equipment that generate, transmit and
distribute electrical energy.
• Electric power systems are one of the largest and more complex systems man has ever built.
• Faults and failures normally occur in power systems.
• Due to the great amounts of energy involved, faults represent a threat to the operation and
security of power systems if the faults are not promptly corrected.
• The rigid interconnectivity existing among modern power systems make them highly
unstable when, faults are not cleared rapidly.
• If a fault in an important transmission line is not identified and removed at the shortest
possible time, it might lead to a widespread damage in the power system.
• Power systems need an auxiliary system that must take corrective actions on the occurrence of
a fault.
• This auxiliary system is known as protection system.
• Power System Protection is the art and science of the application of devices that monitor the
power line currents and voltages (relays) and generate signals to deenergize faulted sections
of the power network by circuit breakers.
• One of the most important equipments employed in the protection of power systems are
protective relays.

Introduction
• These are one of the most flexible, economic and well-known devices that provide reliable,
fast and inexpensive protection.
• The job of the protective relays is to identify and isolate the faults in power system networks.
• The rapid removal of transmission lines’ faults is one of the best measures used to improve
the power systems’ stability, and, implicitly, to ensure an adequate reliability of the grid and
the continuity into energy transmission.
• In order to prevent the damages from spreading to the healthy parts of the power system, the
protective relaying algorithms need to detect the faults within sub cycles of the power system
frequency.
• The unpredictable nature of the power system signals during faults, make their extraction a
challenging job.
• The simplest method for fast fault clearance is that of the decrease of time of the protection
operation.
• Power networks are usually protected by means of two main components, relays that sense the
abnormal current or voltage and a circuit breaker that put a piece of plant out of tension.
• Many types of protective relays are used to protect power system equipments.
• For a good performance of a relay in a power system it must have the following
characteristics; dependability, security, selectivity, sensitivity and speed.

Introduction
• Transmission lines are among the power system components with the highest fault incidence
rate, since they are exposed to the environment.
• Line faults due to lightning, storms, vegetation fall, fog and salt spray on dirty insulators are
beyond the control of man.
• On a transmission system the protective relaying system is incorporated to detect the
abnormal signals indicating faults and isolate the faulted part from the rest of the system with
minimal disturbance and equipment damage.
• Traditionally, power systems problems and applications have been solved by means of purely
analog circuits.
• The latest generations of protective relays be provided with a large capacity of processing
capabilities become more efficient and can perform a numerous number of functions such as
fault locators, integrated monitoring and control functions.
• Rockefeller first presented the implementation of digital relaying in 1969 (Rockefeller, 1969).
• The advances in the very large scale integrated (VLSI) technology and software techniques
led to the development of microprocessor based relays that were first offered as commercial
devices in 1979 (Sachdev, 1979).
• Selective, high speed clearance of faults on high voltage transmission lines is critical to the
stability of the highly complex, modern power system. In this respect, lot of work has been
done to improve the performance of digital protective relays and in the use of intelligent
techniques for analysis of faults and protective relay operations.

Introduction
• When a fault occurs in a power transmission line, the voltage and current signals are severely
distorted.
• These signals may contain dc offsets, high frequency transients, and oscillation components.
• To frame the state of the power system, it is necessary to estimate the fundamental
components of the steady state post fault currents and voltages from these corrupted voltage
and current signals.
• Digital Relays use algorithms for estimating the functioning parameters of a power system
and then use the selected characteristics to make right decisions to disconnect a failed
element, such as a line, transformer or a generator.
• Electric power utilities use electromechanical and solid-state relays for protecting power
systems.
• Current transformers (CTs) are the basic interconnection between the power system and
almost all measurement devices such as protective relays.
• CTs step the primary current down to a nominal secondary level for use by protective relays,
meters, and other monitoring devices.
• One of the practical concerns for the protection engineer is the actual ability of a CT to
replicate the primary current With the advent of microprocessor technology.
• Researchers and designers have made significant progress in designing microprocessor-based
relays that are expected to provide fast and accurate fault detection.

Introduction
• Practically all electrical measurements and relaying decisions are derived from current and
voltage signals.
• Since relaying hardware works with smaller range of current (in amperes and not kA) and
voltage (volts and not kV), real life signals (feeder or transmission line currents) and bus
voltages have to be scaled to lower levels and then fed to the relays.
• This job is done by current and voltage transformers (CTs and VTs).
• CTs and VTs also electrically isolate the relaying system from the actual power apparatus.
• The electrical isolation from the primary voltage also provides safety of both human
personnel and the equipment.
• Thus, CT and VTs are the sensors for the relay.
• CT and VT function like ‘ears' and the ‘eyes' of the protection system.
• They listen to and observe all happening in the external world.
• Relay itself is the brain which processes these signals and issues decision commands
implemented by circuit breakers, alarms etc.

What is an instrument transformer
• An instrument transformer is an electrical device intended to supply measuring instruments
such as meters, relays and other similar apparatus.
• There are two types of instrument transformer: - Current Transformer ( CT ), in which the
secondary current is under normal working conditions, practically proportional to the primary
current and phase shifted from it by an angle close to zero in the appropriate direction for
connections. –
• Voltage Transformer ( VT ) also known as a Potential Transformer ( PT ). In which the
secondary voltage is under normal operating conditions, practically proportional to the
primary voltage and phase shifted from it by an angle close to zero in the appropriate
direction for connections.
• What is the aim of an instrument transformer
• The basic purpose of an instrument transformer is to reduce the voltage and current of an
electrical network to a standardized, non hazardous level.
• They prevent any direct connection between instruments and high voltage circuits which
would be dangerous to operators and would need instrument panels with special insulation.
• They also do away with the need for expensive special instruments when high currents and
voltages have to be measured.

current transformer
• A current transformer is defined as an instrument transformer in which the secondary current
is substantially proportional to the primary current (under normal conditions of operation) and
differs in phase from it by an angle which is approximately zero for an appropriate direction
of the connections.
• This highlights the accuracy requirement of the current transformer but also important is the
isolating function which means no matter what the system voltage the secondary circuit need
to be insulated only for a low voltage.

current transformer
• There are two windings in a current transformer, one of them is a high current primary winding
and the other is a low current secondary winding.
• Unlike in other transformers, the primary winding current in a current transformer is
independent of the secondary winding load.
• The primary winding current depends only on the circuit into which the primary winding is
connected.
• The primary winding is designed to have very low impedance (it often will only have one turn
in the winding) and hence has negligible effect on the main current.
• Therefore, regardless of what change may be made to the secondary winding load, the primary
winding current is always the same as that of the main circuit.
• One may argue that if the load on the secondary winding is changed, something in the primary
winding has to change.
• This is partially true.
• The change is in the relationships between the several components of the primary winding
current.
• The primary winding current is essentially made up of three components; the loss current that
supplies iron and copper loss, the magnetizing current that establishes the flux in the core and
the load current.

current transformer
• However, these currents are not in phase with each other; there are differences in the phase
angles and these angles change when the load changes. The primary winding current, as
always, is the vector sum of these currents.
• The design of the current transformer has to satisfy the equation: IinNin = IoutNout if the
magnetizing and loss currents are not taken into account.
• If accuracy is a concern, then, the magnetizing and loss currents must be kept small.
• The value Iin is determined by the circuit into which the primary winding is connected and
Iout is by the load that is to be supplied.
• Iin verses Iout is the current ratio, which is the same as Nout verses Nin the turn ratio.
• As mentioned earlier, the primary current consists of loss current, the secondary load and the
magnetizing current.
• If high accuracy current transformer is needed, the loss and magnetizing current need to be
kept as low as possible so that the secondary current will truly reflect the primary current
which is also the current to be measured or sensed.
• As the result, a low-loss and high permeability core material shall be chosen.
• .

current transformer
• The toroidal core is the most readily used core geometry for current transformer application.
• There is no air gap in the core so that the magnetizing current will always be small.
• Toroidal core geometry fits perfectly with flux flow in the core, therefore the core material
can be utilized efficiently giving you a small core and small core loss as a result A current
transformer should never be open-circuited while main current is passing through the primary
winding.
• If the load is removed from the secondary winding while the main circuit current is flowing,
most of the primary winding current becomes magnetizing current, but the vector angles
change in such a way as to keep the total current in the primary the same as before.
• Because the main circuit is now mostly magnetizing current, the flux in the core shoots up to
a high level and a very high voltage appears across the secondary.
• Due to the high turn ratio usually found in these transformers, the voltage in this condition
can reach a dangerously high level, which can break down the insulation.
• It also becomes a hazard to personnel. The high flux can saturate the core and result in strong
residual magnetism left in the core, thereby increasing magnetization current and introducing
error in the transformation ratio.
• It is strongly recommend that one put a short on the secondary winding before removing the
secondary load while the main current is flowing through the primary winding.

current transformer
• The current transformer works on the principle of variable flux.
• In the ideal current transformer, secondary current would be exactly equal (when multiplied
by the turns ratio) and opposite to the primary current.
• But, as in the current transformer, some of the primary current or the primary ampere-turns
are utilized for magnetizing the core, thus leaving less than the actual primary ampere turns to
be transformed into the secondary ampere-turns.
• This naturally introduces an error in the transformation.
• The error is classified into current ratio error and the phase error.

Equivalent Circuit of CT
•Equivalent circuit of a CT is not much different from that of a regular transformer.
•However, a fundamental difference is that while regular power transformers are excited
by a voltage source, a current transformer has current source excitation.
•Primary winding of the CT is connected in series with the transmission line.
The load on the secondary side is the relaying burden and the lead wire resistance.

• Total load in ohms that is introduced by CT in series with the transmission line is insignificant
and hence, the connection of the CT does not alter current in the feeder or the power apparatus
at all.
• Hence from modeling perspectives it is reasonable to assume that CT primary is connected to
a current source.
• Therefore, the CT equivalent circuit will look as shown in fig below.
• The remaining steps in modeling are as follows: As impedance in series with the current
source can be neglected, we can neglect the primary winding resistance and leakage
reactance in CT modeling.
• For the convenience in analysis, we can shift the magnetizing impedance from the primary
side to the secondary side of the ideal transformer.

•After application of the above steps, the CT equivalent circuit is as shown in the fig above.
•Note that the secondary winding resistance and leakage reactance is not neglected as it will
affect the performance of CT.
•The total impedance on the secondary side is the sum of relay burden, lead wire resistance and
leakage impedance of secondary winding.
•Therefore, the voltage developed in the secondary winding depends upon these parameters
directly.
•The secondary voltage developed by the CT has to be monitored because as per the
transformer emf equation, the flux level in the core depends upon it. The transformer emf
equation is given by,
Where is the peak sinusoidal flux developed in the core

• If corresponding to this flux is above the knee point, it is more or less obvious
that the CT will saturate.
• During saturation, CT secondary winding cannot replicate the primary current
accurately and hence, the performance of the CT deteriorates.
• Thus, we conclude that in practice, while selecting a CT we should ascertain that it
should not saturate on the sinusoidal currents that it would be subjected to. Use of
numerical relays due to their very small burden compare to solid state and
electromechanical relays, improves the CT performance.
• CT is to be operated always in closed condition.
• If the CT is open circuited, all the current Ip/N, would flow through Xm.
• This will lead to the development of dangerously high level of voltage in secondary
winding which can even burn out the CT.

Performance of CT
• The performance of a current transformer used in protective relaying is largely
dependent on the total burden or impedance in the secondary circuit of the current
transformer.
• The current transformer core flux density (and thus the amount of saturation) is
directly proportional to the voltage that the current transformer secondary must
produce.
• So for a given amount of secondary current, the larger the burden impedance
becomes, the greater is the tendency of the current transformer to saturate.
• Ideally, protective relay systems would ignore current transformer saturation.
• However, that is usually not possible; so it is the task of the relay engineer to
minimize current transformer burden impedance.
• Manufacturers' publications give the burdens of individual relays, meters, and
other equipment.
• Adding the resistance of interconnecting leads and internal resistance of the current
transformer gives the total current transformer burden.
• In modern microprocessor relays with very small burdens, the total relay burden is
often dominated by the lead impedance or internal CT impedance.

Knee Point Voltage of Current Transformer
•Knee point voltage of a current transformer is the magnitude of secondary of current
transformer.
•After or beyond this voltage the linearity between primary and secondary circuit that is the
desired property of Current transformer does not work any more.
• In saturation zone -the error in transformation ratio is high, the secondary current is distorted by
saturation. This is called core saturation of current transformer.
•According IEC, Knee Point Voltage of a Current Transformer is defined as the voltage at which
10 % increase in voltage of CT secondary results in 50 % increase in secondary current.
•This is the significance of saturation level of a CT core mainly used for protection purposes.
•The CT core is made of CRGO steel. It has its won saturation level.
•The EMF induced in the CT secondary windings is
•Where, f is the system frequency, φ is the maximum magnetic flux in Wb.
•T2 is the number of turns of the secondary winding.
•The flux in the core, is produced by excitation current Ie.
•We have a non – liner relationship between excitation current and magnetizing flux.
E2 = 4.44φfT2

Knee Point Voltage of Current Transformer
It is clear from the curve that, Linear relation
between V & Ie is maintained from point A & K.
The point ′A′ is known as ′Ankle Point′ and
point ′K′ is known as ′Knee Point′.
•After certain value of excitation current, flux will not further increase so rapidly
with increase in excitation current.
•This non-liner relation curve is also called B – H curve.
•Again from the equation above, it is found that, secondary voltage of a Current
Transformer is directly proportional to flux φ.
•Hence one typical curve can be drawn from this relation between secondary
voltage and excitation current as shown below,

Equivalent circuit of saturated CT
• One of the major problems faced by the protection systems engineer is the saturation of CT on
large ac currents and dc offset current present during the transient.
• When the CT is saturated, primary current source cannot be faithfully reflected to the
secondary side. In other words, we can open circuit the current source.
• Also, the magnetizing impedance falls down during saturation.
• Then the transformer behaves more like an air core device, with negligible coupling between
the primary and secondary winding.
• The high reluctance due to the air path implies that the magnetizing impedance (inductance)
falls down. The corresponding equivalent circuit is shown in fig below.

• The error of a conventional current transformer is dependent on, whether the core is saturated
or not.
• When the core is saturated the magnetizing current is large compared to the secondary current
and the error is high.
• The degree of saturation depends on the magnitude of fault current, primary time constant,
secondary time constant of the current transformer and the magnitude of the DC component.
• The remanence of the core will also influence the saturation.
• Current transformer saturation can cause both failure to operate and unwanted operation of the
protection depending on the measuring principle.
• Theoretically the saturation and the maloperation of the protection can be avoided by
considering all the negative factors when sizing the current transformer.
• In practice his will often result in unrealistically large and expensive current transformers.

Classification Based on the construction
• Bar Type Wound Type Window Type
•The primary winding is the main conductor passing through the center of the core.
•The secondary winding is uniformly distributed around the toroidal core.
•Essentially, all the flux which links the primary conductor also links the secondary winding.
•The leakage flux, and thus the leakage, reactance is negligible.
•This is a common construction for HV and EHV current transformers

Classification of CTs
• The CTs can be classified into following types:
• Measurement CTs
• Protection CTs
• A measurement grade CT has much lower VA capacity than a protection grade CT.
• A measurement CT has to be accurate over its complete range e.g. from 5% to 125% of
normal current.
• In other words, its magnetizing impedance at low current levels. (and hence low flux levels)
should be very high.
• Rather it is desirable the CT core to be saturated after this limit since the unnecessary
electrical stresses due to system over current can be prevented from the metering instrument
connected to the secondary of the CT as secondary current does not go above a desired limit
even primary current of the CT rises to a very high value than its ratings.
• So accuracy within working range is main criteria of a CT used for metering purpose.
• The degree of accuracy of a Metering CT is expressed by CT Accuracy Class or simply
Current Transformer Class or CT Class.

• Note that due to non-linear nature of B-H curve, magnetizing impedance is not constant but
varies over the CT's operating range.
• It is not expected to give linear response (secondary current a scaled replica of the primary
current) during large fault currents.
• But in the case of protection, the CT may not have the accuracy level as good as metering CT
although it is desired not to be saturated during high fault current passes through primary.
• So core of protection CT is so designed that it would not be saturated for long range of
currents.
• If saturation of the core comes at lower level of primary current the proper reflection of
primary current will not come to secondary, hence relays connected to the secondary may not
function properly and protection system losses its reliability.
• That is why the core of the protection CT is made such a way that saturation level of that core
must be high enough.

• When a CT is used for both the purposes, it has to be of required accuracy class to satisfy both
accuracy conditions of measurement CTs and protection CTs.
• In other words, it has to be accurate for both very small and very large values of current.
• Typically, CT secondary rated current is standardized to 1A or 5A (more common).
• However, it would be unreasonable to assume that the linear response will be independent of
the net burden on the CT secondary.
• For simplicity, we refer to the net impedance on the secondary side (neglecting magnetizing
impedance) as the CT burden.
• It is quite obvious that the driving force required to drive the primary current replica will
increase as this burden increases.
• The secondary terminal voltage rating is the voltage that the transformer will deliver to a
standard burden at 20 times normal secondary current, without exceeding 10 percent ratio
correction .
• If this voltage exceeds the designer's set limits, then the CT core will saturate and hence linear
response will be lost.
• Hence, when we say that a CT will give linear response up to 20 times the rated current, there
is also an implicit constraint that the CT burden will be kept to a low value.
• In general, name-plate rating specifies a voltage limit on the secondary (e.g., 100 V) up to
which linear response is expected.
• If the CT burden causes this voltage to be exceeded, CT saturation results.

• But still there is a limit as because, it is impossible to make one magnetic core with infinitely
high saturation level and secondly most important reason is that although the protection core
should have high saturation level but that must be limited up to certain level otherwise total
transformation of primary current during huge fault may badly damage the protection relays.
• So it is clear from above explanation, rated accuracy limit primary current, should not be so
less, that it will not at all help the relays to be operated on the other hand this value must not
be so high that it can damage the relays.
• for a protection grade CT, linear response is expected up to 20 times the rated current.
• Its performance has to be accurate in the range of normal currents and up to fault currents.
• Specifically, for protection grade CT's magnetizing impedance should be maintained to a large
value in the range of the currents of the order of fault currents.

• Suppose you have one CT with current ratio 400/1A and its protection core is
situated at 500A.
• If the primary current of the CT becomes 1000A the secondary current will still be
1.25A as because the secondary current will not increase after 1.25A because of
saturation.
• If actuating current of the relay connected the secondary circuit of the CT is 1.5A,
it will not be operated at all even fault level of the power circuit is 1000A.
• The degree of accuracy of a Protection CT may not be as fine as Metering CT but it
is also expressed by CT Accuracy Class or simply Current Transformer Class or CT
Class as in the case of Metering Current Transformer but in little bit different
manner.

Instrument Security Factor or ISF of Current Transformer
• Instrument Security Factor is the ratio of Instrument Limit Primary Current to the rated
Primary Current.
• Instrument Limit Current of a metering Current Transformer is the maximum value of
primary current beyond which Current Transformer core becomes saturated.
• Instrument Security Factor of CT is the significant factor for choosing the metering
Instruments which to be connected to the secondary of the CT. Security or Safety of the
measuring unit is better, if ISF is low.
• Suppose one Current Transformer has rating 100/1A and ISF is 1.5 and another Current
Transformer has same rating with ISF 2.
• That means, in first CT, the metering core would be saturated at 1.5X100 or 150 A, whereas is
second CT, core will be saturated at 2X100 or 200A.
• That means whatever may be the primary current of both CTs, secondary current will not
increase further after 150 & 200A of primary current of the CTs respectively.
• Hence maximum secondary current of the CTs would be 1.5 & 2.0 A.

Instrument Security Factor or ISF of Current Transformer
• As the maximum electric current can flow through the instrument connected to the
first CT is 1.5A which is less than the maximum value of electric current can flow
through the instrument connected to the second CT i.e. 2A.
• Hence security or safety of the instruments of first CT is better than later.
• Another significance of ISF is during huge electrical fault, the short circuit current,
flows through primary of the CT does not affect destructively, the measuring
instrument attached to it as because, the secondary current of the CT will not rise
above the value of rated secondary current multiplied by ISF
• Protection class CTs are designed to work in the linear range, with minimal errors
and minimal waveform distortion, only up to 20 times the rated nominal current
with the burden as defined by the relay class (saturation voltage) of the CT per
IEEE Std. C57.13.

Accuracy Limit Factor or ALF of Current Transformer
• For protection current transformer, the ratio of accuracy limit primary current to the rated primary current.
• rated accuracy limit primary current is the maximum value of primary current, beyond which core of the
protection CT or simply protection core of of a CT starts saturated.
• The value of rated accuracy limit primary current is always many times more than the value of instrument
limit primary current.
• Actually CT transforms the fault current of the electrical power system for operation of the protection
relays connected to the secondary of that CT.
• If the core of the CT becomes saturated at lower value of primary current, as in the case of metering CT, the
system fault will not reflect properly to the secondary, which may cause, the relays remain inoperative even
the fault level of the system is large enough.
• That is why the core of the protection CT is made such a way that saturation level of that core must be high
enough.
• But still there is a limit as because, it is impossible to make one magnetic core with infinitely high
saturation level and secondly most important reason is that although the protection core should have high
saturation level but that must be limited up to certain level otherwise total transformation of primary current
during huge fault may badly damage the protection relays.
• So it is clear from above explanation, rated accuracy limit primary current, should not be so less, that it will
not at all help the relays to be operated on the other hand this value must not be so high that it can damage
the relays.
• So, accuracy limit factor or ALF should not have the value nearer to unit and at the same time it should
not be as high as 100. The standard values of ALF as per IS-2705 are 5, 10, 15, 20 and 30.

• ANSI / IEEE classification
• ANSI accuracy class ratings apply only to the full winding.
• Where there is a tapped secondary, a proportionately lower
voltage rating exists on the taps.
• ANSI/IEEE standards classify CTs into two types:
• Class T CT
• Class C CT

Class T CTs
• Typically, a class T CT is a wound type CT with one or more primary turns wound on a core.
• Wound type current transformers
• It is associated with high leakage flux in the core.
• Because of this, the only way to determine it's performance is by test.
• In other words, standardized performance curves cannot be used with this types of CTs.
• Because of the physical space required for insulation and bracing of the primary winding and fringing
effects of non-uniformly distributed windings, flux is present which does not link both primary and
secondary windings.
• The exciting flux should be considered, along with the leakage flux, in determining current transformer
accuracy
•Figure shows one such experimentally calibrated
curve for a CT.
•The letter ‘B' indicates the burden in ohms to which
the CT is subjected.
•It is seen that when burden is less than say 0.1 ohms,
CT meets the linear performance criterion.
•However, as the burden increases to 0.5 ohms, the
corresponding linearity criteria is not met till the end.
•At 4 ohms burden, there is significant deviation
from the linear response.
•A general rule of thumb is that, one should try to
keep the CT burden as low as possible.

Ratio Error:
• CT performance is usually gauged from the ratio error.
• The ratio error is the percentage deviation in the current magnitude in the secondary from the
desired value.
• In other words, if the current measured in the secondary is Is, true or actual value is Ip/N,
where N is nominal ratio (e.g. N for a 100:5 CT is 20) and Ip is the primary current then
• ratio error is given by
• When the CT is not saturated ratio error is a consequence of
magnetizing current IE since
Therefore, % ratio error is equal to

Ratio Error:
• When the CT is saturated, coupling between primary and secondary is reduced.
• Hence large ratio errors are expected in saturation.
• The current in the secondary is also phase shifted.
• For measurement grade CTs, there are strict performance requirements on phase
angle errors also.
• Error in phase angle measurement affects power factor calculation and ultimately
real and reactive power measurements.
• It is expected that the ratio error for protection grade CTs will be maintained within
• The “knee” or effective point of saturation is refined by the ANSI/IEEE standards
as the intersection of the curve with a 45 degree tangent line and for
IEC(international electrochemical commission) defines the knee as the intersection
of straight lines extended from the unsaturated and saturated parts of the exciting
curve. The IEC knee is at a higher voltage than the ANSI knee.

Class C CT
• 'C' indicates that the leakage flux is negligible. Class C CTs are the more accurate bar type CTs.
• In such CTs, the leakage flux from the core is kept very small.
• For such CTs, the performance can be evaluated from the standard exciting curves.
• Also, the ratio error is maintained within 10% for standard operating conditions.
• For such CTs, voltage rating on the secondary is specified up to which linear response is
guaranteed.
• The unsaturated slope is determined by the magnetic core material. The saturated region is the
air-core reactance.
• For example, a class C CT specification could be as follows: 200:5 C 100.
• The labeling scheme indicates that we are dealing with a 200:5 class C CT which will provide linear
response up to 20 times rated current provided the burden on the secondary is kept below
ohm.
• Similarly, a corresponding class T CT may be labeled as 200:5 T 100.
• When the current transformer core is unsaturated, the error due to exciting current is normally
negligible. When the voltage is above the knee of the excitation curve, the current transformer is
said to be operating in its saturated region where the exciting current is no longer negligible.
Therefore, the ratio error of the current transformer becomes much greater beyond the knee.
• For class C CTs, standard chart for versus excitation current on the secondary side is available.
• This provides the protection engineer data to do more exact calculations (refer fig). e.g., in
determining relaying sensitivity.

Example 1:
•
A , C400 CT with excitation curves shown on above fig, is connected to a 2.0 burden. Based
on the accuracy classification, what is the maximum symmetrical fault current that may be
applied to this CT without exceeding a 10% ratio error?
Answer:
CT ratio = 1200/5
Secondary resistance = 0.61
Relay burden = 2
For 20 times rated secondary current, i.e., 100A
Secondary voltage = 100 x (2 + 0.61) = 261 Volts which is less than knee point of the CT.
• Since this voltage is less than 400V, from electrical perspective, linearity will not be lost at
even higher currents.
• Approximate limit on secondary current is given by .
• Hence maximum symmetrical fault current = 36720A.
•

Example 2:
• A ,1200/5 C400 CT is connected on the 1000/5 tap. What is the maximum secondary burden
that can be used and we can maintain rated accuracy at 20 times rated symmetrical secondary
current?
The secondary voltage corresponding to the tap 1000/5,
=333 V
333 = 100(0.51+RB)
Secondary burden = 3.33 - 0.51 = 2.72

Example :2
• The maximum calculated fault current for a particular line is 12,000 amps. The current transformer is rated
at 1200:5 and is to be used on the 800:5 tap. Its relaying accuracy class is C200 (full-rated winding);
secondary resistance is 0.2 ohm. The total secondary circuit burden is 2.4 ohm at 60-percent power factor.
Excluding the effects of residual magnetism and DC offset, will the error exceed 10 percent? If so, what
corrective action can be taken to reduce the error to 10 percent or less?
• The current transformer secondary winding resistance may be ignored because the C200 relaying accuracy
class designation indicates that the current transformer can support 200 volts plus the voltage drop
caused by secondary resistance at 20 times rated current, for 50 percent power-factor burden. The CT
secondary voltage drop may be ignored then if the secondary current does not exceed 100 amps
• N = 800/5 = 160
• IL = 12000/160 = 75 amps
• The permissible burden is given by:
• ZB = NP VCL / 100
• Where ZB = Permissible burden on the current transformer
• NP = Turns in use divided by total turns
• VCL = Current transformer voltage class
• NP = 800/1200 = 0.667 (proportion of total turns in use)
• Thus, ZB = .667 (200)/100 = 1.334 ohms
• Since the circuit burden, 2.4 ohms, is greater than the calculated permissible burden, 1.334, the error will
be in excess of 10 percent at all currents from 5 to 100 amps. Consequently, it is necessary to reduce the
burden, use a higher current transformer ratio, or use a current transformer with a higher voltage class.

Exciting Current
• In an ideal current transformer, the primary ampere turns are equal to
• the secondary ampere turns.
• However, every core material requires some energy to produce the magnetic flux
which induces the secondary voltage necessary to deliver the secondary current.
• Thus, in an actual current transformer, the secondary ampere turns are equal to the
primary ampere turns minus the exciting ampere turns. When the current
transformer core is unsaturated, the error due to
• exciting current is normally negligible.
• When the voltage is above the knee of the excitation curve, the current
transformer is said to be operating in its saturated region where the exciting
current is no longer negligible.
• Therefore, the ratio error of the current transformer becomes much greater
beyond the knee

Remanence
• Remanent flux can be set up in the core of a current transformer under
operating or test conditions.
• During operating conditions, remanent flux can be left in the core when the
primary current is interrupted while the flux density in the core of the
transformer is high.
• This may occur when clearing fault current.
• Testing, such as resistance or continuity measurements, may also leave
remanence.
• The remanent flux in the core depends on many factors.
• The most important ones are the magnitude of primary current, the impedance
of the secondary circuit and the amplitude and time constant of any offset
transient.

Remanence
• Since the impedance of the secondary circuit is generally fixed, the magnitude of
remanent flux is governed by the magnitude of the symmetrical component of the
primary current and the magnitude of the offset transient prior to the primary
current interruption.
• Maximum remanent flux can be obtained under conditions whereby the primary
current is interrupted while the transformer is in a saturated state.
• When the current transformer is next energized, the flux changes required will start
from the remanent value.
• If the required change is in the direction to add to the remanent flux, a large part of
the cycle may find the current transformer saturated.
• When this occurs, much of the primary current is required for excitation and
secondary output is significantly reduced and distorted on alternate half cycles.
• The performance of both C and T class transformers is influenced by this
remanence or residual magnetism.
• Relay action could be slow or even incorrect.

Remanence
• The remanence can be corrected by demagnetizing the current
transformer.
• This is accomplished by applying a suitable variable alternating
voltage to the secondary, with initial magnitude sufficient to force the
flux density above the saturation point, and then decreasing the applied
voltage slowly and continuously to zero.
• If there is any reason to suspect that a current transformer has been
subjected recently to heavy currents, possibly involving a large DC
component, it should be demagnetized before being used for any test
requiring accurate current measurement.

Thermal Ratings
• Current transformer continuous ratings can be increased beyond nominal by use of a
continuous thermal current rating factor.
• This factor is defined in ANSI/IEEE C57.13-1978 as "The specified factor by which the rated
primary current of a current transformer can be multiplied to obtain the maximum primary
current that can be carried continuously without exceeding the limiting temperature rise
from 300C ambient air temperature.
• When current transformers are incorporated internally as parts of larger transformers or
power circuit breakers, they shall meet allowable average winding and hot spot temperatures
under the specific conditions and requirements of the larger apparatus".
• Standard rating factors are 1.0, 1.33, 1.5, 2.0, 3.0, and 4.0.
• As an application example, a power circuit breaker with a 1600 amp continuous rating could
use 1200/5 (maximum ratio) current transformers with a thermal rating factor of 1.33. In this
way the current transformer could continuously carry 1600 amps primary and would
therefore not limit the breaker capability.

CT Saturation and DC Offset Current
• MODERN protective devices depend the phasors of the voltage and current signals.
• Any fault-induced dc offset must be removed from the current signal to estimate the
current phasor accurately.
• Since a dc offset is a nonperiodic signal whose spectrum covers all frequencies, the
presence of such a dc offset may result in a phasor estimation error of almost 20%,
depending on the algorithm used.
• It is well known that the saturation of a current transformer (CT) also has an
adverse influence on the estimation of the current phasor.
• Since dc offset itself is one of main causes of CT saturation, dc offset, and CT
saturation should be considered together when estimating the phasor of a current
signal.

CT Saturation and DC Offset Current

Saturation that occurs primarily as a result of the dc offset component is sometimes referred to
as dc saturation.

Introduction to VT
•Many relaying applications like distance relays, directional over current relays require measurement
of voltages at a bus.
• This task is done by a voltage transformer (VT).
•The equivalent circuit of a VT is similar to that of a conventional transformer.
•Typically, the secondary voltage of the VT is standardized to 110 V (ac).
•Hence, as the primary voltage increases, the turns ratio N1:N2 increases and transformer becomes
bulky.
•

CCVT
• To cut down the VT size and cost, a capacitance potential divider is used (fig 8.2).
• Thus, a reduced voltage is fed to primary of the transformer.
• This reduces the size of VT.
• This leads to development of coupling capacitor voltage transformers (CCVT).
• CVTs are typically single-phase devices used for measuring voltages in excess of one
hundred kilovolts where the use of wound primary voltage transformers would be
uneconomical.

CCVT
• The capacitor voltage transformer is more economical than an
electromagnetic voltage transformer when the nominal system
voltage increases above 66 kV.
• The carrier current equipment can be connected via the capacitor
of the Capacitor Voltage Transformers. Thereby there is no need for
separate coupling capacitors.
• Capacitor Voltage Transformers also serve as coupling capacitors for
coupling high-frequency power line carrier signals to the
transmission line.
• CVTs in combination with wave traps are used for filtering high-
frequency communication signals from power frequency. This forms
a carrier communication network throughout the transmission
network.
• Capacitor type VT is used for voltages 66 kV and above. At such
voltages cost of the electromagnetic type, VT’s tends to be too high.

• The capacitors connected in series act like potential
dividers provided the current taken by the burden is
negligible compared with the current passing through the
series-connected capacitors.
• However, the burden current becomes relatively larger and
ratio error and also phase error is introduced.
Compensation is carried out by ‘tuning’.
• The reactor connected in series with the burden is adjusted
to such a value that at supply frequency it resonates with
the sum of two capacitors. This eliminates the error.
•

CCVT
• It is now obvious that Zth due to the capacitance divider, affects the voltage received by the relay.
• To achieve high level of accuracy, it is therefore necessary to compensate for this voltage drop by
connecting a tuning inductor.
• Under line fault conditions, when the voltage drops and there is no threat of exceeding the knee-
point of the magnetizing characteristic of the step-down transformer, a CVT can be represented
by the equivalent linear circuit as shown in Figure.
• A CVT consists of the following components:
• Coupling capacitors (C1 and C2)
• Compensating reactor (L )
• Step-down transformer
• Ferro resonance-suppression circuit

CCVT
• The coupling capacitors of the CVT function as a voltage divider to step down the line
voltage to an intermediate-level voltage, typically 5 to 15 kV.
• The capacitor divider is made up of many series connected capacitor elements, connected
line to ground.
• A tap is brought out at an appropriate voltage level carefully coordinated with the
intermediate transformer to provide the required output voltages.
• The capacitor elements on the high voltage side of the tap are called C1 and the capacitor
elements on the low voltage side of the tap are called C2.
• To provide the reduced level tap voltage there are many more C1 capacitor elements than
C2 capacitor elements.
• In practice, capacitor C1 is often constructed as a stack of smaller capacitors connected in
series. This provides a large voltage drop across C1 and a relatively small voltage drop
across C2.
• The capacitor elements are housed in hollow porcelain or composite insulators filled with an
impregnating fluid.
• A large metal sheet box at the base encloses the tuning coil intermediate transformer.
• In an electrical power substation, Capacitor Voltage Transformer in combination with Wave
Trap is placed at the sending and receiving ends of the substation. At the receiving end, they
are found just after lightning arrester and before line isolator.

Capacitor Voltage Transformer
The adjustment windings are used for factory calibration of the capacitor voltage
transformer and aren’t used for field use.

• The figure above shows the principle of a capacitive voltage divider on which the
capacitive voltage transformer is based.
• The trimming windings are used for fine tuning the output signal to correspond with
the required accuracy class requirements.
• The compensating reactor compensates the phase angle shift caused by the
capacitive voltage divider.

• The base box is filled with dried mineral oil, protecting the components from
environmental deterioration.
• The high-frequency terminal (4) for the PLC signal comes out of one side through a
piece of resin that separates the capacitive unit from the inductive voltage
transformer.
• The medium voltage inductive voltage transformer is immersed in mineral oil and
housed inside a hermetically sealed metallic tank.
• The secondary terminals are located inside a box (7) enabling connections and has
space with protection elements such as fuses or circuit breakers.
• Ferro-resonance is simply and effectively controlled by the utilization of low flux
density designed magnetic circuitry and a saturable reactor controlled damping
circuit connected across the secondary winding. The Ferro-resonance suppression
circuit does not adversely affect the transient response.

Role of Tuning Reactor
• The compensating reactor cancels the coupling capacitor reactance at the system
frequency.
• This reactance cancellation prevents any phase shift between the primary and
secondary voltages at the system frequency.
• The tuning inductor‘s value is so chosen that it compensates for the ‘net C' at power
frequency (50Hz in India).
• The phasor diagram across resistive load, is as shown in fig 8. 4
• From the corresponding equivalent circuit, it is apparent that,
• If
then voltage drop across C is neutralized and the relay sees the actual voltage to be
measured. (See fig 8.5).
The step-down transformer further reduces the intermediate-level voltage to the
nominal relaying voltage, typically 115/√3 volts.

Ferro resonance
• since the capacitance in the voltage divider, in series with the
inductance of the transformer and the series reactor, could resonant
with the external circuit capacitance and reactance.
• Ferroresonance oscillations may take place if the circuit capacitances
resonate with the iron core nonlinear inductance.
• These oscillations cause undesired information transferred to the relays
and measuring instruments.
• This circuit can be brought into resonance that may saturate the iron
core of the transformer by various disturbances in the network.
• At higher system voltages, the resonance phenomenon usually takes
place on fundamental or on sub-harmonic frequencies, resulting in
voltage transformer heating (finally damages) and non-selective
operations of protective relaying may take place
• The modern CVTs are utilizing the so-called “adaptive” damping
circuits.
•

Ferro resonance
• This phenomenon can also overheat the electro-magnetic unit, or lead to insulation
breakdown.
• All capacitor voltage transformers (CVT’s) need to incorporate some kind of
ferroresonant damping,
• Therefore, a ferroresonance suppression circuit (FSC) is normally included in one
of the CCVT windings.
• The circuit consists of a saturable series reactor and a loading resistor. This circuit
is connected in parallel to one of the secondary cores. During ferroresonance
conditions, high voltages appear, saturating the reactor and turning the damping
resistor on to effectively mitigate the parasitic voltage. During normal system
conditions, the reactor presents high reactance, effectively “switching off” the
damping resistor.
• Possible triggering factors for the ferroresonance phenom
• Circuits tuned at power frequency (Lin parallel with C) and a resistance to ground
have been often used as ferrore-sonance suppression circuits because they damp out
transient oscillations and require small amount of energy during steady-state.
• The inductance L is chosen to avoid phase shifts between v input and v output at
power frequency.
• However, small errors may occur due to the exciting current and the CCVT burden
Zb

CCVT in Power Line Communication
• CCVT is also an economical choice when the transmission line is used for power
line communication.
• The capacitance potential divider also serves the dual purpose of providing a shunt
path to high frequency signal used in power line carrier communication.
• High frequency RF signals can be coupled to the power line for communication.
Filtering of this RF signal is carried out by a parallel R-L-C circuit which is also
known as tuning pack.
• At high frequency, the capacitive shunt impedance is very small and hence these
signals can be tapped by the potential divider.
• To block the path to ground for the RF signal, a small drainage reactor is connected
in series with the capacitance divider.
• Normally, the frequency range of this RF signal is 50 kHz-400 kHz. At this frequency
the drainage reactor offers a high impedance block to the RF signal; while for
power frequency (50 Hz) it appears as a path to ground.
• The high inductance of the reactor and the transformer provides a high impedance
path for the RF signal. Hence it prevents any leakage of RF signal into the
transformer output at 50Hz.

CCVT in Power Line Communication
The resonant frequency of the line trap LC network allows the carrier signal to travel
via the path of the Capacitor Voltage Device (CVT) to the radio transceiver.

Ferro Resonance Problem in CCVT
• The compensating reactor and step-down transformer have iron cores.
• Besides introducing copper and core losses, the compensating reactor and step-
down transformer also produce Ferro resonance due to the nonlinearity of the iron
cores.
• CVT manufacturers recognize this ferro resonance phenomenon and include a
Ferro resonance-suppression circuit.
• This circuit is normally used on the secondary of the step-down transformer.
• This circuit is required to avoid dangerous and destructive overvoltages caused by
ferro resonance.
• Unfortunately, it can aggravate CCVT transients.
• Whether or not this suppression circuit aggravates the CVT transient depends upon
the suppression circuit design.

Transient Response of CCVT
• For faults that cause very depressed phase voltages, the CVT output voltage may not closely
follow its input voltage due to the internal CVT energy storage elements.
• Because these elements take time to change their stored energy , they introduce a transient to
the CVT output following a significant input voltage change.
• These energy storage elements cause the CVT transients.
• CVT transients differ depending on the fault point-on-wave (POW) initiation.
• The CVT transients for faults occurring at voltage peaks and voltage zeros are quite distinctive
and different.
• CVT transients reduce the fundamental component of the fault voltage.
• This decrease in the fundamental voltage component results in a decrease in the calculated
impedance.
• If the fundamental voltage reduction is great enough, Zone 1 distance elements undesirably
pick up for out-of-section faults.
• The transient response of a capacitive voltage transformer is the ability to reproduce rapid
changes in the primary voltage.
• It’s defined as the remaining secondary voltage after a specific time due to a short circuit on
the primary voltage.
• Several factors influence this, they are; the equivalent capacitance of the stack, the tap voltage,
the connected burden, and the type of ferroresonant suppression circuit.

• If a fault is within that portion of line protected by a Zone I element, the resulting distance
calculation decrease due to a CVT transient is tolerable; the protective relay should operate.
• However, if the fault is located outside of that portion of line protected by the Zone 1 element
and the CVT transient causes the Zone 1 element to pick up, then this CVT transient is not
tolerable.

• CVTs provide a cost-efficient way of obtaining secondary voltages for EHV
systems.
• They create however, certain problems for distance relays.
• During line faults, when the primary voltage collapses and the energy stored in the
stack capacitors and the tuning reactor of a CVT needs to be dissipated, the CVT
generates sever transients that affect the performance of Protective relays.
• The CVT caused transients are of significant magnitude and comparatively long
duration.
• This becomes particularly important for large Source Impedance Ratios (SIR — the
ratio of the system equivalent impedance and the relay reach impedance) when the
fault loop voltage can be as low as a few percent of the nominal voltage for faults at
the relay reach point.
• Such a small signal is buried beneath the CVT transient making it extremely
difficult to distinguish quickly between faults at the reach point and faults within
the protection zone.
• Electromechanical relays can cope with unfavourable CVT transients due to their
natural mechanical inertia at the expense of slower operation.
• Digital relays are designed for high-speed tripping and therefore they face certain
CVT related problems.
• CVT transients can affect both the transient overreach (a relay operates during
faults located out of its set reach) and the speed of operation (slow tripping for high
SIRs) and directionality.

•
As can be seen in the fig 8.5, CCVT equivalent circuit is a R-L-C circuit.
If transformer is considered ideal, it can be described by integro differential equation of the
type,
1
( )
t
eq
di
v t Ri idt L
C dt

  

The corresponding differential equation is given by

CVT transients differ depending on the fault point-on-wave (POW) initiation.
The CVT transients for faults occurring at voltage peaks and voltage zeros are quite distinctive
and different.
Two CVT transients for zero-crossing and peak fault initiations are shown in the fig.. For
comparison, the ideal CVT voltage output (ratio voltage) is shown in each figure.
Figure 8.7 shows a CVT transient with a fault occurring at a voltage zero.
Also, notice that the CVT output does not follow the ideal output until 1.75 cycles after fault
inception.
Figure 8.8 shows the CVT response to the same fault occurring at a voltage peak. Again, the
CVT output does not follow the ideal output. The CVT transient for this case lasts about 1.25
cycles.

• Each CVT component contributes to the CVT transient response.
• For example, the turns ratio of the step-down transformer dictates how well a CVT isolates its
burden from the dividing capacitors C1and C2.
• The higher the transformer ratio, the less effect the CVT burden has on these capacitors.
• The different loading on the CVT coupling capacitors due to different transformer ratios
changes the shape and duration of CVT transients.
• Increasing the CVT capacitance value can increase the CVT cost but decreases the CVT
transient response.
• Thus, protection engineers must strike a balance between CVT performance and CVT cost.

CVT components affect the CVT transient response
• Two key CVT components affect the CVT transient response
• The coupling capacitors
• Ferroresonance-suppression circuit.
• The high capacitance value in a CVT decreases the CVT transient in magnitude.
• for a fault initiated at a voltage zero with four times higher value of capacitance

CVT components affect the CVT transient response
• Distance elements calculate a fault apparent impedance based on the fundamental components of
the fault voltage and current.
• The fundamental content of the CVT transient determines the degree of distance element
overreach.
• Figure below shows the fundamental components of the same CVT outputs.
• We obtained the fundamental magnitudes by filtering the CVT outputs using a digital band-pass
filter.
• Notice that the fundamental component of the higher capacitance CVT output voltage is closer to
the true fundamental magnitude than that of the lower capacitance CVT.
• Therefore, any distance element overreach caused by a transient output of a higher capacitance
CVT is much smaller than that caused by the transient output of a lower capacitance CVT .

Ferro resonance-Suppression Circuit
• Two types of ferro resonance-suppression circuits.
Active Passive

Active Ferro resonance-Suppression Circuits
• Active ferro resonance-suppression circuits (AFSC) consist of an LC parallel tuning circuit
with a loading resistor.
• The LC tuning circuit resonates at the system frequency and presents a high impedance to the
fundamental voltage.
• The loading resistor is connected to a middle tap of the inductor to increase the resonant
impedance of the circuit.
• For frequencies above or below the fundamental frequency (off-nominal frequencies), the LC
parallel resonant impedance gradually reduces to the resistance of the loading resistor and
attenuates the energy of off-nominal frequency voltages.

Passive Ferroresonance-Suppression Circuits
• Passive ferroresonance-suppression circuits (PFSC) have a permanently connected loading
resistor Rf, a saturable inductor Lf, and an air-gap loading resistor R.
• Under normal operating conditions, the secondary voltage is not high enough to flash over the
air gap, and the loading resistor R has no effect on the CVT performance.
• Once a ferroresonance oscillation exists, the induced voltage flashes over the gap and shunts
in the loading resistance to attenuate theoscillation energy .
• Lf is designed to saturate at about 150% of nominal voltage to further prevent a sustained
ferroresonance condition.

Ferro resonance-Suppression Circuit Design Affects CVT
Transient Response
• The AFSC acts like a band-pass filter and introduces extra time delay in the CVT secondary
output.
• The energy storage elements in the AFSC contribute to the severity of the CVT transient.
• In contrast, the PFSC has little effect on the CVT transient.
• The majority components of the circuit are isolated from the CVT output when ferroresonance
is not present.
• Figure 7 shows the difference of the CVT secondary outputs for a CVT with an AFSC and a
CVT with a PFSC for the same fault voltage.
• Note that the CVT with a PFSC has a better, less distorted transient response than the CVT
with an AFSC.
• This less distorted transient results in a fundamental magnitude that is closer to the true
fundamental magnitude as shown in Figure 8.
• The PFSC has a permanently connected resistor, which increases the V A loading of the
intermediate step-down transformer.
• For the same burden specification, the CVT with PFSC requires a bigger intermediate step-
down transformer .

Ferro resonance-Suppression Circuit Design Affects CVT
Transient Response

Classification of CCVTs
• CCVTs can be classified into following two types:
• Class 1
• Class 2
• Table 8.1 shows the maximum limit for the ratio and phase angle errors.
• It can be seen that errors of Class 2 type are double than that of class 1 type.

FREQUENCY DEPENDENCE OF A CVT
• The fundamental function of the CVT is resonance between the capacitive and
inductive reactance at rated frequency.
• It can therefore not be expected that the CVT will have the same accuracy for
frequencies deviating from the rated.
• The standards, IEC 60044-5, specifies that for a metering class the accuracy shall
be maintained for a frequency variation between 99-101% of rated frequency and
for protection class between 96-102%.
• The sensitivity for frequency variations is dependent on the equivalent capacitance
and the intermediate voltage.
• High values give a lower sensitivity and smaller variations.
• A purely resistive burden will give error variation only for phase displacement and
an inductive burden gives variation both in ratio and in phase.

ERROR VARIATION FOR TEMPERATURE CHANGES
• The temperature characteristic of a CVT is rather complex and only a few factors
influencing the errors of a CVT will be dealt with here.
• The capacitance of the CVD changes by temperature.
• The size of the change depends on the type of dielectric used in the capacitor
elements.
• The relation between capacitance and temperature can be written:
• Where α Temperature coefficient for the capacitor dielectric, ∆T Temperature change
• The variation of capacitance means two different kinds of temperature dependence.
• If the two capacitors C1 and C2 in the voltage divider can get different temperature
or if they, due to design, have different values of α, the voltage ratio of the CVT
changes by temperature.
• This will have an influence on the ratio error
• It is therefore essential that the design is such that all capacitor elements have the
same dielectric and have the same operating conditions.
• CVT’s having C1 and C2 enclosed in the same porcelain have shown very good
temperature stability.

ERROR VARIATION FOR TEMPERATURE CHANGES
• Another temperature dependence is caused by the changes of the capacitive reactance:
• This temperature dependence influences the tuning of the CVT and gives additional
errors of the same kind as for frequency variations and is proportional to the connected
burden.
• Low temperature coefficient is essential to keep these variations small.
• A third remaining factor influencing the errors, is the change of winding resistance
• in the tuning inductor due to temperature variations.

LEAKAGE CURRENTS AND STRAY
CAPACITANCE
• The capacitor voltage divider is normally built up from a number of series
connected porcelain sections.
• Pollution on the external surface of the porcelains will be equivalent to a parallel
resistance to the capacitor elements and an uneven distribution of pollution between
sections can therefore be expected to have an influence on the accuracy.
• It can though be shown that the higher capacitance in the divider, the smaller
influence on accuracy.
• Measurements on CVTs in dry, wet and polluted conditions have shown that the
influence on ratio and phase errors is very small and can be neglected for CVTs
having a high capacitance.
• In a substation where there is stray capacitance to other objects an influence on
accuracy can be suspected.
• A high capacitance is advantageous in this respect and practical experience show
that this is not a problem in outdoor substations.

COUPLING-CAPACITOR INSULATION COORDINATION
• The voltage rating of a coupling capacitor that is used with protective relaying
should be such that its insulation will withstand the flashover voltage of the circuit
at the point where the capacitor is connected.
• The flashover voltage of the circuit at the capacitor location will depend not only on
the line insulation but also on the insulation of other terminal equipment such as
circuit breakers, transformers, and lightning arresters.
• However, there may be occasions when these other terminal equipments may be
disconnected from the line, and the capacitor will then be left alone at the end of the
line without benefit of the protection that any other equipment might provide.
• For example, a disconnect may be opened between a breaker and the capacitor, or a
breaker may be opened between a transformer or an arrester and the capacitor.
• If such can happen, the capacitor must be able to withstand the voltage that will
dash over the line at the point where the capacitor is connected.
• Some lines are overinsulated, either because they are subjected to unusual insulator
contamination or because they are insulated for a future higher voltage than the
present operating value.
• In any event, the capacitor should withstand the actual line flashover voltage unless
there is other equipment permanently connected to the line that will hold the
voltage down to a lower value.

COUPLING-CAPACITOR INSULATION COORDINATION
• At altitudes above 3300 feet, the flashover value of air-insulated equipment has
decreased appreciably. To compensate for this decrease, additional insulation may
be provided for the line and for the other terminal equipment.
• This may require the next higher standard voltage rating for the coupling capacitor,
and it is the practice to specify the next higher rating if the altitude is known to be
over 3300 feet.

Standard Withstand Test Voltages for Coupling Capacitors

Why non-conventional instrument
transformer(NCIT)
• The conventional instrument transformer is an inductive transformer.
• The basic characteristics of its components determine its limits.
• Taking into consideration the maximal flux density and lack of linearity in the excitation
curve of the iron core, these characteristics will restricts the size and the weight of the
transformers and also the range of applications.
• Accuracy is more significant

What is a non-conventional instrument transformer(NCIT)
• Today a number of different sensors and technologies are used which are gathered under the
general umbrella of non-conventional instrument transformers.
• They range from traditional cores and dividers having an output at lower levels than present
day standards, via air core coils to optical units employing electro- or magneto optical effects.
• Each technology has its advantages and disadvantages.
• The future will probably show that each different technology will be used in the areas where
their specific advantages are useful.
• Today non-conventional current and voltage transformers have achieved high performance.
• In addition, the digital output complies with the most stringent requirements of protective
relays and meters.
• The designs take into account the harshest environmental conditions of temperature,
vibrations and electromagnetic compatibility.
• Another typical feature of modern non-conventional transformers is that the measured signal
in many products is transmitted through an optical fiber from the high potential level to the
instrumentation potential in the substation.
• This significantly reduces the insulation problem associated with traditional transformers.
• They are also called electronic transformers since their hardware is based on electronics.
• have no iron core and have three types of output signals: low energy, high energy, and digital
outputs.
• better transient response due to the lack of an iron core that limits the bandwidths.

Advantages
• Measurement accuracy and protection dynamic range.
• No saturation, ferroresonance or unwanted transients.
• Safety: no risk of explosion, no wired CT secondary circuit.
• Cost reduction: compact CT and VT designs - avoid kilometers of substation wiring.
• IEC 61850-9-2 digital substation.
• Short delivery lead times
• With non-conventional instrument transformers, it is possible to route the signals over a very
long distance.
• It is possible to collect signals in a so-called merging unit which can be read out via remote
access
• Loss reduction
• Space-saving design :
• Installing NCITs in place of conventional equipment reduces the total weight of equipment
and minimizes space requirements of the switchgear.
• Reduced copper cabling:
• The optical connections between NCITs and merging units and the fiber optic Ethernet
network of the process bus, eliminate parallel copper wiring for current and voltage
measurement.
• This reduces not only the number of copper cables in the substation, it also allows the use of
narrower cable trenches.
• Additional environmental benefits :
• The required volume of SF6gas is considerably reduced.
• On-site gas handling can also be reduced, because the NCIT can be installed in the factory
and it does not need to be dissembled for HV testing.

IEC 61850
• While NCITs have been used in medium voltage installations for many years, the absence of a
digital interface standard has slowed the introduction NCITs into the High Voltage (HV)
transmission domain.
• One of the new international standards IEC 61850-9-2, first released in 2004, is providing a
digital interface standard for connection of NCITs to revenue meters, protection and control
equipment, thus providing equipment manufacturers and system integrators a sound basis for
the development of NCIT with electronic interfaces.
• IEC61850 is the new standard for communication networks and systems in substations.
• The IEC 61850 standards are based around the same Ethernet that is used for conventional
office and commercial networking.
• The objective of the standard is to design a communication system that provides
interoperability between the functions to be performed in a substation but residing in
equipment (physical devices) from different suppliers, meeting the same functional and
operational requirements.
• IEC 61850-9-2 specifies a standard communications mapping for Sampled Values (SV).
• The transmission of sampled values according to IEC61850-9-2 LE, through a fiber optic
network, guarantees permanently supervised communication that is immune to electro-
magnetic disturbances and virtually independent of the cable length.

OPTICAL INSTRUMENT TRANSFORMER
• In recent years, electric utilities have been evaluating optical sensors to measure current and voltage.
• These devices are proving their value, especially in applications where accurate measurement over wider
dynamic range, ability to retrofit, and improved safety are of main concern.
• They are well suited for the advanced functionality of leading-edge protective relays and meters and for
compatibility with digital communications in modern substations.
• The NxtPhase NXVCT optical voltage and current sensor, for example, combines voltage and current
sensing (protection and metering) in a single instrument for each of several voltage classes over the range
from 115kV to 765kV.
• Primary advantages of this optical technology over conventional inductive and capacitive measurement
transformers include:
• High accuracy (exceeds IEC Class 0.2 and IEEE Class 0.3 accuracy requirements)
• Wide dynamic range
• High bandwidth
• uses less wiring (all signals are transmitted through a standard fiber optic cable), requires less
maintenance, is easy to install with greater flexibility (bus mounting, horizontal mounting or direct
assembly on the circuit breakers) and easy to upgrade.

• Reduced size and weight
• Safe and environmentally friendly (avoids oil or SF6)
• Low maintenance
• Immunity against electromagnetic interferences (EMI);
• Electrical isolation (the optical sensors are made of dielectric materials);
• Possibility for measuring AC and DC;
• Absence of saturation effects;
• Low power consumption;
• relatively low cost
• During fault conditions a well-known phenomenon called “saturation” occurs in
conventional CTs; the iron core in a transformer “saturates” when high fault
currents induce a large magnetic field.
• In effect, the transformer can no longer accurately represent the primary current in
the current transformer secondary.
• Utilities must therefore select oversized CT ratios in order to avoid false relay
operation.

• An optical CT does not face the same saturation challenges. It uses light travelling
through glass (an optical fibre in the case of the NXCT) to measure the magnetic
field around a current-carrying conductor, which gives a measure of the current
flowing in the conductor.
• If configured correctly, the optical voltage and current sensor has the ability to
measure fault currents exceeding 400 kA peak.
• Additionally, using advanced techniques, both AC and DC currents can be
measured accurately throughout this range.
• Furthermore they are usually connected to optical fibers that have large
communication bandwidths, and due to their very low absorption loss allow remote
detection, high multiplexing capability and data transmission over long distances.
• Also, optical current sensors measure the magnetic field generated by the electric
current rather than the current itself, thus avoiding the electric hazards that the high
voltage measurements imply.

Digital Optical Instrument
Transformers
• DOIT, Digital Optical Instrument Transformers are used for measurement of current
and voltage in power systems.
• Applications are for protection, metering, control and power quality supervision.
• The Instrument transformers combine traditional measuring techniques with digital
optical signal transmission, allowing a purely non-conducting connection between
the transducer part in the switchyard and the interface part in the control room.

Operating Principles of DOCT
• The DOCT, Digital Optical Current Transformer consists of a transducer in the
primary circuit connected by an optical fiber to the interface unit in the control
room.
• In the transducer, the current value is measured with a magnetic current
transformer, a shunt or a Rogowski coil. After sampling and conversion into digital
form by the DOIT electronics, the current value is transmitted as an optical signal
in the fiber to the interface in the control room.
• Power to supply the DOIT electronics is simultaneously transmitted as laser light
from the interface to the transducer, using the same or a separate optical fiber.

• In the DOVT, Digital Optical Voltage Transformer a capacitive or resistive voltage
divider is used to measure the voltage.
• The values are sampled and transmitted to the interface unit in the control room
using identical opto-electronics as in the DOCT.
• The transfer function is better than with conventional voltage transformers and the
risk of ferro resonance is eliminated.

optical sensors
• Conventional high DC current transformers utilize the Hall effect to measure the
magnetic field around the current carrying bus bars.
• Over the years Hall effect based DC current transformers have become very
accurate and reliable.
• High precision transducers, working with magnetic flux compensation, are complex
systems incorporating a magnetic core around the current-carrying bus bars, a
number of Hall elements in air gaps of the core, solenoids to nullify the primary
magnetic field in the core and at the Hall elements, and high gain current
amplifiers to generate the solenoid currents.
• However, they are rather complex systems and demand intricate installation and
commissioning procedures.
• Often an analysis of the magnetic field distribution is necessary in order to place
the transducer head such that errors due to asymmetries in the field and cross-talk
from neighbour currents are minimized.
• Conventional systems weigh up to 2000 kg and consume up to 10 kW of power.
• Special care must be taken to avoid erroneous output due to asymmetric field
distributions and disturbances from neighbor currents or bus bar corners.

optical sensors
• In the fiber-optic current sensor a simple loop of optical fiber around the bus bar replaces
the sophisticated head of the conventional transformer.
• The sensor perfectly integrates the magnetic field along the closed path described by the
sensing fiber.
• As a result, the signal is independent of the particular magnetic field distribution and
only determined by the enclosed current.
• All currents outside the fiber loop are of no influence.
• Sensor placement is therefore uncritical.
• The simplicity of the system reduces the time required for installation and
commissioning to a few hours.
• The sensor overcomes the drawbacks of the classical transducers and offers superior
performance and functionality.

optical sensors
• There are basically two linear effects by which the magnetic field can be measured
by optical sensors: magneto-optic effect (or Faraday effect) and magnetic force (or
Lorentz force).
• The principle of magneto-optic effects is based on the interaction between magnetic
field and the phenomenon of light refraction and reflection in transparent medium
and on its surface .
• Three basic magneto optic effects are know
• Cotton-Mouton effect,
• Kerr surface effect ,
• Faraday effect.
• The most important for current sensor application is Faraday magneto-optic effect.
• Faraday effect causes the electromagnetic wave polarization rotation due to the
magnetic field intensity in transparent material.

• In Faraday Effect current sensors; the current flowing through a conductor induces a magnetic
field that affects the propagation of light traveling through an optical fiber encircling the
conductor. The rotational angle is proportional to the intensity of the magnetic field.
• A linear state of polarization rotates in the presence of a magnetic field because the field
produces a circular birefringence in the glass.
• Birefringence refers to an optical material with two indices of refraction.

Faraday effect
• The sensor makes use of the Faraday effect in the fiber.
• The Faraday effect is the phenomenon that in a medium such as glass right and left
circularly polarized light waves travel at different speeds if a magnetic field is
applied along the propagation direction .
• As a result the waves accumulate a path difference δL or equivalently a phase
difference δΦf = 2 V L H.
• Here, V is a material constant (Verdet constant), L the length of the rod, and H the
magnetic field.

Faraday effect
• The angle of rotation of linearly polarized light is proportional to the strength of the magnetic
field and the cosine of the angle between the field and the propagation direction of the light
wave.
• This rotation can be expressed mathematically by:
• where, V is the material Verdet constant, which is both dispersive and temperature-dependent.
• B is the magnetic flux density vector .
• dl is the differential vector along the direction of propagation.
• This effect is called the Faraday effect or linear magneto-optic effect and can be used to build
optical current sensors.
• Fig. below illustrates the polarization rotation due to a parallel external magnetic field on a
magneto-optical material, such as, glass.
Faraday effect in linearly polarized light

optical CT structure
The magnetic field due to the current carrying conductor changes the velocities of
circularly polarized light waves that travel around the conductor.
Light from a light source, mainly light emitting diode (LED), enters into an optical fiber
polarizer.
The light is polarized and then splits into two orthogonally polarized light waves in the
polarizer and pass through a modulator.
A fibre gyroscope module is employed for interrogation.
The two forward propagating light waves, emerging with parallel linear polarizations
from the lithium niobate phase modulator of the gyro module (grey box), are combined
to orthogonal waves in a polarization maintaining fibre coupler.

• The coupler has a 90°-offset in the fibre orientation at the splice of one of its two entrance
leads.
• Two light waves with orthogonal linear polarizations travel from the optoelectronics module,
which includes a semiconductor light source, via an interconnecting fiber to the single-ended
sensing fiber.
• The sensing fiber forms an integer number of loops, N, around the bus bar.
• A single loop is commonly sufficient for high dc currents.
• At the entrance of the sensing fiber a fiber-optic phase retarder converts the orthogonal linear
waves into left and right circularly polarized light.
• A short section of elliptical-core fiber acts as a quarter wave retarder.
• The circular waves travel through the coil of sensing fiber.
• At the coil end the light waves are reflected and then retrace the optical path with swapped
polarizations.
• The returning orthogonal waves are split at the coupler into the upper and lower branches of
the circuit.
• The waves polarized parallel to the transmission direction of the polarizing modulator, having
travelled along reciprocal paths (vertical polarizations in Fig.), are brought to interference.

• The MOCT consists mainly of electronic and optic part.
• Light from a source enters an optical fibre polarizer, which splits into two linear orthogonally
polarized light.
• Light enters into the sensing head that includes a quarter wave plate .
• Quarter wave plate splits the two linear orthogonal waves into right and left hand circularly
polarized waves.
• Two waves travel at different speeds around the sensing fibre.
• The difference in speed is proportional to the strength of the magnetic field.
• Two waves reflects of a mirror and circular polarization of two waves gets reversed .
• They travelled in the opposite direction of the magnetic field.
• Finally ,two waves again reach the quarter wave plate and linear polarization gets regained.
• Circular polarized lights become linearly polarized, but x-polarization returns in the y-
polarization state and vice-versa.

• The polarization directions of the returning waves are swapped with respect to the forward
propagating waves due to the reflection.
• The waves with nonreciprocal paths (horizontal polarizations) are blocked.
• In addition, these waves are prevented from interference by means of the delay loop in the
lower branch.
• Since the circular waves travel at somewhat different speeds through the sensing fiber if a dc
current, I, is flowing.
• The two returning light waves have accumulated a phase difference given by δΦf = 4 VNI.
The phase difference is proportional to the line integral of the magnetic field along the sensing
fiber and is therefore a direct measure for the current.
• The returning waves are brought to interference in the optoelectronics module.
• The signal processor then converts the optical phase difference into a digital signal.
• A particular advantage of operating the sensing coil in reflection is, besides the simplicity of
the arrangement, the fact that the sensor signal is largely immune to mechanical perturbations
such as shock and vibration.
• While the non-reciprocal Faraday optical phase shifts double on the ways forward and
backward, phase shifts caused by mechanical disturbances are reciprocal and cancel each
other.

• The minimum detectable optical phase shift is a few micro rad.
• With a single loop of fiber a phase shift of 5 micro rad corresponds to a current of about 1A.
• The maximum detectable phase shift is 2 Pi rad and corresponds to a current of 600 kA.
• The bare sensing fibre (diameter 80 mm) resides in a thin capillary of fused silica (inner
diameter 530 mm, inset in Fig.).
• The capillary protects the fibre from external stress.
• The capillary contains a lubricant to avoid internal friction during handling and is embedded
in a thin strip of fibre- reinforced epoxy.
• The strip serves as a robust protection of the capillary and, with the aid of appropriate
markers, allows one to perfectly and reproducibly close the fibre coil, that is to install the coil
such that the retarder and reflector coincide resulting in a perfect closed-loop integration of
the magnetic field.
• The packaged sensing fibre is accommodated in a modular housing, consisting of segments of
fibre reinforced epoxy. The housing is mounted to the current-carrying bus bars.
• The magnetic field on the sensing head is the only physical quantity that affects the difference
between the two light waves.
• This process is called as Faraday effect, and the difference is proportional with the amount of
current passing though the conductor.

• NXCT is divided into four separable elements:
• The opto-electronics chassis, the fiber optic cabling, the sensor head, and standoff.
• The optoelectronics chassis incorporates all the electronics as well as the light source and
optical components up through the modulator.
• This chassis is located in the control room.
• One interesting feature of the fiber current sensor is that the dynamic range of the sensor can
be scaled to fit almost any application simply by changing the number of fiber turns on the
sensor head.
• The first prototypes use four turns of sensing fiber, which allows the sensor to reliably detect
currents over range of 100 mA to 100 kA.
• This range covers the majority of requirements of high voltage metering and relaying
applications.

Sensor heads for
different rated currents
(top) and electronics
(bottom). The outer
diameter of the smallest
head is about 90 cm.

Sensor head
The sensor head is connected to the sensor electronics via a glass fiber cable.
Example of FOCS measuring system for DC
The FOCS sensor head housing is modular
and lightweight.
FOCS maintains high accuracy even in strongly
inhomogeneous magnetic fields.

Comparison of hall effect and fiber-
optic DC current transformers
• Hall effect and optical current transformers are both highly accurate.
• However, the optical sensor offers a number of important advantages
• The high accuracy of the optical sensor is maintained over a very wide operating range of currents.
• The perfect line integration of the magnetic field eliminates erroneous output in case of angled
conductor arrangements, inhomogeneous magnetic fields, and strong neighbor currents.
• The sensor is able to handle bi-directional currents and magnetic fields.
• A local reversal in the field direction, caused by high neighbor currents, does not lead to errors.
• Local field enhancements will not cause saturation.
• The good field integration results in more flexibility in the choice of the sensor placement. Even a
small space on the busbar is sufficient to install the sensor.
• The large bandwidth enables the detection of current ripple and recording of transients.
• The complexity of the sensor head and thus the risk of faults are significantly reduced.
• The sensor electronics is galvanically isolated from the sensor head.
• Both, digital and analog outputs are available to perfectly fit into today’s de-centralized industrial
automation technologies.

Moct - Magneto-optic Current Transducer
• The MOCT system satisfies current sensing needs for revenue metering or protective
relaying in a wide variety of applications. (ABB make).
• The Magneto-Optic Current Transformer for Protection (MOCT-P) is a passive optical
current transducer which uses light to accurately measure current on high voltage
systems.
• The MOCT-P system is suitable for outdoor application and has a continuous current
rating up to 3150 A with an accuracy limit factor of 40 x.
• It meets the protection class accuracy 5TPE, according to IEC 60044-8.
• The optical design enables accurate reproduction of fully offset fault currents with
decaying d.c. component without saturation or other source of distortion.
• The MOCT-P system provides a 200 mV voltage output for use with protective relays.
• Three phases of MOCT-P sensors mounted on polymer insulator columns with
predominated fiber optic cable in the insulator.
• Fiber optic cables for transmission of the light signals between the optical sensors and
the MOCT-P electronic module.
• The MOCT is a current measuring device based on the magneto-optical Faraday
• effect.

• This effect explains the rotation of the plane of polarization of a linear beam of light
in certain materials that become optically active under the presence of a magnetic
field.
• If the magnetic field and light propagation directions coincide, the angle of rotation
(q) is proportional to the magnetic flux density (B), the length of the path (l) and a
constant named Verdet constant (V), which is a property of the material.
• In the equation below I is the current flowing through the conductor and m is the
permeability of the material.

MOCT optical sensor system
• Light is emitted by an LED and transmitted through multi mode optical fiber to the rotator
installed at high voltage.
• The light is polarized as it enters the sensor. It then travels around the conductor inserted
through the opening on the rotator and exits through an analyzer.
• The analyzer is oriented 45degree with respect to the polarizer.
• Subsequently, the light is transmitted back through another optical fiber to the electronic
module where it is converted into an electric signal by a photodiode.
• The signal processing module and precision amplifier circuit provide an analog 1.0 A output
current which is proportional to the primary current flowing through the conductor.

Voltage measurement and the Pockels effect
• The propagation of optical radiation through certain materials while in the presence of an
electric field demonstrates an interesting phenomenon known as the electro-optic effect.
• The effect is defined as a deformation of the refractive indices of a material due to an applied
electric field.
• This deformation gives rise to a change in the way polarized light behaves while propagating
through the material.
• The relationship between the electric field and the change inrefractive index primarily takes
two forms, the linear and quadratic.
• In the linear case the change in the refractive index is proportional to the strength of the
electric field (Pockels effect)
• whereas the change in the index of refraction is proportional to the square of the field strength
in the quadratic case(Kerr effect).
• the Kerr and Pockel’s effects are the electrooptical phenomena used for the DC and AC
voltage measurement.
• The Kerr phenomenon is observed in polar liquids and requires very high electric fields.
• The electrooptical response is nonlinear (quadratic).

• Practical applications of Pockels cells primarily include electro-optic modulators and electro-
optic sensors.
• When used as a sensor, specific characteristics of the transmitted light are measured to
determine the unknown electric field applied to the Pockels cell.
• Once the electric field is known, other quantities such as voltage and current can be readily
determined.
• Unfortunately, Pockel’s effect devices are expensive and need an external laser beam source.
• The Pockels effect can be used in two modes.
• When the applied electric field is normal to the direction of propagation of the incident
light,the transverse Pockels effect is said to occur.
• When the applied field and the propagation direction are parallel, the longitudinal Pockels
effect is said to take place
• Most optical voltage sensors are based on an electro-optic (EO) crystal and longitudinal
Pockels effect.
• The Pockels material used in electro-optic voltage sensors is key in developing a functional
and accurate electro-optic sensor.
• Bulk electro-optic crystals work well and are easily incorporated as Pockels cells if the
appropriate crystal is selected.
• Materials such as KH2PO4(KDP) and LiNbO3are very common choices due to the large
Pockels effect they demonstrate.

• Light transmission to and from the region of voltage measurement is done by optical fibers
which bring inherent immunity to electromagnetic interference and compatibility with low
voltage analog and digital equipment used for metering and relaying applications.
• Works based on the principle that the electric field changes the circular polarization to an
elliptical polarization.
• The voltage sensor operates using a variation of the linear, longitudinal mode electro-optic
Pockels effect referred to as the Quadrature Pockels cell.
• This effect occurs in crystalline materials that exhibit induced birefringence under applied
electric field.
• Linearly polarized light propagating the crystal parallel to the electric field will experience
phase retardation between its components in the slow and fast axis.
• The phase retardation is due to the difference in the velocity of propagation of the light and is
related to different refractive indices between these axes.
• The voltage sensor consists of a crystal placed between high voltage and ground.
• The light emitted by a source is transmitted through multi mode optical fiber to the sensor.
• The beam of light is polarized as it enters the sensor, then it propagates through the crystal in
the direction of the electric field.

• NXVT sensors are placed in a post-type high voltage composite insulator. The post insulator
is built with a fiberglass tube, which is covered on the outside by rubber sheds.
• Inside the insulator tube is a smaller tube, which is a hollow cylindrical resistor used for
shielding.
• Three Pockels cell-based sensors are placed in the inner tube. Dry nitrogen is used for
insulation.
• Two electrodes are placed at the ends of this structure: a high voltage electrode at the top
(connected to the line) and a ground electrode at the bottom
• The voltage on the line creates an electric field between the line and the ground.
• This field is used by Pockels cell-based sensors to measure the voltage
• The effect of the external field is eliminated by the resistive inner tube providing permittivity
shielding.

• Three electric field sensors are located in the inner tube: one in the middle, one near the high
voltage, and one near the ground electrode.
• These sensors are connected to the optoelectronic system by fiber optic cables through the
inner tube.
• The location of these sensors has a crucial importance to the accuracy of voltage
measurement.
• A numerical integration formula(Quadrature Method) is used to define the sensor locations. T
• his method also minimizes the stray field effect for the electric field sensors, caused by the
external electric field.
• The number of electric field sensors is optimized and defined as three for NXVT.
• Both resistive shielding and the Quadrature method help to reduce the electric field effect
caused by neighboring phase voltages.

Current and Voltage Transformers _23_10_2021.pptx

Current and Voltage Transformers _23_10_2021.pptx

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