Introduction to mv switchgear

Introduction to MV
EQUIPMENTS

Prepared by Triyanto Limantoro

Introduction to MV equipments
Basic Magnitude in MV switchgear :
Voltage
Current
Frequency
Short Circuit power
Voltage, rated current and rated frequency:
Single line diagram / Specification
to define the dielectric withstand of the components such
as: CB, insulators, CTs,VTs,etc
Short circuit power :
to choose various parts of a switchgear: withstand
against temperature rises and electro dynamic force.

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- Industrial Division Σ July 2009

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VOLTAGE
Operating/Service Voltage U (kV):
Voltage across the equipment terminals.
example : 22kV, 3.3kV,…
Rated Voltage Ur (kV) : (nominal Voltage)
Max rms (root mean square) value of the voltage that
equipment can withstand under normal operating conditions.
Rated voltage (Ur) is always greater than the operating
voltage.
Rated voltage associated with an insulation level
Examples : Rated voltage 7.2kV, 17.5kV, 12kV and 24kV

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VOLTAGE
Insulation level Ud (kV rms, 1 minute) and Up (kV peak)
Definition: the electric withstand of equipment to switching under operation
over voltages and lightning impulse.
Ud: Over voltage due to of internal switchgear, which accompany all
changes in the circuit: opening/closing CB or Switch, breakdown or shorting
across an insulator, etc…
Simulated in laboratory/factory by the power-frequency withstand voltage for 1 minute.
Example : Ur : 24kV

Ud : 50kVrms/1 min.

Up: over voltage of external switchgear or atmospheric origin
occur when lightning falls on or near a transmission line.
Simulated in laboratory by the lightning impulse withstand voltage.
Examples : Ur : 24kV

Up : 125kVp

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IEC Standard Voltage

20 7.2
28
38
50

12
17.5
24

60

1.2/50us 50Hz

75
95
125

70

36

170

Ud

Ur

Up

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Standard
Schneider MV equipment is conformity with list 2 of the series 1
table IEC 60 071 and 60 298.
Rated
Voltage

kV rms
7.2
12
17.5
24
36

Rated lightning
Rated powerimpulse
frequency
withstand
withstand voltage
1.2/50us 50Hz
voltage
.
kV peak
1minute kV rms
list 1
list 2
40
60
20
60
75
28
75
95
38
95
125
50
145
170
70

Normal
operating
voltage

kV rms
3.3 to 6.6
10 to 11
13.8 to 15
20 to 22
25.8 to 36

Insulation level apply to MV swgr at altitudes of less than 1000
meters, 20 deg.C, 11 g/m3 humidity and press of 1.013 mbar.
Above this ,derating should be considered.

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Derating of the switchgear related to the altitude

2500

Altitude 2500 m

k is equal to 0.85

Impulse withstand of the switchboard must be :125/0.85 = 147.05 kV
Power frequency withstand 50 Hz must be 50/0.85 = 58.8 kV

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Standard
Insulation level corresponds to a distance in air which
guarantees withstand without a test certificate.
Rated
Voltage

kV rms
7.2
12
17.5
24
36

Rated lightning
impulse
withstand voltage
1.2/50us 50Hz
.
kV peak
60
75
95
125
170

Rated powerfrequency
withstand
voltage
1 minute kV rms
20
28
38
50
70

Distance live
to earth in air
.

cm
9
12
16
22
32

lower than this distance, we need simulation/test in the
laboratory to check lightning impulse withstand voltage. Or using
additional insulation material such as heatshring, screen,etc
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Current
The rms value of current that equipment can withstand
when current flow without exceeding the temperature rise
allowed in standards.
Temperature rises authorized by the IEC according to the
type of contacts.

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OPERATING CURRENT : I (Ampere)
Calculate from the load power.
Actual current passes through the equipment.
• generally customer provide its value
• calculate if we know the power of the load
Exercise:
A switchboard with a 630kW motor feeder and a 1250kVA
x’mer feeder at 5.5kV, cos ϕ = 0.85 and motor efficiency η =
90%
How many ampere the operating current of Transformer
and Motor?
In motor = 86.44 A
The answer
In Trafo = 131.22A

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Short Circuit Current
Short circuit power depends on :
Network configuration
(exp: single source, parallel source, network, generators)
Impedance of each equipments or devices.
(exp: lines, cables, transformers, motors)
Power short circuit is maximum power that network or source can deliver
to an installation during a fault,
expressed in MVA or in kA rms at operating voltage.
Exp: Psc = 500MVA @ 20KV or Isc : 31.5kA rms
Determination of the short-circuit power requires analysis of the power
flows feeding the short circuit in the worst possible case.
What is short circuit level for 500MVA @ 20KV ?
14.43kA
Answer
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Short Circuit Current

D

E

Isc at main busbar D when bustie D4 close?
Isc at the outgoing feeder E?
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Minimum short-circuit current: Isc (kA rms.)
Corresponds to a short circuit at one end of the fault point.
This value allows us to choose the setting of thresholds for over
current protection devices(F50/F51) and fuses
Example: Isc: 23 kA rms

Ith
source

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Isc

load

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Maximum short-circuit current: Ith (kA rms. 1 s or 3 s)
Corresponds to a short circuit in upstream terminals of the
switching device,
express in : kA for 1s or 3 s
thermal withstand of the equipment = Ith
Example: Ith: 31.5 kA rms. 1 s or 3 s
It
h
source

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Isc

load

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Peak Value of the max. short circuit current (kA peak)
Value of the initial peak in the transient period
I dynamic (kA peak) is equal to :
2.5 x Isc at 50 Hz (IEC)
2.6 x Isc at 60 Hz (IEC)
2.7 x Isc (ANSI) times the short circuit current calculated at a given
point in the network.
Example: Isc :

25kA

Idyn: 2.5 x 25= 63.75kA peak (IEC 60 056)
Idyn: 2.7 x 25= 67.50kA peak (ANSI), 25kA at
a given point

This value determines the breaking capacity and making (closing)
capacity of CBs and Switches, as well as the electro dynamic withstand of
busbars and switchgear.
Isc value based on IEC: 8 – 12.5 – 16 – 20 – 25 – 31.5 – 40- 50 kA rms

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Frequency fr (Hz)
2 different frequency use in the world:
50 Hz in Europe
60 Hz in the USA
several countries use both frequencies indiscriminately
Instrument Voltage Transformer rated 50 can operate at 60Hz
Instrument Current Transformer rated 50 can operate at 60Hz.
But CT with rated 60Hz can not be operated at 50Hz.

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Introduction to MV equipments
Electrical network can be disconnect, protect and
control by using AIS SWITCHGEAR :
AIR INSULATED SWITCHGEAR (AIS)
METAL enclosed switchgear divided 3 types:
Metal clad : example: MC set,NEX
Compartmented : example: SM6
Block : example Interface/joggle cubicle.

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DIFFERENT ENCLOSURE TYPE (AIS)

LSC2B
metal clad

LSC2A
compartment

GIS

LSC1
Block type
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MV Switchgear to IEC 62271-200
Fully enclosed in metal enclosure and having some current carrying capacity
Loss of Service
Continuity
Class (LSC)
• Architecture based on
“safe compartment
access”

Several levels of
service continuity
during maintenance

LSC 2B
Maintainability of
defined parts
with no need of
cable
disconnection
(separate cable
compartment)

• Safe access to
compartment
• With power flow in busbar
and the other units
• MV Cable in separate
compartment
• Cable of unit under
maintenance can remain
energized

LSC 2A
• Safe access to
compartment
• With power flow in
busbar and the other
units
• MV Cables must be
earthed

Maintainability of
one functional
unit allowing
normal service of
the remaining
units of the
switchboard
(busbar in a
separate
compartment)

MOTORPACT

LSC 1

MCSET or NEX

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• Metal enclosed
not of LSC2 class

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Fully enclosed in metal enclosure and having some current carrying capacity
Partition Class
I or M
• Classification based
on electrical field
presence in safe
access compartment

Partition Class PM
Personnel
comfort during
maintenance

• All partitions and shutters
of safe access
compartment shall be
metallic with some current
carrying capacity

“Metal enclosed”
compliant during
maintenance
Applicable mainly
to withdrawable
system

MCset or PIX
3.110 Shutter
Part of metal-enclosed switchgear and
controlgear that can be moved from a
position where it permits contacts of a
removable part, or moving contact of a
disconnector to engage fixed contacts,
to a position where it becomes a part of
the enclosure or partition shielding the
fixed contacts.

Partition Class PI
• Partitions or shutters may
be partially or totally of
Definition insulating material

Electrical and
mechanical
safety according
to IEC 60466 or
60137

Motorpact

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Fully enclosed in metal enclosure and having current carrying capacity
Internal Arc Class
IAC
• Classification based on
consequences of internal arc on
personnel safety

Accessibility Types
• A : restricted to authorized
personnel only.
• B : unrestricted, including
general public.

IAC classified
Personnel
safety in case • No projection of parts
of internal arc towards accessible
sides
• No ignition of indicators

Motorpact
complies with
AFLR type

Safety in case of
internal fault
during service
condition
Demonstrated by
type tests
(completely
defined by the
standard)

IAC not classified
• No tests performed to
assess behavior of
enclosure under arc
conditions

• Enclosure Identification code:
F - for Front side
• L L - for Lateral side
R - for Rear side

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SWITCHGEAR FUNCTION

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STANDARDS DISTRIBUTION
FEEDERS (AIS)
The MCset range meets the following international standards:
62271-1 : clauses common to high voltage switchgear
62271-200 : metal-enclosed switchgear for alternating current at
rated voltages of between 1 and 52 kV
IEC 62271-100 : high voltage alternating current circuit breakers
IEC 60470 : high voltage alternating current contactors
IEC 60265-1 : high voltage switches
IEC 60282-2 : high voltage fuses
IEC 60271-102 : alternating current disconnectors and earthing
switches
IEC 60255 : measurement relay and protection unit for the
applicable parts
IEC 60044-1 : current transformers
IEC 60044-2 : voltage transformers
IEC 60044-8 : electronic current transformers (for LPCT).

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STANDARDS MOTOR
STARTER / MCC (AIS)
Motorpact meets IEC standards
IEC 62271-1 High-voltage switchgear and controlgear – Part 1: Common
specifications
IEC 62271-200 AC metal-enclosed switchgear and controlgear for rated
voltages above 1 kV and up to and including 52 kV
IEC 60470 High voltage alternating current contactors and contactor
based motorstarters
IEC 60282-1 High voltage fuses: limiting fuses
IEC 62271-102 Alternating current disconnectors and earthing switches
IEC 60044-1 Instrument transformers - Part 1: current transformers
IEC 60044-2 Instrument transformers - Part 2: inductive voltage transformers
IEC 60044-8 Instrument transformers - Part 8: electronic current transformers
IEC 61958 High-voltage prefabricated switchgear and controlgear
assemblies - Voltage Presence Indicating Systems
IEC 60076-11 dry-type transformers
Other specifications
IACS International Association
of Classification Societies

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STANDARDS DISTRIBUTION
FEEDERS (GIS)

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SF6 and Vacuum
SF6 is used for insulation and breaking functions:
That is the only used technique for all voltages, in
secondary distribution (switches, RMU) and in high voltage
up to 800 kV.

Vacuum is limited to the breaking function and only in
medium voltage (mainly up to 36 kV):
The vacuum bottles have dielectric weakness
(NSDD - contact surface state).

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SF6 and Vacuum are two modern breaking
techniques used in Medium Voltage.
They ensure the continuity of service expected by the
users together with complete safety.
The SF6 technique has differentiating advantages :
for specific applications (capacitor banks,
motor breaking, generator , etc …),
for particular network operating modes (e.g. on line
monitoring of breaking medium).

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Equivalent reliability of SF6 and Vacuum CB ’s
Excellent reliability for both techniques:
experience built up by manufacturers and users,
upgrading and optimization of equipment through the use of
modern development methods (CAD-CAM, FMECA, …)
mastering of « sensitive » components such as operating
mechanism and tightness.

The actual failure rate on the installed 180 000 circuitbreakers throughout the world is :
4/10 000 per year ==> MTBF ~ 2800 years.

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Minimum maintenance for SF6 and Vacuum
installed circuit-breakers
SF6 pole-units and vacuum enclosures:
are sealed for life,
are maintenance free,
have mechanical and electrical endurance that is much
greater than actual needs (several tens of times Isc,
10,000 Ir).

Operating mechanism:
is based on the same technology, whatever the technique,
and is a component with high mechanical endurance
(10,000 operations minimum).

The lifetime of the SF6 Merlin Gerin circuit-breakers is 30
years.
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Installation security: assets of SF6.

On-line monitoring of the breaking medium is
possible thanks to a pressure switch .
All the ratings at the pressure switch level.

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No overvoltage having detrimental effect on the
equipment:
No reignition nor restrike, during the switching of capacitors
banks.
No or weak overvoltage during the switching of inductive
loads
(unloaded transformer, starting motor).
No NSDD ’s during breaking, nor multiple prestrikes in
making.

The use of vacuum circuit-breakers requires to have
overvoltage protection (ZnO-RC).

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U source side
U load side

SF6 circuit-breaker (12kV)
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U source side
U load side

45 kV

Vacuum circuit-breaker (12kV)
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Rated characteristics maintained at 0 bar gauge SF6
pressure with breaking once at 80 % or 100 % of the
maximum breaking capacity and a dielectric withstand at
least 80 % of the insulation level,
for example:

SF1 circuit-breaker at 0 bar gauge: 25 kA at 24 kV
125 kV BIL.

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Safety of people related to the switchboards
which the circuit breakers are integrated.

Preponderance of the toxicity of copper vapours present
in all electrical equipment in the event of internal arcing,
whatever breaking technique.
The information is in the IEC report 1634:
Use and handling of SF6 in high voltage switchgear
and controlgear.

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A COMPARISON OF SF6 AND VACUUM CIRCUIT BREAKERS
SF6 or vacuum which one is the best technology in circuit breakers to
the user’s view point ?
• Both can be safe, long lasting, adapted to the utilisation.
• It all depends upon who is the manufacturer.
• You can be confident when he is Schneider (Merlin Gerin-MG) who
is the most experienced maker of MV switchgear with SF6 and an
expert in vacuum.
•But the technologies have different features and merits which are
compared in the attached document.

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DIELECTRIC WITHSTAND
depends on 3 parameters:
The Dielectric strength of the medium
The Shape of the parts
The distance :
ambient air between the live parts
insulating air interface between the live parts

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Dielectric Strength of air depends on ambient
conditions:
Pollution
reducing the insulating performance by a
factor <10. Pollution may occur from external dust, lack of cleanliness,
breaking down of an internal surface, pollution & humidity causes
electrochemical conduction which will worsen discharge phenomena.
Condensation

reducing the insulating performance by a
factor 3

Pressure

related to the altitude, derating performance.

.

Humidity
% of humidity can cause a change in insulating
performances. (liquid always leads to a droop in performance)
Temperature
temp. increases can cause decreases
insulation performance. Thermal shock can be the cause of the micro
fissuration which can lead very quickly to insulator breakdown. Insulator
expands by 5 and 15 times more than a conductor.

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The Shape of the parts
It is essential to eliminate any “peak” effect to avoid disastrous effect on the
impulse wave withstand in particular and on the surface ageing of insulator.
Air Ionization

Generate Ozone

Breakdown of insulator surface or skin

Distance between parts
(there is ambient air between live parts)
For installations sometime we can not test under impulse conditions, the table
below gives the minimum distance to comply with in air either phase to earth or phase
to phase .
The table based on IEC 71-2 according to the rated lightning impulse withstand
voltage and these distances guarantee correct withstand for unfavorable
configurations: altitude < 1 000 m.

Note : the table above does not include any increase which could be required to take account of
design tolerances, short circuit effects, wind effects, operator safety, pollution, etc.
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INSTRUMENT TRANSFORMER

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Current

transformer

Metering transformer applications
Instrument transformers are necessary to provide values
that can be used by these devices which can be analogue
devices, digital processing units with a microprocessor,
after analogue/digital conversion of the input signal (e.g.:
Sepam or Power Logic System).

Current transformers (CT) meet standard IEC 60044-1.
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Characteristics Of Current Transformer:
Based on standard IEC 60044-1.

Insulation
Characterized by the rated voltage:
of the insulation, which is that of the installation (e.g.: 24 kV)
of the power frequency withstand 1 min (e.g.: 50 kV)
of the impulse withstand (e.g.: 125 kV).

Rated frequency
50 or 60 Hz.

Rated primary current (Ipn)
Rms value of the maximum continuous primary current.
Usual values are 25, 50, 75, 100, 200, 400, 600 A.

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Rated secondary current (Isn)
This is equal to 1 A or 5 A.

Rated transformation ratio
Kn = I rated primary / I rated secondary (e.g.: 100 A / 5 A)

Short-time thermal current Ith - 1 second
This characterizes the thermal withstand under short circuit conditions
for 1 second.
It is expressed in kA or in a multiple of the rated primary current (e.g.: 80
x Ipn) for 1 second.
The value for a duration that is different to 1 second is given by:
I’th =SQRT ( Ith^2 / t )
Ith : 16kA/1 sec,
I’th for 2 sec : SQRT (16^2/2) = 11.31kA/2sec

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Short-time thermal current peak value
This value is standardized from Ith - 1 s at:
IEC: 2.5 Ith at 50 Hz and 2.6 Ith at 60 Hz
ANSI: 2.7 Ith 60 Hz.

Accuracy load
The value of the load on which is based the metered current accuracy
conditions.

Accuracy power Pn
Apparent power (VA) that the CT can supply on the secondary for the
rated secondary current for which the accuracy is guaranteed
(accuracy load).
Usual values 5 - 7.5 - 10 - 15 VA (IEC).

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Accuracy class
Defines the limits of error guaranteed on the transformation ratio and
on the phase
shift under the specified conditions of power and current. Classes 0.5
and 1 are used
for metering class P for protection.

Current error ε (%)
Error that the transformer introduces in the measurement of a
current when the transformation ratio is different from the rated
value.

Phase shift or phase error ψ (minute)
Difference in phase between the primary and secondary
currents, in angle minutes
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magnetization curve (for a given temperature and frequency).
This magnetization curve (voltage Vo, magnetizing current function Im) can be divided into 3
zones:
1 - non-saturated zone: Im is low and the voltage Vo (and therefore Is) increases virtually
proportionately to the primary current.
2 - intermediary zone: there is no real break in the curve and it is difficult to situate a precise
point corresponding to the saturation voltage.
3 - saturated zone: the curve becomes virtually horizontal; the error in transformation ratio is
high, the secondary current is distorted by saturation.

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Metering CT
This requires good accuracy (linearity zone) in an area close to the normal service current; it
must also protect metering devices from high currents by saturating earlier

Protection CT
This requires good accuracy at high currents and will have a higher precision
limit (linearity zone) for protection relays to detect the protection thresholds that
they are meant to be monitoring.

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Safety
The CT secondary is used at low impedance (virtually in short circuit).
The secondary circuit should never be left open, since this would
mean connecting across an infinite impedance. Under these conditions,
hazardous voltages for personnel and equipment may exist across the
terminals.
Terminal marking
CT connection is made to the terminals identified according to the IEC:
P1 and P2 on the MV side
S1 and S2 on the corresponding secondary. In the case of a double
output, the first output is identified by 1S1 and 1S2, the second by 2S1
and 2S2.

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CT for metering
Accuracy class
A metering CT is designed to send as accurate an image as possible of
currents below 120% of the rated primary.
Accuracy guaranteed from load 25% and 100% of the accuracy power.
IEC standard 60044-1 determines the maximum error:

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CT for metering
Safety factor: FS
In order to protect the metering device connected to the CT from high currents
on the MV side, instrument transformers must have early saturation
characteristics.
The limit primary current (Ipl) is defined for which the current error in the
secondary is equal to 10%. The standard then defines the Safety Factor FS.
:

This is the multiple of the rated primary current from which the error
becomes greater than 10% for a load equal to the accuracy power.

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CT for protection
Accuracy class
A protection CT is designed to send as reliable an image as possible of the fault current
(overload or short circuit).
IEC standard 60044-1 determines the maximum error for each accuracy class in the
phase and in the module according to the indicated operating range.

For example for class 5P the maximum error is y ± 5% at the accuracy limit current
and y ± 1% at the rated current.
Standardized classes are 5P and 10P. The choice depends on the application.
The accuracy class is always followed by the accuracy limit factor.

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Accuracy limit factor: FLP
A protection CT must saturate at sufficiently high currents to enable sufficient
accuracy in the measurements of fault currents by the protection device whose
operating threshold can be very high.
The limit primary current (Ipl) for which current errors and phase shift
errors in the secondary do not exceed values in the table below
The standard then defines the accuracy limit factor FLP.

In practice this corresponds to the linearity limit (saturation curve) of the CT.

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If ϕ and η are not known, use
approx value cos ϕ: 0.8 and η =
0.8
Capacitor Feeder :Derating
coefficient of 30% to take into
account of temp. rise due to
capacitor harmonic

Bus section
The greatest value of current that
can flow in the bus section on a
permanent basis.
Ips = In bus
Standardized values :
10-12.5-15-20-25-30-40-50-60-75-80
and their multiples and factors
CT must be able to withstand 120%
the rated current
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CURRENT TRANSFORMER
Example:

A thermal protection device for a motor has a setting range of between 0.6 and 1.2 x Ir (CT).
In order to protect this motor, the required setting must correspond to the motor’s rated current.
If we assume that Ir for the motor = 45 A, the required setting is therefore: 45A
If we use a 100/5A CT, the relay will never see 45A , because: 100A x 0.6 = 60A > 45A.
If we use a 75/5A CT, the relay will see , 75 x 0.6 = 45 A
The range of setting will be: 0.6 < 45/75 < 1.2 . This CT is suitable.

RATED THERMAL SHORT CIRCUIT CURRENT (Ith)
Value of the installation max. short circuit current and the duration 1s or 3 s.
Each CT must be able to withstand short circuit current both thermally and dynamically until the
fault is effectively cut off.
Ith = Ssc / (U x V3), Ssc = power short circuit MVA
When the CT is installed in a fuse protected, the Ith = apprx. 80 Ir.

RATED SECONDARY CURRENT:
Local use or inside switchgear Isr = 5A
Remote use or long distance Isr = 1A
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INSIDE MV CURRENT TRANSFORMER

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RATED PRIMARY VOLTAGE (Upr)
According to the design, VT will be connected :
Phase to earth 22.000V/V3 / 110V/V3, where Upr = U/V3
Phase to phase 22.000 / 110V, where Upr = U

RATED SECONDARY VOLTAGE (Usr)
Phase to phase VT, rated secondary voltage : 100V or 110 V
Phase to Ground VT, rated secondary voltage : 100/V3 or
110V/V3

RATED OUTPUT
The apparent power output that VT can supply the secondary
circuit when connected at rated primary voltage and connected to
the nominal load.
It must not introduce any error exceeding the values guaranteed
by the accuracy class . (S = V3. U. I in 3 phase circuit)
Standardized value are:
10-15-25-30-50-75-100-150-200-300-400-500 VA

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ACCURACY CLASS
The limits of errors guaranteed in terms of transformation ratio and phase under the specified
conditions of both power and voltage.

PROTECTION ACCORDING TO IEC 60 186
Classes 3P and 6P (but in practice only class 3P is used)
The accuracy class is guaranteed for values :
of voltage of between 5% of the primary voltage and the max. value of this voltage which
is the product of the primary voltage and the rated voltage factor (kT x Upr)
For secondary load between 25% and 100% of the rated output with a power factor of 0.8
inductive.

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INSIDE MV VOLTAGE TRANSFORMER

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INDEX PROTECTION OF THE SWGR
INDEX PROTECTION

Protection of people against direct contact and protection of
equipment against certain external influences.
Requested by international standard for electrical installations
and products (IEC 60 529)
The protection index is the level of protection provided by an
enclosure against access to hazardous parts, penetration of
solid foreign bodies and of water.
The IP code is a coding system to indicate the protection
index.

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INDEX PROTECTION- first index

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INDEX PROTECTION: second index

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INDEX PROTECTION : third index

Definitions
The protection mentions correspond to impact energy
levels expressed in joules
hammer blow applied directly to the equipment
impact transmitted by the supports, expressed in terms
of vibrations therefore in terms of frequency and
acceleration
The protection indices against mechanical impact can
be checked by different types of hammer: pendulum
hammer, spring-loaded hammer or vertical free-fall
hammer (diagram below).

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PROTECTION INDEX: third index

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Introduction to mv switchgear

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Introduction to mv switchgear