ABB - TRANSFORMERS-PROTECTION-COURSE (2001)

© ABB Power Technology
1_114Q07- 1 -

Transformers
Protection
Protections

AGENDA
 PRINCIPLES
 LINES PROTECTION
 TRANSFORMERS PROTECTION
 INTRODUCTION
 SELECTING A PROTECTIVE SYSTEM
 Differential protection
 Sudden pressure relay
 Overcurrent protection
 Transformer tank protection
 Typical protective scheme for power transformers

 STATION BUS PROTECTION
1_114Q07- 2 -

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 3 -

Introduction. Possible faults in a transformer

 EXTERNAL  INTERNAL
 External Short  Short circuits
Circuits  between turns
 Overloads  between windings
 Overvoltages  Ground faults
 Overtemperature
 Overpressure
 Miss of oil
1_114Q07- 4 -

Introduction. Transformers protections

 INTERNAL  ELECTRICAL
 BUCHHOLZ (SPR)  SURGE ARRESTERS
 THERMOMETER  OVERCURRENT RELAYS
 THERMOSTAT  PHASE
 NEUTRAL
 THERMAL IMAGE
 OIL LEVEL  DIFFERENTIAL RELAY

 PRESSURE RELIEF  THERMAL RELAY
VALVE  TANK RELAY
 BUCHHOLZ-TAP  FUSES

CHANGER
1_114Q07- 5 -

Magnetizing inrush
 When a transformer is first energized, a transient magnetizing or
exciting inrush current may flow. This inrush current, which appears as
an internal fault to the differentially connected relays, may reach
instantaneous peaks of 8 to 30 times those for full load.
 The factors controlling the duration and magnitude of the magnetizing
inrush are:
 Size and location of the transformer bank
 Size of the power system
 Resistance in the power system from the source to the transformer bank
 Type of iron used in the transformer core and its saturation density
 Prior history, or residual flux level, of the bank
 How the bank is energized
1_114Q07- 6 -

Initial inrush
 When the excitation of a transformer bank is removed, the magnetizing
current goes to O.
 The flux, following the hysteresis loop, then falls to some residual value
R. If the transformer were reenergized at the instant the voltage
waveform corresponds to the residual magnetic density within the core,
there would be a smooth continuation of the previous operation with no
magnetic transient.
 In practice, however, the instant when switching takes place cannot be
controlled and a magnetizing transient is practically unavoidable.
1_114Q07- 7 -

Initial inrush
 If the circuit is re-energized at the instant the flux would normally be at
its negative maximum value (- max) as the residual flux would have a
positive value and since magnetic flux can neither be created nor
destroyed instantly, the flux wave, instead of starting at its normal value
(- max) and rising along the dotted line, will start with the residual value
( R) and trance the curve ( L).
1_114Q07- 8 -

Initial inrush
 Curve t is a displaced
sinusoid, regardless of the
magnetic circuit's saturation
characteristics.
 Theoretically, the value of
max is + (| R| + 2| max|).
 In transformers designed for
some normal, economical
saturation density s, the
crest of t will produce super
saturation in the magnetic
circuit.
 The result will be a very
large crest value in the

magnetizing current.
1_114Q07- 9 -

Initial inrush

 For the first few cycles, the inrush current decays rapidly. Then,
however, the current subsides very slowly, sometimes taking many
seconds if the resistance is low.
 The time constant of the circuit (L/R) is not, in fact, a constant: L varies
as a result of transformer saturation. During the first few cycles,
saturation is high and L is low. As the losses damp the circuit, the
saturation drops and L increases. According to a 1951 AIEE report, time

constants for inrush vary from 10 cycles for small units to as much as 1
1_114Q07- 10 -

min for large units.

Initial inrush
 The resistance from the source to the bank determines the damping of
the current wave.
 Banks near a generator will have a longer inrush because the
resistance is very low.
 Likewise, large transformer units tend to have a long inrush as they
represent a large L relative to the system resistance.
 At remote substations, the inrush will not be nearly so severe, since the
resistance in the connecting line will quickly damp the current.
1_114Q07- 11 -

Initial inrush
 When there is more than one delta winding on a transformer bank, the
inrush will he influenced by the coupling between the different voltage
windings. Depending on the core construction, three-phase transformer
units may be subject to interphase coupling that could also affect the
inrush current.
 Similar wave shapes would be encountered when energizing the wye
winding of a wye-delta bank, or an autotransformer. Here, the single-
phase shape would be distorted as a result of the interphase coupling
produced by the delta winding (or tertiary).
1_114Q07- 12 -

Initial inrush
 Maximum inrush will not, of course, occur on every energization.
 The probability of energizing at the worst condition is relatively low.
 Energizing at maximum voltage will not produce an inrush with no residual.
 In a three-phase bank, the inrush in each phase will vary appreciably.
 The maximum inrush for a transformer bank can be calculated from the
excitation curve if available, and Table shows a typical calculation of an
inrush current (used phase A voltage as 0° reference).

Closing Peak value of inrush current wave (p.u.)
S
angle Ia Ib Ic Ia-Ib Ib-Ic Ic-Ia
1.40 0° 5.60 -3.73 -3.73 8.33 -3.73 -8.33
1.40 30° 5.10 — 1.87 -5.10 5.96 5.10 -9.20

1.15 0° 6.53 -4.67 -4.67 10.20 -4.67 -10.20
1.15 30° 6.03 — 2.80 -6.03 7.83 6.03 -11.06
1_114Q07- 13 -

Initial inrush
 From these calculated values it can be seen that:
 The lower the value of the saturation density flux S, the higher the inrush
peak value.
 The maximum phase-current inrush occurs at the 0° closing angle (i.e., 0
voltage).
 The maximum line-current inrush occurs at ± 30° closing angles.
 Because of the delta connection of transformer winding or current
transformers, the maximum line-current in-rush value should be
considered when applying current to the differential relay.
Closing Peak value of inrush current wave (p.u.)
S
angle Ia Ib Ic Ia-Ib Ib-Ic Ic-Ia
1.40 0° 5.60 -3.73 -3.73 8.33 -3.73 -8.33
1.40 30° 5.10 — 1.87 -5.10 5.96 5.10 -9.20

1.15 0° 6.53 -4.67 -4.67 10.20 -4.67 -10.20
1.15 30° 6.03 — 2.80 -6.03 7.83 6.03 -11.06
1_114Q07- 14 -

Sympathetic inrush
 When a bank is paralleled with a second energized bank, the energized
bank can experience a sympathetic inrush.
 The offset inrush current of the bank being energized will find a parallel
path in the energized bank.
 The dc component may saturate the transformer iron. creating an
apparent inrush.
 The magnitude of this inrush depends on the value of the transformer
impedance relative to that of the rest of the system, which forms an
additional parallel circuit.
 Again, the sympathetic inrush will always be less than the initial inrush.
1_114Q07- 15 -

Sympathetic inrush
 The total current at breaker C is the sum of the initial inrush of bank A
and the sympathetic inrush of bank B.
 Since this waveform looks like an offset fault current, it could cause
misoperation if a common set of harmonic restraint differential relays
were used for both banks.

 Unit-type generator and transformer combinations have no initial inrush

problem because the unit is brought up to full voltage gradually.
1_114Q07- 16 -

Recovery and sympathetic inrush may be a problem, but as indicated
above, these conditions are less severe than initial inrush.

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 17 -

Differential relaying for transformer protection
 It compares the current entering the transformer with the current
leaving the element.
 If they are equal there is no fault inside the zone of protection
 If they are not equal it means that a fault occurs between the two ends

87

With internal fault Id > 0 Trip
With external fault Id = 0 No trip
1_114Q07- 18 -


 Alternatively one could form an algebraic sum of the two currents
entering the protected element, which could be termed as differential
current (Id), and use a level detector relay to detect the presence of a
fault.
 In general this principle is capable of detecting very small
magnitudes of fault.
 Its only drawback is that it requires currents from the extremities
of a zone of protection
1_114Q07- 19 -

 Differential relays are the principal form of fault protection for
transformers rated at 10 MVA and above.
 These relays, however, cannot be as sensitive as the differential relays
used for generator protection.
 Transformer protection is further complicated by a variety of equipment
requiring special attention: multiple-winding transformer banks, zig-zag
transformers, voltage regulators, transformers in unit systems, and
three-phase trans-former banks with single-phase units.
1_114Q07- 20 -

 Transformer differential relays are subject to several factors, not
ordinarily present for generators or buses, that can cause miss-
operation:
 Different voltage levels, including taps, that result in different
primary currents in the connecting circuits.
 Possible mismatch of ratios among different current transformers.
 For units with ratio-changing taps, mismatch can also occur on the taps.
Current transformer performance is different, particularly at high
currents.
 30° phase-angle shift introduced by transformer wye-delta
connections.
 Magnetizing inrush currents, which the differential relay sees as
internal faults.
1_114Q07- 21 -


Id
 To prevent miss-operation
percentage characteristics
are used, with line current Operating zone
restraint.

(I1 + I2)/2
1_114Q07- 22 -

 Since the differential relays see the inrush current as an internal fault,
some method of distinguishing between fault and inrush current is
necessary.
 Such methods include:
 A differential relay with reduced sensitivity to the inrush wave (such
units have a higher pickup for the offset wave, plus time delay to
override the high initial peaks).
 A harmonic restraint or a supervisory unit used in con-junction with
the differential relay
 Desensitization of the differential relay during bank energization.
1_114Q07- 23 -

Differential relay for transformer protection
 Induction relays are relatively insensitive
to the high percentage of harmonics
contained in magnetizing inrush current.
 The relay shown consists of a percentage
differential unit and an indication
contactor switch.
 The percentage differential unit, an
induction disc type, has an electromagnet
with poles above and below the disc.
 There are two restraint coils on the lower
left-hand pole; the operating coil is wound
on the lower right-hand pole.
 Both the left- and right-hand poles have

transformer winding, connected in parallel
to supply current to the upper-pole
1_114Q07- 24 -

windings.

 The upper-pole current generates a flux in quadrature with the lower-
pole resultant flux, and the two fluxes react to produce a torque on the
disc.
 Under normal load or in external fault, the currents in the two restraint
windings flow in the same direction.
 These restraining currents are equal (or effectively equal) if auto-
balance taps are used to compensate from mismatch in current
transformer ratios - and if no significant current flows in the operating
coil winding.
 As a result, only contact-opening torque is produced.
1_114Q07- 25 -

 If the taps are mismatched or the main current transformers saturate
unequally on severe external faults, the effective difference between the
currents in the two restraining windings must flow in the operating coil.
 The operating coil current required to overcome the restraining torque
and close the relay contacts is a function of the restraining current. For
an internal fault, the restraining currents are opposite, and restraining
torque tends to cancel out.
 The more sensitive operating coil, however, is energized by the sum of
the two currents. As a result, a large amount of contact-closing torque is
produced.
1_114Q07- 26 -

 In applying the relay, the current transformer ratio error should not
exceed 10% during maximum symmetrical external fault current. The
relay's 50% characteristic satisfactorily handles up to 35% of current
mismatch, including the transformer tap changing on load and current
transformer mismatch.
 The relay's restraining windings have a continuous rating of 10 A; the
operating winding has a continuous rating of 5 A. To prevent
overloading the operating winding, however, no more than 5 A should
be allowed in the untapped restraining winding.
1_114Q07- 27 -

Variable-Percentage Transformer Differential Relay
 This type of relays have a variable-percentage characteristic:
 Percentage is low on light faults, where the current transformer
performance is good, and high on heavy faults, where current
transformer saturation may occur.
 The variable-percentage characteristic is obtained via a saturation
transformer in the operating circuit.
 This transformer also tends to shunt the dc component away from
the operating coil.
1_114Q07- 28 -

 The relay consists of an induction-type differential unit, a dc-indicating
contactor switch, and an optional ac-indicating instantaneous trip unit.
 The induction-type differential unit contains four electromagnets,
operating on two discs fastened to a common shaft.
 Of the electromagnets, one is the operating element and the other three
are restraint elements. On the center leg of each restraint
electromagnet are two primary coils and a secondary coil ; primary coils
are energized from the secondaries of the current transformers
connected to the bank to be protected.
1_114Q07- 29 -

 A 5-A current in the restraint coil will produce restraining torque. The
same 5-A current flowing in two restraint coils of the same restraint
electromagnet will have either additive or subtractive restraining effect,
depending on the polarity of the connection (Figure c).
 This relay is well suited to protect transformer banks not subject to
severe magnetizing inrush, particularly if more than two restraining
circuits are needed. The relay has no built-in taps and generally
requires auxiliary current transformers for current matching. The
operating time of the differential unit is two to six cycles; no setting is
required.
 The faster IIT unit is connected to the differential circuit. It is
recommended for transformer protection in applications where internal
fault current can exceed twice the maximum total current flowing
through the differential zone for a symmetrical external fault. The IIT

unit should be set at 50% external fault current or a value higher than
transformer inrush current, whichever is greater.
1_114Q07- 30 -

Harmonic restraint Transformer Differential Relay
 Since magnetizing inrush current has a high harmonic content,
particularly the second harmonic, this second harmonic can be used to
restrain and thus desensitize a relay during energization.
 The method of harmonic restraint is not without its problems.
 There must be enough restraint to avoid relay operation on inrush,
without making the relay insensitive to internal faults that may also have
some harmonic content.
1_114Q07- 31 -

 In the differential unit, (DU) air-gap transformers feed the restraint
circuits, and a non-air-gap transformer energizes the operating coil
circuit.
 Since the rectified restraint outputs are connected in parallel, the relay
restraint is proportional to the maximum restraining current in any
restraint circuit.
1_114Q07- 32 -

 The percentage characteristic varies from around 20% on light faults,
where current transformer performance is good, to approximately 60%
on heavy fault, where current transformer saturation may occur.
 This variable-percentage characteristic is obtained via the saturating
transformer in the operating coil circuit.
 The minimum pickup is the current that will just close the differential unit
contacts, with the operating coil and one restraint coil energized.
 The continuous rating of the relay is 10 to 22 A, depending on the relay
tap used.
1_114Q07- 33 -

 The harmonic restraint unit (HRU) has a second-harmonic blocking filter
in the operating coil circuit and a second-harmonic pass filter in the
restraint coil circuit.
 Thus, the predominant second-harmonic characteristic of an inrush
current produces ample restraint with minimum operating energy.
 The circuit is designed to hold open its contacts when the second-
harmonic component is higher than 15% of the fundamental.
 This degree of restraint in the HRU is adequate to prevent relay
operation on practically all inrushes, even if the differential unit should
operate.
1_114Q07- 34 -

 For internal faults, ample operating energy is produced by the
fundamental frequency and harmonic other than the second.
 The second harmonic is at a minimum during a fault. Since the HRU
will operate at the same pickup as the DU, the differential unit will
operate sensitively on internal faults.
 For external faults, the differential unit (DU) will restrain.
 The relay operating time is one cycle at 20 times tap value.
 The instantaneous trip unit (IIT) is included to ensure high-speed
operation on heavy internal faults, where current transformer saturation
may delay HRU contact closing.
 The IIT pickup is 10 times the relay tap value.
 This setting will override the inrush peaks and maximum false

differential current on external faults.
1_114Q07- 35 -

Transformer Differential Relay Application
 The following guidelines are designed to assist in selecting and
applying relays for transformer protection.
 When two or more relays appear to be equally suitable, engineering
experience and economics will determine the final choice.
 There is no clear-cut answer to the question of which relay or protective
method to apply.
 As a general rule, however, the induction-disk differential relays are
used at substations remote from large generating sources where inrush
is not a problem and the kVA size of the bank is relatively small.
 The more complex and more expensive harmonic relays are used at
generating stations and for large transformer units located close to
generating sources, where a severe inrush is highly likely.
1_114Q07- 36 -

 In general, the current transformers on the wye side of a wye-delta
bank must be connected in delta, and the current transformers on
the delta side connected in wye.
 This arrangement (1) compensates for the 30° phase-angle shift
introduced by the wye-delta bank and (2) blocks the zero sequence
current from the differential circuit on external ground faults.
1_114Q07- 37 -

 Zero sequence current will flow in the differential circuit for external
ground faults on the wye side of a grounded wye-delta bank; if the
current transformers were connected in wye, the relays would miss-
operate.
 With the current transformers connected in delta, the zero sequence
current circulates inside the current transformers, preventing relay miss-
operation.
1_114Q07- 38 -

 Relays should be connected to receive “in” and “out” currents that
are in phase for a balanced load condition.
 When there are more than two windings, all combinations must be
considered, two at a time.
 Relay taps or auxiliary current transformer ratios should be as close
as possible to the current ratios for a balanced maximum load
condition.
 When there are more than two winding, all combinations must he
considered, two at a time, and based on the sane kVA capacity.
 Only ground one point in the differential scheme, never do multiple-
point grounding.
1_114Q07- 39 -

 The percentage of current mismatch should always be checked to
ensure that the relay taps selected have an adequate safety margin.
 When necessary, current mismatch values can be reduced by
changing current transformer taps or adding auxiliary current
transformers.
1_114Q07- 40 -

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 41 -

Sudden-Pressure Relay (SPR)
 With the application of a gas-pressure relay, many transformers can be
protected by a simple differential relay set insensitively in the inrush current.
 The sudden-pressure relay (SPR), which operates on a rate of rise of gas in the
transformer, can be applied to any trans-former with a sealed air or gas
chamber above the oil level.
 The relay is fastened to the tank or manhole cover, above the oil level. It will not
operate on static pressure or pressure changes resulting from normal operation
of the transformer.
1_114Q07- 42 -

Sudden-Pressure Relay (SPR)
 The SPR relay is recommended for all units of 5000 kVA or more.
 The SPR relay is far more sensitive to light internal faults than the differential
relay. The differential relay, however, is still required for faults in the bushing
and other areas outside the tank.
 The SPR relay operating time varies from 1/2 cycle to 37 cycles, depending on
the size of the fault.
 In the past, large-magnitude through-fault conditions on power transformers
have caused rate-of-change-of-pressure relays to occasionally operate falsely.
There has been reluctance on the part of some users to connect these rate-of-
change-of-pressure relays to trip, and they have therefore used them for
alarming only. Schemes have been devised to restrict tripping of the rate-of-
change-of-pressure device only to levels of current below which the transformer
differential relay cannot operate.
1_114Q07- 43 -

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 44 -

Overcurrent and Backup Protection
 To allow transformer overloading
when necessary, the pickup value of
phase overcurrent relays must be set 50/51
above this overload current.
t
 An inverse-time characteristic relay
Curva trafo
usually provides the best
coordination.
Relé tiempo independ. Relé tiempo inverso

 Settings of 200 to 300% of the
transformer's self-cooled rating are
common, although higher values are t0

some-times used.
i
 Fast operation is not possible, since In n*In

the transformer relays must
coordinate with all other relays they

overreach.
1_114Q07- 45 -

 Overcurrent relays cannot be used for primary protection without the
risk of internal faults causing extensive damage to the transformer.
 Fast operation on heavy internal faults is obtained by using
instantaneous trip units in the overcurrent relays.
 These units may be set at 125% of the maximum through fault, which is
usually a low-side three-phase fault.
 The setting should be above the inrush current. Often, instantaneous
trip units cannot be used because the fault currents are too small.
 An overcurrent relay set to protect the main windings of an
autotransformer or three-winding transformer offers almost no
protection to the tertiary windings, which have a much smaller kVA.
 Also, these tertiary windings may carry very heavy currents during
ground faults. In such cases, tertiary overcurrent protection must be

provided.
1_114Q07- 46 -

 A through fault external to a transformer results in an overload that can
cause transformer failure if the fault is not cleared promptly.
 It is widely recognized that damage to transformers from through faults
is the results of thermal and mechanical effects.
 The thermal effect has been well understood for years.
 The mechanical effect has recently gained increased recognition as a major
concern of transformer failure.
 This results from the cumulative nature of some of the mechanical effects,
particularly insulation compression, insulation wear, and friction-induced
displacement.
 The damage that occurs as a result of these cumulative effects is a function
of not only the magnitude and duration of through faults, but also the total
number of such faults.
1_114Q07- 47 -

 The transformer can be isolated from the fault before damage occurs by
using fuses or overcurrent relays.

50/51N

50/51G
2-3 50/51
1_114Q07- 48 -

Distance Relaying for Backup Protection
 Directional distance relaying can be used for transformer backup
protection when the setting or coordination of the overcurrent relays is a
problem.
 The directional distance relays are connected to operate when the fault
current flows toward the protected transformer.
 They are set to reach into, but not beyond, the transformer.
1_114Q07- 49 -

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 50 -

Transformer Tank protection
 This is a low cost protection against some of the internal faults of the
transformer, which consists of an overcurrent relay which measures
the current flow through the ground connection of the transformer tank.
 It detects hence the ground faults of the transformer and bushings
trough the metallic tank.
 To achieve this the transformer must be completely isolated from
ground (putting some isolating material under the transformer
wheels), and a toroid current transformer is needed surrounding the
only ground connection cable.

64
1_114Q07- 51 -

Transformer Tank protection
 To prevent incorrect tripping (because of possible faults in the
connection cables to fans, etc) it is necessary to take some measure
as the indicated in fig, and to coordinate with the neutral protection.
1_114Q07- 52 -

AGENDA
 PRINCIPLES
 INTRODUCTION

1_114Q07- 53 -

Typical protective scheme for power transformers
 Figure illustrates how a primary
breaker can be used for
transformer protection.
 The basic protection is provided
by the 87T transformer differential
relays.
 Device 50/51, an inverse-time
relay with IIT unit, provides
transformer primary winding
backup protection for phase
faults;
 either device 50G (with a zero
sequence current transformer) or
50N/51N can be used as

transformer primary winding
backup for ground faults.
1_114Q07- 54 -

 Transformer overload, low-voltage
bus, and feeder backup protection
are provided by device 51 on the
transformer secondary side.
 Since the low-voltage side is
medium-resistance-grounded, a
ground relay (51G) should be used
to trip breaker 52-1 for low-side
ground faults and for resistor
thermal protection.
 Device 151G, which trips breaker
52-11, provides feeder ground
backup, whereas device 63, such
as a type SPR relay, offers highly

sensitive protection for light faults.
1_114Q07- 55 -

 The current transformer ratings in
this scheme should be compatible
with the transformer short-time
overload capability: approximately
200% of transformer selfcooled
rating for wye-connected current
transformers and 350% ( · 200%)
for delta-connected current
transformers.
 The neutral current transformer
rating should be 50% of the
maximum resistor current rating.
1_114Q07- 56 -

ABB - TRANSFORMERS-PROTECTION-COURSE (2001)

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