Switchgear and Protection

Switchgear and Protection

Course No.: EEEEC19
(Course Credit – 04)

Lecture 04
Protection of Transformers
 Over-Current Protection of Transformer
• Overcurrent relays are used for the protection of transformers of rating 100 kVA and
below 5 MVA.
• An earth fault tripping element is also provided in addition to the overcurrent feature.
• Such relays are used as primary protection for transformers which are not provided
with differential protection.
• Overcurrent relays are also used as back-up protection where differential protection is
used as primary protection.
• For small transformers, overcurrent relays are used for both overload and fault
protection.
Over-Current Protection of Transformer
 Restricted Earth Fault Protection
• A simple overcurrent and earth fault relay does not provide good protection for a star
connected winding, particularly when the neutral point is earthed through an
impedance.
• Restricted earth fault protection provides better protection.
• This scheme is used for the winding of the transformer connected in star where the
neutral point is either solidly earthed or earthed through an impedance.
• The relay used is of high impedance type to make the scheme stable for external
faults.
• The relay operates only for a ground fault in the transformer.
 Restricted Earth Fault Protection (External Fault)
In case of earth fault on star side of delta-
star transformer, zero-sequence current is
not reflected on delta side. Therefore,
differential protection is less sensitive for
such faults and hence, separate earth fault
protection is required.
During an external fault, the fault current
circulates in the pilot wires and no current
flows through the relay. Hence, relay does
not operate.
 Restricted Earth Fault Protection (Internal Fault)

During an internal fault, the fault current

flows through the neutral CT and not through
the line CTs. Therefore, CT secondary
current flows through the relay and relay
operates if it exceeds pick-up value.
 Differential Protection
• Percentage differential protection is used for the protection of large power
transformers having ratings of 5 MVA and above.
• This scheme is employed for the protection of transformers against internal short
circuits (i.e., phase to phase faults and phase to ground faults).
• The principle of such a protection scheme is the comparison of the currents entering
and leaving the ends of a transformer. The vector difference of currents (I1-I2) passes
through the operating coil while the average current (I1+I2)/2 passes through the
restraining coil. In normal conditions, the two currents at the two ends of the
transformer are equal and balance is maintained. So no current flows through the
operating coil of the relay and relay is inoperative. But when there is phase to phase
fault or phase to ground fault, this balance gets disturbed. The difference current flows
through the operating coil due to which relay operates, tripping the circuit breaker.
 Differential Protection (Circulating current)

No operation of the relay

when fault is external (F1),
while during internal fault
(F2) relay will operate.
Differential Protection (Merz-Price Protection)

The restraining coils are connected

across the C.T. secondary windings
while the operating coils are
connected between the tapping
points on the restraining coils and
the star point of C.T. secondaries.
Differential Protection for Delta Star Transformer
 Factors need to be considered for Transformer Protection
Compared to the differential protection used in generators, there are certain important
points which must be taken care of while using such protection for the power
transformers. These points are,
1. Line current transformer primary rating
In a power transformer, the voltage rating of the two windings is different. Thus there
always exists difference in current on the primary and secondary sides of the power
transformer. Hence if C.T.s of same ratio are used on two sides, then relay may get
operated through there is no fault existing.
To compensate for this difficulty, the current ratios of C.T.s on each side are different.
These ratios depend on the line currents of the power transformer and the connection of
C.T.s.
 Factors need to be considered for Transformer Protection
Due to the different turns ratio, the currents fed into the pilot wires from each end are
same under normal conditions so that the relay remains inoperative. For example if K is
the turns ratio of a power transformer then the ratio of C.T.s on low voltage side is made K
times greater than that of C.T.s on high voltage side.

2. Inherent phase shift of currents in transformer

In case of power transformers, there is an inherent phase difference between the voltages
induced in high voltage winding and low voltage winding. Due to this, there exists a phase
difference between the line currents on primary and secondary sides of a power
transformer. This introduces the phase difference between the C.T. secondary currents, on
the two sides of a power transformer.
 Factors need to be considered for Transformer Protection
Though the turns ratio of C.T.s are selected to compensate for turns ratio of transformer, a
differential current may result due to the phase difference between the currents on two
sides. Such a differential current may operate the relay though there is no fault. Hence it is
necessary to correct the phase difference.
To compensate for this, the C.T. connections should be such that the resultant currents fed
into the pilot wires from either sides are displaced in phase by an angle equal to the phase
shift between the primary and secondary currents.
Power Transformer Connections C. T. Connections
Primary Secondary Primary Secondary
Star Delta Delta Star
Delta Delta Star Star
Star Star Delta Delta
Delta Star Star Delta
 Factors need to be considered for Transformer Protection
3. Bias to cover tap changing facility and CT mismatch
Many transformers have tap changing arrangement due to which there is a possibility of
flow of differential current. For this, the turns ratio of C.T.s on both sides of the power
transformer are provided with tap for of C.T.s on both sides of the power transformer are
provided with tap for their adjustment.

4. Interposing current transformers (ICTs) to compensate for mismatch of line

current transformers
Interposing types can be used to modify the secondary current from an existing line CT.
This is useful for modifying secondary currents without replacing the line CTs.
Factors need to be considered for Transformer Protection
Interposing current transformers (ICTs)
 Factors need to be considered for Transformer Protection
5. Magnetizing Inrush current
When an unloaded transformer is switched on, it draws a large initial magnetizing current
which may be several times the rated current of the transformer. This initial magnetizing
current is called the magnetizing inrush current.
As the inrush current flows only in the primary winding, the differential protection will
see this inrush current as an internal fault.
The harmonic contents in the inrush current are different than those in usual fault current.
The third harmonic and its multiples do not appear in CT leads as these harmonics
circulate in the delta winding of the transformer and the delta connected CTs on the Y side
of the transformer.
5. Magnetizing Inrush current
As the second harmonic is more in the inrush current than in the fault current, this
feature can be utilized to distinguish between a fault and magnetizing inrush current.

Harmonic restraint relay

Working Principle

• The operating principle is to filter out the harmonics from the differential
current, rectify them and add them to the percentage restraint.
• The tuned circuit XCXL allows only current of fundamental frequency to flow
through the operating coil.
• The dc and harmonics, mostly second harmonics in case of magnetic inrush
current, are diverted into the restraining coil.
Magnetizing current compensation
Buchholz Relay
Example-1: A three phase power transformer having a line voltage ratio of 400 V to 33
KV is connected in star-delta. The C.T.s on 400 V side have current ratio as
1000/5. What must be the C.T. ratio on 33 kV side.
Assume current on 400 V side of transformer to be 1000 A.