1) Differential protection compares currents flowing into and out of a protected zone. A difference indicates an internal fault. Modern relays use microprocessors to compare currents. 2) Differential protection is applied to transformers by taking the transformation ratio into account. Ratios of current transformers (CTs) on the high voltage and low voltage sides must match the transformer ratio. 3) Restricted earth fault protection monitors residual current to protect transformer windings against earth faults, providing coverage where overcurrent protection is insufficient.

1. DIFFERENTIAL PROTECTION

2. Differential protection

3. Differential protection application

10. Differential Relay – Operating
Principle
PLANT TO BE
PROTECTED
I1 I2
I1 = I2  Healthy; no trip
| I1 - I2 | > Limit  Fault; trip
“Balanced beam” type of relay – difference current trips beam;
through current restrains it.
Modern relays do the comparison by microprocessor

11. Differential Relay – Operating Principle
.. 2

12. Differential Relay Coil Connection

13. Differential Relay – Transformer
Application
• Transformation ratio has to be taken into account.
Trip if |aI1 – I2| > Limit, where a is trf. ratio (VP/VS)
• Three-phase delta-star or star-delta. The phase
shift and 3 have to be taken into account
• Allow for inrush currents – suppress tripping (2nd
harmonic restraint).
• Allow for ratio changes due to tap-changing (may
be 10-15%)

14. Differential Relay – Transformer
Application .. 2
HV and LV CT
ratios have to be
in the same ratio
as the
transformer

15. Transformer Inrush Current
-Full DC Offset

16. Differential Relay Set-up for Delta-Star
Transformer
CTs in
delta
CTs in
star
Delta
Star

17. Transformer Differential Protection
Example
• 33/11 kV, star-star transformer. Rating = 25
MVA
• Standard CT ratios available are 200:5, 400:5,
600:5, 800:5, 1000:5, 1600:5, 1800:5
• Allow for 15-20% unbalance

18. Transformer Differential Protection
Example .. 2
• Full–load current = 437 A/ph (33kV side),
=1312 A/ph (11kV side)
• Allow 150% overload. Choose 600:5 CT for 33kV.
• Transformation ratio = 3. Choose 1800:5 CT for 11kV
side
• Set HV normal current In = 450 A say
• Set bias to 20%.
• If delta-star transformer, allow for 3 (in LV CT ratio
for old relays – Idelta = Iline/ 3; taken care of in setting
for modern relays)

19. Busbar Differential Protection Scheme

20. Problems
with CT
Saturation in
B-bar Prot’n
Schemes
CT 1 is unsaturated and
CT2 saturated. Note the
distorted CT 2 current i2.
This can create a net
difference current and false
trip the relay.

21. CT
Saturation
Detector
The cct. detects saturation
and reduces the width of
the output square-wave
pulses to the extent the
relay won’t work. Explain
how the cct. works!

22. Limitations with OC Protection in
Transformers
• A simple overcurrent and earth fault relay
will not provide adequate protection for
winding earth faults.
• Need some earth fault protection.
• Degree of earth fault protection is very
much improved by the application of unit
differential or restricted earth fault
systems.

23. Restricted Earth Fault Protection

24. Restricted Earth Fault - How it Works
• On the HV side, the residual current of the 3 line CT’s
is balanced against the output current of the CT in the
neutral conductor.
• For the LV side, earth faults occurring on the delta
winding may also result in a level of fault current of
less than full load. HV overcurrent relays will not
provide adequate protection.
• A relay connected to monitor residual current will
provide restricted earth fault protection since the
delta winding cannot supply zero-sequence current to
the system.

25. Restricted Earth Fault - How it Works ..
2
• Both windings of the transformer can thus
be protected separately with restricted
earth fault.
• Provide high speed protection against
earth faults over the whole of the
transformer windings.
• Relay used is an instantaneous high
impedance type.

26. High Impedance Differential Scheme

27. High Impedance Scheme - How it
Works
The CTs generate currents that create a high voltage across the relay (which
has a high burden).
CTs must generate 2 times the trip voltage to work on an internal fault
If one only CT saturates for through fault, not enough voltage is generated
to trip relay.

28. High Impedance Scheme -
Requirements
• CTs must have identical turns ratios, knee-
point voltages 2 times relay operating voltage,
high accuracy, low reactance (‘PX’ class)
• High internal faults can generate high voltage
across relay - some form of voltage dependent
resistor (“metrosil”) is required to avoid this.

29. High Impedance Scheme - Relay Circuit
Refer worked tutorial example on how the stabilising resistor value
Rstab is calculated.

30. High
Impedance
Scheme -
Example … 1

31. High Impedance Scheme - Example … 2

32. Feeder Differential Protection (Balanced
Voltage)
“Translay” protection
Top - external fault, CT
current balance, no net
voltage across relays
Bottom - internal fault, CT
currents add, net voltage
across both relays.