DIFFERENTIAL-PROTECTION

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.