1. Percentage Differential Protection Prepared by Viren B Pandya

2. Contents Percentage differential relay Connection diagram Construction
Working & characteristics
Simple example
Earth leakage protection for 1-phase load
Earth leakage protection for 3-phase load

3. Percentage differential relay
Simple differential protection relay has drawback of mal-operation at higher through fault due to CT ratio and phase angle error.
It can be made more stable if restraining torque proportional to through fault current could be produced, however operating torque still being proportional to the spill current.

4. Percentage differential relay
This idea has been implemented in percentage differential relay.
Restraining coil is centre taped forming two sections of equal number of turns Nr/2. It is connected in circulating current path.
Operating coil has No turns and it is connected in spill path.

5. Connection diagram

6. Construction – Schematic diagram

7. Working & Characteristics
Let us first get mmf/ampere turn in both coils

8. Working & Characteristics
Torque in electromagnetic relay is proportional to square of flux, hence torque produced by restraining coil is
where M is constant of proportionality
Restraining torque is also produced by spring and given by: Tsp

9. Working & Characteristics
Hence total restraining torque is given by
Similarly operating torque can be obtained as

10. Working & Characteristics
The relay trips if operating torque is greater than the restraining torque.
The relay will be at the verge of operation when operating torque just balances out the restraining torque i.e. when
Neglecting spring torque

11. Working & Characteristics
The result is that:
Where K is ratio of RC turns to OC turns
However if spring tension is accounted

12. Characteristic

13. Working & Characteristics
Operating characteristics of this relay is a straight line with slope of K=Nr/No.
Intercept on y-axis is Ksp.
Which is trip region?
Entire region above straight line
Which is no-operation region?
Entire region above straight line

14. Working & Characteristics
Slope of the relay is customarily expressed as percentage, hence name is percentage differential relay.
Obviously this relay does not have a fixed pick-up value.
Relay automatically adopts its pick-up value to the external fault current.

15. Working & Characteristics
How does this characteristic prevent mal-operation during higher through fault currents?
Observe external/through fault characteristics.
Restraining torque is with summation of currents
Operating torque is with diff. of currents
Hence for external faults, relay restrains
For internal faults, relay operates

16. Simple operation - example
For internal fault in case of a single-end-fed system
What will be current in CT-2?
ZERO
I1-I2=I1
What will be spill current?
(I1+I2)/2=I1/2
What will be circulating current?

17. Earth leakage protection for 1-Φ load
Many times due to insulation failure the chassis of equipment becomes live.
This causes a leakage current to earth from the chassis as the chassis is always connected to earth.
However the leakage current may be too small for an overcurrent relay to detect to operate.

18. Earth leakage protection for 1-Φ load
Human safety is endangered.
Hence special type of differential relay known as earth leakage relay or current balance relay can easily detect such faults.
If chassis is not earthed, will this relay trip? NO
If a person, sitting on his bare feet on earth, touches the chassis, what will happen?

19. Earth leakage protection for 1-Φ load

20. Earth leakage protection for 1-Φ load
Under normal condition, there is no net mmf acting on toroidal core.
Hence flux linked with pick-up coil is zero and no voltage is induced in the pick-up coil.
Under earth leakage, current in phase wire is now greater than that in neural wire.

21. Earth leakage protection for 3-Φ load

22. Any query?