Percentage Differential Protection Prepared by Viren B Pandya

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

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.

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.

Connection diagram

Construction – Schematic diagram

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

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

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

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

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

Characteristic

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

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.

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

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?

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.

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?

Earth leakage protection for 1-Φ load

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.

Earth leakage protection for 3-Φ load

Any query?