Types of Differential Relays

The differential relay picks up when the vector sum of the currents is not zero.

In low impedance differential relay, each CT output is connected to the relay and the vector sum is done by the relay. The relay picks up if the summation is not zero.

In high impedance differential relay, the CT outputs are connected in parallel externally and the vector sum of their current is fed to the relay. Since the relay senses the actual differnce in current and is not dependent on calculation, the high impedance differential relay is more accurate than low impedance differential relay.

Low impedance differential is biased differential protection. CT having different ratio can be used. CT of different class can also be used with modern differential relays ( P740 of Areva)

In high impedance scheme a high impedance ( stabilizing resistor) is used in series with the relay. This is to make this a high impedance one.

Chance of saturation of CT(in the faulty feeder) is more during through fault.A saturated CT secondary is equivalent to a short circuit. The circulating secondary current select saturated secondary path since the other path(relay path) is a high impedane one due to the presence of sabilising resistor. So during through faults no current pass through the relay. Hence this scheme is stable.(PBDCB of Alstom)

High impedance scheme require CT having same ratio on all branches. Class must must be PS(class X). The knee point requird will also be high(depends on the fault level of the system).

Refer busprotection of NPAG for more information

regards