A differential relay is a power system protection relay that operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value. In this article, you’ll learn the differential protection of AC generators.
Suppose we were to measure the amount of current at both ends of every phase winding in a three-phase generator, shown in the following diagram:
Like most large power generators, this unit brings both terminals of each phase winding to external points so that they may be connected in either a Wye or a Delta configuration as desired. In this particular case, the generator’s windings are Wye-connected. So long as we measure current going in and out of each winding individually, it matters little whether those generator windings are Wye- or Delta-connected.
If the circuit is exactly as drawn above, the amount of current entering and exiting each phase winding must be the same in accordance with Kirchhoff’s Current Law. That is to say:
IA1 = IA2 IB1 = IB2 IC1 = IC2
Suppose now that one of the turns within the “C” phase winding were to accidently contact the generator’s metal frame, such as what might happen as a result of insulation damage. This ground fault will cause a third path for current in the faulted winding. IC1 and IC2 will now be imbalanced by an amount equal to the fault current IF:
Another fault detectable by Kirchhoff’s Current Law is a phase-to-phase winding fault, where current flows from one winding to another. In this example, a fault between B and C phases in the generator upsets the balance of incoming and outgoing currents for both phases:
It should be noted that the magnitude of a ground fault or a winding-to-winding fault current might not be large enough to pose an overcurrent threat to the generator, yet the very existence of a current imbalance in any phase proves the winding is damaged. In other words, this is a type of system fault that would not necessarily be detected by an overcurrent (50/51) relay, and so must be detected by some other means.
The relay type designated for this task is called a differential current relay. The ANSI/IEEE number code for differential protection is 87. Differential voltage relays also exist, with the same “87” ANSI/IEEE designation, making it necessary to specify whether the differential quantity in question is voltage or current whenever mentioning an “87” relay.
A simple form of differential current protection for this generator may be implemented by connecting CTs on either side of each winding to operating coils of an electromechanical relay like this. For the sake of simplicity, protection for only one phase winding (C) of the generator will be shown. A practical differential current protective relay system would monitor current through all six stator wires on the generator, comparing currents in and out of every phase:
If the CT primary currents IC1p and IC2p are equal and the CT ratios are equal, the CT secondary currents IC1s and IC2s will be equal as well. The result will be zero46 current through the operating coil (OC) of the differential relay.
If, however, a fault to ground or to an adjacent winding were to develop anywhere within the generator’s “C” stator winding, the primary currents of the two CTs will become unequal, causing unequal secondary currents, thereby causing a substantial amount of current to flow through the differential relay’s operate coil (OC). If this current is sufficient to cause the differential relay to “pick up”, the relay will send a signal commanding the generator’s circuit breaker to trip.
Even with the relay’s pickup value biased to avoid unnecessary tripping, it is still possible that a heavy phase current demanded from the generator may cause the differential relay to trip, due to the impossibility of a perfect match between the two “C” phase current transformers. Any mismatch between these two CTs will result in an inequality of secondary currents that will become larger as phase current grows in magnitude. Large, harmonic-rich inrush currents47 occasionally experienced when a large power transformer is initially energized may also cause false trips in this simple form of differential protection. We do not wish this differential relay to trip for any condition but an internal generator fault in its phase winding, and so a modification is necessary to provide a different operating characteristic.
If we modify the relay to have three coils, one to move its mechanism in the trip direction, and two to help “restrain” its mechanism (working to hold the mechanism in its normal operating position), we may connect these coils in such a way that the two restraint coils48 (RC) are energized by the two CT secondary currents, while the operating coil only sees the difference between the two CT secondary currents. We refer to this scheme as a restrained differential relay, and the former (simpler) design as an unrestrained differential relay:
The general characteristic of a restrained differential relay is to trip on the basis of the differential current exceeding a set percentage of phase current. This photograph shows three differential relays used to protect the windings of a three-phase generator at a gas turbine power plant. Note how one differential current relay is required to protect each of the generator’s three phases:
Modern digital differential relays typically sense CT signals from all three phases, allowing protection in a single panel-mount unit. Digital protective relays offer much more sophisticated approaches to the problem of false tripping based on mismatches between current transformer pairs and/or harmonic currents. The following graph shows the characteristic for a General Electric model 745 transformer protective relay providing differential current protection:
Not only may the pickup value be adjusted by the user, but also the slope of each line segment on the graph, the height of the “kneepoint” step, etc. Note how the term “restraint” is still used in digital relay configuration, even though it originated in electromechanical relay designs.