What's the point of differential protection?

Electrical Apparatus, Apr 2005 by Nailen, Richard L

Why coordination with the motor is essential

ANECESSITY IN ALL THREE-phase motor circuits, overcurrent protection can take two forms. One deals with motor overload, involving a shaft horsepower demand exceeding motor capability, starting that's too frequent or unduly severe, or stalling. A second type of protection also deals with excessive current but at a far higher level, occasioned by insulation breakdown either within the motor winding or in the supply conductors. Opening the circuit quickly requires fault protection.

In low-voltage, medium motor circuits (1 to 500 hp), the commonest method of providing those protective functions is with starter "overloads" (either thermal or electronic) to deal with overcurrent, and upstream fuses or a circuit breaker for faults.

For larger machines at medium voltages, both types of protection are instead provided by relays that control the operation of circuit disconnection devices. Although fuse protection is available, it's well-suited only to fault conditions.

Because of the voltages and currents involved, input to the protective relays is supplied by current transformers (CT's) in the motor supply lines. Figure l(a) illustrates the principle. The CT ratio is chosen so that the expected primary current range-the motor line current-results in secondary amperes between O and 5. Transformer accuracy and design determine whether such a circuit is usable for relaying, or just for monitoring actual line amperes for remote display.

However, the simple scheme of Figure l(a) may not suit all motor applications. The problem is that a motor draws high current during acceleration. To avoid "nuisance trips" during starting, the protective circuit must allow that starting current to flow uninterrupted. However, a low-level fault, caused by a partial ground or short-circuit within the motor winding, may lead to a catastrophic breakdown despite initial overcurrent that is below the normal starting value.

Within one phase of a three-phase winding, as in any single-phase circuit, a ground fault results in a difference in line current between the two sides of the circuit. To sense that, large motors are sometimes provided with differential relaying. As the name implies, the system involves CT's that monitor the difference in current between the two ends of a winding phase. Because that difference is always zero when no fault exists, the relay sensing CT output can be quite sensitive; even the smallest difference indicates trouble. Figure 1(b) illustrates that principle. Normal motor overload or starting, while increasing phase and line current, will not create any difference to produce a CT and relay response.

For a three-phase machine, a typical differential protection scheme is shown in Figure 2. This requires two sets of identical CT's. One is located in the controlling switchgear, sensing current onto the motor supply circuit. The second set is at the motor (which must have both ends of each phase brought out of the winding). The two sets are connected to a common differential relay responsive to any difference in current. A fault either in the motor or on the supply circuit will create such a difference.

A widely used alternative today, to protect the motor only, is the zero-sequence or flux balancing scheme of Figure 3. Here, only one set of CT's is required, at the motor itself.

The illustrations show circuitry for a wye-connected motor winding. Either type of protection is equally applicable to a delta-connected machine, with minor changes to individual phase connections.

Determining when to require either method of differential protection is a decision for the system designer-not the motor manufacturer. No motor or generator standard dictates such protection.

That decision is based on the available fault current (the lower the value, the greater the likelihood that it can be "masked" by motor starting current) and by the economics of motor cost and unit downtime. The choice must be clear when the motor is specified, however, because the motor terminal box construction and the motor lead connections will vary with the CT arrangement.

As ANSI standards point out, differential relaying cannot detect turn-to-turn faults within any winding. The method is said to be superior to other relaying methods of detecting phase-to-phase short circuits, but engineering opinions differ as to the importance of that.

In most motor applications, grounding is much more probable than a phase fault.

As the standards add: "The complete protection scheme must be chosen to achieve optimum service reliability, safety, and protection of equipment at a reasonable cost. It is essential that the ... motor-protection system be coordinated with . . . the supply and . . . the process or load involved. . . ."

"Coordination" with the motor is equally essential. Adding any differential CT's to an existing motor in service, or changing from one CT scheme to another, is seldom possible.

By Richard L. Nailen, P.E., EA Engineering Editor