Breaker Tripping Concepts

The successful clearing of a fault requires a protection relay to detect the fault condition and issue a trip signal and a breaker to operate and clear the fault. Dual-powered relays may use different methods for tripping of the breaker to clear the fault. The method selected for each specific application depends on the specifics of the breaker used and the auxiliary voltage available at the breaker location. Some of the more commonly used methods for breaker tripping by self-powered or dual-powered relays are described in the following sections.

Breaker Tripping Using Striker

A dual-powered relay can trip the circuit breaker by performing a capacitance discharge through a heavy-duty output capable of putting out sufficient power (20mJ at 12V) to a striker that releases the actuating mechanism of the circuit breaker. (Figure 3, Figure 7)

Capacitor Discharge Tripping

Dual powered relays may use either of the above methods. In addition, these particular relays charge an internal capacitor from the current circuit and also from the auxiliary voltage circuit (Figure 7).

This capacitor module has such storage capacity that, in case of loss of auxiliary supply, it can supply sufficient energy to excite a standard trip coil for two consecutive tripping orders without recharge. It may be discharged directly into a suitably sensitive trip coil via one of the programmable output relays. The minimum energy fed to the trip coil is that from the capacitor, but in most cases it will be supplemented by a current from the auxiliary voltage circuit and/or the current circuit.

When energized from current alone, the lowest current for which the relay will operate will be that necessary to start up the power supply. To be able to use lower fault settings an auxiliary supply will be required.

The capacitance discharge circuit is not isolated from the auxiliary supply and to prevent the relay from being damaged, no external ground connection should be made to this circuit.

Figure 8 shows a capacitor module with such storage capacity that, in the case of loss of auxiliary supply, it can supply sufficient energy to operate a standard trip coil for two consecutive breaker trips without recharge.

AC Series Tripping

As an alternative the trip capacitor in the dual powered relays may be discharged into an auxiliary relay. This relay will be de-energized in the quiescent state, with its break contacts short circuiting the trip coils of the circuit breaker. The trip coils are connected in series with the current transformer secondary circuit so that, when the auxiliary relay is operated, the full secondary current is diverted through the trip coils.

To cover all fault conditions, three trip coils are required and may be necessary to limit the maximum energy that can be fed to each coil, by means of saturating shunt reactors.

Testing of dual-powered relays

The testing of dual-powered relays in general is similar to the testing of relays with similar functionality. The only difference is that the tests require the addition of several modes of operation discussed previously in the paper.

The tests of relay performance and characteristics should be performed under the following test conditions:

• Relay powered by AC
• Relay powered by load current above the required threshold
• Relay powered by load current below the required threshold

The threshold of the load current required to power the relay also needs to be tested.