Generator Circuit-Breaker: Emergency Current Without Cooling

Short answer

A generator circuit-breaker carries very high currents and usually depends on a cooling system, its own or one shared with the isolated phase bus. If that system fails, the generator does not have to be disconnected immediately. IEC/IEEE 62271-37-013, 2015 edition, requires the manufacturer to establish emergency values so that the breaker stays in service, with limited time and load. Those values are demonstrated by tests (clause 6.5.101).

What the standard says

Clause 4.4.101 starts from an operating need: rated emergency values must exist so that the breaker remains in service after losing the cooling systems it normally requires. It sets these conditions:

• a) The breaker can be operated for limited periods with parts at a total temperature above the limits set for normal current.
• b) The difference between the emergency temperature limits and the normal operating temperatures gives a defined period during which full load can be carried. After that period, the current must be reduced.
• c) The breaker can remain in service with reduced load, and its value depends on the type of emergency.
• d) If the normal current depends on several independent systems, for example the interrupting medium, the cooling medium or the forced air of the isolated phase bus, the effect of losing each one separately and in combination must be established.
• e) Some plant designs, such as a generator output connected through two circuit-breakers to two step-up transformers, may require special emergency conditions and values.

The three parameters of each emergency

For each emergency condition, the standard requires three parameters to be given:

Figure 9 of the standard illustrates the case in which the loss of two cooling systems, that of the breaker and that of the bus, is studied separately and at the same time. The values determined by that study are:

• I1: rated current with all cooling systems working;
• I2: permitted current if the breaker coolant fails (failure A);
• I3: permitted current if the cooling air of the isolated phase bus fails (failure B);
• I4: permitted current if the coolant flow of the bus and of the breaker fails at the same time (failure C);
• t1, t2 and t3: permitted times without reducing the rated continuous current and without exceeding the maximum permitted total temperature at the hottest spot.

The manufacturer determines the values of t, R and I for each emergency.

How it is demonstrated

Clause 6.5.101 requires the emergency conditions to be demonstrated by tests, which include, where applicable:

• a) the cooling failure: loss of fluid, cooling failure of the isolated phase bus, failure of fans, pumps or air circulators;
• b) the loss of the insulating medium.

Conditions a) and b) are tested separately. If the cooling consists of independent subsystems, the individual failure of each one and the simultaneous failure of all of them are tested.

In each type of test, based on the temperature limits set by the manufacturer, these data are determined:

• the maximum time the emergency condition can last (t1, t2, etc.);
• the rate of decrease of the continuous current (R1, R2, etc.);
• the lowest current value at which the temperature rise of the breaker does not exceed the values of clause 6.5.6 (I1, I2, etc.).

A note warns of a complex case: when the forced cooling of the isolated phase bus and that of the breaker use the same air, losing that air means the bus temperature does not stay constant and also rises. It is suggested to consider that rise in the test. The total temperature limit of the bus is agreed between user and manufacturer, with the practical application in the power system.

What does not change

The emergency values do not modify the rated normal current of the breaker, which follows clause 4.4.1 (the values of the R10 series are only preferred), nor the temperature-rise limits of normal service. They are a temporary regime defined separately. For that normal service, the standard also limits to 50 °C the maximum temperature of the parts the operator handles in routine work (clause 4.4.2).

What to do with these values in operation

The standard defines the parameters, but not the values: each manufacturer assigns them for its equipment. For them to be useful in the plant:

• enter t, R and I for each emergency in the operating procedure and in the control system alarms;
• relate the cooling failure alarm to the generator load reduction ramp;
• keep the emergency test reports, because they are the basis for those values;
• if the generator output uses two circuit-breakers or there is another special arrangement, ask the manufacturer for the emergency values of that arrangement (clause 4.4.101 e).

The generator circuit-breaker classes are explained in generator circuit-breaker: classes G1 and G2. The rated continuous current and temperature rise are covered in rated current and temperature rise. Temperature rise in service is monitored with thermography in substations.

To see how this fits with the rest of the IEC 62271 series, see the standards for primary equipment testing.

What data to ask the manufacturer for

• The list of cooling systems that influence the rated current and their combinations of failures.
• For each emergency, te, Re and Ie, and the test report that supports them.
• The maximum permitted total temperature at the hottest spot and where it is measured.
• The conditions of the isolated phase bus that were assumed, if it shares air with the breaker.

When to call us

At Tiza Group we test the circuit-breakers of your plant before commissioning, with contact and timing measurements, and we check that the alarms and controls are correct, in Colombia or elsewhere on request. See our circuit breaker testing. Write to us and we will send you a quote.

Related guides

• Generator circuit-breaker: classes G1 and G2
• Rated current and temperature rise of switchgear
• Infrared thermography: what it detects and how to interpret it
• Power circuit-breaker: parts, SF6 and vacuum, and testing
• Generator protection scheme: zones, backup and trips

References

• IEC/IEEE 62271-37-013, High-voltage switchgear and controlgear - Part 37-013: Alternating-current generator circuit-breakers, 2015 edition, clauses 4.4.101 (Figure 9) and 6.5.101.

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