Generator Circuit Breaker: Classes G1 and G2

Short answer

A generator circuit-breaker is the circuit-breaker installed between the generator and its step-up transformer. IEC/IEEE 62271-37-013, 2015 edition, treats it as a separate product, because a short circuit between the generator and the transformer has two sources with very different behavior. Classes G1 and G2 describe how asymmetrical the generator-source current is that the breaker must be able to interrupt.

What the standard covers

Clause 1.1 sets the scope:

• three-phase AC circuit-breakers, for indoor or outdoor installation, at 50 Hz and 60 Hz;
• systems above 1 kV and up to 38 kV;
• circuit-breakers installed between the generator terminals and the transformer terminals;
• generators and transformers of 10 MVA or more, specifically covered.

Generator circuits below 10 MVA and pumped-storage installations are considered special applications. The standard states that their requirements are not fully covered. It also applies to the operating mechanisms and the auxiliary equipment of the circuit-breaker.

According to clause 3.4.104, a generator circuit-breaker is the circuit-breaker installed between the generator and its associated step-up transformer. Besides the breaking classes G1 and G2, the standard defines mechanical classes: M1 with normal mechanical endurance and M2 for frequent operation with limited maintenance (clauses 3.4.111 and 3.4.112).

Two short-circuit sources

A short circuit in the generator circuit can be fed from two sides. The standard separates them in clause 4.101:

For the system-source, the rated breaking capability is characterized by the rms value of its AC component and by a DC time constant. For the generator-source, two other values are used, explained below.

What the degree of asymmetry is

Clause 3.7.144 defines it as the ratio, in percent, between the DC component and the peak value of the AC component, determined from the envelope of the current wave at that instant. It is 100 % when the DC component equals the peak of the AC component.

The rated generator-source breaking capability is characterized by two values (clause 4.101.3.1):

• the rms value of its AC component at contact separation, Iscg;
• the degree of asymmetry at contact separation, Asycs.

If the degree of asymmetry exceeds 100 %, the current has delayed current zeros. If it does not exceed 20 %, the rms value of the AC component is enough.

Class G1 and class G2

Clause 3.4.113 defines class G1 and clause 3.4.114 defines class G2. Clause 4.101.3.3 summarizes them:

In both classes, the degree of asymmetry applies regardless of the instant at which contact separation occurs. The practical difference is that a G2 breaker must interrupt, at full current, a more asymmetrical current, that is, one with later current zeros, than a G1.

The standard gives no preferred values for the generator-source AC component, because its maximum is usually lower than the system-source one (clause 4.101.3.2). If the manufacturer assigns a value, it must test the breaker for that capability. At voltages above the rated voltage, no generator-source short-circuit breaking capability is guaranteed. At voltages equal to or below it, it is.

Other generator circuit-breaker values

• Short-circuit making current (clause 4.102). It is based on a DC time constant of 133 ms, and equals 2.74 times the rms value of the AC component of the system-source breaking current. If a generator-source capability is assigned whose making current is higher, the manufacturer assigns a higher value.
• Single-phase earth fault (clause 4.101.4). No specific capability is assigned, because these breakers are designed for systems with high-impedance grounding, where the earth fault current does not exceed 50 A.
• Standard operating sequence (clause 4.106.1). CO – 30 min – CO, with no intentional delay between closing and opening.
• Load current and out-of-phase switching. Each has its own rated capability.

The current asymmetry and the delayed current zeros are explained in generator-source fault current and delayed current zeros. The emergency current on loss of cooling is in emergency current on loss of cooling, and out-of-phase switching is in generator circuit-breaker out-of-phase switching. The generator is introduced in synchronous generator and its protection.

The map of standards for primary equipment testing places this part among the others of IEC 62271.

What to specify when ordering a generator circuit-breaker

• The generator-source breaking class, G1 or G2, and the rms value of the AC component expected from the generator.
• The system-source breaking capability and the rating of the step-up transformer.
• The mechanical class, M1 or M2, according to how many daily operations you expect.
• The rated continuous current and the emergency values if the breaker has forced cooling.
• Whether an out-of-phase switching capability is required, and at what angle.

When to call us

At Tiza Group we test circuit-breakers on site, measuring operating times, travel and contact resistance, and we check the controls before your generation is commissioned, in Colombia or elsewhere on request. See our circuit breaker testing. Write to us and we will send you a quote.

Related guides

• Generator-source fault current and delayed current zeros
• Generator circuit-breaker: emergency current without cooling
• Generator circuit-breaker: out-of-phase switching
• Generator protection scheme: zones, backup and trips
• Synchronous generator and its protection: parts and testing

References

• IEC/IEEE 62271-37-013, High-voltage switchgear and controlgear - Part 37-013: Alternating-current generator circuit-breakers, 2015 edition, clauses 1.1, 3.4.104, 3.4.111 to 3.4.114, 3.7.144 to 3.7.146, 4.101, 4.102 and 4.106.1.

Frequently asked questions