Generator Circuit-Breaker: Out-of-Phase Switching

Short answer

Out-of-phase conditions are those in which the generator and the grid are out of step when the breaker operates. IEC/IEEE 62271-37-013, 2015 edition, defines for the generator circuit-breaker a rated out-of-phase current equal to 50 % of the AC component of its rated system-source short-circuit breaking current. The standard makes it clear that most of these breakers close in that condition, but do not interrupt in full phase opposition, and that damage to the generator must be avoided by preventing incorrect synchronizing.

What out-of-phase means

Clause 3.1.104 defines out-of-phase conditions as abnormal conditions of loss or lack of synchronism between the parts of the system on either side of the breaker. At the instant of operation, the angle between the phasors of the voltages generated on each side exceeds the normal value.

According to clause 6.106.1, that angle can reach 180°, and the resulting current depends on it: it reaches its maximum at 180°, which is full phase opposition.

What current is rated

Clause 4.104 defines the rated out-of-phase making and breaking current as the highest out-of-phase current that the breaker must be able to make and interrupt under the prescribed conditions of use. That current is sought in a circuit with a power-frequency recovery voltage equal to the rated voltage of the breaker and with the transient recovery voltage of clause 4.105. The rated rms value is 50 % of the rms value of the AC component of the rated system-source short-circuit breaking current.

Clause 6.106.2 adds two clarifications. The capability is specified with the maximum value of the out-of-phase recovery voltage for the first-pole-to-clear, which implicitly defines the maximum angle. If the breaker has a rated out-of-phase capability, that capability is based on an angle of 90° at rated voltage.

For the tests, the generator neutral is considered not effectively grounded. The recovery voltage of the first-pole-to-clear is √2 × 1.5 × Ur / √3 = 1.22 Ur, which corresponds to an angle of 90° (clause 6.106.3).

Closing yes, interrupting not always

Clause 6.106.1 makes a practical clarification: most generator circuit-breakers are expected to close, but not to interrupt, under full phase opposition conditions. Only breakers with full interrupting capability, that is, to clear short circuits from either of the two sides, can have a rated out-of-phase capability.

When the user specifies a full-opposition capability, a special breaker may be required, with an interrupting capability that often exceeds the rated short-circuit one. The following are considered:

• a power-frequency recovery voltage equal to Ur, with the transient recovery voltage calculated for the first-pole-to-clear;
• an out-of-phase current of intermediate magnitude between the system-source and the generator-source short-circuit currents, whose value is calculated for each installation.

Why synchronizing is still necessary

The same clause points out a risk that the breaker does not solve. If the sum of the short-circuit reactances of the transformer and of the network, on the transformer side, is lower than the short-circuit reactance of the generator, the full-opposition current would exceed the subtransient current resulting from a short circuit at the generator terminals. The electrodynamic overloading of the generator windings must be prevented by suitable measures, such as avoiding incorrect synchronizing.

That is why the breaker ensures its rated capability but does not replace the synchronizing process. The guide on generator synchronizing explains how voltage, frequency and angle are checked before closing, and the one on generator in parallel with the grid covers parallel operation.

Operating sequence

Clause 4.106.1 sets the standard operating sequence of the generator circuit-breaker: CO – 30 min – CO, with no intentional delay between closing and opening. For the out-of-phase current, clause 4.106.4 keeps that sequence unless the manufacturer assigns shorter intervals.

Transient recovery voltage

The transient recovery voltage for out-of-phase current is given in Table 6 of the standard, as a function of the generator rating. The transient voltage is made up of two parameters, the peak value uc and the time t3, which the standard defines with a two-parameter representation (clause 4.105.1). Clause 6.106.6 requires the prospective transient voltage of the test circuit to meet those values.

What to specify

• Whether the project needs a rated out-of-phase capability, and not only that the breaker closes in that condition.
• The system-source breaking current, because the 50 % comes from it.
• The generator rating, to set the applicable transient recovery voltage.
• The synchronizing scheme and the interlocks that prevent closing out of synchronism.
• The operating sequence and the intervals between operations assigned by the manufacturer.

The generator circuit-breaker classes are explained in generator circuit-breaker: classes G1 and G2. Out-of-phase switching for a general power circuit-breaker is covered in out-of-phase switching of the circuit-breaker.

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

When to call us

At Tiza Group we configure and test the synchronism and protection functions of the generator, and we verify them end to end before the first synchronization, in Colombia or elsewhere on request. See our protection, control and communications configuration service and our protection testing. Write to us and we will send you a quote.

Related guides

• Generator circuit-breaker: classes G1 and G2
• Out-of-phase switching of the circuit-breaker
• Generator synchronizing: conditions and equipment
• Generator in parallel with the grid: protection and utility approval
• 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 3.1.104, 4.104, 4.105, 4.106 and 6.106.

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