Out-of-Phase Switching Capability of a Circuit Breaker

Short answer

Out-of-phase conditions exist when the two parts of the system on either side of the circuit breaker have lost synchronism and the angle between their voltages, at the instant of operation, exceeds the normal value (definition 3.1.115 of IEC 62271-100). Switching in that condition demands more of the breaker, because across its contacts there is not a single source: there are two voltages out of phase.

For that reason the standard treats it as an optional capability. Clause 4.106 of IEC 62271-100 (2008 edition with Amendment 1:2012) states that specifying a rated out-of-phase making and breaking current is not mandatory. If the manufacturer assigns it, the values of the standard apply.

Rated values

The transient recovery voltage follows the tables of the standard according to the system:

• Table 1: breakers in cable systems below 100 kV.
• Table 2: line systems below 100 kV.
• Tables 3 and 4: from 100 kV to 170 kV, with and without an effectively earthed neutral.
• Table 5: from 245 kV.

Conditions of use assumed by the standard

The values hold only if these three conditions of clause 4.106 are met:

• Closing and opening operations are performed according to the manufacturer’s instructions.
• The system neutral earthing is the same as the one with which the breaker was tested.
• There is no fault on either side of the breaker.

How it is tested

Clause 6.110 defines the test. The criteria are:

• The power factor of the test circuit does not exceed 0.15.
• A single-phase circuit is normally used, with approximately half of the applied voltage and of the recovery voltage on each side of the breaker. With the manufacturer’s agreement, other arrangements are allowed, such as two voltages 120° apart or one terminal earthed.
• For systems with an effectively earthed neutral, the applied voltage and the power-frequency recovery voltage are 2.0/√3 times the rated voltage.
• For the other systems, the applied voltage at closing is 2.0/√3 and the recovery voltage is 2.5/√3 times the rated voltage.
• The DC component of the current at contact separation must be less than 20 % of the AC component.

The standard adds that the value of 2.0 per unit is specified at closing because it is the highest to which the first pole to close is usually exposed, which is the one with the longest pre-arcing time.

Duty OP1 may be omitted for breakers whose arc characteristics make critical-current tests unnecessary at a current lower than that of terminal-fault duty T10. In duty OP2, closing is made within ±15° of the peak of the applied voltage, and the making current is the rated out-of-phase making current, with the exceptions set by clause 6.110.3.

When the value in the standard is not enough

Clause 8.103.3 explains the scope. The requirements of the standard cover the great majority of applications of breakers that may switch out of phase. Exceeding them would require combining several circumstances, and since such operations are rare, designing the breaker for the most extreme case would not be economical.

But if frequent out-of-phase operations, or excessive stresses, are expected, the actual system conditions must be considered. The standard mentions two ways out:

• A special breaker or one with a higher rated voltage.
• Reducing the severity of the stress with relays that have coordinated impedance-sensitive elements, which control the instant of tripping so that interruption occurs well after or well before the phase angle reaches 180°.

The loss of synchronism that produces this condition is covered in protection 78. Closing onto a side without voltage is explained in dead line live bus closing. The TRV of this operation is in transient recovery voltage (TRV), and the generator circuit breaker case in out-of-phase operation of a generator circuit breaker.

What not to do

• Do not confuse out-of-phase switching with the synchronism check. This guide covers the breaker capability; the relay that enables closing is explained in the guide on function 25.
• Do not assume that every breaker can switch out of phase: the capability is either assigned or not assigned.
• Do not apply the rated values with a neutral earthing different from the one tested.
• Do not accept the capability without checking the recovery voltage for your type of system: 2.0/√3 if the neutral is effectively earthed and 2.5/√3 in the others.

What to request from the manufacturer

• Whether the breaker has a rated out-of-phase making and breaking current, and its value.
• The type of system for which it was proven: with an effectively earthed neutral or not.
• The test report for duties OP1 and OP2 of clause 6.110.

When to call us

At Tiza Group we analyze with you the points of the network where a breaker could close or open out of synchronism and define the rated values you should request in primary engineering. This design work is delivered remotely, worldwide. See our primary engineering design service or write to us with your single-line diagram and we will send a quotation.

Related guides

• Circuit breaker TRV: transient recovery voltage
• Generator circuit breaker: out-of-phase switching
• Short-circuit test duties T10 to T100a
• Synchronism check 25 (RSYN): how it works
• Out-of-step protection 78 (PPAM): what it is and how it works

References

• IEC 62271-100, High-voltage switchgear and controlgear - Part 100: Alternating-current circuit-breakers, 2008 edition with Amendment 1:2012, clauses 3.1.115, 4.106, 6.110 (Table 29) and 8.103.3.

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