Controlled switching delays the close or open command to each breaker pole so that its contacts act at the point of the voltage or current wave that produces the fewest transients: near voltage zero when energizing capacitors, near the peak for reactors and transformers without residual flux, and with a safe arcing time when de-energizing reactors. The controller compensates for the scatter in the breaker operating time.
What controlled switching is
A circuit breaker that receives a command executes it at any instant of the voltage wave. For most loads that does not matter. For some equipment it does: depending on the point of the wave where the contacts touch or separate, the operation produces an inrush current several times rated, a transient overvoltage or an arc restrike.
Controlled switching, also called synchronous or point-on-wave switching, uses a controller that receives the command, measures a reference voltage or current and delays the command to each pole so that it acts at the optimum instant of its phase. The controller sits between the control or protection command and the breaker coils.
In IEC 61850 the function is modelled with logical node CPOW: it receives the operate command from the CSWI and delivers it to the breaker at the calculated instant.
Why the point of the wave matters
Capacitor banks. A discharged capacitor is a short circuit at the instant of closing. If the pole closes when the system voltage passes through zero, the inrush current is minimal; if it closes at the peak, it is maximal. With back-to-back banks the difference is even greater. The aim is to close near zero voltage across the contacts.
Shunt reactors. On opening, the problem is restrikes: if the contacts separate just before current zero, the gap is small when the recovery voltage appears and the arc restrikes. The controller chooses an arcing time long enough. On closing, the optimum instant is near the voltage peak, to avoid the DC component of the current.
Transformers. Inrush current depends on the difference between the flux imposed by the voltage and the residual flux in the core. Without a known residual flux, closing each phase near its voltage peak reduces the current. With residual flux, the controller must estimate it from the voltage at de-energization and choose the instant at which the imposed flux matches it.
Uncompensated lines. When a long line is energized or reclosed, the overvoltage depends on the voltage across the contacts at the instant of closing, including the trapped charge on the line after an opening. The controller looks for the zero of that voltage. It is an alternative to the pre-insertion resistors described in extra-high-voltage circuit breakers.
| Application | Controlled operation | Target instant |
|---|---|---|
| Capacitor bank | Closing | Near zero voltage across the contacts |
| Shunt reactor | Opening | Arcing time that avoids restrikes |
| Shunt reactor | Closing | Near the voltage peak |
| Transformer | Closing | Imposed flux equal to residual; without residual, near the peak |
| Uncompensated line | Closing or reclosing | Near zero voltage across the contacts |
Breaker scatter and compensation
The controller only hits the target if it knows how long the breaker takes. Closing and opening times vary with:
- ambient temperature,
- the control voltage of the coils,
- hydraulic or pneumatic pressure, or spring energy,
- how long the breaker has been idle,
- accumulated wear.
Modern controllers measure those variables and correct the expected time, and they also learn from previous operations by comparing the actual instant with the target. On closing, pre-strike must also be considered: the arc is established before mechanical contact, when the dielectric strength of the gap falls below the applied voltage. The breaker’s rate of decay of dielectric strength is part of the calculation.
That is why controlled switching requires breakers with stable times. Technical report IEC TR 62271-302:2010 covered circuit breakers with intentionally non-simultaneous pole operation, the basis of this technique; IEC withdrew it on 29 November 2024 without naming a replacement, so today the breaker is specified with IEC 62271-100 and the timing scatter data supplied by the manufacturer.
Where it is used
- High and extra-high-voltage substations with capacitor banks, shunt reactors and long lines.
- Energization of large power transformers, especially in weak networks or near generation, where inrush causes voltage dips or trips.
- Wind and solar plants that energize step-up transformers on networks with low short-circuit power.
- Medium voltage in some frequently switched capacitor banks, with vacuum breakers of stable timing.
How it coordinates with control and protection
- Control commands: manual and remote closing go through the controller, together with the applicable interlocks and synchronism check.
- Protection trips: normally not delayed. Protection opens immediately, except for reactors, where controlled opening is the reason for the scheme and the delay is a few milliseconds.
- Reclosing: if reclosing must be controlled, the automatic reclosing 79 command also goes through the controller.
- Fallback mode: if the controller fails or loses its reference, the command goes straight through and an alarm is raised.
What goes wrong in practice
- Shifted closing instant because the real breaker time changed after maintenance and nobody updated the controller.
- Wrong voltage reference or a reference assigned to the wrong phase, which makes each pole act at a point other than the target.
- Poorly estimated residual flux in transformers, with inrush currents that stay high.
- Controller in bypass with no visible alarm, with uncontrolled operations for months.
How it is tested
In general terms, the voltage reference of each phase is checked, the real breaker times are measured with a circuit breaker timing test and compared with the set values, and real operations are recorded with oscillography to check the closing or opening instant of each pole. The controller keeps its own records, which are useful for trending the times.
We can help with your controlled switching
At Tiza Group we do the secondary engineering that integrates the controller with breaker control, protection and SCADA, and the circuit breaker testing that feeds it, in high and extra-high voltage and in industry, in Colombia or elsewhere on request. Contact us to discuss your application.
Related guides
- Automatic capacitor bank control
- Circuit breaker close permissives
- Substation interlocking: types, levels and logic
- Automatic switching sequences in substations
References
- IEC TR 62271-302:2010, High-voltage switchgear and controlgear – Part 302: Alternating current circuit-breakers with intentionally non-simultaneous pole operation (withdrawn by IEC on 2024-11-29).
- IEC 62271-100, High-voltage switchgear and controlgear – Part 100: Alternating-current circuit-breakers.
- IEC 62271-110, High-voltage switchgear and controlgear – Part 110: Inductive load switching.
- IEC 61850-7-4, Compatible logical node classes and data object classes (CPOW, CSWI, XCBR).
Frequently asked questions
What does a circuit breaker need to support controlled switching?
Independent pole operation, or a three-pole mechanism with a fixed mechanical stagger between poles, and stable, repeatable operating times. IEC TR 62271-302:2010 covered circuit breakers with intentionally non-simultaneous pole operation, but IEC withdrew it in November 2024; today the breaker is specified with IEC 62271-100 and the manufacturer's documentation.
Does controlled switching replace pre-insertion resistors?
In some cases, yes. For line and capacitor bank energization it can replace pre-insertion resistors or current-limiting reactors, with fewer mechanical parts in the breaker. The decision comes from a switching transient study for each application.
What happens if the controller fails?
The design defines the fallback mode: usually the command goes straight to the breaker without delay, so the operation still happens but without control of the instant. That condition must raise an alarm, because the transients are back to those of an uncontrolled operation.