Medium Voltage Switchgear

A 3 kV to 36 kV switchgear system is classed here as medium voltage switchgear or MV switchgear. IEC practice can run MV from 1 kV to 52 kV, so this band is a common indoor and outdoor range rather than the only definition. These switchgears come as metal enclosed indoor type switchgear, metal-enclosed outdoor gear, or outdoor gear with no metal housing. The interrupter may use insulating oil, SF₆ gas or vacuum. It must still interrupt current on a fault, whatever type of CB is fitted in the MV switchgear system. Load switching is extra duty on the same poles.
Medium voltage switchgear, should be capable of,

1. Normal load making and breaking.
2. Short-circuit current interruption.
3. Capacitive current switching.
4. Inductive current switching.
5. Special duties such as furnaces and traction.

Those duties have to be done without injuring people or the rest of the network.

Short Circuit Current Interruption

A circuit breaker is first sized to interrupt the prospective short-circuit current without failing open. How often it sees a real fault depends on the feeder, the network quality and the weather. Where that rate is high, a vacuum circuit breaker is the usual pick. Brochure endurance is often about 100 fault operations at up to 25 kA before a service, against about 15 to 20 shots on many oil or gas poles at the same short circuit current of CB. Rural substations are often outdoor and unmanned, so outdoor medium voltage switchgear that needs little servicing fits. Porcelain-clad vacuum circuit breaker columns cover that duty where indoor kiosks would not.

Switching of Capacitive Current

A capacitor bank is switched in on MV networks to raise power factor. Idle cable and overhead line also carry charging current. The capacitor bank and those idle lines must open without restrike. A restrike in the gap drives an over voltage. Vacuum interrupters are built for restrike-free capacitor class when specified that way.

Closing a capacitor bank also dumps a high inrush through the poles. A liquid Circuit breaker with tulip contacts can pin-weld on that inrush. A Vacuum circuit breaker MV panel sees only a short pre-strike electric arcing interval, so the faces take less melt.

Switching of Inductive Current

Older vacuum circuit breakers (VCB) chopped around 20 A, so a surge protection device was often fitted when they switched transformers. Modern VCBs with CuCr contacts typically chop about 2-4 A, so many unloaded transformers can be switched without extra arresters. Small inductive loads still need a chopping check; a very small transformer can still need a surge capacitor.

Special Application of Medium Voltage Switchgear

Arc Furnace

An electric arc furnace is opened and closed many times a day. Switched current can run from zero to about eight times furnace rating. A typical electric arc furnace at up to 2000 A may see around 100 operations a day at load current. A standard SF₆ circuit breaker, air circuit breaker or oil circuit breaker wears out too fast for that rate. A vacuum circuit breaker is the usual plant choice for that high-count circuit breaker operation.

Railway Traction

Single-phase railway catenary is another medium voltage switchgear duty. The breaker has to clear frequent, often transient, shorts on the overhead wire, so it needs a short break time, a short arc and a fast reclose capability. A vacuum pole does that with a small gap. Single-phase arcing energy is higher than the same current in a three-phase pole, yet vacuum still dumps less energy than oil or air. Catenary faults also outnumber faults on an electrical transmission system. Medium voltage switchgear with a vacuum circuit breaker is the usual traction pick. Where MV trip counts are high, MV Vacuum Switchgear is the practical fit.