High Voltage Switchgear

On this page, high voltage switchgear means gear that handles voltage above 36 kV. IEC lists other HV steps (often 52 kV and up), so 36 kV is a common site cut-off rather than the only one. Switching at these levels draws a long, hot arc, so the poles and tanks have to be designed for that. The high voltage circuit breaker (CB) is the unit that must still open a fault after sitting closed for months. Over recent decades the usual HV interrupters have been the Minimum oil circuit breaker (MOCB), the air blast circuit breaker, and the SF₆ circuit breaker.

A Vacuum circuit breaker is still uncommon at extra-high voltage. Vacuum poles now exist at some HV ratings, but they are not the usual pick for very high short-circuit current at EHV. There are two types of SF₆ circuit breaker, single pressure SF₆ circuit breaker and two pressures SF₆ circuit breaker. The single pressure puffer is the current high voltage switchgear system standard; two-pressure designs are historic. SF₆ gas is still the usual arc medium on high and extra high voltage electrical power system networks. SF₆ gas is also a strong greenhouse gas. IPCC 100-year GWP is about 23,500 times that of CO₂, not 23 times. Leakage of SF₆ gas over the service life must be kept near zero. To cut emission of SF₆ gas, N₂ – SF₆ and CF₄ – SF₆ mixes can cut the mass of pure SF₆, mainly in cold climates or to shrink inventory. They are not a full drop-in yet. Maintenance must not vent SF₆ gas to air.

The SF₆ circuit breaker still wins on long service intervals.
High voltage switchgears are categorized as,

1. Gas Insulated Indoor Type (GIS),
2. Air Insulated Outdoor Type.

Again, outdoor type air insulated circuit breakers are classified as,

1. Dead Tank Type Circuit Breaker
2. Live Tank Type Circuit Breaker

In a dead tank type CB the interrupter stack sits on insulator supports inside an earthed metal tank filled with the insulating medium. In a live tank circuit breaker the interrupters sit on bushings at line potential. Live-tank AIS units usually cost less and need a smaller pad than dead-tank units of the same rating.

Four types of circuit breaker appear in a high voltage switchgear system: the air blast circuit breaker, the SF₆ circuit breaker, the oil circuit breaker and the vacuum circuit breaker, the last still rare at EHV.

Air Blast Circuit Breaker

Compressed air is blown through the arc as the contacts part, timed for current zero when the arc column is least ionized.

Oil Circuit Breaker

Oil poles split into the bulk oil circuit breaker (BOCB) and the minimum oil circuit breaker (MOCB). A BOCB puts the interrupter in an earthed oil tank. Oil then insulates and interrupts. An MOCB cuts the insulating oil volume by putting the interrupters in a live insulating chamber on a porcelain column.

SF₆ Circuit Breaker

SF₆ gas is the usual HV arc medium. Sulfur hexafluoride gas is strongly electronegative, so its dielectric strength and arc cooling are high. That lets the tank and the contact gap shrink. The same gas also makes compact indoor type switchgear (GIS) practical at high voltage.

Vacuum Circuit Breaker

In vacuum, metal vapour after current zero has nothing left to ionize between the open contacts, so the first current zero usually ends the arc. The arc quenching method in a VCB is therefore fast. A VCB built for extra-high voltage still costs more than SF6 or GIS of the same rating, so vacuum is uncommon on EHV boards.

Required Features of High Voltage Circuit Breaker

A high voltage circuit breaker on HV switchgear has to clear, without damage to the rest of the network,

1. Terminal (bus) faults.
2. Short-line faults.
3. Transformer or reactors magnetizing current.
4. Energizing long transmission line.
5. Charging capacitor bank.
6. Out-of-phase switching.

Terminal Fault

Most power system loads look inductive. That inductance stores energy, so when the breaker chops the short-circuit current a high-frequency restriking voltage of a few hundred hertz can appear. That voltage has two parts

1. Transient recovery voltage (high-frequency) just after the arc goes out.
2. Power-frequency recovery voltage across the poles after that oscillation dies.

Transient Recovery Voltage

Right after current zero, transient recovery voltage (TRV) appears across the poles at high frequency and then settles toward the open-circuit voltage. A simple sketch is

The ring frequency follows L and C of the connected network. Circuit resistance damps the swing. A real grid has many L-C loops, so the TRV is a mix of frequencies, not one sine.

Power Frequency Recovery Voltage

Power-frequency recovery voltage is the open-circuit voltage left on the poles after the TRV has died. On a three phase system the first pole to clear sees the highest value. With an unearthed neutral that is about 1.5U (U is phase voltage). With an earthed neutral it is about 1.3U. A switching resistor can cut TRV peak and rate of rise. How fast the gap rebuilds dielectric strength versus how fast TRV rises decides whether a high voltage switchgear system breaker restrikes. In an air blast circuit breaker ionized air clears slowly, so dielectric strength returns slowly. A low-value opening resistor is therefore used to slow the TRV. An ABCB also has a high arc voltage, so the first microseconds of TRV bother it less. In an SF₆ circuit breaker the gas (SF₆) rebuilds dielectric strength faster than air. The lower arc voltage then makes the SF₆ pole more exposed to the initial TRV spike.
In an oil circuit breaker the arc cracks oil into pressurized hydrogen, which rebuilds dielectric strength quickly after current zero. An OCB is therefore hard on both RRRV and the first TRV peak.

Short Line Fault

A short-line fault is a fault only a few kilometres out on the line (often quoted around 5 km). The breaker then sees a sawtooth TRV: source-side oscillation minus line-side travelling-wave voltage, starting from the values that were on the poles just before interruption. Source-side voltage rings at power frequency and settles to the open-circuit value. On the line side the trapped charge launches travelling waves along the transmission line. With no driving voltage left, that line-side voltage decays to zero through line losses.