Switchgear Equipment

Switchgear can be defined as equipment used for controlling, regulating and switching ON and OFF power circuits. Switchgear units range from the small moulded-case unit circuit breakers in a household panel board to the huge, air break switches on 750 kV transmission lines.

Switchgear equipment is generally divided into four groups, namely:

1. Disconnect or isolator switches
2. Load break switches
3. Circuit breakers
4. Contactors

Disconnect/Isolator Switches

Disconnect or isolator switches are used to connect or disconnect circuits at no load or very light loads. They have minimum arc-quenching capability and are intended to interrupt only transmission line charging currents or transformer exciting currents at most. They are normally the least expensive type of switch.

Mechanically disconnect/isolator switches are designed to provide sufficient contact pressure to remain closed through fault currents despite the high mechanical forces these currents may cause. Simple knife switches rely on multiple leaves for contact and frictional forces to maintain contact. Other types have over-center latches, while still others have clamping locks that toggle toward the end of the closing cycle. All operate in air and have visible contacts as a safety provision although low-voltage safety switches relay on handle position. All have provisions for lockout.

Medium and high voltage disconnect switches are available as indoor designs that are typically mounted in metal switchgear enclosures or as outdoor switches incorporated into elevated structures. Both horizontally and vertically operating switches are available in outdoor designs, and most are available with motor operators. Some have optional pneumatic operators.

Load Break Switches

In general, load break switches follow the basic design arrangements of disconnect switches except that they are equipped with arc chutes that enable them to interrupt current they are designed to carry. They aren’t designed to interrupt fault currents; they must remain closed through the faults. Most-operated load break switches can be a lower-cost alternative to circuit breakers in some applications where remote control is needed.

Circuit Breakers

Circuit breakers are the heavy-duty members of the switchgear family. They are rated thermally for a given continuous load current, as well as a maximum fault current that they can interrupt. The arcing contacts are in air with small breakers, but the larger types have contacts in vacuum or oil.

High-voltage utility circuit breakers may utilize Sulfur hexafluoride (SF₆) gas. Most circuit breakers have a stored energy operating mechanism in which a heavy spring is wound up by a motor and maintained in a charged state. The spring energy then swiftly parts the contacts on a trip operation. Usually, the circuit is cleared in 3 to 5 cycles, since the rapid interruption is required to minimize arc heating and contact erosion. Indoor circuit breakers are normally in metal cabinets as part of switchgear line up, while outdoor ones maybe stand-alone units.

Some caution should be taken when specifying vacuum circuit breakers. When these breakers interrupt an arc, the voltage across the contacts is initially quite low. As the current drops to a low value, however, it is suddenly extinguished with a very high di/dt; this current is termed to as the chop current and it can be as high as 3 to 5 A. If the circuit breaker is ahead of a transformer, the high di/dt level can generate a high voltage through the exciting inductance of the transformer, and this can be passed onto the secondary circuits. The required voltage control can be achieved with arresters on the primary of the transformer or metal oxide varistors (MOVs) on the secondary of the transformer. The MOV should be rated to dissipate the transformed chop current at the clamping voltage rating of the MOV. It must also be rated for repeated operations while dissipating the ½ LI² energy of the primary inductance where I is the chop current.

You can also read: Key Components used in Industrial Control

Moulded Case Circuit Breakers (MCCBs)

Moulded case circuit breakers are equipped with thermal and magnetic overload elements that are self-contained. They are rated by the maximum load current and interrupt capacity. The thermal types use selectable heaters to match the load current for overload protection. Larger circuit breakers are operated from the external protective relays that can provide both the overload and short circuit protection through the time overcurrent elements and instantaneous elements. Nearly all the relays are operated from current transformers and most are solid-state.

Because of their heavy operating mechanisms, circuit breakers are not rated for frequent operation. Most carry maximum number of recommended operation before being inspected and repaired if needed. Also, after clearing outdoor a fault circuit breaker, it ought to be inspected for arc damage or any mechanical problems.

Contactors

These are electromagnetically operated switches that can be employed for motor starting and general purpose control. They are rated for many thousands of operations. Contactors can employ air breaks at low voltages or vacuum contacts at medium voltages. Most have continuously energized operating coils and open when control power is removed

Motor starters can handle overload of five times or more, and lighting contactors also have overload ratings for incandescent lamps. The operating coils often have a magnetic circuit with a large air gap when open and very small air gap when closed. The operating coils can have a high inrush current when energized and the control power source must be able to supply this current without excessive voltage drop.

Some types have optional DC coils that use a contact to insert a current reducing resistor into the control circuit as the contactor closes.

Contactors may be mounted within equipment cabinets or as stand-alone units.

Bottom Line

Any piece of electrically operated switch gear whether a circuit breaker or contactor, has inductive control circuits that can develop high voltages in control circuits when interrupted. Good design practice for R/C transient suppressors on operating coils or motors. Metal oxide varistors (MOVs) will limit the developed voltage on opening but will be of no help in limiting the di/dt that can interfere with the other circuits.

Related articles:

• SF6 Circuit Breakers: Features, Operation & Applications
• 5 Causes of High-Voltage Switchgear Failures
• Miniature Circuit Breakers (MCBs)
• Types of Busbar Arrangements in Grid Stations and Substations