An indoor substation houses all electrical equipment within the walls of a building. Indoor substations may either be entirely underground or look similar to other buildings in the neighborhoods that they serve. Outdoor substations have the same equipment as indoor substations but the equipment is located outside where it is exposed to natural elements, rather than in a building. The equipment is usually enclosed within a fence.
In general, a substation is a power system facility that contains power system components such as:-
– circuit breakers and other switchgear – transformers – reactors – capacitors
A substation usually includes a control house that contains equipment such as:-
– protective relays – meters – alarm annunciators – communications equipment
Ground Mat :– Substations usually include a ground mat. A ground mat is a system of bare conductors, on or below the surface, connected to a ground to provide protection from high voltages. One purpose of a substation is to contain the equipment for changing electric energy from one voltage to another. Substations also enable one or more of the following functions to be accomplished:- • Switching operations . Substations connect or disconnect elements of the power system, using circuit breakers and/or switches.
Reactive power compensation – Utilities install synchronous condensers, shunt reactors, shunt capacitors, and static VAR compensators at substations to control voltage. Utilities install series capacitors at substations to reduce line reactance.
In this section we describe the function and operating principles of the following power system components:-
– Switchgear Equipment– Capacitors – Reactors – Ground Switches – Lightning Arresters – Wave Traps
Switchgear is a general term given to switching and interrupting devices. Switchgear equipment is commonly contained in metal-enclosed units. However, at higher voltages, the equipment may or may not be in metal enclosed units. Switchgear equipment performs two separate functions. Under normal conditions, switchgear equipment enables routine switching operations to occur.
For example:- Switchgear equipment disconnects and isolates a piece of equipment so maintenance work can be performed. Under abnormal conditions, switchgear equipment automatically disconnects faulted equipment from the rest of the power system as soon as possible, in order to minimize damage. Under these conditions, switchgear equipment performs a protective function. All switchgear operates by pulling apart electric conductors (contacts). As the contacts are drawn apart while power is flowing through the device, an arc forms between the contacts. The arc is drawn out in length as the contacts open. To interrupt current flow, the arc must be extinguished with a dielectric substance, such as air, oil, or sulfur hexafluoride (SF6).
We will discuss the following types of switchgear equipment: –
* Circuit Breakers *Load Break Switches * Disconnect Switches.
Circuit breakers disconnect circuits or equipment from the power system. A circuit breaker’s primary function is to interrupt current flow under load or fault conditions. • Circuit breakers rapidly isolate faulted portions of the power system. They also provide a means to carry out routine switching operations, such as disconnecting a device to conduct maintenance. To interrupt the current, the circuit breaker must trip.
Let’s examine the mechanics involved in tripping circuit breakers under fault conditions.
• When a fault occurs, relays sense the fault and initiate the opening of the circuit breakers related to the faulted equipment by energizing the circuit breakers’ trip coils. The circuit breakers open for a pre-determined time period based on the reclosing relays settings. After the pre-determined time period, the reclosing relays signal the close coils to close the circuit breakers. The time delay is sufficient to allow the arc to extinguish.
• If the fault still exists after reclosing (e.g., if it is a permanent fault, such as a conductor touching the ground), then the relays signal the circuit breakers’ trip coils to open the circuit breakers again, this time permanently.
• Note: Not all circuit relay schemes include reclosing relays.
Some circuit breakers are equipped with a switch to manually trip in the event of a failure in the electrical controls. Such switches commonly need to be manually reset before the affected circuit breaker can be closed again. A common problem that results in failure to trip or close a circuit breaker is an open-circuit in the trip coil itself or in the DC wiring leading to the trip coil.
To detect an open-circuit condition before it is necessary to trip the breaker, manufacturers connect a lamp on the circuit breaker’s control panel in series with the trip coil and its associated wiring. – If the lamp is on, the wiring and coil are intact. – If the lamp is off, the wiring and coils are not intact and the circuit breaker may not operate when called on to trip.
We know that one function of a circuit breaker is to interrupt fault current. Tripping the breaker to Circuit breakers use many different insulating media to interrupt the arc.
• The most common insulating media include: – air – oil – SF6 – vacuum
There are several different circuit breaker types, including: – oil circuit breakers – air circuit breakers – air blast circuit breakers – gas blast circuit breakers – gas puffer circuit breakers – vacuum circuit breakers
Each type of circuit breaker indicates a method used to interrupt the arc within the breaker.
Air Blast Circuit Breaker:-
Interrupts the arc by: A blast of high-pressure air to stretch and cool the arc. At higher voltages pressurized air is the dielectric media. Energy breaks down air. Opening resistors for some ratings High arc voltage, high current chopping levels. Pressurized porcelain– 300 – 600 psig • Compressor systems. Complex. Noisy . Still built (very cold climates)
SF6 gas blast circuit breaker:-
Interrupts the arc by using Low-pressure SF6 gas as a dielectric. • A blast of high-pressure SF6gas (stored in a reservoir) is used to stretch and cool the arc. In association with a blast of compressed air. SF6gas is compressed in a chamber as the main contacts open. The compressed high-pressure gas is then directed toward the arc to stretch and cool it. • The expanding gas is allowed to exhaust to the inside of the interrupter head tank through the inside of the hollow stationary contact and moving arc horns. • These type breakers are also referred to as “Live Tank” breakers as the tank in which the contacts are located is not grounded.
VCB:-
Interrupts the arc by using the vacuum interrupters use a moving contact and a stationary contact both housed in a sealed, metal envelope. An actuator outside the envelope triggers the movement of the moveable contact. Inside the envelope, a metal or glass condensing shield surrounds the contacts to collect the evaporated contact material that is released when an arc is drawn. Since a vacuum contains no conducting material, the arc is immediately extinguished.
Rating and operations conditions of Circuit Breaker:-
The operating voltage and the amount of current the circuit breaker can interrupt determine a circuit breaker’s rating
A circuit breaker is rated based on the following parameters: –
* maximum voltage * continuous current * interrupting current capabilities
*Voltage capacity for example: – A circuit breaker may be rated as 765 kV maximum voltage, 3 kA continuous current, and current interrupting capability of 63 kA.
*Circuit breakers have a maximum continuous current carrying capability, which is the highest load current that the circuit breaker is designed to carry for extended time periods. • Exceeding this rating may result in overheating that could damage the circuit breaker contacts and insulation. Circuit breakers are also rated according to the maximum fault current that they are capable of interrupting. This is called maximum interrupting current. • Fault currents exceeding the breaker rating may produce arcs with more energy than the circuit breaker can extinguish. In this case, the breaker may fail to interrupt the fault current.
*Interrupting time is the period from the instant current begins to flow through the trip coil until the circuit breaker interrupts the fault. • Typical interruptions are measured in cycles and vary in time. On high-voltage transmission lines, interruptions vary from two to eight cycles.• Closing time is the time it takes to close a breaker from the instant the close coil is energized until current begins to flow through the breaker. • Typical closing times vary from two to 40 cycles.
Independent pole operation uses a separate mechanism for each breaker phase. Independent pole operation allows the tripping of independent poles (phases) which reduce the pos Single pole operation trips one pole for each phase-to-ground fault and trips all poles for all other types of faults.
• Selective pole operation clears only the faulted phase or phases for all types of faults.
