Accessories used in Electrical Substation
Power transformers: These are essential for stepping down the secondary transmission level voltage to primary distribution level voltage and distribute energy / power among feeders.
Bus-bars: A bus-bar in electrical power distribution refers to busbar formed by using line conductors or thick strips of copper or aluminium in control panel within the substation.
Circuit breakers: It is a switching device used to interrupt / make not only the load current but also the much larger fault current, which may occur on a circuit.
Isolators or isolating switches: These are mechanical switching devices capable of opening or closing a circuit:
- When a negligible current is to be interrupted / made or
- Only a small charging current is to be interrupted or
- When no significant voltage difference exists across the terminals of each pole.
Earthing switches: Earthing switches are provided at various locations to facilitate maintenance. Main blades and earth blades are interlocked with both electrical and mechanical means. The earthing switch must be capable of withstanding short circuit current for short duration as applicable to the isolator.
Insulators: It provides adequate insulation for live parts, which is of prime importance for obvious reasons of reliability of supply, safety of personnel and equipment.
Power and control cables: Power and control cables of adequate current carrying capacity and voltage rating are provided at the substation. Power cables are used for 66kV, 33kV,11 kV or LT system to carry load current. The control cables are required for operating and protection system connections.
Control panel: Control panels installed within the building which provide mounting for mimic bus, relays, meters, indicating instruments, indicating lights, control switches, test switches and other control devices. The panel contains compartments for incoming lines, outgoing lines, busbars with provision for sectionalizing, relays, measuring instruments, etc.
Lightning protection: Surge arrestors – Large over voltages that develop suddenly on electric transmission and distribution system are referred to as “surges” or “transients”. These are caused by lightning strikes or by circuit switching operations. Surge arrestor is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current. The surge arrestor which responds to over-voltages without any time delay is installed for protection of switchgear, transformers, associated equipment and lines.
Instrument transformers (CTs and PTs): The substations have current and voltage transformers designed to isolate electrically the high voltage primary circuit from the low voltage measurement and protection circuit and thus, provide a safe means of supply for indicating instruments, meters and relays.
Earthing arrangements: Earthing must be provided for –
− safety of personnel,
− prevention of and minimizing damage to equipment because of flow of heavy fault currents, and
− improved reliability of power supply.
Reactive compensation: It is always a good idea to ensure a power factor correction for transformers. When they are operating on low load (e.g. during the night) they absorb reactive power, which must be compensated to avoid unnecessary loadings and losses.
DC supply arrangement: Station batteries supply energy to operate protection equipment such as breakers and other control, alarm and indicating equipment. The station batteries are a source for operating DC control system equipment during system disturbances and outages. During normal conditions the rectifier provides the required DC supply. However, to take care of rectifier failure, a storage battery of adequate capacity is provided to meet the DC requirements.
Auxiliary supply transformer: An Auxiliary Supply Transformer of adequate capacity is required to be provided for internal use for lighting loads, battery charging, oil filtration plant, etc. The supply should be reliable. In a substation it is normally provided from a station transformer connected on bus bar.
Fire-fighting system: In view of the presence of oil filled equipment in a substation, it is important that proper attention is given to isolation; limitation and extinguishing of fire so as to avoid damage to costly equipment and reduce chances of serious dislocation of power supply as well as ensure safety of personnel.