Electrical General Services Sub-station is a distribution sub-station from which electric supply is distributed to the different users. In a substation there are numbers of incoming and outgoing circuits each having its isolator, circuit breaker, transformers etc. connected to bus-bar system. These equipment are mostly static type. Safety and protection of equipment as well as working personnel is also a considerable factor. Lightening arresters, earthing of equipment and fencing is done for this purpose. Maintenance of a sub-station is essential to ensure un-interrupted electric supply to the using points which involves vigilance, care and well-defined scheme of procedures. Such scheme indicates the authority and responsibility of persons at various levels. It consists of many of periodic schedule maintenance, regular inspections, testing and rectification of defects.
Upkeepment of records of each inspection is essential.
If any replacement is carried out or
adjustment of certain setting is done then these must be entered in a logbook.
A rigid system of maintenance of each
equipment will ensures long life, trouble free service and reduction in unnecessary interruptions.
All work on major electrical installations must be carried out under permit to work system.The “permit to work” is to be issued through permit card only by an authorized person. As the name suggests it authorizes the maintenance supervisor and his team to carry out work. Furthermore, this card will indicate unambiguously the points at which it is safe to work, the time interval when it is to be done, steps to be taken to ensure safety such as earthing, display of danger notices etc. at the nearest live point. It should have the signature of the authorized person. After the work is completed the permit card should be cancelled and it should be taken back. Danger notices should be put up or removed by the responsible supervisor who will take the charge of keys equipment, rooms, etc
1.1
EQUIPMENT IN DISTRIBUTION SUB-STATION
The following
equipment are installed in Electrical
General Services Sub-station:
1.
Distribution Transformer
2.
Circuit breaker
3.
Lightning Arrester
4.
Air Break (AB) switches/ Isolator
5.
Insulator
6.
Bus-bar
7.
Capacitor Bank
8.
Earthing
9.
Fencing
10.
Distribution panel board
1.1.1
Distribution Transformer
The distribution transformer is a main and largest
equipment of Electrical General Service
Sub-station. It is basically a static electrical device which steps down the
primary voltage of 33kV or 11 kV to
secondary distribution voltage of 415-440 volts between phases and 215 volts between phase and neutral
through delta-star windings by electromagnetic
induction without change in frequency.
Transformer consists
of the following parts and components.
·
Primary winding Secondary winding
·
Transformer tank Conservator
·
Cooling tubes Breather
·
Buchholz Relay Explosion
vent
·
Tap changer Oil inlet
valve
·
Oil outlet valve Oil level indicator
·
L.T. terminals H.T. terminals
·
Temperature gauge
F Working of Important Components
·
Conservator – Equipped
with transformer of rating 500 kVA & above
according to heat produced, this expansion and contraction of oil causes the level of the oil in conservator to rise and fall. The aim of conservator is to
- maintain the oil level in tank
- provide space for the
expanded oil.
· Breather
It is attached to conservator tank and contains silica gel, which prevents the moist air from entering into the tank during contraction of oil. When oil is hot there is expansion and gas passes to atmosphere through it. When oil is cooled, it contracts and the air enters in it. It prevents transformer oil from moisture contamination.
•Buchholz Relay
It is protective relay of transformer. This device signals the fault as soon as it occurs and cuts the transformer out of the circuit immediately. This is gas operated protective relay. It is installed in between the pipe connecting the tank and the conservator. This relay works on the formation of excessive oil vapors or gas inside the transformer tank due to internal fault of transformer. It consists of two operating floats A and B. These are operated by two mercury switches separately provided for each float. The float A is for bell alarm and float B is for operating the tripping circuit.
Whenever there is a minor fault or low level of oil, the bell alarm operated by float ‘A’ and whenever there is severe fault in the transformer, float ‘B’ operated due to excessive gases. It trips the circuit breaker and transformer is put out of circuit.
·
Explosion Vent
A major fault inside the transformer causes instantaneous vaporization of oil, leading to extremely rapid buildup of gaseous pressure. If this pressure is not released with in few milliseconds, the transformer tank can rupture, spilling oil over a wide area. An explosion vent provides instantaneous releasing of such dangerous pressure and protects the transformer.
Oil level Indicator
It indicates level of transformer oil at the conservator of the transformer. It has markings on transparent sheet for maximum & minimum levels.
Inlet Valve
It provides passage to pour the transformer oil in the tank during purification or in case of shortage found in the tank.
Outlet Valve
It provides passage to drain the oil during overhauling or as and when required oil sample for testing.
Cooling Tubes
These tubes provide better and effective cooling of transformer oil by increasing the surface area of tank to the atmosphere.
Tape changer
The tape changer is used to regulate the out- put voltage manually according to line voltage. The tapes of transformer can be changed by the tape changer manually. It is provided on HV side so that the voltage on LV side feeding to the load can be maintained. Normally tap selection range is ± 15 % in steps of 2.5 %
Circuit Breaker
The circuit breaker is an equipment which automatically cut off power supply of the system when any fault or short circuit occurs in the system. It detect and isolate faults within a fraction of a second thereby minimizing the damage at the point where the fault has occurred. The circuit breakers are specially designed to interrupt the very high fault currents, which may be ten or more times the normal operating currents.
There are many types of circuit breakers, e.g. Oil, minimum oil, Air blast, Vacuum, SF6, etc. being used at sub-stations. This list is generally in order of their development and increasing fault rupturing capacity, reliability and maintainability. In distribution sub- station, generally oil circuit breakers and air circuit breakers are used.
Lightning Arrester
Air Break (AB) Switch/ Isolator
Insulator
The main function of an insulator is to insulate live conductor or equipment at different voltages with reference to the ground structures as well as provide mechanical support. Provision of adequate insulation in a substation is of primary importance from the point of view of reliability of supply and safety of personnel.