1. Substations receive power transmitted at high voltages from generating stations and transform the voltage to appropriate levels for local use while providing facilities for switching. 2. Typical components of a power plant substation include busbars, disconnectors, circuit breakers, current transformers, voltage transformers, earthing switches, and surge arrestors. 3. Substations are classified based on their function and location as generating, grid, secondary, distribution, and special purpose substations and based on physical features as indoor, outdoor, pole mounted, and underground substations.

1. UNIT V
SUBSTATION, GROUNDING SYSTEM AND
DISTRIBUTION SYSTEM

2. SUBSTATION
SUBSTATION - A station in the power transmission system at which electric power is
transformed to a conveniently used form. The station may consist of transformers,
switches, circuit breakers and other auxiliary equipment. Its main function is to receive
energy transmitted at high voltage from the generating station, by either step-up or step-
down the voltage to a value appropriate for local use and provide facilities for
switching. Substations have some additional functions. Its provide points where safety
devices may be installed to disconnect circuits or equipment in the event of trouble.

3. Typical Components of a Power Plant
Substation (Switchyard)
•A - Busbar
•B - Disconnector
•C - Circuit Breaker
•D - Current Transformer
•E - Voltage Transformer
•F - Earthing Switch
•G - Surge Arrestor
• CONNECT
• CONNECT & DIS-CONNECT
• CONNECT, DIS-CONNECT & DETECT
• DETECT & TRANSFORM
• DETECT & TRANSFORM
• PROTECT & SAFETY
• PROTECT

4. BUSBAR
BUSBAR (or bus, for short) – is a term we use for a main bar or conductor carrying
an electric current to which many connection may be made.
Buses are merely convenient means of connecting switches and other
equipment into various arrangements. The usual arrangement of connections in
most substations permit working on almost any piece of equipment without
interruption to incoming or outgoing feeders. In the switchyard or substation,
buses are open to the air. Aluminum or copper conductors supported on porcelain
insulators, carry the electric energy from point to point.

5. Busbars
(long heavy
tube type)

6. DISCONNECTS
DISCONNECT – is an easily removed piece of the actual conductor of a circuit. The
purpose of disconnects is to isolate equipment. Disconnects are not used to interrupt
circuits; they are no-load devices. A typical use of disconnects is to isolate a circuit
breaker by installing one disconnect on either side of the circuit breaker (in series with
the breaker). Operation of disconnects is one of the most important and responsible jobs
of a power plant operator. One error in isolation of equipment, or the accidental
grounding of line equipment, can be a fatal mistake.

7. A
B
Disconnect Switch
( moving contact rod (A) &
contacts with flexible
fingers (B) )

8. CIRCUIT BREAKER
CIRCUIT BREAKER – is used to interrupt circuits while current is flowing through them.
The making and breaking of contacts in a Oil type circuit breaker are done under oil, this oil
serves to quench the arc when the circuit is opened. The operation of the breaker is very
rapid when opening. As with the transformer, the high voltage connections are made
through bushings. Circuit breakers of this type are usually arranged for remote electrical
control from a suitably located switchboard.
Some recently developed circuit breakers have no oil, but put out the arc by a blast
of compressed air; these are called air circuit breakers. Another type encloses the contacts
in a vacuum or a gas (sulfur hexafluoride, SF6) which tends to self maintain the arc.

9. Circuit Breakers
( Connected in a typical 3-
phase circuit )
Operating
Mechanism
Panel
Position
Indicator

10. CURRENT TRANSFORMER
CURRENT TRANSFORMER – Current transformer are used with
ammeters, watt meters, power-factor meters, watt-hour
meters,compensators, protective and regulating relays and the trip coil of
circuit breakers. One current transformer can be used to operate several
instruments, provided that the combined burden does not exceed that for
which the transformer is designed and compensated. The current
transformer is connected directly in series with the line.

12. VOLTAGE TRANSFORMER
VOLTAGE TRANSFORMER – also know as potential transformer, are used with
volt-meters, wattmeters, watt-hour meters, power-factor meters, frequency meters,
synchroscopes and synchronizing apparatus, protective and regulating relays and the
no-voltage and over-voltage trip coils of automatic circuit breakers. One transformer
can be used for a number of instruments at the same time if the total current taken by
the instrument does not exceed that for which the transformer is designed and
compensated. The ordinary voltage transformer is connected across the line, and the
magnetic flux in the core depends upon the primary voltage

14. EARTHING SWITCH
EARTHING SWITCH – also known as ground disconnect, which used to connects
the equipment to a grid of electrical conductors buried in the earth on the station
property. It is intended to protect people working on the grounded equipment. It does
this by completing a circuit path, thereby reducing the voltage difference between the
equipment and its surroundings. For safety reasons, it is important that ground
disconnects and all associated connections have good contact and low resistance. It is
also important that the protective ground not be accidentally remove, that is why all
the earthing switches, disconnect switches and circuit breakers are all interlocked to
each other and proper/correct sequencing must be followed.

15. SURGE ARRESTOR
SURGE ARRESTOR – are devices used to provide the necessary path to ground for such surges,
yet prevent any power current from following the surge. An ideal arrester must therefore have the
following properties:
1. Ability to remove the surge energy from the line in a min. time.
2. High resistive to flow of power current.
3. A valve action automatically allowing surge to pass and then closing up so as not to
permit power current to flow to ground.
4. Always ready to perform.
5. Performance such that no system disturbances are introduced by its operation.
6. Economically feasible

17. • OVERHEAD GROUND WIRE – by a ground wire is meant a wire, generally
of steel, supported from the top of transmission-line towers and solidly grounded
at each tower. It is considered a preventive device, but it does not entirely prevent
the formation of travelling waves on a line. Furthermore, those lines which are
not equipped with ground wires will be subjected to disturbances which produce
surges that must be allowed to escaped to ground, or the apparatus connected to
the line must be strong enough to reflect or absorb these surges until they are
entirely damped out.

19. PREVENTIVE MAINTENANCE
BUSBARS & OVERHEAD GROUND WIRE
At least once a year
• Visual Inspection & Examination of all wiring connectors.
• Check Insulator , clean or apply HVIC if necessary.
• Check the physical condition of bus (cables or bars)
• For ground wire, check or test the grounding system.
DISCONNECT & EARTHING SWITCHES
At least once a month
• Visual Inspection.
• Check heating resistor located at its control panel for proper functioning.
At least once a year
• Clean contacts of disconnectors as well as earthing switches and apply
electrical contact grease , if necessary.

20. • Check disconnectors and earthing switches, joints and
bearings of the operating linkages for deformed bearing
points.
• Check flexible connections of earthing switches.
• Check all screwed joints for tight fit.
• Clean insulators if necessary, when an excessive amount of
dirt has accumulated.
• Carry-out the maintenance of operating mechanism.

21. VOLTAGE TRANSFORMER
At least once a month
• Inspect the voltage divider to be sure that no oil leak or serious
accumulation of soot, dust or salt composite is present.
• Inspect the intermediate voltage transformer and check the
minimum permissible oil level.
At least once a year
• Check all screwed joints & contact for tight fit.
• Clean insulators if necessary, when an excessive amount of dirt
has accumulated.

22. CURRENT TRANSFORMER
At least once a month
• Visual Inspection to check oil level and defects or possible
oil leaks.
At least once a year
• Check all screwed joints & contact for tight fit.
• Clean insulators if necessary, when an excessive amount of
dirt has accumulated.
• Check primary and secondary connectors and conduct
necessary tightening.
Note: Never open a secondary winding of a CT while on
service.

23. SURGE ARRESTOR
At least once a year
• Visual Inspection & Examination of all wiring connectors.
• Check Insulator and metal circular ring, clean or apply HVIC if necessary.
• Check the physical condition of bus (cables or bars) For ground wire, check or
test the grounding system.
• Note: Arrestors should never be touched unless completely disconnected from
all live lines and equipment and effectively connected to ground at the line
side of the arrestor.

24. FEEDERS DISTRIBUTORS SERVICEMAINS
Distribution
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25. FEEDERS
• These are the cables supplying power in bulk to a selected number of points called
feeding points The feeders run along streets overhead (or underground, in some cases)
and power the distribution transformers at or near the customer premises.
25

26. DISTRIBUTORS
• Distributors are used for current Tapping for the various consumers these
cables are generally having the main street for there route .
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27. SERVICE MAIN
• Service mains are the small cables teed of off from the distributors and taken into the
premises of the various consumers these are low tension cables.
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28. TYPES OF NEUTRAL GROUNDING
Solid grounding
Resistance grounding
Reactance grounding
Are suppression coil or Peterson coil or resonant grounding
Voltage transformer grounding
Zig-zag transformer grounding.

29. Solid grounding:
In solid grounding a direct metallic connection is made as shown in the following
figure. From the system neutral one or more earth electrodes consisting of pipes,
plates or rods are buried in the ground.
When an earth fault occurs between earth and any one phase, the voltage to
earth of the faulty phase becomes zero, but the healthy phase, remains at their
normal phase values.
So lightning arresters of low voltage rating can be used, saving the cost

30. The flow of heavy current If completely nullifies the effect of the
capacitive current Ic at the fault and so no arcing ground or over voltages.
Due to the flow of high fault current, protective relaying is possible.
Increase in earth fault current causes disturbance in the neighboring
communication lines.
Heavy fault current may damage the circuit breaker contacts.
Use of solid grounding is limited to only to system where the normal
circuit impedance is sufficient to prevent very high fault current.

31. Types of substations
As an integral part of the transmission system, the substation as switching station
functions as a connection and switching point for transmission lines. Sub-
transmission feeders, generating circuits and step up and step down transformers.
The objective of substation is to provide maximum reliability, flexibility and
continuity of service and minimise investment cost and safety system requirements.

32. Depending on the service purpose, the substation may be classified
as follows
Generating substations or step-up substation:-
Generating substations are located near the generating plants. The generating
voltages are 11kv,10kv, 6.6kv or 13.8kv respectively, and need to be stepped upto
the transmission voltages in the range of 66of or 110kv or 132kv or 220kv. So that
large amount of power can be transmitted economically over long distance. Each
generating unit is connected to generating transformers to increase the secondary
voltage upto transmission voltage level.
Grid substations:-
Grid substations are located in the intermediate points between the generating
stations and load centres. Load centre receive all the information from the
generating stations and to ensure corrective actions. The main purpose of these
substations is to provide connections of low voltage lines, some compensating
devices etc.

33. Secondary substations:-
These substations are connected with the main grid substations with the help of
secondary transmission lines. The voltages at these substations are stepped down to the
primary distribution voltage or sub-transmission voltage such as 11kv or 6.6kv directly
connected to these substations.
Distribution substation:-
Distribution substations are connected between primary distribution and secondary
distribution. The primary distribution voltage such as 11kv or 6.6kv is to be stepped
down to the supply voltage (400v for three phase and 230v for single phase). These
substations transfer power to the consumers through distributors and service main.

34. Special purpose substations:-
a) Traction substation and mining substation:
These substations transfer bulk power. Special design considerations are required in these
substations such as load distribution in phases in traction substation and safety precautions in the
mining substations.
b)Power factor correction substations:-
These substations are used to improve power factor of the system. These are located at the
receiving end of transmission lines. We use synchronous contenders as the power factor
improvement equipment.

35. c) Frequency changer substations:-
These substations are used to change the supply frequency. Such a frequency change
may be used for some industrial utilisation.
d) Converting substations:-
The converting substations are used to convert a.c power into d.c power. These
substations are required for some purpose like traction, electroplating, electric
welding etc
e) Mobile substations:-
These are used for construction purpose, which are temporary as mobile.

36. Depending upon the physical features, the substation can be classified as follows:
Indoor substations
These secondary substations are of indoor type. The equipment of indoor substations lie
in a room because of economic consideration. The operating voltage are normally 11kv.
These substations are located in big cities.
Outdoor substations:-
The distribution stations are mostly of outdoor type. All equipments required for
switches,circuit breakers, lightning arrestors, potential transformer, current transformers,
capacitor banks are installed outdoor (i.e., open in the air) . Normally, these substations
are used for 33kv voltage and above for cost and safety reasons. The air clearances
required are large. The control and monitoring are performed inside the control rooms.

37. Pole mounted or open or kiosk type:-
These substations are mounted on H-pole (upto 160 KVA) or an iron platform formed by
4-pole structure (upto 400KVA) . These substations are simple and cheap. These are not
required any building for housing the equipments. The operating voltages are upto 11kv.
Electric power is distributed to the consumer from these substations.
Underground substations:-
In highly populated areas, the land required for building equipments is limited and also
the cost of land is very high. Underground substation is located underground. The size of
the substation depends upon the capacity.