Unit-III (Electrical power supply systems)

Comparison between A.C. and D.C. transmission system: D.C transmission A.C. transmission

1. It requires two conductors. 2. There is no inductance and capacitance

effect in d.c. transmission. 3. Due to absence of inductance the voltage

drop in d.c. transmission is less. 4. There is no skin effect in d.c. system. 5. Electric power cannot be generated at high

voltage. 6. D.C. voltage cannot be stepped up or

stepped down.

1. It requires three conductors. 2. There is inductance and capacitance effect

in a.c. transmission. 3. Due to presence of inductance the voltage

drop in a.c. transmission is more. 4. There is skin effect in a.c. system. 5. Electric power can be generated at high

voltage. 6. A.C. voltage can be stepped up or stepped

down by transformer.

Draw the layout of power system. Or

Draw the single line diagram of supply system from generating station to consumers.

Layout of power system

Generation: The generation where the electric power is generated. The standard

generating voltages are 3.3kV, 6.6kV, 11kV and 33kV.

Transmission: To transmit electrical power from generating station to load centre at higher

potential is known as transmission. The standard transmission voltages in India are 33kV, 66kV, 110kV, 132kV, 220kV, 400kV, 765kV and proposing 1200kV.

Distribution: To distribute electrical energy to different consumers is known as distribution.

The distribution voltages are Primary distribution: 11kV Secondary distribution: (i) 440V for 3phase- 4wire (ii) 240V for single phase-2wire

Types of power station or generating station: 1. Thermal power station 2. Hydro electric power station 3. Diesel power station 4. Nuclear power station 5. Gas turbine power station

Name of thermal power station in West Bengal: 1. Kolaghat thermal power station 2. Bakreswar thermal power station 3. Bandel thermal power station 4. Santaldih thermal power station 5. Sagar dighi thermal power station

Hydel power station: 1. Jaldhaka power plant(Siliguri) 2. Ayadha pahar project(Purulia)

Some non-conventional sources: 1. Solar power plant 2. Wind power plant 3. Tidal power plant 4. Photo voltaic cell 5. Geo thermal plant 6. Bio-gas plant

Single phase system: When single turn coil is rotating is magnetic field then the single

turn coil will produce an e.m.f. this e.m.f. is known as single phase system.

Three phase system: A three phase system may be considered as three separate single

phase system and phase displacement from each other by 1200 or 2𝞹/3 radian.

Star connection: The star connection is the connection where,

VL =√3Vph

And

IL = Iph

VL = line voltage Vph = phase voltage IL = line current Iph = phase current

Star connection

Line voltage: Voltage between any two phases is known as line voltage. Phase voltage: Voltage between phase and neutral is known as phase voltage.

Delta connection: The delta connection is the connection where,

VL = Vph

And

IL = √3Iph

Delta connection

Note that in star connection there is a neutral point whereas in delta connection there is no neutral point. Single phase supply is obtained from star connection only but three phase supply is obtained from both star and delta connection.

Domestic power supply

Service connection for house wiring: For single phase service connection from

pole to house the following are required. 1. Pole and pole fuse: Pole carries the supply lines and fuse is to protect any fault. 2. Energy meter: It provides the energy consumption record of the consumer. 3. Service fuse: It protects the energy meter from over loading. 4. Main switch fuse: It isolates the consumer installation from supply and fuse protects

the equipments during fault. 5. Distribution board: It is used to distribute electrical power to different circuit. 6. Earth wire: It prevents shock from touching any metallic body in case of faulty

installation.

Types of house wiring: 1. Cleat wiring: It is used for temporary wiring purpose such as in Puja Pandel. 2. Batten wiring: It is used in damp climate but not in heat or sunlight. 3. Casing and capping wiring: It is most common type wiring used for indoor and domestic

installation. 4. Lead sheathed wiring: It is used in sunrays, rain and in the place of damp. 5. Conduit wiring:

(a) Surface conduit: It is used in work-shop for motor wiring library, auditorium. (b) Concealed wiring: It is used in modern residential building, cinema hall and

auditorium etc.

Conductor used: It may be copper conductor or Aluminium conductor.

Fuse: A fuse is a protective device connected in series with the circuit, when fault occur it

melts and breaks the circuit. The fuse is placed in live or phase wire not in neutral wire.

Some common name of fuse: 1. Re-wireable or kit-kat fuse 2. Cartridge fuse 3. High rupturing capacity fuse (HRC) 4. Miniature circuit breaker (MCB) 5. Moulded case circuit breaker (MCCB) 6. Earth leakage circuit breaker (ELCB) 7. Residual current circuit breaker (ELCB)

Earthing: The earthing is the process of connecting the metallic frame of electrical

equipments or some electrical part of the system to earth.

Why earthing is done? The main function of earthing is to protect from electrical shock in case of leakage current to earth due to faulty installation.

Type of earthing: 1. Plate earthing: It is used for domestic installation. 2. Pipe earthing: It is used for motor installation and sub-station earthing. 3. Rod or spike earthing: It is used in transmission and distribution line. 4. Wire or strip earthing: It is used in hilly areas.

Connection diagram of two way switch: The two way switch is mainly used in

stair case lighting, hall lighting and big workshop where the lamp point is required to be controlled from two places.

Measuring and testing of instruments

Instruments To be measured Unit Connection

Ammeter Current Volt Series

Voltmeter Voltage Amp Parallel

Watt meter Power Watt Series and parallel

Energy meter Energy k.w.h. Series and parallel

Ammeter: An ammeter is a measuring instrument used to measure the electric current is a

circuit. It is connected in series with the load.

The range of the ammeter can be extended by connecting a shunt (very low resistance) in parallel with the ammeter.

An ammeter is a low resistance instrument and voltmeter is a high resistance instrument. If known high value resistance is connected in series with ammeter, then it is used as a voltmeter.

Since ammeter has low a resistance, if it is connected in parallel with the supply, it will draw very high current and the meter will burn out. That is why ammeter is never connected across load.

Voltmeter: A voltmeter is a measuring instrument used to measure the potential difference

across the load in electrical circuit. It is connected in parallel with the load.

The range of voltmeter can be extended by connecting a high resistance called multiplier which is connected in series with the voltmeter.

Circuit diagram for single phase energy meter:

Meter constant:

K = Number of revolution of the disc of energy meter

Kilo−watt−hour

Where, K = meter constant The meter constant of energy meter is 900, it means that 1Kwhr electrical energy is consumed in 900 revolution of the disc of energy meter.

Specification of energy meter: i. Voltage rating ii. Current rating iii. Type( MI or dynamo meter or induction) iv. Type of supply (AC or DC)

The commercial unit of electrical energy is kilo-watt-hour (Kwhr) or B.O.T. If the energy meter is not available then we can measure the electrical energy with the help of watt meter and a stop watch. The electrical energy = Electrical power × Time = Watt meter reading × Time (sec)

= Watt

1000 ×

Second

3600

= Kilo-watt-hour Thus we can measure the electrical energy with wattmeter and stop watch.

Multi meter: A multi meter is a simple instrument which is used for measuring

multipurpose electrical quantity. It requires only dry cell for operation.

Megger: A Megger is a portable insulation tester used for measuring resistance of any

insulation in order of mega-ohm. It consists of an ohm-meter and hand driven d.c. generator which producing 250V, 500V, 1000V and 5000V.

Where, A = voltage coil R1 and R2 = current limiting resistance B = deflecting coil

Application of Megger: 1. Insulation resistance test between conductor and earth 2. Insulation resistance test between conductors 3. Continuity test 4. Polarity test 5. Short circuit test

6. Earth continuity test

Type of instruments: 1. Permanent magnet moving coil (PMMC): Used for D.C. only 2. Moving coil (MI): Used for both A.C. and D.C. 3. Dynamometer type Used for both A.C. and D.C. 4. Hot-wire type: Used for both A.C. D.C. 5. Induction type: Used for only A.C. 6. Electro-static voltmeter: only D.C. voltage

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