Question

 

PLEASE BREIFLY STATE THE DESCRIPTIONS OF FOLLOWING:

Circuit breaker

Relay

Fuse

AutoRecloser

Latching current

Holding current

Thrysitor

Choppter

Inverter

PLEASE BREIFLY DISCUSS EACH AND DRAW THE CIRCUIT DIAGRMAm OF EACH WITH THE EQUATION INVOLVED WITH EXPLANATIONS

Answer 1

1) Circuit Breaker:

An Electrical circuit breaker is a mechanical switching device used for control and protection of Electrical power system. It is capable of making, carrying and breaking currents under normal and specified abnormal circuit working conditions such as short circuits or faults.

A main function of a circuit breaker is to isolate the faulty part of the power system during abnormal circuit conditions such as faults. The circuit breaker isolates the faulty part of the power system after it receives tripping signal from the protective relay. According to the type of insulation medium, circuit breakers can be classified as

1)oil circuit breaker

2)air break circuit breaker,

3)air blast circuit breaker,

4)sulphur hexaflouride circuit breaker,

5) vaccum circuit breaker.

Working principle of Circuit breaker:

A circuit breaker consists of 2 contacts - fixed contact and moving contact, placed in an insulating medium. When a fault has occurred in the power system, the moving contact interrupts the circuit. The contacts are separated,forming an arc between the contacts. The faulty circuit is isolated from the power system after the arc extinguishes.

2) Relay:

Protective relays are installed in the power system for its proper and efficient operation. A protective relay senses and locates the fault, before sending the tripping signal to the appropriate circuit breaker for isolating only the faulty part from the power system. Relay detects the abnormal conditions by continuously monitoring the electrical parameters (voltage, current, phase angle and frequency) of the power system.

Protective relays can be classified into the following categories based on their construction and operation.

i) Electromechanical relay

ii) Static relay

iii) Numerical relay

Working principle of relay:

The operation of the electromechanical relays is due to the mechanical force created by the input quantities on the moving parts, resulting in physical movement of the moving part. The contacts of the relay are closed by the moving part for operation.

In a static relay, a static circuit gives an output signal for tripping of the appropriate circuit breaker by the comparison or measurement of electrical quantities. It employs various semiconductor devices.

Numerical relays operate based on digital devices. They monitor the current and voltage signals through transducers, process the data obtained by sequential sampling of the ac quantities. The data is processed using an algorithm for generating a trip signal.

3) Fuse:

Fuses are used to protect cables, electrical equipment and semiconductor devices against overloads and short-circuits. It carries the working current safely without overheating under normal conditions. It breaks the circuit by melting when the current exceeds a certain predetermined value during abnormal conditions. It possesses inverse time-current characteristic.

Fuses can be classified as under:

i) Rewirable type ( Open and semi-closed type)

ii) Totally enclosed or cartridge fuse

iii) D-type cartridge fuse

iv) High rupturing capacity (HRC) fuse.

4) Auto Recloser:

Auto reclosers are employed in the electrical distribution system to provide uninterruptable power supply in the distribution lines. It is a circuit breaker equipped with a mechanism capable of closing the circuit after the faulty part is opened (isolated) during abnormal (faulty) circuit conditions.

5) Latching current and Holding current:

Latching current is related to the turning ON of the thyristor. A thyristor consists of three terminals- anode, cathode and gate. Latching current can be defined as the minimum current that the anode-to-cathode current has to exceed in order for the thyristor to turn ON.  

Once the thyristor is turned ON, it remains in the ON state till it is turned off. Holding current is defined as the anode-to-cathode current,below which the thyristor turns OFF. The thyristor gets turned OFF if the anode-to-cathode current is below holding current.

6) Thyristor:

Thyristor is a p-n-p-n semiconductor switching device, with four layers and three junctions. It consists of 3 terminals - anode, cathode and gate. They are compact, highly reliable and have low loss.

Thyristor is a unidirectional device that blocks the current flow from cathode to anode. The current flow is blocked until the device is triggered into conduction state (ON state) by applying a proper gate signal between gate and cathode terminals.

7) Chopper:

Choppers are the semiconductor devices which convert fixed dc voltage to an adjustable dc output voltage. This conversion can be carried out by 2 types of dc to dc converters - AC Link chopper and DC chopper.

An AC Link chopper first converts dc into ac by an inverter. AC is then stepped up or stepped down with the help of a transformer, which is then converted into dc by a diode rectifier.

DC Chopper directly converts fixed dc input voltage to variable dc output voltage.

8) Inverter:

An inverter converts dc power into ac power at desired output voltage and frequency. Inverters can be classified as voltage source inverters and current source inverters. In a voltage source inverter, the dc source has a negligible impedance. A current source inverter has the dc source input of high impedance. The rectification of dc input power to ac power is done by diodes or thyristor converter circuits.