Top ten Electrical Switchgear and Protection Questions


1. What are the different methods of controlling voltage in AC supply system?

The different methods used to control voltage in ac supply system are
– excitation control at generating station and by using
– tap changing Transformer
– induction regulator
– booster Transformers
– series compensation
– shunt compensation and
– synchronous phase modifiers

2. What is the operating principle of AVR?

An automatic voltage regulator operates on the principle of detection of error. The output voltage of an ac generator obtained through a PT is rectified, filtered and compared with a difference. The difference of the actual voltage and the reference voltage, known as error voltage, is amplified through an amplifier and supply to the field circuits of the main exciter or pilot exciter. Thus, the amplified error signal controls the excitation of the main or pilot exciter through a buck or boost action. Exciter  output control leads to control of the alternator terminal voltage.

3. What are the requirements of excitation system?

The main requirements of an excitation system are reliability under all condition of services, simplicity of control, ease of maintenance, stability and transient response. The initial cost of excitation equipment should be a secondary considerations in the selection of excitation system. Loss of excitation of a unit on the bus results in a more serious disturbance than that resulting from outage of the generator unit from the bus as the remaining units must not only pick up the load dropped but also supply the large reactive current drawn by unexcited generator. In view of this, an excitation system with better reliability is preferred, even if the initial cost is more. Simplicity of control and ease of maintenance are always preferred for any equipment and excited system is no exception.

4. Why earth wire is provided in overhead transmission lines?

Earth wire is provided above overhead transmission lines for protection from lighting stocks. The ground wires shields phase or line conductors by attracting itself the lighting stocks. Which, in its absence would strike the phase conductors. Besides it, the ground wire reduces the voltage electrostatically or electromagnetically induced in the conductors by the discharge of a neighbouring cloud. It also provides additional protective effect by causing attenuation of travelling waves set in line by acting as a starter circuited secondary of the line and conductors.

5. What is difference between lightning conductor and lightning arrester?

Lightning conductor is installed at the top of the tall structures and is connected to the general mass of earth through a system of connecting conductors and the earth electrodes so that the structures under protection is not subjected to the dangerously high potential of the cloud discharge. While lighting arrester is a device that is connected in parallel with the equipment, under protection from lightning strokes, at the substation.

6. What are the main functions of AVR?

The main functions of AVR are as follows:
– Control of system voltage between prescribed limits and have the operation of the machine near to the steady state stability limit.
– Proper division of reactive load between the alternators operating in parallel.
– Prevention of dangerous over-voltage on the occurrence of sudden loss of load on the system increase of excited and system.
– Increase of excitation under system fault conditions so that maximum synchronism power exist at the time of clearance of fault to prevent loss of synchronism.

7. What is differential protection for generator?

The most common system employed for the protection of stator winding against earth faults and phase to phase faults makes use of circulating current principal. In this scheme of production, current at the end of the protected sections are compared. Under normal operating condition, these currents are equal but may differ on the occurrence of a fault in the protected section. The difference of the currents under faulty condition is made to flow through the relay operating coil. The relay then closes its contacts and make the circuit breaker trip and thus isolate the protected section from the system. Such a production is called a Merge-Price circulating current system. Such a protective scheme is very effective for earth faults and faults between phases.
The relay employed in this protection schemes are generally of electromagnetic types and are arranged for instantaneous operations as fault are expected to be cleared as quickly as possible.

8. What purpose is served by line trap and coupling capacitor in carrier channels?

Line trap is an LC network inserted between the busbar and connection of coupling capacitors to the line and tuned to resonance at the high frequency and are used to confine the carrier currents to the protected sections so as to avoid interference with or from adjacent carrier current channel and also to avoid loss of the carrier current signals in adjoining power circuits for any reason whatsoever.
The coupling capacitor is used to connect the high frequency equipment to one of the line conductors and simultaneously serves to isolate the careers equipment from the high power line voltage.

9. On what factors does the choice of protective gears for transformer depends?

The choice of protective gear for a transformers depends upon server factor such as
– Types of transformer (distribution transformer or power transformer)
– Size of transformer
– Type of cooling used
– System where used i.e. its electrical location in the network and
– The importance of the service for which it is required.

10. What is backup protection for bus work bus bars?

In principle it is the simplest of all to protect the buses with the aid of backup protection of the connected supplying elements which should be respond to any fault appearing on the buses.
When no separate bus protections is provided but distance protections is provided for the feeder connected to the bus, it is possible to cover the bus-bar within zone 2 rich of the distance relay.
Bus backup protection may also mean that in case the breaker fails to operate for a fault on the outgoing feeder, then it must be regarded as a bus faults. It should then open all breakers on that bus. Such a backup protection can be provided with the appropriate time delay through a timer.

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