SESSION:13

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1)       What is Latching current?

Gate signal is to be applied to the thyristor to trigger the thyristor ON in safe mode. When the thyristor starts conducting the forward current above the minimum value, called Latching current, the gate signal which is applied to trigger the device in no longer require to keep the SCR in ON position.

2)       What is Holding current?

When SCR is conducting current in forward conduction state, SCR will return to forward blocking state when the anode current or forward current falls below a low level called Holding current. Latching current and Holding current are not same. Latching current is associated with the turn on process of the SCR whereas holding current is associated with the turn off process. In general holding current will be slightly lesser than the latching current.

3)       Why thyristor is considered as Charge controlled device?

During the triggering process of the thyristor from forward blocking state to forward conduction state through the gate signal, by applying the gate signal (voltage between gate and cathode) increases the minority carrier density in the p-layer and thereby facilitate the reverse break over of the junction and thyristor starts conducting. Higher the magnitude of the gate current pulse, lesser is the time required to inject the charge and turning on the SCR. By controlling the amount of charge we can control the turning on time of the SCR.

4)       What are the different losses that occur in thyristor while operating?

Different losses that occur are

·        Forward conduction losses during conduction of the thyristor

·        Loss due to leakage current during forward and reverse blocking.

·        Power loss at gate or Gate triggering loss. 

·        Switching losses at turn-on and turn-off.

5)       What are the different operation regions of the SCR?

SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit.
Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics similar to diode.
Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor
Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction. By this type of triggering the device damages the SCR. Hence a gate signal is applied before the forward break over voltage to trigger the SCR.

6)       What is the relation between the gate signal and forward break over voltage (VBO)?

Thyristor can be triggered by increasing the forward voltage between anode and cathode, at forward break over voltage thyristor starts conducting. However this process may damage the thyristor, so thyristor is advices to trigger on through the gate pulse. When a gate signal is applied thyristor turns on before reaching the break over voltage. Forward voltage at which the thyristor triggers on depends on the magnitude of the gate current. Higher is the gate current lower is the forward break over voltage.

7)       What are the Application of DC Motors in Thermal Power Plant?

In thermal power plants dc motors are employed for certain control and critical emergency operations which are supplied by dedicated batteries. DC motors operate as backup drives for normal ac drive systems when ac power supply to the plant is lost.
In thermal power plant, the dc motors finds applications for performing control functions such as

Turbine governor motor
Governor limit setting
Motor operated rheostats
Emergency lubrication for the turbines (main, boiler feed pumps)
Generator (H2 oil seal).
DC motor operated valves

DC motors employed in thermal plants are classified in to two types based on the type of application.

DC motors carrying out Control function
Dc motors carrying out Emergency function

Control functions: This category consists of the turbine governor motor, governor limiting setting, motor operated rheostats, etc. These motors are small, about 1/8 hp or less. They are operated quite infrequently for short duration.

Emergency functions: This category consists of turbine-generator emergency (lubrication) bearing oil pumps and emergency seal oil pumps. Such pumps may also be provided for steam turbine drives of feedwater pumps, fans, and other large loads. The lack of lubrication during a shutdown without ac power will ruin the linings of the bearings and damage the shaft.
Hydrogen seal oil pump is provided to prevent the escaping of hydrogen (for large turbine-generators hydrogen cooling is provided for efficient cooling) from the casing by providing a tight seal with high pressure oil.

8)       Why AC systems are more used or preferred over DC systems?

The reasons why AC systems are more widely used and more preferable than DC systems are as follows:

·         It is comparably easy to maintain and modify the voltage of AC electricity for transmission and distribution than it is for DC electricity.

·         The entire plant cost, ie, circuit breakers, isolators, transformers, etc is much lower in comparison to that of DC transmission.

·         All the power stations produce AC electricity, and hence it is advantageous and also easy to use AC rather than converting it and using DC.

·         AC is practically a sine wave current, and hence in case of a large fault in the network, the sine wave will naturally tend to zero at some point and hence it is very easy to interrupt it in an AC system rather than in a DC system.

9)       What are the different substation components?

Different components used in a substation are Circuit Breakers, Disconnecting switches, Grounding switches, Current Transformers, Potential Transformers or Capacitor voltage Transformers, Line Traps, Lightning Arrestors, Power Transformers, Shunt Reactors, Current limiting reactors, Station Buses and Insulators, Grounding system, Series capacitors and shunt capacitors.

10)    For distribution network which type of Switching scheme is employed?

For distribution network Ring Main busbar scheme is employed. It has the provision that if power interruption occurs to one bus section due to maintenance or fault, power can be fed the bus from other side of the power system.