Electronics Question and answers

Electronics

• What is Diode?

Diode is a two-layer semiconductor device, conducts only positive cycles when
applied to anode.

• What is Thyristor?

Thyristor is a four or more layer semiconductor device & having 3 or more
junctions. It is also called Silicon Controlled Rectifiers (SCR). A healthy SCR must
block in both the directions at least 1MΩ resistance, a fused SCR will conducts in
both directions.

• What is material used in making semiconductor?

Silicon & Germanium are the raw materials used for making semiconductor.
Semiconductors are located between conductors & insulators in the resistivity
spectrum & allow current to flow only under certain conditions.

• What is material used in making non-linear resistor & purpose of it in field discharge
resistor?

Silicon carbide materials used for making non-linear resistor. The purpose of this
resistor is to avoid surge voltage when field breaker opens. These resistors are
connected in parallel to the main field winding (Rotor).

• How over voltage is produced in Field breaker?

Over voltages appear if synchronous generators and motors fall out of step inducing
an AC voltage in the field system. Depending on the type of construction of the
machine and the slip this voltage can become un-permissibly high, for this purpose
over voltage protectors are provided in the field breaker cubicle.

• What is firing angle?

The angle in the AC cycle at which the thyristor starts conducting at the application
of positive voltage to gate is known as the firing angle (α)

• What is Inverter operation?

When firing angle a = 90º the positive & negative voltages areas are equal. With a
higher than 90º the negative areas are greater so the total voltage becomes negative.
This condition is termed as 'Inverter Operation".

• What is the purpose of RC network across thyristor?

RC network across each thyristor protects against Hole Storage Effect.

• Why reactors are provided in Thyristor bridges?


Reactors are provided to limit the rate of rise of current (di/dt) in the device, thereby
avoiding possible damage to the device. They also effect a proper sharing of load
among thyristor bridges when connected in parallel.

• What is purpose of connecting thyristor bridges in parallel?

Thyristor bridges are connected in parallel to improve current rating.

• What is purpose of Load angle limiter?

Load angle limiter, it either limits the angle between grid load center and the rotor
axis or generator terminal & rotor axis.

• What is purpose of Rotor angle limiter?

It limits generator voltage & rotor voltage.

• What is purpose of Rotor current limiter?

It limits overloading of rotor.

• What is purpose of Stator current limiter?

It limits the stator current.


• What is purpose of Slip stabilization?

It avoids oscillation of the AC machine (Rotor oscillations).

• What is purpose of reactive power (VAR)?

It is an energy required to built up magnetic field to drive the power.

• What is the advantage of Static Excitation?

Fast response time, high reliability, interchangeability of parts during operation, less
wear & tear due to static devices & less maintenance.

• What is the advantage of field forcing in the rotor?

Field forcing acts for 10 seconds to maintain the generator terminal voltage during
fault condition so as to operated the protection relays.

• When the negative sequence reactance arises?

Negative sequence arises whenever there is any unbalance present in the system.
Their effect is to set up a field rotating in opposite direction to the main field.

• When the Zero sequence reactance arises?

If a machine is operating with an earthed neutral, a system earth fault will give rise to
zero sequence current in the machine.


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