Electrical Engineering Interview Questions – Part 4

Electrical Interview Questions

The gate signal will be applied to the thyristor to turn it on in safe mode. When the thyristor begins conducting forward current over the minimal value, known as latching current, the gate signal used to trigger the device is no longer required to keep the SCR in the ON position.

When the anode current (or) forward current falls below a low level known as the holding current, SCR will return to the forward blocking mode.

Note that the latching and holding currents are not the same. Latching current is related with the SCR’s turn-on operation, whereas holding current is connected with its turn-off. In general, the holding current will be lower than the latching current.

When a number of sections (or) components are linked in a sequence to fulfill a given function, the group of elements that together create a System.

A Control System is one in which the output and inputs are interconnected in such a way that the output quantity (or) variable is controlled by the input quantity.

The output quantity is known as a controlled variable or reaction, whereas the input amount is known as a command signal (or) excitation.

There are 2 primary types of control systems: 

Open Loop Control Systems &

Closed Loop Control Systems

Open Loop Control System

Open loop control systems are those that ensure output quantity has no effect on input quantity. There is no feedback from the output quantity to the input quantity for error correction.

Closed Loop Control System

A closed loop control system provides feedback from output quantity to input quantity for correction in order to maintain the system’s desired output.

During the triggering process of the thyristor from forward blocking to forward conduction state via the gate signal, applying the gate signal (voltage between gate & cathode) increases minority carrier density in p-layer, facilitating the reverse break over of the junction J2, and the thyristor begins conducting. The greater the size of the gate current pulse, the shorter the time necessary to inject the charge & turn on the SCR. The SCR’s turning on time can be controlled by adjusting the amount of charge.

 The different losses occur in: 

  • Forward conduction losses during thyristor conduction, and 
  • Leakage current loss during forward & reverse blocking.
  • Power outage (or) gate triggering loss. 
  • Losses when switching on and off.

Control system feedback occurs when the output is sampled & a proportionate signal is sent to the input for the automatic error correction (change in desired output) and subsequent processing to return the desired output.

The function of feedback in a control system is to return the sampled output to the input & compare the output signal with the input signal for error (difference from the desired result).

Negative feedback improves system stability, rejects disturbance signals, and is less susceptible to parameter fluctuations. In control systems, negative feedback is considered.

Positive feedback is often not employed in control systems since it raises the error signal & causes instability. However, positive feedbacks are utilized in small loop control systems to enhance specific internal signals and parameters.

Knee point voltage is computed for the electrical current transformers & is an essential criteria when selecting a CT. It represents the voltage at which a CT becomes saturated. (CT = current transformer).

Reverse power flow relays are utilized for generating station protection. A generating station is designed to feed power to the grid; however, if the generating units are turned off and there is no generation in the plant, the plant may draw power from the grid. To block the flow of the power from the grid to the generator, we employ a reverse power relay.

Transformers often have high inductance & low resistance. In the case of a DC supply, there is no inductance, hence only resistance will act in the electrical circuit. High electrical current will pass via the primary side of the transformer. As a result, the coil & insulation will eventually burn out.

Free-wheeling diodes minimizes harmonics, sparking, & arching across the mechanical switch, hence reducing the voltage spike seen in an inductive load.

The various techniques for starting an induction motor:

There are several types of starters, including 

  • DOL (direct online), 
  • Star Delta, 
  • Auto Transformer, 
  • Resistance, and 
  • Series Reactor.

At no load, the alternator’s synchronous impedance creates the angle difference. As a result, it should exhibit zero lagging, similar to an inductor.

When the breaker is closed once by the close push button, the anti-pumping contactor prevents it from being closed again by the close push button.

Stepper motors are electrical machines that act in response to input pulses. It is a sort of synchronous motor that operates in steps in either direction rather than in a whole cycle. As a result, it is utilized in automated components.

The motor has a higher load current than the transformer because the motor consumes real power, whereas the transformer just produces working flux and does not consume it. Therefore, the load current in the transformer is low due to core loss.

SF6 – Sulphur Hexa Fluoride Gas

If this gas is employed as an arc quenching medium in a circuit breaker, it is referred to as an SF6 CB.

The output voltage exceeds the input voltage, or the receiving voltage exceeds the sending voltage.

Insulation level refers to a cable’s ability to sustain an applied voltage without rupturing.

  • Near Distribution Transformers, 
  • Outgoing 11KV feeders, 
  • Incoming 33KV feeders, and 
  • Power transformers in substations.