1). What is Latching Current?
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.
2). What is Holding Current?
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.
3). What is a System?
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.
4). What is Control 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.
5). What are the types of Control Systems?
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.
6). Why are Thyristors classified as charge-controlled devices?
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.
7). What are the different losses in Thyristors during operation?
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.
8). What is Feedback in Control Systems?
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.
9). Why is Negative Feedback preferred in the Control System?
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.
10). How does Positive Feedback affect the System’s stability?
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.
11). What is Knee Point Voltage?
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).
12). What is a Reverse Power Relay?
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.
13). What happens if a DC supply is applied to the primary of a transformer?
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.
14). What is the difference between Isolators and Circuit breakers? What is a bus-bar?
| Isolators | Circuit Breakers |
| Isolators are mostly used for switching purposes under normal settings, however they cannot function in fault conditions. | Circuit breakers is engaged under fault conditions based on the fault detected. |
| However, Isolators were utilized to isolate the CBs during repair. | A bus bar is just a junction via which power is routed to independent loads. |
15). What are the benefits of free-wheeling diodes in a Full Wave rectifier?
Free-wheeling diodes minimizes harmonics, sparking, & arching across the mechanical switch, hence reducing the voltage spike seen in an inductive load.
16). What are the procedures for starting an Induction Motor?
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.
17). What is the power factor of an alternator with no load?
At no load, the alternator’s synchronous impedance creates the angle difference. As a result, it should exhibit zero lagging, similar to an inductor.
18). What is the function of Anti-Pumping in the Circuit Breakers?
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.
19). What is a stepper motor and what are its applications?
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.
20). There are transformers and induction machines. Those two share the same supply. Which device will have the maximum load current? And why?
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.
21). What is an SF6 Circuit Breaker?
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.
22). What is the Ferranti Effect?
The output voltage exceeds the input voltage, or the receiving voltage exceeds the sending voltage.
23). What is the insulation voltage in cables?
Insulation level refers to a cable’s ability to sustain an applied voltage without rupturing.
24). What is the difference between MCB and MCCB? Where might they be used?
| MCB | MCCB |
| MCBs – Miniature Circuit Breakers | MCCB – Molded Case Circuit Breaker |
| MCBs are thermally activated and used to prevent against short circuits in circuits with low current ratings. | MCCBs that operates thermally for overload current and magnetically for instant trip in short circuit conditions. |
25). Where should lighting arrestors be installed in Distribution Lines?
- Near Distribution Transformers,
- Outgoing 11KV feeders,
- Incoming 33KV feeders, and
- Power transformers in substations.
