# BHEL Electrical Interview Questions

## 1). What are the different conditions for parallel operation of Transformers?

To run the transformers in parallel, it is mandatory to satisfy the following conditions so that the load sharing and the stability are taken to consideration.

These include:

Same Voltage Ratio: The first voltage ratio must be equal to the second voltage ratio of the transformers Transf.

Identical Phase Sequence: Typically it should be the same and furnished with some kind of insulation to help prevent the phase displacement.

Similar Impedance: Transformers should possess the same impedance since this determines how the load is divided.

Same Polarity: These circulating currents or short circuits are damaging when the polarity is incorrect.

Phase Angle Difference: One must ensure that there is phase angle difference between the transformers and they should not be more than ‘0’.

## 2). What are V and inverted V curves in Synchronous motor and their significance?

The characteristics of V type and Inverted V type curves of synchronous motors are explained in this unseen footage.

The curve V and inverted V show the armature current with reference to the field current in a synchronous motor.

In the V curve of the armature current, a minimum value is obtained at the synchronous reactance and the power factor shown in the inverted V curve increases as we increase the excitation till it becomes leading.

These are important curves, which help to regulate the value of the power factor and stability of the motor.

## 3). Explain electrical corona? And methods to reduce the electrical corona?

Corona is a discharge of electricity in air or other gases surrounding the conductor whenever the electric strength of the system exceeds a certain value. It leads to power dropout, electromagnetic coupling and the noise heard as hum and buzz.

Measures to counter it include the use of conductors with bigger cross-section, smoother outer surface of the conductors, proper spacing between conductors, and the corona rings.

• A P (Proportional) controller is a type of controller that is based on the level of error and it modulates the output proportional to the level of error. P controllers are much uncomplicated and may entail steady-state errors.
• A PI (Proportional-Integral) controller applies the proportional control and integral control at the same time, thus eradicating steady-state errors. PI controllers reduce steady state errors but can cause overshoot.
• Derivative action is used in the PID (Proportional-Integral-Derivative) controller increases system stability and decreasing time response. Of the different control systems available, PID controllers give the best control but are difficult and time-consuming to set.

## 5). What are the different braking methods used in dc motors

DC motors use various braking methods, including:

• Regenerative Braking: It serves as a motor and as a generator which sends current back to the power supply system.
• Dynamic Braking: Energy is given out in form of heat through a resistor.
• Plugging: Since the supply voltage polarity of a linear motor is changed, there is a Reverse Torque.

## 6). What is water hammering phenomenon?

Water hammering is defined as a pressure surge in a pipe whenever a fluid, for one reason or another has to be stopped, or to change direction. It may result in noise making; pipe wear and/or bursting. It is experienced in the flowing systems such as water supply systems and steam distribution systems. Methods of mitigation include slow opening and closing of the valves, air chambers, and surge tanks.

## 7). Advantages of DC motors over AC motors?

DC motors provide smooth speed adjustments with speed range, so they are useful with speed changing appliances. They have a high starting torque as well as they consist of relatively uncomplicated control circuits.

DC motors are used whereas speed control and torque control is mandatory like in electric vehicles, in hoist and elevators etc.

## 8). What is Load factor, Demand factor, Installed capacity, spinning reserve?

• Demand Factor: How much the maximum consumer demand is in relation to the total connected load demand.
• Installed Capacity: For power station, total installed capacity to generate power is defined by Kulkarni as.
• Spinning Reserve: More spinning reserve which is an excess generating capacity that can be obtained from the units that are already connected to the consumer interface.

## 9). Which power plants can be used as a peak load plants. Reason?

Peaking power plants like gas turbines and hydroelectric plants are used in peak load condition as they offer the capability of starting quickly and to meet fluctionating loads.

Hence, they have a major role of servicing high demand on electricity thus can easily respond to a high demand.

## 10).What is String Efficiency in Insulators?

String voltage equivalence refers to the extent of the voltage across the insulator string. It is expressed as the voltage across the complete string divided by the number of discs times the voltage which is applied to the disc which is closest to line. Greater string efficiency results in a more balanced supply of voltage and consequently a reduced threat for flashover occurrences.

## 11). What is Transient stability and methods to improve transient stability?

Transitory stability is the ability of a power system to remain synchronized following a disturbance for instance a short circuit.

Measures for improving dynamics comprise fast tripping of the circuits with use of breakers, application of high-speed relays, augmenting inertia of the system and utilizing FACTS.

## 12). What is single line diagram and the assumptions required?

A single line diagram electrical drawing is kind of notations used to represent the three-phase power system. This envelops all the three phases in just a single line. Some of the assumptions of the electrical power system are: These comprise equal and equal but opposite components so that minor details are excluded in order to have easy analysis of loads and designs.

## 13). What is Step Potential and Touch Potential in substations?

Step potential is the voltage difference as the person’s one foot touches the ground.

Touch potential is the difference in voltage between the ground and whatever an individual comes into contact with.

These potentials appear during the fault conditions and may produce electric shocks. Then there are measures that talk of precautions such as grounding and other factors that have to be considered in order to reduce risks and incidences to the minimum.

## 14). What are different switching schemes employed in substations?

Different switching schemes include:

• Single Bus: Non elaborate but very rigid in its outlook.
• Double Bus: Provided safeguard and choice.
• Ring Bus: It has a very high reliability, it also contains the ability to isolate faults.
• Mesh: Allows for power to flow in multiple ways increasing its durability.

## 15). How Speed of DC shunt motor is controlled?

The speed of a DC shunt motor may be regulated either by changing the field current and/or by changing the armature voltage. It also affords accurate speed control for various functions needed in various operations.

## 16). What is the difference between Microprocessor and Microcontroller?

Feature Microprocessor Microcontroller
Definition A computer component that is usable for general processing data and involves arithmetic and logic operations. A chip that contains a control processor, memory, and handlers that are used in specific control functions.
Components Comprises of a CPU, may include external memory, input/output peripherals, and other parts. Micro controller has CPU, memory (RAM, ROM) input/ output port, timers and some other peripherals integrated in to a single chip.
Purpose Initial architecture intended for computations which are good for use in PCs, servers, and other high-performance machines. It is designed for control tasks that are carried out in embedded systems like in home appliances, vehicles, and numerous connected devices.
Memory Such components require additional components, which are located outside the microprocessor: RAM, and ROM. On-chip memory is integrated including Random Access Memory (RAM) and Read Only Memory (ROM).
Cost Tend to be generally more costly mainly because they require some external components and they are high-performing. Overall less expensive mainly because of their integration and call for lower performance.
Power Consumption Typically uses up more power, optimal for systems with an abundant supply of power. Reduced power loss, favoured by portable applications requiring power from batteries.
Performance An ability to make many calculations within a short span of time. A reduced capability for processing information and mainly designed for the execution of certain control functions.
Flexibility Extremely portable, may be used for a large number of programs and operating systems. Mainly for specific control tasks sometimes it even has its own firmware.
Applications Desk-Top, portable computers, specialized computers, servers, high end client devices. Consumer electronics, avionic systems, automobile systems, life support systems and medical instruments.
Developmental Complexity Generally complex development environment, may demand state-of-art operating systems and a number of software tools. Development is relatively easier often using RTOSs or no OSs at all.
I/O Ports Generally has few I/O ports; depends on companion chips to communicate with different modules. Usually will contain many I/O ports and peripherals through which the unit can directly interface with other devices.

## 17). What is SIL and its significance

Sound intensity is defined by means of the Sound Intensity Level (SIL).

SIL in electrical context means Surge Impedance Loading which is the maximum amount of power a transmission line is capable of being supplied before it becomes unstable.

It is considered as important for system security and power quality as well as for keeping the system stable to transmit the power.

## 18). Realize Ex-OR using NAND gates

An Ex-OR gate can be implemented using NAND gates and it requires placing four NAND gates in a particular way. Ex-OR gate is the type of gate that produces ‘1’ only when the number of ‘1’s in the input is odd.

## 19). What is Race around condition?

Race around condition takes place in flip-flops when the output is active more than once during the clock pulse time period, it thus produces unpredictable outcomes.

The occurrence was identified in JK flip-flops when both J and K inputs are at HIGH state. Techniques available are edge-triggered flip flops, master/slave flip flops.

## 20). Why single phase induction motor is not self-starting and methods to start single phase induction motor?

Single phase induction motors must be started with the help of some external system because they do not possess the rotating magnetic field.

Starting methods include:

• Split-Phase: They include; utilizing an auxiliary winding with phase shifting capacitor.
• Capacitor Start: The addition of a capacitor in series with the auxiliary winding also.
• Shaded Pole: Applying a shading coil for attaining a time delayed magnetic field. These procedures assist in developing the needed primary torque for starting the motor.