Most Common Electrical engineering interview questions

in this post we are going to explain most Common Electrical engineering interview questions. The question here will focus on Overhead Power Transmission Lines. Following are some of the important and most Common Electrical engineering interview questions

Why use high-voltage instead of low Voltage for transmission?

The best answer to that question is that high-voltage transmission lines transport power over long distances much more efficiently than lower-voltage distribution lines for two main reasons.

First, high-voltage transmission lines take advantage of the power equation, that is, power is equal to the voltage times current. Therefore, increasing the voltage allows one to decrease the current for the same amount of power.

Second, since transport losses are a function of the square of the current flowing in the conductors, increasing the voltage to lower the current drastically reduces transportation losses.

Additionally, reducing the current allows one to use smaller conductor sizes.

What are Touch potential and step potential?

Step potential and touch potential are both electrical safety terms related to the risk of electric shock in high voltage environments, such as near power lines or electrical substations.

Touch Potential:

Touch potential refers to the voltage difference between an energized object or surface and a grounded object that a person may touch simultaneously. When a person touches an energized object while also in contact with a grounded surface, they can create a path for electric current to flow through their body, resulting in an electric shock. Touch potential is a concern when there is a voltage gradient between different surfaces that a person may come into contact with, such as when working near electrical equipment.

Step Potential:

Step potential is the voltage difference that can occur between a person’s feet when they are in contact with the ground at different points. In the event of a fault or electrical discharge, such as a lightning strike or a ground fault in a high voltage system, the ground can develop a voltage gradient. If a person’s feet are in contact with the ground at different points with a significant voltage difference between them, they can become part of an electric circuit and receive an electric shock. Step potential is a concern in situations where there is a voltage gradient in the ground, such as during a fault condition or in areas with high ground resistance.

Why do we need live line Washing?

Live line washing is a maintenance technique used to clean insulators and other components of high voltage power lines while the lines are energized. This process helps prevent the buildup of contaminants such as dust, pollution and salt which can reduce the effectiveness of the insulators and lead to electrical arcing or flashovers. By keeping the insulators clean, live line washing helps ensure the safe and reliable operation of the power transmission and distribution system.

Hot line washing of transmission lines is very risky and that needs only to be done by highly responsible and technically qualified people with complete understanding with regard to safety. it is very necessary to ensure the efficiency of power transmission lines, reliability of power grids, and to reduce power outages.

What are the main Parts of a Transmission Tower?

The main supporting unit of overhead transmission line is transmission tower. Transmission towers have to carry the heavy transmission conductor at a sufficient safe height from ground. In addition to this, all towers have to sustain all kinds of natural stresses.

Main Parts of a Transmission Tower

A power transmission tower consists of the following parts:

Peak of Transmission Tower
Cross arm of Transmission Tower
Boom of Transmission Tower
Cage of Transmission Tower
Transmission Tower Body
Leg of Transmission Tower
Stub/Anchor Bolt and Base plate assembly

1. PEAK OF TRANSMISSION TOWER
The portion above the top cross arm is called peak of transmission tower. Generally, earth shield wire is connected to the tip of this peak.
2. CROSS ARM OF TRANSMISSION TOWER
Cross arms of transmission tower hold the transmission conductor. The dimension of cross arm depends on the level of transmission voltage, configuration and minimum forming angle for stress distribution.
3. Boom of Transmission Tower
Boom is a rectangular beam of the cross section in a horizontal configuration tower. It is used to support transmission conductors in the horizontal configuration.
4. CAGE OF TRANSMISSION TOWER
The portion between tower body and peak is known as cage of transmission tower. This portion of the tower holds the cross arms.
5. TRANSMISSION TOWER BODY
The portion from bottom cross arm up to the ground level is called transmission tower body. This portion of the tower plays a vital role for maintaining a required ground clearance of the bottom conductor of the transmission line.

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What is a synchronous condenser?

A synchronous condenser is a special type of synchronous machine used to provide reactive power support, voltage regulation, and improve power factor in electrical grids. It operates in a similar manner to a synchronous motor or synchronous Generator but without driving any mechanical load or prime mover. The main purpose of a synchronous condenser is to stabilize the voltage in a power system by either absorbing or supplying reactive power.

What are the various excitation conditions for synchronous condensers?

Under-excitation (low DC excitation): The synchronous condenser absorbs reactive power from the grid (acts as an inductive load).

Over-excitation (high DC excitation): The synchronous condenser supplies reactive power to the grid (acts as a capacitive load).

How a Synchronous Condenser Absorbs or Supplies Reactive Power?

The key to reactive power control in a synchronous condenser is the DC excitation supplied to the rotor:

Supplying Reactive Power (Over-excitation):

When the DC excitation to the rotor is increased beyond normal (over-excitation), the rotor’s magnetic field becomes stronger than the stator’s rotating magnetic field.This causes the synchronous condenser to behave like a capacitor, and it starts supplying reactive power (positive MVAR) to the grid. This increases the voltage in the system.

Output behavior: No real power (kW) is supplied, but the machine provides reactive power (MVAR) to the grid.

Absorbing Reactive Power (Under-excitation):

When the DC excitation to the rotor is decreased below normal (under-excitation), the rotor’s magnetic field becomes weaker than the stator’s rotating magnetic field.The synchronous condenser now behaves like an inductor, and it starts absorbing reactive power (negative MVAR) from the grid.This reduces the voltage in the system.

Output behavior: Again, no real power (kW) is absorbed, but the machine absorbs reactive power from the grid.

By adjusting the excitation of the rotor, the synchronous condenser can either absorb or supply reactive power to the grid.

What challenges do renewable energy sources introduce to grid stability and voltage regulation, and how can these challenges be addressed to ensure reliable power system operation?

1. Challenges in Renewable-Dominated Grids:

Lack of Inertia:

Conventional power plants (e.g., coal, gas, nuclear) use large rotating generators that provide inertia to the power system. This inertia helps stabilize grid frequency by resisting sudden changes in demand or generation.
Renewable sources like wind and solar don’t have such large rotating masses (or have much smaller ones in the case of wind turbines). As a result, these sources provide little to no inertia to the grid. Without enough inertia, the grid becomes more vulnerable to frequency fluctuations during sudden load changes or disturbances (like faults or short circuits).

Lack of Reactive Power:

Traditional synchronous generators inherently generate reactive power alongside active power, which helps maintain voltage stability.
Renewable energy sources (like wind turbines and solar PV inverters) typically operate using power electronics, which lack the inherent ability to provide reactive power unless they are specially designed to do so. This results in a deficiency of reactive power, which can lead to voltage instability in the grid.

What is the major Role of Synchronous Condensers in Renewable Grids?

Synchronous condensers (SCs) help mitigate both of these challenges by:

Providing Inertia:

Synchronous condensers are essentially large rotating machines that add inertia to the grid. This inertia helps buffer the system against sudden frequency changes caused by variability in renewable generation. For instance, if there’s a sudden dip in solar power output due to cloud cover, the SCs’ inertia slows down the rate of frequency change, giving system operators more time to respond.

Supplying Reactive Power:

SCs generate or absorb reactive power, which helps maintain grid voltage within acceptable limits. When there is a demand for more reactive power, synchronous condensers can supply it, and when there is too much, they can absorb it. This is critical for voltage control in a renewable-heavy grid, where conventional generators that usually provide reactive power might not be present.

Short-Circuit Power Contribution:

SCs also contribute to the short-circuit power of the system. This is particularly important because fault levels in a power system must remain above a certain threshold to ensure that protection systems (like relays and circuit breakers) operate properly.
In a grid with fewer conventional generators, the overall short-circuit level may be reduced, which can make it harder for protective devices to detect and clear faults. SCs help raise the short-circuit power contribution, ensuring that fault conditions are properly handled.