# Electrical Interview Questions: Series-4

## Electrical Interview Questions

In this series of Interview questions, we will learn about the following major differences in Electrical
Engineering.

1. What are the major differences between the transformer and Induction motors?
2. What is the difference between Neutral and Ground Potential?
3. What is the difference between a bus coupler and Interconnecting transformer?
4. What is the difference between Measured and Derived Earth faults?

## Major Differences in Electrical Engineering:

1. What are the major differences between the transformer and Induction motors?

1. Transformer is an electrical device that transfers electrical power from one circuit to another without changing its frequency. whereas an Induction motor is a machine that converts the given electrical energy into mechanical energy.
2. Transformer contains no moving parts and mostly Iron core is the medium for the passage of flux. whereas the Induction motor must contain air as the medium between the stator and the rotor.
3. In the induction motor, due to the air gap, it draws more lagging current to magnetize the rotor field at starting and during operation. so it always operates at a lagging power factor. whereas the transformer can be operated at any type of power factor depending upon load.
4. As there are no moving parts in the transformer the efficiency of a transformer is high. whereas in induction motors mechanical losses are always present so their efficiency is low.
5. In a transformer, frequency is constant for primary and secondary induced emfs. whereas in an induction motor, the rotor-induced emf frequency is slip times the stator frequency.

### 2. What is the difference between Neutral and Ground Potential?

The ground is always considered at zero potential. Whereas the neural potential is the sum of ground potential and voltage drop in the Resistance or Reactance between ground and neutral.

The neutral may or may not be at the same potential as the ground.

In case of an unbalanced condition of the circuit, zero sequence currents flow through the grounded impedance and cause a voltage drop. So Neutral Potential is not equal to the potential at the Ground.

However, in the case of a balanced condition, there are no current flows through the Resistance or Reactance. In this case, Ground Potential is equal to Neutral Potential which is zero.

### 3. What is the difference between a bus coupler and Interconnecting transformer?

Bus Coupler:

It is a breaker that connects two different buses.

• Purpose of bus coupler in double bus bar system:

It can allow the power transfer from one bus to another bus if surplus power is available in one bus. The bus coupler is always on. In case of failure of one bus, the bus coupler must be made off to isolate the faulty bus.

• Purpose of bus coupler in switchgear:

The bus coupler is used with two incoming buses. In this case, under normal conditions, the bus coupler is made off. In case of failure of any incoming bus, the bus coupler is turned on for charging that portion from the healthy incomer.

Interconnecting Transformer:

It is a transformer which connects two different substations through buses which are in the same premises.

It interconnects two different system voltage levels. For example 132 kV to 66 kV, 220 kV to 132 kV and 400 kV to 220 kV, and so on.

Purpose:

In case of failure of supply to any one bus, to maintain continuity of supply, it takes power from another bus to which it is interconnected.

This transformer is an auto-transformer and it is bi-directional.

### 4. What is the difference between Measured and Derived Earth faults?

In Relays Earth fault current can be calculated in two ways.

1. Derived Earth Fault:

Earth Fault current is equal to the vector sum of the three line current values.
It is calculated using the formula:

where,
IE: Earth Fault current
IR: Current flowing through the R phase
IY: Current flowing through the Y phase
IB: Current flowing through the B phase

2. Measured Earth Fault:

Earth fault current can be measured in any one of the following ways.

• Direct Ground Current Measuring:

Core-Balance Current Transformer is used for earth leakage and sensitive Earth Fault conditions. CBCT encircles the three-phase conductors and measures zero sequence (ground) current directly. This is used as input by the Relay to measure Earth’s Fault current.

This method is well suited for use on low resistance grounded systems, but not well suited for use on solidly grounded systems

• Residual Ground Current Measuring:

Measuring of ground current by means of an overcurrent relay located in the common return conductor of the Current Transformer secondary current.

This method is appropriate for solidly grounded power systems, but not well suited for low-resistance grounded systems.