Electrical Interview Questions: Series-4

Electrical Interview Questions-Series-4

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

Electrical Interview Questions

Major Differences in Electrical Engineering:

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

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

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

Resistance Grounding Connection
Resistance Grounding

However in case of balanced condition there is 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 bus coupler and Interconnecting transformer?

Bus Coupler:

It is a breaker which 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:

Bus coupler is using 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 turn 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 same premises.

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

Derived Earth Fault Current Equation

where,
IE: Earth Fault current
IR: Current flowing through R phase
IY: Current flowing through Y phase
IB: Current flowing through 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 an input by the Relay to measure Earth Fault current.

Direct Ground current measuring using CBCT

Direct Ground Current Measuring

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 over current relay located in the common return conductor of the Current Transformer secondary current.

Residual Ground current measuring using Over current relay

´╗┐Residual Ground Current Measuring

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

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