Electrical Interview Questions: Series-1

Important Electrical Interview Questions:

For preparing any Electrical job interview the knowledge on various technical components is required. we have started a series of Electrical questions to help the interview participants who are looking job in the core sector.

1. What are the necessary conditions required for parallel operation of transformers?

The various conditions that must be fulfilled, for the successful parallel operation of 3-phase transformers, are as follows:

• The line voltage ratios of the transformers must be same.
• The transformers should have equal per unit leakage impedances.
• The ratio of equivalent leakage reactance to equivalent resistance should be same for all the transformers.
• The transformers should have the same polarity.

In addition to these four conditions, two more essential conditions that must be fulfilled for parallel operation of three-phase transformers are as follows:

Relative-phase displacement:

The relative phase displacement between the secondary line voltages of all the transformers must be zero i.e. the transformers to be connected in parallel, must belong to the same group number.

YY with ΔΔ transformers cannot be connected in parallel.

Note: Transformers of group numbers 3 and 4 can be successfully operated in parallel.

For Vector groups of transformer refer Vector Groups for 3-phase transformers

Phase sequence:

An improper phase sequence would give unbalanced voltages at the load terminals consequently parallel operation is not possible.

2.How many times more the Induction motor will be heated during starting than running?

An induction motor usually draws six times the full load current for which it is rated when starting. This high stator current induces a comparably high current in the rotor. The rotor resistance at zero speed is three times the rotor resistance when the motor is at rated speed. Thus, the I²r heating in the rotor is approximately 6²*3=108 times the I²r heating when the motor runs normally. So the manufacturer has to consider the maximum locked rotor time and locked rotor amps while designing the motor.

3.What is Capacitive voltage indicator? Where it is used and how it works?

Capacitive voltage indicators are installed in the panels of medium voltage switchgear for simple voltage checking. Capacitive voltage indicator tells either “voltage present” or “no voltage present” only. If there is potential on the bus bar the LEDs are flashing else no LED flash.

For some applications, there is no need to measure the voltage for connecting further measurement or protective equipment like Voltmeters and Relays. Example likes to detect system to verify safe isolation from supply. The voltage-free conditions on the equipment, provide guarantee the personal safety during operation and work on the switchgear panel.

If system is isolated from supply there should be no voltage present. For such applications, equipping the switchgear panel with Potential Transformers is very expensive.

For such situations, voltage detection using Capacitive voltage indicators offers an efficient and cost-reducing alternative, and is ideally suited for medium-voltage switchgear systems.

Principle of operation of capacitive voltage indicators:

Capacitive voltage indicators worked based on capacitive voltage dividers. These consist of capacitors connected in series.  The voltage across individual capacitors is divided in inverse proportion to the ratios of their capacitances. The smallest value capacitor coupled to the bus bar. The largest value capacitor is connected to the LED circuit. If any voltage is present LED flashing occurs otherwise no LED flashing. Like this with the help of Capacitor voltage divider circuits, Capacitive voltage indicators shows the presence of voltage on the bus bars.

4.What are IPBD and SPBD?

These are the different bus ducting methods.

IPBD: Isolated Phase Bus Duct

Application is to form alternator power lead out. The conductors between Generator Transformer, Unit Auxiliary transformer are enclosed in hallow, tubular aluminium enclosures. These enclosures are continuous and are connected in star and earthed at each end. The conductors are supported on porcelain insulators within the enclosure as shown in below figure.

IPBD is used because of its highest reliability and short circuit resistance. IPBD preferably used to reduce electrodynamic forces between conductors during short circuit conditions, hence simplified insulator design.

View Single Line Diagram of Power plant for identifying the running path of IPBD.

SPBD: Segregated Phase Bus Duct

The three phase conductors are inside an enclosure but segregated from each other. SPBD is used to avoid the occurrence of phase to phase short circuit. SPBD runs between UAT and Unit Auxiliary Bus and also Station Transformer and Station Bus.

The conductors of the three phase are in a common metal enclosures with metal barriers between them as shown in above figure.

View Single Line Diagram of Power plant for identifying the running path of SPBD.

5.What are different types of C.T testing methods?

This is one of the common Electrical questions asked, You should have the knowledge of each test.

• Polarity test
• Winding and Insulation resistance tests
• Ratio test
• Knee point voltage test

Also go through all the series of Electrical questions for Interviews and give your valuable suggestions to improve further writings.