Welcome to EEE Interview Tips

For Your Pre-Professional Preparation!

Download EEEBooks

To Download Electrical and Electronics Ebooks click the "EEE Library" menu! Enjoy!!

EEE Interview Tips

Leave Your Comments, Views And Questions.

Learn to make Electronics

Burn things out, mess things up - that's how you learn to Make Electronics! Enjoyy!!

EEE Forum

Drop your Ideas, Thoughts, Questions and Comments!

July 12, 2013

What are the Functions of Substation Equipment?

Functions of Substation Equipments:

Sl. No.

Incoming & outgoing circuits Connected to bus-bar

Circuit Breaker
Automatic switching during normal or abnormal conditions

Disconnection under no-load condition for safety, isolation and maintenance.

Earthing switch
To discharge the voltage on deadlines to earth

Current Transformer
To step-down currents for measurement, control & protection

Voltage Transformer
To step-down voltages for measurement, control & protection

Lightning Arrester
To discharge lightning over voltages and switching over voltages to earth

Shunt reactor
To control over voltages by providing reactive power compensation

Neutral-Grounding resistor
To limit earth fault current

Coupling capacitor
To provide connection between high voltage line & PLCC equipment

Line –Trap
To prevent high frequency signals from entering other zones.

Shunt capacitors
To provide compensations to reactive loads of lagging power factors

Power Transformer
To step-up or step-down the voltage and transfer power from one a.c. voltage another a.c. voltage at the same frequency.

Series Capacitor
Compensation of long lines.

What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

What are the errors in Current Transformer?

(a) Ratio error
Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100
The value of transformation ratio is not equal to the turns ratio.

(b) Phase angle error:
Phase angle =180/π[(ImCos δ-I1Sin δ)/nIs]

What is an arc? Give the two methods of arc interruption?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.

Methods of arc interruption: 
  1. High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished 
  2. Current zero method:-The arc is interrupted at current zero position that occurs 100 times a second in case of 50Hz power system frequency in ac.

What do you mean by current chopping?

When interrupting low inductive currents such as magnetizing currents of transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

How to calculate voltage drop?

How to calculate voltage drop:
In situations where the circuit conductors span large distances, the voltage drop is calculated. If the voltage drop is too great, the circuit conductor must be increased to maintain the current between the points. The calculations for a single-phase circuit and a three-phase circuit differ slightly.

Single-phase voltage drop calculation:

Three-phase voltage drop calculation:

OR 3øVD = (SQRT(3)*L*R*I)/1000
VD = Voltage drop (conductor temp of 75°C) in volts
VD% = Percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2(A)(4) and throughout the NEC.
L = One-way length of the circuit's feeder (in feet)
R = Resistance factor per NEC Chapter 9, Table 8, in ohm/kft
I = Load current (in amperes)
Source voltage = The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480 V.

Related Link: How much voltage drop is Acceptable?

How much voltage drop is Acceptable?

The national Electrical Code (NEC) recommends that the combined voltage drop of the electrical system (brunch circuit and feeders) not exceed 5% for optimum efficiency. This recommendation not only can improve safety but can ensure proper equipment operation and power efficiency. Foe Example, In a 220 volt 5 ampere circuit, this means that there should be no more than a 11 V drop (109 volts) at the furthest outlet when the circuit is fully loaded.

Related Link: How to calculate voltage drop?

July 5, 2013

What are the Components of the power system?

The various components of power system are:
  • Generator (alternator), 
  • Transformer, 
  • Transmission line, 
  • Induction motor,
  • Synchronous motor,
  • Resistive and 
  • Reactive loads.

Related Posts Plugin for WordPress, Blogger...