Power Systems MCQ Quiz - Objective Question with Answer for Power Systems - Download Free PDF

Explanation:

Distance Relay for Fault Protection

Definition: A distance relay is a type of protective relay used in power systems to detect and isolate faults. It operates based on the impedance between the relay location and the fault location. The primary function of a distance relay is to protect transmission lines from faults by measuring the impedance, which is inversely proportional to the distance to the fault.

Distance relays measure the voltage and current at the relay location and calculate the impedance (Z = V/I). When a fault occurs, the impedance changes significantly. If the calculated impedance falls within a predetermined zone, the relay identifies the presence of a fault and triggers the protection mechanism to isolate the faulty section.

Correct Option Analysis:

The correct option is:

Option 1: It operates based on the impedance between the relay and the fault.

This option correctly describes the functioning of a distance relay. The relay measures the impedance between its location and the fault, and if the impedance falls within a predetermined range, it indicates a fault condition and initiates the protection mechanism.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: It is used only for short-circuit protection.

This option is incorrect because distance relays are used for various types of fault protection, not just short-circuit protection. They can detect phase-to-phase faults, phase-to-ground faults, and other abnormal conditions on the transmission line.

Option 3: It operates based on the current only.

This option is incorrect because a distance relay operates based on both voltage and current measurements to calculate the impedance. Relying on current alone would not provide the necessary information to determine the distance to the fault accurately.

Option 4: It operates based on the voltage at the fault location.

This option is incorrect because, although voltage measurement is part of the relay's operation, the primary basis for its operation is the impedance calculation, which involves both voltage and current. Voltage alone cannot determine the distance to the fault.

The correct answer is: 4) It detects the difference in voltage between two or more points and trips the system if the voltage difference exceeds a set threshold.

Explanation:
The primary function of a voltage differential relay is to:

• Compare voltages at two or more points in the system (e.g., across a transformer winding, busbar, or transmission line).
• Detect imbalances (differences) in these voltages.
• Trip the circuit breaker if the voltage difference exceeds a predefined threshold, indicating a fault (e.g., short circuit, open circuit, or insulation failure).

Option Analysis
Phase angle differences in currents → This describes a phase comparison relay, not a voltage differential relay.

Impedance measurement → This is the function of an impedance relay (used in distance protection).

Current entering/leaving a zone → This describes a current differential relay, which operates based on Kirchhoff’s current law (not voltage).

Thus, the correct choice is 4).

Explanation:

The Time Setting Multiplier (TSM) of a Relay

The Time Setting Multiplier (TSM) is a critical parameter in the operation of protective relays, especially in the context of overcurrent and differential protection schemes. The TSM adjusts the time delay characteristic of the relay, which influences how quickly the relay responds to fault conditions. This adjustment is crucial for coordinating the protection devices within an electrical system to ensure selective tripping and minimize disruption to the power supply.

Analysis of Other Options:

Option 1: Adjust the Operating Current Threshold of the Relay

This option is incorrect because the operating current threshold, also known as the pickup current, is typically adjusted using the current setting parameter of the relay. The TSM is concerned with the time delay characteristics rather than the current threshold.

Option 2: Increase the Current Setting of the Relay

This option is incorrect as well. Increasing the current setting of the relay would mean changing the current level at which the relay starts to operate (pickup level), which is not the function of the TSM. The TSM specifically adjusts the time delay after the relay has picked up a fault current.

Option 4: Decrease the Time Delay of the Relay

While this option is partially correct, it is incomplete. The TSM can indeed decrease the time delay of the relay, but its primary function is to modify the time delay based on fault severity, meaning it can both increase and decrease the time delay as required for proper coordination. Therefore, option 3 is more comprehensive and accurate.

The correct answer is: 3) The ability of the relay to detect and respond to very small fault currents.

Explanation:

Sensitivity in protective relays refers to:

• The minimum fault current (or power, voltage, etc.) that the relay can reliably detect and respond to.

• A highly sensitive relay can operate for very small fault currents, ensuring protection even for minor faults.

Tarapur Atomic Power Station:

• Tarapur Atomic Power station is located in Tarapur, Maharashtra.
• It was the first commercial atomic power station of India commissioned on 28th October 1969.
• It was commissioned under 123 agreements signed between India, the United States and International Atomic Energy Agency. 
• The station is operated by the National power corporation of India.

About Tarapur Atomic Power Station:

• Tarapur Atomic Power station is located in Tarapur, Maharashtra.
• It was the first commercial atomic power station of India commissioned on 28th October 1969.
• It was commissioned under 123 agreements signed between India, the United States, and International Atomic Energy Agency. 
• The station is operated by the National power corporation of India.

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The minimum clearance distance that equipment should be kept away from power lines of different voltage levels is shown in below table.

Transmission lines are classified based on three criteria.

a) Length of transmission line

b) Operating voltage

c) Effect of capacitance

The table below summarizes the classification of transmission lines.

Concept:

Load factor:

The load factor is the ratio of average energy consumed to maximum demand.

Load factor = average energy consumed / maximimum energy consumed

Calculation:

Given load factor = 0.5

Average energy consumed at 0.5 load factor = 600 kWh

Maximum energy consumed =  = 1200 kWh

Now maximum energy consumed is constant and load factor is increased to 0.8

Average energy consumed = load factor × maximum energy consumed

= 0.8 × 1200

= 960 kWh

Load factor 

Average demand = (50) (0.6) = 30 MW

Plant capacity factor 

Plant capacity 

Reserve capacity = Plant Capacity – Maximum Demand = 60 - 50 = 10 MW

CONCEPT:

Nuclear reactor:

• It is a device in which a nuclear reaction is initiated, maintained, and controlled.
• It works on the principle of controlled chain reaction and provides energy at a constant rate.

EXPLANATION:

• The moderator's function is to slow down the fast-moving secondary neutrons produced during the fission.
• The material of the moderator should be light and it should not absorb neutrons.
• Usually, heavy water, graphite, deuterium, and paraffin, etc. can act as moderators.
• These moderators are rich in protons. When fast-moving neutrons collide head-on with the protons of moderator substances, their energies are interchanged and thus the neutrons are slowed down.
• Such neutrons are called thermal neutrons which cause fission of U235 in the fuel. 

The following types of cables are generally used for 3-phase service:

1. Belted cables - up to 11 kV

2. Screened cables - from 22 kV to 66 kV

3. Pressure cables - beyond 66 kV

Belted cables:

• These cables are used for voltages up to 11 kV but in extraordinary cases, their use may be extended up to 22 kV
• The belted type construction is suitable only for low and medium voltages as the electrostatic stresses developed in the cables for these voltages are more or less radial i.e., across the insulation
• For high voltages (beyond 22 kV), the tangential stresses also become important
• These stresses act along the layers of paper insulation
• As the insulation resistance of paper is quite small along the layers, therefore, tangential stresses set up leakage current along the layers of paper insulation
• The leakage current causes local heating, resulting in the risk of breakdown of insulation at any moment

Recovery Voltage:

The RMS voltage that appears across the circuit breaker contacts after final arc interruption (when breaker opens) is called “recovery voltage”

Restriking Voltage:

It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction

Active Recovery Voltage:

It may be defined as the instantaneous recovery voltage at the instant of arc extinction

Arc Voltage:

It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rises rapidly to a peak value and current reaches to zero.