Protective Relays MCQ Quiz - Objective Question with Answer for Protective Relays - 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.

Concept:

The pick-up current of a relay is given by:

Pick-up current = Rated secondary current of CT × Current setting

Calculation:

Given, the CT ratio = 400/5

Secondary current = 5 A

Relay setting = 12.5%

Pick-up current = 5 × 0.125

Pick-up current = 0.625 A

Negative sequence relay:

• It protects generators from the unbalanced load by detecting negative sequence current.
• A negative sequence current may cause a dangerous situation for the machine.
• Phase to phase fault mainly occurs because of the negative sequence component.
• The negative sequence relay has earthing which protects from phase-to-earth fault but not from phase-to-phase fault.

Note: 

Concept:

Plug setting multiplier:

Plug setting multiplier is the ratio of fault current and the product of plug setting and CT ratio.

Plug setting multiplier \(= \frac{{fault\;current}}{{plug\;setting \times CT\;ratio}}\)

Calculation:

Plug setting = relay setting × secondary CT current

= 0.5 × 5 = 2.5

CT ratio \(= \frac{{400}}{5} = 80\)

Concept:

Plug setting multiplier is the ratio of fault current and the product of plug setting and CT ratio.

Plug setting multiplier \(= \frac{{fault\;current}}{{plug\;setting \times CT\;ratio}}\)

Calculation:

Plug setting = relay setting × secondary CT current

= 1.5 × 5 = 7.5

CT ratio \(= \frac{{400}}{5} = 80\)

Distance Relay:

• A distance protection relay is a name given to the protection, whose action depends on the distance of the feeding point to the fault.
• The time of operation of such protection is a function of the ratio of voltage and current, i.e., impedance.
• This impedance between the relay and the fault depends on the electrical distance between them.
• Types of distance relays are impedance relays, reactance relays, and the mho relays.

Important Points

• Reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
• The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
• Ohm relay is suitable for medium transmission lines.

Distance Relay:

This type of relay is used for the protection of the transmission lines.

Depending upon the length of the transmission line, the distance relay is divided into:

1.) Impedance Relay:

• This relay is a voltage restrained overcurrent relay.
• This relay operates when the impedance seen from the fault point is less than the relay setting (Z).
• It is used in the protection of medium transmission lines.

​2.) Reactance Relay:

• This relay is a current restrained overcurrent relay.
• This relay is used for the protection of short transmission lines.
• Reactance relay is independent of resistance value.

3.) Mho Relay:

• This relay is used for the protection of long transmission lines.
• Mho relay is least affected by power surges.
• Mho relay is inherently a directional relay as it detects the fault only in the forward direction.

Shortcut Trick

Concept:

Plug setting multiplier is the ratio of fault current and the product of plug setting and CT ratio.

Plug setting multiplier \(= \frac{{fault\;current}}{{plug\;setting \times CT\;ratio}}\)

Calculation:

Plug setting = relay setting × secondary CT current

= 0.5 × 5 = 2.5

CT ratio \(= \frac{{500}}{5} = 100\)

Mho Relay:

A Mho relay measures a component of admittance |Y| ∠ θ. But its characteristic when plotted on the impedance diagram is a circle passing through the origin. It is also known as angle impedance relay.

The relay is called Mho relay because its characteristic is a straight line, when plotted on an admittance diagram.

Important Points:

• Impedance relay is a voltage restrained overcurrent relay.
• Reactance relay is an overcurrent relay with directional restraint.
• Mho relay is a voltage restrained directional relay.

• Reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
• The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
• Impedance relay is suitable for medium transmission lines.

Additional Information Mho Relay: Also known as a polarized distance relay or admittance relay, it is inherently directional and operates based on both the magnitude and angle of impedance. The term "Mho" is derived from the unit of admittance (the inverse of ohm), and it is characterized by a circular or oval operating characteristic on the impedance plane. The Mho relay is directionally sensitive and thus incorporates the angle of impedance in its operation, making it an angle impedance relay.

Given the network is:

In the given network,

R₂, R₄, and R₅ are directional overcurrent relays. R₁, R₃ and R₆ are non-directional overcurrent relays.

For the fault on line 2 i.e. L₂, R3 and R4 must be operated. If R₄ is not operated then R₁ and R₆ will operate.

Therefore, back up for R₄ are R1 and R6

The relay R₃ is directly connected to relay R₄, so that relay R₃ can’t provide remote backup.