Protective Relays MCQ Quiz - Objective Question with Answer for Protective Relays - Download Free PDF

Induction Type Directional Overcurrent Relay

Definition: An induction type directional overcurrent relay is a protective device used in electrical power systems to detect overcurrent conditions while ensuring the current flows in a specific direction. This relay combines the principles of overcurrent protection and direction discrimination, making it suitable for applications where fault directionality is critical, such as in ring main systems or parallel feeder systems.

Net Torque at Balance Point:

When an induction type directional overcurrent relay is on the verge of operating, the net torque acting on the relay must be analyzed. In this condition, the relay is balanced, and the mechanical torque generated within the relay coil system reaches equilibrium. This balance point is crucial for determining the relay’s operating threshold.

Correct Option: The net torque at the balance point is equal to zero.

Explanation:

The torque in an induction type directional overcurrent relay is developed by two fluxes: the flux produced by the current coil and the flux produced by the voltage coil. These fluxes interact with each other, inducing currents in the relay’s disc or rotor, which ultimately produce a resultant torque. This torque is responsible for moving the relay’s disc or rotor to operate the relay contacts.

At the balance point, the relay is on the verge of operation, which means that the torque produced by the interaction of the fluxes is in equilibrium. In this state:

• The driving torque, which tends to move the disc or rotor, is exactly balanced by the restraining torque, which opposes the motion.
• As a result, the net torque acting on the relay is zero.

This balance ensures that the relay does not operate unnecessarily during normal conditions or minor disturbances but operates reliably when the fault current exceeds the threshold and flows in the specified direction.

Correct Option Analysis:

The correct option is:

Option 4: Equal to zero.

This is because, at the balance point, the driving torque and restraining torque cancel each other out, resulting in no net torque on the relay’s disc or rotor. This equilibrium condition ensures that the relay is stable and does not operate until the system conditions necessitate it.

Additional Information

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

Option 1: Equal to K2.

This option is incorrect because the net torque at the balance point is not a constant value such as K2. The balance point is defined by the condition of zero net torque, where the driving and restraining torques are equal and opposite.

Option 2: Equal to VI cos (θ - α).

This option represents the torque equation in terms of the product of voltage (V) and current (I) with a phase angle difference (θ - α). While this expression is related to the torque produced in the relay, it does not define the net torque at the balance point, which is zero.

Option 3: Equal to the pick-up torque of the relay.

The pick-up torque is the minimum torque required to initiate the motion of the relay’s disc or rotor. However, at the balance point, the net torque is zero, as the driving and restraining torques are in equilibrium. Therefore, this option is incorrect.

Conclusion:

The net torque at the balance point of an induction type directional overcurrent relay is equal to zero. This equilibrium condition ensures the relay remains stable under normal operating conditions and operates reliably when the fault current exceeds the threshold and flows in the specified direction. Understanding the torque balance in such relays is essential for designing and applying protective systems in electrical power networks.

Explanation:

Balanced Beam Type Relay

Definition: A balanced beam type relay is an electromechanical device used primarily for protective purposes in electrical systems. This type of relay operates using a pivoted beam that is balanced under normal working conditions and tilts or moves when an abnormal condition arises, such as a fault or overload. The beam's movement triggers the relay to perform its intended protective action, like tripping a circuit breaker.

Working Principle: In a balanced beam type relay, the beam is held in a horizontal or balanced position during normal working conditions. This is achieved by a spring or similar mechanical component that ensures stability and prevents the beam from tilting unnecessarily. When a fault condition occurs, the current through the relay coil changes, creating a magnetic force that overcomes the spring's resistance and causes the beam to tilt. This tilt activates the relay's contacts, initiating the protective mechanism.

Advantages:

• Simple construction and reliable operation.
• Quick response time during fault conditions.
• Efficient in detecting abnormal conditions like overcurrent or short circuits.

Disadvantages:

• Requires precise calibration to maintain the balance under normal conditions.
• Limited application range compared to modern electronic relays.
• Susceptible to mechanical wear and tear over time.

Applications: Balanced beam type relays are commonly used in older electrical systems for overcurrent or fault protection. They are also employed in certain specific applications where mechanical simplicity is preferred over electronic complexity.

Correct Option Analysis:

The correct option is:

Option 4: The beam is held in a horizontal position by the spring.

This option correctly describes the normal working condition of a balanced beam type relay. During normal conditions, the spring ensures that the beam remains in a balanced, horizontal position. This stability prevents the relay from activating unnecessarily, ensuring reliable operation of the electrical system. The spring's role is crucial as it counterbalances the magnetic force generated by the current flowing through the relay coil. Only during abnormal conditions, like a fault, does the magnetic force overpower the spring's resistance, causing the beam to tilt and activate the relay.

Additional Information

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

Option 1: The current through the coil causes the beam to tilt downward.

This option is incorrect because it describes the behavior of the relay under abnormal or fault conditions, not normal working conditions. Under normal conditions, the spring holds the beam in a horizontal position, counteracting the magnetic force produced by the current. The beam only tilts downward when there is a fault or overload, triggering the relay's protective action.

Option 2: The beam oscillates between horizontal and tilted positions.

This option is incorrect as it misrepresents the behavior of the relay. Under normal conditions, the beam remains stable in the horizontal position, held by the spring. Oscillation between horizontal and tilted positions would indicate instability or improper functioning of the relay, which is not characteristic of a balanced beam type relay.

Option 3: The spring compresses completely to maintain stability.

This option is incorrect because the spring in a balanced beam type relay does not compress completely to maintain stability. Instead, it provides a calibrated counterforce to the magnetic force generated by the current through the coil, ensuring the beam remains horizontal under normal conditions. Complete compression of the spring would compromise its ability to counterbalance the magnetic force, leading to improper relay operation.

Option 5: (No description provided)

Since option 5 lacks a description, it cannot be analyzed or considered correct. The absence of information makes it irrelevant to the question.

Conclusion:

Understanding the operational characteristics of a balanced beam type relay is crucial for its proper application and maintenance. The relay's beam remains in a horizontal position under normal conditions due to the counteracting force of the spring, ensuring stability and preventing unnecessary activation. The detailed analysis of the options highlights the importance of distinguishing between normal and abnormal working conditions to correctly interpret the relay's behavior.

Explanation:

Numerical Solution: Protective System Multiplier (PSM)

The given problem involves calculating the Protective System Multiplier (PSM) for a relay connected to a current transformer (CT) with a given fault current and relay setting percentage. Below is the detailed solution:

Step 1: Understanding Protective System Multiplier (PSM):

The Protective System Multiplier (PSM) is defined as the ratio of the fault current (secondary current of the CT during a fault) to the relay pickup current. It is mathematically expressed as:

PSM = (Fault Current in CT Secondary) ÷ (Relay Pickup Current)

Where:

• Fault Current in CT Secondary: This is the current flowing through the secondary winding of the current transformer during a fault condition.
• Relay Pickup Current: This is the threshold current at which the relay starts to operate. It is calculated using the relay setting percentage.

Step 2: Given Data:

• Current Transformer Ratio: 400/5
• Fault Current (Primary Side): 2.4 kA = 2400 A
• Relay Setting: 150%

Step 3: Calculate Fault Current in CT Secondary:

The fault current in the secondary winding of the CT can be calculated using the CT ratio:

Fault Current in CT Secondary = (Fault Current in Primary Side × CT Secondary Rating) ÷ CT Primary Rating

Substituting the values:

Fault Current in CT Secondary = (2400 × 5) ÷ 400

Fault Current in CT Secondary = 30 A

Step 4: Calculate Relay Pickup Current:

The relay pickup current is determined using the relay setting percentage. Relay setting is given as 150%, which means:

Relay Pickup Current = (Relay Setting × CT Secondary Rating) ÷ 100

Substituting the values:

Relay Pickup Current = (150 × 5) ÷ 100

Relay Pickup Current = 7.5 A

Step 5: Calculate PSM:

Now, the PSM can be calculated using the formula:

PSM = (Fault Current in CT Secondary) ÷ (Relay Pickup Current)

Substituting the values:

PSM = 30 ÷ 7.5

PSM = 4

Step 6: Final Answer:

The Protective System Multiplier (PSM) is 4. Therefore, the correct answer is Option 3

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.

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.