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

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).

Thermal Overheating Protection in a Transformer

​Transformer overheating protection involves strategies to prevent damage caused by excessive heat, typically using temperature sensors, alarms, and forced ventilation. These measures ensure the transformer operates within safe temperature limits and prevents damage from overloading or other faults. 

Thermal Overheating Protection in a transformer is specifically designed to:

• Monitor the internal temperature of the transformer.
• Prevent damage due to overheating, which can degrade insulation and reduce transformer life.
• Trigger alarms or shut down the transformer if the temperature exceeds safe thresholds.

Percentage Differential Protection of an alternator

• It is a protection scheme primarily used to detect and protect against internal faults within the alternator windings.
• It uses a "percentage" or "bias" principle to avoid tripping during external faults or minor CT errors, making it stable and selective.
• It works by comparing the current entering and leaving the alternator windings, and operates only when there is a significant difference between them, indicating an internal fault.
• A differential relay monitors the difference between these currents. If the alternator is healthy, I₁ ≈ and I₂, so no differential current flows → the relay does not trip.
• If there's an internal fault, part of the current leaks to ground or between phases → I1 ≠ I2, producing a significant I_diff and relay trips.

Stator Earth Fault Protection

• Stator earth fault protection in alternators involves preventing damage to the stator windings when an insulation failure causes a ground fault. This is typically achieved by limiting the ground fault current and detecting and tripping the circuit during a fault. 
• Stator windings can fail due to insulation breakdown, leading to a short circuit between a winding and the machine's frame (ground). 
• Stator earth fault protection in generators limits ground fault current to minimize damage to the stator core and windings.
• It's achieved by grounding the generator's neutral point with a limited impedance (e.g., a resistor) and using protective relays to detect and respond to earth faults.
• Different methods, like 20Hz injection or third harmonic monitoring, are employed to detect and protect various percentages of the stator winding. 

Inter-Turn Fault Protection

• In this method of protection, currents at the two ends of the protected section are compared.
• Under normal operating conditions, the currents are equal. Under abnormal or fault conditions, the current becomes unequal. The difference of the unequal currents under fault conditions is arranged to pass through the operating coil of the relay.
• The relay then closes its contacts to isolate the protected section from the system.

Purpose of 'Inter-Turn Fault Protection'

• Detect short circuits between adjacent turns of the same phase winding in the stator of an alternator.
• These faults can cause localized heating, insulation damage, and even lead to catastrophic winding failures if not detected early.
• Since these faults do not always cause a phase-to-ground or phase-to-phase fault, standard protections may not detect them making inter-turn fault protection critical.

∴ current through the operating coil = 3.125 – 2.75 = 0.375 A

The correct answer option 3): Pair 3

Concept:

The differential relay actually compares between primary current and secondary current of the power transformer,

if any unbalance is found in between primary and secondary currents

the relay will actuate and inter-trip both the primary and secondary circuit breaker of the transformer.

For differential protection of the power transformer, CT s must be connected in the star in the delta side of the transformer and in the delta in the star side of the transformer.

 The given connection for the transformer is Y -Δ. The delta side must be connected to the star. Therefore, the connection for the CT must be Δ- Y connection. Therefore, the primary CT must be a star and the secondary must be a delta.

Merz-price differential protection scheme:

• Merz-price differential protection is nothing but a percentage differential protection.
• It works under the principle of the circulating current scheme.
• Merz-price differential protection is used to protect the transformer from internal short circuits, Internal ground faults, and inter-turn short circuits.

Percentage of Winding unprotected = (I₀ × R_n / V_ph) × 100

Where,

I₀ = Minimum pick up current of relay × CT ratio

R_n = Neutral resistance

V_ph = Phase voltage of the generator

Percentage of Winding protected = 100 - Percentage of Winding unprotected

Calculation:

Given,

Minimum pickup current of relay = 2 A

CT ratio = 500/5

R_n = 4 Ω

V_ph = 11 / √3 kV

I₀ = 2 × (500 / 5)

= 200 A

Percentage of winding unprotected = (200 × 4 × √3 / 11 × 10³) × 100

= 12.6 %

Percentage of winding protected = 100 - 12.6

= 87.4 %

Therefore, the percentage of generator winding will be protected by the percentage differential protection scheme is 87.4%

Generator Stator Protection:

• These faults occur in the generator due to the insulation failure of the stator windings.
• The following fault can cause insulation failure in the stator winding.

1. Fault between phase and ground
2. Fault between phases
3. Inter-turn fault

• The stator winding faults are considered the most dangerous fault, therefore, automatic protection is much necessary to clear the stator winding fault as soon as possible.
• For the protection of alternators against stator winding fault, the Differential method of Protection or, Merz-Price Protection is most commonly used because of its greater sensitivity and reliability.
   

Differential Protection of Alternators:

• The most common system used for the protection of stator winding faults is the circulating-current principle.
• In this method of protection, currents at the two ends of the protected section are compared.
• Under normal operating conditions, the currents are equal.
• Under abnormal or fault conditions, the current becomes unequal.
• The difference of the unequal currents under fault conditions is arranged to pass through the operating coil of the relay. The relay then closes its contacts to isolate the protected section from the system.
• The schematic arrangement of current differential protection for a 3-phase generator is shown.

• Two identical current transformer pairs CT₁ and CT₂ are placed on both sides of each phase of the stator windings.
• The secondaries of each set of current transformers are connected to the star.
• The two neutral points and the corresponding terminals of the two-star groups being connected together by means of a four-core pilot cable.
• This pilot cable provided an independent path for the current circulation in each pair of current transformers and the corresponding pilot P.
• The relay coils are connected in star, the neutral point is connected to the current-transformer common neutral, and the outer ends one to each of the other three pilots.
• The burden on each current transformer is the same as the burden of Relay.
• The relays are connected across equipotential points of the three pilot wires and these equipotential points would naturally be located at the middle of the pilot wires.
• The relays are used in this method of protection is instantaneous electromagnetic type.

Operating force of relay coil = n_o(|i₁|-|i₂|)

Opposing force produced by restraining coil \(= {n_r}\left( {\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}} \right)\)

At balanced (or) boundary condition

\(\begin{array}{l} {n_o}\left( {\left| {{i_1}} \right| - \left| {{i_2}} \right|} \right) = {n_r}\left( {\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}} \right)\\ \frac{{{n_r}}}{{{n_o}}} = \left( {\frac{{\left| {{i_1}} \right| - \left| {{i_2}} \right|}}{{\frac{{\left| {{i_1}} \right| + \left| {{i_2}} \right|}}{2}}}} \right) = bias \end{array}\)

Where n_o and n_r are the number of turns on operating (or) relay coil and restrained coil respectively.

Biased differential relay:

In this relay, there are two types of coil: one is the operating coil and the other is the restraining coil.

The bias ratio is the ratio of the number of turns of the restraining coil to the operating coil. It is given by:

Bias = \( {N\over N_o}\)

MMF produced in operating coil = No(i1 - i2)

MMF produced in restraining coil = \( {N \over 2}(i_1+i_2)\)

At balanced or equilibrium condition:

\(N_o(i_1-i_2)= {N \over 2}(i_1+i_2)\)

\( {N\over N_o} = \dfrac{2(i_1 - i_2)}{(i_1 + i_2)}\times 100\)

% Bias = \(\dfrac{200(i_1 - i_2)}{(i_1 + i_2)}\)

• Protection devices perform their purpose by keeping a faulty section isolated from the remaining healthy system to make it work without any disturbances.
• The function of a protection system is not to prevent faults as its name suggests, rather it minimizes repair costs as it senses fault because it only acts after a fault occurs.

Protection Schemes:
There are several protection schemes invented along the line as protection engineers face new challenges with the advancement in power systems. Here, we will discuss the most basic ones.

• Overcurrent Protection Scheme 
• Differential Protection Scheme
• Distance Protection Scheme
• Directional Protection Scheme/ Merge Price Protection

Protection Schemes and respective protected parts or machines: 

Merz-price differential protection scheme:

• Merz-price differential protection is nothing but a percentage differential protection.
• It works under the principle of the circulating current scheme.
• Merz-price differential protection is used to protect the transformer from internal short circuits, Internal ground faults, and inter-turn short circuits.

The CTs connection in the Merz-price protection:

CTs are connected to every phase of the system.

In order to nullify the phase shift of the Power transformer (PT), The Current transformers (CT) connected in a reverse configuration.

The star-star power transformer is protected by the current transformer having delta/delta connections.

Biased Differential Relay:

• The relay whose operation depends on the phase difference of two or more electrical quantities is known as the differential protection relay.
•  It works on the principle of comparison between the phase angle and the magnitude of the same electrical quantities.
• In a biased differential relay, the bias is defined as a ratio of the number of turns of restraining and operating coil.

​​Operation of biased differential relay:

The MMF produced in the operating coil = N_o(I₁ - I₂)

The MMF produced in the restraining coil = \( {N_r\over 2}(I_1+I_2)\)

At balance condition:

\( {N_r\over 2}(I_1+I_2)\) = = No(I1 - I2)

Bias = \( {N_r \over N_o}\) = \( 2{{(I_1-I_2)}\over (I_1+I_2) }\)

Differential relay:

• A differential relay is one that operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value.
• Thus a current differential relay is one that compares the current entering a section of the system with the current leaving the section.
• Under normal operating conditions, the two currents are equal but as soon as a fault occurs, this condition no longer applies.
• The difference between the incoming and outgoing currents is arranged to flow through the operating coil of the relay.
• If this differential current is equal to or greater than the pickup value, the relay will operate and open the circuit breaker to isolate the faulty section.

Applications of differential relay:

• Generally, differential relays are used to protect the equipment against internal faults in large pieces of equipment.
• Merz price protection scheme is one form of the differential relay which is used to protect the stator winding of the Alternator from the internal faults and transformer.
• Differential relay is also used for the protection of transformer winding.
• We can make use of differential relay in the protection of the Transmission line and Bus bar protection also.
   

Note: Reactance Relay, Admittance Relay, Impedance Relay are Distance Relay.