Test: Protective Relays - Electrical Engineering (EE) MCQ

Concept:

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

Plug setting multiplier 

Calculation:

Plug setting = relay setting × secondary CT current

= 1.5 × 5 = 7.5

CT ratio = 400/5 = 80

Plug setting multiplier 

Differential relay:

• Differential relay operation depends on the phase difference of two or more electrical quantities.
• It works on the principle of comparison between the phase angle and the magnitude of the same electrical quantities.
• The differential relay is used for the protection of the feeder, large busbars, etc.

Distance relay:

• Distance relay is widely used for the protection of high-voltage AC transmission line and distribution lines.
• Distance protection schemes are commonly employed for providing the primary or main protection and backup protection for AC transmission line and distribution line against three-phase faults, phase-to-phase faults, and phase-to-ground faults.

Under frequency relay:

• Under frequency relay is used to protect the alternator when the frequency drops below the operating frequency.
• It is a backup protection for over fluxing (V/F) protection.
• Under frequency occurs due to turbine low speed, grid frequency fluctuation, etc.
• The generator can tolerate moderate under frequency operation provided voltage is within an acceptable limit.

Buchholz relay:

• Buchholz relay protects the transformer from internal faults.
• It is the gas actuated relay.
• The Buchholz relay is placed between the main tank and the conservator.
• Buchholz relay is used in the transformer having a rating higher than 500KVA.
• It is not used in a small transformer because of economic considerations.

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.

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.

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 

Calculation:

Plug setting = relay setting × secondary CT current

= 0.5 × 5 = 2.5

CT ratio = 400/5 = 80

Plug setting multiplier 

Silicon oil has the highest breakdown strength of all types of oil, followed by:

• Capacitor oil
• Cable oil
• Transformer oil

Here is the explanation of why silicon oil has the highest breakdown strength:

• Silicon oil has a high dielectric constant. This means that it can store a lot of electrical energy before breaking down.
• Silicon oil is very hydrophobic. This means that it repels water and other impurities, which can lower the breakdown strength of oil.
• Silicon oil has a high viscosity. This means that it flows slowly, which helps to reduce the risk of arcing and breakdown.

Silicon oil is used in a variety of electrical applications, such as transformers, capacitors, and circuit breakers, where its high breakdown strength is essential for safety and reliability.

Zero factor protection: This option doesn't correspond to known electrical protection systems. The term "zero factor" does not define a type of standard protection that is used for electrical lines.

Lewis defense: This term is not associated with electrical protection systems. It might be confused with a strategy in game theory or related to military use but is not applicable in this context.

Direct protection: It generally refers to methods that protect equipment or lines directly, like using fuses, circuit breakers, or relays. However, these don't necessarily refer to specific methods to protect LT lines passing through fields.

PVC protection: It's a common practice to use PVC (Polyvinyl Chloride) for covering the LT (Low Tension) lines in agricultural fields. The PVC casing or conduits provide insulation, preventing direct exposure to the conducting wire, reducing the risk of electrical shock, and protecting the wires from damage due to weather conditions or farm-related activities. So this is the correct answer amongst the provided options.