Test: Switchgear & Protection - Electrical Engineering (EE) MCQ

As the power levels remain same at the two sides of transformer,
√3*400*1000 = √3*33000*IL2
IL2 = 400/33
Current through the secondary of CT on the primary side = 5A
Current through the pilot wire = 5√3 A
So CTs on the secondary side being star connected will have 5√3 A.
CT ratio on 33000V side = 400/(33*5√3 ) = 7/5.

The primary CT current for the relay operation = 1.5*1000/5 = 300 A
% winding unprotected = (Is*RV*100%)
= (300*5√3)/11000*100
= 23.6 %.

There will be flow of inrush current at the energization of transformer which will let relay may operate.

The transformer having heavy inrush current has 2nd harmonic components.

The line voltage determines the arc de ionisation in a circuit breaker.

As the power levels remain same at the two sides of transformer,
√3*400*1000 = √3*33000*IL2
IL2= 400/33
Current through the secondary of CT on the primary side = 5A
Current through the pilot wire = 5√3 A.

% winding protected = 76.4 %
resistance required to protect 90% of the winding
10 = Is*RV*100
R = 2.12 Ω.

The carrier frequency has to be high in the case of carrier-current-pilot scheme.

Basic principles of transformer protection

• Gas protection
• Longitudinal differential protection or current quick-break protection
• Backup protection for phase-to-phase short circuit
• Zero sequence protection of grounding short circuit
• Overexcitation protection

​Explanation 

• The Buchholz relay is normally not provided in small distribution transformers.
• Buchholz relay, however, is a gas-actuated protection device that is typically used in oil-filled transformers of larger sizes.
• It detects internal faults like short circuits and insulation breakdowns by sensing gas accumulation in the oil. This type of protection is generally not provided in small distribution transformers due to their simpler design and lower cost.
• Overfluxing protection and overcurrent protection are commonly provided in transformers, including small distribution transformers, to protect against overvoltage and excessive current conditions, respectively.

Rating of Circuit Breaker:

A circuit breaker should be rated with a short circuit-making capacity. As the rated short circuit making current of the circuit breaker is expressed in peak value, it is always more than the rated short circuit breaking current of the circuit breaker.

Making capacity of a circuit breaker is equal to 2.55 times symmetrical breaking current.

Breaking capacity is always expressed in RMS value.

Both breaking and making capacity are generally expressed MVA.