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MGVCL Exam Paper (30-07-2021 Shift 1)
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An RLC series circuit consists of R = 16 Ω, L = 5 mH and C = 2 μF. Calculate the quality factor.

For series RLC circuit,

Q = (1/R)*√(L/C)

Q = 1/16*√(5 mH/2 μF)

Q = 50/16

Q = 3.125

MGVCL Exam Paper (30-07-2021 Shift 1)
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Three equal star-connected inductors take 8 kW at 0.8 power factor when connected to a 460 V three - phase 3-wire supply. If one of the inductor in phase – Y is short circuited, then the line current in phase-Y is ___ Assume RYB sequence.

Here, Y phase is shorted,

Equation of 3-phase power = √3*VL*IL*cosφ

IL = P/(√3*VL*cosφ)

= 8000/(√3*400*0.8)

= 12.55 A

Phase impedance = Vp/Ip

= 460/(√3*12.55)

= 21.16 ohm

cosφ = 0.8, So φ = 37 degree.

Here Impedance of Y phase is shorted,

So, Neutral point and Y phase at same potential,

Three line voltages are given by,

Vry = 460∟0°

Vyb = 460∟120°

Vbr = 460∟-120°

Here N and Y phase at same potential,

Ir = (460∟-120°)/(21.16∟37°)

= -21.7∟83° A

Ib = (-460∟120°)/(21.16∟37°)

= 21.7∟-157° A

Ib = -(Ir + Iy)

= -21.7∟83° + 21.7∟-157°

= 37.6∟53° A

MGVCL Exam Paper (30-07-2021 Shift 1)
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The normal percentage bias setting of differential relay used for transformer protection will be

In bias differential protection for transformer,

There are two basic settings required to setup the percentage biased differential protection.

1. Minimum pickup

2. The slope.

The minimum pick-up covers the transformer magnetizing current typically 1-4% of transformer rating.

A pick up of 0.2 to 0.3 times the TAP is recommended for general cases.

The slope is defined by determining the potential sources of differential current.