EE MCQ

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.

Related Question

MGVCL Exam Paper (30-07-2021 Shift 1)
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A balanced delta connected load of 8+j6 Ω per phase is connected to a 415 V, 50 Hz, 3-phase supply lines. If the input power factor is improved to 0.9 by connecting a star connected capacitor bank, then the required kVAR of the bank is

Current flow through the circuit,

I = V/Z

= 400/10

= 41.5 A

Power factor = 8/10

= 0.8

After connecting a capacitor bank, the reactive power component of current changes but the active power component of current is unchanged.

I1*cosφ1 = I2*cosφ2

41.5*0.8 = I2*0.9

I2 = 36.88 A

Q1 (before connecting capacitor) = √3*VL*IL*sinφ1

= √3*415*41.5*sin(36.86)

= 26.847 kVAR

Q2 (after connecting capacitor) = √3*VL*IL*sinφ2

= √3*415*36.88*sin(25.84)

= 15.867 kVAR

kVAR supplied by capacitor bank = Q1 - Q2

= 26.847 - 15.867

= 10.98 kVAR

MGVCL Exam Paper (30-07-2021 Shift 1)
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The operating time of an inverse definite minimum time (IDMT) overcurrent relay corresponding to plug-setting multiplier (PSM) = 1 and time-multiplier setting (TMS) = 1 will be

Equation of operating time for IDMT relay (t) = 0.14*TMS/[(PSM^0.02) - 1]

Here,

PSM = TMS = 1

So,

t = 0.14*1/(1 - 1)

t = infinite

MGVCL Exam Paper (30-07-2021 Shift 1)
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The power factor angle of a purely inductive circuit is

Power factor for purely inductive circuit is 0 and power factor angle is 90 degree.

Power factor for purely capacitive circuit is 0 and power factor angle is 90 degree

Power factor for purely resistive circuit is 1 and power factor angle is 0 degree

MGVCL Exam Paper (30-07-2021 Shift 1)
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An RC series circuit with R = 150 Ω and C = 2 μF is excited by a DC source of 15 V by closing the switch at t = 0 with an initially uncharged capacitor. The initial and final voltages across the capacitor respectively are

For t = 0 in RC circuit,

After t = 0+ condition, capacitor is behave as short circuit.

Voltage across the it 0.

At final time,

Capacitor is behave as open circuit, voltage across the capacitor at final time is equal to 15 V.

Capacitor opposes the change of voltage.