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.
Current flow through the circuit,
I = V/Z
= 41.5 A
Power factor = 8/10
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
= 26.847 kVAR
Q2 (after connecting capacitor) = √3*VL*IL*sinφ2
= 15.867 kVAR
kVAR supplied by capacitor bank = Q1 - Q2
= 26.847 - 15.867
= 10.98 kVAR
Equation of operating time for IDMT relay (t) = 0.14*TMS/[(PSM^0.02) - 1]
PSM = TMS = 1
t = 0.14*1/(1 - 1)
t = infinite
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
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.