Relay Setting Calculation

Capacity of the Transformer, Sref in MVA System Voltage in kV Transformer % Impedance HV Side CT - primary in Amps HV Side CT - secondary in Amps LV Side CT - primary in Amps LV Side CT - secondary in Amps HV Side Voltage LV side Voltage OLTC Range Vector Group Rated current of the Transformer HV side in Amps Rated current of the Transformer at HV Side in Secondary, Iref Amplitude matching factor of HV side Rated current of the Transformer LV side in Amps Rated current of the Transformer at Sec. Side in Secondary, Iref Amplitude matching factor of LV side The Recommended seting Idiff> Idiff>> m1 m2 The base value for m2

1

The stability check is performed for the following cases 1.The 20 MVA Transformer is fully loaded at -7.5% tap 2.The20 MVA Transformer is fully loaded at +7.5% tap 3. The 20 MVA Transformer through fault stability at -7.5% tap & +7.5% tap. Stability of 20MVA Transformer at full load and under operation at -7.5% tap: Transformer load Transformer HV Side (33kV) current in Amps Equivalent secondary current at HV Side (33kV) in Amps Transformer HV Side (33kV) current in Iref, Amp Transformer LV Side (6.9kV) current in Amps

20 33/6.9 10 1000 1 2500 1 33 6.9 7.50% Dyn11 350 0.35 2.857142857 1673.5 0.6694 1.493875112

0.2 8 0.25 0.5 2.5

20 0.378291371 0.378 1.079946 1673.5

Equivalent secondary current at LV Side (6.9kV) in Iref, Amps Transformer LV side (6.9kV) current in Iref Differential current at -7.5% tap position Restraining current at -7.5% tap position Considering the recommended setting the operating current calculation. The differential current = 0.075A & the Bias current = 1.042A The Differential current required for the relay to operate = Idiff> (1-0.5 * Bias setting) + (Bias Setting *Bias Current) The Differential current required for the relay to operate in Amps

Stability backup calculation in through fault condition: The Fault MVA Through Fault current on the HV side in Amps Equivalent secondary current on the HV side in Amps Through fault Current of HV Side in Iref in Amps Through fault current on the LV side in Amps Equivalent secondary current on the LV side in Amps Through fault LV side current in Iref Tap Position +7.5% Through fault current on HV side at Max. tap position in Amps Equivalent secondary current on the HV side in Amps Through fault Current of HV Side in Iref in Amps Differential current in Max. tap position, Idiff in Amps Restraining Current in Max. tap position in Amps Considering the recommended setting the operating current calculation. The differential current = 0.675 A & the Bias current = 9.63 A The Differential current required for the relay to operate = Idiff> (1-0.5 * m1) + (m2 * Bias current) +(IR,m2 * (m1-m2)) The Differential current required for the relay to operate in Amps REF PROTECTION CT References for LV REF Protection:

0.6694 1.0041 0.075846 1.042023

0.3884

200 3.499195185 3499.2 A 3.4992 9.9972144 16.73528132 16735.2A 6.69408 9.9741792

3.255065288 3255A 3.255 9.299535 0.675 9.6365

Transformer Capacity in MVA Transformer % Impedance LV Phase side CT reference- primary LV Phase side CT reference- secondary LV Neutral side CT reference- primary LV Neutral side CT reference- secondary LV Side Voltage reference in kV REF Settings: Maximum Fault Current in Amps Equivalent Fault Current in Sec. in Amps Maximum Earth fault current in Amps (As per Grounding Resistor rating)

20 10 2000 1 2000 1 6.9 16.73528132 16735.2A 8.3676 400 10% of maximum earth fault current

REF Pickup current in Amps 40 Equivalent secondary current in Amps Is 0.02 Rstab calculation: Rct value of CT in Ohms(from drawings) Rct 8 Assuming a lead length of 100mtrs of 2.5 sqmm copper cable with a resistance of 7.81 Ohms/km. Lead resistance in Ohms RL Voltage developed during fault across the relay Vs in Volts Fault current * (Rct + 2RL) 79.93832 Stabalizing resisitor value in Ohms Rstab Vs/Is 3996.916 Requirment of metrosil Vp PEAK VALUECAN BE ESTIMATED BY Vf voltage due to internal fault equivalent to secondry fault

2*under root of(2*Vk(Vf-Vk)) If(Rct+2Rl+Rstab) 33492.668

Vp

9215

f 7.81 Ohms/km.

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