STABILIZING RESISTOR CALCULATION

Introduction

=Let,

- Voltage in Volts

- Setting value of the relay as secondary value

=Let,

- Maximum through fault current

- CT Secondary side current in Amps

- CT Primary side current in Amps

- CT Secondary winding internal resistance in Ohms

- Total resistance of wiring, connections and etc.,

Calculation

Lead Resistance

Distance from CT to relay 100 mtr

Typically = 8.87 Ω/km for 2.5 Sq.mm, armoured copper cable

Rl = 8.87 Ω/km

Number of runs to be used = 1

Lead resistance Rl = 0.887 Ω

Since the application is for REF, we will conisder 2xRl as the total lead resistance

= 2 x Rl Ω

= 1.774 Ω

For a Typical Case, Calculating Isc (max) through fault current for Transformer:

Transformer Details:Rated MVA (MAX, i.e., with cooling) 250 MVAVoltage lvl where relay is mounted 400 kV%Z 10%Isc 3608.44 Amps

Rstab VS / ISet

VS

ISet

VS Imax * (CTSec / CTPri) * (RCT + RWdg)

Imax

CTSec

CTPri

RCT

RWdg

R L is the Lead resistance of the Cable

RWdg

= 16000 Amps

= 2000 Amps

= 1 Amps

= 6 Ohms

= 1.774 Ohms

= 10 %age of setting value

= 0.1

= Volts

= Volts

= 62.192 Volts

= Ω

Ω

= 621.92 Ω

Summary

= 621.92 Ω

Imax

CTPri

CTSec

RCT

RWdg

ISet

ISet

VS Imax * (CTSec / CTPri) * (RCT + RWdg)

VS

VS

Rstab VS / ISet

Rstab

Rstab