- #1

- 51

- 0

Hi,

I am going through some texts but it was not very clear.I hope some experts here can help me on this.

The max voltage across the relay and stabilising resistor is given by;

(If/N)(Rs+Rb)

where If - max short circuit rating, A

N - current transformer, CT ratio

Rs - CT internal resistance

Rb - max lead burden

Relay impedance at setting current is given by VA burden/(I setting)2

Total relay circuit impedance = V max/I setting

Stabilising resistance = total relay circuit impedance - relay impedance at setting current.

Now if I have Isc=2000A

CT ratio=200:5

Rs=0.6 ohm

Rb=0.4 ohm

Hence max voltage calculated from the equation above = 50V.

Now here is the problem to find relay impedance at setting current,

Do I assume the relay setting and the VA burden or if these two parameters can be calculated from the above information?

Thank you.

p/s: I dont think we need to look at the circuit diagram because it just shows the protection scheme without going anywhere related to the calculation.

Edit:Posted the circuit diagram of the high impedance circulating current scheme.

I am going through some texts but it was not very clear.I hope some experts here can help me on this.

The max voltage across the relay and stabilising resistor is given by;

(If/N)(Rs+Rb)

where If - max short circuit rating, A

N - current transformer, CT ratio

Rs - CT internal resistance

Rb - max lead burden

Relay impedance at setting current is given by VA burden/(I setting)2

Total relay circuit impedance = V max/I setting

Stabilising resistance = total relay circuit impedance - relay impedance at setting current.

Now if I have Isc=2000A

CT ratio=200:5

Rs=0.6 ohm

Rb=0.4 ohm

Hence max voltage calculated from the equation above = 50V.

Now here is the problem to find relay impedance at setting current,

Do I assume the relay setting and the VA burden or if these two parameters can be calculated from the above information?

Thank you.

p/s: I dont think we need to look at the circuit diagram because it just shows the protection scheme without going anywhere related to the calculation.

Edit:Posted the circuit diagram of the high impedance circulating current scheme.

Last edited: