High impedance circulating current scheme(calculate stabilising resistor)

AI Thread Summary
The discussion centers on calculating the stabilizing resistor for a high impedance circulating current scheme using provided equations and parameters. Key values include a maximum short circuit current (Isc) of 2000A, a current transformer (CT) ratio of 200:5, and resistances Rs and Rb of 0.6 ohms and 0.4 ohms, respectively. The maximum voltage across the relay and stabilizing resistor is calculated to be 50V, but there is confusion regarding the relay impedance at the setting current and the VA burden. Additional data from the relay manufacturer is necessary for accurate calculations, and the importance of the CT ratio is emphasized for determining primary current. The final calculated value for the stabilizing resistor is stated to be 400 ohms.
StealthRay
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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 don't 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.

circulatingcurrentprotect.jpg
 
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This looks very much like an incomplete version of a homework or coursework problem, presented without the schematic diagram which originally accompanied the text.

Some readers may be sufficiently familiar with this material to determine the original question, but it would be much better if you would present the question clearly in its original form.
 
Adjuster said:
This looks very much like an incomplete version of a homework or coursework problem, presented without the schematic diagram which originally accompanied the text.

Some readers may be sufficiently familiar with this material to determine the original question, but it would be much better if you would present the question clearly in its original form.

Aww,man,this is not a homework.As you can see,all the equations are there.I can even provide the step by step answers.It just that the text plainly stated the VA burden as 1 VA and relay setting as 0.1A,which I don't understand where they came from.

The final answer would be 400 ohm for the stabilising resistor.

I just need the explanation and this is not homework.

Thank you.
 
StealthRay said:
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 don't think we need to look at the circuit diagram because it just shows the protection scheme without going anywhere related to the calculation.

Why are you talking about a current transformer ratio for a relay? This is confusing. Posting the diagram would help a lot.
 
berkeman said:
Why are you talking about a current transformer ratio for a relay? This is confusing. Posting the diagram would help a lot.

Ok,I have posted the circuit diagram.Thank you.
 
To Berkeman:- CT ratio needed as primary (not secondary) current given.

To All:- I have a software package developed that calculates stabilizing resistor values as well as much more, it's designed for HV Commissioning Engineers. If anyone's interested please send me a PM.
 
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