Current transformer for protection question

AI Thread Summary
The discussion focuses on determining the RR value to ensure relay current does not exceed 20mA, neglecting relay resistance. It references the Merz-Price theorem, noting that identical current transformers (CT1 and CT2) are affected by leads and secondary winding resistances, disrupting the balance. The RR is necessary to prevent unwanted relay operation when the current is below the threshold. The original poster expresses uncertainty about the formula needed for the calculation but later indicates they have resolved their query. The thread concludes with a request to close the discussion.
bizuputyi
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Homework Statement



Determine RR value if the relay current has not to exceed 20mA. Neglect relay resistance.

2. The attempt at a solution

I believe the solution is behind Merz-Price theorem, where the voltage system is balanced. CT1 and CT2 are identical current transformers, however, RL2 (leads resistance) and RS2 (secondary winding resistance) upset the Merz-Price balance, therefore the need for RR I guess. The relay pulls in when current through it exceeds 20mA. RR is there to prevent unwanted relay operation (relay is to remain off under threshold current).
Now, without voltage and other resistance values I don't see the trick.

Is there a formula for this?

Thank you for your help.
 

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I believe I sorted out. The thread may be closed. Thank you.
 

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