Current transformer for protection question

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SUMMARY

The discussion centers on determining the RR value for a relay current that must not exceed 20mA, based on the Merz-Price theorem. The participants identify that identical current transformers, CT1 and CT2, are affected by leads resistance (RL2) and secondary winding resistance (RS2), disrupting the Merz-Price balance. The RR value is crucial to prevent unwanted relay operation when the current threshold is not met. The user concludes that they have resolved their query and suggests closing the thread.

PREREQUISITES
  • Understanding of the Merz-Price theorem in electrical engineering
  • Knowledge of current transformer (CT) operation and characteristics
  • Familiarity with relay operation principles and current thresholds
  • Basic concepts of electrical resistance, including leads and secondary winding resistance
NEXT STEPS
  • Research the Merz-Price theorem and its applications in protection systems
  • Study the characteristics and calculations involved with current transformers
  • Explore relay design and operation, focusing on current thresholds and protection schemes
  • Investigate methods to calculate RR values in relay protection circuits
USEFUL FOR

Electrical engineers, protection system designers, and students studying relay operation and current transformer applications will benefit from this 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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