Influence of mutual coupling between parallel circuits on the setting of distance protection

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The significant mutual-coupling impedances between the two circuits in double-circuit overhead-line construction are considered. The impedances measured by distance-protection relays are examined, both for stability for faults outside the protected zone and for operation for faults within the protected zone. This is done for different primary-system-circuit arrangements and for the condition where one circuit is out of service and earthed at both terminals. Some proposals for a practical setting policy are made.