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Recently the commissioning part of our company started a new project in a gas turbine power plant with six individual power generation units, each 160 MW.
Each power unit includes one auxiliary medium voltage switchgear to supply internal consumptions. Also common switchgear is considered for power plant common consumers and a long interconnection cable (approximately 700 m) between each MV and common panel for emergency using. This interconnection path is used instead of incoming units when their incomings can not be energized via normal path.
One day, the head of electrical commissioning group called the office engineers to help. He said,” we can not run unit 2 switchgear via normal path, and we have decided to use interconnection of common switchgear. But when we close the related circuit breaker, its relevant sensitive earth fault trips it”.
The setting of mentioned relay is similar to the other same relays in output feeders and it is adjusted at 2 A ( in primary side ) for all sensitive earth fault; because that medium voltage system is resistance grounded with maximum 10 A earth fault currents. Also all those earth fault relays are connected to special core balance current transformers.
He said they have never had any problem with other output feeders. They didn’t have enough time, and wanted permission to increase the setting of earth fault relay in commissioning duty. They knew well this activity may be accompanying a great risk.
Can you take that risk of operation?
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