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Protection Riddle No.9 - Trip relay mal-operation
In a bus bar protection scheme applied for a double bus high voltage substation, protection systems design group have chosen high speed trip relay with operation time equal to 4 ms and a 2.5W DC bobbin; because they want to decrease the thermal stresses on output contacts of protection relays.
As you know in high impedance bus bar protection scheme they must start several trip relays in bus bar faulty conditions simultantinusly.Therfore, dc circuit currents specially the circuits that involve to protective relays, must be considered carefully in detail design step.
But in practice they met some mal-operation of trip relay in pre-commissioning stage. According to pre-commissioning reports,
mal-operation had been accompanied with a DC battery system alarm for earth leakage.

Can you explain the reason of that mal-operation?
Author : Hamid - From: Iran
Tue, December 4, 2007 - 17:03
Trip relays should be of high wattage type .

The trip relay should pass a test of "Non operation " for discharge current of 10 microfarad charged to the 1.2times rated voltage value through the relay.

This is specified in some British electricity standards EB2.

This is essential to avoid tripping when the other side (the one which is connected to the contact and not negative )of the trip relay coil is accidentally earthed (resulting battery earth leakage relay pickup) , during which time the capacitance due to contact connected to that trip relay coil discharges through the coil of the relay .

Author : S.Muthukrshna
Tue, December 4, 2007 - 17:04
Dear sir,

Thank you very much for your wonderful reply.
Author : Hamid
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