Operation of Translay Protection - pilot cable failure

[JohnMurrell]

I am looking at an event where both the 11Kv cable which is part of a ring circuit and the pilot cables failed simultaneously time (within a few ms). The cable is protected by Translay protection

Can you confirm if the Translay Protection would have operated in this event.

If not what protection (if any) would normally be provided for this sort of failure ?

Thanks

 

[Bung]

Depends what you mean by "Translay" - if it is real Translay, it is a balanced voltage system, and it is stable for open circuit pilots. It will maloperate with short circuited pilots. A circulating current scheme (Translay-S or MBCI)will be stable (ie not operate) for a short circuit pilot, and will maloperate on open circuit pilot.

Pilot wire supervision is available which can be used to take whatever action you like. It is also possible to use overcurrent checks relays to supervise the differential operation.


Bung
Life is non-linear...

 

[RalphChristie]

What kind of relays do you use?

Like Bung said, certain schemes incorporate pilot supervision in order to alarm or trip faulty pilots. This can be done by injecting a current from the aux. source into the pilot circuit. Failure is detected by monitoring the supervision current and the necessary action taken when the pilotwires are faulty. (alarm or trip)
Certain schemes also incorporate an instantaneous overcurrent check relay. The current setting of this relay prevents operation for normal load currents, but not for fault currents. The diff relay will still operate for a fault on the line it is protecting, but also for a through fault.

Another thing to consider (when you have a faulty pilot) is to reduce the time settings of the back-up protection until the faulty pilots are repaired. Due to the much slower clearance time of back-up protection, which can result in substantial damage, such a decision is not to be taken lightly. To prevent slow clearance of a fault, the backup time could be reduced, but keep in mind that there will not be sufficient discrimination between various backup relays.