Fault contribution by Synchronous motor

[dfdt]

It's well known that large SM(synchronous motors) do contribute to fault current to some degree for finite time.My question is that should we be providing some sort of relaying to trip circuit breaker of SM in order to prevent it's[pre][/pre] infeed to the fault? Other option is to do nothing as fault contribution from SM anyway will die down in 2 to 3 cycles

 

[oldfieldguy]

The number of cycles of contributed current depends on the inertia of the motor's load.

One of my clients had this issue with the local utility company. I have seen an oscillography report of its actions under fault conditions. It did indeed contribute several (more than five or six) cycles of voltage and current back into the faulted line, with magnitudes diminishing. Because it was, in effect, supporting the voltage on the incoming line, an undervoltage protection relay was not sufficient to open the breaker.

It was 'cured' by installing a reverse power relay to open the motor feeder breaker.

old field guy

 

[waross]

If either a synchronous motor or an induction motor is being over driven it will be regenerating. If the grid protection trips before the motor protection, then the motor may continue to contribute to the fault as long as it is being driven.
The solution is a reverse current trip and brakes or a resistor bank to stop the load. In the case of a downhill conveyor, stop the material feeder, connect the resistor bank to the motor and then trip the motor off line. (Still connected to the resistor bank.) The resistor bank must be adequate to prevent the motor running away. With no more material loaded on the belt, the resistor bank will avoid a runaway until enough material has unloaded that the belt stops.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dfdt]

Thanks Gents for your advice and personally speaking I would not be worried too much to provide reverse power protection simply because motor contribution would well and truly die down by 120 ms which is allowed fault clearance time as per the Grid code.