Uf & df/dt relay timer setting for load shedding

[sramesh]

Good day to you

Our consultant recommends U/f & df/dt relays setting with 100msec time delay for load shedding scheme (4 stages of load shed from 48.5 to 47.8 @ 1 Hz/sec)to avoid black out in case one/few generators sudden trip.( our electrical system is with own generation and no link with Grid)

My query is, Delay of 100 msec, will it be sufficient? will it cause spurious trip during a system equipment fault ( short circuit or groud fault), which may take more than 100 mSec to clear the faulty cicuit.

Thanks in advance for your clarification

 

[mgtrp]

A fault will usually (not quite always!) result in an overfrequency event.

 

[ScottyUK]

A load fault resulting in a short circuit will result in the machines speeding up as load comes off them. A good AVR will act to correct this by boosting the field but the effect will still happen to some extent.

A generator trip in an islanded group will definitely result in a slowing of the remaining units as they pick up the additonal load. The time delay and df/dt pickup is usually dependent on the type and relative size of prime mover and generator and the governor loop dynamics. A system comprising sets with little stored rotational energy will slow rapidly: for example a small steam turbine driving a turbo-alternator has far less rotating mass than a a low speed hydro machine with a salient pole generator. Similarly losing one set of a group of three is far more serious in terms of recovering from the event than losing one set out of a group of ten. Fo these reasons it is pointless to look at numerical settings without fuller details of the system.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[sramesh]

Good day to you

Thanks for the replies

I forget to mention( apologies), the u/f & df/dt Load shedding is proposed trip the out going feeders (based on its criticality to the plant plant operations) connected to our main distribution 33KV Bus to protect the running Generators not to trip on its u/f trip, which is set at 47 Hz.

Each 33kV feeder panel relay is set for u/f & df/dt trip based on it priority.

Regards
Ramesh

 

[slavag]

Hi.
Its very dificult say.
80-120ms is a minimum of time for the df/dt function, setting.
1Hz/sec its usualy loss of supply indication, up to 0.5-0.6Hz/sec its loss of generation indication.

OK, such setting seems not bad, I assume its some industrial power plant according to your data?
Please don't worry about maloperation trip of U/F and df/dt function in case of short circuit, you don't see start of those functions.
4 step is good, but from my expirince all problems are finished on the second step.
I would like recommned to you use in such application adaptive load shedding system with on-line monitoring of power balance.

Good Luck.
Slava