Feeder protection for temporary Generator Application

[rockman7892]

I wanted to get others opinion on protection functions for a 4.16kV Switchgear feeder breaker that feeds a provision for an external roll-up generator.

Usually in a situation with permanent generator the feeder breaker may have all gen protection functions (87G, 32, etc..) however in this application the portable/temporary generator has an internal breaker with its own protection and control. With that being the case I'm thinking that the feeder breaker in the switchgear simply becomes a feeder breaker with 50/51 functions only that simply coordinate with any 50/51 or 51V functions internal to the generator?

There is also the possibility that on rare occasions the generator will operate in parallel with an incoming utility source to the switchgear. The feeder breaker would close to provide the utility voltage source to the generator and the generator would then have its own internal sync check to sync to utility and close its breaker. Thus I am thinking that no sync check (25) is required on feeder breaker?

If there is a case however where the generator is operating in an islanded condition and the utility source comes back online there will need to be a sync check function at the main breaker, in order to transfer back to utility to keep gen online.

Based on this do others agree that only 50/51 is needed at gen feeder, and main breaker should have 25 sync function?

 

[krisys]

First of you have to develop a system operating philosophy. Identify the synchronization point. Establish suitable protection measures based on the type of generator (steam turbine generator, Diesel Generator, Gas Turbine generator etc.) and its rating.

Take the prior approval from the utility. Some utilities don't allow any source at customer end. Then mutually agree the system operating philosophy and proceed further.

 

[ScottyUK]

Agree that you can let the generator look after the generator-specific functions using its on-board scheme, then just provide relatively basic protection on the feeder.

For paralleled operation take a good look at how your earth fault protection will behave with a second source flowing the 'wrong' way through your network, especially if it is an impedance-earthed scheme. The effect of adding the generator will depend on how and where you earth the generator star point.

Good luck getting a 51V to coordinate with upstream 50/51 relays.

 

[waross]

There are too many "It depends" factors to answer without a lot more information.
One thing, is this a specific generator or provision for a rental generator?
If you are going to use a "Roll up" rental generator don't depend on anything in addition to over-current protection on the generator panel.
I have seen some fairly large rentals with not much in the way of protection.
Standby raises more questions.
Running in parallel with the grid raises more questions.

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

 

[rockman7892]

This is a military application so there is a specific generator that is well documented. Operating manual shows unit as having internal control and protection functions including Overcurrent, Ground Fault, O/U Voltage, O/U freq etc.... It also shows it as having synchronization functions for closing its internal contactor/breaker onto utility or other paralleled generators.

My main concern is weather or not I need to provide any sync check capability with feeder breaker or main breaker in Switchgear. Manual shows generator as having an automatic syncing mode where it automatically senses bus voltage and synchronizes and then closes contactor to operate in parallel. But there is also a manual sync mode where it senses bus voltage synchronizes and then prompts the operator to manually operate control to close contactor. Seeing that the operator can manually close this contactor (not sure of any internal overrides) tells me that perhaps there is a need for a sync check function at the feeder breaker.

What about phase reversal if the generator is connected with an opposite sequence from the utility? I don't believe a sync check relay would see this and there is still the case where the generator can operate islanded so phase sequence needs to be checked with perhaps a phase sequence relay 47? With phase sequence relay would you make that a permissive for closing feeder breaker or just provide tripping function if gen came online with opposite sequence.

Generation capacity is only 800kW so utility has stated that they have no interconnect requirements at the main breaker such as 32, 67, etc.. which would be typical.

Scotty UK - I think your comment suggesting looking at how the earth fault current behaved flowing in opposite direction was a case where fault current through a relay could be large coming from a strong source in one direction and small coming from a weak source in the other direction? Isn't this usually handled with a 67 and 67N? Both utility source and generator are solidly grounded.

 

[Hoxton]

Quote from an old, gold engineer: "When all else fails, you have to engineer it!"

This is a summary of the comments above. I have seen them making similar comments on similar questions.

Sorry, you can ask general questions here, but we cannot offer specific guidance on specific problems, I think. Sorry I am not a supervisor of this site, but that is my advice.

 

[waross]

You should have synchronizing gear on any breaker that may be the last one closed.
Sync check on the generator won't do any good if the feeder breaker is closed out of sync.
A mechanical interlock is a good idea on a transfer switch.
If you must trip the main incoming breaker and than connect the generator with a feeder breaker that will not be mechanically interlocked with the main, consider at least two different electrical interlocks.
Consider a fault on the feeder when you are running parallel. The fault will be fed by both the generator and by the grid.
You may want full protection on both the generator breaker and the feeder breaker in this event.
General comments only.

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

 

[rockman7892]

Waross

When you refer to "full protection" for a fault on the feeder are you referring to something more than just 50/51 settings no both the feeder breaker and generator breaker?

 

[ScottyUK]

rockman7892,

Yes, directional relays would be one way of doing it. Earth fault coordination with the generator connected low down in the distribution system will be a problem because power flow is the 'wrong' way through the system compared to the direction it was designed for. The normal rule that relays closest to the source operate last will be reversed unless you start changing settings further away from the generator. If this generator is only energising a local switchboard and not feeding back into a larger distribution system then the problem may be less significant although you should still take a look at it.