Feeder protection vs. Differential protection on Distributed Generatio 2

[CanuckEngineer]

Hello all.

I have read many postings/threads on this site over the last few years.

I have decided to join, not only to further my own personal knowledge but to be able to help others, which I believe is more important, because you help others your understanding grows that much more.

I work as an electrical design engineer up here in Toronto, Canada. I work for a dealer that sells generator and switchgear equipment, for the Standby and Prime POwer industries.

Lately I have been getting into more and more complex projects, involving multi paralleled gensets to Utility, peak shaving, relay protection etc.

Slowly learning all the ins and outs of this industry.

I have the following question.

We have a multiple generator set appliation. Six to be in fact which are to be paralleled and tied to a common bus. The generators are somewhat large, in termns of reciprocating type, they are rated at 2250eKw at 13.8kV voltage.

Now, we have Sr489 protection relays for each genset on the aux. switchgear.

The consultant has come back and asked what is protecting the feeder or cable connection between the generator and the switchgear.

They are asking if their is an internal fault, i.e. upstream of the protection relay and its associated CT's. What will trip the gen. breaker and what would shut down the genset.

I was reading up on this, and I wanted to ask you guys on whether Differential GEnerator protection would apply here, 87G?

We do have 6 lead genset and we do have 3 CT's mounted on the generator side for this differential protection, with the matching set on the switchgear, both sets feeding into the SR489.

Will differential protection on the SR489, i.e. 87G, be sufficient in short circuit or over current situations right on the generator?

What could be some of my other options here.

Much thanks guys,

CanuckEngineer.

 

[dpc]

If the differential zone includes the cable which it sounds like it does, based on your description, it should be well protected against phase faults. Ideally, your generator differential zone would extend from the neutral side CTs on the generator to the bus side CTs on the generator breaker, fully including the connecting cable.

Ground fault protection will depend on how the generator is grounded. For resistance-grounded units, the fault current may be low enough that the generator differential might not reliably operate. But presumably, the generator ground fault protection will detect a ground fault in the feeder. The generator overload protection should protect against cable overloads/overheating.

 

[slavag]

Hi CanuckEngineer.
Im agree with Dave.
But you have take in account, in all cases 50, 51 overcurrent protection needed and you have it in this relay.
Best Regards.
Slava

 

[CanuckEngineer]

Thanks dpc.

Much appreciated.

Regards,

RK.

 

[CanuckEngineer]

Thanks Slava.

The relay scheme will incorporate the 50/51.

Regards,

RK.

 

[rob46]

You may consider GFA300 ground fault relay between generator neutral and ground.

 

[marks1080]

CanuckEngineer:

I am also based in Ontario, and work for the transmission utility as a P&C field engineer.

You should also be aware that the utility here does not allow the DG's to island. This might be something you need to consider as your gen. breaker may trip via a Transfer Trip or Remote Trip from the T.S. the feeder gen sets are connected to. I am assuming this is a DG project.

Rgds,
marks

 

[CanuckEngineer]

Thanks marks

This application is simpler.

Pure standby, so basically went utility goes out, the ats's switch over, utility brekaer opens.

Gens power up and come online.

However as I learn more about different modes, ie.. Islanding etc. I will take that into account.

 

[davidbeach]

marks1080, I trust that your statement about islanding only applies to islands that would include some portion of the utility system. I can't see how or why the utility would care if the PCC breaker opens and the generation supplies some portion of the customer's load as an island.

 

[marks1080]

david:

It's a policy decision as far as i understand it. If we have generation connected to a feeder, if that feeder breaker trips we always send transfer trip to the generator breaker.

 

[dpc]

transfer trip to the generator breaker.

Normally, the transfer trip would trip the utility tie breaker, if there is one, to allow the generators to continue to provide power to the customer's internal loads, if desired.

 

[marks1080]

I'm not sure there is a 'normal' case when it comes to distributed generation.

The utility i work for has a policy, that right now is very striaght forward: If the feeder breaker trips, than you make the DG breaker trips as well. The only acception is with DG units sized such that their G/L ratios would automatically take them down.

That being said, the same utility has long term plans to develope the grid into a system which can support islanding. It's probably ten years away, but it is coming.