Generator Wye connected to ground causes high circulating current?

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ed_wins

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Hello to everyone.

We have a four existing diesel drive generators with the following data;

Kw 1200, Kv 6.6, Amps 131.2, Kva 1500, Pf .8, Hz 50, RPM 1000.

These Generators are running/connected to step up transformer of Delta - Y, 6.6 Kv - 11 Kv respectively and this is for the overhead lines.

Now, we have this new Caterpillar Diesel generator set with the following data:

Kw 1200, V 400, Amps 2309, Kva 1600, Pf .8, Hz 50, RPM 1500.

Since the rated output voltage is only 400 volts,
we installed a new step-up transformer Y - Delta, 400 - 6.6 Kv respectively, in order to match with the existing generators with rated voltage output which is 6.6 Kv.

As required, the new generator have all the synchronizing control and protection, the synchronizing control, its sensors such as CT's and the main breaker are intalled between
the Generator and the 400 v - 6.6 Kv( Y - Delta) transformer.The secondary of this transformer is permanently connected with the existing bus of 6.6 Kv.

Take note that this new generator is a Wye output and the Nuetral terminal was connected to the ground and to the nuetral in the primary side of(400V-6.6Kv)transformer

Now the problem is we can not put it online with the other generators, the breaker will only switch on momentarily and tripped, with the errors in our synchrinizing software of Reverse Power.

We able to test this Caterpillar generator alone, online in a dead bus, but the Kvar reading is 200% with 91% effective power.

Is there a possiblity of high circulating current because of this Nuetral to ground connection from the generator?

Can we remove this Nuetral line from generator terminal, anyway we are only stepping up its rated 400v to 6.6 Kv.

Take note: the old generators set have no nuetral out, but only the 3 phase lines going to the 6.6 Kv - 11 Kv transformer.

I appreciated very much of your time and kind help.

Ed
 
Couple questions:

1. Is the breaker that is tripping simply an overcurrent breaker for phase overcurent, or does it have some other function?

2. Have you been able to use phasing sticks to make sure the output of the new step-up transformer was in phase with the 6.6 kv of the other generators? Seems like you should not have any phase shift between the two, but with mixing delta and wye, like you are, I can't be 100% sure.

3. Is the new step-up transformer designed and built for that purpose, or did you install a transformer that was designed for use as a step down?
 
Just humour me here; the syncroniser is seperate and dedicated to this generator, yes, and is trying to syncronise from the 400V genset output to the 400V transformer input, and not to the 6.6KV bus?

I'm fairly convinced your neutral concerns on the primary side are unfounded; the transformer isolates what is a SDS formed by the generator, and the power system on the secondary of the transformer.

The other thing that occurs is that you are suggesting getting odd readings, maybe there really is an instrumentation error sufficient to make the system not capable of working?
 
Quote: "As required, the new generator have all the synchronizing control and protection, the synchronizing control, its sensors such as CT's and the main breaker are intalled between the Generator and the 400 v - 6.6 Kv( Y - Delta) transformer.The secondary of this transformer is permanently connected with the existing bus of 6.6 Kv.
"

The wye-delta transformer has a 30 degree phase shift between primary and secondary so the synch for the new generator must be designed to take this into account.

Note: edited for clarity, then for content
 
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I know generators moderately well, specifically Cat generators, but aside from that, I want to point out that this topic is outside of my normal knowledge base. View this information with that in mind.

I don't think the problem lies with the generator per se, but with the delta transformer. I recall hearing many years ago that a delta-source can have internal eddy currents if it is imbalanced. Even though it is not truly a source, it is nevertheless acting as a source, and I suspect it could therefore suffer the same internal eddy currents.

That being said, this imbalance with the transformer could be the result of a voltage imbalance of the generator. The reason why this may not have been noticeable with a delta-configured generator, is because that generator would have experienced the same imbalance and limited itself due to mechanical limits on the engine.

I don't know if this is feasible on all generator packages, but years ago I was chatting with a Cat mechanic who was reconfiguring a Cat gen set for Delta output at the time. I thought it was odd that he was doing this, but it was a client request, and maybe it was a similar situation to your own.
 
whillis said:
The wye-delta transformer has a 30 degree phase shift between primary and secondary so the synch for the new generator must come from the secondary of the 400 v - 6.6 Kv step-up xformer and not the primary where you have it now.

Note: edit for clarity
Click to expand...
Oh, never mind my previous babbling. I didn't notice this 30 degree phase shift.
 
As i understand you have gone for star on LV side ( 400 volts) and Delta on HV side ( 6.6kv).

We have 16 gens ( 400 volts) in sync all feeding to a step up transformer 400 > 13.8 kv but the type we have used is delta on LV side and star on HV side. All generator neutrals are bonded to ground.

The star point of stepup transformer is also bonded to earth.

We do not have any issues with circulating current.

I propose transformer using delta on LV side( 400 volts) and star on HV side( 6.6kv), which is similar to what you did for older generators.

Good luck
 
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