Voltage Step up/Step down Scheme

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Sajid khan

Sajid khan

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Pakistan
Hi All,

We have a project where we are using synchronising of 2x500kVA LV (400V) Generators on a common bus, the supply is then fed to 1250kVA step-up transformer to step up voltage(11kV) to transmit it to 350M away where the supply voltage is again step down by 1250kVA to 400V for utilisation.
My questions are,
1. What can be the implications of this system generally?
2. Will it be problematic to generators, to overcome Sudden load increase or decrease, as there are transformation of voltages at two levels(step-up & step down? What i m thinking is that the generators will experience a delay response upon load variation due to double transformation of voltages.

Valuable Suggestions are required

Thanks in advance


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You of course know the reason for such a setup: to reduce the voltage drop caused by conductors on the run. A quick back of the envelope suggests that you need some 30 parallel 1000 kcmil Al conductors to maintain 2% voltage drop at full load. Stepping the voltage up means your conductors can be much smaller, limited by ampacity rather than voltage drop.

The transformers won't introduce a delay in the generator response; but they do introduce inductive impedance between the generator and the load. This will introduce a voltage drop that is dependent upon the power factor of the load.

At a distance of 350 meters I question that the cost of wire saved could really balance the cost of the transformers.

-Jon
 
We can safely assume that moving the generators 350M is impossible, correct?

Transformer losses translate to fuel costs, but someone has likely already crunched those numbers.

I've done something similar for large pumps that were 1 mile from the nearest power lines, it was no problem, but I was not dealing with synched generators. That's something I have no real expertise in. I used 2 synched 4160V generators and step-up transformers on a test stand for a 12kV 2500HP cryogenic pump once and the pump load starting across-the-line caused an issue with the synch panel, so the breakers on the generators both tripped. But after we added a soft starter on the 4160V side, it worked fine.
 
winnie said:
You of course know the reason for such a setup: to reduce the voltage drop caused by conductors on the run. A quick back of the envelope suggests that you need some 30 parallel 1000 kcmil Al conductors to maintain 2% voltage drop at full load. Stepping the voltage up means your conductors can be much smaller, limited by ampacity rather than voltage drop.

The transformers won't introduce a delay in the generator response; but they do introduce inductive impedance between the generator and the load. This will introduce a voltage drop that is dependent upon the power factor of the load.

At a distance of 350 meters I question that the cost of wire saved could really balance the cost of the transformers.

-Jon
Click to expand...

Yes we have done cost analysis of such a setup and its Economical than the low voltage distribution. I will definitely share the numbers once i will join my office.


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zbang said:
350 meters (1200 feet), not 350 miles? Just confirming.

For that distance, I'd really wonder if the cost and complexity of the transformers, MV wiring, etc is really cheaper than more wire. (My math tells me it's under 1500 amps/leg, but it's late and I'm tired.)
Click to expand...

Its 350M.. and is really cheaper as step up and step down require two transformers and MV cable Compare to many LV cables for various loads at a distance of 350M.


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Jraef said:
We can safely assume that moving the generators 350M is impossible, correct?

Transformer losses translate to fuel costs, but someone has likely already crunched those numbers.

I've done something similar for large pumps that were 1 mile from the nearest power lines, it was no problem, but I was not dealing with synched generators. That's something I have no real expertise in. I used 2 synched 4160V generators and step-up transformers on a test stand for a 12kV 2500HP cryogenic pump once and the pump load starting across-the-line caused an issue with the synch panel, so the breakers on the generators both tripped. But after we added a soft starter on the 4160V side, it worked fine.
Click to expand...

Jraef its true we cannot move Generators setup near to the load due to space limitations. If it would have been possible then surely we would not bother this setup.


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1500 amps for 350 meters seems like you could just use a bunch of paralleled aluminum conductors, or move the generators closer as someone else suggested. Or get a couple of mv generators to start with and then only need one step down xfmr.
 
petersonra said:
1500 amps for 350 meters seems like you could just use a bunch of paralleled aluminum conductors, or move the generators closer as someone else suggested. Or get a couple of mv generators to start with and then only need one step down xfmr.
Click to expand...
I did some quick dirty math, and of we assume about an actual 700 amps, one would need 10 sets of 500 AL or about 35,000 feet with no neutral for about 3% VD. Perhaps OP could turn up the voltage a bit and get down to say 5 sets? 500 AL is maybe 2.50/foot? What is a 1250 MVA transformer, maybe 30k each?
 
Question: In the location where this system will be used, what is the highest voltage that the generators can be operated at without moving to a different rule structure? Has the evaluation been done for operating the generators at that voltage, and then using a single step down transformer at the load?

For example, could the generators be re-connected in star (presuming they normally are used in delta) to up the voltage to 690V?

In the US in this situation, my guess is that people would be looking closely at 600V. But the economics of voltage higher than the utilization voltage often depends more on the applicable rules rather than physics.

-Jon
 
Sajid khan said:
Hi All,

We have a project where we are using synchronising of 2x500kVA LV (400V) Generators on a common bus, the supply is then fed to 1250kVA step-up transformer to step up voltage(11kV) to transmit it to 350M away where the supply voltage is again step down by 1250kVA to 400V for utilisation.
My questions are,
1. What can be the implications of this system generally?
2. Will it be problematic to generators, to overcome Sudden load increase or decrease, as there are transformation of voltages at two levels(step-up & step down? What i m thinking is that the generators will experience a delay response upon load variation due to double transformation of voltages.

Valuable Suggestions are required

Thanks in advance


Sent from my iPhone using Tapatalk
Click to expand...
Interesting that you are are using SI units. The voltage will be 11 kV at 50Hz. Also the distance is usually m for metres, not M for miles...................:)
 
winnie said:
Question: In the location where this system will be used, what is the highest voltage that the generators can be operated at without moving to a different rule structure? Has the evaluation been done for operating the generators at that voltage, and then using a single step down transformer at the load?

For example, could the generators be re-connected in star (presuming they normally are used in delta) to up the voltage to 690V?

In the US in this situation, my guess is that people would be looking closely at 600V. But the economics of voltage higher than the utilization voltage often depends more on the applicable rules rather than physics.

-Jon
Click to expand...

Generators voltage are 400V that will be stepped up to 11kV to transmit it 350M and then step down again to 400V. This Generators supply is a backup supply that will be ATS with Utility supply(11kV/400V).
Cost evaluation has already been done and this scheme is feasible rather than running multiple LV conductors of 400A,630A, 250A, 150A, 100A etc.


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Besoeker3 said:
Interesting that you are are using SI units. The voltage will be 11 kV at 50Hz. Also the distance is usually m for metres, not M for miles...................:)
Click to expand...

Thanks for pointing it out.
Yes 11kV 400V 50HZ. Its 350meters.
I worked in both SI and FPS and NEC and IEC thats why mixing terminologies some time..


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Sajid khan said:
Generators voltage are 400V that will be stepped up to 11kV to transmit it 350M and then step down again to 400V. This Generators supply is a backup supply that will be ATS with Utility supply(11kV/400V).
Cost evaluation has already been done and this scheme is feasible rather than running multiple LV conductors of 400A,630A, 250A, 150A, 100A etc.


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Click to expand...

Understood.

My question was if an intermediate higher voltage was evaluated which would permit the scheme to be done with step down transformers only.

If the 400V generators can be operated at 690V, that reduces the cabling required by 66%. Then you only have a step down transformer. Similar scheme, but using the generator itself as the initial step-up.

-Jon
 
winnie said:
Understood.

My question was if an intermediate higher voltage was evaluated which would permit the scheme to be done with step down transformers only.

If the 400V generators can be operated at 690V, that reduces the cabling required by 66%. Then you only have a step down transformer. Similar scheme, but using the generator itself as the initial step-up.

-Jon
Click to expand...

I have to check Generator specifications, if it could be operated at 690V or nearby.


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ramsy said:
Did lineman provide estimate for aerials?

Lines strung between poles 350m apart use smaller wire and less labor than underground raceways.
Click to expand...

Overhead is not possible as there are buildings in between. Underground is also cheap too as we are using 11kV 3C- 300sq mm Aluminium Underground feeder, this has additional Excavation cost only compare to overhead system which would includes poles and a clear way also..


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ramsy said:
Did lineman provide estimate for aerials?

Lines strung between poles 350m apart use smaller wire and less labor than underground raceways.
Click to expand...
This isn't exactly my line, but wires strung on poles with 350 meter spans? That sounds awful far to me, but what do I know? :D
 
1/0 in raceway only handles 150A from T.310.16 vs 230A blowng in the wind from table 310.17.
 
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