Voltage regulator VS capacitor bank

[JLuc]

Hi all,

here's the situation; a 20MVA generator will be shutdown for 3 years at a papermill. This generator was providing all power factor correction needed by the plant. And about half of the load. (Total load is 12MW, 5.7MVar)

When the generator is down, pf at service entrance drops down to 0.87 and voltage on 13.8kV bus drops to about 13kV, and causes all kinds of problem.

Primary voltage is 22.9kV, and they use a 25kV/13.8kV transformer (16MVA), on the lowest tap (22.8kV/13.8kV). So no help here to raise the voltage.

NOTE: no pf penalities at all, they pay kw-h only!

I see three possible options:

-install a capacitor bank on 13.8kV bus: but these capacitors will be useless when the generator is put back on in three years.

-install a voltage regulator transformer(with automatic OLTC) upstream of the existing one

-change the existing transformer for a 22.9kV/13.8 kV with automatic OLTC

I would like to have your opinions about this project.

Thanks!

 

[waross]

It is hard to comment without knowing the cost of the various options. I imagine that any installation will be superfluous in three years. An OLTC may provide more operating flexibility than a capacitor bank, in the future. A capacitor bank may be easier to sell if you decide to remove it when the generator is back online.
I would be checking the warehouse and talking to the head electrician about hiding places. It is possible that there may be an old, unused, synchronous motor available somewhere. A 4 MW motor would bring you up to 0.95 pf.
It's a long shot but worth checking. Also, of course, if you find an alternator that may be motored as a synchronous condensor that is also an option.

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

 

[davidbeach]

The capacitors could continue to be used once the generator is back on-line; the generator could then run closer to unity power factor.

 

[JLuc]

That is true davidbeach,

but since it is a 20MVA generator supplying only about 6 MW, there is no benefits to run it at 1.0 pf.

I think it will all come down to pricing...

Capacitor banks needs maintenance, protection study, harmonic study.. etc.

Voltage regulator tranformer might be more expensive but it is "plug and play" , plus, I think it would be maintained by the utility.

Thanks for the feedback.

 

[itsmoked]

JLuc; I'm very curious as to why a generator would "go away" for three years.?

Could you just lease one and drop it somewhere on site for the interim 3 years? No studies. It would be easy to sell later too, I'd think.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

20MVA generators don't just get leased and dropped in. My guess would be that something is getting rebuilt.

 

[JLuc]

itsmoked:

It is a steam powered generator, the steam comes from a gas boiler that will be replaced by three bark boilers. The steam generated by this gas boiler is also used in the process.

During the time of the project (3 years), the steam required by the process will be supplied by some other boiler.

When the three bark boilers are all set and hooked up to the generator, they will restart de generator.

So, they need to buy all power from utility during this replacement (3 years max).

 

[itsmoked]

I see.
Most interesting!

Thanks JLuc.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[QBplanner]

The bottom line is one thing should be kept in mind, VR/OLTC can not provide you additional Var but only obsorb var from the system u connect.
Shunt capacitor bank does provide u additional Vars.

I would suggest if the system you connect is strong, then get a VRor/OLTC and bring Var from the system if no penalties. Of course you have to compare the cost.
Cheers

 

[LionelHutz]

Are you sure there will be no kVAR charges? Once the utility sees the plant will be requiring 5.7MVAR for 3 years they just might want to charge for supplying it. If the plant has a power contract stating no power factor penalty it just might be dependent on them correcting their power factor xx% of the time.

Have you determined that by eliminating most of that 5.7MVAR with a capacitor bank that the new voltage drop will be acceptable? You might correct the voltage drop from 6% you have now to say 3% so is that 3% suitable?

Changing the transformer could be very close to "plug and play".

Installing a new transformer or a capacitor bank are both large projects so to me the decision mostly comes down to cost. But don't forget delivery time too.

 

[waross]

Have you considered getting a price from Cat or Onan/Cummins on rental gensets? They can get about 1.5 MW in a semi trailer. Check with them as to how many VARs you can produce without becoming unstable. It may only take one or two units to raise your voltage to an acceptable level.
Even if the cost is too much, your boss may be impressed with your diligence in evaluating all possible solutions.

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

 

[jghrist]

Have you considered getting a price from Cat or Onan/Cummins on rental gensets? They can get about 1.5 MW in a semi trailer. Check with them as to how many VARs you can produce without becoming unstable. It may only take one or two units to raise your voltage to an acceptable level.

You will find that generating vars with diesel fuel may not compare favorably with using scrap wood chips!

 

[waross]

It doesn't take much fuel to generate VARs.

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

 

[apowerengr]

Cap bank is the inexpensive answer since your volt drop is likely due to the power factor of the load and the voltage regulation of the transformer. 1200-1800kVAr will likely provide the required voltage rise at minimal cost compared to the other options you suggest. 22.9 sounds like northern MN - you could rent a set of 400 amp 14.4kV vregs for three years if you're close and are interested in a rental.

 

[JLuc]

apowerengr:

the plant is in connecticut, the rental of 400 amp vregs could be a good option.

any information about companies that could sell/rent this kind of equipement? i'm in discussion with local utilities about this, but I would like to have a couple of options.

Thank you.

 

[QBplanner]

I think Coper power system produces VR-32 series

 

[JLuc]

What would be the typical impedance of a voltage regulator?

Exemple: 400A , 14.4 kV ,±10%, (576 kVA)

Since it is an auto-transformer, I expect the impedance to be lower than a regular transformer.

Any typical impedance?

JL

 

[QBplanner]

It is typically very small compare to the transformer.
In our base case it was given 25kV 19.2MVA VR has a 0.08p.u. /100MVA base. it is about 0.097 Ohms. For your reference only.