I was hoping someone would be able to answer this question for me. I work at a production plant and we have three G.E. steam turbine generators. Units 1 and 2 are rated at 12,500 kva and unit 3 is rated at 25,000 kva. Each generator feeds a separate bus that ties into the grid. My question is this, we want to close a tie breaker that will basically tie units 1 and 3 onto the same bus, putting them in parallel. We will be running unit 1 at 5 MW and unit 3 at 10MW (still on the grid). However, in the past when we have done this the two generators have 'fought' with one another. Everyone seems to have a different opinion as to why. I believe that we need to run each of them at the same line voltage (about 13,200-13,400) but when we have the line voltage the same the power factors are different. I believe that we need to adjust the reactive current compensators' to make the power factors the same. Am I wrong? Thank you for any input into this.

 

When you say they "fight" each other please explain what you mean. Is the load (MW) unstable or is the VAr meter unstable, or both?

Also, when you tie these two generators together are they connected together before or after step-up transformers?

 

Sounds like you need reactive droop on both generators, if you are operating in parallel with the utility. Then they will share the reactive load appropriately.

Basler, a major supplier of generator voltage regulators, has a paper that may help you, at http://www.basler.com/downloads/VR_parallel.pdf .

 

> When you say they "fight" each other please explain what you mean. Is the load (MW) unstable or is the VAr meter unstable, or both? <

If you adjust load one the voltage becomes unstable. The load really doesn't wander too much

> Also, when you tie these two generators together are they connected together before or after step-up transformers? <

Before the step up transformer

 

> Sounds like you need reactive droop on both generators, if you are operating in parallel with the utility. <

All three generators have reactive current compensators, wouldn't this be reactive droop?

 

Genman...do the three alternators have the same pf rating?

Regards, Phil Corso

 

Yes, units one and two are rated at 12,500 kva at a .8 power factor, unit three is rated at 25,000 kva at a .8 power factor

 

Genman... putting short-circuit duty and economic strategy aside for the moment, following is what I have gleaned, thus far:

A) The lead-in to your thread say you want to parallel a 10MW unit with a 20MW unit.

B) Your post says there are qty 2, 12.5MVA units and qty 1, 25MVA Unit, all rated at 0.8 pf.

C) All, apparently, are connectible to the grid with individual 13(?)kV step-up Xfmrs.

D) You want to parallel unts #1 & #3 via a tie-breaker between their 13(?)kV busses.

E) You want #1 to export 5MW & #3 to export 10MW to the grid via their to be paralleled Xfmrs.

My questions are:

(Q1) What happens with unit #2?

(Q2) What is the local (in plant consumption) in MW and MVAr?

(Q3) How large is grid capacity relative to your qty 3 gen plant!

(Q4) What is capacity and %impedance of step-up Xfmrs?

(Q5) Are the Xfmrs equipped with tap-changers?

(Q6) If the answer to(Q5) is yes, then what are the tap intervals?

Regards, Phil Corso

 

I will try to answer these questions the best I can. I don't know anything about the step up transformers. These generators feed the load at the production facility, we rarely push out to the grid, but we can (can also bring power in) So when unit one is making 5 megs and unit 3 is making 10 megs, that is just to meet our power needs. Each generator feeds its own bus which powers the equipment we use, further on downstream units 1 and 3 connect (synched) to a substation(public service) unit 2 also connects (synched) to a different substation on the other side of town. Unit 1 powers 'a' bus. Unit 3 powers 'c' bus. we are closing a tie breaker that connects 'a' and 'c' bus. We keep the voltage between the two as close as possible (pf's unequal) but during load changes the voltages and pf between the two will swing. I believe that the power factors should be equal, along with the voltage. The only way I can see to do this is with the reactive current compensators( each generator has one in the exciter cabinet) The reactive current compensators haven't been adjusted in 30+ years so no one knows anything about them. Each one has a dial, and according to the manual when you adjust it the pf will swing towards unity on that unit(that's about all it says) To answer your first question unit 2 is synched to a substation on one side of town, unit 1 and 3 are synched to completely different substation about 5 miles away.

Hope that makes it more clear, Thanks

 

Genman... thank you for the quick reply. In my opinion, you have a "Tail Wagging the Dog" situation. There is lack of data to offer a meaningful solution!

To summarize, there are three gensets, separated by cables that introduce relatively large impedances. And, a grid connection that seems to serve only as a frequency and voltage reference.

Furthermore, I've concluded from your statements that little engineering has done (in many years!) For example, is there a Single-Line-Diagram showing detailed electrical equipment parameters? Has a Load-Flow-Analysis been carried out? How about a System-Stability-Study? What are the "on-site" Load Characteristics?

I don't believe you can get a fix eye-balling the problem over the internet (refer to my 2o-Nov-10 (21:20 reply). Much more is involved than just tuning three reactive-KVAr controllers!

I urge you to seek out a competent Electrical Power consultant/contractor to help!

Regards, Phil Corso

 

Thank you for your time. I am kind of alone on this, so I am doing the best that I can. But would you agree with me that when we parallel units 1 and 3 that the voltages should be equal, and the power factors too? Can you tell me where to get information on the reactive controllers? I do agree that we need to contract the services of an expert, but they won't spend the money. We only do this every few years.

 

Also, the only reason we are tied to the grid is for stability, we can separate ourselves or an under frequency relay will separate us. The main purpose of being on the grid is in case we lose a generator or overhaul a generator we can still pull in 18 megs from public service, just thought I would clear that up.

 

Genman... the grid connection serves to provide backup, not stability! Can you send a simplified Single-Line-Diagram of your plant’s configuration?

Phil Corso (cepsicon [at] aol [dot] com)

 

> Can you send a simplified Single-Line-Diagram of your plant's configuration? <

Ok thanks, I'll send one.

 

Genman... has your question been resolved?

Regards, Phil Corso