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system stability (25kv versus 260kv)

system stability (25kv versus 260kv)

system stability (25kv versus 260kv)


All the electrical experts, please help me find out some references that talks about overall stability of the system,. Here is what I am fighting with.

We have a potential design that involves two 50MW of power generation at 13.8kv (total 100MW). This power needs to be shipped at a distance of about 1500m. and then come back at 25KV to feel the facility (several circuits). There are two options we currently have and need your opinion as to which one is the best and why?. Thanks in advance for your help;

1. First option involves a 15KV main-tie-main switchgear @ 6000 A ( I have found one) right after the generators  and synchronize it and then up convert it to 260Kv (two transformers for two gens) and then ship it to the Sub (@ 260kv). This scheme gives cable and transformer redundancy. But in tshi scheme you are exporting 1005 power and then getting it back from the Sub at 25kv. There are cable losses, costs of cable...etc.  

2. Second option is to step up the 13.8kv generation to 25kv and then install a local 25KV switchgear close from the generators. feel all the local 25Kv load (almost 60% of 100MW) and then transport the remaining power to the substation at 25KV. This potentially adds two distribution buildings (one is the SUB an done is this 25KV SWGR). This will now require more relaying and communication with the SUB.

In option one the generators are synchronized before the step-up transformers at 13.8kv and in option two the generators are synchronized after the step-up transformers. I need to understand which choice is a better one and which system would be more stable.  I have done skm studies and fault level calculations. can some one throw their experience on system stability, generator system grounding with respect to each option. Any references or documents will be highly appreciated. Thanks,

RE: system stability (25kv versus 260kv)

Number 2.  Clearly.

Generator unit connected to the step-up transformer and you don't need a 13.8kV breaker, just a 25kV breaker.

Then you're only looking at one 25kV circuit between the plant and the substation instead of a 25kV circuit and another circuit at a different voltage.

RE: system stability (25kv versus 260kv)

And some savings on the transformation and transmission:
Solution 1 requires:
100 MVA of 13.8/260 kV Txmer
60 MVA 260/25 kV transformation at the substation
1.5 km of 260 kV line
4.5-6 km of 25 kV line (3-4 circuits)

Solution 2 translates in:
100 MVA of 13.8/25 kV Txmer
saving of 40 MVA of 260/25 kV transformation at the substation
3-4.5 km of 25 kV line (2-3 circuits)

Do the maths!

RE: system stability (25kv versus 260kv)

It's gonna be interesting to see 100 MW generation stable on 25 kV cable.
You need to study this properly, but as a rule of thumb you need an evacuation path about 3 x gen output to maintain stability.
I've never seen 2x 50 MW generators to feed into 25 kV network before, usually this amount of power should be evacuated into a robust 110/132 kV network, at least.

May you grow up to be righteous, may you grow up to be true...

RE: system stability (25kv versus 260kv)

Be careful of what people suggested. They suggested only from equipment or design perspectives. You need to run the stability analysis to prove there are no swing out risk during and after the large disturbances.
This is so called IPP interconnection study here in North America.
Nobody can give you any reference of how our system will work or not. You need at least detailed geenrator/exciter models and run the study either using PTI or PSLF or other software. You can go to the website and download some IPP interconnection study reports
Be careful here are some bad examples too.

RE: system stability (25kv versus 260kv)

Agree with m3ntsan and QBplanner.  You need to design on capable to get your power out under maximum generation and minimum load conditions.  You have 1500m distance to the grid, correct?  @ 25kV, consider you get 100% output to the grid, be aware of your line losses and over heating!

RE: system stability (25kv versus 260kv)

power plant and 24kV subestation  belongs to same company or are independent: generating produtor and utility?

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