how to calculate and avoid the Voltage drop at starting of gas compressor

live11 · Sep 21, 2011

in a gas power plant There are 2 main transformers ( st1 , st2 ) taking from the grid 380 kv and step down to 13.8 kv ,and then feeding the main bus which it is divided into 2 buses ( A Feed by st1 , B Feed by st2 ) between them bus tie

st1 = 380 kv/13.8 kv
st2= st1

Bus A feeding gas compressor (1, 3,5 ) by 13.8 kv , in addition h2 plant , admin building and ccb
Bus B feeding gas compressor (2,4,6) by 13.8 kv , same above

gas compressor : OP=5200KW , I=260.2 normal operation and for starting current 4.5*260.2
and the voltage is 13.8kv

our problem ( at starting operation of gas compressor there is a drop of voltage accured for 20 second to admin building , central control building , h2 plant )
how can avoid this drop and calculate it
do you think it is from the cables ? if yes how can calculate it since i now cables size , material inside etc

Knobhead · Sep 21, 2011

This is not a very helpful reply, but you would be much better employing a local consultant. From the limited information you give it would be pure guesswork here in the UK.

My first guess would be the reactance of the 13.8KV feeder transformers combined with harmonic disturbance by whatever starting method. The quickest way to check this it check the voltage at the relevant bus during a compressor start. If there’s no appreciable drop then it’s time to look at volt drop in the cables.

Pure guesswork on my part.

malcolmsanford · Sep 21, 2011

Again no help at all Tony but I surprised that there are admin building and other areas on the same systems, as when those babies start up they will draw huge loads.

Engineer54 · Sep 21, 2011

malcolmsanford said:
Again no help at all Tony but I surprised that there are admin building and other areas on the same systems, as when those babies start up they will draw huge loads.

Sounds as if, this could be in your part of the world Malcolm!! lol!!

malcolmsanford · Sep 21, 2011

Your right mate 380Kv is here, but I leave those voltages to you and the Tony, lol never felt the urge to dabble in that, leave it to the real men lol

But even here mate I would be surprised to see facility buildings on the same sub as these babies, but as you know mate stranger things have happened in the desert.

Engineer54 · Sep 21, 2011

Going from what i hear, ....They are starting to fill quite a few of those Engineering positions with home grown Engineers. What they don't take into consideration is that none of them come with any meaningful experience. I'm afraid that's going to take a few more years, that's if those Engineers are prepared to take the time to gain that experience!! You know yourself, most want, and think they are managers as soon as they have that bit of paper in their hands, .....some don't even want to wait for the bit of paper!! lol!!!!

Knobhead · Sep 21, 2011

To be honest I think this is a student’s imagination running wild.

6000HP OK but no mention of the starting method. He states 4.2X FLC for starting, what kind of a starter is that! It’s not far off DOL, the mechanical stresses would wreck most machines. He also states 20S start time, even with a high inertia load combined with 4.2 starting current, it’s unrealistic. I’ve worked on motors the same size but with LRS control, they would draw at max 1.3X FLC and they would be at full speed in 35S.
The feeder transformers seem unlikely at 380/13.8KV in one step. I could be wrong on that but it’s not normal practice as the fault levels would be horrendous! But it would solve most volt drop problems as the bus.

Would be interesting if he comes back with any more figures. I can’t remember where my books on MV system management are.