Make before Break transfer switch 2

[nkn5]

I have a 400V switchboard with two incomers : one from emergency diesel generator (400V) and second from main power (gas turbine power @11kV stepped down to 400V thru couple of other switchgear in the overall system).

I would like to have load transfer from emg incomer to the main power incomer with out interruption ( make before break)provided the two sources are synchronized. Can you give me any idea on these questions I have

1) How much time typically is required for this transfer to happen ?
2) What fault current rating should I consider for the bus ? As I am not running the sources in parallel I do not want to addup the fault currents from the two sources.
3) Do I need any additional equipment to do this? or the breakers / IEDs programmed are enough to do this ?

Can you also share any practical problems / issues you have seen in your experience in doing this.

Thank you

 

[ScottyUK]

1) A few seconds for the diesel set to load until the main source is in a neutral state, neither importing nor exporting power or reactive power. You could just open the main supply as soon as the diesel set is on the bars but this will probably result in a frequency disturbance as the diesel goes from no load to loaded instantaneously: depends how badly your load will react to that.

2) You are running in parallel if you have make before break transfer. Most codes will require you to design accordingly. It's up to you if you choose to ignore the code requirements.

3) You'll need a synchroniser on at least machine. If you want a graceful load / de-load then you need power measurement, which means both VTs and CTs.

 

[xnuke]

nkn5,

I recommend the article in this link, as I think it answers all of your questions. I've done this sort of closed transition trip before, with an incoming generator significantly larger than the emergency generator, with no issues. I did a hard transfer, without waiting for any load to be transferred from the emergency generator to the incoming generator, since the emergency generator load was quite small. This was no problem, as the larger machine picked up almost all the load nearly instantaneously. After the sources were synchronized (synch check relay was used to prevent problems) and the breaker closed, I had a separate hardwired timing relay trip the emergency generator breaker within 100 ms. I used two "a" contacts from the breakers in series to start the timing function. When both breakers indicated closed, the timing relay would ensure the emergency breaker opened within 100 ms.

If the machines were similar in power rating, I probably would have done a soft transfer where the incoming generator picked up load while the emergency generator shed it before opening the emergency generator breaker. I still would have limited the time in parallel to as short as possible, certainly not exceeding 10 s.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[nkn5]

Thanks xnuke,

That was helpful. In the earlier case you did a transfer with in 100ms ( with out a soft transition switch), the fault rating shall be based on the bigger generator (40kA) as a source but not from both (40kA+60kA). Am I correct on this?

Even I would add up both, the switchgear would be okay for 100kA@100ms which was already rated for 60kA@1sec.

Any thoughts on this?

Thanks again

 

[nkn5]

xnuke,

I mean, they way I want to consider " Even I would add up both, the switchgear would be okay for 100kA@100ms which was already rated for 60kA@1sec.
" is okay as per the conclusion made at the link you provided. Please correct if I am wrong.

 

[FreddyNurk]

A couple of considerations, partly as per ScottyUk's point 1.

If the only application you ever have for the closed transfer is to go from the (much smaller...) diesel set to the GT, and you're happy with dumping all the essential load straight onto the GT, then chances are you'd get away with a 100ms transfer, provided appropriate consideration is given to not allow more than 100ms transition.

However, if this is an essential services generator, most of the time its a good idea to test them, and most of the time the best way to do this is on the load its meant to power. Whilst it should normally be sized to cater for dumping the essential load on it at once, during testing its much better to load it slowly, as per xnuke's second suggestion.

Such requirements would mean that much more than 1/10th of a second is needed to transfer the load, and exposure time for paralleled sources starts to increase.

 

[ScottyUK]

Check the X/R ratio at the switchgear and ensure that the breaker can deal with the long time to first current zero. You shouldn't just look at the numerical value of breaking capacity and assume it will be ok. Distribution breakers frequently get mis-applied in generator applications and perform fine until they have to interrupt a big fault when they suffer a disruptive failure.