Genset Soft-Loading

[karalahana]

Hello, I want to ask you about a system in which there are three transformers and 3 gensets all have the same KVA respectively, I mean the back-up gensets are rated the same KVA with the tranformer to which they belong, Of course xformer-genset groups have different power ratings from each other in this case.Three gensets feed the same synchronisation busbar and the output of this busbar feeds the load busbars which is being fed by xformers separately, a classical system except that the xformers feed their loads separately and have no tie-breaks to connect them . I wonder if it is possible to have a soft-loading scheme here if the xformers have the same output voltages so that they can be paralleled (if they are fed from the same source and xformers have the same characteristics such as X/R ratio, connection type, etc) and they have separete busbars . I think it is impossible to have a soft-loading with a case in which xformers couldnt be paralleled, I mean paralleling of xformers is a stipulation to have this system soft-loading but I am not sure if the xformers busbars are separate.

 

[rbulsara]

What you describe ( if I understand it correctly) is a common setup. Transformer secondary parallleing is not a requirement for soft loading/unloading of the gen on the primary common bus.



Rafiq Bulsara

http://www.srengineersct.com

 

[karalahana]

imagine it please, we have three gensets paralleled while we have three transformers which have separate buses. So there is a unique busbar and its voltage conditions while three separate transformers voltage conditions. As far as I know soft-loading is a mains-genset parallelling isnt it? ?s there a contradiction if we say soft-loading can be achieved in such a scheme?

 

[rbulsara]

No. Unless your definition of soft loading is different than what is commonly understood. It may help if you state why do you think it could be a contradiction or a problem. Generators only see the loads on their terminals.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

Note that soft loading/unloading is always between two different sources, such as generators and utility or another gen power bus. Load is supposed to be up and running on one of the sources, before it gets transferred to the other.

Rafiq Bulsara

http://www.srengineersct.com

 

[karalahana]

no I mean with soft-loading a scheme in which a load is fed by both the utility and genset at the same time and say a genset is operating and utility is off and due to the reappearance of utility for example we take the load from genset slowly and transfer it to the utility in the final stage, and vice versa ,(if an interruption is informed to the user in advance and no transfering is sensed). So I ask you if we need to have all three transformers be paralleled or at least ready to be paralleled so that we can use soft-loading or soft unloading features to take advantage of this situation.

 

[rbulsara]

No. In your scenario you will be partly off loading the generator to utility, upon its return. I am not sure why you think what you think.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

That type of transfer is done with a combination of frequency control and governor control.
Normally, when a generator is paralleled with the grid, the generator will be running at no load with the throttle almost closed. The frequency is set slightly above grid frequency. When the generator output is passing through the synchronizing zone, the breaker is closed and the generator picks up a small portion of the load, just enough to maintain stability. The throttle is then opened (by advancing the governor setting or by a generator control panel) until the generator is delivering the desired output.
In your case, you are going in the other direction. The generator is already loaded, you may sync at a slightly higher frequency and export some energy or at a slightly lower frequency and drop a little load on the grid. Your protection scheme may dictate this choice. Once you are connected with the grid, the throttle may be opened or closed to either export power or transfer the load to the grid.
The choice of doing this as a group or each generator independently may be dependent on the synchronizing and control equipment that is installed.
A good operator may transfer as a group or independently manually. The possibility of an automatic transfer either way, group or independently, may depend on the available features of your Automatic Transfer Switch (ATS) and/or your generator control panel.


Bill
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"Why not the best?"
Jimmy Carter