multiple generator modeling

[tonyflair]

If I'm considering a distribution system that is solely supplied by synch. generators and I have 3 of them running at once......is there any way to model these in terms of reactance (sub-transient, zero sequence, etc.) as one large generator ? Is it as easy as considering them connected in parallel (which they are) and then adding the reactances as if the were in parallel too ? (As well as adding up there power)

Thanks in advance

 

[cuky2000]

Similar to motors, generators could be combined as one equivalent unit with a capacity equal to the sum of the generators in service. The subtransient reactance of the equivalent unit needs also to be adjusted. For instance, two identical generators in parallel are half of one generator.

 

[JJHorak]

I know people and fault calc programs treat groups of gens as a single source, but you at touching on a real tough one. The Xd, Xd', Xd", Xq...., all those time constants, are rough modelling of a generator, and further, the excitation system response and feedback from loads (motors, capacitances.... Most doing the fault calcs just give up and go for the crude answer of considering them all one big machine with everything in parallel.

 

[CJFlatters]

Tend to agree with brokenhandle, I would model the fault current characteristics of each machine separately, they will have different time constants as well as reactances.



_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[cuky2000]

If the "exact" answer is required, precise model and complex calculation will be required. The key here is to determine the level of precision and compromising between complexities, time and cost in making decisions.

One parameter to define the accuracy of the model could be the margin between the design short circuit levels compared with the interrupting rating of the protective device(s).

For large margin, let's say above 20%, the experience often could help to make short cuts selecting either a simplify or complex model to determine the withstand capability of the system.