Back Liveming from Diesel Generators

[Rodmcm]

We have been asked to assist with a project (designed, installed but not commissioned - but thats another story) that backlivens an isolated 11kV distribution system from 2 x 750kVA 415V diesel generators. Step up is from 415V to 11kV 1500kVA and then there is 4,250KVA of connected transformers (multiple 11kV/415V 250kVA and 500KVA transformers) and a total standing load of about 600kW at turn on all spread over four sites. The problem is that we cannot find any literature or obtain from the gen supplier any information that proves that the generators will actually hold up the voltage at turn on. Can anyone assist?

 

[davidbeach]

Define what you are concerned about. Voltage dip during transformer inrush? Steady state voltage? Something else?

Could be a bit ugly trying to start 3 times as much transformation as you have generation. One way to avoid the inrush is to connect all the dead transformers, with the secondaries open, to one generator before starting the generator. If you build up the transformer voltage at the same time you build up the generator voltage you skip the inrush. Once that generator is up to speed and voltage, synchronize the other one to it, then add load.

 

[Rodmcm]

What I am concerned about is whether the AVR/exciters will hold up the voltage on inrush. In the present installation none of the 415V breakers are controllable (old installation) so while your idea has merit, is an expensive solution for us.

 

[waross]

Your best bet will be to use the 1500KVA transformer as a Generator Step-up Unit. Start one generator connected to the 1500 KVA transformer and bring them up together. Then start and sync the other generator. Then close the high voltage and connect the transformer to the line.
Use the Permanent Magnet Generator option on the generators.
I had experience in a small plant where a 1000KVA transformer bank was regularly energised from a 600 KW generator.
When you close the connection to the power line, expect to see a lot of black smoke, generators leaning on their mounts and ammeters trying to bteak their needles.
The better your voltage holds up, the worse the other issues will be.
Actually you may consider self excited generators and take advantage of the greater voltage dip to reduce the other issues somewhat.
It will all pass in a few cycles in any event, why not let the voltage dip?
If you must have full voltage from the first cycle you will have to either sectionalize the line or better yet use davidbeaches suggestion.
Personally, I would compare the cost of High voltage sectionalizing switches, adding undervoltage release and motor operators to the 415 volt breakers, and comparing the cost and the operating implications with adding a circuit to intentionally allow the terminal voltage to drop so as to limit the combined transformer inrush. A resistor across a CT will generate a voltage that can be added to the sense voltage so as to fool the AVR into thinking that the terminal voltage is higher that it actually is, and thus reducing the excitation and the terminal voltage.
Use a relay to take it out of thee circuit once you are online.
When you compare the relative costs of avoiding a voltage build-up on energization and planning to intentionally use a planned voltage build-up to reduce stress on equipment, you may find that your custormer is willing to live with a voltage build-up.
respectfully

 

[Rodmcm]

Just for further general education I have recieved manufacturer's voltage dip curves. These show a 35% voltage dip at 4pu current at 0pf.