System Stability. Help !!!!!!!!!!!!!!

[maypot]

Hi ,

We are designing the system network of a new factory. The estimated load is around 10 MW and a nearby IPP will be supplying us power @ 11 k V . The IPP is requesting us to install a 11 / 6.6 k V ( or 1 / 1 isolating transformer ) transformer on our side with the ultimate idea to isolate their network from ours which is understable from the stability point of view.
However, as we have got a very tight budget , we are proposing the IPP to feed directly our 11 k V cell and we will be limiting the ICC through suitable reactor on their 11 k V system. It is understood that the system stability on both our sides and the IPP is questionable as we will be having a common earth .
The second solution will result in considerable saving as we have already in the group transformers , MV panels etc.
I trust that through grading and co-ordination, the second option is as valuable as the first one.
Suggestions ?

Bob

NB Is it possible to insert attachment file to a post ?

 

[alehman]

IPP's normally feed into a public utility system, from which you in turn draw power. In that case, the IPP interface to the utility should follow the utility company's standards. I would be concerned with the reliability of having my plant fed only from an IPP.

If they are feeding your plant directly (without interface to a utility company), I can understand why they may want a transformer for ground isolation or limit fault current. It seems like if they plan to be in the business of selling power, they should provide the necessary equipment to interface with their customers. Keep in mind though, that one way or another, you as the customer will be paying for the equipment whether you buy it or they buy it.

 

[mc5w]

The reactors will work just as well as a transformer and at higher efficiency and at lower cost. All that a transformer does is to act as an inductance that happens to change the voltage.

If all of your 11 KV transformer primaries are delta there will be no noticable difference under normal conditions. The only normal flow of neutral current would be from your own step change voltage regulators if you have them.

Downstream of the voltage regulators you might want to have a trifiliar wound inductance that acts strictly to increase the zero sequence impedance that any ground faults will see.

 

[maypot]

The IPP, a sister company , also is a newly built one and they will be exporting power 54 MW to the utilty via a transmission network 11/66 kV . The intention to feed straight from the IPP is strictly commercial.
The 11 kV transformers are all delta connected on the 11 k V sides and we will be having both overcurent and earth fault protection relays on the 11 kV cell .

Bob

 

[dpc]

There is a big difference between a reactor and a transformer when it comes to ground faults. A delta-wye transformer will isolate your ground fault current from the IPP. This greatly simplifies ground fault protection on both sides.

If the IPP is generating directly at 11kV, this is even more important.

Will it work without the transformer? Probably. But it is a really good idea.

 

[mc5w]

Cleveland Electric Illuminating Company's Eastlake power plant has some 13,200 volt 3-wire distribution directly from the power plant buses to a limited number of local customers. Some of these customers are quite a few miles away. This is partly because that when recovering from a blackout some customer load has to be connected right away so as to provide a minimum load to stabilize (damp) the speed governors and field (voltage) regulators. Having some amount of reactive power usage from customers also helps offset the charging current from the transmission lines when they are connected. A few distribution substations are then connected to provide more stability and to act as a sink for the bump that occurs when synchronizing to lines that did not get blacked out. Synchronization is never perfect.

In other words, a few lucky devils get their power back early when the utility is recovering from a blackout.

I did not know that you were this close to the generating facility. As long as you are using an overhead lightning wire you should not have any trouble with operating directly from generation. The generation buses should have resistance grounding or a zig-zag set to provide solid grounding. If the 11/66 KV transformers are YDY with a 6.36 KV delta tertiary winding then the 11 KV generator buses can be solidly grounded or resistance grounded using the step up transformers.

An alternative way to connect voltage regulators is to use the open delta configuration of two single phase units.