Hello I'm an electrical protection engineer.

I was trying to look for information about why the voltage transformers are open delta connected in a Ge design and if it has something to do with the LCI start up? maybe it has something to do with the 89nd switch and the need to be unearthed during the start up?

The reason why the machine working as a generator is grounded is because they need to limit fault current to a low value for a phase-ground fault to avoid damages to the machine(they limit to a typical value of 10A)?

The reason why it can not be ungrounded is because in case of a fault high transient over voltages appears in the machine deteriorating insulation materials. So in order to protect the generator they reduce both voltage transients and current fault values?

I hope this helps could you guys also answer my question?
Thanks in advance

 

How would you connect the potential transformers (I'm presuming you're talking about the ones used for synchronization purposes)? Isn't open delta sufficient for synchronization purposes? Doesn't it reduce the amount of wiring required to just three wires for two voltage sources?

As for the rest of the post, I'm not sure if you're asking or commenting. I think you're mostly correct on most of the items.

 

Serp1981...

Reur your question about open-delta xfmrs...
the reason is economic, that is, only two transformers are required instead of three!

Reur question about disconnecting the NGR... see: http://control.com/thread/1240942296!

Regards, Phil Corso

 

Thanks for your answers the second part of the initial answer was the reply to an old post so don't worry about it.

I've heard about the open delta connection and know how it works, it is more common in USA, I'm used to wye-wye connected voltage transformers for electrical protections but when the supply of a gas turbine comes from GE usually they use that connection in the voltage transformer a custom and an economic issue I think you are right.

A friend of mine told me the LCI needs a system totally ungrounded, so the voltage transformers cannot be wye connected and the neutral earthing must be removed too for starting up the gas turbine I'd like to know if this is points is true or if it depends on the manufacturer. If you could help me It would be ok

 

Serpil981... the PTs should not be disconnected.

Only the NGR is disconnected for the acceleration period during which the the generator operates as a synchronous motor!

Regards, Phil Corso