Generator Neutral Surge Arrester

[Schwatzernov]

Hi,

I have seen the standard generator single line diagram of Siemens always shows the surge arrester between generator neutral grounding transformer and ground for gas turbine generator started with static frequency converter.

The question is that why the arrester is need to be connected bewteen NGT and ground ?

I have not seen other generator manufacturers connect such arrester like this.

Can anyone explian why it is need for machine started with SFC ?

Thank you very much.

Schwatzernov.

 

[RRaghunath]

In case there is open circuiting in the NGT primary-to-ground circuit for what so ever reason, the generator system becomes ungrounded with a possibility of over voltages during Single phase-to-ground faults. I think the Surge arrester is to short the generator neutral to ground and thus protect the system under such an event.
This may not have any thing to do with SFC start or any other starting method of Gas turbine.

 

[Schwatzernov]

Raghun,

It is really related with SFC, I had already checked with many plants for CCGT which all gas turbine and steam turbine generators supplied from Siemens and conclusion is that all gas turbines started with SFC will be installed with the neutral arrester. Gas turbine which is started by cranking motor is not installed with neutral arrester.

However in case of other generator manufacturers, there is no neutral arrester installed.

I do not think that the arrester at the neutral is used for discharging the temporary overvoltage when ground fault happens, if so, why it is not installed at the generator terminal instead. Installing arrester at the neutral may be subjected to discharge current passing through the generator winding.

However it is not the basic principle in applying arrester for discharging the temporary overvoltage when ground fault happens. Basicly under ground fault condition the arrester shall not discharge but when the surge comes. The winding of generator can withstand rated power frequency voltage(which is always higher than 1.75 p.u.) up to one minute.

The lightning surge or switching surge can be discharge by generator terminal surge arrester and capacitor.

So I can not find the reason that why the surge arresrter is installed at generator neutral but Siemens really did for GTG started with SFC.