Generator neutral voltage 2

[niallnz]

I have recently installed a GE Multilin 489 generator protection relay onto a 4MVA 11kV 4pole synchronous generator, with brushless excitation. The star point of the generator is conected to earth via the primary of a 100:1 11/root3 transformer. The secondary of the transformer is connected directly to the Multilin Vneutral input (E10, F10).

When recommissioning the generator I noted that with the turbine running a speed and unsynchronised the measured neutral votage was approximately 300V. This was acceptable has the Multilin also displayed that they were some differences in the phase voltages. The generator was synchronised and loaded to 1MW with out adjusting the AVR. the neutral voltage dropped to 0V, as the system is strong and balanced. When the generator was manually tripped off, the measured neutral voltage when up to almost 3000V. Although the phase voltages were the same as before synchronisation. I turned off the AVR, and then turned it back on and the neutral voltage returned to its earlier value of 300V.

There have been occasions since, that when the AVR has been turned on the Multilin has tripped due to high neutral voltage (Fundamental). If the AVR is cycled the neutral voltage returns to normal.

As anyone got any ideas or tests that I can run. Thanks in advance.

 

[ScottyUK]

NiallNZ,

You don't mention a burden on the secondary side of the grounding transformer. Is there one, other than the Multilin?

 

[dpc]

As Scotty says, there is generally a secondary resistor on the grounding transformer. This is needed to avoid resonance problems. The relay may be too high an impedance to pin down the neutral voltage. The resistance seen by the primary side should be greater than the capacitive reactance on the primary.

Without this burden, the generator isn't really grounded.

 

[niallnz]

Thanks guys, good points. There isn't a secondary resistor, as the protection relay the Multilin replaced was a moving disc type which was directly connected to the transformer via a series resistor. Looking at the specs of the old relay, (VDG14/VAA) it's input was not rated for the transformer maximum secondary voltage, so I figured the resistor was there to limit the voltage applied to the relay, forgot to consider the burdon it was putting onto the transformer.

 

[jbartos]

Suggestion: Normally, the generator neutral grounding system grounding adheres to industry standards, e.g. IEEE C62.92-1989 IEEE Guide for the Application of Neutral Grounding in the Electrical Utility Systems, Part II - Grounding of Synchronous Generator Systems.
Fig 3 Distribution Transformer Neutral Grounding
It shows the secondary of the distribution transformer connected to Z and voltage relay in parallel