Generator Trip as result of neutral displacement fault

[Northseatom]

One of our 6kV generators tripped as result of neutral displacement fault (59N). Starpoint of generator is grounded through resistor. Protection unit is Alstom LGPG111. Neutral displacement protection is enabled on this unit. Settings are Ve>0,55kV, t1=0,5sec., t2=6sec, and t2 reset=0sec.
Know about neutral displacement protection in un-earthed networks. Network is relative small. Measurements are currently undertaken to generator.
Two questions:
1) Is there anyone with some experience in mentioned set-up and/or faults.
2)Is use of neutral displacement protection in resistor earthed network common?

 

[busbar]

Typically, zero-sequence voltage relaying is a means of detecting insulation failure in stator windings and stepup-transformer lowside, including breaker and bus connecting the two.

Unit-breaker tripping limits evolving insulation failure and ground-resistor overheating.

 

[kannanR]

An ungrounded generator neutral will shift depending on the harmonic content of the generator. Based on this neutral the continuous current rating of the Grounding resister is designed. hence if there is no fault in your system as discussed above you may check your neutral grounding resister.

 

[busbar]

Small aside — The 59G relay typically is third-harmonic restrained. That is an easy verification to do with a relay test set.

 

[jbartos]

Suggestion: Visit

http://www.ieee-pcic.org/Subcommittees/safety/1999_agenda_items/1

http://www.geocities.com/SiliconValley/Pines/3406/high_r.html

http://www.beckwithelectric.com/powerlines/articles/i40/hybrid.htm

(for Hybrid System Grounding of generator.)
etc. for more info

 

[AusPowEng]

Northseatom,

To address your second question, it is my experience that neutral displacement relays on non-effectively earthed (ie. impedanced earth) systems are used as backup protection to standard sensitive earth fault protection to clear prolonged faults and, become primary protection should the NER (or NET & NER combination ) fail due to open circuit.

I believe the LGPG111 has zero sequence harmonic restraint (ie. triplens 3rd, 9th, 15th) but I'll check that out.

I assume you have checked co-ordination between the LPG reponse of 0.5sec and your other EF protection responses?

Regards,

AusPowEng

 

[rbulsara]

Pages 24-26 of The Alstom manual for this LGPG111 relay describes the application of 59N function and recommendation of the settings in detail.

I am not sure what more can anyone help without looking at your complete wiring diagram of the protection scheme.

Plus each realy software have slightly diffrent way of inputting the setting data in them. You may be better off consulting an application engineer of ALSTOM.

To your second question, the answer is yes, 59N is commonly applied to impedance grounded or ungrounded generators.