Distribution Power Transformer Neutral Grounding Reactor

[eamonkerrigan]

I am having a hard time finding information about nuetral grounding reactors. Hopefully you can help me with a few issues.

I'm replacing two 10/12.5 MVA auto-transformers with one 15/20/25MVA Delta-Zigzag. Voltage is 23-13.8kV. Planning needs a really low impedance on XFMR(2.18%) which is making my ground fault currents really high. So I need a neutral reactor to compensate.

We don't have a spec so I'm trying to develop one based off of our CLR reactor spec.

However, it seems to me that C57.16 does not apply to neutral reactors (correct me if I am wrong).

It seems like neutral reactors fall under C62.92 which is neutral grounding in Utility Systems. However, there is very little talk about reactors (except for generators which I am not!)

There is also IEEE32, but again neutral reactors are barely a footnote.

My big questions are:
-BIL...Do I just calculate the voltage drop accross reactor? or do I match the XFMR nuetral BIL?
-No requirements about dielectric ratings in any standard I can find. It is very important for CLR's but not for neutral reactors?
-Continuous current rating (IEEE32 seems to say percentage of thermal current rating?)
-Finally, a real stumper. There is a note in IEEE32 which states third harmonic current shall not exceed 15% of rated continuous current. I know I have a lot of 3rd harmonics (that's actually why we're putting in the delta-zigzag), so should this drive my continous current rating? I don't know where this requirement is coming from? Thermal issue?

 

[pie314]

IEEE32 is the only one which covers Neutral Grounding reactors. The continuous current rating is just a way of ensuring that the reactor can meet the short time current. For a 10 second time IEEE32 states a continuous rating of 3%.Your Spec needs to state the short time ground current, and the length of time. A company which makes neutral grounding reactors can take it from there. Typically the reactors do not have a continuous current.

 

[odlanor]

1- I suggest IEEE Std 142-1991). IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems.
By IEEE Std-142-1991 reactor shall be sized for a minimum ground fault current of 25%
and prefer 60% of three-phase fault current , for avoiding hard transients overvoltages. It refers to 3 < X0/X1 < 10.
2- You should use neutral BIL of zigzag transformer grounded with X0/X1> 3.
3- Technical Caracteristics:
- rated whitstand voltage at industrial frequency
- rated whitstand voltage at lightning impulse
- Indutance (in mH)
- Current (through fault)
- duration of current (10 s.)
4- You should made all calculation of impedance in ohms.
After that you convert to indutance units.
Is is convenient to check with manufacture about dimensions of indutance coil.