Grounding of primary neutral of a step up Transformer in Solar Generaton Sytem 1

[Rwelagedara]

Dear All,
We are having a transformer with a vector group Dyn11(low voltage primary connected in star with a neutral).This is to be used as a step up transformer in a Solar Generation System. The inverter output is having just 3 wires with no neutral.

We planed to connect the primary neutral of the transformer directly to earth.But found in C57.159_IEEE Standard for Transformers for Solar Generation Systems that a neutral in transformer LV side and should not be connected to earth if available.Should we keep the LV side neutral isolated? What adverse effect will happen if we connect it to ground?

 

[xnuke]

This has been addressed in excellent fashion on this site in this Link thanks to waross.

 

[Rwelagedara]

Hi xnuke,Thanks.Yes I saw that excellent post by waross.But as per that explanation "grounding the wye point makes no difference UNLESS the source wye point is also grounded. Then the connection becomes a neutral connection that is incidentally grounded".

However , what was stated in IEEE standard is that ,"it is recommended that the neutral point is neither the grounded nor connected to other ground points". It seems like saying not to ground the neutral anyhow.

And if there is no grounded source why point available and neither the transformer why point is grounded, what will be the path for fault currents in case of a line to ground short circuit in primary side. Won't that be a problem?

 

[mgtrp]

Be careful comparing the descriptions in the linked eng-tips post (explanations from waross) and the IEEE standard--I believe that waross's explanations largely relate to transformers with a wye-connected HV and delta LV (i.e., Ynd), while the IEEE standard is referring to transformers with a delta-connected HV and a wye-connected LV (i.e., Dyn).

For what it's worth, I'm used to seeing an earthed LV on solar farms, and indeed this is as recommended by multiple inverter manufacturers. A few examples:
ABB
SMA
Yasakawa
These may differ from your application, as they are each four-wire inverters. I'm not sure why the discrepancy with the IEEE recommendations.

Cheers,
mgtrp

 

[mvcjr]

"And if there is no grounded source why point available and neither the transformer why point is grounded, what will be the path for fault currents in case of a line to ground short circuit in primary side."

For a single LG fault in the primary side (LV side), no fault current will flow if the trf neutral is isolated. If the neutral is grounded, a single LG fault current will be supplied from the HV side but the inverter will not supply fault current.

 

[prc]

I think better to go by the advice of inverter manufacturer. In your case it seems clear- inverter gives only three wires and hence may be not to ground the transformer LV neutral. Transformer manufacturer will bring out the neutral, unless asked not to do so.

What will happen if you earth? I know a case in India where inverters were getting faulty frequently. When the LV transformer neutral grounding was taken out, the problem disappeared.

 

[RRaghunath]

Delta on inverter side and star with earthed star point on distribution side (or grid side) has an advantage that in case of an earth fault in the star windings or the cables out of transformer, the inverter will be able to sense and block the inverter output and protection will be able to open the Star side circuit breaker.
Thus, Star connected winding on grid side seems apporiate to me.