Effective grounding

[AIRVIPER00]

I have a solar interconnection on a 1.2MWac facility with a 12.5kV y grounded primary and 600V y grounded secondary. The utility tells me that we need to have an effectively grounded system even though the transformer is Y-Gnd/Y-Gnd. One utility engineer tells me that this transformer is ok and is effectively grounded, but another reviewer says it is not. Nothing like contradicting "opinions".

Any ideas what to do here? or should I add a grounding transformer to the 1600A switchboard?

 

[dpc]

I think there may some misunderstanding or miscommunication. Is the 12.5 kV system solidly grounded? With a grounded wye - grounded wye transformer, absent any grounding impedance, the 600 V system is effectively grounded. The zero sequence reactance will be mainly the zero sequence reactance of the utility system plus the transformer. Adding a grounding transformer on the 600 V will result (most likely) in a lower zero sequence impedance and higher ground fault current at 600 V, but it will also potentially impact the relay protective scheme of the utility. I would ask the utility for specific reasons why they think you need a grounding transformer.

 

[9wearSuave]

I agree with the above. It doesn’t seem to be in the best interest of the utility to add a ground further into your system as that would desensitize their relays.

 

[Kiribanda]

A grounding system to be effective following conditions have to be satisfied.
1) Sync. Machine based generation: When the generator is connected or embedded into the 12.5kV utility system,
X0/X1<3 & R0/X1<1

Since more & more inverter based generation is coming such as PV,
2) For inverter based generation:
0.01<R0/X0<0.3 & 1<(Z0/ Z1load)<2

where
X0 = zero sequence reactance
X1 = positive sequence reactance
R0 = zero sequence resistance
Z0 = (R0+jX0) is the impedance of the supplemental grounding
Z1 load is the equivalent impedance of the grounded load of the network.

So you have to perform system studies & prove (2) mentioned above. Otherwise during a ground fault on the 12.5kV utility system,
line to ground voltages of the healthy phases will exceed 138% of its nominal line-to-ground fundamental frequency voltage for a duration exceeding one fundamental frequency period.
It will impact the surge arresters & VTs installed on the 12.5kV system.

 

[jghrist]

I agree with dpc. If the 12.5 kV system is effectively grounded (which it undoubtedly is), then the 600 V system will be effectively grounded without an additional grounding transformer.

 

[wayne440]

...The utility tells me that we need to have an effectively grounded system...One utility engineer tells me that this transformer is ok...but another reviewer says it is not...Any ideas what to do here?

Before spending much time or money, find out which of those two people has the last word with the utility. If the utility can't decide, respectfully ask them to do so.

 

[wcaseyharman]

In our utility, the requirement for the connecting generation to be effectively grounds stems from the possibility of the generation to become islanded under fault conditions (substation breaker opens) while customers are attached. In those cases if the generation becomes islanded during a fault, customers could see damaging overvoltages.

When I looked into it for inverters, it was tricky because embedded in the assumption for effective ground was that Z2 is equal or less than Z1. However, for inverters that is not necessarily true. Some data sheets we have gotten show the effective Z2 as a very high number, which makes it impossible for the inverter system to be effectively grounded regardless of adding a grounding transformer.

I think my utility is still working these details out, I dropped off that effort and am not sure where they ended up.