Effective Grounding - Zig-Zag Transformer

[pcanning87]

I have a utility requesting "Effective Grounding" for a system behind a Grounded Wye - Grounded Wye distribution transformer. In terms of installation, do we just need to tie the A, B, C phases of the Zig-Zag to the distribution transformer secondaries and then ground the derived neutral of the Zig-Zag?

Does the distribution transformer neutral remain solidly grounded? I'll admit I don't fully understand the purpose of a Zig-Zag transformer used in this application.

 

[ron]

Sounds like the utility transformer has a grounded Y secondary. Why do you need the zig-zag? The secondary of the utility transformer is already grounded.

 

[pv_n00b]

I have a utility requesting "Effective Grounding" for a system behind a Grounded Wye - Grounded Wye distribution transformer. In terms of installation, do we just need to tie the A, B, C phases of the Zig-Zag to the distribution transformer secondaries and then ground the derived neutral of the Zig-Zag?

Does the distribution transformer neutral remain solidly grounded? I'll admit I don't fully understand the purpose of a Zig-Zag transformer used in this application.

The purpose of effective grounding is to limit over-voltage from the PV inverter(s) during fault conditions. Effectively grounded is not the same as solidly grounded.

There are a number of steps to putting together the effective grounding system all of which require some experience. If it's your first time I would recommend working with an engineer who has experience in effectively grounding PV systems or with the inverter manufacturer if they are helpful.

To give a basic outline:

1. Selecting a grounding transformer
   1. Must have the correct X0/X1 and R0/X1 values, a very debated subject for static inverters.
   2. Must be sized for the expected neutral current caused by any utility voltage imbalance. I have seen many effectively grounded systems with a steady state 50A on the neutral from the utility.
   3. This probably requires specifying the correct neutral impedance device to get the correct ratios and limit neutral current.
2. Set up the protection system.
   1. Generally this is a ground fault relay on the grounding transformer sensing the neutral current and tripping the PV interconnection OCPD on a ground fault.
   2. The transformer protection OCPD will also generally trip the PV interconnection OCPD when opened.

Do all this knowing that in the real world it is most likely wasted on PV inverters because the whole effective grounding system was designed for rotating generators with high fault currents and high inductance. I have read several studies that show it does not help reduce over-voltage from inverters. But utilities like to throw the requirement out there because they treat all generators the same.

There are a few good resources for learning about effective grounding as applied to PV inverters. One is here: https://solectria.com//site/assets/files/1484/solectria_effective_grounding_for_pv_plants.pdf I would also recommend checking with your inverter manufacturer since they may have designs for doing this with their inverters.

A lot of the stuff you will find on the internet when looking for information about effective grounding relates to rotating generators and that's of little help to us PV folks trying to specify equipment.

​

 

[CJarosch]

We have had many issue with the interconnection of zig zag transformers and PV string inverters. The utility sets the sizing requirement calculations, which our engineer calculates. Zig zags are only required on systems larger than 100kW.

The problem is this; we are required to provide a single phase test on all three phase interconnected PV systems. If we happen to have a utility required zig zag zag transformer on the line, the test will fail more often than not. We turn off a breaker creating the single phase event. The inverter immediately stops producing power. So far no problem. The inverter starts a 5 minute count down.

During this part, the tester is taking voltages and current numbers.

We re-establish the lost phase conductor. Now the inverter is not supposed to start for 5 minutes. It starts 3 minuted later (the amount of time it takes to jot down voltage and current). Thus the test fails.

I believe what is happening is that upon loss of phase, there is a voltage spike that trips the inverter. The inverter does not see a phase loss. I believe the zig zag is creating voltage on the lost phase wire, tricking the inverter into thinking it can start after 5 minutes. If the inverter is left with a lost phase, it will try to start after the 5 minutes, but will see a voltage spike again, thus repeat. It does start to produce power in the 2 seconds, thus test will fail.

I have heard neutral ground reactors, instead of zig zags, do not have this problem.

Just for fun, I disconnected the neutral from inverters and connected them directly to zig zag XO. The inverters reacted fine in single phase testing.

Has anyone had this issue, and even better, solved it?

 

[electrofelon]

Is this national grid? They have that zig zag/ effective grounding BS requirement. It has come up before on here, and no one can make sense of it it seems.

 

[CJarosch]

Xcel Energy requirement.

 

[jim dungar]

The original post was from several years ago. Please start a new thread if you would like to continue old discussions.