Problems with Solar PV and Zig Zag Transformers

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CJarosch

Member
Location
Hudson, WI
Occupation
Master Electrician
The company I work for has had many issues 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?
 

pv_n00b

Senior Member
Location
CA, USA
I would have to know more about your system design to provide any useful input. Typically a grounding transformer is required when a WYE-Delta transformer is used to connect the PV system to the service and the PV system is not effectively grounded. It does not allow the inverter to detect a single phase loss at the service. A WYE-Delta transformer isolates the PV inverter neutral from the service neutral and will prevent it from accurately detecting phase loss at the service. To do that you have to use a WYE-WYE transformer so the phase loss on the service side is mirrored on the inverter side.
 

CJarosch

Member
Location
Hudson, WI
Occupation
Master Electrician
Oddly enough, EE at IREC has report stating in all cases, 3 phase inverters will detect phase loss. The issue with the grounding transformer is to detect ground faults on the grid, which puts a high voltage from other phases to ground. A zig zag will become imbalanced, letting current to flow, thus tripping a contact breaker. That in turn trips the shunt trip breaker for the system. Otherwise, say in the case go a synchronous generator, the generator will continue to provide 3 phase power on a faulted grid. That is why it has been standard practice to use grounding transformers on synchronous generation. Inverters are much more sophisticated. So there are persons working to get the IEEE requirements to loosen on this front.

We cannot control what transformer is on sight. Most systems we do are 1.5MW or smaller on rooftop. Which means we use the utility configuration available.

I am looking into using neutral grounding reactors instead. I understand they cause less issues.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
The term "neutral grounding reactor" to me refers to a reactor that limits the fault current through a neutral in a low impedance grounded system. But this would not be connected to the ungrounded phases, and so it cannot limit transient overvoltages from faults such as grounding transformers can do by creating a low impedance for zero-sequence waveforms.
But I'd be interested if there's something other than a zig-zag or wye-grounded/delta that could be used in such an application.
 

pv_n00b

Senior Member
Location
CA, USA
CJarosch, which report from IREC is that? I'd like to read it.
The purpose of effective grounding is to clamp transient voltage spikes to an acceptable level if the PV system becomes the only generation source on a distribution system. This could happen if a substation isolated a distribution section the PV system is on and the PV system keeps supplying the section until the anti-islanding circuit detects loss of the substation and shuts the PV system down. This protects the other utility customers on the section from damaging voltages, the utility does not really care about what happens to the PV system.
Effective grounding can be done in several ways and a zig-zag grounding transformer is one of them. The grounding transformer can also be used to detect ground faults but that is a bonus in addition to the effective grounding function, not the primary purpose. Most grounding transformers need a neutral grounding reactor to meet the ratios that make them effectively grounded. It's a complex balance between reactance, resistance, and the utility grid in the section. As far as I know, to create an effectively grounded system requires a transformer of some type.
 
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