Zigzag Transformers for Islanding TOV on PV Projects

[cyriousn]

I'm trying to understand why grounding transformers are put on solar projects where the transformer is Yg-Yg. Is it due to the condition where the ground source is lost if the breaker at the local substation is tripped and the PV site is islanded with the distribution circuit until the PV site shuts down in 2 seconds or less due to islanding detection?

I'm working on a project where there is a zigzag transformer connected on the 600V side of the 23kv/600v Yg-Yg Transformer. The zigzag is fed from a 400A electronic breaker in a switchboard that shunt trips the 2500A MCB. Typically I see Yg - Delta grounding transformers on solar projects so this is a new one for me.

Wouldn't the grounding transformer, under steady state conditions, be a grounding source for the local distribution circuit for imbalances? The rating of the transformer is 867A continuous on the neutral. Is there an easy way to determine what conditions on the distribution circuit could trip the circuit breaker and dump the project?

 

[pv_n00b]

The effective grounding XFMR is not used under normal steady state operation, it is used when there is a L-G fault on the utility distribution system that causes it to be isolated at the substation. This removes the ground reference for the distribution system in the substation and now you have a PV system feeding an ungrounded distribution system with a L-G fault until the inverter realizes something is wrong and shuts down. During that period it could cause a TOV event on the distribution system that may damage other customer's electrical systems. The effective grounding transformer will limit the TOV voltage and prevent that damage. That's the short explanation.

 

[pv_n00b]

Wouldn't the grounding transformer, under steady state conditions, be a grounding source for the local distribution circuit for imbalances? The rating of the transformer is 867A continuous on the neutral. Is there an easy way to determine what conditions on the distribution circuit could trip the circuit breaker and dump the project?

This is true and why it is an effective grounding transformer (EGT) and not a grounding transformer. EGTs are not designed to provide a solid ground like grounding transformers are. They are designed with specific impedances to limit TOV and at the same time limit ground fault current through them so the utility protection system is not desensitized.
The CB protecting the EGT is linked to the main CB so that if the EGT is isolated the PV system cannot back feed the utility distribution system. The CB for the EGT should not trip unless there is a fault on the EGT circuit. Sometimes the neutral current of the EGT is monitored by a relay and if ground fault current over a set point is detected the PV system main CB is opened to take the system offline.

 

[cyriousn]

This might be a stupid question but could the 400A breaker rating be too high for protecting the grounding transformer? Is there a condition where the 867A continuous neutral rating is violated from having more than 289A on each phase? We could dial down the breaker rating but I didn't know if we would be looking at any potential nuisance tripping of the solar site.

 

[ggunn]

This might be a stupid question but could the 400A breaker rating be too high for protecting the grounding transformer? Is there a condition where the 867A continuous neutral rating is violated from having more than 289A on each phase? We could dial down the breaker rating but I didn't know if we would be looking at any potential nuisance tripping of the solar site.

Phase currents do not add to make line current.

 

[pv_n00b]

This might be a stupid question but could the 400A breaker rating be too high for protecting the grounding transformer? Is there a condition where the 867A continuous neutral rating is violated from having more than 289A on each phase? We could dial down the breaker rating but I didn't know if we would be looking at any potential nuisance tripping of the solar site.

You should size the OCPD using Art. 450.5 for grounding autotransformers. Find out the inrush current and stay above that to prevent nuisance tripping on startup.

 

[electrofelon]

The effective grounding XFMR is not used under normal steady state operation, it is used when there is a L-G fault on the utility distribution system that causes it to be isolated at the substation. This removes the ground reference for the distribution system in the substation and now you have a PV system feeding an ungrounded distribution system with a L-G fault until the inverter realizes something is wrong and shuts down. During that period it could cause a TOV event on the distribution system that may damage other customer's electrical systems. The effective grounding transformer will limit the TOV voltage and prevent that damage. That's the short explanation.

This makes no sense to me. Almost every grounded distribution system is an MGN, so its going to have tons of ground reference on the line. Furthermore, why does the substation lift the grounded conductor when the line trips out? I Wouldnt think they would regardless of if its an MGN or uni-grounded.

 

[pv_n00b]

This makes no sense to me. Almost every grounded distribution system is an MGN, so its going to have tons of ground reference on the line. Furthermore, why does the substation lift the grounded conductor when the line trips out? I Wouldnt think they would regardless of if its an MGN or uni-grounded.

It requires a bit of a deep dive. When there is a L-G fault on the distribution system there are several levels of protection that are used to clear it if it is temporary and isolate it if it is not. The end result is that with the distribution isolated from the utility generation the only thing left energizing the faulted line would be DG on the distribution system. Eventually the DG will notice that the utility is no longer energizing the line and trip off but during that period between when the utility is isolated and the DG trips off there is a danger of TOV. That is because the DG is typically an ungrounded source. The L-G potential of the unfaulted phases can rise to the L-L potential and damage anything connected to the distribution system. The purpose of the effective grounding transformer is to ground the DG during this period and limit the L-G potential on the unfaulted phases.