Interfacing Solar Array to 480 3-Phase delta grid

[MurphyLuth]

I am installing 2 separate solar PV arrays to two meters at the same location. One array is 13 KVA and the other is 35 KVA. Both arrays would be interfaced to 480 VAC 3-phase delta grid power with an earth ground. Most solar inverters want a Wye grid connection. I have contacted the inverter manufacture and they told me that I need a grounding transformer between the grid and inverter. What would be the specifications for this transformer?

 

[jumper]

Welcome Murphy.

I moved your thread to our PV forum to help get more responses to your question.

 

[Smart $]

Need more details before commenting further...

You say the grid source is grounded 3? delta. Exactly how is it grounded? Center tap of one phase (480/240V 3? 4W) or corner grounded (480V 3? 3W). If the former, the neutral (center tap connection) is required to be brought to the service disconnecting means.

What is the output configuration and voltage of the PV inverters? 480V delta, 480/277V wye, or other (elaborate). If 480/277V wye, is a neutral connection to a grid source neutral required in the inverter's instructions?

The more details you can provide the better. May help tremendously to provide make and model of the inverters, link to online version of instructions...

 

[GoldDigger]

Most of the single phase string inverters that I have seen are designed to be used with three phase systems via a line-to-line (one side of a delta) configuration, but some may want to see a connection to a neutral for control purposes. All require an EGC.
You are talking about single unit three phase inverters, I presume?

The specifications for the transformer would basically be the input delta primary to match the POCO voltage and the output wye voltage to match what the inverter wants to see.

Note that, as discussed in another thread (http://forums.mikeholt.com/showthread.php?t=164362&highlight=reverse+use), you want to use a transformer which is designed to be used as a step up or step down or same voltage transformer with the POCO side as the primary. Even though the power will be flowing from the GTIs to POCO, all of the other characteristics of the transformer should be chosen as if the POCO side is the primary winding.

 

[ggunn]

I am installing 2 separate solar PV arrays to two meters at the same location. One array is 13 KVA and the other is 35 KVA. Both arrays would be interfaced to 480 VAC 3-phase delta grid power with an earth ground. Most solar inverters want a Wye grid connection. I have contacted the inverter manufacture and they told me that I need a grounding transformer between the grid and inverter. What would be the specifications for this transformer?

The inverter manufacturer's tech support/field applications engineer is your friend. Whatever you are doing they have done a bunch of; get them to spec your transformer.

 

[MurphyLuth]

http://files.sma.de/dl/21561/STP12_15_20_24TL-US-IA-en-15W.pdf

The 3 phase power from the grid is 3 wire 480 VAC. The ground is established at the meter with a grounding rod.

One system will be 30KW and use two SMA America Model 15000 TL-US inverters (link to installation manual above). The other system will be 15KW and use one SMA 15000 TL-US. I looked at the instruction manual and talked to SMA technical and the output of the inverter needs a neutral. SMA told me that I need a grounding transformer between the inverter and the grid. They said they could not give me the specs for this transformer and referred me to a transformer manufacturer. The transformer manufacture told me that I need a Delta/Wye transformer which seems to make sense.


My question now is: Can a grounding or zig-zag transformer also be used. I have read a little bit on the web regarding these but do not understand a lot about them in this application. One problem with the Delta/Wye transformer is the efficiency (estimate 97%). The solar system already has enough inefficiency as it is. From what I have read the grounding transformer would be more efficient.

 

[Smart $]

The 3 phase power from the grid is 3 wire 480 VAC. The ground is established at the meter with a grounding rod.

Still doesn't answer the question of how it is grounded. Perhaps it won't matter in the end, but still a relevant detail at this point. Let's put it another way... Is it a 3-wire or 4-wire service?

The latter is much more common IMO. Has a center-tap neutral. 480/240V 3? 4W

The former would have one line grounded, so that line would be 0V to ground. This line would also not have any inline OCPD. 480V 3? 3W

One system will be 30KW and use two SMA America Model 15000 TL-US inverters (link to installation manual above). The other system will be 15KW and use one SMA 15000 TL-US.

Seems to me you have a 45kW system using three (3) SMA 15000 TL-US inverter...???

 

[texie]

http://files.sma.de/dl/21561/STP12_15_20_24TL-US-IA-en-15W.pdf

The 3 phase power from the grid is 3 wire 480 VAC. The ground is established at the meter with a grounding rod.

As Smart$ mentioned, the setup of the delta service system is important. Lack of thorough understanding of this can lead to serious safety issues and potentially letting the smoke out of it's cage.
The mere presence of a GES (the "grounding rod") does not indicate the type of system grounding or if it is grounded at all as all systems must have a GES.

 

[MurphyLuth]

I went back and took a closer look at the service entrance. The entrance head at the top of the pole has 4 wires, one white with green tape (neutral I assume) and three black. Both meters have recently been labeled with a sticker 277/480. However, on both meters, the neutral ends at the meter and is not passed through to the disconnects. On one service there are mainly 480 3-phase pumps. However, there is a 5KVA transformer supplying some 120VAC outlets. This service was installed about 35 year ago and it is difficult to get information off of the transformer. If the neutral is not passed through, how am I getting the 120V? Since I never saw a neutral in the disconnect conduit, I thought the service was 3 wire delta. Thanks Smart $ and texie for insisting we get the type of service right before moving forward.

Now, if the service is actually Wye, do I need to change the disconnect to interface the inverters or just extend the neutral from the meter?

 

[Smart $]

..

Now, if the service is actually Wye, do I need to change the disconnect to interface the inverters or just extend the neutral from the meter?

Under current Code, bringing the neutral to the service disconnecting means is required. There has been some debate whether a disconnect for PV System line-side connection is actually a service disconnecting means... but in your case, it don't matter because you need the neutral. No transformer is needed from what I gleaned off a quick look at the inverter technical data.

 

[MurphyLuth]

More Questions

More Questions

Thanks Smart $. I have a few more questions regarding the existing installation:

When I bring the neutral to the service disconnect, can I use the existing 3 pole fuse/disconnect or do I need a 4 pole disconnect? One meter has a 3 pole fused disconnect and the other has a 3 pole circuit breaker.

One of the meters services a home that is about 1200 ft away from the pole/meter. There is no neutral at the 480 3-phase disconnect and only the three legs are run to a 3 pole circuit breaker at the house. The three wires from the cb are connected to a delta/wye step down transformer with a 4 wire output. In this situation it seems like the grid wye power is acting like delta power from the meter and then changed back to wye at the house. Do you see anything wrong with this installation? It would be a real PITA to extend the neutral at this point. This installation has been working well for 20+ years; but, going through this exercise has me curious now.

Thanks again for your information. You have been a tremendous help!

 

[Smart $]

Thanks Smart $. I have a few more questions regarding the existing installation:

When I bring the neutral to the service disconnect, can I use the existing 3 pole fuse/disconnect or do I need a 4 pole disconnect? One meter has a 3 pole fused disconnect and the other has a 3 pole circuit breaker.

One of the meters services a home that is about 1200 ft away from the pole/meter. There is no neutral at the 480 3-phase disconnect and only the three legs are run to a 3 pole circuit breaker at the house. The three wires from the cb are connected to a delta/wye step down transformer with a 4 wire output. In this situation it seems like the grid wye power is acting like delta power from the meter and then changed back to wye at the house. Do you see anything wrong with this installation? It would be a real PITA to extend the neutral at this point. This installation has been working well for 20+ years; but, going through this exercise has me curious now.

Thanks again for your information. You have been a tremendous help!

3-pole disconnect is all that is necessary. The neutral just connects directly, line to load, and bonded to GES and EGS.

Run to house is compliant on load side of service disconnect.

 

[shortcircuit2]

One of the meters services a home that is about 1200 ft away from the pole/meter. There is no neutral at the 480 3-phase disconnect and only the three legs are run to a 3 pole circuit breaker at the house. The three wires from the cb are connected to a delta/wye step down transformer with a 4 wire output. In this situation it seems like the grid wye power is acting like delta power from the meter and then changed back to wye at the house.

Do you see anything wrong with this installation?This installation has been working well for 20+ years; but, going through this exercise has me curious now.

You have 480volt service to a house?

 

[Smart $]

One of the meters services a home that is about 1200 ft away from the pole/meter. There is no neutral at the 480 3-phase disconnect and only the three legs are run to a 3 pole circuit breaker at the house. The three wires from the cb are connected to a delta/wye step down transformer with a 4 wire output. In this situation it seems like the grid wye power is acting like delta power from the meter and then changed back to wye at the house.

Do you see anything wrong with this installation?This installation has been working well for 20+ years; but, going through this exercise has me curious now.

You have 480volt service to a house?

Service yes... but stepped down for house usage. See highlighted text in quoted statement. This is not all that uncommon for houses that are uncommonly distant from service point.

 

[MurphyLuth]

"you have 480 service to your house?"--I get that asked all of the time . When the house was started in 1986 it was necessary to extend service from the existing utility pole 1200 feet away. This meant either running overhead service (unsightly), underground HV service (expensive especially because of mountainous rocky terrain), or low voltage underground requiring shallower trenching. We opted for 480 3-phase to reduce conductor and conduit size. The 480 is stepped down but some 480 is used to run 10KVA of air conditioning to reduce the load on the transformer.

 

[epm]

Corner

Corner

This is complicated for my little brain but will throw this out there.
Most 480 volt delta systems are corner grounded by the utility and a 4th ground wire is brought to the service. Since one phase is already grounded this 4th wire is somewhat redundant.
However this grounded leg will be 480 volts phase to phase but no voltage to ground at the service and the inverter. A small grounding transformer to generate a 277/480 neutral wire will provide a neutral reference wire for the inverter, but how will that affect inverter output on that grounded phase? Output current would still have to be phase to phase right? How can that happen when one phase is still grounded? Only the neutral is delivered to the inverter out of the grounding transformer.
This probably needs a sharp electrcial enginer review but what may be needed is a full size delta/wye transformer between the service and the inverter.

 

[jaggedben]

This is complicated for my little brain but will throw this out there.
Most 480 volt delta systems are corner grounded by the utility and a 4th ground wire is brought to the service. Since one phase is already grounded this 4th wire is somewhat redundant.
However this grounded leg will be 480 volts phase to phase but no voltage to ground at the service and the inverter. A small grounding transformer to generate a 277/480 neutral wire will provide a neutral reference wire for the inverter, but how will that affect inverter output on that grounded phase? Output current would still have to be phase to phase right? How can that happen when one phase is still grounded? Only the neutral is delivered to the inverter out of the grounding transformer.
This probably needs a sharp electrcial enginer review but what may be needed is a full size delta/wye transformer between the service and the inverter.

Well, I'm not a sharp electrical engineer, (that is, I'm not an electrical engineer ), but here goes...

Three phase inverters that are set up to output on 480 volts typically will not put any current on the neutral. They don't have to. Essentially they act to put power on the transformer they are hooked up to in a similar manner as the primary side does. The transformer always takes care of the neutral current. The inverter may or may not require a neutral connection, but if it does it is purely to measure voltage and phase to make sure that both the utility and the inverter stay within the proper ranges. As such, I'm doubtful that a corner grounded delta on the primary side of the transformer presents any issue for this type of inverter. The same is true of split-phase inverters.

Now, some single phase inverters can also be hooked up line-neutral on a wye service, at 277V on a 480/277 wye system. In that case of course the inverter does put current on the neutral. I believe that delta-wye transformers are typically set up so that line-neutral current on the wye side corresponds to line-line current on the primary side, but I could be wrong about that. A corner grounded delta may or may not present an issue in such a case. Same with an open delta. That would require an EE to answer.