PV System Backfeeding to Utility thru padmounted transformer 1

[Omarelec]

I have a PV grid-tied system at 480/277volt tied to the service entrance switchgear. The secondary of the pad mounted transformer will be 480/277volt solidly grounded system. What configuration the transformer needs to be at the primary? what concerns I have to worry about when backfeeding?

 

[Omarelec]

PRC, this is office building and the transformer I'm asking about is the one tied to the utility to step-down service to the building to 480/277v. Most of the time the building will draw excess power needed by loads from utility. in some cases solar will backfeed to utility. I don't think I want to have ungrounded wye tied to the 480v switchgear.

 

[catserveng]

Most of the inverters I come across in the field require a 4 wire connection, but there have been exceptions, Every inverter type I've seen so far does publish their requirements for the low side of the transformer. In the areas I work in most, the utility defines the high side and protection required for interface.

MikeL.

 

[waross]

Thanks Mike. I suspected that the four wire connection may be needed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

These guys seem to be showing up in a lot of sites in my part of the world,

http://www.blueoakenergy.com/blog/distribution-and-substation-transformers-for-utility-solar-p

I have done a couple of projects with one company that had a published spec for their preferred inverters,

https://solectria.com//site/assets/...50xtm_isolation_transformer_specification.pdf

One site recently had a mix of inverters and the solar contractor was not diligent in his review of inverter requirements, he bought all the same connection style of transformer, only to find that his smaller paralleled inverters didn't work very well on that particular connection. So we had to scramble to find a properly configured transformer to minimize job delays.

At least from my end in the field, I'm not seeing a "one size fits all solution". California utilities all seem to have their own ideas, even if we end up doing pretty much the same in San Diego and Sacramento, it still has to go thru a pretty complex and time consuming review, probably more to do with how the utility connection is going to interact with the system rather than the system itself, but a lot of these solar contractors want to slam and go, so it gets pretty interesting at times.

MikeL.

 

[prc]

Omarelec, the PV must be feeding power to 480 V through some transformer. I was talking about this one. The DT that you are considering is always Dyn in our part of the world.

 

[LionelHutz]

OK, you're saying this transformer will feed an office building and will also have a PV system connected to it in a net metering type of system. Well, this means the requirements for the transformer are fixed by the office building needing a properly installed code compliant service. Most likely delta primary and Yn secondary on the transformer. The Yn secondary is a basic service requirement in NEC land. You need to double check with the utility, but the transformer used to step down to a customer service is usually delta primary.

The PV system will have to be capable of connecting to this building service. You don't change the type of building service into something that isn't code compliant for an office building to suit the PV system.

 

[waross]

what concerns I have to worry about when backfeeding?

Doing this without notifying the utility is more of a concern than the transformer connection.
You may create a danger to utility workers. You must check with the utility and meet their requirements.
Doing this without the permission of the utility may invoke fines and, in the event of an injury to a utility worker, criminal charges and even jail time.
You have not responded to comments concerning the utility requirements.
You must have an agreement with the utility before you export any energy.
The rate for exported energy is often less than the rate for use of energy.
Do you choose to ignore that which you do not understand.
I could say;
"Do so at your own peril."
But in this case you may be putting others at peril also.
Bear in mind also the cost.
Modern KWHr meters do not run backwards when you export energy, they sum the outgoing energy with the incoming energy.
With the default programming of modern KiloWattHour meters, you will automatically be charged for all energy that you export unless the utility becomes involved and reprograms the KWHr meter.
In the event of an incident you may void the building insurance.
There are invertors available that monitor the current drawn from the grid and at times of light loading limit their output so as to never export power.
Please tell me that your invertors have this feature.
PS. You should be able to tie a bank of invertors rated at 277:480 volts to the secondary of a building supply transformer with no regard to the primary winding;
However, in the event that you have a heavy unbalanced load in your building at any time, you may find a resulting circulating current in the delta primary.
The utility may choose to ignore this issue or they may have strong opinions on the matter.
CONSULT YOUR LOCAL UTILITY.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Kiribanda]

Omarelec,
When the PV or any other IPP is connected to Utility (parallel feed), it is a requirement to maintain the Utility
network as an effectively grounded system from ground fault protection point of view. In order satisfy this requirement
the Utility (high side) of the transformer should be WYE and not DELTA. Therefore, the transformer connection is YNd.
If your system interconnection studies prove that a YNynd transformer also provides an effectively grounded system
when the PV is connected then, you can have a YNynd (tertiray delta) too. But normally it is YNd. Hope this helps.

 

[Omarelec]

Thanks all for your responses.
Bill, we are coordinating with utility during the design phase. The utility wants us to provide the transformer and my question and concerns about the transformer. My inverters have that feature.
My question is about the configuration of the transformer. I see most answers is to have what typically is provided for this type of building delta grounded Wye where the inverters connected to the GY side

 

[waross]

If the utility wants you to replace the service transformer, ask them for their requirements.
The utility may or may not have an issue with a delta high side.
The invertors may have an issue with a delta or three wire wye connection on the low side.
Ask the utility what they want for the high side.
Consult the invertor instructions or manufacturer for the low side requirements.
I suspect that if the utility wants you to supply a transformer they may want a wye high side.
Not all invertors will work with a delta low side.
If the utility will accept that your invertor will not backfeed into the grid there should be no reason to change the existing transformer.
As you see there is no common agreement as to the proper transformer configuration for a PV installation.
A delta facing a generator is preferred , but not all invertors will work with a delta transformer connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Omarelec]

Bill
This is new installation so there is no transformer to replace. I agree I have to check with utility about the preferred primary configuration but I suspect I'll get an answer or the correct answer.
Thanks

 

[prc]

Omarlec, To my knowledge inverters are never connected to yn (ie grounded Y) always to y (ungrounded). That is why I thought an intermediate transformer between inverter and yn.

 

[Omarelec]

prc, I have to check with the inverter manufacturer on that but you cannot have ungrounded neutral to the 480/277v serving the building.

 

[waross]

prc; A clarification please. Those working with utilization voltages, (120/240V, 120/208V, 277/480V, 347/600V) make a clear distinction between a ground connection and a neutral connection.
Those working with distribution voltages tend to use the term ground to describe neutral connections.
You may have a yn connection that is either grounded or ungrounded.
Do mean neutral connected but ungrounded?
Also, in North America we have code considerations.
We are required to ground any circuit incorporating a neutral.
This rule will apply to buildings falling under the scope of the CEC or the NEC and most co-generation type systems.
This rule may not apply to large scale stand-alone PV installations.
Some AHJs may not allow an inverter system to be run ungrounded if a neutral is present.
Some AHJs may allow an isolating transformer and an ungrounded neutral.
Some AHJs may require an inverter that does not incorporate a neutral if the inverter is to be used ungrounded.
It depends.
Check with your local AHJ early in the design stage.
This is a general suggestion and not aimed at you or a criticism Omarelec.
If your inverters are suitable for use with a grounded neutral then all is good.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Inverter as just a bunch of power electronics needs a transformer on the output. Inverter as a packaged, listed, piece of equipment most likely includes that transformer within the overall enclosure.

 

[Omarelec]

Bill,

I agree Yn doesn't tell me if the neutral is grounded or not. We will have grounded neutral at the service entrance.

AHJ is adopting NEC 2017 although it is outside US. Luckily it is not my local AHJ

 

[prc]

Agree Bill. May be a better designation may be yng. Solar transformers are manufactured as Dyn ,but n is kept floating ie ungrounded. In one solar firm, it was reported that inverter units are failing frequently. When I checked, it was found, yn was kept grounded as normal practice in similar step down distribution transformers. When yng was made to yn, the failure stopped.

david, solar transformers are pretty big. Normal size in India is 2-5 MVA with two secondarys. There are ratings as high as 12.5 -16.5 MVA with 4 LVs ie 4 inveter units feeding to a single transformer. Interestingly these large units are made with two transformers inside a single oil filled tank with one HV terminals and 4 pairs of LV terminals brought out. Such ratings are required when country is planning to add 100 GW of solar power to grid.

 

[waross]

Thanks for the explanation prc.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FreddyNurk]

Actually, prc's latest statement made me think that perhaps there is a misinterpretation as to what function the transformer is performing, and where its located. This is the same as dpc's statement about inverter transformers.

The utility in question (generally) only cares about what gets connected to its network, and in my experience these end up being normal distribution (i.e. Dyn) transformers, with grounded neutral accepted as being required. Utilities will generally then have requirements on the protection scheme in use (e.g. anti-islanding inbuilt in the inverter, NVD on the high side of the transformer). Its likely different in different locations (hence the 'ask the utility' statement), and its entirely possible that for this connection some utilities may request a YNd arrangement, as per expected practice to feed into a network.

What made me think of it is prc stating 'solar transformers' - the inverter I came across needed an isolating transformer to connect it to the network, which is entirely in line with prc stating the neutral isn't grounded. However, the transformer that might couple the inverter to the site's distribution system may or may not be the same as what is used to connect the site's distribution to the utility's network.

At what waross called utilisation voltages though, I'd expect it'd be a Dyn transformer for the utility connection, with possible additional utility protection being required, but that is up to the governing utility.

 

[waross]

By code a 277:480 Volt system must be grounded if the neutral is used. There are not many buildings where the neutral is not used. I have seen lots of 277 Volt lighting but never 480 Volt lighting in a building.
If Omarelec's inverter must have a floating neutral there will need to be an isolating transformer.
Remember that this is a system intended to supply part of the energy needs of a building.
prc is sharing his experience with much larger systems and higher voltages.
It may be apples and oranges.

Bill
--------------------
"Why not the best?"
Jimmy Carter