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?

 

[davidbeach]

Those are questions you need to asking the serving utility. The answers one utility gives will be different from what another utility will give.

 

[Omarelec]

What about if the pad-mounted transformer is owner provided and not by the utility? are there specific requirements for the transformer?

 

[davidbeach]

At least in the US, the serving utility has to be involved in the process. Even if the connection is deep in the customer’s system. There is also no universally “right” answer.

 

[waross]

Forget the transformer. There are specific requirements for back-feeding no matter what type of transformer or the ownership of the transformer.
Each utility will have their own requirements.

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

 

[Omarelec]

The project is outside the US. The utility is asking to comply with NEC 2017 and IEEE standards. The utility is not providing the transformer and asking to be customer provided.
Typically we specify delta-wye padmounted transformer but I'm not sure how it will react during backfeeding.

 

[davidbeach]

Then you’ve already got it “wrong”. Absent any other requirements you’re describing a GSU and a GSU should always be delta on the generator side and grounded wye on the system side. If you really think you need a grounded eye on the generator side the best choice would be a wye-delta-wye.

 

[davidbeach]

And, as implied earlier, there is no consensus around that answer. How does your nearest neighbor with a similar system do it?

 

[Omarelec]

Apologies I should have made clear. This is not GSU setup. The PV system is mainly to reduce dependence on utility. We will still draw current from utility during peak loads but during light loads, we will backfeed to utility. We are not storing the extra energy produced.

 

[Omarelec]

I'm not aware of any similar setup over there.

 

[Omarelec]

So I'm thinking in my case in lieu of using delta (utility side) to grounded wye (building side), is to use grounded wye to grounded wye

 

[waross]

Electronic revenue meters have been around for quite a few years now.
Among other features they typically measure power flow in both directions. The KWHr IN is saved in one register and the KWHr OUT is saved in a second register.
The default setting will add both registers and display the sum of both registers. This is the value that the meter reader will see. This is the value that your charges will be based on.
You may end up paying for all the power that you export if you do not have an agreement to export power to the grid.
Years ago I saw a small wind turbine (60 kW) installed and connected without notifying the utility or getting permission.
It worked well until the owner received his next utility bill and found that he had been billed for all the power that he had exported as well as all the power that he had consumed.
As well as billing issues, there are issues with backfeeding into a fault and backfeeding into a dead line.
It takes a little more than NEC compliance.
A delta winding distributes a single phase load more evenly on a generator.
An unbalanced load may be considered as a balanced load plus a single phase load on one or two phases.

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

 

[catserveng]

https://solarprofessional.com/artic...?v=disable_pagination&nopaging=1#.W3jbM85KjRY

See if this helps, MikeL.

 

[Omarelec]

Thanks all for your input. Mike thanks for sharing
Based on the website I'll go with delta_grounded wye

 

[Mbrooke]

GSU and a GSU should always be delta on the generator side and grounded wye on the system side

I beg to disagree- I know of many cases where a cogen back feed was wye-wye or delta-delta at a hydro station despite the system being solidly grounded wye.

 

[davidbeach]

Absent any information about the surrounding system, I'll stick with my statement about GSUs. Doesn't mean that's always the way it happens for smaller systems.

Many inverters have an output isolation transformer in the package that is delta on the electronics side and grounded wye on the terminal side. A lot of wind farms have tower transformers with delta windings on the collector circuit side and the need for separate grounding transformers on the collectors. I've not seen a good explanation as to why the tower transformer can't be wye on the high-side. There's not a good place for the grounding transformers; put them at the switchyard so you can monitor currents and you don't have a sufficient ground reference at the far end of the collector. Put them out on the collector so they provide a better ground reference and they're much more difficult to monitor.

Like I said, no consensus. Lots of opinions though.

 

[waross]

With a wye:wye facing the generator and the wye point connected to the neutral, the maximum safe single phase KVA load is 1/3 of the set KVA rating.
With wye:delta facing the generator, the maximum safe single phase KVA load is 2/3 of the set KVA rating.
At less than full load, this translates into less internal voltage drop, and less phase to phase voltage variation with unbalanced loads.
The generators that I am familiar with often require about twice the excitation current at full load as at no load.
Worst case: With one phase fully loaded and the other two phases unloaded the unloaded phases may have a ratio of as much as 2:1 compared to the loaded phase. Depending on the AVR sensing scheme, one or all the voltages may be far from normal.
A delta facing the transformer will place an equal KVA loading on all three phases. Two of the phases will be loaded at a 50% power factor.
While this may not apply to solar generation, it is the reason that a delta facing a generator is the preferred arrangement.

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

 

[waross]

David. The delta in the wye:delta:wye you suggest will go a long way towards spreading the current, equalizing the voltages and balancing unbalanced loads across the three phases.
It will be good but how good will depend on the relative impedances of the generator and the transformer windings.

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

 

[prc]

The type of transformers required for this application are fully covered in IEEE Standard C57.159-2016 Transformers for DPV power generation systems. Most popular connection for these transformers is Dy ie D =HV to grid, LV to PV . This is quite opposite to normal GSU where it will be YNd( Yn to grid, d to generator). Please note that LV is y and NOT yn. It is always ungrounded. Occasionally YNd or YNy are also used for this application.

 

[waross]

It is always ungrounded.

Is this a three wire connection or a four wire connection to the inverter? The wye point may be connected yet still be ungrounded.

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