Wye not delta?

[fsck]

First, I appreciate my fellow's engineer's patience on what, to people who build MW power systems, must seem like a gnat's worth of watts.

Here's another view of the proposed system. The issue is the backup inverters do not supply 277 P-N, just 120V. So to feed the long cable, we need a step-up transformer of [208/120:480/277].

The 480 will be wye-fed by the poco, but when we move to backup, that's out of the picture.

The xfmr LV will be wye, given the source. To avoid the 480 floating, the HV side would be wye as well. (Y:Y)

True? Or should we just use delta on the HV, and create a reference above ground [three resistors?]?

Here's the other shoe. When in PGE mode, I'd like to switch [not shown] the transformer around to step 480 down to 120 for several functions. (One use is feeding the inverters to recharge their battery plant.) Is that a feasible idea, and if so, does it alter the wye-delta picture?

 

[rbulsara]

Seems like you need to hire a professional engineer.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Also, when referring to Pacific Gas & Electric, please use PG&E. PGE is Portland General Electric. When talking about utility requirements it is really helpful to know which utility.

 

[fsck]

davidbeach: I'm displaying my slant. To me, it is firstmost "pge.com" which is Pacific Gas & Electric. My bookmark is "pge"... Until you'd mentioned it, I'd never even heard of Portland General Electric.

The one I always found confusing was when living in Florida, where's there is Florida Power and Light [FPL], and also another "Florida" power company to the north.

 

[davidbeach]

So, they can't include the "&" in their URL, but that doesn't make it any less a part of their name. If you want answers that apply in the service territory of a particular utility, you have to make sure everybody knows exactly which utility you are talking about. Getting the right answer for one utility could cost you a lot of money when it is the wrong answer for a different utility. Engineering is about precision, not sloppiness.

 

[jghrist]

You can connect a delta 208V winding to a 208/120Y source.
See

http://www.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

 

[DTR2011]

fsck-

If you check where davidbeach works, you may understand a little better.

 

[fsck]

If you check where davidbeach works, you may understand a little better.

Noted. I've been editing my chicken-scratch one-lines so if I ever need to post one again, I hopefully won't insult him...

 

[PHovnanian]

fsck, two things will affect your selection of deltas and wyes. First is the need to support unbalanced loads, or loads with harmonic components. The other is the behavior of the inverters themselves. Even if you have balanced loads, the inverters may no be 'happy' if they feed into a system that won't pass their output harmonics. You'll have to check with the vendor on this requirement.

The switching of the 120/208 and 277/480 sides of the transformer to support multiple source configurations is certainly possible. Harmonic and zero sequence current flows must be considered for each configuration. One other thing to consider: Power flows through the transformer may reverse upon switching from utility to inverter power. In addition to providing the proper transfer switch configurations to meet code and utility requirements, attention to the possibility of multiple power sources into panels and proper labeling to ensure NEC compliance must be considered.

 

[fsck]

PHovnanian

Thanks for your insight. I'm trying to get a better read on the inverters re: several issues. I'm awaiting a response from the manufacturer.

The harmonics issue bothers me and has since I started thinking about the house. I have no good idea how bad a problem it is in general in households, but would SWAG it's getting worse and worse; here, with VFD's on the water pumps and blowers, and lots of switcher supplies for computers.....it's bound to be above average.

But in all configurations, we have a delta:wye transformer between the source and the loads; or the load is a delta motor. How effective is same in reducing them in the contemporary landscape?

 

[Zogzog]

Last I checked Portland GE was not a utility, I think everyone knew what he meant. When I worked for ABB a lot of people, mostly in the south, called it AB&B but I never got all upset about it.

 

[waross]

I once designed, installed and commisioned a system where I had to match invertors, DC loads, solar panels, battery chargers and 12V automotive alternators, with one of three 120 VAC generators.
Yes, it is possible to field fabricate an approved, three way transfer arrangement for small generator installations.
Then the customer operated his system not based on design, economics or good practice but on a basis of convenience.
Despite my protests and suggestions he managed to trash his battery bank in about two years.
I lost interest in further design for this customer.
Your design seems to be a series of "push the envelope" compromises.
I foresee a system that the owner has to "Live with" and adapt his life style to the electrical system, rather than the electrical supply being user transparent.
That's the best case.
Worst case is a general disaster.
I like a challenge but I am losing interest in this one.
Good luck.

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

 

[fsck]

Your design seems to be a series of "push the envelope" compromises.

I foresee a system that the owner has to "Live with" and adapt his life style to the electrical system, rather than the electrical supply being user transparent.

I'm hoping to give him the opposite. The big compromise is to have the BU inverters, battery plant & 25KW generator not at the house; but back at the solar shed. The upside is: quieter ops, no 1000 gal propane tank or battery bank at the house, and most importantly, PV array contribution during grid failure.

The downside from his PoV is it will not be no-break transfer. From mine, I have to crack some tough nuts like the inrush.

He's very happy with those tradeoffs.

Thank you for your help to date.

jghrist: Thank you for a intense and informative paper. I'll read it several more times before thinking I get more than a small percentage of it.

 

[1000V]

Give the guy a break. What's the point in splitting hairs and getting teary over an '&'.

 

[davidbeach]

Last I checked Portland GE was not a utility

Then what do you call a company that owns and operates generation facilities, owns and operates transmission facilities, and owns and operates a distribution system, serving more customers in Oregon than any other utility?

Not getting teary over an '&' at all, but the interconnection rules for PG&E are significantly different than they are for PGE. Hard to know what makes sense for one installation if you don't even known where it is.

 

[fsck]

I'm not upset at davidbeach's stance. As engineers, we should be pedantic when it is needed to prevent conclusion.

Hereafter, I'll endeavor to use "PG&E" or "pg&e" even on my own sketches.

 

[busbar]

An aside…there are 208?:480Y/277V transformers for this application. An example is jeffersonelectric.com/s3/site/catalog/3phaseVent.pdf 

 

[1000V]

Hey calm down davidbeach... we've all had a drink.

 

[waross]

I had high hopes going in to my project and thought that I had delivered a very good overall system. The customer proved that my design wasn't fool proof.
You may consider using the generator to bring the transformer online, sync the inverters and then drop the generator until it may be needed.
I would also be considering using more inverters so that the solar energy may be delivered directly to the grid and avoid the expensive and high maintenance batteries.


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

 

[fsck]

There are two functions to the system:

1) Grid-tie sell-back to PG&E.

2) Provide backup power from solar, battery & generator during PG&E outages.

3) It's NOT to sell stored energy back to PG&E; in fact they forbid same.

The batteries are short term; unless he gets something insane like

http://www.relocationspecialists.org/bat/cell.html

they won't hold very much....


Maybe I'm underestimating the issues, but I have a very clueful owner who may not have a EE degree, but have a technology education and career, understands limitations and compromises, and is willing to live with such. While this house has a far higher % of paid-for help; the previous one he & his spouse built themselves: dug foundation, supervised pour, had logs set, and did all the wiring, plumbing and finish work themselves. (I got involved on the controls and HVAC.)