Grid Tied System to Primary Voltage OK?

I'm currently proposing a 1 megawatt system to a food processing company that has a 1200A 3-Phase 12,000v main (Utility Company Primary Voltage). The main has a large capacitor and a large step down transformer to create 4000A of 277/480v 3-phase. There is plenty of room on the 4000A main to back feed into without overloading the bus bars. My question is if there is any problem with doing this with regards to the step down transformer or capacitor? I'm assuming the step down transformer acts in a fashion similar to that of a utility owned transformer? Does anyone have experience with this kind of system?


  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Grid Tied System to Primary Voltage OK?

    This scale of system needs licensed professional engineering, including the electrical end. The Engineer you hire will be required to provided answers such as this.

    In addition, this scale with require utility involvement and approval that is very different than smaller than 10K or 100K system have.

    The Capacitor is to improve the power factor as commercial customers get billed for power, whether the current is in sync with the voltage or not.
  • bottlefed67bottlefed67 Registered Users Posts: 6
    Re: Grid Tied System to Primary Voltage OK?

    I plan on using an electrical engineer in the future if the job comes to reality but at this stage of the game i'm just trying to cover any extra unforseen expenses that are obvious to someone more experienced.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Grid Tied System to Primary Voltage OK?

    My point is this IS the point you have to do all of your research, not later. If your hesitant to start with using professionals at this point, my guess is your pitching a project and not a customer requesting a RFP.

    Your interconnect panel/transform ect is a pretty minor thing compared to what the utility will require engineering wise, also, in case your not aware, on systems of this scale its not net metering, at best you might get wholesale rates which basically make projects of this size have poor ROI's at best unless there is signifigant grant money involved or above market purchase agreements for excess production.

    For a DIY thing, speak with the utility as that's likely the major roadblocks/issues and costs will be, next the building department, a 1MW system is a huge undertaking
  • BB.BB. Super Moderators, Administrators Posts: 32,813 admin
    Re: Grid Tied System to Primary Voltage OK?

    Pay special attention to what Solar Guppy says about billing rates and how the utility may account for the solar array's power production...

    In California, typically, commercial pays two different items on the bill...
    1. They pay a $$/kWHr -- just like residential
    2. They pay a reservation charge -- basically it is a charge based on the highest 15 minute power consumption in the last year (your reservation).
    3. There is also a "multiplication factor" based on "Power Factor" -- Basically if you have a bunch of motors that are 60% power factor (convert 60% of the current used into real work)--Their bill may be increased by 1/60%=1.67x the kWH reading on the monthly bill (the reason for the large capacitor bank at the building power panel--get the PF as close to 1.0 as practical).
    For a typical business, reservation charges are very roughly 50% of the bill. So, at best, a solar array may offset only about 1/2 of the bill (if net metered, less if they pay you the wholesale power price). [this is my understanding of how it works in California--things change and I certainly am not the expert here).

    Second, a solar array may actually increase reservation costs--remember a company may use "average power" 24 hours per day--A solar system outputs most of its power in a 4-6 hour period--potentially increasing the reservation charges by 2x or more (reservation charges don't care if you are consuming or generating the power--they charge both ways).

    Here is a thread I posted a while ago about a San Diego school district that found out they ended paying more for power when they installed solar arrays at a couple school--So much, that they stopped the program from doing further installs.

    In the end, much of the reason solar grid tie may make economic sense is because of special regulations that favor Solar RE power over other power sources. If you do not follow the rules exactly--and/or do not understand the consequences behind them (such as reservation charges)--you could end up with a costly white elephant.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bottlefed67bottlefed67 Registered Users Posts: 6
    Re: Grid Tied System to Primary Voltage OK?

    Actually this system would be a net metered installation. This user is running between $40,000.00-$70,000.00 per month in electricity charges, 20% of which comes from the current 1,200 kw demand charge. The PV system would not produce more than their current peak demand so upping the reservation portion of the bill is not an issue. The kwh savings based on peak, part peak & offpeak output will result in an annual savings of about $165,000.00. (31% of the annual kwh usage portion of the energy bill.) Not a lot when considering that the top line cost of the project is in the range of $5 million. However, after tax credits, the utility administered solar incentives, federal depreciation, and state depreciation values the net system cost is about $1 million and the payoff is in about 7 years.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Grid Tied System to Primary Voltage OK?

    I doubt your going to have 80% of the installed system cost recovered by tax credits and incentives. Your calculations appear to not be using NPV calculation for ROI.

    You gave a 165K annual saving, so I'm not sure what you expect to produce from a 1MW system or what your expecting for the utility to pay for excess energy produced, which can easily be 50% or more of the generation as big loads cycle on and off during the PV production over a day.

    BB would know about CA, here in FL, you will only get wholesale value for a commercial account, the Electric company isn't going to be paying retails for any kWhr from a system over 10kw here in Florida.

    And 5 dollars a watt installed seems below costs, that might cover the hardware, engineering and insurance for installation, but not the labor. If it really was that low cost, EVERY company in CA would be doing it, and they are not.
  • bottlefed67bottlefed67 Registered Users Posts: 6
    Re: Grid Tied System to Primary Voltage OK?

    According to the California solar iniative website the 862kw (CEC-AC rated) system should produce about 1.5 million kwh annually as installed.

    The utility will pay full retail rates based on time of use or production, for excess energy produced throughout the year as long as you are not a Net Energy Producer for the year.

    Under $5 a watt installed, yes. Sharp panels, SMA central inverters, and pro-solar ground mount racking system. I use my proven sub contractors for all the labor ie, concrete, rack erection, panel placement, electrical etc.,. It's proven to be quite efficient. The economics of scale helped a lot too with this one.
Sign In or Register to comment.