Electrical Engineering Questions

GMD83GMD83 Registered Users Posts: 5
I work in sales and I'm not an engineer by trade. In order to talk with engineers at utilities, I need to understand the basics of how engineers think with regards to solar energy. The thing is, I don't understand the electrical side, therefore I don't know what questions to ask or how to ask them. I need help, in basic terms. Here is the best question I can come up with that sums up everything. I hope it makes sense:

Question: Given the phase, volts at the site, the main breaker amps (main service?), and transformer kVA, how would an engineer determine a solar array size in kW? Example:
  • 3ph - 480V - 600A main breaker: what would be the max system size that can be installed at this site? How did you arrive at that number?
  • If the site required more solar than your suggested size, what solutions would you try/consider so we could install a larger solar system?
    • Load side/ line side tap? How would this work? (I've heard engineers talk about these, but I don't know what they mean)
    • Replace the conductors? (Also, I don't know what this means)
    • Tie into the nearest 3ph line? How would this work? (Seriously, no clue what that means either)
    • Upgrade the transformer? What good would this do? (I'm out of breath)
    • Tie into the substation? How would this work? (My brain is about to explode)
Someone please help me!! Thank you for answering.

Comments

  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    GMD83 wrote: »
    how engineers think with regards to solar energy

    Short answer....They think about ELECTRICITY first and source second. They also think about the Watts and Kilo-Watts being used

    480V times 600 A = 288Kw, and they apply a time factor, hours, so now you have Kwh or Kwatts used in an hour and it is cumulative...
    a simple start...

    Is this 3 phase power for a water pump or??
     
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  • GMD83GMD83 Registered Users Posts: 5
    Everything is hypothetical in my posts. However, these are real world questions. That being said...

    Based on the client's utility usage, I can size the system easily. However, once I size the system, the utility (local CoOps) typically don't have the capability of "taking on" a large solar system. They claim the transformer is too small, or the conductor size is too small, or there's no 3ph line to the site, etc. In order to bypass all the phone calls and emails, if the client gives me the phase/volts on site/main breaker amps/transformer size kVA, how would I determine the absolute most/largest solar system size, regardless of the usage?

    I've been told that the size of the transformer, multiplied by the power factor, equals the max size of the potential PV array.
    • 167kVA x 0.8 = 133kW
    Also, I came across this calculation. When would I use this calculation?
    • (volts x amps x SqrRt 3 x pwr fctr) / 1000
      The SqrRt 3 would be for the 3ph and the power factor .8
      • I'm assuming the volts and amps in the equation refer to the transformer size?
      • Or do the volts and amps refer to the particular site? 480V x 600A? As in my example.
    Does this make sense? I'm doing my best in attempting to convey my confusion.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    GMD83 wrote: »
    I work in sales and I'm not an engineer by trade. In order to talk with engineers at utilities, I need to understand the basics of how engineers think with regards to solar energy. The thing is, I don't understand the electrical side, therefore I don't know what questions to ask or how to ask them. I need help, in basic terms. Here is the best question I can come up with that sums up everything. I hope it makes sense:

    Question: Given the phase, volts at the site, the main breaker amps (main service?), and transformer kVA, how would an engineer determine a solar array size in kW? Example:
    • 3ph - 480V - 600A main breaker: what would be the max system size that can be installed at this site? How did you arrive at that number?
    • If the site required more solar than your suggested size, what solutions would you try/consider so we could install a larger solar system?
      • Load side/ line side tap? How would this work? (I've heard engineers talk about these, but I don't know what they mean)
      • Replace the conductors? (Also, I don't know what this means)
      • Tie into the nearest 3ph line? How would this work? (Seriously, no clue what that means either)
      • Upgrade the transformer? What good would this do? (I'm out of breath)
      • Tie into the substation? How would this work? (My brain is about to explode)

    Someone please help me!! Thank you for answering.

    Hi GMD83,

    Well I could probably easily write 50 pages answering your questions, but Ill try to take a quick stab at it. First, note that the an existing service size is often not the limiting factor to how much PV one can install. I would say that much more often than not, tax/rebate structure, electrical usage, finances, and/or available space for panels determines the system size rather than electrical service capacity. For example, I worked on a 1 meg (DC) PV system last winter. It occupied about 4 acres (200X800), was around 3200 panels. Despite being a 1,000 KW system, the actual inverter capacity was 720 KW (due to losses, real world conditions, etc) and this is only 866 amps at 480 volts - really not a very big service.

    Another thing to note is that usually the term "system size" refers to the sum of the STC rating of all the solar modules, it is the max inverter output current that you would size to an existing service. As you may have noted in the first paragraph, DC size often exceeds actually inverter capacity by quite a bit.

    Yet another thing to note is that you usually dont have to think about transformers - that serving transformer is under utility control and they will take care of that its their problem. You generally wouldnt connect through customer owned transformers as you would purchase inverters that are matched to the service voltage.

    There are two ways you can connect an inverter's output to an electrical system: one is to connect to a circuit breaker in a panel ("load side"), or you can tap into the service entrance conductors before the main disconnect(s) ("line side tap"). The former is where you run into the "120%" rule which basically says that the sum of all of the overcurrent devices supplying a conductor or busbar cant be more than 120% of the rating of the busbar or conductor. The fear is that one could theoretically exceed the rating of the panel - usually the breaker feeding the panel is sized to protect the panel from an overload but if you supply it with another source such as PV you could possibly overload it. No one really knows how they came up with the 120%. Basically in practice this limits the size of a system that you can connect to a breaker so medium to large size systems require a supply side connection and in that case you can go up to the rating of the service which is the ampacity of the service entrance conductors. IT can be a bit tricky because the main service disconnect rating can exceed the amp rating of the conductors and often with larger services you will have multiple service disconnects so you really have to look at the size of the service entrance conductors to get an accurate figure. But in any case, the max inverter output current would be the ampacity of the service entrance conductors times the voltage times 1.732 (ignore the 1.732 if it is single phase). Actually it would 80% of that because PV is a continuous load so you can only use 80% of the conductor ampacity.

    If the service isnt large enough, you have a few options 1) upgrade the electrical service. Often one could add another set of service entrance conductors and another service panel rather than rebuilding the whole service - this is when having an electrician who knows his stuff is critical. 2)bring in a second service although the voltage or phasing would have to be different than the existing service and the utility may have a "1 service" policy.

    Hope that helps!
  • BB.BB. Super Moderators, Administrators Posts: 31,153 admin
    Many of the questions you ask are really something that your Network Engineers need to answer.

    It is a bit strange--But the energy flow through the AC mains is very similar to how the Battery in an car works with the loads and the generator (really alternator).

    More or less, the Solar GT system simply pumps energy "backwards" into the AC panel. And if the Loads are greater than the GT power, the power meter turns "forwards" (charges customer for AC power used). If the GT solar outputs more energy than the local loads, the meter turns "backwards" (the customer is "credited" with energy generated).

    The details matter... For example, in our area (as I understand) that for three phase power systems, GT solar needs to be "balanced" across the three phases within 10 kWatts (i.e., the largest single phase GT inverter would be 10 kWatt maximum on one phase. Otherwise, the rest of the GT inverters need to be "balanced" across all three phases--within 10 kWatts or less between phases).

    Also, there are different types of service (delta, Wye, or even open wye/delta, etc.). Each has its own requirements.

    http://www.programmablepower.com/support/FAQs/DF_AC_Distribution.pdf

    Also, just to give you an idea of sizing the system... Say you get around 6 hours per day of solar power. And you have a facility that takes roughly constant power over 24 hours (refrigeration or water pumping or similar). To have a "net zero" power usage, the solar power system would need to be ~4x larger peak power than the loads (sun is 6 hours per day vs 24 hours per day loads).

    For the network engineer--This means that the AC mains may need to be 4x larger now that solar has been added. And for many commercial customers (at least in our region), there are two major components to the bill... Once component is kWH hours used. The second is a "reservation charge" based on the top 15 minute period of power usage in the last 1 or 12 months.

    A typical plan may have have ~50% of charges based on kWH costs, and the other 50% of the bill may be based on reservation charges... So, it is possible for a commercial utility bill to be higher with GT solar power vs their original power bill (i.e., the "credit" on the power bill from GT solar is less than the 4x increase in cost of the reservation charges).

    So--A large GT solar plant/installation may require the network engineer to "up size" the Utility's infrastructure to handle the increased power flow.

    In a few areas (like downtown San Francisco--as I understand)--There are parallel AC transmission line feeds (redundancy/extra power feed)... And these can have "back feed" prevention switches (to prevent one AC transmission system feeding backwards into the second/parallel transmission system).

    And if you look at various residential / commercial billing plans--You will see that there are lots of complexity for commercial, agricultural, time of day, seasonal, power factor, differential charges/credits (my charges are: I buy retail, I sell retail, with time of day usage pricing, and tiered energy usage).

    As Ethan says, this is a 50 page explanation about the various issues. And sometimes things happen that are just out of left field--There have been times where wind farms generate more power than the utility can carry (and ignore utility phone calls to shut down generation)--Or in other places (with smaller networks) where a major GT Solar installation can cause major voltage regulation issues for the network (sun up, network over voltages).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GMD83GMD83 Registered Users Posts: 5
    Thank you all for the input. I have a pretty good grasp on net metering, system sizing based on the kWh, stringing inverters, types of inverters, etc. The only area I have a hiccup on is the electrical side. Here is a live project I am working on that might be a better example:
    • The client used in the last 12 months 717,300kWh for their farm
    • The utility is a local coop
    • At the farm it is: single phase, 120/240V, and a 200A main breaker (assuming there is enough room)
    • According to the utility, the transformer at the site is 167kVA
    To size the system, I understand the math: 717,300kWh/365days/5sun hours/.77eff factor = 510kW needed to offset potentially 100% of their usage.
    • Or you can just take the kWh and divide it by 1.4 to get roughly the same answer = 512kW. So for sake of argument, let's just make it 500kW for a round number.
    Will a 500kW system work on this site? Reasons? I suppose, according to Ethan, it would depend on the service entrance conductors. But if we replace those with a larger conductor (minimal charge I hope?) would this proposed system size work? If it won't work, what solutions would you propose?

    In all cases, this will go into engineering before it gets installed. I'll let them worry about breakers and additional equipment. So as to not waste time with the client, I just what to give the client a high level estimate on what we can do for them. I don't want to spin my wheels telling the client and future clients "sure we can put up a 500kW system, no problem" when in reality we can only install a 20kW system or something, based on what's at the site.

    There is a lot of information that can indeed be 50 pages long. I'm not looking to get into deep deep engineering, just a superficial understanding of what can and cannot work given the parameters of the site.

    Or you can put it this way: you were given this information above and were told, create a proposal. What size system would you propose? Why? How did you arrive at that size? Would you need more information about the site? What questions would you ask the utility?
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    Something is not right with your numbers. 717000 KWH is an average of 340 amps at 240 single phase, yet you say they have a 200 amp main....?

    You will have a demarcation point in an electrical service called the "service point". On one side of that point is the customer equipment where the NEC would apply, and on the other side is the utility side which is under utility rules and their control. The size of the utility transformer is not your concern - utilities usually will overload their transformers up to 140% and usually go by an operating temperature calculation not KVA size. So in short, if you want to install a system of XXX KW, you would take that to the utility and they would either say yes we can do that with the existing infrastructure, or no we cant, and it will cost you $XXX for us to upgrade it. That is the bottom line.

    As far as the customer side, if that equipment is not large enough (max size being ampacity of service entrance conductors X .8 = max inverter output current, but then in theory you can put as much PV as you want into those inverters) then as I said you need an upgrade. What I will add from my last post is that if this is a ground mount system and your utility allows virtual net metering, you could bring in another service just for the PV which could be a great option if say the array was in a field and there were power lines close by. Then you would give your load/supply data to the utility and they would come back with a cost to supply that service.
  • GMD83GMD83 Registered Users Posts: 5
    First off, thank you so much for posting and helping me out, everyone! I'm just some knucklehead trying to figure out how to get by and streamline the sales process. So, thank you!!
    So in short, if you want to install a system of XXX KW, you would take that to the utility and they would either say yes we can do that with the existing infrastructure, or no we cant, and it will cost you $XXX for us to upgrade it. That is the bottom line.

    This is what I have been doing, so it's good to hear it from a professional. At least I'm on somewhat of a right track :) The 200A main, I forgot to put, was an assumption. I figured 200A because it's a 120/240V single phase site....I suppose we can say for certain that my assumption was wrong lol. How did you arrive at an average 340 amps?

    When I talk to the utilities, these are the questions I typically ask:
    • Given the site information, we want to install xxx kW of solar. Will this work?
    • What kind of upgrades would be needed, if needed?
    • What phase runs out to the site (if I'm close to the site, I'll go out there and it usually says the phase on the meter or transformer)
    • What size kVA transformer is at the site? (I know you just said this isn't my concern, but every time I talk to the utility, they tell me the transformer needs upgrading)
    • What would be the cost of upgrades if needed?
    • And then the typical: net metering agreement, interconnection agreement, avoided cost, contract length, etc.
    The thing is, when I ask these questions, the engineers don't understand what I'm saying. Am I not asking the right questions? Perhaps not asking them the right way? They virtually look at me like I'm some dumb hick (not far from the truth). We had a job where it was potentially going to be a 40-60MW solar farm. I talked to the engineer at the utility and at first it seemed like it would be feasible. But after a month of talking off and on, the engineer was even concerned with 500kW, let a lone 40MW. Well why the hell didn't he tell me this from the start instead of wasting a month of talking?!?! His concern was about having too much load? on his line or service or something; that they would have to build a substation. Where I work, money isn't an issue. Can it be done?....great. It's gonna cost this much?....perfect. Done and done, let's roll....what's the hold up? Drives me nuts!!!

    I suppose I just want to minimize my talking to the engineers, because I typically get nowhere. Their terminology confuses me and I don't usually understand what they are saying half the time. This was the reason for this post from the start. I can tell what the client would need (size kW array), but what can physically fit on the site I'm unsure. This is where I need guidance.

  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    GMD83 wrote: »
    How did you arrive at an average 340 amps?

    717300 kwh ÷ 356 days = 965 kwh per day

    965 kwh per day ÷ 24 hours = 81.9 kwh per hour = 81.9 kw = 81900 watts

    81900 watts ÷ 240 volts = 341 amps

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • BB.BB. Super Moderators, Administrators Posts: 31,153 admin
    And to break even with solar, you would need around 4x solar current based on around 6 hours of useful sun per day.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GMD83GMD83 Registered Users Posts: 5
    vtmaps wrote: »

    717300 kwh ÷ 356 days = 965 kwh per day

    965 kwh per day ÷ 24 hours = 81.9 kwh per hour = 81.9 kw = 81900 watts

    81900 watts ÷ 240 volts = 341 amps

    --vtMaps

    So based on that math, the main breaker amps would have to be larger than 340? Am I understanding that correctly? Or is the right terminology the main service would have to be larger than 340A...?
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    GMD83 wrote: »

    So based on that math, the main breaker amps would have to be larger than 340? Am I understanding that correctly? Or is the right terminology the main service would have to be larger than 340A...?

    The service size is how many Amps the utility is willing to commit to deliver. Although they rarely have the equipment in place to do that since they know that the NEC overestimates the actual user demand.
    You can put a smaller breaker or fuse on it, but POCO will not allow you to put in a larger breaker than the nominal service size.
    If there are multiple main breakers, then all restrictions on the sum of the breaker sizes is off but the calculated load cannot be larger than the service size.
    SMA SB 3000, old BP panels.
  • Ethan BrushEthan Brush Solar Expert Posts: 231 ✭✭
    GMD83 wrote: »

    So based on that math, the main breaker amps would have to be larger than 340? Am I understanding that correctly? Or is the right terminology the main service would have to be larger than 340A...?

    Right. 340 amps is the average power consumption, and of course it will be (presumably) higher during the day, and lower at night. I would guess you have something like a "600 amp" service which could be a 600 amp main, but the service entrance conductors could be three sets of 2/0 copper for a total of 525 amps which is allowed by the "next size up" rule (NEC 240.4(B)). Or maybe it is three sets of 3/0 copper or 250 aluminum which would give you a "full" 600 amps.
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