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  • Utility smart electric meters, are they measuring differently?

    Hello, my name is Mattias and I work with R&D at an energy Company in Sweden. I work with a lot of questions but try to stick to PV-related questions as much as possible. We have seen a "problem" here in Sweden that different electric grid owners configures their smart electric meters differently. I am preparing a contribution to a PV Conference in Washington and I want to present a connection between my research and how the situation is in North America.

    In Sweden, all houses including single family houses are connected to all three phases. If you install a PV system and the internal load or maybe the production of electricity is not even between the phases the Electric meters records different values. For example, if the use (import) of electricity on phase one and two is 1 kW momentarily and the export on phase three is 2 kW. Then one type of electric meter records momentarily 2 kW import and 2 kW export. The other type records 0 kW (1+1-2) on import and exports. The momentarily values are then summed to an hourly value that is collected by the grid owner. I think it is 15 minutes in USA. The collected value is not netted somehow and can be both imported and exported values for that timeframe.

    My question is: Is it the same in North America? I cant see a national regulation on this and then I assume that there might be different configurations. For a split-phase Connections, I assume that both legs are measured separatly and the same problem can occur there as well.

    Please use your gathered expertize and help me. I have e-mailed NREL but this seems to be new to them (?) and I have not get any answer from them more than that they are looking at the problem and will return with information...

    Best regards!

    Mattias G, Sweden

  • #2
    Hello MattiasG and welcome to Solar Panel Talk

    While I am not 100% sure I believe that most if not all US homes are powered by 120v/240v single phase (3 wire) 2 phase wires & 1 neutral/ground wire system.

    The kWh energy that is imported or exported (from a solar pv system) is measured across the 240v (2 wires) "phase to phase" path and not each of the 120v "phase to neutral/ground" paths.

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    • #3
      Thanks, and thanks for a fast answer, it sounds reasonable that it is measured across the 240V. I know this is not a big topic for you since most of you can gain the net-metering scheme but when it ends and there is a different value between imported and exported electricity this can make a difference and as we have seen in Sweden, quita a large difference as well.

      Comment


      • #4
        Originally posted by MattiasG View Post
        Thanks, and thanks for a fast answer, it sounds reasonable that it is measured across the 240V. I know this is not a big topic for you since most of you can gain the net-metering scheme but when it ends and there is a different value between imported and exported electricity this can make a difference and as we have seen in Sweden, quita a large difference as well.
        I guess I am surprised that the pv inverter being used over in Sweden for the home isn't a 3 phase model which should "export" equally on all 3 phases at the same time like they do here in the US at the commercial or utility solar farms. .

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        • #5
          For larger systems the production is distributed between the phases. PV systems on private houses are often sold "as cheap as possible" and it is not uncommon that the systems offered have a single phase inverter. A single family house use almost only equipment connected to one phase and the power use then varies a lot between the phases so a three-phase inverter decrease the "problem" but does not solve it.

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          • #6
            Originally posted by MattiasG View Post
            For larger systems the production is distributed between the phases. PV systems on private houses are often sold "as cheap as possible" and it is not uncommon that the systems offered have a single phase inverter. A single family house use almost only equipment connected to one phase and the power use then varies a lot between the phases so a three-phase inverter decrease the "problem" but does not solve it.
            I would say that a typical US home is a mixed bag of 120v and 240v loads. The 240v loads would be the larger ones like: air conditioner & heating system, stove, cloths dryer, water heater and for me a pool pump. All other loads (lighting, appliances powered from a receptacle, fans, clocks, etc.) are 120v.

            Even with this mixed load the electric meter calculates the kWh used across the 240v and not the individual 120v circuits.

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            • #7
              Originally posted by SunEagle View Post
              Even with this mixed load the electric meter calculates the kWh used across the 240v and not the individual 120v circuits.
              It's been a while since I've had to do much with transformers - but I think this makes sense from a technical sense too.
              I think a split-phase transformer that has 3kW and -3kW on the two halves will give the same loading (from the view of the power company) as a transformer that has 0kW load.
              (Or in practice at least close enough to the same that it's noise in the system)
              I think with a 3-phase system the power company sees a difference at the pole when the phases are unbalanced. Most likely the amount of being unbalanced winds up largely averaging out once you look at a larger group (ex. a city block of homes; or in the US a city block of businesses)

              I believe that most if not all US homes are powered by 120v/240v single phase
              I think the vast vast majority of single-family houses in the US are 120V/240V.
              There are a few farm houses that are 3-phase. (3-phase was installed for the farm and the house gets the same) And probably a few other cases.
              And there are some apartments/condos that are 3-phase.
              (Probably other cases too - those are ones I know from family members having that)

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              • #8
                Originally posted by SunEagle View Post

                I would say that a typical US home is a mixed bag of 120v and 240v loads. The 240v loads would be the larger ones like: air conditioner & heating system, stove, cloths dryer, water heater and for me a pool pump. All other loads (lighting, appliances powered from a receptacle, fans, clocks, etc.) are 120v.

                Even with this mixed load the electric meter calculates the kWh used across the 240v and not the individual 120v circuits.
                Yes something is wrong and stinks. If you have 3-phase would indicate a vary large load indeed. Switzerland uses 240 VAC 50Hz single phase like most of Europe.

                OP are you sure Swedes use 3-phase? News to me. At what voltages. Here in the USA is either 488/277 for commercial usage, and 208/120 for small commercial and mega house mansions. Most all residential are single phase 240/120.
                Last edited by Sunking; 01-10-2017, 06:31 PM.
                MSEE, PE

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                • #9
                  Thanks for all discussion/information. Here in Sweden and some other countries there are three phase connections to all buildings and the voltage is 400/240. The problem is of course that the own consumed electricity differs depending of the electric meter configuration and apart from the economic value the new building code also use own consumed electricity when calculating the energy performance of the building so it is not only a strict economic value of produced electricty that is affected. This problem has been seen in Sweden and Denmark (as I have seen) and also in Australia: http://reneweconomy.com.au/3-phase-t...seholds-17282/

                  Comment


                  • #10
                    You might find a paper interesting from someone in your country http://www.gke.org/pdf/Electricity%2...202010%201.pdf He likely has knowledge on this issue. a fairly heavy poster on an engineering forum and seems willing to share.

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                    • #11
                      Originally posted by SunEagle View Post

                      Even with this mixed load the electric meter calculates the kWh used across the 240v and not the individual 120v circuits.
                      '
                      Actually the typical US meter sums the current in the two hot legs (changing sign of one) and multiplies by the line to line voltage divided by two.
                      That is an exact measure of the power drawn regardless of the nature of the loads IF the neutral wire is always exactly halfway between the two hot leads.
                      The meter has no connection to the neutral to measure either current or voltage.

                      In reality, if you have a large unbalanced load the neutral voltage will be shifted away from ground by a few volts, so the meter is slightly biased in favor of the Power Company. (POCO).
                      SunnyBoy 3000 US, 18 BP Solar 175B panels.

                      Comment


                      • #12
                        Originally posted by inetdog View Post
                        '
                        Actually the typical US meter sums the current in the two hot legs (changing sign of one) and multiplies by the line to line voltage divided by two.
                        That is an exact measure of the power drawn regardless of the nature of the loads IF the neutral wire is always exactly halfway between the two hot leads.
                        The meter has no connection to the neutral to measure either current or voltage.

                        In reality, if you have a large unbalanced load the neutral voltage will be shifted away from ground by a few volts, so the meter is slightly biased in favor of the Power Company. (POCO).
                        My home is 240V and the AHJ and utility approved my installation of a 120V 3kw inverter on just one leg. The solar credits all add up to my (heavily) measured system.
                        OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                        Comment


                        • #13
                          Originally posted by ButchDeal View Post

                          My home is 240V and the AHJ and utility approved my installation of a 120V 3kw inverter on just one leg. The solar credits all add up to my (heavily) measured system.
                          And the same small meter error that works in favor of POCO on load measurement will work against you on the reverse power measurement too.
                          Your inverter will be working against a higher line to neutral voltage than what the meter assumes.
                          SunnyBoy 3000 US, 18 BP Solar 175B panels.

                          Comment


                          • #14
                            Originally posted by inetdog View Post

                            And the same small meter error that works in favor of POCO on load measurement will work against you on the reverse power measurement too.
                            Your inverter will be working against a higher line to neutral voltage than what the meter assumes.
                            possibly some there but with an all electric home there is already considerable imbalance. It was approved as it is such a small system in relative comparison to the home though. I have several meters and the production I measure is always very close to the grid calculations.
                            Though most of the big loads are 240V, I have moved many of the 120v loads (that are not on the emergency panel that hangs off the inverter) onto the opposite leg to make a bit more balanced.

                            So I have on the emergency panel the several circuits
                            Kitchen Fridge
                            freezer
                            well pump (converter to 120V with autotransformer)
                            garage door
                            two lights / plugs circuits

                            My Neutral has non to extremely small shift.

                            OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

                            Comment


                            • #15
                              Originally posted by inetdog View Post
                              '
                              Actually the typical US meter sums the current in the two hot legs (changing sign of one) and multiplies by the line to line voltage divided by two.
                              That is an exact measure of the power drawn regardless of the nature of the loads IF the neutral wire is always exactly halfway between the two hot leads.
                              The meter has no connection to the neutral to measure either current or voltage.

                              In reality, if you have a large unbalanced load the neutral voltage will be shifted away from ground by a few volts, so the meter is slightly biased in favor of the Power Company. (POCO).
                              Thanks. That is a better description of how a US POCO meter works.

                              Comment

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