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Looking for some technical details on Volt / Var and Frequency / Watt modulation.

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  • Looking for some technical details on Volt / Var and Frequency / Watt modulation.

    I understand that there are two ways solar inverters can regulate power, one is called Volt / Var and the other one Frequency / Watt.
    But my electricity knowledge has limits so I have the following questions for each.

    Volt / Var.
    My understanding is that this is used in reaction to Grid Voltage rise, and when that happen, based on a predefined curve, the inverter will start raising the reactive power component of the AC output.
    If the inverter is raising the AC output Reactive Power component to modulate the Real Power output, does that mean that it has no effect on the DC input power. The DC production will be untouched, just the AC out component will be manipulated.

    Frequency / Watt
    I am less clear on what happens in that scenario. I understand, the Grid can change the frequency to force an inverter to reduce its output.
    Is the frequency change used as a signal, that in turn makes the inverter throttle the DC output, or is the power reduction done differently.

    I apologize if any of these are stupid question J

  • #2
    Originally posted by scrambler View Post
    I understand that there are two ways solar inverters can regulate power, one is called Volt / Var and the other one Frequency / Watt.
    But my electricity knowledge has limits so I have the following questions for each.

    Volt / Var.
    My understanding is that this is used in reaction to Grid Voltage rise, and when that happen, based on a predefined curve, the inverter will start raising the reactive power component of the AC output.
    If the inverter is raising the AC output Reactive Power component to modulate the Real Power output, does that mean that it has no effect on the DC input power. The DC production will be untouched, just the AC out component will be manipulated.

    Frequency / Watt
    I am less clear on what happens in that scenario. I understand, the Grid can change the frequency to force an inverter to reduce its output.
    Is the frequency change used as a signal, that in turn makes the inverter throttle the DC output, or is the power reduction done differently.

    I apologize if any of these are stupid question J

    Both apply to newer grid-tied inverters (GTI) and they only produce AC output so I am not sure what DC output you are talking about. GTI only have DC input from PV panels and no DC output. Volt-Var control is supposed to be used by POCO's to leverage GTI to help prop up grid voltage/stability when needed. Freq-Watt (and also Volt-Watt) control is supposed to be used by POCO's to modulate power from GTI in case of grid anomalies (e.g. too much power exported for the grid, etc.). In any case, the DC voltage of PV panels are not directly controlled by these control mechanisms.

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    • #3
      I meant DC input (output of the solar array).
      I wanted to confirm if in either case, the Array DC production is throttled, or if the power modulation only affects the AC output of the inverter.

      Comment


      • #4
        Originally posted by scrambler View Post
        I meant DC input (output of the solar array).
        I wanted to confirm if in either case, the Array DC production is throttled, or if the power modulation only affects the AC output of the inverter.
        The AC output is definitely modulated. I have only heard speculation about what happens on the DC side. It may be unique to the firmware of each inverter. Some people have speculated that the MPPT algorithm is tweaked. Others have said that it is the same as if the inverter was not connected to the panels but that would only be the case if AC power is zero. I have AC coupled micros and an AC coupled GT inverter with optimizers. I have occasionally tried to tell if all the micros or all the power optimizers are modulatating or if only some are offline and the string output is reduced. I am curious as well but it does work when needed.
        9 kW solar. Driving EVs since 2012

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        • #5
          It seems to be very hard to find specific info about that....

          As I mentioned, for Volt / Var, given it is a manipulation of the AC current, I don't see why that would affect DC production, but who knows.

          And for Frequency / Watt, I cannot find what exactly it causes the inverter to do to modulate power, so....

          Comment


          • #6
            Originally posted by scrambler View Post
            It seems to be very hard to find specific info about that....

            As I mentioned, for Volt / Var, given it is a manipulation of the AC current, I don't see why that would affect DC production, but who knows.

            And for Frequency / Watt, I cannot find what exactly it causes the inverter to do to modulate power, so....

            Not sure what you mean by DC production but PV panels are basically constant DC current sources within their operating voltage range at any given solar irradiance. So, GTI connected to a PV panel typically adjust its DC input voltage operating point based on various factors (e.g. MPPT, freq-watt, islanding, etc.) to extract desired power from the PV panel. There is typically a DC-DC boost converter in the GTI to do the actual conversion from low voltage DC to high voltage DC required by the DC-AC inverter to generate AC voltage. So, the DC-DC boost converter does modulate the power input from attached PV panel. As example, if freq-watt control specifies a particular power level then the DC-DC boost converter would adjust the DC input voltage operating point to get the necessary input power level from attached PV panel in order to satisfy the freq-watt control.

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            • #7
              Any regulated, switching power converter has the ability to control the current
              drawn from the power source, usually by changing its internal duty cycle. With
              solar panels the voltage drops off considerably as more current is drawn. An
              MPPT control will attempt to operate at a current which maximizes the power
              drawn from the panels, which is somewhere between maximum DC current and
              maximum DC voltage.

              To avoid overload and failure, GTIs monitor the output AC power delivered. As
              the programmed maximum power rating is approached, the control will move
              away from MPPT operation, toward less power converted. The general method
              is to draw less panel current, causing voltage to rise.

              As for frequency, the power grid has an inertia akin to the earths rotation. A GTI
              will have essentially no influence.
              Bruce Roe

              Comment


              • #8
                Originally posted by solardreamer View Post
                if freq-watt control specifies a particular power level then the DC-DC boost converter would adjust the DC input voltage operating point to get the necessary input power level from attached PV panel in order to satisfy the freq-watt control.
                If you know that for sure, then it answers my second question, which is that a freq / watt control command from the grid would result in a throttled DC production of the panels.
                Do we know what is the correlation between the frequency variation and the power variation (I assume as a percentage of the max)?
                I think I read in some PG&E doc that there is no power modulation between 59.9 and 60.1, then there is a 5% reduction for each 0.1Hz above that.

                Now all I need is confirmation of the fact that in a Volt / Var situation, the DC production of the Panels is unaffected.

                Comment


                • #9
                  Originally posted by scrambler View Post

                  If you know that for sure, then it answers my second question, which is that a freq / watt control command from the grid would result in a throttled DC production of the panels.
                  Do we know what is the correlation between the frequency variation and the power variation (I assume as a percentage of the max)?
                  I think I read in some PG&E doc that there is no power modulation between 59.9 and 60.1, then there is a 5% reduction for each 0.1Hz above that.

                  Now all I need is confirmation of the fact that in a Volt / Var situation, the DC production of the Panels is unaffected.

                  What is your concern? For full detail, you should read CA Rule 21 specs on smart inverters. Freq-watt control is linear between 60.1 (100%) and 62.0Hz (0%) for 100% name plate output control range but other ranges have different specs. Also, I would not assume Volt-VAR control does not modulate DC input. The specs require GTI to maintain 0.9 power factor AC output while changing VAR so that implies DC input modulation could be required under some conditions. These control mechanisms exist but I highly doubt they could be fully utilized in the real world because many of the POCO grid generation equipment will likely fail long before grid anomalies become severe enough to fully utilize the control mechanisms. As example, the Texas grid generation equipment started to shut down when the grid frequency fell to 59.4Hz and was minutes away from total grid outage when the grid frequency fell to 59.3Hz for a few minutes.

                  Comment


                  • #10
                    Originally posted by solardreamer View Post
                    Also, I would not assume Volt-VAR control does not modulate DC input. The specs require GTI to maintain 0.9 power factor AC output while changing VAR so that implies DC input modulation could be required under some conditions.
                    That does confuse me, as that would mean that Volt / Var would only be used until PF reaches 0.9, then would become a Volt / Watt type modulation???

                    If you have a link to the specs describing that, I would be grateful..

                    Comment


                    • #11
                      Originally posted by scrambler View Post

                      That does confuse me, as that would mean that Volt / Var would only be used until PF reaches 0.9, then would become a Volt / Watt type modulation???
                      No, Volt-VAR is not supposed to change real power from the GTI. So, in general, GTI needs to adjust apparent power to satisfy volt-VAR control which likely requires DC input modulation.

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                      • #12
                        Originally posted by solardreamer View Post

                        No, Volt-VAR is not supposed to change real power from the GTI. So, in general, GTI needs to adjust apparent power to satisfy volt-VAR control which likely requires DC input modulation.
                        You like to confuse people with contradictory statements

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                        • #13
                          Originally posted by scrambler View Post

                          You like to confuse people with contradictory statements
                          what's contradictory?

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                          • #14
                            Originally posted by solardreamer View Post

                            what's contradictory?
                            Unless I am not getting what you are saying
                            "Also, I would not assume Volt-VAR control does not modulate DC input. The specs require GTI to maintain 0.9 power factor AC output while changing VAR so that implies DC input modulation could be required under some conditions."
                            vs
                            "No, Volt-VAR is not supposed to change real power from the GTI."

                            If Volt / Var modulates the DC input (array DC production), Then it affect the real power

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                            • #15
                              Originally posted by scrambler View Post

                              Unless I am not getting what you are saying
                              "Also, I would not assume Volt-VAR control does not modulate DC input. The specs require GTI to maintain 0.9 power factor AC output while changing VAR so that implies DC input modulation could be required under some conditions."
                              vs
                              "No, Volt-VAR is not supposed to change real power from the GTI."

                              If Volt / Var modulates the DC input (array DC production), Then it affect the real power

                              DC input modulation does not necessarily change AC output real power. GTI can modulate DC input and adjust AC output apparent power and/or power factor within limits at same AC output real power to satisfy Volt-VAR control.

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