Question about grid-tied systems versus grid-tied systems with battery backup

Dusty
Dusty Solar Expert Posts: 271 ✭✭✭
in Grid Tie and Grid Interactive Systems #1
How is it that straight grid-tied systems are able to feed power to the grid without the need for a battery bank to stabilize a/c ripple, while battery backup systems rely on such a large battery bank to stabilize the inverter output? :confused:
XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #2
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Dusty wrote: »
    How is it that straight grid-tied systems are able to feed power to the grid without the need for a battery bank to stabilize a/c ripple, while battery backup systems rely on such a large battery bank to stabilize the inverter output? :confused:
    The grid is so huge compared to your generation system and the loads in your house, nothing your inverter does can change the line voltage and frequency as long as you are connected to it. In fact, your inverter monitors what is on the grid and tries to match it precisely, and if it can't, it shuts down. It is the same whether you have batteries or not and when you are connected to the grid and the grid is up there is no difference between the two types of system.

    The only time that batteries are a factor is when you are running off-grid, and it's got nothing to do with AC ripple. It's because the power generation curve over the course of a day and your power usage curve over the same period of time do not match. You want to run your lights at night, right? You have to have someplace to store energy when your system is producing more than you are using and someplace to draw from when your production is less. Hence, batteries.
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #3
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    I was told in a previous thread that the rule of thumb was 100Ah of battery capacity for every 1Kwatt of PV power. This was to prevent a/c ripple from being created when selling to the grid and shutting down the inverter with faults. I've reduced my sell amps to only what my PV system is capable of harvesting, and I haven't had an issue with system faults yet.

    I was just wondering why it's an issue with a battery-based GT systems, while GT systems that have no batteries at all don't have a/c ripple problems selling to the grid. How is a non-battery GT system's output kept stable when selling without having batteries to act as a sort of ballast for the inverter?

    I understand the need for ample AH capacity with an off-grid system, and days of autonomy with a grid-tied system in case the grid goes down, but my grid is generally very stable. I can always fire up the generator if the batteries reach the limit of their depth of discharge. In the last couple years, I've only lost power a few times--the longest was 12 hours when hurricane Isabel came through back in 2003.

    I went with the XW 6048 for a couple reasons: I wanted to learn about solar power, reduce my reliance on the grid (Chesapeake VA is pretty close to the Outer Banks of NC--a hot spot for hurricanes), not have to upgrade to a larger inverter if I expanded the PV capability later, and have a whole-house UPS and not have to rely on a bunch of small UPS's for the computer, router, cable modem, etc. I didn't need a lot of battery backup capacity since most times I only lose power for less than an hour.
    ggunn wrote: »
    The grid is so huge compared to your generation system and the loads in your house, nothing your inverter does can change the voltage and frequency as long as you are connected to it. It is the same whether you have batteries or not and when you are connected to the grid there is no difference between the two types of system.

    The only time that batteries are a factor is when you are running off-grid, and it's got nothing to do with AC ripple. It's because the power generation curve over the course of a day and your power usage curve over the same period of time do not match. You want to run your lights at night, right? You have to have someplace to store energy when your system is producing more than you are using, and someplace to draw from when your production is less. Hence, batteries.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    There is actually a fairly significant difference in the way the two types of grid-tie inverters function.

    The straight grid-tie units take whatever power the panels produce and turn it into 60 Hz AC at a Voltage higher than the utility, causing it to try and push current against the electrical inertia of the grid.

    The hybrid-battery based units run from the batteries and above a certain Voltage level and try to do the same. These are the ones that suffer from the AC ripple effect and require minimum battery bank size to overcome the problem. There's quite a few threads discussing this with some excellent explanations by Solar Guppy and RCinFLA.

    One example: http://forum.solar-electric.com/showthread.php?12529-Minimum-Battery-Bank-for-XW6048/page2
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #5
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Thank you for that excellent thread on a/c ripple issues with a battery-based GT system.

    I thought that there was a significant difference between the two types of GT systems, and I was merely curious about how a non battery-based system differed from a battery-based system in how it pushed power to the grid without the a/c ripple problems that a battery-based system has to contend with.

    Many thanks!
    Cariboocoot wrote: »
    There is actually a fairly significant difference in the way the two types of grid-tie inverters function.

    The straight grid-tie units take whatever power the panels produce and turn it into 60 Hz AC at a Voltage higher than the utility, causing it to try and push current against the electrical inertia of the grid.

    The hybrid-battery based units run from the batteries and above a certain Voltage level and try to do the same. These are the ones that suffer from the AC ripple effect and require minimum battery bank size to overcome the problem. There's quite a few threads discussing this with some excellent explanations by Solar Guppy and RCinFLA.

    One example: http://forum.solar-electric.com/showthread.php?12529-Minimum-Battery-Bank-for-XW6048/page2
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #6
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Grid Tied inverters use high voltage capacitors to store energy... They only have to store enough energy for a 1/2 cycle of power (the operating voltage is probably on the order of ~360 VDC--which is much less current than a battery bank running at 12-48 VDC).

    Typically, the high voltage capacitors are electrolytic--Hence one of the weaknesses of a GT inverter--These types of capacitors do have a limited life (and the hotter they run, the less life they have).

    Off grid inverters use the battery bank for energy storage, that is why the bank has to be large enough to manage the "ripple" current needed to run a single phase AC inverter. (note, that is 100 AH @ 48 volt battery bank recommended per 1kW of power output).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #7
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Cariboocoot wrote: »
    There is actually a fairly significant difference in the way the two types of grid-tie inverters function.

    The straight grid-tie units take whatever power the panels produce and turn it into 60 Hz AC at a Voltage higher than the utility, causing it to try and push current against the electrical inertia of the grid.

    The hybrid-battery based units run from the batteries and above a certain Voltage level and try to do the same. These are the ones that suffer from the AC ripple effect and require minimum battery bank size to overcome the problem. There's quite a few threads discussing this with some excellent explanations by Solar Guppy and RCinFLA.

    One example: http://forum.solar-electric.com/showthread.php?12529-Minimum-Battery-Bank-for-XW6048/page2

    Ah, my experience is mostly with Sunny Island/Sunny Boy systems, which operate differently (I surmise) from the Xantrex system Dusty has (which I see now from reading another thread). With an SI/SB system, the SI with batteries is essentially out of the circuit when the grid is up, so it operates as a straight grid tied system. I should have qualified my remarks as they pertain to that type of system; I confess that I don't know much about the way Xantrex systems operate. They seem hideously complex to me. :D

    And FWIW, an inverter does not need to raise its voltage to push against the grid; it is just another source element in a massively parallel circuit of sources and loads. All the sources can feed the loads just fine with the same output voltage.
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #8
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Bingo! That completely answers my question. Thank you Bill! :D

    -Dusty
    BB. wrote: »
    Grid Tied inverters use high voltage capacitors to store energy... They only have to store enough energy for a 1/2 cycle of power (the operating voltage is probably on the order of ~360 VDC--which is much less current than a battery bank running at 12-48 VDC).

    Typically, the high voltage capacitors are electrolytic--Hence one of the weaknesses of a GT inverter--These types of capacitors do have a limited life (and the hotter they run, the less life they have).

    Off grid inverters use the battery bank for energy storage, that is why the bank has to be large enough to manage the "ripple" current needed to run a single phase AC inverter. (note, that is 100 AH @ 48 volt battery bank recommended per 1kW of power output).

    -Bill
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    ggunn;

    Yes the Sunny Island system has that built-in AC coupling advantage. :D Apparently the XW's can do this too, to some extent, but they don't have that wonderfully integrated system that SMA has.

    Now if only it was fiscally sensible to install GT here. But even with BC Hydro's 17% rate hike (over 3 years) it doesn't work out. There's no feed-in premium or rebates or tax incentives, and all the equipment costs more because it's Canada (free trade? HA!). :cry:
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #10
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Speaking of Sunny Islands, I have been hearing rumors for a couple of years now about a coming 8-10 kW 240VAC split phase SI for the US/Canada market, but so far it has not materialized. It seems to me that SMA could sell a bunch of them.
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  • jagec
    jagec Solar Expert Posts: 157 ✭✭
    #11
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    ggunn wrote: »
    And FWIW, an inverter does not need to raise its voltage to push against the grid; it is just another source element in a massively parallel circuit of sources and loads. All the sources can feed the loads just fine with the same output voltage.

    Well, it has to be slightly higher to compensate for the wire resistance between the source and the load. But that's extremely low.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #12
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    jagec wrote: »
    Well, it has to be slightly higher to compensate for the wire resistance between the source and the load. But that's extremely low.
    True enough, but the idea that an inverter's output voltage has to be higher than the grid voltage to push power onto the grid is incorrect. An inverter is a current source; for a given amount of input on the DC side it pushes out the same amount of AC power, whether it goes to local loads or the grid. There is, as you say, a bit of voltage rise at the output terminals to compensate for the voltage drop in the conductors, but in a correctly installed system it's usually minuscule. In an ideally modeled parallel system where the wire resistance is zero, the voltages on all the sources are the same.

    This was confusing to me at first as well because I was thinking in terms of a battery powering a load. A battery is a voltage source, though, not a current source (actually no source is purely either a voltage or current source, but let's not go there), and delivers whatever current is necessary (within reason) on demand to try to maintain the same voltage at its terminals. An inverter is a current source and behaves differently. It tries to deliver the same amount of current and adjusts its voltage accordingly. Luckily, the grid is so massive compared to an inverter that its voltage and load characteristics remain the same no matter what a grid tied inverter does (neglecting, as we said, voltage drop in the conductors) or how many lights you turn on, so it can just chug merrily along at whatever output level it chooses irrespective of what the local load is.

    Your utility company generators have to behave as voltage sources and therefore deliver current on demand. They must keep the voltage the same, so they have to respond to changes in demand by changing the amount of current they produce. This is why some utilities have started to complain about the introduction of solar and wind sources to their grid; solar and wind are current sources which respond to local environmental conditions, and when they turn off and on they force the power company to ramp their output current up and down to compensate so that the line voltage stays the same.

    BTW, this is also why you can't run an inverter off grid without batteries. An inverter tries to put out the same current (power, actually) no matter what, so if it were to be connected only to local loads and no batteries (and if it would not shut down when this happened), the voltage would change radically every time you switched a load off or on. An inverter cannot supply constant voltage power on demand.

    Ohm's Law is a bee-yatch. :D

    PS: I know I have played a bit fast and loose with power and current. That's because we tend to think in terms of constant voltage - we've got a 12VDC system, or we are connected to 240/120VAC. In a constant voltage system, power and current are sort of interchangeable; since power is the product of voltage and current, when you hold voltage constant and change the power, you change the current by the same factor.
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #13
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    I know I'm missing something. Won't be the last time, either! :blush:

    So if the utility grid has a constant voltage and an almost unlimited amount of current potential, how is it that my little inverter set to a "sell" amperage of 10 amps is able to "push" anything out to the grid at all???
    ggunn wrote: »
    True enough, but the idea that an inverter's output voltage has to be higher than the grid voltage to push power onto the grid is incorrect. An inverter is a current source; for a given amount of input on the DC side it pushes out the same amount of AC power, whether it goes to local loads or the grid. There is, as you say, a bit of voltage rise at the output terminals to compensate for the voltage drop in the conductors, but in a correctly installed system it's usually minuscule. In an ideally modeled parallel system where the wire resistance is zero, the voltages on all the sources are the same.

    This was confusing to me at first as well because I was thinking in terms of a battery powering a load. A battery is a voltage source, though, not a current source (actually no source is purely either a voltage or current source, but let's not go there), and delivers whatever current is necessary (within reason) on demand to try to maintain the same voltage at its terminals. An inverter is a current source and behaves differently. It tries to deliver the same amount of current and adjusts its voltage accordingly. Luckily, the grid is so massive compared to an inverter that its voltage and load characteristics remain the same no matter what a grid tied inverter does (neglecting, as we said, voltage drop in the conductors) or how many lights you turn on, so it can just chug merrily along at whatever output level it chooses irrespective of what the local load is.

    Your utility company generators have to behave as voltage sources and therefore deliver current on demand. They must keep the voltage the same, so they have to respond to changes in demand by changing the amount of current they produce. This is why some utilities have started to complain about the introduction of solar and wind sources to their grid; solar and wind are current sources which respond to local environmental conditions, and when they turn off and on they force the power company to ramp their output current up and down to compensate so that the line voltage stays the same.

    BTW, this is also why you can't run an inverter off grid without batteries. An inverter tries to put out the same current (power, actually) no matter what, so if it were to be connected only to local loads and no batteries (and if it would not shut down when this happened), the voltage would change radically every time you switched a load off or on. An inverter cannot supply constant voltage power on demand.

    Ohm's Law is a bee-yatch. :D

    PS: I know I have played a bit fast and loose with power and current. That's because we tend to think in terms of constant voltage - we've got a 12VDC system, or we are connected to 240/120VAC. In a constant voltage system, power and current are sort of interchangeable; since power is the product of voltage and current, when you hold voltage constant and change the power, you change the current by the same factor.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #14
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Dusty wrote: »
    I know I'm missing something. Won't be the last time, either! :blush:

    So if the utility grid has a constant voltage and an almost unlimited amount of current potential, how is it that my little inverter set to a "sell" amperage of 10 amps is able to "push" anything out to the grid at all???
    It's because it's (mostly) a current source, not a voltage source.

    An ideal voltage source (which does not exist in the real world) has the same voltage across its terminals no matter what. The current it puts out is totally dependent on the load (Ohm's Law), and a dead short drives the current to infinity. A big lead acid battery is a pretty good approximation of an ideal voltage source. Anyone who has dropped a wrench across the terminals of a car battery has a tactile understanding of this.

    An ideal current source (which also does not exist in the real world) has the same current through it no matter what. The voltage on its output terminals is totally dependent on the load (Ohm's Law again), and an open circuit drives the voltage to infinity.

    A PV driven inverter behaves much more as a current source than a voltage source; it cannot function as a constant voltage power source the way that a battery can. A grid tied PV inverter has its output voltage clamped by the the grid, so the only thing it can do is deliver power at that voltage (the definition of current). How much power that is is determined by its efficiency and the amount of power the PV is feeding it. PV is a current source over most of its IV curve (the flat part off to the left of the MPPT knee - you see how the current is pretty much the same no matter what the voltage?), and that is what is actually driving this process; a battery driven inverter (i.e. a Sunny Island) has no problem with delivering constant voltage power on demand because it gets its power from a near ideal voltage source.

    Look at it this way: Your inverter is determined to put out that 10A no matter what, and your household loads and everyone else's out on the grid are all the same to it. It doesn't know or care who else is also sending current through those loads.

    Don't feel bad; I have a degree in electrical engineering and it took me a while to get this through my thick skull. It's because for the longest time I could not help thinking of the inverter as a voltage source, which is not surprising since virtually all the power sources we encounter (batteries and utility AC power) are constant voltage power sources.
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #15
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Thanks again for the explanation, and also for your patience!

    I guess what was causing me confusion--I always thought that in order to move current from one source to another, you had to create a difference in potential. My thinking was to have a higher voltage potential on one source would force current to move through the meter to the other source (the grid). Of course, since you have to synch up the frequency, you could only change the voltage potential only so much to induce current flow from the PV inverter to the grid. I certainly can't create more current than is possible from the grid, which is why I thought it had to be a subtle difference in voltage to cause the current flow.
    ggunn wrote: »
    It's because it's (mostly) a current source, not a voltage source.

    An ideal voltage source (which does not exist in the real world) has the same voltage across its terminals no matter what. The current it puts out is totally dependent on the load (Ohm's Law), and a dead short drives the current to infinity. A big lead acid battery is a pretty good approximation of an ideal voltage source. Anyone who has dropped a wrench across the terminals of a car battery has a tactile understanding of this.

    An ideal current source (which also does not exist in the real world) has the same current through it no matter what. The voltage on its output terminals is totally dependent on the load (Ohm's Law again), and an open circuit drives the voltage to infinity.

    A PV driven inverter behaves much more as a current source than a voltage source; it cannot function as a constant voltage power source the way that a battery can. A grid tied PV inverter has its output voltage clamped by the the grid, so the only thing it can do is deliver power at that voltage (the definition of current). How much power that is is determined by its efficiency and the amount of power the PV is feeding it. PV is a current source over most of its IV curve (the flat part off to the left of the MPPT knee - you see how the current is the same no matter what the voltage?), and that is what is actually driving this process; a battery driven inverter (i.e. a Sunny Island) has no problem with delivering constant voltage power on demand because it gets its power from a near ideal voltage source.

    Look at it this way: Your inverter is determined to put out that 10A no matter what, and your household loads and everyone else's out on the grid are all the same to it. It doesn't know or care who else is also sending current through those loads.

    Don't feel bad; I have a degree in electrical engineering and it took me a while to get this through my thick skull. It's because for the longest time I could not help thinking of the inverter as a voltage source, which is not surprising since virtually all the power sources we encounter (batteries and utility AC power) are constant voltage power sources.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #16
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Dusty wrote: »
    Thanks again for the explanation, and also for your patience!

    I guess what was causing me confusion--I always thought that in order to move current from one source to another, you had to create a difference in potential. My thinking was to have a higher voltage potential on one source would force current to move through the meter to the other source (the grid). Of course, since you have to synch up the frequency, you could only change the voltage potential only so much to induce current flow from the PV inverter to the grid. I certainly can't create more current than is possible from the grid, which is why I thought it had to be a subtle difference in voltage to cause the current flow.

    Exactamundo. That's how a voltage source works, which is what a PV inverter isn't. You gotta hold your mouth a certain way to see it. :D

    Look at it this way: A difference in potential causes current flow, but current flow causes a difference in potential.
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  • Dusty
    Dusty Solar Expert Posts: 271 ✭✭✭
    #17
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    That makes total sense. I just didn't know you could do it that way. Kinda like the cart before the horse in my limited way of thinking.

    By lowering a resistor value in a basic circuit, are you increasing current or decreasing voltage? Both! But which happens first..... :D
    ggunn wrote: »
    Exactamundo. That's how a voltage source works, which is what a PV inverter isn't. You gotta hold your mouth a certain way to see it. :D

    Look at it this way: A difference in potential causes current flow, but current flow causes a difference in potential.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
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  • Lee Dodge
    Lee Dodge Solar Expert Posts: 112 ✭✭
    #18
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    ggunn-

    Thanks for this discussion about grid-tied inverters and how they work as current sources, and how that is connected to the fact that grid-tied systems cannot be made to operate with just a simple tickler to simulate the grid.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #19
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Lee Dodge wrote: »
    ggunn-

    Thanks for this discussion about grid-tied inverters and how they work as current sources, and how that is connected to the fact that grid-tied systems cannot be made to operate with just a simple tickler to simulate the grid.

    De nada. Happy to help.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #20
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    Dusty wrote: »
    That makes total sense. I just didn't know you could do it that way. Kinda like the cart before the horse in my limited way of thinking.

    By lowering a resistor value in a basic circuit, are you increasing current or decreasing voltage? Both! But which happens first..... :D
    Not exactly. In a simple circuit with a battery (voltage source) connected to a resistor (assuming that the resistor is of a large value compared to the resistance of the wires), if you decrease the resistance the current increases but the voltage drop across the resistor remains the same. In that same circuit with an ideal current source instead of a battery as the power supply, lowering the resistance lowers the voltage drop across the resistor but the current through it stays the same. Ohm's Law.

    Like I said, don't feel bad that it takes some mental exercising to wrap your brain around this stuff. It is not surprising that most folks visualize current flow from the perspective of constant voltage sources, since virtually all power supplies we come into contact with hold voltage constant and supply current on demand.
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  • DanS26
    DanS26 Solar Expert Posts: 269 ✭✭✭
    #21
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Great thread guys, I've really learned a lot. As a corollary to this discussion, I've always wondered why my Fronius 7.5 GT inverter outputs AC voltage at a constant .2 to .3 volts higher than the grid. I believe the Fronius documentation did state that the inverter voltage would be higher but it did not elaborate or explain why this is SOP.

    I don't mean to hijack the thread with this question, but it does seem pertinent to the discussion.
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #22
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Two reasons...

    The first, there is always a level of uncertainty when making any sort of measurement... 0.25 volt error / 240 VAC = 0.001 = 0.1% difference...

    Usually, f you are within 2-3% between meters (uncertified), you are "close enough" for government work. Any better accuracy, you have to go with much more expensive equipment.

    The other, as current forced down the wire, you will have V=I*R ... Which will increase the voltage at the inverter vs the main panel...

    Lastly, the way AC voltage is measured is "root mean square"... Basically, the shape of the wave form and area under the curve equates to the "equivalent" energy level.

    There are inexpensive meters that measure "peak AC voltage" and assume Sine Wave (1/sqrt(2). And if you don't have a perfect sine wave (such as MSW--modified square wave), the square root of two factor is no longer correct. So, you will see a fairly large difference between a "cheap" AC DMM and a True RMS reading meter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • DanS26
    DanS26 Solar Expert Posts: 269 ✭✭✭
    #23
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Bill,
    Are you saying I'm just seeing a measuring error? I don't think so.

    I believe the Fronius inverter is purposely outputting a slightly higher voltage, I just don't understand why.



    BB. wrote: »
    Two reasons...

    The first, there is always a level of uncertainty when making any sort of measurement... 0.25 volt error / 240 VAC = 0.001 = 0.1% difference...

    Usually, f you are within 2-3% between meters (uncertified), you are "close enough" for government work. Any better accuracy, you have to go with much more expensive equipment.

    The other, as current forced down the wire, you will have V=I*R ... Which will increase the voltage at the inverter vs the main panel...

    Lastly, the way AC voltage is measured is "root mean square"... Basically, the shape of the wave form and area under the curve equates to the "equivalent" energy level.

    There are inexpensive meters that measure "peak AC voltage" and assume Sine Wave (1/sqrt(2). And if you don't have a perfect sine wave (such as MSW--modified square wave), the square root of two factor is no longer correct. So, you will see a fairly large difference between a "cheap" AC DMM and a True RMS reading meter.

    -Bill
    23.16kW Kyocera panels; 2 Fronius 7.5kW inverters; Nyle hot water; Steffes ETS; Great Lakes RO; Generac 10kW w/ATS, TED Pro System monitoring
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #24
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    Electricity, like water, has to run "down hill". So, for a GT inverter to put current into the grid, the voltage at the inverter has to be a bit higher than that of the power panel/pole transformer.

    Say you have a 5 kW GT inverter system, and you measure the voltage with it off, and again with it on. Say it is 250 vac with no current, and 255 vac at 5kW... A little algebra (see if I can get this right):
    • V=IR; P=V*I
    • I=P/V = 5000/250 = 20 amps
    • R=(change in voltage)/(change in current) = (255v-250v)/(20a-0a)= 0.25 ohms for length of cable

    Note, if you see the 0.2 to 0.3 volt rise both early in the morning and at noon (same voltage rise), then you are just seeing measurement error. If you are seeing ~0 volt difference in the morning (no current flow), and 5 volt rise at mid-day (with my sample 5kW inverter and 5 volt rise), then you are seeing the voltage rise of the current through the home/utility drop wiring.

    You can "calibrate" your DMM against the GT inverter by measuring the voltage when the inverter is just turning on (or off). But remember, you might also have temperature compensation issues too (cool meter from inside home vs cold/hot GT inverter system).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #25
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    BB. wrote: »
    Electricity, like water, has to run "down hill". So, for a GT inverter to put current into the grid, the voltage at the inverter has to be a bit higher than that of the power panel/pole transformer.
    This is not correct. If wire resistance were zero, the system would work just fine with the voltage exactly the same (actually, it could not be anything BUT exactly the same). What he is seeing is voltage rise at the inverter output due to conductor resistance between the inverter and the grid. If what you were saying were correct, then the amount of current from the inverter would depend on the voltage difference between the inverter and the grid and minor changes in voltage would result in large changes in output current. Such is not the case; the inverter puts out the current it "wants to" irrespective of which loads are local and which are on the grid and the voltage differential is dependent on the current flow, not the other way round.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,654 admin
    #26
    Re: Question about grid-tied systems versus grid-tied systems with battery backup

    But we are not talking about zero resistance (or, more correctly, impedance) wiring. R (Z) is always non-zero (except for specific lab/electronic/etc. setups).

    So, there will always be some voltage drop required to make current flow in the correct direction (ignoring magnetic fields, etc.).

    I was not trying to say that the inverter output current is dependent on the resistance of the home wiring, only that V=I*R (or everything in vectors V=I*Z with little arrows on top of all the letters).

    Assuming a "fixed" value for R (Z), then the voltage difference between the inverter and the main panel is dependent on the amount of current flowing (and why I asked earlier if the OP saw a difference in voltage "rise" between sunup vs noon time sun).

    Yes, GT inverters are current sources (and actually constant power devices where P=I*V -- so current is affected by output voltage) with their output power determined by how much solar energy is available from the panels at any point in time.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #27
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    BB. wrote: »
    ...there will always be some voltage drop required to make current flow in the correct direction (ignoring magnetic fields, etc.).

    My point is that the voltage drop/rise (depending on which way you look at it) is caused by the current flow, not the other way round. The inverter does not raise the voltage, it pushes current.

    A lot of folks fixate on whether their system and house are sourcing or sinking power, which is important from a metering standpoint but not from an electrical one. To the inverter there is no difference between the loads in the house and the loads out on the grid. It's all massively parallel.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #28
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    ggunn wrote: »
    My point is that the voltage drop/rise (depending on which way you look at it) is caused by the current flow, not the other way round.

    They're really the same thing. No voltage increase, no current; Ohm's Law always applies.

    For a more analytical approach, consider that at a given output impedance, a Thevenin equivalent circuit (i.e. a voltage source with series resistance) looks exactly like a Norton equivalent (a current source with a shunt resistance) to the load.
    The inverter does not raise the voltage, it pushes current.

    It does both; it has to. You can't have the same voltage on both sides of any wire (other than a superconductor) and have a current flow through it.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #29
    Re: Question about grid-tied systems versus grid-tied systems with battery backup
    bill von novak wrote: »
    They're really the same thing. No voltage increase, no current; Ohm's Law always applies.

    For a more analytical approach, consider that at a given output impedance, a Thevenin equivalent circuit (i.e. a voltage source with series resistance) looks exactly like a Norton equivalent (a current source with a shunt resistance) to the load.



    It does both; it has to. You can't have the same voltage on both sides of any wire (other than a superconductor) and have a current flow through it.
    I know. A voltage gradient causes charge to move and charge moving causes a voltage gradient. I didn't mean for this to become a "thing". It's just that realizing that a PV driven inverter is a current source (it can't hold voltage virtually constant and deliver current on demand like a battery can) is central to understanding why you can't run off grid without batteries.
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