Diversion Load Running after PV Array Shuts Down

Hi All,

I have a new PV grid tied system that seems to work fine, except for my additional Xantrex C-40 controller continues to divert power from the batteries to a water heater element when the sun goes down.

My system consists of a Xantrex 4548 Inverter/Charger, a Xantrex MPPT 60 Charge Controller, with 18 Canadian Solar 250 watt panels. 16 of the panels are on the MPPT 60 and two panels are on the C-40 in diversion mode, diverting to a water heater with a 500 watt, 48 volt element.

All is well when the sun is up, but when the system shuts down at night, the C-40 continues to divert power to the water heater element. This is also true if I just pull the PV array cut off switch in the daytime. If I turn off the breaker to the C-40, the diversion load stops.

I have followed the manual to the letter and re-checked all the wiring and controller settings. All of the voltage settings are set the same on both controllers. Both have temp sensors attached to the battery negative posts.

Any ideas?

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Diversion Load Running after PV Array Shuts Down

    Welcome to the forum.

    I have a question: why would you have a diversion load on a grid-tie PV system? There is no need as any 'surplus' power produced by the system would be sold to the grid.

    Not sure what you mean by "two panels are on the C-40 in diversion mode" as that is not a normal configuration either. When a controller is used in diversion mode it is connected to the battery bank to 'bleed off' excess power (to a load) and maintain a fixed battery Voltage. This is not normally done with PV as it is unnecessary, but is a standard practice for wind/hydro turbines.

    You have 4500 Watts of panels. On a 48 Volt system that would produce around 72 Amps maximum current, which is clearly more than the 60 Amp MPPT controller could handle. For that matter the sixteen 250 Watt panels will 'max out' that controller, being capable of around 64 Amps.

    The panels are probably Vmp around 30, so they won't do much for a 48 Volt system through a PWM controller like the C40 unless two were in series (array Vmp 60). Individually the panels would only provide about 8 Amps of current, so a 40 Amp controller would be overkill. They certainly wouldn't need one each.

    How much of a battery bank do you have? It should be at least 450 Amp hours (@ 48 Volts of course) and it wouldn't hurt for it to be larger.

    I don't suppose you could show us a wiring diagram or power flow chart for this? Frankly the configuration as described doesn't make sense.
  • Emerald PowerEmerald Power Registered Users Posts: 5
    Re: Diversion Load Running after PV Array Shuts Down

    Hi,

    Thanks for responding to my question. I will try to answer yours.

    I am new to this game and willing to be wrong and learn all I can.

    I am using the C-40 in diversion control mode because I will be adding a wind generator and hydro to the mix and I want to use the C-40 to control these along with the two additional panels. In my research, I have seen many times that folks use a PWM controller such as a C-40 to integrate PV, wind, and hydro energy sources. The diversion load keeps a torque on the wind generator for over spin reasons. My local utility only pays back .02 per Kwh, so in reality I am not trying to get a return from them. My main goal is to power my own house during the day, lowering my bill, and have a back up for when the grid goes down, which happens frequently where I live.

    I realize the MPPT 60 is maxed out, hence the reason the two additional panels are on a seperate controller.

    The two additional panels are in series and producing 60 volts @ 8.3 amps. The C-40 handles this with ease and will allow me to add the wind and hydro input in the near future. The reason for the C-40, is that this is the only 48 volt unit that Xantrex makes, that I could find.

    My battery bank is 450 ah, consisting of 8 Trojan J185H 12 volt units configured for 48vdc.

    So, is it possible to have two controllers on one battery bank? Or, have I done something wrong?

    Kind Regards,
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Diversion Load Running after PV Array Shuts Down

    It is definitely possible to have two controllers on one battery bank. But it doesn't always work smoothly.

    When you connect a controller for handling a diversion load the input of the controller is connected to the battery bank and the output connected to the load. The controller monitors the battery Voltage and connects the load as needed to regulate the battery Voltage by 'bleeding off' excess power.

    If you are using one in such a mode with two 'extra' solar panels then those panels are connected directly to the batteries, yes? As is the controller input, and then the heating element is connected to the output of the controller.

    The controller will try to regulate battery Voltage to the level it is set at, regardless of where that power is coming from. If the two 'extra' panels produce nothing but the ones on the MPPT controller are pushing the battery Voltage up the diversion controller will try to pull the Voltage down if it thinks it is too high.

    Herein lies the problem; getting the controllers to work together and agree on battery Voltage and charging stage. If the C40 is continuing to draw off Voltage and send it to the heater after dark, then it most likely has its Voltage level set too low. The settings on these are not profoundly accurate (and you need to change the indicators when using the different mode). Try edging the Voltage setting up until you see the current to the heater shut down and check the battery Voltage at that point.

    I wouldn't get too excited about the possibility of adding wind or hydro either. Wind in particular disappoints most of the time.
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Diversion Load Running after PV Array Shuts Down
    I have a new PV grid tied system that seems to work fine, except for my additional Xantrex C-40 controller continues to divert power from the batteries to a water heater element when the sun goes down.

    What voltages are you seeing when this happens? Are your bulk and float settings being hit?

    To test it I'd do the following:

    Take the C40 out of the circuit. Connect its input to a power supply. Connect its output to a small diversion load (a 120 volt light bulb or something.) Ramp the power up and down. At voltages above bulk and float the controller should be on; below bulk and float it should be off. (In between a timer is active so you'd have to wait a while to see the bulk to float transition.)
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Diversion Load Running after PV Array Shuts Down
    Hi,

    Thanks for responding to my question. I will try to answer yours.

    I am new to this game and willing to be wrong and learn all I can.

    I am using the C-40 in diversion control mode because I will be adding a wind generator and hydro to the mix and I want to use the C-40 to control these along with the two additional panels. In my research, I have seen many times that folks use a PWM controller such as a C-40 to integrate PV, wind, and hydro energy sources.

    Your problem is twofold:
    1. Right now you do not need to have a diversion load at all, and
    2. The voltage at which the diversion load kicks in is set too low. It should turn off when the battery voltage. with or without load, drops below roughly the normal Float voltage value for your batteries. If you set the diversion voltage correctly, it should do what you expect when the PV goes away, although it may take a short time to pull the battery voltage down from the high level it was at during charging.
    Or maybe 3. The controller is defective.
    SMA SB 3000, old BP panels.
  • Emerald PowerEmerald Power Registered Users Posts: 5
    Re: Diversion Load Running after PV Array Shuts Down

    Yes those two panels are connected directly to the batteries.

    The C-40 has 2 volts of adjustment range for the "float" level, and what Xantrex calls the "bulk" level. At first, I set these to Trojans specs of 59.2 for the bulk. (Trojan calls this the "daily" charge) And 52.8 for the float level. After your suggestion last evening to nudge up the bulk level, the amps and volts dropped off the diversion load, and reached .000 when I hit 60 volts on the dial. This is as high as it will go. I confirmed the voltage reading with a meter on pot on the circuit board. The MPPT 60 controller was also set for 59.2 as well as the XW4548's grid powered charger. I checked it a few times during the night and the diversion load stayed at .000. This morning when the XW MPPT 60 woke up, it raised the voltage to 59.8 (even though it was set for 59.2) and the C-40 was load shedding about 4 amps. Since I can't increase the C-40's voltage any higher, I lowered the voltages on the inverter.charger and the MPPT 60 to 58.8 and it dropped the diversion load a couple of amps. I don't know if I should drop the voltage any more to try and balance the two controllers.
  • Emerald PowerEmerald Power Registered Users Posts: 5
    Re: Diversion Load Running after PV Array Shuts Down

    The float voltage stays around 53, with the controller being set at 52.8. They do not seem to fluctuate, as the grid charger is holding them there quite nicely even when the C-40 was bleeding off some current to the heater element.

    I may try the isolated test if I can't get it working otherwise. Thanks!
  • Emerald PowerEmerald Power Registered Users Posts: 5
    Re: Diversion Load Running after PV Array Shuts Down

    Thanks for your input. It helps with my understanding of how things work.

    It would be nice if the C-40 had a pot that adjusted just the diversion loads dump point.
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