2 inverter system power problems

[James Leenders]

Hi Everyone.
I have come across a challenge with my borehole system:

Initial power comes from 8 x 300 watt high voltage panels, (2 strings of 4 panels in series (300vdc) connected in parallel). Total output 300vdc and 2.4 kW in full sun. This powers a TopSolar mppt inverter which is rated at 300va and 230vac output. This charges 2 x LiFePo4 battery packs at 24vdc and 5 kWh capacity which means I have power 24/7.
This in turn powers a VEGA DY220SL photovoltaic pump controller (at 230vac input) through an isolation relay operated by a float switch in my water storage tank. The VEGA unit runs a 230vac 3∆ borehole pump rated at 0.75 kW (1 HP).
The VEGA unit consumes 1350 watts soft start (single phase) which should relate to 1.5 kW ie; within the capacity of the TopSolar (3,000va = 2.55 kW). However, the TopSolar unit is actually running at a constant 105% (overload) = 2.68 kW.
If I run an electric kettle (1.5 kW resistive load) on the TopSolar unit it reads 56% load and no overload.
Please can anyone explain why there is this huge difference? I know it must be due to a reactive load as opposed to a resistive load but I don't know why. Or if I can resolve this issue. ie; just buy a bigger primary inverter or is there some way to make the TopSolar unit see the VEGA unit differently.. without running a constant overload.??
Best regards and Many thanks. James Leenders

 

[JBertok]

Being personally unfamiliar with Vega, I read up on some specs for it.

There appears to be several power levels of the DY220SL available:


Every version of the DY220SL appears to be intended to be directly powered by a PV array with a voltage range between 220-370V DC.
Your original post says the PV connects to the MPPT inputs on a Top Solar inverter which has a 24V battery bank.
I am not sure how the DY220SL is getting power from the Top Solar inverter, it seems to be designed to take PV directly. If the DY220SL is just functioning in strictly an AC-input only mode where is is only converting single phase to 3-phase, it seems like overkill for this application.
Is it possible to connect 4 panels directly to the MPPT input on the DY220SL? That would be exactly right for its rated array working voltage of 305VDC.

 

[James Leenders]

Being personally unfamiliar with Vega, I read up on some specs for it.

There appears to be several power levels of the DY220SL available:
View attachment 215422

Every version of the DY220SL appears to be intended to be directly powered by a PV array with a voltage range between 220-370V DC.
Your original post says the PV connects to the MPPT inputs on a Top Solar inverter which has a 24V battery bank.
I am not sure how the DY220SL is getting power from the Top Solar inverter, it seems to be designed to take PV directly. If the DY220SL is just functioning in strictly an AC-input only mode where is is only converting single phase to 3-phase, it seems like overkill for this application.
Is it possible to connect 4 panels directly to the MPPT input on the DY220SL? That would be exactly right for its rated array working voltage of 305VDC.

 

[James Leenders]

Hi J Bertok
Yes, the DY220SL-001 will work quite happily from the solar panels which supply 305vdc at 2.4 kW in full sun. The challenge arrises at night or during overcast days when the direct solar power isn't enough to power the pump. The use of the primary inverter gives the advantage that it will slowly charge its batteries even on overcast days and then, when needed, the batteries can power the system with stored power. It is possible that the DY220SL-001 is feeding back a harmonic frequency into the Top Solar inverter which (by distorting the sine wave) then overloads. But; I just don't know...or if I can do anything about it, or cure it in any way. If I purchase a larger primary inverter it might be that exactly the same thing happens again. That's why I'm hoping that a real expert could give me a reasonable definitive answer.
I have thought of using a transformer to isolate the two inverters from each other?
So...if you've got any ideas..I'm all ears.
Many thanks and best regards James

 

[JBertok]

Ahh, understood. So you're not using the MPPT or the DC-to-AC inverting capabilities of the DY220SL-001, it's converting single phase to three-phase for the submersible like I thought could be the case.

750 Watts is about right for a 1HP AC induction motor mechanical output rating, but that number seems low for the power consumption rating at 1HP. I would think that might be closer to 1000W. But... even factoring in the extra 250W, 2680W AC from the inverter feeding the phase converter and pump just doesn't add up. That missing 1700W would have to be dissipating as heat from the components - imagine almost the heat output of a typical 1800W space heater at full blast if it really is taking this much power.

Is it possible the TopSolar's internal metering is generating a false measurement caused by the reactive load of the phase converter circuitry? Induction motor loads are notoriously problematic for high-frequency inverters, but that motor isn't being powered directly from the Top Solar. I think with calculating amperage in a single to three phase conversion you just multiply the 3-phase amperage by 1.73 to get your single phase amps assuming a 100% conversion efficiency, but your Top Solar inverter is reporting in Watts not Amps, as you've said. That inverter is rated for 2400W output continuous, and allegedly 99.5% efficiency. (that would be amazing if that spec is real) The power output just isn't not adding up as you've concluded. I guess you could meter the 24VDC going into the inverter and do the math on that number to verify the inverter really is putting out close to 2.7kW.
So, 2680 watts x 1.005 (99.5% inverter efficiency) / 25.6 volts (battery) should be about 105Amps DC. If it's a lot less than that then something is up with how AC output is being metered. Even if false, the inverter is reporting that 2.7kW output meaning it "thinks" it's running over 100% duty cycle, and that could trigger protection shutdown.

Regardless of the measurements, 2400W should be plenty of capacity to start and run a 1HP induction motor. With soft-starters I'd even argue it should power TWO of them, and actually I think soft-start functionality is built into the DY220SL. I'm out of ideas with this one, wish I could be more help, sorry.

 

[James Leenders]

Thank you so much! You have basically confirmed what I have worked out. The best thing is that ; I'm not going crazy: the numbers don't add up. I can hold my hand under the fans of the TopSolar and, although slightly warm, there's no way they're dissipating 1700 watts of energy. Craig (inverter repairer) had the theory about a harmonic feedback that would distort the sine wave and create a false overload and I think there could be some credence to this theory.
One of the reasons for not using the DC to AC mode DY220SL is that during low sun days it continually tries to soft start the pump, essentially pulsing it but not actually moving any water, thus just adding wear to the pump. There was no way of preventing this so I then came up with the idea of running it on AC to AC mode, (easier switching 230 AC on and off) and the battery storage enabled 24 hour service whenever water usage tripped the float switch in the tank.
Craig is currently repairing an SRNE SR-EOV24 inverter that was damaged by a lightning pulse, when fixed I think I might try this in place of the TopSolar unit; the SRNE is a full 3 kW output and very sturdy machine (more expensive and higher quality) at least I can then compare the two.
Thank you so much. Many thanks.
Best regards James