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Kirk B avatar image
Kirk B asked

Negative AC input .... How so?

Twice now I have experienced a negative AC input. I'm sure it is happening more frequently and has just gone unnoticed. I have two multiplus 24/3000 inverters in parallel. I have defined two AC inputs. Shore power is connected to the master AC input. When there is a load (eg, Air Conditioner, water heater), I'm seeing the AC input go negative, a significant drain on my batteries which appear to be supplying power for both the AC Output and the suspicious AC Input (feedback???). After the load drops, the system appears to return to normal and recharges the battery bank. I have no assistants loaded and have not defined a country code for feeding back to the grid. How can this be???


MultiPlus Quattro Inverter Charger
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2 Answers
Kirk B avatar image
Kirk B answered ·

Daniël,


Yes, you read correctly, this is a parallel system. When I built the system I received sign off from you guys that defining two separate inputs was ok. However, and this might be on me, I was running split phase but due to limitations only having 3000va per leg. We moved over to a parallel setup to ensure we could pull more amps per leg. Essentially we have 3 air conditioners. 2 on one leg and 1 on the other. Parallel allows me to run any two of the 3. Where as, split phase restricted me to one AC per leg restricting the ability to run the two air conditioners that are on the same leg at the same time. Hope that makes sense :)

If the asymmetrical wiring on the AC input is indeed a problem, that is an easy fix.

Also, sorry no accurate diagram and pictures right now will be tough as we are traveling. Inverters are in a storage compartment that has a number of other items stored in front of my inverter / battery enclosure.

Thanks!

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Hi Kirk,

Yes as in the documents: All AC and DC wiring must be symmetric.

So you'd need a transfer switch, or for the time being just use a plug to connect to either shore power or generator.

In a split-phase situation you can indeed use two separate inputs.

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Kirk B avatar image Kirk B Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

This is great news honestly And likely explains a few other anomalies I’ve had. Thanks!

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Kirk B avatar image Kirk B Daniël Boekel (Victron Energy Staff) ♦♦ commented ·

Daniël,


Giving this some big picture consideration. A no feedback system should mean no feedback. If there were a mechanical failure in the wiring that went unnoticed, the results could be catastrophic. Especially if power is being provided by a generator. Considering this is in an RV, a mechanical failure that does not result in a short (eg, ac input wire breaks loose but does not touch ground) is very possible. If this scenario results in a parallel system feeding power back, imho this is dangerous. Is that safe to say?

If so, a Quattro 5k May be the better solution.

Thanks, Kirk.

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ Kirk B commented ·

Hi Kirk,

All loose wires, etc. are (potentially) dangerous.

All power electronics can be dangerous. A couple of hundred amps can melt things quickly.

That's why we don't advocate inexperienced users working with these sort of systems, as is explained in various warnings in both manuals and software, like here:

https://www.victronenergy.com/live/ve.bus:manual_parallel_and_three_phase_systems#warning

Victron installers are trained by Victron, and even 'power users' can be allowed to follow training:

https://professional.victronenergy.com/learn/events/


Feeding back power into a generator: if it's a 'mechanical' generator (non-inverter type), chances are nothing breaks: voltage / rpm shoots up and inverter stops with overvoltage / hertz alarm. But yes things could break.


It is also sometimes difficult for us to give proper advice on this forum, as we have to feel if users are 'up to the task' and sometimes have to advice to get an experienced installer to check out / correct a system.

about one bigger inverter vs two smaller ones:
Yes we'd always advice to use one bigger unit, the Quattro has the added benefit of two AC inputs, making life much easier in a dual power source setup.
Also the chance of wiring mistakes are much smaller.

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Daniël Boekel (Victron Energy Staff) avatar image Daniël Boekel (Victron Energy Staff) ♦♦ Kirk B commented ·

quote:

Considering this is in an RV, a mechanical failure that does not result in a short (eg, ac input wire breaks loose but does not touch ground) is very possible.

No. this should not be possible on a well installed system.

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Daniël Boekel (Victron Energy Staff) avatar image
Daniël Boekel (Victron Energy Staff) answered ·

Hi @Kirk B

This usually has to do with wiring issues.

Please read this document:

https://www.victronenergy.com/live/ve.bus:manual_parallel_and_three_phase_systems

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Kirk B avatar image Kirk B commented ·

Thanks Daniël. I did follow that guide when I did the installation. I am however using only 1 breaker for the AC output but I would not imagine that would cause a back feed on the AC input.


Question: Am I really back feeding power on the AC input or is this just an artifact of something else? This seems rather dangerous and implies that the AC in and AC out are not isolated.


I added a second multi as my power needs exceeded my original design. Would I be better off selling the multi's and going with a Quattro?

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