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Participant

Join Date: Feb 2015
Posts: 3

Multitasking Solar

02/03/2015 11:48 AM

There is a current boom in solar sales and installation in Louisianna USA at this time. However most of the systems are a central inverter grid tyed type. I am looking for a 3 position disconnect/switch to transfer the solar panel feeds to a stand alone battery backup system as a retro fit to these systems. Outback, Midnight solar and others, already make charge controllers that work at the voltage amp output of single or multiple strings of these systems. The engineering is done, I need an inexpensive way to easily and safely without violating the warranty of the inverter system to divert the feed from the panels to the charge controller. I know a 3 position hp rated disconnect will work however they are bulky and expensive. All of the central inverters have DC rotary switches already but I am having problems locating the manufacture. The manufactures I have contacted have not provided an answer.

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Power-User
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#1

Re: Multitasking Solar

02/03/2015 12:10 PM

The big issue is that your switch needs be rated to switch high voltage DC. In working with high voltage DC one must be aware of and deal properly with arcing. This is why the switches that you will find will be large, bulky and expensive. It's also one of the more expensive parts in a commercial inverter.

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#2
In reply to #1

Re: Multitasking Solar

02/03/2015 1:39 PM

and...almost certainly will have a handle like a lever, not just a turn knob so the switch will have a "positive" open or close or the contacts can weld if stopped momentarily.

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#5
In reply to #1

Re: Multitasking Solar

02/04/2015 5:43 AM

GA

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Participant

Join Date: Feb 2015
Posts: 3
#3

Re: Multitasking Solar

02/03/2015 1:57 PM

update I have been able to find an industrial control 600v dc auto- off- hand switch that is only rated for 20 amps, I am looking for one rated 30amps, Similar to the ones in the existing inverters.

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#4
In reply to #3

Re: Multitasking Solar

02/04/2015 1:53 AM

You don't say what voltage your panels are working at, but if it's grid feed I would expect somewhere around 300 - 400v. Your current though, is pretty small at only 30 amps.

Have you considered an electro-mechanical solution?

At that current, you could reasonably expect good arc quenching with a pair of contacts in series and possibly a capacitor - resistor across them.

Therefore, a possibility would be two four pole n/o 50 amp AC contactors (easier to get than DC ones at that voltage). A single four pole c/o contactor will not give you the off state that you specify.

One contactor would use 2 poles in series for each of +ve and -ve from the panels to the inverter, and the other contactor would use the same from the panels to the charge controller (I assume your charge controller is an MPPT type and can handle the input voltage).

Control of the contactors could then be automatic (battery state of charge based) or manual (small 3 position switch) if preferred.

Coil voltages would be of your choosing dependent on control method, AC coils will work fine on DC, just keep the applied DC about 30% lower than the rated AC coil voltage either by coil selection or a suitable series resistance.

Interlocking to prevent both contactors being energised concurrently will be mandatory and easily managed either via the auto/manual operators or a set of auxillary contacts.

If you go down this path, use contactors, not relays, as the contacts break on both sides (larger arc gap) rather than the reed type operation of a relay. They also have greater spring load so will disengage on DC far better than will a relay.

The above could be achieved electronically using either Mosfets or IGBTs but you would likely pay a fair bit to get it done whereas the contactors would be an easy fix.

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Guru

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#6

Re: Multitasking Solar

02/04/2015 10:21 PM

Friend,

One of my friends is called "Mr. Solar" at the Baton Rouge chapter of ABC (Associated Builders and Contractors). A call there may get you reliable help.

--JMM

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