The self destruct switch

[DanB]

One of our neighbors has a really nice system.  He's got 2 Outback inverters, 1.2kW of solar on his roof top, hooked in through an MX60 charge controller. He's also got a 20' diameter wind turbine that we helped him build.   All of this charges a (roughly) 800kWh 'rolls' battery bank and it's monitored with a trimetric meter.

I was up watching the system a few weeks ago and noticed the trimetric wasn't monitoring current in, or out - he'd mixed up some of the wires on the shunt.  Easy fix!

He's got 1 big breaker in between the batteries and everything else (his power panel was ordered and came that way). Seems like lots of off grid systems are setup this way.

So the other day it was quite breezy and sunny and he decided he'd rewire the trimetric so it worked...

To shut down the system he shut off the 1 big breaker between the batteries and... everything else.  He forgot to shut down the solar array and the wind turbine.  In less than 1 second he lost both inverters, the MX60 and the trimetric meter due to the high voltage....  probably from the wind turbine.

I wonder what the solar array through the MX60 does under these conditions - does anybody know what sort of voltage might be at the output end of the MX 60 under these conditions (the array voltage is around 100V)?

Expensive mistake - in such high winds he's fairly lucky I think that the unloaded/free spinning wind turbine wasn't damaged.

It should be such that this sort of thing can never happen... it should've been wired so that there is no way to expose the inverters/rest of the system directly to high voltage from the array, and the wind turbine, it would've been easy to setup that way.

As it is, he's lucky there was a spare 48V inverter in the canyon to get him by while all this stuff is being mailed off for repairs.  That will at least keep his fridge going and save a lot of generator time!

[TimS]

wow.  Sorry to hear that.  I'm not sure what the Mx60 does without a load, I'm surprised that that happened.

[electronbaby]

without load, the MX should only have a max of 110vdc if the PV array was designed correctly. You could push it a little higher, but it advised not to.

The problem stems from the fact that the battery breaker disconnected the batteries and not the inverters. The systems we do have two sets of breakers. One located at the bank itself (which you normally never shut down) and another set of breakers at the inverters (which you would use to shut down the system). If you are to shut down the inverters, always shut off the MX first with its own dedicated breaker. The wind should never be disconnected from the battery bank, and normally would be shut down by shorting (before the rectifier!!)

Im not sure if the MX would allow the high voltage DC to pass through to the output, as there is some isolation there, and this would be a question for boB. Was he running a mate?

The wind definitely could have produced enough high voltage to cook the inverter. Make sure not that the wind is always connected to the battery bank (or at least turned off) when inverters are shut down.

Sorry to hear about the damage  :-/

[dbcollen]

The MX60 is usually self protecting, it will say "Unloaded", or "Got Battery" and limit the output voltage to the charge setpoint. If you have enough PV to cover the load, the inverters will be happy to run off the MX60 output directly. I have done that to keep the basement lights on while working on the battery bank. The Windmill, or Hydro are the problem in that scenario, and need to be TURNED OFF, as most diversion loads will not react fast enough to keep the voltage down.

Sorry to hear about the loss.

Dustin

[richhagen]

In the computer lab I helped BThumble with in Fiji, a similar problem occurred.  The teacher disconnected the battery and left the turbine connected, burning up the dump load controller and the laptop power supplies.  Apparently the laptops were ruined one by one as the teacher tried to get one to work.  Ben rewired it so that if the batteries are disconnected, the turbine was shorted, and the next year after that the turbine was put in storage for lack of anyone local to maintain it.  We had sent a replacement for one of the guys which was in horrible shape from corrosion, but no one had bothered to put it in and we were worried that if no one maintained it, it would wind up toppling over.  Probably a very good thing since there is now no air service to that island, and no reasonable way to get there within short time frame as one now has to either hire a boat, or ride the freighter which takes several days each way making its rounds. Rich

[richhagen]

I'm sure these are not unique situations, and the moral or warning of the story is to disconnect the turbine and short it prior to disconnecting the batteries.  Solar is probably less problematic if the system was designed such that the open circuit voltage is within the tolerances of the charge controller and the controllers output voltage is limited under those conditions as electron baby pointed out.  The voltage of the turbine is proportionate to its RPM, and unloaded, most can go to a multiple of the system voltage in gusty winds.  If I had such a system, I would try to rewire it to make it more idiot proof, and would at least post conspicuous warnings in case I was not the one operating it.  At least, I would connect the turbine directly to the battery bank, with its own shorting switch on the turbine side of the rectifiers, and then have a shutdown switch or breaker between these and the rest of the system.  To shut everyting down, one would need to throw two switches, but it seems to me it would be less prone to this type of disaster.  As it is, I live in the middle of a big city, so while I would love to have one, I would have even more issues with a big Hawt.  Rich

[halfcrazy]

I did that same thing here by mistake I turned off the battery breaker and the pv input breakers but forgot the turbine  witch fed the Classic close to 200 volts and in turn smoked an FM60 charge controller and a Xantrex XW charge controller it strangely didn't hurt the Magnum inverter and the 2 classics survived as they are safe to a lot higher voltage.

[bob golding]

i have fried a laptop several inverters and a charge controller in the past by doing this. my system is still a lash up with croc clips at the moment. i did wire it all up properly once then fried my inverter/charger  from the genny  end! cant win. i am back to the lash up until i get around to finishing playing with it. i just make sure i disconnect things in the right order and try not to disconnect anything when there is any wind. this sort of thing is so easy to do. sorry to hear about their problems. hope it all gets fixed soon. maybe a poll to see how many have people have done this. might make people feel they are not the only ones to have done this?

bob

[GWatPE]

Sorry to hear about the damage, but I would have thought the windmill overvoltage protection would have protected the system, if the loading was removed.

Gordon.

[halfcrazy]

Windmill over voltage protection? On these home brew machines the only overvoltage protection is the battery bank and the dump load but disconnecting the battery breaker unhooked the turbine from the battery

[scoraigwind]

I have a standard label that I like to display at the battery terminals: "Never disconnect the battery while the wind turbine can run.  Always brake the wind turbine before working on battery connections."  It's such an expensive mistake and it goes on happening.

I never use a single fuse or breaker that disconnects the wind turbine from the battery and leaves it connected to the inverter, controller or even volt meter.  I always use a separate fuse for the wind and solar inputs.

If you need a device that disconnects the battery in one movement, then a good way to do this is to use a multi-pole device.  For example you can get fuseholders with multiple fuses that are all pulled out at the same time by one big lever.

If you are using a switch to disconnect the wind turbine then it's a good plan to use a 'changeover' switch wired so that it short circuits the turbine in the other position.  This will short circuit the DC of course, whereas an AC short circuit brake is also a good thing to to have.

The best way to wire the wind turbine is to bring the AC wiring straight to a short-circuit switch (usually 2-pole with 2 phases being shorted onto the third) as it enters the building - then go from there to the rectifier and then the battery via an ammeter and a fuse.  Use the biggest fuse you dare because you do not really want it to blow.  Connect nothing else to the wind turbine wiring.  Wire everything else to its own battery fuse or breaker.  Wired like that you can short circuit the turbine and then work safely on the rectifier or anything else in the system without risk of it starting up.

[wooferhound]

I will change to poll soon to ask, what kind of failures we have had.

[BX]

I am currently designing the "back office" for my system: 10' turbine, 48 volt design from the book. Doesn't this wiring help to avoid the problem ?

[Bushwhacker]

Would a simple double knife switch work on a DC turbine? In the charge position it would feed turbine output to the batteries and when in the stop position it shorts the output wires from the turbine? Thinking about it, a single knife switch out to be able to do that. I think that's what Hugh said??? Yes, I'm still a DC gen cave man.

Not trying to hijack the thread, just 'electrically challenged' and learning. :s

[scoraigwind]

Yes a nicaraguan knife switch is ideal for that job.

[hydrosun]

I didn't see anyone comment on the wiring of the main breaker in the original posting. In all the Outback boxes I've ever seen the main breaker only shuts off the inverter. A jumper wire from the battery side of the breaker goes to the positive bus bar or directly to the other dc breakers. So you have to individually shut off the loads or dc sources. Makes it more obvious when you disconnect something.

Chris

[Vtbsr]

I found in my Trace sw inverter owners Manual Page 110. It is talking about self inductance in battery cables.  This is called flyback effect and can get into thousands of volts range if the battery is suddenly removed from the circuit. By keeping your battery to inverter cables as short as possible, and taped or twisted you can cut down the inductance ( in micro-henries). The battery cables, build up a magnetic field when a current passes. When you break the circuit the  magnetic field must jump and create the voltage spikes.  With my Harris hydro you have to shut down the turbine before you can disconnect it from the battery bank. Anyone else think this might be the problem? I wonder what would happen on the grid if a large part was suddenly removed with out a shutdown of a power plant.

[bob golding]

this!

http://www.lifeaftercoffee.com/2006/05/23/worlds-biggest-jacobs-ladder/

remember the energy is not from the grid just what is stored in those big inductors to the right of the switch.