[RE-wrenches] Remote battery disconnects
bob at midnitesolar.com
bob at midnitesolar.com
Wed Feb 1 19:06:49 PST 2023
SPDs or (MOVs) on the PV input side of a charge controller are not
wired to snub excessive PV voltage.
They are wired from PV+ to GND and the other from PV- to GND and only
the two in series, at double the
MOV clamp voltage would have any effect on clamping the PV+/PV- voltage.
SPDs are wired to keep the charge controller's insulation system to
below the Hi-Pot voltage the unit is
tested with. i.e. They are wired for common-mode and not
differential. That voltage is much higher
than maximum controller or inverter input voltage.
What *IS* in place that might help limit PV voltage is the PV array
itself. Any PV voltage that would try go
go above the array's Voc would be clamped across the array. This
would be during the day or at night and
of course that Voc is determined mainly by PV cell temperature. Solar
panels are just a bunch of diodes
in series so will be forward-biased when Voc is exceeded by an inverter
or charge controller.
PV modules make great space heaters and can even melt snow, ya know. 😁
As for charge controllers blowing up when charging hard and then
disconnecting, I am very anal in
designing in safe guards to keep this from happening on the MidNite
Classic at least. That was done
several years ago now.
But even then, electronics, especially power electronics, break once in
a while and it is sometimes very
hard to know why it happened.
With our new HV MPPT controllers, their E-Panels also include
remote-trip PV breakers, triggered by the controller's
hardware in order to reduce battery side over-voltage in case
something goes wrong.
So, I understand that we have a remote battery disconnect device
basically done but have held off due to
not knowing how important it would actually be. 300 amps from an
external switch closure or open.
Our older BD module basically went away with the bird house and bird
next shut down system.
There are NEC requirements that are not enforced and some installers
don't worry about including,
but from this discussion, it sounds like this battery disconnect better
be finished sooner than later.
Now we know. Thanks !
🌜boB🌛 MidNite Solar
On 2/1/2023 2:49 PM, Brian Mehalic via RE-wrenches wrote:
> Most firefighters are not electrical engineers; nor are most insurance
> agents. However both of those groups are stakeholders with influence
> over codes and standards. Some may say too much influence, but the
> fact of the matter is that code making is a group/consensus process,
> and very often none of the groups get exactly what they want (in fact
> sometimes a group accepts exactly what it doesn't want to leverage
> that for something they do; yes it often smacks of politics). I would
> appeal to y'all to submit a proposal for the 2026 NEC® - they are due
> by September of this year. Better yet would be to circulate language
> that this, or some other, group can work on, agree on, write a solid
> technical justification for, and sign their names to and submit - the
> more stakeholders that support a proposal the more weight it has.
>
> It seems that the PV Industry Forum, or another similar body, may
> again begin work to develop PV and storage industry
> stakeholder-consensus PIs to submit; in the past this was a very
> successful endeavor. I'll post on this list if and when that happens
> so that anyone that is interested can get involved, but don't let that
> possibility keep you from submitting any PIs on your own.
>
> Maybe in the meantime, a control system that shuts off the PV, and
> then the batteries, would be worth designing.
>
> Brian Mehalic
>
>
>
>
> On Wed, Feb 1, 2023 at 2:10 PM James Jefferson Jarvis via RE-wrenches
> <re-wrenches at lists.re-wrenches.org> wrote:
>
>
>
> On 2/1/2023 2:58 PM, Starlight via RE-wrenches wrote:
> > Have you (or any) installed surge suppression on the PV input
> side to
> > clamp the open circuit voltage surge?
>
> That's not where the issue is and isn't really a solution. As I
> explained in my previous e-mail, the issue is the open circuit on the
> output leaves no where for the energy in the MPPT controllers
> inductor
> to go and the only option is for the voltage to rise until it finds
> somewhere to go.
>
> Surge suppressors, such as sold by Midnite Solar, are nothing more
> than
> Metal Oxide Varistors. Or in the case of Delta, they are pieces of
> wire
> in sand. They require large rise in voltage before they conduct.
>
> MOVs are sometimes found on the battery side of inverters and charge
> controllers, but their clamp time and voltage before clamping
> requires
> additional circuitry that works faster to protect fragile
> semiconductors.
>
>
> If you are proposing using a surge supressor to regulate the
> voltage of
> an island between a charge controller and an inverter when the
> battery
> is disconnected, that's also a no go. Once the MOV clamps, it stays
> clamped until voltage goes essentially to zero or until it blows up.
> It's not a regulator.
>
> -
>
> To further stir the pot, it is definitely possible to design
> electronics
> that can deal with load dumps and survive most anything. Aerospace
> electronics come pretty close. But all that costs money and takes
> space
> to do. Are we comfortable with requiring $16,000 charge
> controllers so
> they can handle load dumps reliably from a poorly thought out NEC
> requirement? I think the fossil fuel industry would be very happy
> with
> the cost of renewable energy electronics being an order of magnitude
> more expensive.
>
>
>
>
> >
> > Larry Crutcher
> > Starlight Solar Power Systems
> >
> >
> >
> > On Feb 1, 2023, at 1:23 PM, William Bryce via RE-wrenches
> > <re-wrenches at lists.re-wrenches.org
> > <mailto:re-wrenches at lists.re-wrenches.org>> wrote:
> >
> > I can also backup what Jim has said, and have seen the same gear
> > destroyed by removing the battery abruptly when the controller
> is under
> > heavy load. They can die, and sometimes die spectacularly.
> >
> > Have seen SolArk inverters integrated MPPT controllers blow up when
> > lithium battery BMS disconnects. Not a field fixable issue.
> >
> > Just flip off the breaker when the solar is working hard and and
> the
> > SolArk will give up the smoke.
> >
> > Like I originally said, it’s the non talked about issue that is
> a big
> > issue depending on what gear your using.
> >
> > On Wed, Feb 1, 2023 at 2:49 PM James Jefferson Jarvis via
> RE-wrenches
> > <re-wrenches at lists.re-wrenches.org
> > <mailto:re-wrenches at lists.re-wrenches.org>> wrote:
> >
> >
> >
> > On 2/1/2023 12:25 PM, Alex MeVay via RE-wrenches wrote:
> > > Although our controllers are probably smaller than what
> you would be
> > > considering in this discussion, surviving a load dump
> (suddenly
> > > disconnected battery) is an engineering requirement for
> us, and
> > likely
> > > would be for other responsible MFG's on this list
> (boB?). The charge
> > > controller can either handle full input voltage on the
> output, or
> > > there is a comparator that will shut the controller down
> instantly
> > > when the output voltage gets too high.
> >
> > So that's part of the picture.
> >
> > The other part of the picture is the rest of the system. The
> discussion
> > and what NEC is mandating is that the battery be
> disconnected. As
> > far as
> > I can tell, there isn't an explicit requirement in all cases
> that
> > all of
> > the power inputs into the system are all going to go away at
> exactly
> > the
> > same time .... or ever. This leaves things like charge
> controllers
> > getting input power potentially from PV or wind or grid or
> generator or
> > something else. And nothing in the NEC, as far as I can tell, is
> > mandating that all of the DC loads be disconnected. So the
> loads are
> > online. Now the regulation circuit (switching power supply
> in the case
> > of a MPPT controller) is regulating variable loads and
> nothing is
> > providing substantial resistance to change. Normally the
> battery is
> > acting like a very very very big capacitor. But without the
> battery,
> > there isn't enough damping in the control loops and voltage
> stability
> > will suffer. This is where you get 250 volts on your
> normally 48 volt
> > battery bus. Or 3 volts. Or -80 volts. Or all of those in a
> fraction of
> > second. This sort of thing is hard on electronics and will cause
> > failures.
> >
> >
> > Alex's comment about Genasun's controllers handling full
> input voltage
> > on the output is probably unique to their niche product.
> Looking at
> > their biggest controller, it appears to support VOC of 34
> volts. At 34
> > volts, he can use 50V or 100V rated components on his
> output. For a
> > 150V
> > input controller, you probably can. But you wouldn't because it
> > would be
> > too expensive. But on a 600V or 1000V controller, there's
> just no way
> > that you do that because 1000V rated components are big and
> expensive
> > and their spacing requirements are huge compared 100V level
> sort of
> > stuff. So the way bigger MPPT controllers deal with load
> dump is with
> > transient voltage supression or other diodes to handle the
> voltage
> > spike
> > caused by the inductor when the load goes away. When operated in
> > parameters, these parts don't wear out. But it can be
> interesting to
> > size them adequately to account for inductance elsewhere in
> the system
> > adding to voltage and energy that has to be absorbed.
> >
> >
> > There is a very simple experiment that anybody can perform
> to see how
> > equipment handles a load dump: Simply wait for a sunny day
> and turn off
> > the battery breaker. If no magic smoke was released, turn
> back on
> > battery breaker. If still no magic smoke was released, then
> great, your
> > system survived a load dump.
> >
> > If you are at all uncomfortable doing this and/or your
> system gets
> > destroyed in the process, think back to my earlier security
> comments
> > about having a self destruct switch on the outside of your
> building
> > allowing anyone walking by to do this experiment for you.
> >
> > I have personally destroyed Outback, Midnite, and Morningstar
> > controllers inadvertently or intentionally doing load dumps
> by shutting
> > off their output breaker. Typically the TVS diodes short out and
> > secondary over current protection (circuit breaker) trips
> before things
> > catch on fire. Usuaully the UL94V0 rating on the circuit
> board and the
> > box the circuit board is in prevents fire from spreading
> when things do
> > get wild.
> >
> > But I've also seen all of those brands survive a load dump.
> >
> > I have multiple customer who have fielded lithium battery
> systems to
> > cold locations and have had battery BMS disconnect the
> battery from the
> > rest of the system. Ten's of thousands of dollars of
> equipment has been
> > destroyed in these islanding events.
> >
> >
> > Some thoughts, for what they are worth.
> >
> > -James Jarvis
> > APRS World, LLC
> >
> >
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