Small scale off-grid inverter blowing internal fuse/OCPD question

alexj · December 2017

I am working with an organization that builds small-scale offgrid solar systems for running lighting and small appliances in the developing world. I am new to the organization and I am trying to improve the designs of the organization's systems. We have been using Xantex 300W X-Power inverters that are internally fused and have had quite a few fuses blow in very remote locations - a situation that is really difficult to address. The customer cannot fix the situation and we have to send a tech out, which can involve hours of travel in some cases. The internal fuse is a 40a blade-style fuse. I have a few questions that I am hoping some folks here can answer:

1. If the inverter has built in overload protection that, in my experience, typically works to shut down the inverter before the fuse blows, then why are these fuses blowing? In no cases so far has it been a short or installation issue. What could we improve in our design or our discussions with our clients to prevent this from happening? We obviously discuss not connecting too large of loads, but what are we missing?
2. There is already overcurrent protection in the design, in the form of a 40 amp breaker between the battery and the inverter, but the breaker has not prevented the internal fuse of the inverter from blowing in these cases. The 40amp breaker is oversized in my mind - we should be fine with a 35amp breaker, but in any case, after looking at the trip curves for a blade-fuse, it is clear that they will open far faster than Midnite's breakers. In an ideal world, I would have some form of OCPD that would open before the inverter's internal fuse does, but that will not trip as the inverter surges.
-Is there a DC breaker solution that isn't cost prohibitive (Midnite's breakers, at $20, are around the pricepoint we are looking at) that would work for this application?
-Would a 40a blade fuse right near the battery open before the internal fuse in the inverter? Or would they both open? Or would it just be a crap shoot because they are so close and there might be variances in manufacturing?

Links to datasheets:
Littelfuse blade fuses: http://www.littelfuse.com/~/media/automotive/datasheets/fuses/passenger-car-and-commercial-vehicle/blade-fuses/littelfuse_atof_datasheet.pdf
Midnite Q series breakers: http://www.midnitesolar.com/pdfs/Q-Series(1).pdf

Cheers!

BB. · December 2017

Automotive fuse holders and fuses are well known for poor reliability.

Holders that have poor electrical connections is probably the number one issue. This leads to overheating holders and possible fire hazard.

Less clear is if the fuses themselves and or Tyne overheating fuse holder causes the fuses to pop at well less than rated current.

Ideally, using magnetic circuit breakers and wiring rated at 1.25x (nec 1/0.80 derating) at minimum battery bus voltage will also help a lot. For example a 600 watt 12 volt inverter:

600 Watts × 1/0.85 inverter eff × 1.25 nec derating × 1/10.5 batt shutdown = 84 amp minimum branch circuit rating.

That will give you a reliable system design.

Bill

BB. · December 2017

Thermal breakers can work ok.

Hot installations (ambient temperatures) can derate a thermal breaker and years of running near rated current can accelerate aging (probably will still work well for many years).

Bill

Estragon · December 2017

Just a thought... this appears to be a small MSW inverter. Is there any chance it's being used to backfeed an AC distribution or other load with a neutral-ground bond?

This type of inverter is designed to power AC loads directly plugged in to the outlet on the inverter.

BB. · December 2017

If the loads are inductive (motors, transformers) or other poor power factor, that can high (circulating) currents on the ac and possibly dc sides of the inverters. MSW inverters are worse wth poor power factor loads.

Bill

mcgivor · December 2017

@alexj
My view on the situation is that the fuses are more than likely doing what they are designed to do, protect the inverter. Not included are the loads and battery capacity, my understanding is that the mentioned inverter is a 12VDC input 230VAC output. Consider a load of 1 amp on the AC side, this will equate to ~20A on the DC side with a simple resistive load, inductive loads would be more. Depending on the capacity of the battery, this could result in a DC voltage sag, which in turn will increase current, so having a marginal battery capacity may be the root cause of the problem. Using a 12V inverter to achieve 230V is particularly difficult due to the approximate 20 : 1 current conversion ratio. The inverters internal overload protection is not over current protection, it is to prevent overload on the AC side assuming input voltage is stable, if the input voltage is low current will increase and the fuses will blow.

A few questions, are the fuses blowing immediately or after a period of time, have voltage measurements been taken at the battery and the inverter input under load, has the current been measured and are any motors part of the loads?

It would be very helpful if the battery capacity, conductor size (battery to inverter ) loads and any other details are provided, without which one can only speculate.

Photowhit · December 2017

You should already have some feedback as to what the loads have been when the fuse has blown. Could you relay that information?

BB. · December 2017

Fuses and breakers are there to reduce the chances of overheated wiring and components from catching fire.

Fuses and breakers are not near fast enough to save the FET/Transistor in an inverter from failing from over current. Inverters, in general, have electronic limits that will shut down an inverter if there is an overload on the AC output much faster than a fuse/breaker will.

If you are having an internal inverter fuse fail--That sounds like the inverter itself failed (shorted output transistors) or, possibly, your inverter is running near (or a bit over) rated output power with low input voltage. Inverters are "constant power" devices:

Power = Voltage * Current

If, you have a 600 Watt load, the inverter will draw more input DC current as the input voltage falls. The input voltage can fall as the battery bank discharges, gets old, failing cell, excessive voltage drop on DC battery bus, etc.

Some more details on the the hardware (inverter brand/model/size, battery bus voltage, AH rating of battery bank, unusual conditions like high/low ambient temperatures, etc.).

-Bill

alexj · December 2017

@Photowhit - I am sure you know how it is with customers: they will never tell me if they did something that led to the problem as they fear having to pay of the damage. I wish I could give more info, but all I ever get is that nothing out of the ordinary was happening - that the system just suddenly failed.

alexj · December 2017

mcgivor

Interesting points - that explanation of overcurrent protection vs. overload on the AC side makes good sense to me. I would love to get some more info on what is happening with the systems and intend to do so here in the near future through installing some monitoring systems at various locations. I haven't been able to visit these ones that are having issues yet, although hopefully soon. I can personally try to run some experiments here in the near future with a test setup and see how the voltage sags. Here is some more information on the design:
The fuses have been blowing after months of usage typically.
The system is floating.
#8THHN run from battery to 40amp breaker to inverter.
75ah AGM battery
Loads on AC side typically shouldn't be anything other than small electronics. But I do know that some people do run blenders.
Regarding potential heating issues : we do put these small inverters, with a charge controller, and bus bars into a metal box with very small vents that is roughly two feet wide by three feet tall. More ventillation is on my list of things to address and to get information on the temperature inside that box under usage.

Thanks!

Alex

alexj · December 2017

Bill - thanks for your help as always on this site. Over the years I have always been on the lookout for your posts regarding whatever I am trying to figure out.

The inverter is a MSW Xantex 12v 300W inverter. The inverter is wired to the inverter directly with a 40amp breaker in between - no bus. No system has more than 6' of #8THHN between the battery and inverter. These systems are installed at between 3500m and 4300m typically. Season/temp doesn't seem to correlate with failures.

Let me know if there is anything else that could be useful.

alexj · December 2017

Taking a look at amperage. The Xantrex 300w cuts out at 10v. The Xantrex 300W at 85% efficiency = 352w worth of DC. Let's throw in the 125% continuous duty derate. We would then get 352 * 1.25 / 10 = 44a.....

alexj · December 2017

@mcgivor

Thanks for answering my question regarding why the inverter would still blow a fuse, but fail to overload on the AC side. I intend on getting out to the sites soon to be able to get some more information, but no I don't have voltage/amperage measurements currently and don't have a testing setup - I will build one here soon. We are also working on installing a monitoring platform on a few of our installs here in the near future to collect data. Here is a bit more information that might be useful in figuring out this problem:
-The systems are all floating.
-The inverter is MSW.
-The loads are typically just small electronics, but there are some people that I think are using blenders that push the limits of the inverter.
-The fuses have been failing months after the system has been installed.
-The wire from battery to inverter is #8THHN, run less than six feet, through a 40a breaker.
-The batteries are 75Ah AGM.
-The system is installed in a two foot by three foot metal box with limited venting, along with the charge controller. I intend to test internal temperature under conditions of use here in the near future and think that we should probably be venting it better. Heat may be an issue.

I know that the best answers will come from testing this at some point in the near future and collecting realworld data. I just don't have the time at this very moment and wanted to bounce the question off of your brilliant minds.

Thanks!

Alex

BB. · December 2017

4,300 meter elevation (~14,000 feet)? That is high. Most electronics need to derate their max power at high altitudes because of reduced cooling. For example:

http://outbackpower.force.com/articles/Electricity_101/High-altitude-derating-factors-for-electrical-systems

Altitude	Derating Factor
1,000 m	1.00
1,500 m	.94
3,000 m	.82
4,000 m	.78

http://outbackpower.force.com/servlet/fileField?id=0BE80000000GmtV

Is this your inverter?

http://xantrex.com/power-products/power-inverters/xpower-inverter-international.aspx

When you have a internal fuse (breaker) pop, is the inverter still good or has it failed? Any chance of lighting strikes? Have you ever had the external DC breaker trip? Any signs of debris in the inverter (i.e., dust, bugs, etc. on circuit/control boards--A common high current failure is when both "high' and low transistors turn on at the same time--either a transistor fail shorted, or control board failure that turns both transistor at the same time). Generally this causes major internal damage and is not fixed with just a fuse replacement. Dust/debris internal plus high humidity (rain, major ambient temperature drop) can short a control board/transistor drive circuit.

One "difference" with MSW inverters is that if you ground both the DC input (negative or positive ground the DC input) and have a short from (either terminal) of the AC output to the same safety ground--That will short out the AC inverter and let out the magic smoke. Most MSW inverters do not have galvanic (aka transformer) isolation of the AC output with respect to the DC input.

If you DC ground reference the battery bus (common safety measure to prevent short circuits of the DC bus from doing strange things and divert lighting energy)--You have to float both the AC connections. If you have surge suppressors on the AC output, that would seem to offer a potential grounding path for MSW inverter damage.

Your 40 amp breaker would seem to be "OK" (assuming that the AC inverter is not subject to a constant near rated output for hours on end).

There are fast blow/trip and slow blow trip fuses/breakers. For fuses, simply more thermal mass on the fuse (i.e., a measured blob of solder on a fusing strip) can make the fuses "slow blow". Slo-blow fuses are typically used when you have high starting current (such as motor) where a few seconds of 2x rated load power is to be expected.

Most TSW/PSW (true/pure sine wave) AC inverters have transformer isolation and can have both ground referenced DC battery bus and an AC output ground referenced neutral.

If you have a couple sites that are more likely to have internal fuses blown, you might want to try and get a decent quality TSW inverter and see if that reduces the failures.

I am trying to figure out if the internal 40 amp fuse is blowing because of normal operation (too large of AC loads), interaction with AC loads (highly inductive AC loads, AC to DC power supplies with traditional AC rectifier front end--very power power factor, loads that do not "like" MSW wave form). Or something went wrong (Inverter failure, lightning, surge suppressor trip).

One odd ball failure that we had here--People living in a region with 230 VAC 50 Hz AC power--But many times used US type AC power strips (rated for 120 VAC at 60 Hz) to plug in their 120-240 VAC 50/60 Hz computer gear. The wide voltage rage AC computers where happy at either 120 or 230/240 VAC but the US 120 VAC surge suppressors where just at the limits of tripping the surge MOV. These 120 VAC MOV trips were (as I recall) were vary hard on the 230 VAC inverters (a MOV trips "shorted" and relies on up stream fuses/breakers to cut the power, or an inverter over current trip). MOV trips can short line to line or line to ground.

Do you have any MOV (metal oxide varistor) surge suppressors on the AC output (lightning suppressors, AC power strings with suppressors, etc.)? Typically a MOV that trips on normal AC mains (or direct lightning strikes) will have obvious signs of over current/voltage (browning, exploding MOV, etc.). A MOV tripping on a 300 Watt AC inverter may not show any major signs of damage if it did trip.

-Bill

alexj · December 2017

-We have a few inverters with failed fuses that still operates after replacement. And then three of them that do not and are clearly damaged by either an equipment failure or lightning.
-No issuse with bugs/water/significant debris. Our design has kept the box relatively sealed for this reason as the places where we install the systems are pretty rough conditions for equipment. It is likely that there is some dust and I think that significant temperature drops and humidity shifts are the norm in the Andes.
-The systems are floating, so I don't have many concerns in that area. Nor any surge suppressors or lightning arrestors or anything with varistors.
-We install PSW roughly half of the time with other systems. None of these have had any issues.
-Nobody that I know of is trying to use heavy inductive loads or anything that really doesn't like MSW inverters. We only sell them to folks who have no intention of using any loads that don't like MSW. This doesn't mean that they aren't doing it though.

After bouncing these ideas around a bit I think that the most likely scenario is that it has to do with heat. Lightning I assume would also likely damage the charge controller?

BB. · December 2017

The forum's original founder (now retired) had said that in his experience, that the most common lighting induced failure was on the output stage of the AC inverter. Of course, if you have an output stage failure (failed shorted transistors), that would pop DC input fuse(s).

One thing with MSW inverters is the the "square wave" fronts. That high rate of voltage change (from zero to peak AC voltage), can cause igh peak currents too. If your loads are "typical" computer / Florescent twisty bulb / LED power supplies with poor power factor power supplies (typically use a diode charging a high voltage capacitor or even using capacitors to limit current to lighting), this can cause high peak current wave forms (when the square wave switches on). Because P=I^2*R -- High peak current (short current peaks vs low average current over entire wave cycle). This can cause fuses (which are just resistive heating elements, that when get "too hot", melt), to run hot (in some cases, high peak currents cause magnetic "impulses"--Which can "ring" metal cases, windings, and I guess even fuse element).

A TSW inverter is much less likely to create high peak current pulses, and the fuses might run cooler.

A filament lamp (or resistance electrical heater) does not have these high peak wave forms (i.e., PF~1.0).

While I am not sure this is your customers' problem--The fact that PSW/TSW inverters seem to not fail--It can be a possibility (fuses, and other inverter components "running hot").

This is a fairly complex subject (MSW vs PSW, non-power factor corrected vs Power Factor Corrected loads, etc.), the mix of source inverters and loads seems to be an issue here. If temperature is an issue--Measuring fuse/inverter component temperatures with different loads (i.e., a desktop computer vs 300 watts of filament lamps vs 300 Watts of LED/Twisty Floresent bulbs) could be interesting.

Yours is not an isolated issue (MSW vs PSW vs various types of AC generator wave forms). This is a big issue with the Film industry too... Here are some interesting links that may help explain:

http://www.cinematography.com/index.php?showtopic=43872
http://www.screenlightandgrip.com/html/emailnewsletter_generators.html

For installations where nearby lightning is an issue--PSW/TSW AC inverter + DC ground bonding + AC ground bonding (always read inverter manuals--Almost all PSW/TSW inverters can have AC neutral ground bonding and almost all MSW inverters must have floating AC output--Keyword is "almost"). Add MOV surge suppressors on/near the AC inverter output (there are DC surge suppressors too): is a good start at hardening the system. Midnite makes very nice MOV surge suppressors (the Delta spark gap type of are of iffy value):

https://www.solar-electric.com/marine-rv/circuit-protection/surge-protection.html
http://midnitesolar.com/products.php?menuItem=products&productCat_ID=23&productCatName=Surge Protection Devices

Midnite SPD information (Surge Protection Devices)

Keeping wiring run in pairs next to each other (or in metal conduit) helps. Avoid big loops of wires which act like antenna (laying excess cables in pairs of L1+L2 or + and - in "figure eights" is OK).

-Bill

NANOcontrol · December 2017

I'd be using polyswitch thermal resetable fuses. These change resistance when they heat up from current and stay "open" till power is removed for a time. Assume you have an on off switch. Not fast acting but would work in those cases where they are just overloading the inverter. Several can be used in parallel to get the necessary current. Sure beats a service call.

BB. · December 2017

It has been decades since i have used polyswitches, but they were not good for high current applications. Mostly used in circuits with a maximin of a couple of amps.

Bill

alexj · December 2017

@BB.

Bill - I am going to try to do some experiments around the MSW inverters/PSW inverters soon. It would be interesting to really get into it. It is hard deploying technology given the constraints that we have here regarding cost/quality/availability, so field testing or in our "lab" is really our only option to not make mistakes on a larger scale.

With that in mind, a question for you regarding lightning protection: It is definitely an issue here and something that the communities where we work would like us to address in our systems. Do you know of a way to build an effective lightning protection system for a reasonable cost on this scale? Or even a system that is cheap and would offer some protection, but maybe not be as solid as a system build around Midnites equipment?

My thinking has been that as no system guarantees protection and the cost is relatively high to get proper equipment - both in terms of surge protection and building an adequate grounding system that would be functional with low enough resistance - that replacing individual components that are damaged in individual systems is the way to go. The answer obviously depends on how frequently equipment is damaged by strikes: it only amounts to a few systems out of hundreds each year. If there was a cheap way to address the issue, it would be great though.

Thanks!

Alex

BB. · December 2017

Here are some older threads:

BB. said:

Re: Working Thread for Solar Beginner Post/FAQ

A couple threads about Lightning:

Off Grid Grounding Technique?
Another Question, this time about Lightning

Note, the above are discussions, not a do A, B, and C--and you will be "safe". There probably is no such thing with lightning. Several different techniques are discussed--and a few of those posters even have experience with lightning. :cool:

And our host's FAQ:

Lightning Protection for PV Systems

From other past posts here, Windsun (admin/owner of NAWS), he said that most of lighting induced failures he saw were in the Inverters' AC output section.

Towards the end of this thread is a very nice discussion of proper generator grounding.

-Bill

http://forum.solar-electric.com/discussion/352031/direct-lightning-strike-what-will-the-damages-be/p2

I don't know of any cheap and easy solutions.

Installing wiring in metal conduit (especially AC wiring if it goes "any distance" through the structure). Lightning rods to intercept lightning before it strikes the structure, Surge Suppressors on AC and DC wiring (where it enters the structure), on AC inverter output, etc. all help.

Breaking it into two questions. Keeping people safe is one. The other is the costs of construction vs repairs (i.e., lightning hardening vs just leaving an "inverter replacement" kit for local installer...

-Bill

NANOcontrol · December 2017

I think it is worth investigating. I've used multiple parallel 4.6A ones for 20A automotive applications with success. Specs are like nailing jelly to a tree. Lead length, airflow and temp have an effect.. Shoot for about 25A and they will take short surges. 5A are also available.
https://www.ebay.com/itm/100Pcs-New-PolySwitch-Resettable-Fuse-16V-9A/192392683304?hash=item2ccb7f5f28:g:FJcAAOxyXDhSdkV4

alexj · December 2017

@BB. @NANOcontrol

Thanks for the additional info. I'll do some more exploring and will hopefully have some interesting results to share some day.

westbranch · December 2017

I would seriously look into replacing the 'problem children' with TSW inverters and use MidNite SPDs....
Yes , surge protection is a monster topic but from more 'experienced' lightning sufferers have posted, it's a crap shoot, and if one item fails there will be others and the whole system is eventually replaced...
Lots to read about here on the forum, there was one fellow whose Array was a good 400m>/ft? away and he had a NEMA plug about 30 feet from the house he disconnected when the sparks threatened or would be away for a while ...

good luck...

oh Ya, put undersized CBs in that deters any high loads from damaging the new TSR inverter...
and consider going to 24V system

clockmanfran · December 2017

Firstly Mods, I am not trying to sell anything here, and if I can help folk who have a need then I will try and support them, so my apologies if this posting upsets anybody.

Hi alexj,

Inverters in the real World and in the Third World, will always be an issue in todays terms.

Over the years I have had several purchased manufactured Inverters, all of which are not ideal for general normal use.

In 2014-15 with help from 'Oztules' (yea! I know he once was on here). I pushed Inverter technology backwards but went forwards with the latest driving control tech.

Firstly we use a toroid core, we stack them together and wind the core to our calculations depending on the cross sectional area. We use a decent size matched inductance on the Primary windings so that idle power use/loss comes down to about 38/40 watts for a real PSW 6kW Inverter.

The power board and the control board are designed for real harsh environments and manufactured with large tracks, large pads, to stops substrate separation and importantly all components are through hole type, so in the field repairs are relatively easy. Especially is you have insect/moisture infestation. We also put all component values on the boards silk screen, so its "painting by numbers" and therefore very easy to repair without dragging out paperwork to find out what goes where.

We use the 8010 SMD chip, (the only SMD) and run at 23.4KHz modulation frequency. We have several methods of soft starting the large toroid and this is a key issue. We have also all the normal things, like overtemp shut down and auto restart, and can be 50Hz or 60Hz.

In the end I produced a 'How to make a OzInverter' book as a non profit basis. that's lists and shows everything and we also supply the bare Power Board PCB, OzControl Board PCB and the OzCooling PCB as an extra option.

I have replied to questions from the third world, and have advised folk that these OzInverters could easily be manufactured in the thousands, if you have a good Engineer and EE and a good team of assemblers.

In the future I might explain to my young boys whats what, and produce a kit of parts. Manufacturing them myself for resale in this day and age and this present Western culture is a NO NO just to many blasted hoops to jump through.

I say keep it 'Simple', make it 'Robust', and importantly keep it 'Cost Effective'.

Using new componets etc, about $600, scavenging cores, cases, cable etc as low as $250.

If you want to see the project development right up to today.....

http://www.anotherpower.com/board/index.php?PHPSESSID=h7jd5l4edljl6k4ls18pgfj4i1&topic=1116.510

I trust this helps.

NANOcontrol · December 2017

Honestly, there is virtually no chance this is being caused by lightning strikes. Anything to take out a 40A fuse several times will definitely destroy the electronics. Lightning damage of that magnitude is very obvious. Something that should be considered is the quality of the fuses. Some manufacturers put anything they can find to make the fuse. I've seen some made out of just aluminum. Even a good fuse has some variability. Again this is not likely the cause, but is something to be aware of.

mike95490 · December 2017

alexj said:

.......
-Would a 40a blade fuse right near the battery open before the internal fuse in the inverter? .......

Automotive blade fuses reqiure a really good holder to maintain a good, low resistance connection. My 120V charger uses the MAXI version of the blade fuse. https://www.solar-electric.com/dcblatmma.html Maybe that can be retrofitted, if the fuses are heating from poor connections and failing. But a more "robust" fuse may hold in long enough to fry FET's in the inverter.
I like the Blue Seas MRBF https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A
fuses for mounting to batteries, but replacement fuses are expensive, but not as costly as melted wires or a fire

Small scale off-grid inverter blowing internal fuse/OCPD question

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