Charge controller malfunctioning

ahmad090

Hi members, i was wondering if someone could help me with my PV problem. I have 6 panels, 2 each in series hence 3 parallel strings. Panel ratings are 250 watt, Isc=8.83 A and Voc=37.5V. The inverter is 3 kva with 2 batteries in series.
So the problem is, 1 year after use my charge controller overcharged batteries to 30V. I got it repaired under warranty but now its the 4th time that it has happened that i need to get the charge controller repaired. Together with this i have a 60A dc breaker between inverter and array which keeps tripping as well. The problem only occurs in summer around noon.
I got a backup inverter and it has faced the same fate. What could be the problem? Any one array shorting? How do i diagnose which one is it?

BB.

Sorry to read about your problems Ahmad.

Can you tell us a bit more about your system?

What is the brand/model of charge controller?

What is the battery bus voltage (12/24/48 vols)?

What is the the battery brand/model/type/AH capacity (i.e., Trojan flooded cell, 220 AH @ 6 volts, etc.)? How are they wired (all series)?

What is the brand/model of AC inverter? Do you have any other chargers (AC battery charger, inverter-charger, etc.)?

-Bill

ahmad090

Hey Bill, i have a Voltronics Axpert solar inverter (3KVA) with built in charge controller.
The battery bus voltage is 24V. I have 2 dry batteries of JTE in series @150 AH each.
I dont have any other charger, I ve tried replacing complete inverter but the problem persists.
I doubt that the problem is with my PV array.

Thanks

Estragon

Is there a breaker between batteries and inverter?  If so, what size? 

What wire sizes from batteries to inverter, inverter to combiner, and combiner to strings? 

Are strings individually breakered/fused? 

Is the DC (negative?) grounded?

Is this grid-connected in any way, or totally offgrid?

Is there a remote temperature sensor attached to the batteries from the inverter?

ahmad090

@Estragon No it is the breaker between solar array and the inverter.
The solar pv array isnt individually breakered. I only have one dc breaker rated 60A between the complete solar array and inverter. 
It is connected to grid but doesnot feed power back to grid, grid charges batteries over-night and no there isnt any temperature sensors as well.
The dc negative isnt grounded. The two wires from panels go into the pv port at inverter through my dc breaker.
I think in summers when output is relatively high, over current causes breaker to trip and also causes charge controller to break down but the reason for this high current is unknown to me.

Estragon

Is the 60adc breaker polarized, and if so, is the line terminal connected to the pv or  the inverter. 

What is the voltage rating on the DC breaker?

Is the breaker and/or inverter in the sun or otherwise hot location?  

billybob9

I just drew up your system ( not knowing ) and figured that if your battery bank was two 12v batteries in series that would be a 24v system. Controller 12v - 24v that's good but if your 250w panels are in series that makes them 500w which I think is too much. I get beat up all the time here so I'm used to it but could this be.

littleharbor2

They're 500 watts regardless how they're wired. 
If the system is set up as 24 volt nominal you can run TWICE the solar thru it compared to a 12 volt set up.

BB.

Well--I always like to run a bit of math so that people can check and update their systems themselves, without need me to hold their hands (but always feel free to ask questions--I just like to give you the basics in one fell swoop).

Some assumptions:

• You are somewhere around Lahore Pakistan.
• 250 Watt array has Vmp~30 volts (for power sizing, we use Vmp and Imp ratings).
• 250 Watt panel (Pmp) / 30 Volts Vmp = 8.33 Amps Imp (Voltage and Current maximum power)
• 2x 12 volt batteries @ 150 AH each -> for 24 volt @ 150 AH battery bank
• Batteries: http://www.soherwardi.com/dry-battery-prices.html (I am not having much luck getting detailed specifications)
• Across the pond, we probably call "dry Lead Acid Batteries" "GEL" type (electrolyte mixed with silica dioxide to make "GEL").
• 3,000 Watt Inverter. Perhaps this one https://solarwalas.com/product/axpert-mks-3kva-inverter/ , 24 volt around (25 Amp max for solar, 30 amp max for 230 VAC 50/60 Hz (and some other specifications)

First, your system sizing is more for a emergency backup system (UPS, Uninterruptible Power Supply) vs powering a home or office for hours (or possibly days) with no AC power, just solar.

A 3,000 Watt @ 24 volt inverter would draw from the Battery Bus:

• 3,000 Watts * 1/0.85 AC inverter eff * 1/21.0 volt minimum DC bus voltage = 168 Amps

Your 150 AH GEL batteries, while very good for short term surge current, they will only supply somewhere around 20-50 minutes at 3,000 Watts (and draw 2x 3,000 Watts for starting surge).

Next, need to find out more about your dry batteries... In the USA, we generally limit them to 5% rate of charge (and it would take 24-48 hours at 14.2 volts to fully recharge). GEL batteries do not like to be charged too quickly or too high of voltage:

• 150 Amps * 0.05 rate of charge = ~8 amps @ 24 volts charging.

So, depending on your Inverter-Charger and battery specifications, you may be charging them with too much current:

• 3 parallel string s* 500 Watts * 0.77 panel+controller derarings * 1/28.4 volts charging = 41 Amps typical maximum current from your solar array (I may have the wrong inverter-charger--limited to 25 amps the one I found), and upwards of 30 amps from your AC charger block.

Generally GEL batteries are for "float operation"--Basically they sit unused until you have a power outage... If you try to use them daily for an off grid power system, they generally, do not last very long (perhaps 1 year +/- or even much less). You need to look at the battery manual(s) and see how they should be charged (voltage, current, time, profile). They may be OK for your needs--I just do not know.

You need to tell us what your energy needs are from this system... UPS (once a month or less, 2-4 hours summer afternoon power outages, running a few lights and a laptop, server farm, etc.).

General, for a "solar power system" we would be aiming at C/20 to C/8 discharge rate, and discharge from 100% to 50% state of charge. For example say you want 4 hours of backup power (discharge to 50% or a bit less State of charge):

• 150 AH * 0.50 max planned discharge * 0.85 AC inverter eff * 24 VDC nominal bus voltage = 1,530 Watt*Hours of "useful energy"
• 1,530 WH / 4 hour discharge rate = ~382.5 Watts of 230 VAC loads (4 hours to 50% battery capacity, recharge >90% by next afternoon).

The Solar Panel input range for you 2x 250 Watt panels is a bit difficult (Vmp~30 volts). You have a min/max solar input of 30-66 VDC (operational)... Ideally you probably need to put these two panels in series for Vmp-array of ~60 volts (and Voc-std conditions ~ 75 VDC @ 25C... If it gets cold there, Voc-cold could, in theory, damage your solar charge controller inputs).

And how much energy can your 1,500 Watt array harvest (all things being equal, which they rarely are):
http://www.solarelectricityhandbook.com/solar-irradiance.html

Lahore
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 58° angle (from vertical):
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
4.72	5.66	6.16	6.50	6.68	6.34
Jul	Aug	Sep	Oct	Nov	Dec
5.52	5.46	6.00	6.43	5.73	4.91

For an off grid power system (battery based), you should only "count" on 50-65% of your predicted daily harvest (for many reasons):

• 1,500 Watt array * 0.52 off grid system system eff * 6.34 hours of sun (June Average) = 4,952 WH of energy per "average June day"

I will stop here... There is a lot of data for you to think about. 

-Bill

billybob9

Littleharbor

Your right and that's the assumption I was going by. If a 12 volt system should only take 500w before it starts getting hot and it doubles for a 24 volt system. That would be 1000w. The system 0f Ahmad is 1500w as he has three 500w strings. I don't know if his system is 24 volts but I'm hoping he will tell us. 

billybob9

Bill

When I see your Math I always think of the guy at the blackboard saying what your saying and writing the algebraic equation as he goes along. Then just like in the movies he and another person will put it in scientific notation form crossing out exponents and converting big numbers into powers of 10. So if we took your last post and filled in all the correction for temperature, barometric pressure, and even gravity we'd be at the bottom of the blackboard. I'll bet you've done this. I really like what you are doing here and hope everyone appreciates it as much as I do.  BG 

Estragon

Seems to me there are two issues here.

First, the CC "overcharging to 30v".  This could just be an auto-EQ setting in the controller.  Depending on temp, 30v could be a perfectly reasonable Eq (or even absorb, but maybe not in June in Pakistan) voltage for a 24v nominal bank.

The other issue is the 60a breaker flipping.  As described, I don't see how 3 x 9a Isc strings flips a properly wired 60adc breaker.  My guess is the breaker is borked, or there's a wiring problem.

ahmad090

@Estragon the voltage rating is 500V. And the breaker isnt in any hot location. 
Since my charge controller has burnt away 4 times, i think it is safe to assume that the breaker is not faulty rather there is a short circuit in solar wiring or the solar panel itself. How do I diagnose which panel is it?

ahmad090

@BB. All of your assumptions are correct.
I would like you to think along the lines of over current from solar array which is causing the breaker to trip and simultaneously damaging my charge controller.
I dont see any issues with the batteries themselves.
Can you help me diagnose what kind of short circuit is it and how do I locate the faulty array of panels.
Thanks

Estragon

If you have a DC clamp meter you could measure current coming from each string.  If not, you could try disconnecting each string in turn, and see what the resulting current into the controller is.

The thing is, with 3 strings and Isc of <9a per string, that's ~27a max - not enough to flip a properly functioning 60a breaker.  It's possible for the CC to fail in a way that creates a short from batteries to pv though, which is why a polarized breaker should be installed with the line terminal connected to the CC (normal current flows from pv to CC, but fault current can go the other way).  DC breakers are supposed to survive opening under load, but it's possible repeated arcing has damaged it.  If you have an infrared temp sensor, you might check to see if the breaker is heating up under load.

ahmad090

@Estragon thanks I will try your suggestions to check panels and the dc breaker.

BB.

Here are some AC+DC Current Clamp meters (this is India, close as I could find quickly near Pakistan):

https://www.amazon.in/s?k=dc+current+clamp+meter

Note, there are AC Current Clamp Meters + AC/DC volt reading.... You want an AC+DC Current Clamp DMM (Digital multimeter) type that can measure DC current with the clamp.

In the above link, you will find both type... AC Current + AC/DC voltage, and AC+DC Current Clamp DMM (sometimes you have to do some very careful reading to find out which it is).

In general, the 60 Amp breaker between the solar array and charge controller is not doing you much good (other than as a "Good" on/off switch--Which is very handy when working on the system).

For protection, you need to use a series protection fuse/breaker for each series string in your array--I would guess 15 Amp rated fuse/breaker for your panels (the spec. sheet should tell you exactly). These fuses are there and required when you have 3 or more parallel solar panels in an array... Keeps one shorted panel/string from getting too much current from all of the other panels in the string (3 parallel strings is "close" as to needing the fuse or not).

You may need to work with your inverter-charger-solar charger and see what they say. In theory, the solar array is larger than the inverter-charger can support--But it does depend on what type of solar charge controller is in the inverter-charger.... A PWM charger, over paneling will cause cause excessive current flow in the solar charger and damage/ruin the controller. A MPPT type controller can control the "over powered charger" and limit itself to a safe level of current.

-Bill

ahmad090

Thankyou for the suggestions @BB. I will now monitor individual array currents

BB.

Solar panels don't really "overcurrent" into a short circuit--The short circuit current (Isc and Imp--current maximum power) is proportional to the amount of sunlight hitting the panel. You are not going to see >>Isc or Imp ever out of those panels... You are going to see (guessing) less than 9 amps peak current ever from a panel/series string (3x9=27amp which is much less than your 60 Amp breaker).

Depending on the quality/source of your circuit breaker(s), they could simply be poorly designed/manufactured.

-Bill

billybob9

If your panels did do the rated watts then on my 24v system I have 640w on panels going to my controller that limits my battery voltage to 28v . The amperage would be 22.8a and below the controllers maximum of 40a. If I increased my panel wattage to 1500w this would go over my controllers allowed amperage. Or am I missing something here ? 

Tecnodave

Billybob9,

1500 watts of panels will generate about 52 amps out of a good controller at peak power, which will rarely occur. A good 40 amp controller can handle this kind of "over paneling"  the controller will limit the maximum current to 40 amps, the input voltage will rise slightly and not all the available power will be harvested.

I over panel my MidNite controllers at about 30-50% depending on my local weather, but I'm right on the left coast, just miles from the Pacific Ocean, sometimes having 2000-2500 feet of "marine layer" so thick you cannot see the sun's location in the sky. But I do switch off extra array's when the sun pops out in full glory. Also when leaving system unattended. 

ahmad090

@Tecnodave i have a set of 3 pv module in parallel with each module having 2 panels in series which makes system Voc = 75V and Isc=26A (individual voc and isc mentioned in my thread above). So theoretically the current should never exceed 30A and hence the 60A breakerr shouldnt trip. However not only the breaker is tripping but the solar charger is being damaged too.

mcgivor

Questions
Do the fans run in the inverter controller?
Is there natural ventilation? 
What are the ambient temperatures where the equipment is located? 

Because the problem occurs during summer only, it is possible the circuit breaker is a thermal  type which generates heat, without adequate ability to dissapate the heat, in an enclosure for example, they tend to trip prematurely, I have some that do exactly that when ambient temperatures exceed 40°C, have to leave the enclosure door open to prevent the occurrence.

The controller may be operating at higher temperatures than designed for, my controllers are designed to derate themselves as temperature rises to prevent component damage, both are passively cooled but without fan assistance , the heatsink temperatures rise to > 80°C, with a fan they stay at or around ambient. Looking at the specifications for your unit, there is no mention of self derating, not to say there is none.

Just seems like the perfect recipe, summer, noon with possible maximum current, hottest time of day, heat is a killer of all things electrical,

ahmad090

Yes the fabs do run.
Inverter is located inside house in ambient temp of around 35 degrees.
However, thanks for the suggestions. I will try installing a secondary fan with the inverter as well.

Tecnodave

Anmad090,

It keeps coming back to that DC breaker between the array and the combo charge controller/inverter. I think that it is faulty. There is no way that your array can possibly output more than 30 amps and you repeatablly trip a 60 amp breaker. Something wrong there. I do not have any idea what kind of breakers you have in Pakistan. Here in America the preferred breakers are the Carling C series also marketed by MidNite Solar as the MNDC breaker. These are magnetic/hydraulic, unlike common breakers which are thermal. Also suitable is the MidNite MNEPV breaker which is also magnetic/hydraulic. These are manufactured in South Africa by CBI, Circuit Breaker International and are recertified by MidNite Solar to 150 volts DC 

Normally we we use a combiner which has a breaker for each string of panels. Suggested combiner would be the MidNite MNPV-3 with 3 each MidNite MNEPV-15 amp breakers. This will allow you to switch on or off individual strings of panels as a diagnostic tool as well as shutting down one string when there is too much solar power available for the charge controller to handle.

As as I have never been to Pakistan I do not know what is available there so I point to the sources as much as I can. 

I  hope that you will find this information useful

ahmad090

@Tecnodave thankyou so much. Iam gonna add individual breakers for arrays and replace the current breaker as well from certified company. Will share results on the thread as well. 
Need to get the charge controller fixed first 😋

bill von novak

ahmad090 said:

Hi members, i was wondering if someone could help me with my PV problem. I have 6 panels, 2 each in series hence 3 parallel strings. Panel ratings are 250 watt, Isc=8.83 A and Voc=37.5V. 

The Voltronic Axpert MKS 3K-24 has a max input voltage of 75 volts.  Your voltage at STP (25C) will be 75 volts.  If it ever gets colder than 25C, then the voltage will be greater than 75 volts and may damage the controller.  A possible failure is a failed-closed pass element which will overvoltage the batteries.

ahmad090

@bill von novak what do you suggest then? 
Adding all panels in parallel to reduce voc would result in more than 50A of current.
Secondly the problem only occurs in summers when its 40 degrees outside not in winters when its 10 degrees.

Estragon

Wiring in parallel may also be an issue in hot weather, Vmp will be lower.  Mppt controllers like 20-30% of voltage "headroom" to buck down from string voltage to charging voltage.  28.8 x 120% = 34.6v.  If much distance from CC to array, and/or wire size is marginal for the higher current, and/or heat drops Vmp much, the mppt may struggle?

Which problem only happens in summer (breaker opening, or overcharging)?  Or do they always happen together?

Overvoltage is most likely to happen first thing is the morning before the sun hits the panels.  There will be little current in the early dawn light, but enough light to get high pv voltage on the still cool/cold panels.  It could happen in spring/summer too, but you'd think it more likely in winter.

Is it possible the damage is actually happening in winter, but not being noticed until summer when the long days allow for overcharging?  Does the unit do daily logging in which it records maximum pv input voltage?

ahmad090

@Estragon the problem only occurs in summer afternoons.so i believe its not an issue with high voltage rather high current or high ambient temperatures