Blocking Diodes between Series String before combiner box.

[johngalt]

I have 9 x 12volt 145watt panels installed on my shed making 3 strings of 3 panels each in series. These 3 circuits will then be combined into a breaker box where each circuit will have it's own 15 amp breaker. From that bus bar the combined current will then go to a TriStar MPPT 60 charge controller and eventually to the battery bank.

I understand that the controller will block reverse current going back to the panels at night from the batteries but I'm not so sure about the panels themselves.

Would it make sense to put a blocking diode on each circuit before going into the combiner box to prevent current back feeding from one series of panels to another?

My friend had two small arrays on opposite sides of his house and he said he had to put a blocking diode between them because when one side was shaded the other side would try to back feed to the shaded side. All of the 9 panels I have are side by side on my shed facing due South. The only way they can be fully shaded at the same time is at night. There could of course be various levels of shading with clouds but all 3 circuits would be affected to some degree at the same time. I really can't see a situation where one of the 3 series of panels would be fully shaded while the other two were not.

Just trying to decide if installing a blocking diode on each circuit is a necessary step or just over kill for a problem that may never exist.

Any thoughts on the matter are welcome.

Thanks

[Mike90250]

Panels facing the same orientation, feeding a MPPT controller, do not need blocking diodes. Even at sunrise/sunset, as some strings get shaded, power production will be low, and most good commercial panels have low "dark" leakage.


Your friend, with 2 different arrays, had to use a blocking diode, to keep the MPPT from being confused as it tests the array for power. Even better would have been 2 separate MPPT controllers, because there can be losses and failures in the blocking diode. (heat sink, long hours of high current, diode not sized properly)

[johngalt]

Makes sense. That's what I was hoping. Just wasn't sure.

Thanks

[inetdog]

Makes sense. That's what I was hoping. Just wasn't sure.

Thanks

You have hit on a very contentious subject, not well understood by forum members (both here and on the "other forum") or even by some CC or GTI manufacturers.
The following is my personal opinion only, but I am, of course, quite convinced that I am right:

1. Blocking diodes should not be necessary on each string, even if they are differently shaded. Except in 100% moonless darkness, a panel will still have approximately the same Voc. The single blocking function provided by the CC will take care of the total darkness situation.

2. As long as the panel strings are identical, you do not need two MPPT inputs or even two blocking diodes even if the two string are facing in opposite directions, AS LONG AS THERE IS NO PARTIAL SHADING.

3. If you have two panel strings with Vmp values differing by more than 10% they should get separate MPPT inputs for maximum efficiency.

4. If either string gets partial shading, it will have the same effect as having different Vmp voltages, and you may also need two MPPT inputs in this case too.

[johngalt]

More good info -thanks

Here is a diagram of what I am building. A few things have changed since I drew this up like the inverter box, I may not use one with this system and just use the 12 volt system for lights and a few small 12v applications. In the house I will have 4 x Sun Xtender PVX-1530T in parallel. (153Ah each 612Ah total at 12 v)

My main question was about the blocking diode which I think you guys have answered. The other is whether the total wattage of the array is too much for the controller. What happens to the extra wattage if it is available but not used? I wanted to make the array larger for cloudy days and longer harvest window throughout the day but I don't want to burn anything up either. Also wanted higher voltage for longer cable run.

Another piece of info - These panels are on the shed about 80 feet from the house. The total wire run may be about 100 feet using 10 AWG stranded PV wire. I may also terminate the 3 strings inside the shed using a 3 way knife switch on each string. That way I can divert the power to the shed if the batteries in the house are full. If I do this I could divert 1, 2, or all 3 strings to the shed. Maybe run a well pump or add some more batteries and inverter to run power tools, I don't know yet. I'm just trying to make the setup as modular and as flexible as possible so I can change and expand in the future without too much reworking.

Feel free to pick it apart and tell me where I screwed up.

[Mike90250]

Feeding extra wattage to an MPPT controller, it throttles back to it max output current rating, or it's thermal limit. So a little extra wattage helps, but a lot would just be wasted.
60A @ 55V = 3300 watts output, so the most I'd want to feed that with would be maybe 4500W of panel, and you will run the controller at it's max limit for a couple hours a day. Will it last?

[johngalt]

Feeding extra wattage to an MPPT controller, it throttles back to it max output current rating, or it's thermal limit. So a little extra wattage helps, but a lot would just be wasted.
60A @ 55V = 3300 watts output, so the most I'd want to feed that with would be maybe 4500W of panel, and you will run the controller at it's max limit for a couple hours a day. Will it last?

Yeah, I sent an email to Morningstar and asked them about it. They said that the MPPT-60 will not draw more than it can use. The max output to a 12 volt battery system is 800 watts. I have 1305 nominal available but doubt I will have that much available in reality. Maybe in the dead of winter it will help me out the most. Even if the array was only putting out at 65% I would still have enough wattage to run the charger at it's maximum capability.

Once the system is online if I find that I'm wasting too much wattage I can divert one of the strings to the tool shed and run the MPPT-60 off the two remaining. I've got a smaller controller I can hook up in the shed for other things.

[Naptown]

Yeah, I sent an email to Morningstar and asked them about it. They said that the MPPT-60 will not draw more than it can use. The max output to a 12 volt battery system is 800 watts. I have 1305 nominal available but doubt I will have that much available in reality. Maybe in the dead of winter it will help me out the most. Even if the array was only putting out at 65% I would still have enough wattage to run the charger at it's maximum capability.

Once the system is online if I find that I'm wasting too much wattage I can divert one of the strings to the tool shed and run the MPPT-60 off the two remaining. I've got a smaller controller I can hook up in the shed for other things.

Keep in mind that you will have to isolate the array's and CC's
I think you have this in mind if I remember correctly but you cant have 2 charge controllers on one array. If you can turn off the array to one and turn on the other that would work.

[johngalt]

Keep in mind that you will have to isolate the array's and CC's
I think you have this in mind if I remember correctly but you cant have 2 charge controllers on one array. If you can turn off the array to one and turn on the other that would work.

I plan on splitting out the strings between the cc's. only one cc uses the string at a time.

[Naptown]

I plan on splitting out the strings between the cc's. only one cc uses the string at a time.

What is the rating on that switch. Not being a spring loaded switch I could see a lot of arcing going on in there if voltage and load are high enough.

[johngalt]

What is the rating on that switch. Not being a spring loaded switch I could see a lot of arcing going on in there if voltage and load are high enough.

The switch is rated at 32amps and the 3 panel string connected to it should not be able to generate more than 10.5 amps max. (Isc 8.37 x 1.25 = 10.46 amps)

[Naptown]

At what voltage which is more important. DC will arc a long way depending on the voltage. You will have almost 60V at Vmp

[inetdog]

I plan on splitting out the strings between the cc's. only one cc uses the string at a time.

Interesting knife switches. They look like they would be fine for switching the panels between CCs, etc and also acting as disconnect switches. But I would not be comfortable trying to switch them while current was flowing in the circuit without knowing more about their rating.
A lot of battery switches are designed for a maximum of 12 or 24 volts.

[johngalt]

The knife switch label reads:

Double Power Load
Switch Blade
Rated Voltage: 380/220V
Rated Current: 32A
Frequency: 50Hz

I connected it to my bench power supply and put a volt meter on each side. I could only turn up the power supply to 32volts. When I moved the blade to one side I saw the voltage and again when moving to the other side but nothing while changing positions. I only got voltage when the blade engaged one side or the other.

Just to be on the safe side I will connect a string to the switch with volt meters on each side and do the same kind of test at the full panel voltage. When I first tested the strings they were running between 64 to 66 volts each.

Max possible should be 22.3 x 3 x 1.25 = 83.62V

It's much colder out now from when I first checked the string so maybe I'll see some of that extra voltage.

From looking at the rating, seeing the ruggedness of the switch up close, and my preliminary test, I don't think it will be a problem to use as intended. From the blade to the poles is over an inch in each direction. I don't think I can get 84 volts to jump an inch.

[inetdog]

The knife switch label reads:

Double Power Load
Switch Blade
Rated Voltage: 380/220V
Rated Current: 32A
Frequency: 50Hz


From looking at the rating, seeing the ruggedness of the switch up close, and my preliminary test, I don't think it will be a problem to use as intended. From the blade to the poles is over an inch in each direction. I don't think I can get 84 volts to jump an inch.

1. The switch is rated for a high enough voltage, but not rated for DC use at all.

When we talk about arcing in this context, we are not referring to a breakdown of the insulating properties of the switch to allow current to start to flow between disconnected terminals. We are talking about an arc which starts as the contacts of the switch are opening while carrying current. As the contacts begin to separate the voltage gradient across the microscopic gap, just starting to widen, is high enough to create an arc. The ionized air and/or metallic particles from the contacts combine to allow the arc to continue to carry current as the gap widens.
In the case of AC, the current will fall to zero every 100th or 120th of a second and, deprived of power, the ionized conducting path will dissipate enough that the arc cannot restart as the voltage rises. For this reason it is not essential that an AC switch separate the contacts quickly or make other provisions for extinguishing the arc.
For high DC voltages (including the up to 600 volts used in PV input to a grid-tie inverter), the final separation of the contacts of an AC-only switch may not be large enough to actually interrupt the arc, causing a very hazardous situation.

2. You switch appears to have a plenty wide contact separation, but when opening the contacts under load with high voltage DC the resulting arc can be scary and can also lead to physical damage to the switch contacts. In the worst case, this damage may increase the contact resistance of the closed switch to a dangerous level, or even result in a repeated arcing of the supposedly closed contacts (called a serial arc fault.)

3. The final gap that the arc can cross and the ease of arc formation will rise with the voltage, and the heat and contact damage will rise with the current being interrupted.

4. You may seem to get away with it just fine, but we cannot recommend anybody using an AC-only switch, circuit breaker or fuse in a DC circuit.