How to keep VOC within limits of Charge Controller

[digimart]

I have come across Xantrex MPPT Charge controller, its Maximum No load Voltage limit (VOC) is 150 VDC.

I have 10 solar panels in hand with VOC of 36V each. As quantity of panels is 10, so I can have combination of two arrays of 5 panels each, or 5 arrays of 2 panels each.

As I would like to have maximum volts on panels side, so that to reduce amperage . But if I connect panels in series of 5 each, the combined VOC goes to 180V. Its way above the MPPT VOC limit.

Is there any kind of dummy load type of thing which I can attach, so that the VOC never crosses the upper limit of Xantrex CC.
Is a "Shunt" same thing as dummy load ?

[daz]

I have come across Xantrex MPPT Charge controller, its Maximum No load Voltage limit (VOC) is 150 VDC.

I have 10 solar panels in hand with VOC of 36V each. As quantity of panels is 10, so I can have combination of two arrays of 5 panels each, or 5 arrays of 2 panels each.

As I would like to have maximum volts on panels side, so that to reduce amperage . But if I connect panels in series of 5 each, the combined VOC goes to 180V. Its way above the MPPT VOC limit.

Well, you need to have a look at that prior to purchasing solarpanels!
You may have to end up doing a 3x3 array (3 panels in series, and 3 panels in parallel). That will give you a VOC of 108V, and you may actually get more powr out minus the one panel, as opposed to having all 10 panels connected with a lower voltage (due to losses on low voltage).

Is there any kind of dummy load type of thing which I can attach, so that the VOC never crosses the upper limit of Xantrex CC.
Is a "Shunt" same thing as dummy load ?

No, there isn't. To make sure that you do not exceed the controllers VOC, you only install enough panels to be able to stay within your VOC. In your case the maximum you can have in series is 3 panels, or 108V VOC.

[digimart]

At another website, I saw some post the following :
" There is a way to limit Voc of 3 panels in series from reaching above 140V limit by making a voltage clamp circut. Its a pretty simple device that uses zener diode driving a high powered pass transistor that is connected in series with large power resistor for heat dissipation. It connects in parallel with solar array. Because Vmp of 3 panels in series will be 32.2V * 3 * 0.8 = 77.3V the clamp circuit will never see maximum power point current, so there is no need to size the circuit to handle full PV power. There is a way to assembly how much current the panels will produce at 140V. That current will be dissipated by clamp circuit. "

Please comment on the above way / method to bring down the VOC of panels.

[digimart]

Well, you need to have a look at that prior to purchasing solarpanels!
You may have to end up doing a 3x3 array (3 panels in series, and 3 panels in parallel). That will give you a VOC of 108V, and you may actually get more powr out minus the one panel, as opposed to having all 10 panels connected with a lower voltage (due to losses on low voltage).




No, there isn't. To make sure that you do not exceed the controllers VOC, you only install enough panels to be able to stay within your VOC. In your case the maximum you can have in series is 3 panels, or 108V VOC.

At another website, I saw some post the following :
" There is a way to limit Voc of 3 panels in series from reaching above 140V limit by making a voltage clamp circut. Its a pretty simple device that uses zener diode driving a high powered pass transistor that is connected in series with large power resistor for heat dissipation. It connects in parallel with solar array. Because Vmp of 3 panels in series will be 32.2V * 3 * 0.8 = 77.3V the clamp circuit will never see maximum power point current, so there is no need to size the circuit to handle full PV power. There is a way to assembly how much current the panels will produce at 140V. That current will be dissipated by clamp circuit. "

Please comment on the above way / method to bring down the VOC of panels.

[daz]

At another website, I saw some post the following :
" There is a way to limit Voc of 3 panels in series from reaching above 140V limit by making a voltage clamp circut. Its a pretty simple device that uses zener diode driving a high powered pass transistor that is connected in series with large power resistor for heat dissipation. It connects in parallel with solar array. Because Vmp of 3 panels in series will be 32.2V * 3 * 0.8 = 77.3V the clamp circuit will never see maximum power point current, so there is no need to size the circuit to handle full PV power. There is a way to assembly how much current the panels will produce at 140V. That current will be dissipated by clamp circuit. "

Please comment on the above way / method to bring down the VOC of panels.

They do not know what they are talking about. Well...they do know, but that is not practical!

Let's go through some numbers....
32.2V x 3 = 96.6V VOC

Now they want to limit VOC to 80% total, so;
32.2V x 3 x 0.8 = 77.28V VOC

Now, here is the part they didnt tell you....
96.6V - 77.28V = 19.32V drop,

let's assume you have 200W solar panels @ 32.2V VOC each, which means each panel puts out 200W / 32.2V = 6.2A.
Assuming all the panels are in series;
power lost = volt drop x current
power lost = 19.32V x 6.2A
power lost = 119.79W

Here they say.....

...powered pass transistor that is connected in series with large power resistor for heat dissipation...

have you ever seen a power resistor rated at 120W??

You are effectively dissipating about 60% of a 200W panel's energy, and hoping that your "voltage clamp circut" does not fail at any time...whatsoever. The moment that fails, you have toasted your charge controller. Is it worth it? Not to mention easily finding a power resistor rated at that value (you will more than likely have to connect multiples in parallel).

Is all the above worth it? IMHO, just leave one panel out of the array...or just go buy another two!

[noshali]

Hi,
I am in a smilar situation and if you take a volt meter...when the sun is really shining...these panels tend to give 80 plus volts. Please find the link to Schneider sizing tool. It is not recommended to use more than 2 such panels.

http://www.se-renbu.com/support/xwsizing/disclaimer_SE.aspx

Regards,
Ali Raza

[Sunking]

Simple solution. Use a Midnite Solar Classic 200. One neat thing about all of Midnite Solar CC's, the Voc for say the Classic 200 is 200 volts + Nominal Battery Voltage. So if you have a 48 volt battery it is 248 volts.

[bcroe]

Voc

I once tried clamping VOC to get a controller started. It just got confused.

Yes I have seen a 100/200W resistor, think the biggest one I have (not liquid
cooled) is 400W. Bruce Roe

[inetdog]

At another website, I saw some post the following :
" There is a way to limit Voc of 3 panels in series from reaching above 140V limit by making a voltage clamp circuit.
...
That current will be dissipated by clamp circuit. "

Please comment on the above way / method to bring down the VOC of panels.

That way seems pretty difficult and wasteful of power. If you are going to lose power anyway (although at a time when the panels will not be producing much power even though the voltage is high), why not just use a relay to short out one of the three panels. The relay will not dissipate any power, since the voltage will be close to zero. The panel will not be damaged. And you could even use a solid state relay for the job.

As to using the Midnight Classic with HyperVOC, once again when the Voc is between the nominal operating limit and that limit plus battery voltage, the CC will not produce any power, but it will not be damaged either. The hope is that at the time when the panels are cold, they would not have been producing much power anyway. But comparing with shorting out one panel in the string, 2/3 of not very much is still more than none of not very much.

[bcroe]

V oc Clamp

I for one, would like to hear who has successfully used a
voltage clamping clamping circuit, to knock down V oc
until V mp can be utilized. My own attempts found the
power into the MPP device wasn't nice & smooth. If
a relay shorting a panel wasn't in perfect sync, V oc
would pulse back & shut things down.

Using a continuous (zener emulating) clamp, my MPP
device seemed to be confused. In the end something
broke, and the device I was trying to protect blew up.

My conclusion was, this problem would take some
serious research time (with a scope) and perhaps
money, to sort out. Given it was a kludge anyway, I
decided it would be more productive to write it off &
arrange the equipment as it was designed to be used.
Bruce Roe