MX60 Max Array Voltage? [RE-wrenches]

[Christopher Freitas - OutBack Power]

Could you post this to the wrenches group? I'm not allowed to post 
there, 
but I wanted to answer this guy.

Daryl wrote:

>Second is it possible to build a circuit that uses
>pulse width modulation to bypass any of the extra
>power to a resistor if the voltage exceeds the
>maximum? This would be handy for inverters also, such
>as the sunny boy. I am considering using this in my
>next project that is going to use an MX60, the exact
>problem that previous posts have described.

I have designed such a circuit for use with a high voltage panel (old 50 
volt panel) and a Microsine, which has a max OCV of 48 volts. I did it 
by taking two taps off the panel, the 24V nominal tap and the 30V 
nominal tap, and switching between the two of them depending on OCV. 
When voltage exceeded 46 volts, the circuit switched to the lower tap, 
and remained there for at least a few seconds (to prevent oscillations.)

An "ordinary" PWM circuit won't do what you want. As PWM is defined in 
charge-controller terms, it simply chops the voltage at a medium 
frequency (below 10 hz) to get an average voltage lower than the OCV of 
the panel. That means that half the time you get the full OCV (which can 
damage things) and half the time you get zero volts. This will not only 
not protect things like a SunnyBoy, it will drive its peak power 
tracking algorithm nuts.

A high speed switchmode PWM circuit will do what you want, but now 
you're talking an inductor, transistors, a controller IC, catch diodes 
etc. Also, if you're going to go to all that trouble, you've basically 
designed the power part of a PP tracking regulator anyway.

>In my case the peak power voltage is going to be about
>110 volts but the open circuit will be almost 200
>volts, the peak power current about 10 amps. I figure
>a 20 kHz PWM and a 25 ufd capacitor would keep the
>voltage spikes in line.

This might work, but:

1) you'll probably need to increase the capacitor size to give you a 
decent time constant and not exceed the cap's ripple current limit - 
would depend on array current

2) you would certainly confuse the RV Power Products charge controllers 
by limiting the OCV; not sure if the MX60 will care

3) you'll likely need a dummy load to prevent the off-resistance of your 
switching devices from slowly charging the cap beyond your voltage 
limits

4) the additional capacitance might confuse PP tracking algorithms since 
there is now a lag between increased current draw and reduction in array 
voltage

>I have a similar problem with a SMA install where the
>open circuit voltage is above 600 just a little, maybe
>700, and of course the peak power is well below 600.
>So I would like a PWM circuit that could limit this
>voltage to protect the SMA as I have heard of failures
>where the 600 volts is exceeded on open circuit.

Why not just electronically change the array taps to decrease the size 
of the series array under no load conditions? 600 volt arrays are 
typically composed of many seriesed panels, and it's easier to switch 
once to limit voltage than to switch at 20khz.

-- Bill von Novak

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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