diy solar

diy solar

PV String DC Voltage Limiter

Canxane

New Member
Joined
Nov 21, 2020
Messages
71
Need a way to limit PV string DC voltage to that allowed by a hybrid inverter.

Unique scenario. Imagine a situation where you are unable to change your panels or the string size of the panels (permits, zoning, etc) and you need to limit the DC voltage to a lower value than the string can theoretically produce. If your existing inverter strings theoretically can produce up 550V DC on a cold sunny day, and the current inverter allows 600V DC you are okay. But what if you want to switch in a hybrid inverter that only allows 500V DC per string? You are okay 360 days of the year, but a few winter days might exceed the inverter string voltage limits.

Is there a way to limit the DC voltage of a PV string to protect the inverter on those few days it might be a problem?
 
If you can't reconfigure the array.
I think that you should buy an inverter that can handle it.
 
Remote controlled switch (cheap enough) with a bigger relay to bypass the last panel(s) on the string?
-
 
No, it wouldn't :·)
You just wire the relay so it draws from the last but one (or two, whatever) instead of the last.
They usually have normally open and normally closed contacts...
If it looks like the temperature is going to drop below safe, you click the remote.
-
 
Only single large inverter that comes close to a drop in hybrid replacement is the solark 15kw inverter. The strings max out at 500V DC. The solark manual says to not exceed 550V or damage will occur. The current Fronius 11.4kw allows 600V DC and the strings were sized to fit that inverter. Looking for a way to switch without having to restring and change the panel layout.

There is nothing wrong with the fronius (other than noise). Will be used to do another array where batteries not needed.
 
Personally, I wouldn't worry about permits or zoning.
Just rewire it for lower voltage strings.
Nobody knows what you are doing, unless you tell them.
 
Personally, I wouldn't worry about permits or zoning.
Just rewire it for lower voltage strings.
Nobody knows what you are doing, unless you tell them.
Unless the neighbor is the utility engineer who approves solar and you are trying to make a change without him knowing. He sees you taking panels off the roof to change strings it will be an issue. Big steep roof - lift and other equipment needed. Maybe if we knew his vacation schedule.

Working on the assumption we can't change the panels - is there any way to just limit the voltage to 500V DC on those days it might go over?
 
Not that I know of. But, that doesn't mean that it doesn't exist.
Good luck.
 
The following idea was pitched by @Supervstech in my thread hashing out my array configuration.


It should be possible to use a low-temperature charging disconnect to disconnect the array bellow the temperature your concerned about. If you wire it up at night it would fly under the radar of your neighbor and get the job done ;)
 
Only single large inverter that comes close to a drop in hybrid replacement is the solark 15kw inverter. The strings max out at 500V DC. The solark manual says to not exceed 550V or damage will occur. The current Fronius 11.4kw allows 600V DC and the strings were sized to fit that inverter. Looking for a way to switch without having to restring and change the panel layout.

There is nothing wrong with the fronius (other than noise). Will be used to do another array where batteries not needed.
Until I saw your post about your Fronius strings at 500 vdc, I honestly didn't think to check the VOC, since the Fronius manual listed normal operating voltage at 420 vdc. My solar panel strings configuration VOC is 470 vdc maximum. I found the warning message about VOC > 550 volts. Solar panels do degrade about 1% per year. My panels are 3 years old, so my actual VOC is probably 470 x 97% or 460 volts. The Sol-Ark Limitless 15 KW hybrid inverter is listed at 500 volts, so my VOC even at 470 vdc maximum shouldn't void the 10 years warranty.
 
Last edited:
Here''s a simple approach: Use a thermostat to short circuit the last panel in the string when temp falls below T degrees.
It would need to be rated for the panel's Isc
 
You could dump some of the output to resistors (basically how windmills and hydroturbines gets regulated for the simple installations), ideally into something useful like room heaters or a water heater, if the voltage gets too high. Something very simple to be able to do that is a voltage sensing relay.
 
Use the dc panel voltage to heat a lot of water , that will pull the panel down.

You could build voltage regulators on the panel output but it’s a lot of heat to get rid off.
 
Last edited:
Back
Top