High VOC and charge controller, please help

[Paulcheung]

Hi guys,

My name is Paul Cheung, I live in Jamaica. I have an off grid PV solar system, it has 12 Panels, The panels eight of them is SUN-ND250HAC from Sun-Elec from Miami. VOC is 38.3 volts, Vmp is 29.8 volts, Imp is 8.4A, Isc is 8.9A. The controller is Midnite Classic 150. The panels are wired in 2 series of 4 panels each series. The battery is 48 volt bank. The other 4 panels is Intenergy INE-240-60; VOC is 37.35 volts, ISC is 8.38A, VMP is 30.89 volts, IMP is 7.77A. these four panels wired in a single series also connected to the Classic 150 controller.

The system installed from last year November 2013 with an Outback FM60 controller. Since this year March 2014 I bought 4 more of the Intenergy panels, I bought a classic 150 controller as the current go over the 60 amps sometime. It works ok so far, but as I learned more about PV system design, my system has a major fault, The VOC on the string is over the controller's limit. We don't have extreme cold weather in Jamaica. But it can be a bit of cold in December in some years as I live in the colder part of the Island.

My question is how does Classic 150 controller deal with high VOC? Does it just cut off the current and switch off until the VOC drop down to safe range? Or it will damage the controller if the VOC go over the 150 Volts in the early morning in the colder months? Should I go through the trouble to rewire the strings with 3 panels? do I have enough volts to deal the voltage loss with 3 panels in a string run about 75 to 100 feet to the controller, and absorb the 48 volts battery bank with 60 volts absorption setting?

Thank you in advance.

[boB]

Paul, the Classic is made so that it can withstand that temporary high input voltage.

We call that feature, "HyperVOC" and this is exactly what it is for.  This works especially well
for 48V batteries like you have.

If you don't want the Classics to ever turn off, you can certainly re-wire them for lower input
voltage which can also help the Classic to run cooler.  The decision to run lower input voltage
depends on how far away the PV solar array is from your power center...  i.e. how long the
wires are.  If the PV array is fairly close, I would wire the array for lower voltage.  If the
PV array is say, hundreds of feet away, I would stay at the higher input voltage because
the power loss in the wire/cable will be lower at high input voltage.   It depends on which
loss is less offensive.  A cooler running charge controller will also be good for its long
life but maybe it is not all that hot running as it is now ?  If you can touch the Classic,
it's probably cool enough.

boB

[Vic]

Hi Paul,

Welcome to the Forum.

The two different PV modules are close enough in Vmp to be wired in parallel strings on a single Classic 150.

The Classic Charge Controllers (CC) are unique,  in that they have Hyper Voc.  This where the CC simply rests when the input voltage from the PVs reaches or exceeds the number value of the specific Classic -- 150 V,  in your case.  The Classic will make NO power when this 150 V input is reached,  but should not be  damaged,  unless the input voltage exceeds the voltage rating 150,  plus the nominal value of the battery voltage -- 48 V,  or a total of 198 volts.  Above 198 V,  the Classic may be damaged,  and the warranty would probably be voided.

Your MPPT CCs will operate less efficiently with your four PVs in series,  verses running PV strings of three.  This lower efficiency results in added heat that the Classic must rid itself of.

So,  YES you should rewire your PVs in strings of three,  and you will only be able to use 15 PVs,  having one in unused PV in reserve.

You may want to use the Classic String Sizing Tool to confirm that your total power is comfortable for the Classic -- about 3960 W STC,  which is probably OK.

Classic String Sizer:
http://www.midnitesolar.com/sizingTool/displaySizing.php

More Later,   Vic

[Paulcheung]

Thank you guys, so I shouldn't be worry too much about damage to the controller. The wire is about 90 to 100 feet between the panels and the controller, if I rewire to 3 panels, would the better efficiency compensate the lost of 1 panel power?

I am not worry about the lost of power when the voc over 150 volts because that is rarely happen, I just don't want it get damaged if on cloud edge when the rain stop and the panel cool.

Thanks again.

[boB]

If you don't want the Classics to ever turn off, you can certainly re-wire them for lower input
voltage which can also help the Classic to run cooler.

What the heck was I thinking here about not wanting the Classics to "ever turn off" ??

I don't know !

[vtmaps]

The wire is about 90 to 100 feet between the panels and the controller, if I rewire to 3 panels, would the better efficiency compensate the lost of 1 panel power?

Welcome to this forum, I've been following your discussions about batteries on another forum.

As mentioned, 3 panels in series would be optimal from the controller's perspective.  But lower voltage means higher cable losses.  In many (not all) systems the over all efficiency of the higher and lower string voltages is the same, but in the higher voltage system more of the inefficiency (and heat production) is in the controller and in the lower voltage system more of the inefficiency (and heat production) is in the cable. 

Here's a recent analysis of higher vs lower string voltage for another system:
http://midniteforum.com/index.php?topic=2008.msg18796#msg18796

Does this analysis and conclusion apply to your system?  Depends... what size cable do you have installed?  If you have a small cable and want to add panels to it, you may need high voltages.  If lower voltage is practical (cable size, distance) it is a better design.

--vtMaps

[dgd]

Thank you guys, so I shouldn't be worry too much about damage to the controller. The wire is about 90 to 100 feet between the panels and the controller, if I rewire to 3 panels, would the better efficiency compensate the lost of 1 panel power?

I am not worry about the lost of power when the voc over 150 volts because that is rarely happen, I just don't want it get damaged if on cloud edge when the rain stop and the panel cool.

Thanks again.

I would doubt the minor increase in overall efficiency of a 3 serial PV string and 15 PVs in total in five parallel strings, would make up for the loss of one PV panel.
Sixteen panels in four strings or 4 in series would be the way I would go.
These 30v mpv panels would make about 120v, this is easy for the Classic without significant overheating.

dgd

[vtmaps]

I would doubt the minor increase in overall efficiency of a 3 serial PV string and 15 PVs in total in five parallel strings, would make up for the loss of one PV panel.

I suspect that you are correct, but sometimes its not just about getting the most power.  The OP is expanding to a 4000 watt array.  The Classic 150's maximum power handling is 4438 watts with an input voltage of 120 volts, and 4847 watts with an input voltage of 90 volts.  The reason the Classic is derated by 409 watts at the higher input voltage is because it produces so much extra heat that it must derate itself to avoid damage.

I believe that a good conservative design does not push the equipment near to its limits all day.  The OP is in a warm-hot climate with long sunny days.  The equipment will be happier (cooler) and last longer with the lower input voltage. 

We still haven't heard back from the OP about what size cable is in place and whether he will upgrade it... he may have to stay at the higher input voltage.

--vtMaps

[dgd]

The Classic 150's maximum power handling is 4438 watts with an input voltage of 120 volts, and 4847 watts with an input voltage of 90 volts.  The reason the Classic is derated by 409 watts at the higher input voltage is because it produces so much extra heat that it must derate itself to avoid damage.

I see from the panel specs provided that the max power would be 4Kw with sixteen panels,
BUT with warm-hot climate the OP would probably see somewhere about 75-80% of this once the PVs get hot
Ok, he may get full rating with MPV early morning or on a cool day with clear (or slight clouds) but the 150's rated 4438 watts is probably enough margin.

I believe that a good conservative design does not push the equipment near to its limits all day.  The OP is in a warm-hot climate with long sunny days.  The equipment will be happier (cooler) and last longer with the lower input voltage. 

Yes I would completely agree with this, it would be interesting to know what sort of daytime power is generated from the existing twelve 250w pvs when they get hot, also, as you have asked, what is the guage that 100feet of wiring.

dgd

[Paulcheung]

Sorry I didnot know someone did more reply after I did my last post.

The wires are the same size wire that come with most grid tie panels. (4mm2?) it has 3 wires from the combiner box going to the charge controller down stairs. 2 positive with 2 strings of 4 panels each wire and all the negative wire are connected to 1 wire, the output current to the battery bank is around 40 to 55 amps at about 100 to 110 volt input, the classic controller does not report incoming currents.

I live in a place that has alot of clouds passing by even the weather is not overcast. the panels are aim to sun for winter season because we got exceesive sun hours during summer months. I got the highest haveast in one day is about 18KWH with 16 panels and around 14KWH with the 12 panels. with long sunny day like that the controller do goto absorb mode and cut off some of the current.

I only see the panels name rated power right after the cloud edge. the time is hot even in winters so I take the chance leave the panels in 4 in a series.

[zoneblue]

I lean the other way guys. Having studied as many controller outut curves as i can find, and watched those who have struggled with hot classics at high voltages, i suggest 3S strings would be better for you. When i did the math on our system i was going to lose more watts in the classic than in the cable when comparing strings of 2 to strings of three. Even if its in the same or near ballpark, that amount of heat is a LOT easier to disipate in teh cable than in the controller.

However... for the home run 4mm2 for 90 feet is not really suitable. Copper is expensive but its not that expensive. Off grid home run cables are typically in the order of 16-20mm2 (6awg to 4awg). You can model and compare these losses very roughly by assuming that the controller will be 2% less efficient for each addition to the string length. Thats an unofficial measure that i beleive originates from the FM controller curves. It would be helpful if midnite published there curves, (they have done all the testing anyway.) And make string design a much simpler process.

With mppt there is a tiny bit of a contradiction, unfortunately, that while mppt allows longer string lengths the efficiency is decreased undoing some of the gain of mppt in the first place.  On balance the sweet spot is an array vmp of  somethng like twice the system voltage.

The weak point in all electronics is electrolytic capacitors, and they have notably short lifes, and accelleratingly so with each degree in extra temperature.

[vtmaps]

When i did the math on our system i was going to lose more watts in the classic than in the cable when comparing strings of 2 to strings of three. Even if its in the same or near ballpark, that amount of heat is a LOT easier to dissipate in the cable than in the controller.

Total agreement.  I've also done the math. 

Another point in favor of the lower voltage string is that the power loss in the cable goes as the square of the current in the cable... therefore at typical (less than max) power levels the cable loss is much much lower than what you calculate at max power.

On the other hand, the controller inefficiency persists at all power levels.

--vtMaps

[Paulcheung]

What is the most efficient input voltage for the Classic 150 with 48 volt battery system that the battery bank voltage is mostly at 52 to 60 volts?

[Vic]

Hi again Paul,

IMO,  there is a sweet spot at around 90-ish Volts for Vin for the Classic 150 on a 48 volt battery.

Three PVs in series per string,  is what I run on some systems here,  and would run,  if I were you.   But  this would leave one PV unused.

There is one additional issue with running strings of four 60 cell (30-ish volt Vmp) PVs.   This is,  that  when later stages of Absorb,  and in Float,  the PVs are often lightly loaded (unless there are largish loads on the inverter/s),  this makes the PV voltage into the CC rise,  closer to Voc for the temperature of the PVs.  This makes the Vin even higher than it was when the PVs were at temperature-corrected Vmp for the solar input.  You could be seeing about 130 or 135 volts into the Classic when in Float.   This results in a lot of extra power lost in the CC,  even when considering the relatively low output current from the CC under these conditions.

You might well have some Opportunity loads,  like A/C  that could help load the PVs,  and reduce a relatively high Vin when in Float,  but from direct personal experience,  MPPT CCs do not run efficiently with Vins fairly close to the maximum specified for that CC.

FWIW,  opinions,   Vic

[vtmaps]

IMO,  there is a sweet spot at around 90-ish Volts for Vin for the Classic 150 on a 48 volt battery.

That is certainly correct for typical 'grid-tie' panels with a Vmp of 30 volts. 

I think that if you were constructing your array from true 12 or 24 volt panels, you could aim for a Vmp that is about 76 volts as a ever-so-slightly sweeter spot.   

Of course, every system is different... if you live in a very hot climate and refrigerate your batteries, a Vmp of 76 volts might not be high enough to give the controller the headroom it needs.

--vtMaps