Getting Your Grid Tied Solar Array To Work When The Grid Is Down

NOV 26 2014 BY GEORGE BOWER 56

Grid Tied Solar PV System

Grid Tied Solar PV System

Slide 2 - A View To The East

SlideA View To The East

If you have an EV chances are you either already have a Solar array or you are thinking about getting one. Also, if you already have a system you may have one that uses micro inverters (usually Enphase inverters). In a micro inverter type grid tied (GT) system there is an individual inverter under each panel as shown in figure 4.

Preface:

I’ve been looking into using some Leaf battery Cells instead of my current deep cycle lead acid batteries. Everything in this article still applies as far as the low cost DC TAP goes. The only thing that changes is I need a different charge controller. But believe it or not the MPPT  terminology applies between both lead acid batteries and Lithium.

 (There is also an excellent article  in the My Nissan Leaf Forum – Very impressive with Leaf modules)

Looking West

Looking West

Figure 4

Figure 4

Most grid tied system owners are aware that their grid tied inverters shut down when the grid is down.

Figure 5

Figure 5

This is a disturbing situation. When the grid is down is when you need them the most. Sure one could use a portable gen set but what if there is an extended grid outage? Wouldn’t you want your solar system to be available?

The most obvious solution would be to fool the inverters into thinking the grid is up by feeding them a pure sine wave at 220 volts. They would see this sine wave and power back up….problem solved.

Figure 6

Figure 6

The Enphase paper in figure 6 references a MagnaSine inverter that is designed to fool the inverters by creating an AC coupled system that consists of the MagnaSine inverter and a battery bank. The problem is that the inverter alone is $2,500 (USD) and that is not all you need. You also need a critical load panel….and of course the batteries. I already have a small battery bank and inverter system so I’m not that keen on spending $2,500 on an emergency power system that may never get used.

You can read the Enphase AC coupling paper just google “AC Coupling of Enphase Micro Inverters, 2014”.  Things get complicated if the grid is down and all your critical loads are off and you’re batteries are fully charged. Apparently the enphase inverters don’t like being dead headed. They need a place to send their AC output. So in the enphase MagnaSine inverter system the MagnaSine inverter shuts down the enphase micro inverters by shifting the fake grid signal slightly. The enphase inverters sense an out of spec signal and shut down.

If you have not yet installed a grid tied system you should consider the Outback AC coupled system (or Sunny Boy). These grid tied inverters are not Micro inverters though. You still bring back DC to one central inverter as shown in figure 7.

Figure 7

Figure 7

The problem with the outback solution is it’s expensive. You need additional relays and the critical load panel again for a system that may never be needed. A schematic for the Outback system is shown in figure 8. Hooking your generator into this system is also possible. It uses relays instead of frequency shift to decouple the GT inverter in the case of no critical loads and a full battery.

Figure 8

Figure 8

All these AC coupled solutions have a high price tag. Personally I can’t justify it. I wanted a solution that is low cost. The low cost solution I have come up with is fairly simple. It involves tapping into the DC output of the PV panels upstream of the micro inverters as described in figures 9 and 10.

Figure 9

Figure 9

Figure 10

Figure 10

Since the grid is down, the enphase inverters are shut down and tapping into the DC upstream of them should make no difference. The problem would be if the grid came back up with the DC diverted to the battery charge controller. In order to fail safe this situation the DC shut off (DC disconnect) that will be installed at the end of the DC run will have a padlock. So if the grid goes down and you want to tap into the DC for battery charging, you remove the padlock from the DC disconnect and go to the AC solar Panel disconnect and switch that AC disconnect off and padlock it in the off position.  The main purpose of the emergency DC tap out is to charge batteries. In my particular case I will have 4 deep cycle batteries to be charged so the DC tap out needs to go to a battery charge controller. I also have a DC to AC inverter.

In the process of designing this low cost DC tap out system I was forced to learn some basic wire sizing and charge controller guidelines that I would like to share. The good folks at “Northern Arizona Wind and Sun” helped me with the design. Their website is an excellent place to go for all kinds of info when you are designing a system. I plan to go to them to buy the hardware I will need to finish this project in return for all the time they spent with me.

Here are the guidelines I wanted to share. The first guideline has to do with wire sizing. When sizing your DC run you must use short circuit panel amps times 1.25 as shown in figure 11.

Figure 11

Figure 11

When sizing your battery charge controller you must use open circuit panel voltage times 1.25 as shown in figure 12

Figure 12

Figure 12

Another thing I learned in this design project was that all charge controllers are not created equal. There are 2 basic types: PWM and MPPT chargers. There is a good tutorial at the Northern Arizona Wind and Sun web site but the bottom line is that I need to use an MPPT charge controller. That is because my grid tie solar panels are somewhat of an odd ball voltage as they are specifically designed for grid tied inverters. They are 60 open circuit Volts and most battery systems are 12, 24 or 48 Volts. If you charge a 24 Volt battery pack with a 60 Volt Panel with a PWM type charge controller you are wasting almost half of your Voltage. In order to use a PWM charge controller you need to match your battery voltage to your panel voltage.

The charge controller selected for my system is an MPPT charge controller that can charge at 12, 24 or 48 Volts and 60 amps as shown in figure 13.

Figure 13

Figure 13

As I mentioned earlier I plan to use 4 deep cycle batteries at 24 Volts. That will give me 4 kwh of storage as shown in figure 14.

Figure 14

Figure 14

Your reaction may be that 4 kwh isn’t enough. I also have a Honda eu2000i generator that is very fuel efficient because it is variable speed. I am currently modifying it for tri fuel capability. I have a 250 gallon propane tank that is used for heating so the plan is to tap into that tank for another source of emergency power.

Figure 15

Figure 15

The low cost DC wire tap system is not finished yet. A few things still to do before installing it as shown in figure 15. When it is finished I’ll publish another article.

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56 Comments on "Getting Your Grid Tied Solar Array To Work When The Grid Is Down"

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Leptoquark

I thought people put in a transfer switch to grid-tied systems to temporarily take them off the grid, and hence make them active during a power outage. See http://www.aeesolar.com/grid-tied-solar-systems-with-backup-power
for example.

Remember the reason grid-tied solar PV systems turn off during an outage: it’s so your system doesn’t electrocute the lineman working on the wires.

Does this design account for that?

Kosh

+1

And what are these long term grid outages anyway? Seems like a lot of hassle for at most a couple hours. In my experience, that usually happens at night anyway…..

Just pick up a good book and a flashlight.

Aaron

This past year, in the city of Dallas, we had 3 days without power after a storm in October. We lost all the food in our freezer/refrigerator. Being able to switch over to solar only (even if it’s a manual cut-off switch) would have saved that food and allowed us to do other things in the house that required power.

Spec9

But how many times has that occurred in the last 10 or 20 years.

For me, the last time I had a power outage of more than 4 hours was the 1989 San Francisco Earthquake.

Dan Hue

Here in PA, it happens quite a bit. Lots of trees and suspended power lines make for a bad combination. One tree can cause a short term outage, but hurricane remnants, which commonly pass us over from the south, and of course the dreaded ice storms can wreak havoc on the system.

Spec9

I’m with you, Kosh. People are not very good at analyzing risks. We freak out over Ebola despite the fact that more people have married Kim Kardashian than have died of Ebola in the USA.

Unless you have a remote off-grid cabin or a notoriously extremely unreliable local grid, you are far better off to spend that extra money on more solar PV panels for your system than to waste the money on batteries & charge controllers that will rarely ever get used. The world will be better off too.

Grendal

And let’s be very clear that marrying Kim Kardashian is far more unhealthy for you than catching Ebola.

Chase

basically remembering to manually disconnect the grid is this designs only protection against energizing the secondary side of the utility transformer…. Well designed backup systems are expensive for a reason. Peace of mind.

Kalle

Arent they shuting down to not electricute some poor electrishian working on the system down the line?

kompot

Yea, that is exactly what I was told by my provider. They dont want any electricity coming back to the grid, so they can safely work on repairs.

MrEnergyCzar

Interesting work around. For those with low energy use homes, you can use your Volt for extended power outages. Here’s a short video explaining….

https://www.youtube.com/watch?v=T0CSs3yorFs&index=5&list=PL3C7D1EBDF618528E

Bill Howland

Some of us already have off-grid power from our Solar Arrays.

My 38 panel system in Buffalo probably makes less juice yearly than your 14 panel system in Arizona, for various reasons, but mainly since we get as much sun as Alaska.

Using just spare parts, I have 2- 1500 watt Secure Power System outlets providing up to 3000 watts in theory during a bright sunshiney day. Wintertime operation, when most power failures occur, limit the actual power to a few hundred watts here.

The feature is included at no extra cost in my two SMA 4000 watt transformerless inverters.

Spec9

I’ve got an Enphase microinverter based system as well. However, I think that although an off-grid back-up system that works during a power outage is an interesting hobby project, it is really just pointless waste of money if you have a reliable grid.

You are far off better served by spending that extra money on more solar PV panels and more inverters until you generate more power than you use.

However, since the whole battery-thing appears in the news all the time (which it really shouldn’t because it doesn’t matter until we get like 30% of the grid solar PV and right now it is less than 1%), I believe Enphase has been working on their own system. Just searched . . . yep, the Enphase AC Batter or Enphase Energy Management system. Looks like a way to easily add a battery to an Enphase microinverter based system.

ModernMarvelFan

I thought you can just do this by hook up a generator to your panel (2 phase) and then cut the main breaker (to cut the power from the grid). Then you already have a local system.

Based on the generator/inverter, it should keep your house at a steady 240V that should turn on the grid tied solar inverters.

Am I missing something?

QCO

The author’s mistake was to deploy those silly Enphase inverters in the first place.

A central inverter would have given him the flexibility to do several things, including a DC tap to charge a separate battery and inverter set or an integrated grid tie inverter with battery bank changer that are now becoming available.

The best solution for him now is to charge a set of batteries from the grid (which you are solar charging anyway via grid tie) and use them with a dedicated inverter for key loads during outages. Maybe put up an extra panel dedicated to the battery bank, if you feel the need to last a bit longer.

But if you want the “whole house” UPS solution, get rid of the Enphases and build a float charge system using conventional charge controller, battery and inverter components.

Spec9
Meh. I’ve previously installed a central inverter but went with microinverters instead for several reasons: 1) It is easier to install. No dealing with High-voltage DC. Very plug & play. 2) Works great with shading . . . shading on a panel only affects that panel. However, situation can be now be remedied with DC-optimizers I guess. (They were not available when I designed my system.) 3) Easy to expand the system. I designed my system such that I can easily add another 8 PV panels or so if I want more power. 4) Any single panel or inverter that dies doesn’t take out the system, it only takes out that one panel. 5) My local building department demanded DC disconnects on the roof. I wasn’t sure what to buy for that and how I’d install them. And since my system is split into two parts, I’d need at least 2 of them. And then I’d end up with these ugly switches on my roof instead of my nice clean install that uses Solodecks so all the conduit & wiring is hidden in the attic. 6) I’m guessing it is a bit safer to have 240VAC than 400+VDC running in… Read more »
David Hrivnak

Another option is the newer SunnyBoy SMA inverters have a switch to put them in stand alone mode. Each inverter can output 120V 1500 watts of power when the sun is shining. I tried it and it was able to charge my EV and keep the refrigerator running. It is isolated so the line-men are safe.

io
CAUTION — George, your planned rewiring (fig 10) is NOT SAFE, on many levels. I completely understand your desire to add a low-cost minimal emergency power capability to your grid-tie PV system (in fact, I’m planning the same stuff), but let me describe a few safety issues I see here: 1) You effectively connect many PV modules directly in parallel (all the time, not just when micro-inverters are off). Even if you size the additional DC wire correctly, the module’s aren’t: their connectors, wiring and innards can usually only take 15 A max. If a short develops in one module, it will be sinking the current from all the remaining ones. Very unsafe conditions like overheating conductors or arcing will likely result. For that reason, you must not put more than 2 modules in parallel without protection diodes or fuses for each module. 2) You also connect multiple micro-inverters’ inputs in parallel. They are absolutely NOT designed to be wired this way. Even if somehow they manage to not interfere with each other (I suspect that at the very least their MPPT is going to get seriously confused), feeding them multiple modules violate their specifications. A malfunction like an internal… Read more »
Chase

Whatever Mickey Mouse hook up you do to get power in an outage just remember a transformer works in reverse… I believe it’s your right to survive and provide for you and your family by any means but don’t be an idiot in the process and endanger the people trying to repair the grid.