DIY Grid Tied Inverter (doesn't Feed the Grid) UPS Alternative

This is a follow up post from my other Instructable about making a grid tie inverter that doesn't feed back into the grid, since it is now always possible to do this in certain areas as a DIY project and some places do not allow feeding into there grid ( and its pretty obvious why the grid doesn't want just anyone feeding into the grid as i will explain a little later on)

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The Concept

Many people want to have solar panels to reduce their impact on the environment or reduce the cost of their electricity and there two ways about doing this, going fully off grid that requires a large bank off batteries and a decent inverter or subsidising your electricity with both grid and and renewable energy using grid tied inverters that feed your power back into the grid.
The problem is that going off grid is not always possible, designing a system that would power everything you want without issue would be very difficult and unreliable. and with grid tied inverters you would need a qualified electrician to install the inverter so it conforms to regulations when feeding back into the grid which is not very cost effective for everyone or ideal for your application. so my solution is a small scale solar system with a "grid tied non feedback inverter" using basic components that are easily available. this allows you to produce and use your own electricity without feeding into the grid but still able to use the grid power when you run out.

Step 1: The Basic Principle (an Overview Explaining the Changes)

So a few things have changed since the last time upload about grid tied inverter, one being that i am no longer using ups (uninterruptible power supply), This is for a couple of reasons, the main few being that i was not able to pull the amount of electricity I needed without overloading the "UPS" at which point it safety features would cut in and shut it down, not great when you need to unlock several items to turn on one ups! another issue i was running into was the dc current was exceeding the rating of the relays meaning that i had to double them up to try and reduce the current but was still to warm.

I have also moved away from the complex switching system that was using relays and ssr, simply because when you run into problems fault finding can be difficult and tricking a "UPS" to switch over isn't the most flawless way for electricity, and that when I was pulling allot of current you would notice things like light flicker and some devices didn't like it, mostly computers!

so as a result i have done away of UPS and Relay circuits and simplified it with common items that are just as accessible to most people, and is currently my preferred way of controlling my system.

Step 2: Parts Used and Brief Description

So Finally We can get into Everything i have used on this project, and this time i will be more in depth!

But first, a little safety disclaimer, This project involves AC (alternating current) and DC (Direct Current) electricity both of which are extremely dangerous and can cause harm or even kill if not installed correctly. If You DO Not Know What you are doingor understand it fully then do not attempt this or other electrical projects of a similar nature. That being said anyone can learn this stuff, just seek help from people who do understand it and stay safe!

Parts (linked to places to buy) :

Victron Energy Solar Controller MPPT
Sun Tech 275w PV Panel
20A PWM Charge Controller
20W Solar panels
100AH 12V PowerLine Leisure battery
APC 16A ATS (Automatic Transfer Switch)
Victron Energy Battery Protect 65A
12v 500w Pure Sine Wave Inverter
Sonoff Wifi Controller
2 way consumer unit with RCD Main switch

Quick Description:

Victron Energy Solar Controller MPPT
- Controls the charging of the batteries from the 275w pv array it turns the 30v output down to 13v to charge up the batteries and stops charging them when there full.
Sun Tech 275W PV Panel
- They convert sunlight into 30v dc which then goes to a charge controller to charge batteries, I Brought damaged ones cheap then repaired them with a clear resin.
20A PWM (Pulse Width Modulation) Charge Controller
- Does the same as the first charge controller, just for a lower current, this is used to take power from my 20w solar array and is wired in parallel with the first charge controller.
20W Solar panels
- Takes power from the sun and converts it to dc power
100AH 12V PowerLine Leisure battery
- This is where all the electricity produced is stored
APC 16A ATS (Automatic Transfer Switch)
- This device is the brains and switches between an inverter and the grid (more in depth explanation further on)
Victron Energy Battery Protect 65A
- Is a low voltage cut off to stop you from running batteries completely flat
12v 500w Pure Sine Wave Inverter
- Converts 12V DC and turns it into 230v Ac Sine wave (like grid power)
Sonoff Wifi Controller
- not overly necessary, but allows you to wireless control circuits attached to it via WiFi
2 way consumer unit with RCD Main switch
- Protect you and your circuits from faults that may occur either from devices you plug in or fault that an inverter might cause (will go into more depth why you should have this)

Step 3: Simple Diagram to Show How This Works and Explanation

So, to cut a long storey short, instead of using an "ups" im now using an automatic transfer switch and inverter. The automatic transfer switch allows for two power inputs and switches to the other when one fails, you are also able to choose which one is default.

These switches are mainly used for large servers to switch from one ups to another seamlessly and allow up to 16 amp switching. Which when you take into consideration a normal radial circuit wired in 2.5mm cable is attached to a 16 amp MCB in the UK will be more than enough for my switching needs, and since this is all contained in one device makes it a lot safer and simpler .

so the way i have wired this system, is that the inverter is connected to the battery protector, this turns off the dc voltage to the inverter when there isn't enough power in the batteries. The inverter is then wired into the automatic transfer switch along with the grid and i have set the "ATS" to use supply one as default (this is the inverter) now when the battery protect turns off the inverter the "ATS" will seamlessly transfer to grid, and back to the inverter once the batteries have recharged.

***Added Feature***

The sonof wifi switch runs off the 12v battery bank, and is wired into the remote control of the battery protector, this means i can control whether the inverter is on or off via Alexa or my phone, i have set up some timers on it as well since i am not home during most the day the inverter wont actually be turned on till about 2:00pm this means that for most the morning my batteries are charging up and i can stay on battery bank power late into the evening making the most of the energy ive generated. and that i can automatically control inverter without having direct access to it.

Step 4: Types of Inverter

Why i chose to use a pure sine wave instead of a cheaper modified sine wave.

Well the truth is, I didn't get a choice. I originally did this set up with a 2000W modified sine wave and ran into issues because the automatic transfer switch wasn't able to switch seamlessly it had to cut power completely then restart every time it switched over, not to mention modified sine wave makes an irritating buzzing sound on everything you plug into it. so i had to switch out to a pure Sine wave and the "ATS" worked perfectly.

After looking into this a bit more, i found out that the reason the modified sine wave inverter didn't work with the "ATS and grid power because of what is called "phase mashing" which is when the "ATS" tries to push a modified sine wave onto a load that is already accepting a perfect sine wave, and when you look at a picture of a modified sine wave and pure sine wave you can see why devices wont like switching instantly. pure sine inverters work because its the same as the grids wave.

And its because of phase mashing that the grid doesn't just want anyone feeding into it and that you need permission so they can see your system and make sure it works correctly and would shut down safely and not feed in to there power if there was a power cut. its all down to keeping people safe.

Step 5: Using an Consumer Unit

The consumer pictured above is the one i installed after the automatic transfer switch this is because unlike a ups the transfer switch can not detect faults on a system, so if the breaker feeding the grid side of your Ats trips because of a fault on the load side of this system it could potentially switch to inverter and your fault would still be live and dangerous, the point of an rcd is to protect against imbalances in current so offers the most protection to people, were as the mcb protects the circuit from damage.

its always good to keep in mind that, most protective devices are there to protect the circuit not you, this is another reason why its good to have this consumer unit on the load side of an "ATS" becuase it will protect the inverter from short circuits and overloads, as well as devices that might become faulty.

As an (Apprentice) electrician are goal is for there to be discrimination between circuits ,having a consumer unit protects your solar loads from affecting normal grid appliances, since this is only a small scale system and its more likely to trip this first before your house breaker. always better to be over kill and safe!

if i were to improve this system i would switch the mcb for rcbos since they offer the maximum protection for you and protection your equipment