First, let's explain why this happens.
Why your inverter has to trip on over voltage
The Australian Standard for Solar Inverters AS4777.1 mandates that an inverter must disconnect from the grid if:
- the average AC voltage over any 10 minute period goes over 255V
- the voltage at any time goes over 260V
So if your inverter trips on an 'over voltage' error, the voltage where the grid connects in to your inverter has breached one or both of these limits.
Why your inverter goes into 'voltage-dependent power reduction' mode
In marginal cases your inverter may not trip off, but may reduce its power output instead as a way to cope with grid voltages that are a little too high.
When your inverter reduces its power due to high grid voltages it is in what's called "Volt-watt response mode". It has to do this to comply with the latest version of Australian Standard AS4777.2.
I knocked out this sketch to show what happens. Your inverter will start reducing power at 250V and reduce it linearly down to 20% as the voltage increases, tripping if it hits 265V.
This is a grid protection feature, and is important to maintain grid quality for everyone.
Why the overvoltage tripping or power reduction occurs
It may be one of the following reasons:
1. Your local grid is already operating outside the Australian Standard voltage limits. AS 60038 specifies 230 volts as the nominal grid voltage with a. +10%, -6% range, so an upper limit of 253V. If this is the case then your local DNSP has a legal obligation to fix the voltage. Usually by modifying a local transformer.
2. Your local grid is just under the 253V limit and your solar system, although installed correctly and to all the standards, pushes the local grid just over the 255V tripping limit. Your solar inverter's output terminals are connected to a 'point of common coupling' with the grid by a cable. This cable has an electrical resistance that creates a voltage across the cable whenever the inverter exports power by sending electrical current into the grid. We call this a 'voltage rise'. The more your solar exports the bigger the voltage rise thanks to Ohm's Law (V=IR), and the higher the resistance of the cabling the bigger the voltage rise.
The Australian Standard 4777.1 says that the maximum voltage rise in a solar installation must be 2% (4.6V).
So you may have an installation that meets this standard, and has a voltage rise of 4V at full export. Your local grid may also meet the standard and be at 252V.
On a good solar day when no one is home, the system exports almost everything to the grid. The voltage is pushed up to 252V + 4V = 256V for over 10 minutes and the inverter trips.
3. The maximum voltage rise between your solar inverter and the grid is above the 2% maximum in the Australian Standard, because the resistance in the cable (including any connections) is too high. If this is the case then the installer should have advised you that your AC cabling to the grid needed upgrading before solar could be installed.
How To Diagnose If Your Issue is Reason #1, #2 or #3
Test Your Grid Voltage
To test your local grid voltage, it must be measured while your solar system is powered off. Otherwise the voltage you measure will be affected by your solar system, and you can't lay the blame on the grid! You need to prove that the grid voltage is high without your solar system operating. You should also turn all the big loads in your house off.
It should also be measured on a sunny day around noon - as this will take into account the voltage rises caused by any other solar systems around you.
First - record the instantaneous reading with a multimeter. Your sparky should take an instantaneous voltage reading at the main switchboard. If the voltage is greater than 253V, then take a photo of the multimeter (preferably with the solar supply main switch in the off position in the same photo) and send it to your DNSP's power quality department.
Secondly - record the 10 minute average with a voltage logger. Your sparky needs a voltage logger (i.e. Fluke VR1710) and should measure the 10 min average peaks with your solar and big loads switched off. If the average is above 255V then send the recorded data and a picture of the measurement setup - again preferably showing the solar supply main switch off.
If either of the above 2 tests is 'positive' then pressure your DNSP to fix your local voltage levels.
Verify the voltage drop in your installation
The company that sold you the solar system must, if asked, provide their Voltage Rise Calculations (VRC) between your Inverter & Point of Common Coupling (PCC) on the grid in writing. Be prepared to give these to your DNSP if asked.
Ensure they show a voltage rise of less than 2% and that they have been signed off by a CEC Accredited Installer. If you are worried that they might be 'fudged', then consider a second opinion from a well regarded local CEC accredited installer. Be prepared to pay for the second opinion.
If the calculations show a voltage rise of more than 2% then you will need to upgrade the AC cabling from your inverter to the grid connection point so the wires are fatter (fatter wires = lower resistance).
Final Step - measure the voltage rise
If your grid voltage is OK and the voltage rise calculations are less than 2% then your sparky needs to measure the problem to confirm the voltage rise calculations:
- With PV off, and all other load circuits off, measure the no-load supply voltage at main switch.
- Apply a single known resistive load e.g. heater or oven/hotplates and measure the current draw in the actives, neutral and earth and the on load supply voltage at main switch.
- From this you can calculate the voltage drop / rise in the incoming consumer main and service main.
- Calculate the line AC resistance via Ohm's Law to pick up on things like bad joints or broken neutrals.
Now you should know what your problem is.
If it is problem #1- grid voltage too high- then that is your DNSP's problem. If you send them all the evidence I've suggested they will be obliged to fix it.
If it is problem #2 - grid is OK, voltage rise is less than 2%, but it still trips then your options are:
- depending on your DNSP you may be allowed to change the inverter 10 minute average voltage trip limit to 258V. Get your sparky to check with the DNSP if you are allowed to do this.
- If that is not possible then, if you have 3 a phase supply, upgrading to a 3 phase inverter usually solves the issue - as the voltage rise is spread over 3 phases.
- Otherwise you are looking at upgrading your AC cables to the grid or limiting the export power of your solar system.
If it is problem #3 - max voltage rise over 2% - then if it is a recent installation it looks like your installer has not installed the system to the Australian Standard. You should talk to them and work out a solution. It will most likely involve upgrading the AC cabling to the grid.