Solar Voltage Rise – why you should care

Solar Voltage Rise can significantly reduce your solar production, but the problem is often ignored. It’s one thing to use a quality inverter and panels, but if solar voltage rise is not considered by your solar installer, then your solar may produce significantly less than it should have.

In part one, I’ll explain what voltage is, why solar voltage rise occurs, and then show three methods for solar voltage rise calculation. In part two we’ll look at why you should want to minimise voltage rise. In part three I’ll explain four ways a quality solar electrician will do that. I’ll also reveal an inverter preset “handbrake” that you may be able to turn off.

Part 1: Why does voltage rise?

In Australia, the nominal grid voltage is either 230 or 240 Volts. The grid voltage levels will vary and fluctuate throughout the day depending on how much power is being drawn from the grid, and how much solar is being sent back. It’s common to see voltage fluctuations of 10 volts throughout the day. It’s the job of the electricity distributor to maintain your voltage – between about 217 volts and 254 volts. But keeping the voltage below 254 is becoming more of a challenge for the electricity distributors.

Solar Voltage Rise

When your solar system is producing more power than your home is using, it sends the excess back to the grid. In order for power to flow from your home to the grid, the voltage from the solar inverter has to produce a voltage that is a couple of volts higher than the grid voltage. Voila, Solar Voltage Rise.

In the ideal situation, the voltage rise is not a problem: the inverter increases the grid voltage from 240 volts to 242 volts. The problem arises when the customer’s cables between the inverter and the grid are too small for the size of their solar system. Let’s get back to basics to understand why.

VOLTAGE = CURRENT × RESISTANCE

Volts, amps, ohms, current, resistance. How do they relate to one another?

•   Voltage is the muscle that gets the current through. In Australia, that muscle is around 240 volts (V).
• Current is the rate of flow. A 5kW inverter will pump out around 20 amps at lunchtime. We measure current in Amps (I).
• Resistance prevents current from flowing. Resistance depends on the cable’s diameter, length, and whether it is made of copper or aluminium. We measure resistance in ohms (Ω).

In the illustration, if the resistance pulls harder, either the current needs to get thinner or the voltage needs to get stronger. In 1827 Georg Simon Ohm egotistically called this co-dependent relationship “Ohm’s law”.

Ohms Law

Ohm’s law is the mathematical relationship between voltage, current and resistance:

• V= I×R
• Voltage  = Current  x Resistance
• Volts = Amps x Ohms

To keep the equation balanced, if the resistance in your property’s cable is high, either the voltage from your inverter will have to be higher, or the current to the street will have to be lower.

But reducing the current is a stupid idea. If your inverter wants to send 20 amps back to the grid, then we should “let it flow”.

The only way left to balance the equation is to increase the voltage even more.

The higher your cable’s resistance is, the higher the voltage must be to force the current to the street. Solar Voltage Rise starts becoming a problem.

Voltage Rise and Voltage Drop

Solar Voltage Rise is a relatively new issue that is causing problems with solar systems and grid voltages around Australia. The more solar that is installed in your street, the higher the grid voltage gets at lunchtime.

While Solar Voltage Rise is a relatively new problem, the opposite problem has been well known since Thomas Eddison lit up New York City streets. It’s called voltage drop.

Voltage drop is the same phenomenon as voltage rise, but it is seen from the grid side rather than the household side.  Assuming there was no solar to account for, the transformer in your neighbourhood may be set to pump out voltage at 250 volts. By the time that voltage gets to the last house on your street, the resistance of all the cables may have dropped the voltage to 230 volts.  By the time the voltage gets to the welder in his shed down the back paddock, it may have dropped to 215 volts.

Electricians minimise voltage drop by choosing the correct cable for the anticipated load. The same formulas and tools we use to work out voltage drop can be used in reverse for solar voltage rise calculation.

Solar Voltage Rise Calculation

According to the Australian Standards AS/NZS 4777, the voltage rise between a solar inverter and the street can be no more than 2 per cent (about 5 volts). In theory, you can use ohms law to calculate the voltage rise of a cable if you know the resistance and reactance of the cable. But there are much more realistic ways to work out what size cable we need to use, or how big of a solar system we can install on your home.

Voltage Rise Calculation Method 1

Work it out old school. Buy a copy of AS/NZS 3008.1.1 for 300 bucks and use one of the four methods it lists. The most common method is described in section 4.2. First, find the correct “millivolts per amper metre” figure from the correct table. Next insert that figure it into the formula: Vd = [L x I x (mV/A.m)] / 1000. Then account for the number of phases and turn it into a percentage of the voltage. Keep trying till you get the desired cable size, length or current.

While this is the hard way, it’s the best way if you want to get a solid understanding of voltage rise calculation. But there is a cheats way to calculate solar voltage rise effectively and it doesn’t cost a cent.

Voltage Rise Calculation Method 2

The easy way to work out voltage rise is to download a solar voltage rise calculation app. You just need to know the size and length and type of cable. The app I use is called WireWizard by Bambach Cables. The app is only on iPhone but it’s also available online. I’ve compared the app’s results with the AS/NZS 3008 pen and paper method, and it worked spot-on every time.

Voltage Rise Calculation Method 3

But sometimes we can’t be confident of the overhead cable sizes or length and route of the existing underground cable. And what happens if there is a high-resistant joint in the cable? In this case, it’s best to get an actual measurement of the voltage drop. Get a couple of voltmeters and a clamp meter and make an actual measurement of the voltage on the street and the voltage at the house while drawing the known current with a clamp meter.

We regularly use Method 2 and Method 3 for voltage rise calculations to be sure we are not installing a solar system that will have a solar voltage rise issue.

Part 2: Why should I care about Solar Voltage Rise?

There are three reasons you may want to reduce household voltage. It’s all about the money.

1. Inverter cycling

In part one, I explained that the fatter the current is, the stronger the voltage needs to be to push that current back out to the grid. Guess when your solar system is running at the fattest current? At lunchtime, when it is producing maximum power!

So if the current is at its highest at lunchtime, the voltage will have to be at its highest so that it can muscle all of that current back to the grid. The problem is, half the people on your street have solar too. Everyone is lifting the grid voltage at the same time. The voltage from your inverter needs to keep getting higher and higher. Inverter cycling begins.

When your inverter is off, you are not making energy. When you are not making energy, you are losing money. It’s all about the money.

2. Burning out appliances

If your solar inverter regularly spends it’s lunchtime cycling off and on and reaching high voltages, it won’t last as long. Many appliances, particularly older 3-phase motors, older fluorescent lights and incandescent lamps will fail quicker with higher voltage. Fortunately, modern houses with inverter air-conditioners and led lights are less susceptible to damage from higher voltages.

3. Conservation voltage reduction

An idea sometimes promoted in the solar industry is to sell homeowners a voltage regulator to reduce their power consumption. The idea is that if you run your appliances at 225 volts instead of 255 volts, you’ll reduce your energy consumption. Well yes, but no.

Conservation voltage reduction is a concept employed by Electricity Distributors to reduce the strain and power loss on the grid by reducing grid voltage.  Factories with multiple older-style 3-phase inductive motors and older-style fluorescent lights will also see savings by installing a voltage regulator. However, most modern household appliances will see minimal or no energy consumption savings by reducing your voltage.

Minimising your household voltage may prevent older appliances from burning out prematurely, and it may save you a smidgeon on our power bill. But the main reason you would want to keep the voltage low at your place is just so your inverter will stay online and that sunshine can make you money. It’s all about the money.

Part 3. How do we minimise Solar Voltage Rise?

Electricity Distributors are continually adjusting grid voltage to attempt to keep it within the required limits. But high grid voltage isn’t a problem that can be solved so easily. The real world has shown that Electricity Distributors can’t always keep your voltage below 253 volts. It’s the electrician’s responsibility to ensure the solar voltage rise at your place no higher than 2 per cent (5 volts). But the less solar voltage rise we contribute, the less chance you will have of your inverter turning off at peak production times. There are various methods we can use to reduce solar voltage rise.

1. Use a three-phase inverter

One solution is to install a 3-phase inverter. A 3-phase inverter will divide the current over 3 different cables. Instead of 21 amps being forced to the grid through one cable, you have 7 amps being sent back through 3 cables. Ohm law has shown us if we lower the current, we lower the solar voltage rise proportionally.

It’s often cost-prohibitive to upgrade an older home to 3 phase. But if you already have 3 phase at your house, we recommend spending a little more on a 3-phase solar inverter. If you are building a new home, it won’t cost much more to install 3 phases during the build. (Installing 3 phase will also allow you to install a much larger solar system)

DID YOU KNOW… if you only have single phase, you can still install up to 13kW of solar panels in Qld. Read Ben’s post about installing a 10kW inverter on single phase.

2. Increase your cable size

A 5kW inverter produces a maximum of 21 amps. Any electrician can tell you 2.5mm2 is big enough for 21 amps. After all, 2.5mm² cable is used to connect power points in every home in Australia. But after doing a solar voltage rise calculation, most solar electricians will use a minimum of 4mm2 cable to keep your voltage rise below 2 per cent. At MC Electrical, we know how important it is to keep your voltage drop as low as possible. Our standard cable for a 5kW single-phase inverter is 6mm2. If the inverter is a long way from your switchboard and our solar voltage rise calculations are approaching 2 per cent, we will run parallel 6mm2 cables just to be sure.

2. Install your inverter near your Switchboard

1. Installing a 5kW inverter 5 meters away from your switchboard. A voltage rise calculation shows a 0.3 per cent voltage rise when we use our standard 6mm2 cable.
2. Install the same 5kW inverter 25 meters away. Increase your cable to 10mm2. Your voltage rise issue would increase to 0.9 per cent.

Installing your inverter further away from your main switchboard means your material cost goes up, your labour cost goes up, and your solar voltage rise goes up.

4. Correctly program volt response modes

The 2015 update of the Australian standards for solar inverters (AZ/NZS 4777.2) detailed a bunch of grid support functions to help inverters reduce the overvoltage problem created by solar. While some of these functions were only a recommendation, almost every inverter manufacturer I contacted had implemented the recommended settings.

The problem is the pre-set values act like a really nasty handbrake on your solar system. You’ll want your installer to adjust them. These volt-response settings differ slightly from one Electricity Distributor to the next. The below example shows the standard preset values and the more flexible values permitted by Energex and Ergon (Qld). If your sparkie is not up to speed with the correct voltage settings and leaves your inverter with its preset values, then your inverter will reach the overvoltage values prematurely.

Volt response mode settings

Volt-Watt Response Mode

This outdated function is pre-set in your inverter to reduce in its maximum production capacity in a linear fashion between 250 volts and 265 volts. This means that if the voltage at your inverter is a legal 256 volts, then your inverter will be limited to 68% of its capacity. This will happen even if you are consuming all of your solar production and not contributing to voltage rise. That’s not splitting hairs – that’s like pulling on your handbrake while you’re cruising down the highway.

The CEC has pointed out that Volt-Watt response mode is a “blunt instrument”. I suggest in Qld we should be turning Volt-Watt off or increasing the lower limit to 255 volts.  These values may be specified by your Electricity Distributor so it’s important to check that first. In Qld, the Volt-Watt setting is not required.

If the voltage at your inverter is a legal 256 volts, then your inverter will be limited to 68% … that’s not splitting hairs, that’s like pulling a handbrake …

Volt-Var Response Mode

Volt Var response mode will be OFF by default. If your sparkie takes the time to correctly enable Volt-Var mode, your inverter will gradually limit the production capacity when the voltage gets high. This can help prevent your inverter from tripping from overvoltage. (The values required in Qld are; between 248 volts and 253 volts, your inverter will reduce to 90% of its maximum capacity or a 0.9 power factor).
If the wrong mode is chosen, your inverter will be reduced to 90% of its maximum capacity even at normal operating voltages (eg, a 5kW inverter will only ever get to 4.5kW max).

Sustained overvoltage

Your inverter reaches 257 volts for 10 minutes – your inverter will turn off (a Qld Setting limit).  If your sparkie ignores this adjustment, the inverter will turn off at 255 volts after 10 minutes.

Overvoltage #1

If your voltage reaches 260 volts for more than 1 second – your inverter will turn off.

Overvoltage #2

Your voltage reaches 265 volts. Your inverter will turn off.

The hurdle

Now, this is all fine and dandy if your installer is up to speed with these settings and takes a few minutes to adjust your Fronius inverter while onsite. However many inverters require you to have a laptop with windows and the correct adaptor to adjust these settings (a pain for us Macbook users). Enphase and SolarEdge require us to jump high hoops so their engineers will create new grid profiles.

At the end of the day, if your sparkie leaves your inverter at the inverter’s preset values, then your inverter will be programmed to turn off at conservative preset voltages. You’ll potentially be driving down the road with the handbrake on.

Conclusion

Solar Voltage Rise is a problem created by solar and a problem that we need to proactively address. The relationship between voltage current and resistance means if you have a small cable between the street and the inverter, you will have a solar voltage rise problem. While high voltage can cause various issues, the biggest problem is it causes inverters to ramp down or shut off and stop producing solar. The practical ways to combat voltage rise include using a three-phase inverter, using a larger cable, installing your inverter near your switchboard, and setting the inverter’s volt response mode function correctly. Of course, to do this, you really want to choose a sparkie who understands voltage rise calculations and how to minimise it.

Mark Cavanagh