When Nikola Tesla invented three-phase power nearly 140 years ago, he wouldn’t have thought that it would create complications for homeowners with a solar power system and three-phase supply. But here we are!
Both three-phase electricity and solar panels are hugely beneficial, but their combination can cause issues in places with phase-accurate billing, such as here in New Zealand. But the much-respected inverter brand Fronius now offers a solution to these issues. Here’s a closer look at the problems with having a solar power system on a three-phase home, and how Fronius tackles these problems.
Issues With Having a Solar Power System on a 3-Phase Home
Three-phase power is a beautiful invention of electrical engineering. It uses three different phases of alternating current instead of one, allowing us to have higher transmission voltage and safer, more reliable systems, particularly where power-intensive appliances are used.
Here’s a schematic of how 3-phase power works:
(source: GenServe)
The total available power in such a system is the sum of all three phases, but having three phases offers better consistency, smooth power delivery, and safety. In a large factory, where most machines have high power consumption, all three phases are powering every machine. For a home, on the other hand, since appliances consume low to moderate amounts of power, different phases can power different appliances. For example, your fridge, TV, and lights would be connected to one phase, AC and computers to another, and so on. Now let’s turn to why three-phase and solar have troubles when paired together.
Problem 1: Lower Profitability Because of Symmetric Generation
Three-phase systems are ‘load-balanced’, which means every phase must have the same amount of power flowing through it for optimum performance. When you have a solar power system on a 3-phase home, your installer will probably use a single-phase inverter with your solar panels.
Typically, these inverters have symmetric generation, where the power output is divided equally among the three phases. For example, consider a home solar system generating 3 kW of power that’s converted from DC to AC in a 3-phase, symmetrical inverter. As a result, you have 1 kW of power output for each phase.
So far, so good - we’ll just use the 1 kW each for whatever appliances are connected to each phase. Now consider this scenario, which is typical in NZ - your system is interconnected with the grid, and all the available power in each phase isn’t always utilised. Here’s a diagram representing this case:
Here, one of your phases has a washing machine connected to it and uses 1.5 kW, so it uses the 1 kW available from solar and takes 0.5 kW from the grid. No worries here. But look at the other two phases - one uses only 0.5 kW, while the other uses just 0.7 kW. So the remaining 0.5 kW and 0.3 kW flow into the grid.
The problem with this arrangement is that instead of sending that 0.8 kW back to the grid, you could have used it for your first phase where you’re short of 0.5 kW. Unfortunately, in NZ, energy sent to the grid (exported) and energy bought from the grid (imported) are not valued at the same rate, in which case it would not have mattered how much power goes where.
According to the Sustainable Energy Association of New Zealand (SEANZ), the difference between imported and exported power can be over 20 cents per kWh. In other words, if you could use the power that flowed back into the grid instead of buying power from the grid for another phase, you could save 20 cents for each unit of energy. This number is so large that you can see a huge impact on your monthly bills, drastically reducing the worth of your solar power system.
Here’s how Fronius describes this whole issue in brief: ‘In standard inverters, power is symmetrically generated but the site is imbalanced due to household load variations, leading to the risk of purchasing expensive electricity from the grid on certain phases.’
Problem 2: Lost Revenues Due to Symmetrical Export Control
Export control or export limiting is a system that limits how much energy you can send into the grid. In some areas, utility companies prescribe export limits to prevent grid overload due to excess solar power.
In NZ, you may choose to limit your inverter’s export to zero watts, which means no unnecessary export. However, in a symmetric inverter, the weakest phase will have a natural zero export, but the other two phases may be limited to the same generation even though load requirements are higher. This means you end up importing energy from the grid for the other two phases. Take a look at this example below:
In this example, the second phase needs the lowest amount of energy - 5 kW, so the inverter limits the output to 5 kW to avoid any export. However, because its operation is symmetrical, the inverter also limits both other phases at 5 kW. Now, the loads connected to these two phases require more than 5 kW of power, and they end up importing it from the grid. Ultimately, we have a system where enough power was available and yet we ended up buying expensive energy from the grid.
Possible Solutions
Both the above-discussed problems with three-phase systems occur because of symmetrical power delivery through all three phases. One way to address this problem is to attach your solar power system and inverter to just one phase and self-consume all power through this phase. However, this needs a complex, possibly more expensive installation process. Plus, it may also cause network reliability issues due to unbalanced circuits.
Another solution is to use a 3-phase inverter, which will add to your system cost by a few hundred dollars. Lastly, you can use three single-phase inverters, but this too will boost your pricing. Now let us look at how Fronius helps you tackle these issues without spending hundreds more on your system.
How Fronius Solves the Above Issues
As mentioned above, both issues occur because of symmetric generation, and Fronius has a single stone to kill both the birds - asymmetric generation, which is offered by Fronius's Gen24 Symo Inverters and the latest Verto Inverters. By having asymmetric generation, some of your phases can draw more power than other phases, eliminating the need to import power from the grid while also stopping any unnecessary export. Below is a pictorial representation of the same:
Here, you can see that while the first phase draws 1.5 kW, the other two aren’t forced to draw 1.5 kW, as some of it would be exported since the load consumption is lower (0.5 and 1 kW). The same solution applies to export limiting, as seen below:
As mentioned in the green box above, the inverter can shift power delivery between phases as per load requirements. The first phase uses 7.5 kW, so some power from the other two phases shifts to this phase, particularly as the other phases consume low power. Note that the export is limited to 0 W, as the user has no interest in the low buy-back rates and plans on using solar for self-consumption only.
And if you do have any excess energy left after fulfilling your loads on all three phases, your inverter can send the excess power to the grid, provided you haven’t set any export limits. Here’s how it would look:
As you can see, the three phases need 8, 6 and 2 kW (16 kW in total), but the available power at this moment is 20 kW, so after fulfilling the total requirement of 16 kW, the inverter exports the remaining 4 kW to the grid. Because after all, some return for your surplus energy is better than none.
Fronius Symo’s Limitations
The Fronius Symo is a game-changer in New Zealand’s solar landscape, but like all of us, it has its flaws! One notable limitation of the Symo is its per-phase power cap. In other words, although different phases can have different power flows, there is a limit to the percentage of total power that can flow from any one phase.
For instance, a 6kW inverter is limited to 2kW per phase. This means that if you’re generating 5kW but have a 3kW demand on one phase, you’d still need to draw the extra 1kW from the grid. A potential workaround might be to oversize the inverter, but this solution comes with added costs, making it less accessible for many users.
Nevertheless, the Symo will largely reduce the number of times you import energy from the grid, thus reducing your energy expenses. As for the limitation, Fronius and other brands need further innovation to fully tackle this issue.
A More Complete Solution
Obviously, technology can solve policy-related problems only to a certain extent. The broader solution to this problem lies in policy reform. New Zealand and the Czech Republic are the only two countries that lack a fair ‘net metering scheme’—a system that could entirely alleviate the challenges described above.
SEANZ, New Zealand’s solar association, has been pushing for an ‘instantaneous net metering scheme’ with the Electricity Authority for years to address these gaps and reduce reliance on technological fixes like the Symo feature.
While the case for net metering remains strong and SEANZ continues to champion the cause, the Fronius Symo Inverters are a step in the right direction. Fronius deserves credit for tackling this problem with New Zealand’s unique energy landscape in mind, even if the solution isn’t perfect.
Summing Up
Having a solar power system on a 3-phase home can seem like a deal-breaker because of the symmetrical operation that unnecessarily exports some power and then imports some, instead of using all available power for self-consumption.
Thankfully, Fronius’ Symo inverters tackle this with the simple solution of asymmetric generation, which offers much-needed flexibility of power availability on all three phases. If you have a 3-phase home, the asymmetric generation feature of Fronius Symo inverters is a game-changer, making it a smart investment for your solar setup.