If your inverter sees a grid voltage that is too high for too long, Australian Standards mandate it disconnects from the grid. Before the voltage is so high it disconnects, your inverter may also reduce its power output in response to high grid voltages.
There’s a lot of fear-mongering about how the rise of renewables threatens our power grids, but a real problem getting real attention from the industry is how voltage rises on our mostly old and inflexible infrastructure stops customers from getting the most out of their solar PV installations.
Anyone who’s trained in this stuff already knows this of course, but I’d guess most consumers don’t realise the relationship between the voltage at the inverter and the voltage on the grid is very important. When things go wrong, the customer gets a bill showing far less electricity shipped to the grid than they expected, and someone – a solar installer, an electricity retailer, or a network – gets an angry phone call.
At a recent Clean Energy Council webinar, all four speakers – the CEC’s James Patterson, Solar Analytics’ Stefan Jarnason, SA Power Networks’ Travis Kausche, and SMA’s Piers Morton – agreed over-voltage problems are a big contributor to consumer complaints that they’re not getting value-for-money out of their grid-connected solar power systems.
The inverter has to be running at a higher voltage than the grid, so it can push power out (current flows from a point of higher voltage towards a point of lower voltage, never the other way around). The problem is every solar installation pushing power into the system lifts the network voltage just a little – and with tens of thousands of systems coming online on SA Power’s network each year, some systems are confronted with a grid with voltage outside inverter tolerance (the AS/NZS 4777.1 standard limits inverter voltage to 255V).
It’s worth noting solar power systems aren’t the only cause of overvoltage issues – as Solar Analytics founder Stefan Jarnason remarked, enough overvoltage issues occur at night-time to prove that.
SA Power Networks strategist Travis Kausche told the webinar the state currently has homes feeding 1 GW into the grid; 163 MW of that came online in 2018, and there’s up to 300 MW in the pipeline for 2019.
Kausche said the growth of solar energy “makes the dynamic range [the difference between the highest and lowest voltages seen on the network – Editor] much greater than if there was only load on the network”.
What happens when the inverter has to back off? The customer starts complaining, usually to their installer, that network feed-in tariff payments are falling short of their expectations.
Everybody can do the right thing, but the system doesn’t work right (Image: Clean Energy Council seminar)
That can be fixed, but it’s always better to try and avoid problems than to have to rescue the relationship with a cranky customer – and preventative measures, as well as remediation, were the focus of the webinar.
Solar Analytics’ Stefan Jarnason said his company’s analysis of 30,000 customers showed 50 percent of feeders had overvoltage issues 50 times a year or more, when scanned for voltages exceeding 253V for more than 5 minutes (undervoltage was, by comparison, rare, affecting only 2 percent of customers experiencing 50 or more events a year).
The more granular the data-logging, Jarnason showed, the easier to see the impact on the customer. Solar Analytics’ capture at five-second resolution clearly shows an inverter shutting down because its voltage is too high, trying to reconnect, shutting down again, and so on.
The orange line shows a system shutting down because of voltage rise on the grid, then trying to restart (Image: Clean Energy Council seminar)
The challenge for networks is some of the available fixes are hard to implement on old infrastructure.
Imagine a long feeder serving many customers, meaning that to maintain voltage in-tolerance for the last customer, the voltage closest to the transformer will leave the inverter very little headroom.
As Kausche explained, the network provider can only adjust the voltage by changing the tap on the transformer if the transformer is new enough and has lower voltage taps available – but the majority of SA Power Network’s 70,000 low voltage transformers don’t have that capacity, and transformer upgrades are expensive. Substation transformers are much more capable, but don’t offer granular voltage management.
“We have these legacy old assets that can’t really go down,” Kausche said.
The webinar highlighted the role solar installers can play in remediation, with all three speakers saying that getting inverter installation and configuration right plays a big part in avoiding complaints.
SA Power Networks has changed its inverter rules to require all inverters installed on its network have the Volt-VAR setting enabled (listed as optional in the AS/ NZS 4777.1 standard), so the inverter is more responsive to network conditions.
The network is also working with other providers to produce a standard set of solar inverter settings, so at some point OEMs and solar installers have a standard configuration profile Australia-wide.
SA Power Networks is also trying to move more customer loads to the middle of the day. If an off-peak hot water system, for example, can be remotely configured, adjustment is easy, and from 2020 there will be a “sponge” tariff to encourage consumers to schedule loads such as washers, dishwashers and pool pumps towards the middle of the day.
The CEC’s Patterson pointed out system design, including cable gauge, is also important to manage voltage rise. The CEC surveyed participants to see if they liked the idea of an online calculator tool to help installers design their solar power systems – and received a 100 percent “Yes” vote; so we can probably expect to see that offered fairly soon.
SMA’s Piers Morton said the impact of voltage rise emphasised the need for remotely-manageable solar inverters, something SMA will be introducing in the near future, and said installers can also help by paying more attention to balancing systems across different phases.
If you are solar power system owner with a voltage rise problem Finn has written a handy troubleshooting guide here.