A three-year trial that tested the value to networks of residential solar and storage systems uncovered the hidden value in reactive power – and the potential for inverters to really flex their muscles.
Most people think about clean energy sources wind and solar in one of two ways: as erratic performers that will shut down the grid at their whim, or as utterly logical solutions to our energy needs that, cleverly managed, will power us forever more.
In Australia, an enormous number of homeowners have chosen solar PV as a clever way to push down their power bills. No-one can vouch that all 2 million of them believe the Earth’s heating up, but their investment in solar cuts our reliance on coal. And that’s good.
Behind the scenes, however, the energy technologists are looking at their screens with worried faces. On networks where solar penetration is high, voltage levels are rising as inverters export unused electricity to the grid. Then, as night approaches, demand is ramping up ever steeper towards resistantly high peaks.
Clean energy comes with a little pain, and if networks get stressed about it you can be sure they’ll pass their frustrations along in the form of higher charges for all customers – or bar exports, as they do for commercial and industrial solar systems.
Network services, made at home
A solution to rising voltage levels needs to be found, and researchers at the Institute for Sustainable Futures (ISF) at the University of Technology Sydney think customer inverters can help, whether they are solar or battery inverters.
If customers provide network services, networks can avoid upgrades and supply charges will surely fall. After all, voltage regulation in solar and storage systems can be accessed, so why not do so? “The standard means you have to have that capability, but it doesn’t require that that capability be turned on,” says ISF research principal Dani Alexander. The benefit of working with networks to find a solution is that the capability could be turned on, if customers are offered an incentive, she says. “You can’t just tap into people’s solar or storage systems without them agreeing and being appropriately compensated for it.”
What does it mean to regulate voltage in a spindly, complex NEM? When the voltage rises, inverters connected to solar PV systems on that part of the network shut down. Not every owner of a solar system is really that engaged, so they may be totally unaware that their inverter has suddenly switched itself off. There have been many instances where inverters don’t reset when voltage falls back into an acceptable band – and need to be reactivated manually. Those owners may go months not knowing their panels are doing nothing at all. “One of my friends lost six months’ of supply without knowing,” says Geoff James, a research principal at the ISF. “They’re not energy experts, so how would they know? The system is going to rely on this customer solar contribution, so we want it to achieve maximum value.”
Some serious investment is taking place in commercial and industrial solar systems around Australia but the market has a very long way to go to catch up to the levels of penetration in residential solar, where it’s widely quoted that around 20% of homes are topped off with some panels. “They are more critically dependent on the economics of their solar,” says James, “and if it’s not going to be able to generate then they won’t get the return they need and they won’t do it.”
Real versus reactive
As more solar is exported to the grid the range in voltage is getting wider and the ramp rate is steepening. For the past three years ISF has worked on an approach to finding a solution with network partners Essential Energy in NSW and AusNet Services in Victoria, with each settling on a slightly different methodology. Essential was more interested in “real power” impacts, whereas AusNet took a different approach, expecting solar inverters would be more useful than battery inverters and eager to explore the consequences for “reactive power”.
“Reactive power is relatively unexplored from solar inverters,” James says. “The type of network in each trial was similar, but the approach was somewhat different.”
Real power is about ramping systems up and down; choosing when to inject energy from batteries or switch off solar. Reactive power is about changing the type of energy output, from kWh to kVAR.
Reactive power, a property of AC, is the phasing of the voltage and the current. Normally, voltage and current are in phase, to produce real power. When they are out of phase, some real power is sacrificed. The combination of real power and reactive power, says James, “has a profound effect on voltage.”
The higher the ratio of reactive power to real power, the more profound the effect on voltage.
The researchers found out that tier one residential inverters used in the trials with stated reactive power limits around 20% could handle much higher amounts. “We used them way beyond their specs, safely and within the confines of the trial, and they performed very well,” James says. “Even for fairly low levels of solar power they could still get heaps of reactive power, and that could manage the voltage.”
It’s clear the industry, given the right signal, could very easily produce inverters with much more flexible specifications, he says, and that are well adapted to providing these services. “That was encouraging.”
The survey sample
In Yackandandah, north-east Victoria, AusNet tested 14 solar-and-storage sites along a SWER feeder in Ben Valley and 35 solar-only sites. In and around Collombatti, north-east NSW, Essential Energy tested 32 solar-and-storage and nine solar-only sites.
The NSW system was controlled by smart software developer Reposit and Mondo Power’s Ubi system was used in Victoria. The $5 million research included $1.6 million in funding from ARENA and some state funding.
Some participants accepted an ARENA-funded subsidy to install a battery and solar-only participants were given a new tier one inverter that could be controlled by a third party. Batteries are useful and extremely flexible, the researchers say, but also expensive. For many households they are not an economic investment.
Knocking on a solar owner’s door and asking them to, in some cases, spend money so they can take part in a trial to provide “network services” sounds a bit like asking an office worker to take a pay cut to measure the effects on Australia’s GDP. The researchers admit it wasn’t easy making the pitch to homeowners. “Because it’s all new it takes a lot of explaining,” James says. “We ran barbecues multiple times … there was quite a lot of work.” But generous subsidies helped get the required numbers over the line.
In NSW, $7,200 subsidies were offered on 10kWh batteries and hybrid inverters, with Reposit offering payments of $1/kWh for “control events”. So far, owners of 6.5kWh batteries are earning average annual payments from Reposit of $260 and owners of 9.8kWh batteries up to $330 a year.
In Victoria, participants were paid a flat $200 a year.
“In Victoria we did a calculation and said, OK, if we operate your system at maximum capacity … you’re still going to be less impacted [than the $200 earned] in terms of your energy revenues,” James says. “You’ll be better off – and you’re actually helping the network that might allow you to put more solar on.”
Although the two trials linked more than 80 residential energy systems, the researchers are reluctant to call them virtual power plants. “I don’t call them virtual power plants … although they are the same technology,” says James. For a start, the rules don’t allow a network to provide power on the grid. “But if you have a system that can manage an aggregation of customer supply then definitely there are people who are going to want to use that as a power plant,” he says. “The same control system can be used to supply network services – and that’s what we were doing.”
The Networks Renewed trial saw AusNet, Essential and the Institute for Sustainable Futures awarded the Clean Energy Council’s Innovation Award at the Australian Clean Energy Summit in July.
Balance of power
As the results came in, the researchers saw the value of reactive power outweighed real power. “For the same voltage impact, you can have a much smaller amount of real power constraint,” James says. When voltage is high, it makes more sense for solar systems to supply reactive power than to be cut off. “If you got cut off you would miss out on revenue, but if you instead diverted some of your active power to reactive power it costs you less,” Alexander says.
Maybe it would be just as effective for a smaller sample of participants to supply reactive power so everyone else can benefit by providing real power? “What a fascinating question,” says James. “We are really interested in exploring an equitable way to provide these services across all the customers.”
Inspecting parts of networks under laboratory conditions would allow a designer to decide the best places to inject reactive power when needed, which of course might not be where solar and storage assets are distributed. “There’s more to be discovered in the data that we have,” James says. “People in some locations would have found their services called on more than in others, and that’s because of the physical nature of some parts of [any] network. Typically, at the end of the feeder the voltage varies more than at the beginning, so that customers would be expected to be called on more than people at the transformer end, the substation end.”
It’s evidence that a controlled system would be superior to a mandated system. A solution that allowed flexibility for inverter settings for customers would be more technically efficient. “At several levels the mandated, same-size-fits-all approach is not something we’d advocate.”
Alexander anticipates future joint research with another institution may reveal an approach to inverter dispatch optimisation that will share the benefits. “You can talk about it in a social finance sense, but actually there are potentially technical solutions to this problem that we can test,” she says.
The research, dubbed Networks Renewed, only includes customer-owned assets but networks have all sorts of ways to manage the problem of voltage irregularities themselves: transformer taps, power electronic voltage regulators, re-conductoring and upgrading distribution transformers, etc. For the Collombatti trial, solutions available to Essential Energy range from $1,185,000 for power electronic voltage regulators to the no-cost option of refusing any further connections of solar or storage – which of course is not going to happen as the entire NEM slowly lurches away from reliance on large thermal generators.
When the assets are already there on people’s rooftops, it makes far greater sense to learn how to use them. The challenge is to access the legion of assets equitably, Alexander says.
No matter what the research says about Collombatti and Yackandandah, networks will assess each problem on its own merits and apply the solution that fits, says James. “There’s always going to be a combination. There’ll be some feeders that are best approached by network investment, or they might take advantage of high solar penetration. ‘I’m going to use the customer solution on this feeder and I’m going to put a voltage regulator that I buy on that feeder.’ That’s how the system should work; it’s a rational decision.”