More solar power on the grid has its challenges. Lots of people (and governments) are claiming the challenges can be solved with batteries and inverter control. Those solutions have a part to play, but a new study suggests there is important work to be done upgrading local networks.
While electricity distributors emphasise the importance of seeing “behind the meter” to manage the impact of renewable energy on low voltage (LV) networks, a study published by ARENA suggests network upgrades are going to be more effective in the long term.
As we’ve discussed recently, utilities will increasingly need to look behind the meter so they can dial back the power fed from household solar PV and spare the low voltage network from unwanted voltage rises.
The undesirable side-effect of this is that curtailment reduces the money the homeowner makes from their solar power installation – and unless something is done, as PV penetration rises so will curtailment at times of peak generation.
A study authored by Citipower/Powercor (CPPAL) and published by ARENA, “Future Grid For Distributed Energy” (PDF) says a range of LV network upgrades to manage grid voltages from the transformer side will provide important voltage management into the future.
The report is designed to give the industry tools to help forecast the hosting capacity of their LV networks, and to identify the mitigations that best keep grids stable as distributed energy resources (DER) become ubiquitous.
The report notes networks are already experiencing the effects of DER; particularly rooftop solar PV, increasingly augmented with behind-the-meter batteries. As well as the well-known voltage rise issues (too much power from the rooftops can take the network voltage above its 230V-plus-10% maximum), the CPPAL report also mentions some distribution network assets can suffer “thermal issues” when power is flowing upstream from the household.
The report says:
“In Australia, these issues are resulting in reduced power quality, involuntarily reduced PV generation (also known as ‘curtailment’) and distribution businesses sometimes delaying or refusing prospective PV connections”.
Faced with the question “how much DER can we install on our network?”, it looks like the industry’s answer right now is “who knows?”, so CPPAL’s study is designed to give the industry a consistent answer:
“This study’s purpose is to develop a replicable methodology that CPPAL and other energy industry stakeholders can use to improve their understanding of LV networks’ PV hosting capacity. This study also explores efficient enablement of further PV uptake without risking quality of supply to customers.”
CPPAL (and its partner in preparing the report, ENEA Consulting) settled on three metrics:
- The percentage of the maximum reference PV penetration level when the first breach of the maximum voltage limit or equipment thermal constraint occurs on the LV network.
- The annual average hours per day in breach of a voltage limit or equipment thermal rating as PV penetration increases.
- The increase in annual maximum voltage level as solar PV penetration increases.
CPPAL selected ten low-voltage networks as representative of the more than 80,000 LV networks it operates. It’s also worth observing the distributor reported nearly 150,000 new solar panel installations in 2019 alone.
As we know, all that new solar power capacity landing on the LV network is causing trouble. In the report, CPPAL said seven of the ten LV networks experienced power quality issues with just 25% of the network’s baseline PV hosting capacity installed. Just two networks reached maximum penetration with no issues.
The modelling also showed six networks would spend as much as eight hours a day in breach of voltage parameters at 40% penetration, while just three would reach PV saturation with low breach conditions (just 10 minutes a day).
Voltage rise would be a problem on nine out of the ten networks. The modelling suggested that if solar power inputs are entirely unmanaged, at penetrations above 40% three networks would experience a theoretical voltage rise of 50V, and four would experience a 6.5V rise at saturation.
Mitigations
The project also assessed five mitigation measures:
- Upgrading transformers with extra boost/buck taps to change the output voltage, to cope with changing network conditions;
- Adding an on-load tap changer (OLTC) to a distribution substation transformer, for automatic voltage adjustments; and
- Adding a low voltage regulator to manage the voltage on the LV network;
- At the customer side, adding a smart inverter to manage what’s sent to the grid; and
- Batteries behind the meter.
In this specific context, mitigating grid voltage rise, household batteries were assessed as offering little value – they have only a small impact on managing voltage rise or reducing time in breach.
The report found the smart inverter is at its most valuable in LV networks with low DER penetration. It’s a low-cost solution that’s easy to implement.
At high DER penetration, the report found, network-side mitigations are necessary:
- Transformer upgrades and OLTCs support higher DER penetration before a network reaches a breach condition; and
- At high DER penetration, automatic voltage regulators are the best way to manage LV network voltage.
About Richard Chirgwin
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Cannot understand why these desirable upgrades are just being recommended. How could it be sensible to subsidize solar for consumers and then look for ways to block unused feedback to the grid when the voltage rises in an outdated grid design.
It makes little sense at all considering that for years the grid was “gold plated” at consumer expense with none of these people even testing its future capabilities under these circumstances, knowing full well what was coming.
Ah the joys of capitalism. Spend big on “gold plating” when guaranteed a return from consumers. Spend nothing when more solar from consumers means less return to companies.
If only Bob Brown’s Greens weren’t so pure and bloody minded 11 years ago we would have had a carbon trading scheme up and running for a decade and none of these carbon wars. I haven’t voted for them since.
WTF has a carbon trading scheme got to do with the management of high DER penetration. What governments didn’t realise was the high uptake of solar and should have installed voltage regulators. Ships generate their own power and certainly have voltage regulators.
Huh! Well, I’d have thought the “gold-plating” was to get them ready for the increased DER?
What I still don’t understand – looking at the Supply and Demand widget data for SA, is what is supposed to happen when the total DER generation exceeds total demand, no matter how much “gold” is used to upgrade the grid capacity to shuffle energy around without causing excessive voltage rise or curtailment, etc., etc., – once the batteries are all full?
I note that when the wind blows, SA is nearly self-sufficient already – from this source alone. It aslso already has a high penetration of both rooftop, and large-scale PV. Now add in a windy sunny day in summer – when both are operating at high utilisation. Surely, if generation greatly exceeds demand and the batteries (including behind the meter) are all full, the excess energy has to go somewhere?
Export it, you say. To where I ask – if all adjacent States reach SA’s level of DER, and so also don’t want, or can’t use the excess. Do we really have hydro capacity sufficient to store the excess for 6 months, so it may be got back during the other 6 months around winter?
So far, I see SA running gas to stabilise their grid, exporting “green” energy across the border when excess renewables are being generated, then “apparently” filtering the electrons generated by 80% coal and gas when importing at night, for example. Seems like “creative accounting” to me.
We ALL subsidise coal. That appears to be acceptable though.
The real problem is we have a government which has done everything in its power to shut down the renewable energy movement. Doing nothing to prepare the grid for the future is the sort of vandalism one expects for a government which appears to be run by the fossil fuel industry. That’s why you continually hear talk of ‘new coal fired power generators’. This is the voice of corruption at work.
This article seems to be pushing untruths. The assertion that power companies need to intrude behind the meter to avoid line overvoltage is bunkum. For some time, new inverters have had the capability to stop exporting at a preset line voltage. That setting is by now presumably prescribed for all new installations and existing ones with the capability.
That is self-correcting, through automatic export curtailment.
And automatic transformer tap changers will reduce curtailment, again without the need for any domestic intrusion by the power companies. It is just a matter of setting the voltage thresholds for inverters and tap changers to elicit the desired automatic behaviour. Hysteresis and time delay are just sofware, and cost nothing.
The real reason for manoevering to seize control of domestic inverters is not being revealed, but talk in other articles of “shutting down” inverters rather than just stopping export, suggests that the aim is to deprive solar households of self consumption as well. Whatever the excuses and obfuscations, the result will be good for battery sales as households go off-grid due to this machiavellian overreach.
The ‘gold plating” network expenditure clearly did nothing to prepare the network for current and future needs. It also did little or nothing to protect the state against the death and destruction wrought by massive raging bushfires lit by decrepit line infrastructure. Did it all go on booze and canapes? When will there be a start on genuine investment in the network, to actually provide for current and future needs?
Mind you, the inadequate network has led to throttling of solar farm output as well as rooftop generation. And then there’s the new solar farms stalled because of the threat of throttling due the inadequate network. So the fossil polluters are laughing all the way to the bank.
Maybe Edison was right, moving from DC to alternating current added Westinghouse’s transformers for long distance transmission and Tesla’s efficient synchronous AC motors for large equipment, but also added too much overall complexity and complication at too high of a cost. Automated tap changers in California wear out quickly as they just weren’t designed to switch up and down for every cloud.
Homes, neighborhoods, and local microgrids could have a lot of batteries and DC to DC converters for the cost of all those inverters and transformers.
Or not, and our centre-tapped single phase and your three phase systems are still about as good and as cheap as is reasonably possible overall, as a system with a lot of centralized generation and very little storage.
We have the only PV system in a mile radius, and I saw line voltages vary from the 230’s to the 250’s even before it was installed. Maybe all that’s needed to sort it out, are ever ‘smart’-er meters that bill / pay on a sliding scale from a high price on the lower end of voltage right down to free for overvoltage.
Or maybe the power grid needs to be rebuilt something like the natural gas system here. The hundreds of thousands of well collection points are gathered into a few thousand processing and half dozen central storage systems, then delivered *separately* into the local distribution systems. But I’m not an engineer, probably a DC bus system has its own challenges, and building separate electricity storage and transmission systems is prohibitively expensive.