New modelling exposed grid problems that forced solar farms to cut output

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Modelling around a potential line trip led to concerns about system strength in western Victoria and led to severe curtailment of five big solar farms.

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New modelling and grid simulations conducted by the Australian Energy Market Operator are apparently at the heart of the decision last week to dramatically curtail the output of five big solar farms in Victoria and New South Wales by 50 per cent.

The decision to impose the constraints on the Karadoc, Wemen, Bannerton and Gannawarra solar farms in Victoria, and the Broken Hill solar farm in NSW, has taken the industry by surprise, and led to concern that other installations may also be affected. Or that these solar farms already affected may be further constrained.

The constraint has also blind-sided energy professionals because the solar farms affected have been operating for some time, and in the case of the 53MW Broken Hill solar farm located near the town of the same name in NSW, since 2015.

But that may exactly the reason why these farms have been targeted, as it is understood that all were approved before so-called “system strength” rules were implemented in 2017, and connection requirements further tightened earlier this year.

That means that the solar farms were not subject to system strength impact assessments which are now in place, and it is the lack of system strength, and potentially dangerous (to the system) level of voltage changes that is now causing concern in what is known as the “west Murray” section of the grid.

The problems were identified as part of a series of ongoing “simulations” that the Australian Energy Market Operator is conducting across the grid, and particularly in areas where it is concerned about potential weaknesses.

In this case, the modelling suggested dramatic voltage changes after the potential trip of a 220v transmission line. As a result,  AEMO imposed the constraint on the five solar farms while it sought to find a solution to the problem with the local network operators, the solar farm owners, and the equipment suppliers.

AEMO – as are the solar farm owners – are confident that a solution can be found soon. But exactly what that might be, and how much it would cost and what time would be needed to fix, is not yet finalised.

Possibilities include an upgrade or changes to the inverter technology or settings, or even the addition of a synchronous condenser, which are becoming a common and expensive addition to the grid as more wind and solar farms are built.

Many wind and solar farms in Queensland, NSW and Victoria have been required to add syn-cons, a decades-old technology that basically amounts to a spinning turbine with little or no fuel burned – at an estimated average cost of $25 million to ensure their connection and commissioning goes smoothly.

South Australia is installing four such machines around its grid, but that is being done by the network operator because AEMO had formally identified an issue with system strength, and therefor won regulatory approval. The addition of the syncons in South Australia will mean less dependence on gas plants running in reserve, and lower costs.

There is a chance that AEMO may declare a similar system strength issue in west Murray, although because of the time taken for regulatory approvals of such installations, the solar farms will likely want an earlier, or interim solution to allow them to return to their full output.

Most of the solar farms have pre-agreed contracts, supplying customers such as the Victoria tram network and corporate customers such as Carlton & United Breweries.

And it should be pointed out that this is not about the variability of solar, or the need for storage. Gannawarra has a co-located Tesla big battery, and it is also constrained like others in the areas.

A spokesperson for AEMO said the issues in the West Murray area have not previously been experienced to this level.

“AEMO, the generators and the network businesses are working through a number of technical variables, however the situation remains dynamic, so constraints are required to protect the power system while we are collaboratively pursuing a range of solutions for current and future network users,” the spokesperson said.

“Ultimately, AEMO’s aim is to generate outcomes that are acceptable from a system security perspective but, as far as possible, are reasonable and proportionate for existing users and those seeking to connect.”

The issues appear to be unique. “As you know, Australia is at the forefront globally of incorporating electronically coupled generation (renewables) to its national grid, and while the industry has broken new ground on managing phenomena such as system strength, reserve management and distributed resource integration, the learning curve for the entire industry remains very steep,” AEMO said.

AEMO says that the voltage oscillations  have the  potential to damage customer connected equipment and loads. “The constraints imposed on the solar farms limit the oscillations to a point where the risk should be mitigated. As further assessments are undertaken and more information is provided to AEMO by the generators, these limits may be tightened or relaxed.”

The issue has also raised new concerns about the status of Australia’s energy market rules and regulations. AEMO says on one hand it is relying on getting the best modelling information from generators, who say on the other hand their modelling is often hamstrung by their lack of access to network-wide data.

The solar industry has been facing increasing hurdles over the last couple of years, some of which has led investors to re-consider their commitment to Australia, as the head of New Energy Solar said last month.

The industry has been afflicted by connection and commissioning delays, resulting in a blow out of costs and claims of damages to construction firms, as well as big changes to marginal loss factors, which has impacted revenues for many solar and wind farms.

Others have been required to spend lots of money on synchronous condensers or other machinery., and to add to their woes, many solar farms in Queensland and South Australia have been forced to switch off during periods of negative pricing, either because they are required to do so under their off-take agreements, or because they are not willing to pay others to take their output over sustained periods.

And, of course, there has been a lack of federal policy and the threat of government intervention in the market which is also dissuading investors.

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