The search for stability

1 May 2020

As “conventional” power generation declines, additional measures are needed to ensure the stability of the grid.

Above: Ansaldo synchronous condenser


The Energiewende is, for example, prompting the four German transmission system operators to implement measures to bolster grid voltage, in particular by installing systems that compensate for the loss of reactive power previously provided by the inertia of large rotating masses at conventional power plants. Reactive power is particularly required for the AC transmission of large amounts of energy over long distances, which is likely to figure prominently in Germany’s energy future of Germany.

The German TSO Amprion and Siemens plan to develop and install what they describe as the “world’s first rotating asynchronous phase shifter”, with an output of about 300 MVA. Called ARESS (Asynchronous Rotating Energy System Stabilizer), the new machine can supply “far more rotational energy, especially when providing momentary reserve, and also over a longer period” than a synchronous phase shifter (aka synchronous condenser). ARESS is an “extremely responsive and powerful” piece of electrical equipment, Amprion/Siemens say, able to make a significant contribution to frequency stability, complementing Statcom (static synchronous compensator) systems and synchronous condensers already in use/being implemented.

The ARESS project was launched on 26 March with the signing of an agreement by Dr Klaus Kleinekorte, CTO of Amprion, and Dr Jochen Eickholt, managing director, Siemens Energy. Due to the Coronavirus pandemic, the documents were signed via video conference.

Amprion and Siemens believe that ARESS can be more compact and cost-effective than alternative technologies.

They expect the project to run for four years, culminating in the operation of a pilot system. Meanwhile in Italy, Ansaldo Energia and ABB have won an order from TSO Terna worth over 70 million euros to improve the quality of the electric power system by providing synchronous condensers, following a European call for tenders.

Ansaldo Energia’s scope includes the design, supply, commissioning and maintenance of three synchronous condensers, each rated 250 MVAR, for installation in central and southern Italy, plus options for two further synchronous condensers to be exercised no later than 36 months from the order date.

The installation of the first three units is due to be completed in 2022.

They will provide reactive energy, short-circuit power and inertia, says Ansaldo, which is particularly important in a grid with a high share of renewables, which are “characterised by low or no inertia”’ the company notes.

The synchronous condensers will be equipped with Ansaldo’s flywheel technology, which increases the overall inertia of the rotating system while minimising mechanical losses thanks to a vacuum chamber.

ABB will supply, install and commission the systems that connect the synchronous condensers to the grid, including protection and control electronics and monitoring and diagnostics.

The contract also includes maintenance and support over twenty years.

In the UK, National Grid ESO (Electricity System Operator) has agreed contracts with five organisations – Drax, Rassau Grid Services (Welsh Power), Statkraft, Triton and Uniper – for provision of grid stability services by building new facilities or modifying existing assets.

The contracts are worth £328 million for delivery of services over a six-year period and are the first stage of what NG ESO calls its System Stability Pathfinder initiative, an “innovative and world first approach to managing the stability of the electricity system”, with further tenders expected to be invited in subsequent stages.

NG ESO estimates that the new approach will save consumers up to £128 million over the six-year period, “reducing the costs currently associated with managing system balancing and stability, with the reduced carbon emissions a significant step towards the ESO’s ambition of being able to operate the GB electricity system carbon free by 2025.”

The key service to be provided is inertia, helping to keep the electricity system running at the right frequency, says NG ESO. “This has traditionally been provided by using the kinetic energy of the spinning parts of large generators when they were supplying electricity to the grid. Under the new approach inertia will be provided without having to provide electricity – allowing more renewable generation to operate and ensuring system stability at lower costs.”

In total, the contracts amount to the procurement of 12.5 GVA seconds of inertia, says NG ESO, the equivalent of the inertia provided by approximately five typical coal fired power stations.

NG ESO says the inertia contracts provide one illustration of how the tools it uses to balance the grid are “developing in order to be able to operate with zero carbon by 2025”, noting that in 2019 it launched a new suite of “response products to better manage frequency” and implemented “changes to widen access to the balancing mechanism, making it easier for a more diverse range of technologies and generation providers to participate in the electricity market. NG ESO says it has plans “to explore further the delivery of inertia by new and innovative technologies.”

Under its inertia contract with NG ESO, Uniper says it will “repurpose redundant steam generators at Killingholme, and build two new synchronous compensation units at Grain, which will start delivering inertia services and voltage control to the grid from April 2021.”

Uniper will provide these services “without the need for combustion of fuel”, as already noted, independent of “generating assets on site, whilst making use of existing infrastructure.”

Under the terms of the provisional contract awarded to Drax, one of the four turbines at its 440 MW Cruachan pumped storage hydro power station in Argyll and Bute, Scotland will cease power generation and only be used to provide grid support services.

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