The Elering tender that took seven months to prepare and another five to see through went the way of Siemens Energy whose €83.5 million bid was the cheapest. According to the network operator, the most expensive bid was five times higher.

The total cost was calculated in three parts – construction, maintenance over the first eight years and lifetime operating expenses.

Siemens and Elering signed the construction contracts in mid-December.

The condensers, sometimes also called synchronous capacitors or compensators, will be installed at three 330 kV junction stations: the Püssi, Kiisa and Viru substations. The Püssi substation was picked due to its proximity to the starting point in Estonia of the EstLink 2 undersea power cable to Finland and the Viru station because it is near Eesti Energia's Narva Power Plants that will have to be closed eventually after which additional voltage adjustment capacity will be needed. The third condenser will be built in Kiisa as it hosts two Elering emergency reserve power stations.

Should the worst case scenario manifest and the Estonian grid go completely dark, it will be the Kiisa emergency plants and synchronous condenser where the gradual restarting of the grid will begin, Elering said.

The design work on the Püssi condenser started on February 1 and the construction in June. General construction wort, installation and adjustment of the condenser is underway. Construction work should be completed this summer, with Elering set to test the entire station around New Year's.

The planning phase of the Kiisa condenser started on January 1 this year, with the Viru device set to follow on June 1. Construction work will take 12-14 months and will be followed by an approximately six-month test period. The final synchronous condenser should be completed by late 2024.

Crucial for system operation

The condensers are necessary for disconnecting the Estonian and Baltic systems from the Russian grid in 2025. Every Baltic country will build three synchronous condensers.

A synchronous condenser is essentially a giant flywheel that helps maintain system frequency at 50 hertz. The latter differing just 0.5 hertz from the nominal 50 hertz frequency of the European grid would pose an acute threat to small automatic power plants and could lead to shutdowns and the collapse of the entire grid.

Maintaining the frequency requires a balance between production and consumption at any given moment. However, since abrupt changes to production capacity and consumption do occur from time to time, efforts need to be made to maintain the frequency. Control devices in Russia currently maintain the 50 hertz frequency as part of the BRELL agreement, while this will become impossible once the Estonian system is disconnected from the Russian grid in 2025, Elering said.

Elering CEO Taavi Veskimägi said that without the inertia delivered by these devices, the risk of the system collapsing is great. "It is a vital investment in the first line of defense of the power system," he said.

A synchronous condenser is a device that slows frequency changes to give other frequency control devices time to make adjustments. The condensers to be built in Estonia are roughly the size of a bus and can work as motors or generators. When a condenser is drawing energy from the system, it is working as a motor, while it functions as a generator providing energy once its rotation speed starts to fall.

Synchronous condensers are not constantly working and need to be switched on when needed. They are mainly meant to help maintain the system during outages of major grid components. It takes around 15 minutes to start a condenser.

Technical specifications of Estonian synchronous condensers:

• Inertia 1,750 MWs
• Short circuit power 900 MVA
• Reactive compensation +50Mvar/-50Mvar
• Startup time 15 minutes
• Startup reliability 95 percent (the condenser must power up 95 times out of 100 after the operator presses the button)

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