Synchronous Condenser

A synchronous condenser, also called a synchronous compensator, is a synchronous machine that spins in step with the grid without driving a load or being driven by a turbine. By adjusting its field excitation it supplies or absorbs reactive power to control voltage. Unlike power electronic devices, it also provides physical inertia and short-circuit current, two properties grids lose as coal, gas and nuclear plants retire.

A unit is brought up to speed by a starting motor or a static frequency converter, synchronized with the network and then kept spinning by a small amount of active power drawn from the grid to cover its losses. The excitation system raises field current to produce reactive power and lowers it to absorb reactive power. During a fault the machine responds immediately and inherently, delivering several times its rating in short-circuit current, which helps protection operate correctly and helps nearby inverter-based plants ride through. Many units add a flywheel to increase the inertia they provide.

Synchronous condensers were common in the mid twentieth century, fell out of favor when SVCs arrived, and are now being installed again in large numbers. Grids with high shares of wind, solar and HVDC, including those in Great Britain, Germany, Denmark and South Australia, procure them to restore system strength. Some operators convert retired generators into condensers by decoupling the turbine, reusing the existing machine and grid connection.

Procurement models vary. Some transmission operators own condensers as regulated network assets, while others contract third parties through stability or system strength tenders and pay for availability over long terms. For investors, these contracts create a class of grid service revenue that does not depend on energy prices.