Degradation of system strength because of inverter-based resources (IBRs) is a major concern facing the zero-carbon transition. Blackouts like the one in Iberia on April 28 highlight this concern. While the full report on this event is yet to be produced, the initial response of the system operator has been to require increased amounts of synchronous generation on the system by curtailing wind and solar power. This tends to confirm speculation that the large amounts of IBRs had decreased system stability.

Lack of synchronous inertia is most often cited as the destabilizing issue, although there are many options for fast-frequency response, which can help with the “real power” problem that synchronous inertia solves by reducing rates of change of frequency (ROCOF). Less focus has been given to the “reactive power” aspect of system stability which is most economically addressed by the short-circuit current (or “fault current”) capacity of synchronous machines. This is typically 10 times rated current for short durations. The windings of synchronous generators (or compensators) in an AC grid are electro-magnetically coupled to each other in short-circuit across large geographic distances. This enables them to respond to system voltage and frequency changes, and holds the inertial energy of their rotors together, at microsecond timescales.

Therefore, the displacement of synchronous generators by IBRs is a problem. It is the fault current capacity of synchronous machines which is irreplaceable for system stability, more so than the inertia.

For example, IEEE 2800:2022 states that adding synchronous condensers is “presently the primary solution for adding system strength because of multifaceted benefits including large capability to supply fault current, inertia and voltage support capability”. Without these, IBRs in South Australia, Ireland and other places have had to be curtailed, causing significant financial pain and planning uncertainty to wind farm developers there.

Where synchronous condensers have been installed, to date it has been at the expense of the network companies. However, it is foreseeable that it will not be long before solar and wind farms will be required to have some degree of synchronous compensation.

The authors have previously presented work on Type 5 wind turbines, i.e. those that drive a directly-connected synchronous generator, without inverters, through one of three variants of hydraulic variable speed system. Their 2024 paper explained that the SyncWind Type 5 power-train is the most cost-effective of these three, because its hydraulics are only rated at 5% of turbine power, giving it a total turbine cost less than that of a Type 3 turbine.

This paper will detail the various control sub-systems of the power-train, explaining how it:

• achieves very fast torque control to protect the drive-train and adjust output power using an hydraulic relief valve
• maintains turbine efficiency cost-effectively with its low-variable speed operation
• exports or imports real and reactive power control during normal operation, with and without wind
• imports or exports reactive power during fault conditions
• stabilises voltage
• provides synchronous inertia from the generator rotor during frequency excursions
• provides fast-frequency response from the wind turbine rotor’s kinetic energy.