In order to further improve the inertia estimation algorithm, the planned HVDC transfers and emergency reserves from HVDC links needs to be included into the algorithm. Furthermore, an extension of the Finnish thermal inertia constants would allow the current aggregated thermal production to be separated into smaller production types and thereby reducing the error in the estimation.
As the number of dimensioning faults increases, it is suitable to present incentives or grid codes to ensure frequency stability, especially during scenarios of medium to high wind, high import and low load conditions.
The action of the FCPA during high wind speeds should be further evaluated to ensure that the added reference signal to the pitch actuator does not cause increased mechanical stresses and over speeding of the rotor. This is mainly due to the derivative of the power coefficient with respect to the change in pitch angle is lower at high pitch angles than at low pitch angles. A dynamic FCPA that adjusts the gain of the controller at different operational points of the VSWT could improve the situation.
A coordinated response from controlled inertia algorithms together with an FCR-D set point change from conventional generation should be investigated in order to reduce the prolonged frequency swings. The fast power electronics should be utilized to handle the first initial dip with a defined inertial power. This power can then through communication be sent to other slower generators. These generators can then increase their output power to compensate for the additional power needed during the recovery phase of the VSWTs.
Topology investigations regarding the possible storage systems could deliver a more holistic approach. This would open up for fully detailed switching models and small signal stability analysis of controller interactions between the grid connected converter and the storage DC-DC converter.
Even though the Nordic TSOs assume a rather low integration of solar units until 2025, fre- quency control through these units, especially during island operation, should be further stud- ied.
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