The fluctuation of electricity prices and the increase in reserve prices have heightened interest in hybrid power plants in Finland. Traditionally, hybrid power plants have been implemented with a top-level park controller that manages the individual controllers of different plant sections, such as wind farms and batteries. However, practical experience has shown that this approach is challenging to implement. Particularly modelling the power plant has been challenging. Additionally, the use of two overlapping controllers, sometimes from different manufacturers, increases uncertainty regarding the plant's compliance with requirements. Another possible approach has been to build the other plant section as its own plant with its own main transformer and connection. Even though building the other plant section with its own transformer would not require a top-level controller, this implementation could be costly since it requires purchasing a new main transformer and connection.

Updated Finnish grid code has allowed an alternative approach for converting plants into hybrids, where the plant sections independently regulate the voltage of the common medium-voltage bus. This approach moves the point for voltage control from the main transformer high-voltage side to the medium-voltage bus. This approach does not require a common top-level park controller but only a slow controller that limits active and reactive power to prevent overloading components such as the main transformer. This enables the addition of plant sections behind the same main transformer without the need for a new transformer or connection. Similar implementation is already in use, for example, in hydropower plants, where all generators regulate the voltage at their own terminals.

This study examines the feasibility of such a hybrid plant using PSCAD simulations with manufacturer-specific black box models. The research focuses particularly on a battery + BESS hybrid, as wind farms are the most common converter-connected plants in Finland. The study investigates, among other things, the operation of voltage control, reactive power capacity, and the effects of the main transformer tap changer.

Additionally, the paper examines the interaction between grid forming control and inverter level fast reactive current response mandated by the updated Finnish grid code. Specifically, it investigates whether problems or cross-regulation can arise between these, as the medium-voltage bus is a weaker point than the high-voltage side of the main transformer.

Preliminary simulations indicate that voltage control can be implemented in the proposed way. However, the tap changer must be kept locked to maintain reactive power support during longer over- and under-voltages.