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Introduction
In recent years, the renewable energy generation started to rise dramatically for most
countries. In Europe, following the introduction of the renewable energy directive
2009/28/EC, the region increased the share of renewable energies in energy
consumption to 20% by 2020, according to Eurostat [1]. Out of such energy
consumption, 23% was consumed by the electricity sector as the second highest source
of energy consumption [1]. Nevertheless, renewable energy generation is expected to
continue rising in the coming years as part of the decarbonization plan. The directive
EU/2018/2001 has set the renewable energy target to reach 32% by the year 2030 [2].
Subsequently, a provisional agreement was accepted to raise that target to at least
42.5%, with an aim for 45% [2]. The Continuous rise of renewable generation in the
electricity sector can cause some problems for the grid, which need to be addressed.
In traditional power systems, the synchronous generator is the main source offering
support to the grid through its kinetic energy and governor control. The synchronous
machine has the capability to participate in the primary frequency control using the
governor speed control mechanism and dampen the system dynamics through its
inertia. However, the power converters connecting renewables with the grid do not
possess such capabilities. In fact, their control structure is more focused on extracting
the maximum power from the renewable source. With the growth of renewable energy
sources, the use of synchronous machines is expected to decline, hence decreasing the
system inertia and support [3]. Remarkable efforts are focused on developing a control
approach that allows power converters to mimic the behavior of a synchronous
machine. A captivating control family has emerged, called ‘Grid-forming Inverters,’
allowing the inverters to provide some of the synchronous machine functionalities like
primary frequency control, oscillation damping, and contributing to system inertia.
This control methodology is best suited to be implemented with batteries. The high
ramp rate along with power and energy characteristics of batteries ensures compliance
with the control output power signals. The grid-forming presents multiple control
approaches [10], with the utmost attention focused on the so called “Virtual
Synchronous Machine”. It directs the power converters to act in a similar manner to a
synchronous generator, thus providing all its functions mentioned above.
