Testing characteristics of grid forming converters part I: specification and definition of behaviour

On the path towards an energy system powered entirely by Renewable Energy Sources (RES), power electronic converters will have to take over more and more functionalities from Synchronous Generators (SG) to ensure a stable and secure operation of the power grid. Moreover, it is widely recognized that the use of Grid Forming Converters (GFC) is necessary to fully meet these requirements. Over the few last years, different concepts have been developed to achieve grid forming characteristics of static power converters.
The next essential step is to agree on an exact definition and specification of GFC electrical behaviour as well as to define a suitable conformity assessment procedure. For this purpose, standardized testing guidelines for GFC are needed to assess those functionalities, which are relevant for dynamic grid stability. As a British-German joint work of the two research projects Battery-VSM and VerbundnetzStabil, a first draft of such guidelines is being currently developed.
In this first part, we summarize the requirements from GFC based generation from system perspective, review the current requirements on power electronic converters and line out the specifications determining whether a converter is grid forming or not. We define the behaviour of a GFC in the normal operational range such as voltage source behaviour, power quality impact and provision of inertial response. Furthermore, specifications to evaluate the grid-stabilizing behaviour of GFCs during grid-fault or overload situations, which exceed the converter’s current limits, are given.