Testing characteristics of grid forming converters part III: inertial 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 to ensure a stable and secure operation of the power grid. Moreover, it is widely recognised that the use of Grid Forming Converters (GFC) is necessary to fully meet these requirements. Over the 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, standardised 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, the first draft of such guidelines is being currently developed.
One of the necessary characteristics which makes converter a GFC is its ability to provide inertial response during the dynamic frequency changes in the system. This paper focuses on how to demonstrate and quantify an inertia-equivalent behaviour of a GFC. The response to a transient system event is quantified in terms of a damping factor D as well as an equivalent inertia constant H. A few alternative methods are proposed for empirical estimation of those parameters accompanied by selected laboratory test results and practical considerations.