# Influence of Load Dynamics on Converter-Dominated Isolated Power Systems
## 1 Introduction  
Battery energy storage systems (BESS) are being installed in geographical islands with large shares of converter-interfaced renewable energy sources (CI-RES) in order to provide fast power-frequency regulation reserve. This solution contributes for mitigating the displacement of the traditional thermal-based generation units in face of CI-RES increase, as this trend significantly reduces synchronous inertia and degrades power-frequency control capabilities [1, 2]. Currently, the power converter interfaces used in CI-RES and BESS are typically controlled as grid-following units, thus requiring to be installed in systems where other elements operated as voltage sources assure the definition of the voltage and frequency waveform [3]. Thus, in order to achieve operating scenarios with up to 100% RES integration, the existence of power converters with grid-forming control capabilities is mandatory. The operation of this type of converter requires an energy buffer--typically a BESS--coupled to it [4]. Recently, several works have been carried out with respect to grid-forming operation modes of power converters. The concept of the virtual synchronous machine (VSM) is a widely recognized approach which aims to emulate in a power converter the inherent operational characteristics of a synchronous machine (SM) [5, 6, 7, 8]. In power system stability analysis, the focus is traditionally associated with power-generating unit modeling, while load models are relegated to a second plan despite recognizing their influence on network stability [9]. This becomes even more relevant when the generation system is also being tremendously modified and converter-based units become the dominant sources. In fact, different load models could lead to different system stability assessment conclusions, which might affect the network planning and operation [10]. This was emphasized in [10], where the authors stressed the importance of including induction motor (IM) load models in simulation platforms in order to best fit numerical simulation results to event recordings in large interconnected systems. The obtained results led to the conclusion that a level of 20% to 30% of IM loads is required to be considered to best fit simulation studies to the recorded events. The emergence of power electronic interfaced loads is increasing the interest in load modeling and parametrization. Within the scope of a CIGRE working group [9], the authors presented the results of a survey addressing load modeling practices in more than 160 utilities and system operators. It was concluded that, while for steady state studies there is a dominant load model type used by the inquired utilities, for the case of dynamic power system studies, no clear trend regarding the adopted models was able to be clearly identified. A load model stablishes the mathematical representation of the relationship between its active and reactive power response with respect to time and/or grid quantities (in particular, frequency and voltage). The load models can be generally divided into two main categories: static and dynamic. Static load models express their active and reactive powers in any instant of time as functions of the bus voltage and frequency, with ZIP and exponential models being typical implementations associated to this category. The dynamic load models express the time dependence of active and reactive powers, being generally represented by IM load or exponential recovery load models [11]. There are two distinct approaches for identifying the load model parameters: measurement-based and component-based. The component-based method consists of the knowledge of physical behaviors of loads. However, such information is not always possible to obtain. On the other side, the measurement-based method consists in collecting a measurement's dataset which is used to identify the load model parameters that best suit the measurement data [11]. More recently, distinct load modeling strategies have been proposed in the literature.