Quantitative transient stability assessment in microgrids combining both time-domain simulations and energy function analysis

Abstract

Hybrid microgrids of the future comprise synchronous generators and inverter-based photovoltaics, battery storage systems etc. Interconnection of off-grid hybrid microgrids can provide economic benefits and increase security of supply. Both in the short-term and long-term planning phase as well as during operation, transient stability assessment (TSA) of such clustered hybrid microgrids is essential at different operating points. Quantitative TSA helps in, for example, optimal generation dispatching, adjusting clearing times in the protection systems, optimal clustering of microgrids etc. However, the existing five hybrid methods developed for transmission systems with certain assumptions and limitations, taking the advantages of both time-domain simulations and transient energy function (TEF), cannot be directly applied for microgrids. Although there could be less than ten synchronous generators in clustered hybrid microgrids, a detailed TSA in such systems is too complex to begin with. Hence, two basic power system models – one-machine infinite-bus system (OMIBS) and two-machine system (TMS) – with classical 2nd order and detailed 5th order modelling of diesel generators (DG) with and without controllers are studied in this paper. One of the aims is to get a better knowledge of the dynamic behaviour and analyse kinetic energy (KE) and potential energy (PE) profiles. The main goal is to develop a new hybrid method to define a stability margin (SM) and a stability degree (SD) for both the types of generator modelling. The stability analyses in the OMIBS show that, the influence of the generator modelling on the end results, i.e., SM and SD, is significant. The duration of the sub-transient state in DG might be of just several tens of milliseconds after the fault incident, but the indirect effect of the sub-transient short-circuit currents especially on speed governors until the end of the forward swing is substantial. In the TMS, the synchronous operation of the DG is strongly dependent on each other. The TEF and its components like KE and PE, which are applied individually to each DG, are inversely proportional to the inertia constants of the DG. It was shown not only graphically, but also by postulating and proving two propositions. Therefore, an important foundation regarding the TEF has been laid for the quantitative TSA in multi-machine clustered hybrid microgrids.