Hierarchical Risk Assessment of Transmission Network Considering the Influence of Micro-grid

Transmission network risk is calculated by multiplying occurrence probability and consequence of contingencies. Since power flow in the traditional distribution system is unidirectional, its loads are usually aggregated into an equal load and connected to the transmission network through a substation in the traditional risk assessment. And, all loads are simply treated as constant-power type. Details of distribution network and loads are usually ignored. However, today a growing number of distributed generators (DGs) based on renewable resources being connected to modern power systems. How to properly evaluate their influence on risk assessment of transmission network should be considered. Micro-grid has been regarded as a good way to integrate various renewable resources into distribution network. It can increase DG's penetration and reduce its negative impact. However, micro-grid can be considered as a controllable load or a small virtual power plant, so it can significantly change characteristics of distribution loads and operational modes of distribution network. In this paper, microgrid is used as a tool to reduce load curtailment under some critical contingencies. And its influence on risk assessment of transmission network is then analyzed and discussed. In the study, a linear optimization method is used to determine the load curtailment. Discrete probability model of DGs in micro-grid and detailed model of distribution system are taken into account in the optimal process, such as island, tie switches, links, etc. Risk calculation of whole power system is executed though iterations between transmission network and distribution system. Load curtailment expectation instead of absolute load curtailment is used to risk calculation. The correctness and effectiveness of the proposed method is finally validated by RTS and RBTS test system.