Abstract: To address the security operation risks caused by the complex uncertainties arising from the integration of massive distributed renewable energy sources into low-voltage grids, a multi-layer risk assessment and coordinated optimization method is proposed that accounts for the dispatchable region of microgrids and the security region of transmission-distribution networks. First, a multi-level risk assessment index system and framework are established from the perspectives of the transmission, distribution network, and microgrid. Second, stochastic models for key uncertain factors such as distributed wind power, photovoltaic generation, load, and component failures are developed, and a set of system operational states is generated using sequential Monte Carlo simulation. Then, a collaborative operation optimization model for the transmission, distribution, and microgrid is constructed based on the dispatchable domain of microgrids and the security domain of main and distribution networks. The stochastic power flow is rapidly solved using the semi-invariant method and Cornish-Fisher series expansion, and an opportunity-constrained optimization scheduling model is introduced to ensure economic operation while effectively managing operational risks. Finally, the method is validated using an improved transmission-distribution-microgrid system based on IEEE 9-bus and IEEE 33-bus configurations. The results demonstrate that the proposed method achieves optimal economic benefits while effectively controlling operational risk levels at all levels, providing theoretical support and methodological references for the collaborative operation and risk assessment of main-distribution-microgrids with high renewable energy penetration.

Key words: transmission-distribution-microgrid, risk assessment, dispatchable domain, security domain, collaborative optimization, stochastic power flow calculation, chance-constrained optimization