Enhanced Dynamic Stability Control for Low-Inertia Hybrid AC/DC Microgrid With Distributed Energy Storage Systems

Abstract

Hybrid ac/dc microgrids (MGs) integrated with traditional diesel generators, distributed energy storage systems (ESSs), and high penetration of renewable energy sources (RESs)-based distributed generators (DGs) have become an attractive power supply solution for isolated remote areas and islands, which can effectively reduce environmental protection pressure and improve power supply reliability. However, in such inherent low-inertia systems, randomness and fluctuation of the output power of RESs and uncertain load consumption can easily incur dynamic stability issues, such as transient power impact, unacceptable frequency deviations, and operation mode transitions for security. To solve the above problems and enhance the dynamic stability of the system, an enhanced dynamic stability control (EDSC) scheme with locally measured signals only is proposed in this paper. In this control scheme, the bi-directional interlinking dc-ac converter uses the ac frequency in ac MG as the reference value of dc voltage and adopts the current feedforward control to control the dc voltage in the dc MG to be consistent with the ac frequency. By electrically coupling dc voltage and ac frequency, power disturbances in ac or dc sides will cause almost identical variation degrees in both ac frequency and dc voltage. Under the proposed EDSC scheme, the distributed ESSs in both ac and dc sides are then automatically coordinated and controlled by the unified droop control to balance transient power disturbances and smooth output of diesel generator under normal condition, which can effectively improve stability and controllability of such low-inertia systems. Furthermore, in an emergency such as failure of diesel generator, the operation mode can be switched seamlessly with the proposed EDSC scheme. The detailed theoretical analysis including control system design, small signal model analysis of key parameter influence on system dynamics, and simulation verifications in the PSCAD/EMTDC environment is presented to verify the effectiveness and practicality of the proposed EDSC scheme.

Keywords

• Low-inertia hybrid ac/dc microgrids
• distributed energy storage systems
• enhanced dynamic stability control
• transient power sharing
• seamless transition