A Novel Frequency Reference Compensation Method in Decentralized Secondary Frequency Control for Parallel Operation of Multiple Generators in an Islanded Microgrid

Abstract

Conventional centralized and distributed structures for secondary frequency control (SFC) in an islanded microgrid rely on communications. However, this may degrade the reliability of the system if communication delays or failures occur. As a potential alternative, several decentralized integral controls have emerged for SFC of islanded microgrids; however, these methods are limited by their inability to account for the unknown biased errors of real systems, which can accumulate in the controller of each generator causing clock drift and fighting among the generators. To overcome these issues, this paper proposes a novel SFC method for diesel generators in an islanded microgrid. With the proposed method, the frequency can be maintained at almost the nominal value by “frequency reference compensation control (FRCC)” with only local measurements that do not require telecommunications. Therefore, this approach prevents catastrophic circumstances due to communication failures and delays in islanded microgrids. The proposed controller has improved robustness against unknown biased errors, thus mitigating fighting among multiple diesel generators. To ensure system stability, small-signal stability analyses and common quadratic Lyapunov method were conducted with switched state-space modeling. The small-signal stability analyses show that the proposed method shifts the unstable eigenvalue at the critical point of the system matrix to the negative real part, ensuring the system is asymptotically stable. Control parameters are achieved via heuristic optimization called particle swarm optimization (PSO). Hardware in the loop simulations were performed to verify the feasibility of the proposed control method.

Keywords

• Clock drift
• decentralized control
• islanded microgrids
• secondary frequency control
• switched state-space modeling