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It is expected worldwide that the increase in renewable penetration will worsen the voltage profile in contrast to the conventional synchronous generators. This is because it is not proper to authorize all independent power producers&#x2019; renewable sources to control grid voltages. Although there has been a bunch of research to improve voltage stability, each has the problem of huge computational effort, limited grid reflection, calibration, etc. Thus, the indirect voltage control by droop can be a substantial solution. Then, it is the very one of the critical issues to determine the proper droop ratio, which might be strongly non-linear due to the complexity of the power system. This paper proposes the online nonlinear P-Q droop estimation of distributed generations. It improves voltage stability, which might get worse after the connection of renewable energies or energy storage devices through inverters. First, power sensitivities between multiple P and Q are derived and used to determine the initial state of the linear P-Q droop required for the initial operation that gets data for precise estimation of the P-Q droop. Thus, it enhanced the estimation performance by reducing the required data yet without the extreme P-Q range. Then, the nonlinear P-Q droop estimation is conducted with the Kalman-filter algorithm. The performance is verified by applying it to the real distribution power system of an island in Korea. For the verification, the distribution power system is modeled at the EMT level and simulated using the power system computer-aided design and electromagnetic transient and DC (PSCAD/EMTDCTM). The voltage stability was improved by the proposed nonlinear P-Q droop estimation compared to the cases using fixed droop or Q of zero.

Online Nonlinear P-Q Droop Estimation of Distributed Generations Based on Kalman-Filter Algorithm to Improve Voltage Stability.