가상 동기 발전기 기반 그리드 포밍 컨버터의 새로운 유효전력 제어기법

Abstract

Conventional ac power system stability relies on the rotating mass of the synchronous generator (SG). However, as the penetration level of power converter based renewable energy source (RES) to the grid increases, it is inevitable for the inertia in power system should be reduced. Inertia plays a vital role in maintaining frequency stability in power system. A low-inertia system can experience a rapid frequency drop in response to disturbances such as generator or transmission line failures, posing a risk of wide-area blackouts and simultaneous disconnection of large-scale power generation facilities. As a result, there is an increasing demand for various studies on the inertia emulation and stability contribution of grid-connected voltage source converters (VSCs). The conventional grid-tied VSCs, operating based on grid-following (GFL) method, which involves operating VSC as current source, cannot provide inertia to the power system and have stability problem in weak grid condition. In order to overcome the problem, grid-forming (GFM) method, which involves operating VSC as voltage source, has been proposed to contribute to the voltage and frequency stability of the power system. The virtual synchronous generator (VSG) method is one of the GFM methods used to emulate the inertia of SG. It can provide inertia to the power system by utilizing swing-equation. However, the VSG method exhibits droop characteristics and is coupled with inertia emulation in response to frequency variations in the power system. In another way to emulate the inertia, proportional-integral (PI) or integral-proportional (IP) structure power controllers have been proposed. However, a problem arises where the measured active power is affected by the output frequency. This can lead to unstable frequency variations, deteriorating the frequency stability of the power system. This paper proposes a novel power control method for GFM converters that mitigates output frequency variations and ensures frequency stability of the power system. The proposed power control method can reliably regulate the output frequency in various grid conditions and contribute to the frequency stability by implementing appropriate inertia emulation. The validity of the proposed power control method for GFM converters has been validated through offline simulations and Hardware-in-the-Loop (HIL) experiments.