Abstract
There exists serious frequency instability in the UHVDC islanded operation system consisting of a large capacity UHVDC and its corresponding power source with hydraulic power generators. Ultralowfrequency oscillation (ULFO) is one of the frequency instable problems, which will happen because of the inconsistent frequency control features between UHVDC and hydraulic power generators. Through the frequency-domain analysis, there exists a frequency band with negative damping in the hydraulic turbine control system, and UHVDC is negative damping for frequency deviation with constant power control mode. There are four negative damping contributing factors as hydrowater starting time constant, hydrogenerator governor forward gain, islanded system total inertia, and UHVDC power. These four factors are analyzed in this article in the frequency domain. Based on the detailed ULFO damping analysis, effective frequency control strategies for UHVDC islanded system are proposed in this article by withdrawing a part of primary frequency control functions of generators or coordinating the frequency controls between hydraulic generator governors and UHVDC frequency limitation controller (FLC). Through the proposed frequency strategies, islanded system frequency is mainly controlled by UHVDC FLC, which can successfully suppress the ULFO. Validation simulations are performed on the RTDS platform, and the simulation results prove the proposed strategies.
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