Abstract
Electrical resonances may compromise the stability of HVDC-connected offshore wind power plants (OWPPs). In particular, an offshore HVDC converter can reduce the damping of an OWPP at low-frequency series resonances, leading to the system instability. The interaction between offshore HVDC converter control and electrical resonances of offshore grids is analyzed in this paper. An impedance-based representation of an OWPP is used to analyze the effect that offshore converters have on the resonant frequency of the offshore grid and on system stability. The positive-net-damping criterion, originally proposed for subsynchronous analysis, has been adapted to determine the stability of the HVDC-connected OWPP. The reformulated criterion enables the net damping of the electrical series resonance to be evaluated and establishes a clear relationship between electrical resonances of the HVDC-connected OWPPs and stability. The criterion is theoretically justified, with analytical expressions for low-frequency series resonances being obtained and stability conditions defined based on the total damping of the OWPP. Examples are used to show the influence that HVDC converter control parameters and the OWPP configuration have on stability. A root locus analysis and time-domain simulations in PSCAD/EMTDC are presented to verify the stability conditions.
Highlights
H ARMONIC instabilities have been reported in practical installations such as BorWin1, which was the first HVDC-connected Offshore Wind Power Plant (OWPP) [1], [2]
The CIGRE Working Groups suggest that series resonances can be found in the range of a few hundred Hz
These resonances can interact with the offshore HVDC converter control leading to system instability
Summary
H ARMONIC instabilities have been reported in practical installations such as BorWin, which was the first HVDC-connected Offshore Wind Power Plant (OWPP) [1], [2]. The impact that low frequency series resonances have on the voltage stability of HVDC-connected OWPPs is analyzed and discussed. This paper furthers the initial contributions of [19] by providing a formal framework for the analysis of electrical resonance stability in HVDC-connected OWPPs. An impedance-based representation is used to identify resonances and to assess stability considering the effect of the offshore converters. The alternative approach to the positive-net-damping criterion is demonstrated using the phase margin condition This criterion defines the relation between the damping at electrical series resonances and system stability. The relationship between the total damping and resonant frequencies with the poles of the system is demonstrated This relationship shows that the pole analysis and the positive-net-damping criterion provide the same information about resonance stability. The examples presented in this paper are complementary to those included in [19]
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