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This paper presents stability analysis and control strategies to guarantee stable operation of type-III wind turbines with different types of transmission systems, including a weak inductive grid, a series-compensated transmission line, and an HVDC converter in offshore wind farms with HVDC transmission. Turbine-grid system stability is examined through impedance-based analysis based on the latest impedance models of type-III turbines that account for the effects of dc bus dynamics and coupling over frequency to provide accurate prediction of subsynchronous resonance (SSR), especially with weak inductive grid. A general method to damp SSR with weak inductive and series-compensated grid is proposed by placing an active damping filter in parallel with the current controller. HVDC converters operating in ac voltage control mode are shown to exhibit capacitive output impedance in the subsynchronous frequency range, which can lead to SSR with type-III turbines as series-compensated grid does. The proposed damping method is further demonstrated to be effective in damping the SSR in this case. Detailed simulations are also presented to support each case studied in this work through impedance analysis. [ABSTRACT FROM AUTHOR]

This paper presents methods to model and solve high-frequency resonance problems in HVDC and wind energy systems. Delay is identified as a root cause for such resonances and is shown to create a negative damping in turbine and HVDC converter output impedances. A systematic method to include control delays in small-signal sequence impedance models is presented and the developed models are used to study the effects of delay on system stability. The method is applied to type-IV and type-III turbines as well as HVDC converters based on modular multilevel converters. In addition to full models that can predict the associated impedance and negative damping behavior accurately, simplified models are also presented to gain insights. Damping of high-frequency resonance by passive and active methods are also discussed. [ABSTRACT FROM AUTHOR]

This paper examines the possibility of subsynchronous resonance (SSR) between voltage-source converters (VSC) and the power grid under different configurations and in different applications. SSR between Type-III wind turbines and a weak grid with series compensation has been discussed in the literature. The purpose of this work is to understand whether a VSC operating as an inverter for Type-IV wind turbine and PV application could form SSR with the grid, especially a weak grid with or without series compensation. The analysis is based on analytical impedance models of the VSC and is performed in two steps. In the first step, dynamics of the dc bus voltage are ignored, allowing the use of relatively simple models of the VSC to be used. In the second step, the effects of dc bus dynamics on the inverter output impedance are considered. In each case, both an inductive grid and a capacitive grid (when series compensation is used) are considered, and design guidelines to avoid SSR and possible mitigation methods when SSR occurs are also presented. It is found that, with an ideal dc bus, SSR could happen but is unlikely if there is no series compensation in the grid. A soft dc bus, on the other hand, may form resonance with the grid through the inverter and, when such a resonance occurs, the inverter output current typically exhibits a supersynchronous as well as a subsynchronous resonance at frequencies that are symmetrical about the fundamental frequency. The impedance-based small-signal analysis is verified by detailed circuit simulation. [ABSTRACT FROM AUTHOR]

This paper presents a summary of the recent work by the authors on the modeling and mitigation of subsynchronous resonance (SSR) between Type-III wind turbines and the grids that uses series compensation. The basis for the work is analytical impedance models describing small-signal behaviors of the turbine from subsynchronous to supersynchronous frequencies. The method and general procedure to develop such impedance models are outlined, followed by a complete set of models for Type-III turbines employing standard control architectures. Use of the models to predict SSR when the grid is equipped with series compensation is then illustrated and validated. The analytical impedance models also provide a basis for the development of SSR mitigation solutions. As an example, damping of SSR through control of either the rotor-side or stator-side converter to reshape the overall turbine output impedance in the subsynchronous frequency range will be discussed and a case study utilizing active damping on rotor-side converter is presented. [ABSTRACT FROM AUTHOR]