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Field experience has shown that sub-synchronous oscillation (SSO) and harmonic resonance can occur between wind farms (WFs) and high voltage dc (HVDC) systems. The oscillations can appear in the presence of background harmonics due to the interaction between the wind energy conversion system's (WECS) converter controller, HVDC converter controller and the impact of the interconnection system impedance. However, the root causes of these oscillations observed in the field are not entirely understood and they can be attributed to various sources within the components and controllers of the interconnected system. This paper explores the possible causes of these oscillations by investigating the impact of controllers and components in the wind farm and in the voltage source converter (VSC)-based HVDC transmission system. In order to understand this phenomena, the impedance of both the wind farm and the HVDC from the offshore ac collection point are analytically derived to identify potential resonance points. The impedance frequency responses of the wind farm and the HVDC converter indicate the potential resonance at low frequency. The origin of these oscillations can be attributed to the propagation of the WECS resonance through the WECS full converter dc link and the interaction between the WECS and the HVDC system. Once the source and the load impedance are identified, an impedance-based stability method is adopted in order to determine the stability. In an attempt to improve the oscillatory phenomena, an active damping scheme is implemented on the offshore HVDC rectifier. An analysis and time domain simulation results with its respective harmonic spectra show that the implemented active damping is very effective in eliminating the oscillations observed in the interconnected system. Moreover, this paper presents the role of the ratio between the bandwidths of the interconnected areas, as having an essential role in the root cause of the instability. The general rule is observed that when the bandwidth of the HVDC rectifier (which is the source) is faster than the bandwidth of the load (WFs inverter); the system operates stably.

Abstract-The high voltage dc (HVDC) systems are appearing more and more, and it is becoming a requirement that the HVDC voltage source converters (VSCs) operate both as an inverter and a rectifier without changing the controls to provide the flexibility of having power flows in both directions. It is observed that the HVDC system operates stably when the power flow direction is from the power controlled-converter to the dc voltage controlledconverter and it becomes unstable when the power flow direction has been altered. In order to analyze such instability problem and to design the local control, an impedance-based method is proposed. Identifying the source and the load impedance are prerequisite to apply the impedance-based method. Existing method of determining the source and the load impedance cannot predict the stability when the power flow direction is altered; therefore a method based on the power flow direction has been presented to determine the source and the load impedance. The converter which injects power to the dc system is the current source represented with its Norton equivalent parallel impedance while the other converter impedance is considered as the load impedance. The stability of the system is determined by the ratio of the load impedance to the current source impedance. Once the source and the load impedance are analytically obtained, the impedance-based Generalized Nyquist Stability Criteria is applied to determine the stability. The system stability for the two power flow directions is well predicted from the Nyquist plot of impedance ratio. A two-terminal HVDC system has been developed in MATLAB/Simulink to demonstrate the application of this method and the results are compared with the experimental results.

Estimation of critical control parameters is a desirable tool feature for stability analysis and impedance shaping of high voltage dc (HVDC)-connected wind farms. Accurate estimation of such parameters would be enabled by access to detailed models, which is not always the case in real wind farms. Industrial secrecy is one of the main factors hindering the access to such models. This paper proposes a Grey-box method that, with basic assumptions about the control structure of the wind energy conversion system (WECS), can estimate the parameters of its controllers. The method is based on the measurements of frequency domain equivalent impedance combined with nonparametric impedance identification used in the solution of an inverse problem. The method makes possible to specify which part of the equivalent WECS impedance has a major impact on the stability of the system and according to this, re-shape the impedance to enforce stability. Once the critical controller bandwidth is identified with this method, an instability mitigation technique is proposed based on reshaping the impedance by re-tuning the critical controllers of the interconnected converters. In order to avoid interaction between the HVDC rectifier and the WECS inverter, the controllers of both converters need to be re-tuned in such a way that the q-axis impedance magnitude of the HVDC system is kept lower than the q-axis impedance magnitude of the wind farm at the frequency of the phase-lockedloop bandwidth. The results show that the method ensures the stability of the system by re-tuning only the critical controller parameters.

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