Jing Lyu

Frequency Domain Stability Analysis of MMC-Based HVdc for Wind Farm Integration

This paper investigates the stability of offshore wind farms integration through a modular multilevel converter-based high-voltage dc (MMC-HVdc) transmission system. Resonances or instability phenomena have been reported in between wind farms and MMC-HVdc systems. They are arguably originated due to interactions between the MMC and the wind power inverters. However, the nature of these interactions is neither well understood nor reported in the literature. In this paper, the impedance-based analytical approach is applied to analyze the stability and to predict the phase margin of the interconnected system. For that, analytical impedance models of a three-phase MMC in a compensated modulation case and a direct modulation case are separately derived using the small-signal frequency domain method. Moreover, the impedance models of the MMC take the circulating current control into account. The derived impedance models are then verified by comparing the frequency responses of the analytical model with the impedance measured in a nonlinear time-domain simulation model developed in MATLAB. The results show that the potential resonances or instability of the interconnected system can be readily predicted through the Nyquist diagrams. In addition, the analysis indicates that the circulating current control of the MMC has a significant impact on the stability of the interconnected system. Finally, the time-domain simulations validate the theoretical analysis.

Oscillatory phenomena between wind farms and HVDC systems: The impact of control

Field experience has shown that sub-synchronous oscillation (SSO) and harmonic resonance can happen due to the interaction of wind energy conversion systems (WECS) converter controller, HVDC converter controller and impact of system impedance. This paper investigates the operation and control of WECS employing full-scale converter connected to the electrical network through voltage source converter (VSC)-based high voltage dc (HVDC) transmission system. SSO and harmonic resonance are observed depending on the control implementation of the ac collection bus side HVDC converter. The origin of these oscillations can be attributed to the propagation of the WECS 15 Hz resonance through the WECS full converter DC link and the interaction between the WECS and the dc link dynamics of the HVDC system. An active damping scheme is proposed to improve the system performance. The active damping is implemented to an offshore HVDC system installed with the purpose of integrating two wind farms. An analysis and time domain simulation results and its spectral analysis are presented to show how effective the application of the proposed active damping is.

Impedance modeling of modular multilevel converters

The impedance modeling of modular multilevel converters (MMCs) is a prerequisite for applying the impedance-based stability analysis method to an MMC-based high-voltage direct current (MMC-HVDC) transmission system. The objective of this paper is to develop impedance models of a three-phase MMC in order to assess its stability. The analytical ac-side and dc-side impedance models of the three-phase MMC with and without circulating current control (CCC) are derived separately with consideration of the MMC internal dynamics. In doing that, a control strategy for the MMC in an application of wind power integration through MMC-HVDC is taken into consideration when calculating the ac-side impedances. The analytical impedance models derived for the MMC are verified by the method based on injecting a small-signal perturbation in the detailed nonlinear time-domain simulation model of the MMC.

Impact of Power Flow Direction on the Stability of VSC-HVDC Seen From the Impedance Nyquist Plot

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-controlled converter, and it becomes unstable when the power flow direction has been altered. In order to analyze such an 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. The 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 that 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 criterion 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.

Impedance modeling of modular multilevel converter based on harmonic state space

This paper presents a novel small-signal model of modular multilevel converter (MMC) based on harmonic state space (HSS). With this model, the impedance of MMC can be easily obtained. Whats more, the accuracy and complexity can be balanced well. In the end, the simulation and analytical results are compared to verify the feasibility of the proposed small-signal model of MMC.

Optimal Design of Controller Parameters for Improving the Stability of MMC-HVDC for Wind Farm Integration

A subsynchronous oscillation (SSO) phenomenon has been observed in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration in the real world, which is independent of the type of wind turbine generator. This kind of oscillation appears different from those in doubly fed induction generator-based wind farm with series-compensation line or wind farm integration through two-level voltage-source converter-HVDC transmission system, because the internal dynamics of the MMC may have significant impact on the oscillation. By far, however, very few papers have reported it. In this paper, the generation mechanism of the SSO phenomenon in an MMC-HVDC transmission system for wind farm integration is revealed from an impedance point of view. The harmonic state-space modeling method is applied to model the multifrequency behavior of the MMC, based on which, the ac-side small-signal impedance of the MMC is analytically derived according to harmonic linearization theory. As a general rule, the controller parameters of the wind power inverter and the HVDC converter are designed separately, to meet the performance requirements of the single converter under ideal conditions, but this practice does not guarantee the stability of the interconnected system. Therefore, an optimal design method for controller parameters is proposed in this paper in order to guarantee the small-signal stability of the interconnected system from a system point of view. Finally, time-domain simulations validate the effectiveness of the theoretical analysis and the proposed optimal design method.

Stabilization control methods for enhancing the stability of wind farm integration via an MMC-based HVDC system

The subsynchronous oscillation (SSO) phenomenon may occur in a modular multilevel converter-based high-voltage dc (MMC-HVDC) transmission system for wind farm integration, which has already been observed in the real world. In this paper, the generation mechanism of the SSO phenomenon in the interconnected system is revealed by means of impedance-based analysis. On this base, from the HVDC converter control point of view, stabilization control methods are proposed to guarantee the stability of the interconnected system. Both frequency-domain analysis and time-domain simulations are carried out to validate the effectiveness of the proposed stabilization control methods.

Impedance modeling of modular multilevel converters by harmonic linearization

The impedance modeling of modular multilevel converters (MMCs) is the prerequisite for applying the impedance-based stability analysis method to MMC-based power electronic systems. MMC has complex internal dynamics, e.g., harmonic circulating currents and capacitor voltage ripples, which might have a significant impact on operation stability of the interconnect system based on MMC. The harmonic linearization method, which is able to capture the internal dynamic behaviors of an MMC, is used to develop the ac-side small-signal impedance models of the MMC in this paper. An inherent low-frequency resonance phenomenon in the ac-side impedance of the MMC is revealed. Furthermore, the influence of the closed-loop control on the ac-side impedance of the MMC is also examined. Finally, the analytical impedance models are validated by the measured results in the time domain simulation by MATLAB/Simulink.