Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Representations of AC power systems by frequency dependent impedance equivalents is an emerging technique in the dynamic analysis of power systems including power electronic converters. The technique has been applied for decades in DC-power systems, and it was recently adopted to map the impedances in AC systems. Most of the work on AC systems can be categorized in two approaches. One is the analysis of the system in the dq-domain, whereas the other applies harmonic linearization in the phase domain through symmetric components. Impedance models based on analytical calculations, numerical simulation and experimental studies have been previously developed and verified in both domains independently. The authors of previous studies discuss the advantages and disadvantages of each domain separately, but neither a rigorous comparison nor an attempt to bridge them has been conducted. The present paper attempts to close this gap by deriving the mathematical formulation that shows the equivalence between the dq-domain and the sequence domain impedances. A modified form of the sequence domain impedance matrix is proposed, and with this definition the stability estimates obtained with the Generalized Nyquist Criterion (GNC) become equivalent in both domains. The second contribution of the paper is the definition of a Mirror Frequency Decoupled (MFD) system. The analysis of MFD systems is less complex than that of non-MFD systems because the positive and negative sequences are decoupled. This paper shows that if a system is incorrectly assumed to be MFD, this will lead to an erroneous or ambiguous estimation of the equivalent impedance.

No abstract

No abstract

The small-signal impedance modeling of modular multilevel converter (MMC) is the key for analyzing resonance and stability of MMC-based power electronic systems. MMC is a power converter with a multi-frequency response due to its significant steady-state harmonic components in the arm currents and capacitor voltages. These internal harmonic dynamics may have great influence on the terminal characteristics of the MMC, which, therefore, are essential to be considered in the MMC impedance modeling. In this paper, the harmonic state-space (HSS) modeling approach is first introduced to characterize the multi-harmonic coupling behavior of the MMC. On this basis, the small-signal impedance models of the MMC are then developed based on the proposed HSS model of the MMC, which are able to include all the internal harmonics within MMC, leading to accurate impedance models. Besides, different control schemes for the MMC, such as open-loop control, ac voltage closed-loop control, and circulating current closed-loop control, have also been considered during the modeling process, which further reveal the impact of the MMC internal dynamics and control dynamics on the MMC impedance. Furthermore, an impedance-based stability analysis of the MMC-HVDC connected wind farm has been carried out to show how the HSS based MMC impedance model can be used in practical system analysis. Finally, the proposed

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.

No abstract

This paper demonstrates how the range of stable power transfer in weak grids with voltage source converters (VSCs) can be extended by modifying the grid synchronisation mechanism of a conventional synchronous reference frame phase locked loop (PLL). By introducing an impedance-conditioning term in the PLL, the VSC control system can be virtually synchronised to a stronger point in the grid to counteract the instability effects caused by high grid impedance. To verify the effectiveness of the proposed approach, the maximum static power transfer capability and the small-signal stability range of a system with a VSC HVDC terminal connected to a weak grid are calculated from an analytical model with different levels of impedance-conditioning in the PLL. Such calculations are presented for two different configurations of the VSC control system, showing how both the static power transfer capability and the small-signal stability range can be significantly improved. The validity of the stability assessment is verified by time-domain simulations in the Matlab/Simulink environment.Peer ReviewedPostprint (published version