Jiabing Hu

Direct Active and Reactive Power Regulation of DFIG Using Sliding-Mode Control Approach

This paper presents a new direct active and reactive power control (DPC) of grid-connected doubly fed induction generator (DFIG)-based wind turbine systems. The proposed DPC strategy employs a nonlinear sliding-mode control scheme to directly calculate the required rotor control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any synchronous coordinate transformations. Thus, no extra current control loops are required, thereby simplifying the system design and enhancing the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the designs of the power converter and the ac harmonic filter. Simulation results on a 2-MW grid-connected DFIG system are provided and compared with those of classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC. The proposed DPC provides enhanced transient performance similar to the LUT DPC and keeps the steady-state harmonic spectra at the same level as the VC strategy.

Direct Active and Reactive Power Regulation of Grid-Connected DC/AC Converters Using Sliding Mode Control Approach

This paper proposes a new direct active and reactive power control (DPC) for the three-phase grid connected dc/ac converters. The proposed DPC strategy employs a nonlinear sliding mode control (SMC) scheme to directly calculate the required converters control voltage so as to eliminate the instantaneous errors of active and reactive powers without involving any rotating coordinate transformations. Meanwhile, there are no extra current control loops involved, which simplifies the system design and enhances the transient performance. Constant converter switching frequency is achieved by using space vector modulation, which eases the design of the ac harmonic filter. Simulation and experimental results are provided and compared with those of the classic voltage-oriented vector control (VC) and conventional lookup table (LUT) DPC strategies. The proposed SMC-DPC is capable of providing enhanced transient performance similar to that of the LUT-DPC, and keeps the steady-state harmonic spectra at the same level as those of the VC scheme. The robustness of the proposed DPC to line inductance variations is also inspected during active and reactive power changes.

Improved Voltage-Vector Sequences on Dead-Beat Predictive Direct Power Control of Reversible Three-Phase Grid-Connected Voltage-Source Converters

This paper presents a dead-beat predictive direct power control (DPC) strategy and its improved voltage-vector sequences for reversible three-phase grid-connected voltage-source converters (VSCs). The instantaneous variation rates of active and reactive powers, by applying each converter voltage vector in 12 different sectors, are deduced and analyzed. Based on the power variation rates, it is found that the values of the predicted duration times for the two conventional active converter voltage vectors are less than zero when the grid-connected VSC operates as either a rectifier or an inverter. In order to solve this issue, two new alternative vector sequences are proposed and compared. Experimental results on a 1.5 kW reversible grid-connected VSC system are presented to validate the feasibility of the proposed voltage-vector sequences on the dead-beat predictive DPC strategy.

Investigation on Switching Patterns of Direct Power Control Strategies for Grid-Connected DC–AC Converters Based on Power Variation Rates

This paper investigates the switching patterns of direct power control (DPC) strategies for three-phase grid-connected voltage-sourced dc-ac converters. The instantaneous variation rates of active and reactive powers by applying each converter voltage vector in 12 different sectors are deduced. In terms of hysteresis look-up-table DPC, based on the power variation rates, modified switching tables are presented to alleviate the active power pulsations, and an asymmetrical hysteresis controller is proposed for active power control to eliminate its steady-state errors. As to predictive DPC, according to the power variation rates, the cause is identified to that the values of the predicted duration times for the auxiliary active voltage vector become negative. Consequently, to solve this problem, two new alternative vector sequences are proposed and compared. A simple compensation method is further added to the basic control scheme of predictive DPC to deal with grid voltage unbalances. Experimental results on a 1.5-kVA grid-connected dc-ac converter are presented to validate the feasibility of the improved switching patterns and the unbalanced compensation method on the DPC strategies for dc-ac converters.

Improved Nearest-Level Modulation for a Modular Multilevel Converter With a Lower Submodule Number

This letter proposes an improved nearest-level modulation (NLM) method to enhance the quality of output voltage of a modular multilevel converter (MMC) with the low amount of submodules, as well as restrain the voltage fluctuation of the submodules. By adding a small offset, which is alternating at the double fundamental frequency to the reference signals, a small phase shift of the step-changing moment between upper and lower arms voltage emerges. As a result, an odd-level difference between the output voltage of lower arm and that of upper arm occurs, which can increase the level number of output voltage from N + 1 to 2N + 1, where N is the number of submodules per arm. With the proposed method, the total harmonic distortion (THD) of the output voltage is mitigated without increasing the switching frequency of IGBTs or changing the average voltage of submodules capacitor. In addition, within a special range of power factor angle, the circulating current can be reduced by choosing the proper phase between the fundamental and the double fundamental frequency. Simulation and experimental results verify the effectiveness and validity of the proposed NLM scheme.

Electrical machines and power‐electronic systems for high‐power wind energy generation applications: Part II – power electronics and control systems

Purpose n n n n– Power‐electronic systems have been playing a significant role in the integration of large‐scale wind turbines into power systems due to the fact that during the past three decades power‐electronic technology has experienced a dramatic evolution. This second part of the paper aims to focus on a comprehensive survey of power converters and their associated control systems for high‐power wind energy generation applications. n n n n nDesign/methodology/approach n n n n– Advanced control strategies, i.e. field‐oriented vector control and direct power control, are initially reviewed for wind‐turbine driven doubly fed induction generator (DFIG) systems. Various topologies of power converters, comprising back‐to‐back (BTB) connected two‐ and multi‐level voltage source converters (VSCs), BTB current source converters (CSCs) and matrix converters, are identified for high‐power wind‐turbine driven PMSG systems, with their respective features and challenges outlined. Finally, several control issues, viz., basic control targets, active damping control and sensorless control schemes, are elaborated for the machine‐ and grid‐side converters of PMSG wind generation systems. n n n n nFindings n n n n– For high‐power PMSG‐based wind turbines ranging from 3 MW to 5 MW, parallel‐connected 2‐level LV BTB VSCs are the most cost‐effective converter topology with mature commercial products, particularly for dual 3‐phase stator‐winding PMSG generation systems. For higher‐capacity wind‐turbine driven PMSGs rated from 5 MW to 10 MW, medium voltage multi‐level converters, such as 5‐level regenerative CHB, 3‐ and 4‐level FC BTB VSC, and 3‐level BTB VSC, are preferred. Among them, 3‐level BTB NPC topology is the favorite with well‐proven technology and industrial applications, which can also be extensively applicable with open‐end winding and dual stator‐winding PMSGs so as to create even higher voltage/power wind generation systems. Sensorless control algorithms based on fundamental voltages/currents are suggested to be employed in the basic VC/DPC schemes for enhancing the robustness in the entire PMSG‐based wind power generation system, due to that the problems related with electromagnetic interferences in the position signals and the failures in the mechanical encoders can be avoided. n n n n nOriginality/value n n n n– This second part of the paper for the first time systematically reviews the latest state of arts with regard to power converters and their associated advanced control strategies for high‐power wind energy generation applications. It summarizes a variety of converter topologies with pros and cons highlighted for different power ratings of wind turbines.

Electrical machines and power‐electronic systems for high‐power wind energy generation applications: Part I – market penetration, current technology and advanced machine systems

Purpose n n n n– Wind energy has matured to a level of development at which it is ready to become a generally accepted power generation technology. The aim of this paper is to provide a brief review of the state of the art in the area of electrical machines and power‐electronic systems for high‐power wind energy generation applications. As the first part of this paper, latest market penetration, current technology and advanced electrical machines are addressed. n n n n nDesign/methodology/approach n n n n– After a short description of the latest market penetration of wind turbines with various topologies globally by the end of 2010 is provided, current wind power technology, including a variety of fixed‐ and variable‐speed (in particular with doubly‐fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG) supplied with partial‐ and full‐power converters, respectively) wind power generation systems, and modern grid codes, is presented. Finally, four advanced electrical‐machine systems, viz., brushless DFIG, open winding PMSG, dual/multi 3‐phase stator‐winding PMSG and magnetic‐gear outer‐rotor PMSG, are identified with their respective merits and challenges for future high‐power wind energy applications. n n n n nFindings n n n n– For the time being, the gear‐drive DFIG‐based wind turbine is significantly dominating the markets despite its defect caused by mechanical gears, slip rings and brush sets. Meanwhile, direct‐drive synchronous generator, especially utilizing permanent magnets on its rotor, supplied with a full‐capacity power converter has become a more effective solution, particularly in high‐power offshore wind farm applications. n n n n nOriginality/value n n n n– This first part of the paper reviews the latest market penetration of wind turbines with a variety of mature topologies, by summarizing their advantages and disadvantages. Four advanced electrical‐machine systems are selected and identified by distinguishing their respective merits and challenges for future high‐power wind energy applications.Purpose – Power‐electronic systems have been playing a significant role in the integration of large‐scale wind turbines into power systems due to the fact that during the past three decades power‐electronic technology has experienced a dramatic evolution. This second part of the paper aims to focus on a comprehensive survey of power converters and their associated control systems for high‐power wind energy generation applications.Design/methodology/approach – Advanced control strategies, i.e. field‐oriented vector control and direct power control, are initially reviewed for wind‐turbine driven doubly fed induction generator (DFIG) systems. Various topologies of power converters, comprising back‐to‐back (BTB) connected two‐ and multi‐level voltage source converters (VSCs), BTB current source converters (CSCs) and matrix converters, are identified for high‐power wind‐turbine driven PMSG systems, with their respective features and challenges outlined. Finally, several control issues, viz., basic control targ...

Sliding mode current control of grid-connected voltage source converter

This paper presents a new sliding mode current control (SMCC) of three-phase grid-connected voltage source converters (VSCs). The proposed SMCC strategy employs a nonlinear sliding mode control scheme to directly calculate the required converter control voltage so as to eliminate the line current errors in the stationary reference frame without involving any synchronous coordinate transformation. Constant converter switching frequency is achieved by using space vector modulation (SVM) which eases the design of the ac harmonic filter. Simulation results on a 3kVA grid-connected VSC system are provided to validate the feasibility of the proposed SMCC strategy during normal and faulted grid voltage conditions.

Imbalance Mechanism and Balanced Control of Capacitor Voltage for a Hybrid Modular Multilevel Converter

Due to different charging and discharging characteristics of full-bridge submodules and half-bridge submodules in hybrid modular multilevel converters (MMC), capacitor voltage imbalance will occur under boosted ac voltage or reduced dc voltage conditions. To address this issue, the mechanism of capacitor voltage imbalance is carefully studied, with three main factors—modulation index, power angle, and hybridization ratio—summarized and their effect on capacitor voltage imbalance analyzed. Further, a control strategy based on fundamental frequency reactive circulating current injection is proposed to keep the capacitor voltage balanced in the hybrid MMC. The amplitude and phase angle of the injected circulating current are calculated and their influence on the energy fluctuation in the submodules’ capacitors and the semiconductors’ current stress is explored. Experimental results under boosted ac voltage and reduced dc voltage conditions demonstrate the feasibility and validity of the proposed scheme.

Fundamental-Frequency Reactive Circulating Current Injection for Capacitor Voltage Balancing in Hybrid-MMC HVDC Systems During Riding Through PTG Faults

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results.