An optimized SVPWM switching strategy for three-level NPC VSI and a novel switching strategy for three-level two-quadrant chopper to stabilize the voltage of capacitors

Voltage source inverter (VSI) can produce single and three-phase (3P) AC voltages from a constant or variable DC voltage. There are many ways to control the VSI output voltage. Each control way produces some harmonics at the VSI output voltage. The space vector pulse width modulation (SVPWM) may be more effective than other modulation methods, e.g. harmonic injection, phase shifting, multi career pulse width modulation, in decreasing the low order harmonics (LOH). Different switching strategies (SSs) of power electronic devices in SVPWM have some specific advantages and disadvantages with regard to one another. In this paper, a comparative study between different SVPWM SSs is carried out by considering some objective functions such as total harmonic distortion (THD), power and switching losses, the ratio of the harmonic components to the fundamental component, distortion factor (DF). It is also shown that by selecting an optimized and appropriate SS for SVPWM, the harmonic orders, which are the multiples of the frequency index (FI), are eliminated. Then, to investigate the impact of variations of the capacitors voltage and switching frequency on power quality criteria, the most appropriate and optimized SS is applied to a 3P three-level (3L) neutral-point-clamped (NPC) VSI to supply a 3P load.     

燃气轮发电机三电平变频起动装置的研制

提出了一种用于燃气轮发电机启动的二极管中点箝位型(NPC)三电平静止变频器(SFC),并对三电平SFC进行了仿真和试验研究.结果表明:三电平SFC应用一种准双极性空间矢量脉宽调制(SVPWM)技术解决了三电平输出电压突变的问题,并采用基于零序分量叠加的控制技术有效抑制了中点电压波动,解决了电流型SFC起动装置存在的严重谐波污染、起动转矩小和附加机械振动等问题.此外,对该装置功率器件的水冷散热器进行了合理设计.     

Multiphase space vector pulse width modulation

Pole-phase modulation adjusts the pole-phase ratio of an induction machine and requires a multileg, multiphase inverter. This paper analyzes an n-leg, n-phase inverter, and presents techniques for n-phase space vector pulse width modulation (SVPWM). In particular, nine-phase SVPWM is developed and implemented on a nine winding induction machine. The nine-phase SVPWM is compared to nine phase sine-triangle PWM in terms of DC bus utilization.     

光伏模拟器用死区消除PWM整流器运行方式

针对光伏模拟器用单相脉宽调制（PWM）整流器提出电压平方外环电流内环的控制策略,同时揭示直流侧存在2倍基波频率的纹波电压以及此纹波电压对网侧电流的影响。PWM整流器采用的基于H桥死区消除SPWM,在网侧电流非过零区域,各桥臂上下两开关管中,必定存在其中一个开关管驱动信号为低电平,而在电流过零处推导出存在的天然死区时间。由微控制器（MCU）与现场可编程门阵列（FPGA）相结合的控制系统可实现提出的运行方式。实验结果验证了理论推导的正确性以及提出运行方式的有效性。     

A survey of shaft voltage reduction strategies for induction generators in wind energy applications

Common mode voltage generated by PWM inverters and the parasitic couplings of the machine structure in high frequencies create a model for the system which leads to an induced voltage on the shaft. Shaft voltage became a dominant side effect of power electronic converters since they are widely used in wind turbine applications to prepare desirable frequency and suitable control on active and reactive power. This voltage is known as the main cause of many unwanted problems such as leakage current, ball-bearings damages and reduction of generator's life time. In this paper, pulse width modulation strategies have been presented for two-level and three-level back-to-back AC–DC–AC converters in order to reduce or eliminate common mode voltage of these converters for reduction or elimination of shaft voltage in squirrel cage and doubly fed induction generators. Applying these techniques lead to complete elimination of shaft voltage for squirrel cage Induction generator with back to back AC/DC/AC converter and a 66 percent reduction of the shaft voltage generation in the Doubly Fed Induction Generator. Simulation results and mathematical analysis have been presented to investigate proposed techniques.     

基于DSVPWM的PMSG并联型GS-NSC研究

为提高九开关变换器直流侧电压利用率,减少开关损耗,采用一种不连续空间矢量脉宽调制（DSVPWM）,同时将九开关变换器用于直驱式永磁同步风力发电机（PMSG）网侧构成并联型网侧九开关变换器（GS-NSC）。在对并联型GS-NSC控制策略、故障穿越方案进行理论分析的基础上,建立PMSG并联型GS-NSC仿真模型,设计多种电网电压故障工况对其进行仿真研究。结果表明,在电网电压正常工况下,并联型GS-NSC可维持电网电流的正弦波特性。在电网电压跌落工况下,并联型GS-NSC可向电网注入无功电流辅助电网电压的恢复,并通过提升并网电流幅值减少卸荷电路的功率损耗,降低PMSG散热负担。在电网电压骤升工况下,并联型GS-NSC可动态分配直流母线电压,避免因直流侧过压而导致PMSG退出运行。     

一种基于PSCAD/EMTDC仿真平台的SVPWM逆变器控制算法

空间矢量脉宽调制法(SVPWM)比传统的正弦脉宽调制法(SPWM)具有较高的直流电压利用率以及较低的输出波形失真度。介绍了三相逆变器的SVPWM控制算法,建立了基于PSCAD/EMTDC仿真平台的三相逆变器模型,并给出了基于PSCAD/EMTDC仿真平台的SVPWM优化控制算法。通过建立一个控制系统来进行仿真和研究,给出了仿真线电压及相电压波形。仿真结果充分显示了SVPWM技术应用于大功率场合的优点。     

Control of a grid-connected direct-drive wind energy conversion system

This paper investigates the current control for a grid-connected direct-drive wind energy conversion system (DDWECS) with a permanent magnet synchronous generator (PMSG), which utilizes a back-to-back pulse width modulation (PWM) converter. For the machine-side, the controller adopts a current vector control method based on the rotating reference frame (RRF) and the maximum power extraction (MPE) is realised through the tip speed ratio (TSR) method. For the grid-side, a novel controller is proposed for the first time to be successfully used for the DDWECS, which combining a proportional complex integral (PCI) current inner loop based on stationary reference frame (SRF) for regulating the grid-side current with a dc voltage outer loop for stabilizing the dc bus voltage and compare with the proportional resonant (PR) controller. A system simulation model is established by using the Matlab/Simulink to simulate the performance of the DDWECS and a prototype system has been build and tested to verify the validity of the developed control methods for both machine-side and grid-side and the excellent performance of the DDWECS.     

一种能够抑制共模电流的单级单相Buck-Boost光伏逆变器

该文给出一种能抑制共模电流的单级单相Buck-Boost光伏逆变器,该逆变器是通过将H桥逆变器与Buck-Boost斩波电路级联以及对相关的元件进行复用并对电路进行简化得到的,可同时实现升降压以适应宽输入范围的直流侧电压,无电解电容应用且具备有效抑制漏电流的能力,保证系统的可靠性和安全性,非常适用于中小功率光伏系统。此外,利用一种基于调制波重构的非线性调制策略,能显著降低直流侧储能电感大小,有效提高功率密度,实现均衡升降压。详述其工作原理,通过共模分析验证其能对阴阳两极漏电流实现有效抑制以及新型调制策略下可实现均衡升降压控制。在理论分析基础上,完成逆变器并网控制的仿真和实验,结果与理论分析相契合。     

Performance improvement of a slip energy recovery drive system by a voltage-controlled technique

This paper introduces the performance improvement of a slip energy recovery drive system for the speed control of a wound rotor induction motor by a voltage-controlled technique. The slip energy occurred in the rotor circuit is transferred back to ac mains supply through a reactor instead of a step up transformer. The objective of the voltage-controlled technique is to increase power factor of the system and to reduce low order harmonics of the input line current. The drive system is designed and implemented using a voltage source inverter in conjunction with a boost chopper for DC link voltage, instead of a conventional drive using a 6 pulse converter or a Scherbius system. The slip power is recovered by the help of a voltage source inverter (VSI) based on a space vector pulse width modulation (SVPWM) technique. In order to keep the speed of the wound rotor induction motor constant over a certain range of operating conditions, the servo state feedback controller designed by a linear quadratic regulator (LQR) is also introduced in this paper. The overall control system is implemented on DSP, DS1104’TMS320F240 controller board. The performance improvement of the proposed system is tested in comparison with the conventional Scherbius system and the modified conventional Scherbius system by a 12 pulse converter in conjunction with a chopper at steady state and at dynamic conditions. A 220 W wound motor is employed for testing. It is found that the motor speed can be controlled to be constant in the operating range of 450–1200 rpm at no load and full load. It is also found that the efficiency of the proposed system is remarkably increased since the harmonics of the input ac line current is reduced while the ac line input power factor is increased.     

A novel sensorless current shaping control approach for SVPWM inverter with voltage disturbance rejection in a dc grid–based wind power generation system

A novel sensorless current shaping (CS) control strategy is proposed to avail better power quality (PQ) of a dc grid–based wind power generation system (WPGS) used on a poultry farm by generating an appropriate reference current for space vector pulse width modulation (SVPWM) inverter. The proposed CS strategy also offers adequate control for parallel operation of multiple generators and inverter applications, without requiring voltage and frequency synchronization. Further, to control the poultry farm–based WPGS, a two‐stage control loop is implemented such as energy flow control loop (EFCL) and harmonic control loop (HCL). The first loop is used to regulate the power flow, and the second loop is used to compensate harmonics. A mathematical current decomposition technique is suggested for an appropriate resistance emulation to realize a better power flow, higher harmonic rejection, and better inverter operation. In this planned approach for attaining constant wind speed, an electric ventilation fan in the poultry farm is used. A combined hybrid dc and ac grid approaches are suggested for facilitating variable load integration in a poultry farm–based microgrid system. Moreover, for achieving better power management during the islanded mode of operation, the battery energy storage (BES) device is integrated with the dc grid through a bidirectional converter. The proposed WPGS design and control approach has been simulated through MATLAB/Simulink software under various test conditions, to demonstrate the operational capability, to achieve better PQ, and to increase the flexibility and reliability in the microgrid operation.     

三相联网逆变器电流控制策略研究

通过MATLAB/SIMULINK建立了联网逆变器的3种控制策略(滞环控制、基于SPWM调制的电流控制和基于SVPWM调制的电流控制).通过对3种控制策略的4个方面(稳态响应、电网电压谐波影响、动态响应和直流电压)来仿真对比分析3种控制策略.仿真结果表明,基于SVPWM调制的电流控制在3相联网逆变器中应用优于滞环控制和基于SPWM调制的电流控制.     

不平衡电网电压下T型三电平光伏并网逆变器的有限集模型预测控制

为实现电网电压不平衡时对T型三电平光伏并网系统输出功率和电流质量的控制,以达到入网功率平稳或电流正弦为控制目标,结合光伏阵列输出功率前馈,在两相静止坐标系下提出一种直流母线电压外环PI控制、并网电流内环有限集模型预测控制的控制策略,并在电压外环中引入2倍频陷波器以获得平滑的入网功率参考值。仿真结果表明:当电网电压不对称时,采用所提控制策略能够实现对入网有功、无功功率2倍频脉动及负序电流的分别抑制或协调控制,且并网电流谐波畸变小、入网电能质量高,同时实现T型三电平逆变器的中点电位平衡。     

A multi-function grid connected PV system with three level NPC inverter and voltage oriented control

In this paper a grid connected photovoltaic (PV) system is presented. The grid integration of the PV system is carried out via a three phase three level neutral point clamped (NPC) inverter. To control the inverter a modified version of voltage oriented control (VOC) method and the space vector pulse width modulation (SVPWM) technique have been applied. With the proposed modification the PV system operates as a shunt active power filter (SAPF), a reactive power compensator, and a load’s current balancer simultaneously. In this way the PV system operates more efficiently compared to the conventional PV systems and offers ancillary services to electric power system. The effectiveness of the proposed control scheme is established through simulation results with Matlab/Simulink in steady state and transient response of the electric power distribution system.     

Design and performance analysis of the three-level isolated DC-DC converter with the nanocyrstalline core transformer

In this study, an isolated three-level DC-DC converter is proposed for high power and high conversion ratio applications such as fuel cells. The proposed system consists of a single phase three-level inverter, a medium frequency transformer and a diode rectifier unit. In the proposed system, a DC supply voltage is converted to a medium frequency AC voltage via a three-level inverter instead of the conventional two-level inverter. Since the three-level inverter generates an AC waveform with multiple steps, lower voltage harmonics and lower EMI levels than conventional two-level inverter are achieved. Thus, the three-level inverter provides higher efficiency value. The medium frequency transformer enables high voltage conversion ratio and provides galvanic isolation as well. The output voltage of the medium frequency transformer is converted to the DC voltage and thus the DC-DC conversion is achieved. According to simulation and experimental results, it is seen that the proposed DC-DC converter structure provides higher power density and higher efficiency values than conventional system.     

1MW直驱风力发电系统建模与仿真研究

建立了1 MW直驱风力发电系统电磁暂态仿真模型,在PSCAD/EMTDC环境下采用电压空间矢量脉冲宽度调制技术(SVPWM)真实地反映变流器的动态开关过程和谐波特性,实现了直驱风力发电系统的机侧最大风能跟踪以及永磁发电机定子电流的解耦控制和网侧直流母线电压稳定的控制。文章还对网侧变流器的软件锁相环(SPLL)进行了优化设计,使之能够快速、准确地跟踪电网基波正序电压,有助于改善网侧变流器在电网不对称故障期间的动态控制性能。     

New management structure of active and reactive power of a large wind farm based on multilevel converter

This paper proposes a system of supervision and operation of a new structure wherein a large wind farm is connected to an electrical grid. The farm is managed in such a manner that it can produce the power needed by the grid system. The supervision algorithm is used to distribute the active and reactive power references to the wind turbines proportionally. Based on the aerodynamic power and wind speed of each turbine, the active and reactive power references are produced individually. By using the vector field oriented control, each doubly fed induction generator is controlled through the rotor, which is connected to the two-level pulse width modulation converter. The close loop control is used to provide a constant DC voltage using a five-level neutral point clamped converter. The five-level neutral point clamped converter allows also the adaptation of the voltage level to the electrical grid with better resolution waveform. The analysis of the simulation results shows the effectiveness of the proposed system.     

采煤机变频器的软件设计与实验研究

为提高采煤机利用电能的效率,针对SPWM脉宽调制的变频器直流电压利用率低、输出电流的谐波含量高的问题,以数字信号处理器TMS320F2812为核心,CPLD为辅助控制电路,DSP完成算法设计,CPLD实现逻辑控制与键盘显示,最大发挥各自的性能.采用软件法实现空间矢量脉宽的调制.实验结果表明：该方法比硬件法谐波含量相对减少,电机电流的谐波小,实现了频率调节范围为1～50 Hz,低的电机高频噪声,高的电压利用率.     

DSP based inverter control for alternate energy systems

This paper presents a DSP based algorithm to control inverters used in interfacing alternate energy systems with the electric utility. Since a constant and ripple free dc bus voltage is not ensured at the output of alternate energy sources, the main aim of the proposed algorithm is to make the output of the inverter immune to the fluctuations in the dc input voltage. In this paper a modified space vector pulse width modulation (SVPWM) technique is proposed which will maintain the quality of the ac output of the inverter, regardless of the ripple present at the inverter input. The principle is explained qualitatively and extensive experiments have been carried out to verify and validate the proposed algorithm. A 16-bit fixed-point C2000 family DSP from Texas Instruments was used as the controller to implement the proposed control algorithm.     

用于海上风电场直流输电的新型变换器

针对永磁直驱风力发电机变流技术的特点,并根据柔性直流输电系统的电压和功率控制要求,提出了一种新型的直流变换器。该变流器采用三电平拓扑结构、内环电流峰值控制,以及适用于三电平Boost变换器电流峰值控制的双梯形波补偿控制方法,经Matlab／Simulink仿真,研究结果证明该变流器具有功率开关电压应力小、电抗器电流脉动小,以及运行可靠、动态响应性能好等优点,适用于海上风电柔性直流输电等大功率、高电压场合。