摘要
在大功率电力电子装置的研究和应用领域,多电平变换器因其比两电平变换器具有更好的输出电压波形和可获取更高的电压等级等突出优点而广受关注,已成为一种代表性的解决方案。然而目前并没有一种完美的多电平变换器拓扑,现有的拓扑在实际应用中均存在一定的局限性。二极管钳位型逆变器既不需要多个独立的直流电源,也不需要笨重的电容及其预充电系统,同时还具有开关控制易于实现、保护电路简单等优点。对这一拓扑的应用造成阻碍的是其直流侧分压电容在逆变器运行时会发生电压偏移,使得逆变器性能下降甚至不能正常工作。本文的工作就致力于从调制方案这一硬件结构最简单同时也最节省系统成本的途径来研究解决多电平二极管钳位逆变器的直流电容电压偏移问题。
论文分析了传统SVM算法在多电平二极管钳位逆变器中实现的原理和步骤,指出了传统SVM算法需要进行大量三角函数运算或查表操作、因而运算效率低的不足。通过引入基于Kohonen竞争性神经网络的分类算法,提出了一种改进的多电平二极管钳位逆变器SVM算法。改进的算法并不需要对神经网络进行训练,在整个实现过程中不再需要任何三角函数计算或查表操作,而只需要进行简单的四则运算,所节省的运算时间可用于完成其他耗时的工作如实现电容电压均压等。对三电平和五电平的二极管钳位逆变器进行时域仿真,结果验证了文中数学分析的正确性和所提算法的可行性,同时表明改进的算法是一种对多电平二极管钳位逆变器通用的算法,将它应用于不同电平数的逆变器时不需要进行任何修改。
为了能够利用调制算法消除多电平二极管钳位逆变器直流电容电压的偏移,论文从理论上对这一现象产生的原因进行了深入的分析。首先研究了三电平二极管钳位逆变器的调制矢量的特点,通过具体的图例分析了四类调制矢量对中点电位的影响,明确了三电平二极管钳位逆变器能够利用小矢量调节电容的电压。其次以五电平二极管钳位逆变器为例分析了电平数超过3时的多电平逆变器的电容电压偏移问题,分析表明这类拓扑无法在全调制比范围内利用小矢量控制电容电压。由于逆变器直流侧电容电流平均值为零时,直流电容电压才能保持平衡,因此推导出在SPWM调制算法下五电平二极管钳位逆变器直流侧电容电流平均值与调制比和交流侧功率因数之间的函数关系。分析结果说明在不采用辅助电路的情况下,传统的SPWM技术不能维持直流电容电压平衡；但SVM方法却有可能利用冗余开关状态、在不设置辅助硬件的前提下实现直流侧电容均压。在分析电容电压偏移的本质原因的基础上,说明了无论采用哪种PWM算法来维持多电平二极管钳位逆变器的电容电压,都存在一个由调制比和交流侧功率因数所限定的稳定运行范围,因而电容电压平衡SVM算法主要适合应用于二极管钳位逆变器进行无功功率交换的场合。
根据多电平二极管钳位逆变器的最小能量特性建立了五电平逆变器的能量函数,研究了通过能量函数来选择恰当的冗余开关状态从而实现电容均压的方法。为了计算在不同开关状态下的能量函数值,推导了逆变器中间支路电流平均值与各个扇区开关状态之间的关系,得到了五电平二极管钳位逆变器的综合电流数学模型。在此基础上建立了多电平二极管钳位逆变器的通用电流模型,从而得到了对任意电平通用的电容电压平衡SVM算法。仿真研究表明基于能量函数的电容电压平衡算法能够在对称负载、不对称负载和畸变负载等稳态条件下使得偏离的电容电压回归到标准值。
基于能量函数的平衡算法计算量非常大,且未考虑对开关频率的优化,因而不可避免地造成逆变器运行时开关频率较大。针对该算法的不足,本文提出了种新的基于有功电流的SVM算法。新算法利用有功电流来判断二极管钳位逆变器交流侧的能量传输方向,通过分析能量传输方向与不同的开关序列对电容电压的影响,从一组预定义的开关序列中选取使得偏离标准值最多的电容电压尽可能回归标准值的开关序列。新算法使逆变器在运行中摆脱了大量的乘法运算,只需几次比较运算就能得出调制结果,大大降低了运算量、提高了调制速度；同时算法对开关器件工作频率进行了优化,使得在一个开关周期内的器件开关次数最少,能有效降低开关频率从而降低开关损耗。大量不同工况下的仿真研究证明了基于有功电流算法的有效性以及它的性能优势。
为了进一步考察电容电压平衡算法的应用效果,本文研究了一个基于五电平二极管钳位逆变器的STATCOM系统。该STATCOM的直流侧电容没有辅助平衡电路,它是完全依赖空间矢量调制算法来控制电容电压的。建立了STATCOM的数学模型,在此模型的基础上分别设计STATCOM交流侧电流控制器、直流端电压控制器和公共连接点电压控制器,再将控制器与电容电压平衡算法相结合,构成了完整的STATCOM控制系统。对STATCOM系统的性能进行了仿真研究,结果表明STATCOM的控制器能有效的控制STATCOM输出的无功功率、直流端电压和公共连接点电压。对基于能量函数和基于有功电流的电容电流平衡算法的性能进行了仿真对比,结果证明两种算法都能有效的在稳态和动态条件下将五电平二极管钳位逆变器的直流电容电压维持在标准值,但本文提出的基于有功电流的算法具有更好的动态性能。
In the field of high power electronics research and application, multilevel converters have drawn wide attention and become a representative solution for their outstanding advantages of better output voltage waveforms and higher voltage rates. However, there is no perfect multilevel converter topology without any practical limitation. The diode-clamped inverter (DCI) possesses some of the desirable features like the easiest switching control and the least complex protection circuit among the multilevel inverters while it doesn't need isolated DC sources and bulk capacitors and their precharge circuits. What hinders DCI's development is the divergence of DC capacitor voltages resulting in poor performance or even collapse of the inverter. This paper is dedicated to solving the capacitor voltage divergence problem of DCI in the most cost efficient way of modulation scheme.
Space vector modulation stands out in the application of multilevel converters because it offers significant flexibility to optimize switching waveforms and it is well suited for implementation on a digital computer. The topology features and function principles of multilevel DCIs are elaborated and the theory and steps of the conventional SVM is analyzed, and then the shortcomings of the conventional SVM are pointed out. To improve the conventional SVM, a classification algorithm based on Kohonen's competitive NN is applied. Although the algorithm is an NN-based one, it does not need a training stage. The proposed SVM switching strategy only needs to carry out simple mathematical operations instead of the time consuming trigonometric functions that are required for the implementation of conventional SVM strategies. Thus, adequate time is saved for other tasks, e.g. achieving DC capacitor voltage balancing. The validity of mathematical analysis and the feasibility of the proposed algorithm are verified by time-domain simulation studies in the MATLAB/SIMULATION environment for both a three-level DCI and a five-level DCI. The simulation studies also verify that the proposed algorithm is a generalized one for an n-level DCI and does not need any modification as the number of levels increases.
To eliminate the capacitor voltage drift of multilevel DCIs via modulation, the cause for the phenomenon is studied in depth. The switching vectors of a three-level DCI are analyzed and classified into four groups, that is, zero vector, small vector, medium vector and large vector. Graphic illustrations demonstrate that the small vectors are able to control the capacitor voltages for a three-level DCI. Behaviors of DC capacitor voltages of an n-level (n>3) DCI is analyzed through an example of a five-level DCI, and a conclusion is drawn that for such a topology, it is impossible to obtain balanced capacitor voltages for the full modulation index region only by use of small vectors. Derivational studies show that average values of capacitor currents are functions of the modulation index and the AC-side power factor when the inverter is modulated by a SPWM strategy. Consequently, traditional SPWM technology is unable to maintain the capacitor voltages without the help of additional balance circuits, while a SVM scheme can possibly achieve the goal by delicately select redundant switching states. Analysis also shows that no PWM strategy can guarantee voltage balance of capacitors of a passive-front-end DCI with more than three levels, under all possible operating conditions. The limit of the operational region which guarantees balanced capacitor voltages is reached primarily due to a large modulation index and/or a high AC-side power factor. Therefore a DCI is more suitable for reactive power compensation.
Based on the forementioned SVM algorithm proposed in the paper, a mathematical basis for the balancing strategy is developed for a five-level DCI. A quadratic cost function, that is associated with the voltage deviations of the DC capacitors, is used to select the best adjacent switching states over each sampling period. To calculate the cost function, relations between the averaged values of the DC-side intermediate branch currents and AC currents for different switching states in six sectors respectively are studied and a comprehensive current model for the five-level DCI is built. A generalized current model for an n-level DCI is then achieved and a voltage balancing SVM scheme based on the cost function is established. Effectiveness of the strategy under balanced, unbalanced and distorted operating conditions of a five-level DCC, based on time-domain simulation studies in the MATLAB/SIMULINK environment, is evaluated.
The quadratic function based SVM scheme has to carry out enormous calculations and it pays little attention to the reduction of switching frequency. To overcome these shortcomings, a new SVM strategies based on active current is proposed in this paper. The new strategy judges the direction of the AC-side active power flow by means of active current, and based on the impact analysis of the active power flow and different switching sequences on the capacitor voltages, selects the best sequence to draw the most diverted capacitor voltage to its normal value. During this process, only comparison operations are needed, so the on-line modulation process is greatly relieved from a heavy calculation burden as no multiplication operations are involved. In addition to the voltage balancing ability, the proposed strategy implements switching frequency optimization via switching sequence arrangement so as to ensure the minimum number of switching transitions. The effectiveness and advantages of the proposed active current based SVM strategy are verified by means of comparison with the cost function based scheme within the stability boundaries. Simulation studies also demonstrate that the proposed strategy is capable of voltage balance under unbalanced or distorted load conditions or other conditions related to the parameters of actual capacitors.
This paper investigates the feasibility of the DC-capacitor voltage balancing strategy for application of a five-level DCI as a STATCOM unit. In comparison with the existing five-level DCI based STATCOM systems, the salient feature of the proposed STATCOM is that the capacitor voltage balancing task is achieved with no requirements for additional power circuitry. A mathematical model of the STATCOM unit is developed. Then, based on the developed model and the SVM balancing strategy, the AC-side current controllers, the DC-bus voltage controller, and the load voltage controller are designed to control reactive power flow, DC-bus voltage and load voltage of the STATCOM respectively. Performance of the designed controllers is verified based on time-domain simulation studies. Effectiveness of the two capacitor voltage balancing strategies based on cost function and active current respectively under both steady-state and transient conditions are investigated and compared. Simulation results show that both balancing strategies effectively maintain the capacitor voltages at the normal value, but the active current based one has better dynamic performance than the other.
引文
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