The Research On Multilevel Diode-clamped Inverter Capacitor Voltage Balancing SVM Strategy And Its Application

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.