Abstract.
In this paper a novel modular digital controlalgorithm for multi level inverters is introduced and discussed.This algorithm is essentially based on the duty cycle expressionand allows a great simplification, because of its modularity andits simple implementation. The algorithm is particularly suitablefor the Multi Point Clamped (MPC) inverters with no constrainton the number of the voltage levels and results faster than themost recent solutions proposed in literature.The modulation techniques considered in the paper are the suboscillation PWM and, above all, the Space Vector Modulation(SVM), but the proposed algorithm may be employed for themain carrier based PWM control techniques including the “Zerosequence injection”. Simulation of a MPC inverters, controlledwith the proposed algorithm, have been made with the help ofthe MATLAB-SIMULINK® package. For these simulations amathematical model already known in literature, but adapted tobe used within the MATLAB-SIMULINK® environment, hasbeen used. The simulation results show the effectiveness of theMPC model formulation and of the proposed control algorithm.
Keywords: Multilevel inverter; Space VectorModulation; Digital modular control.
1. Introduction.
The recent progresses in the semiconductortechnology made available fast commutating electronicspower devices to be used with high voltages and currents,so extending the application field of Voltage SourceInverters (VSI). Actually, it is not still possible to employtraditional VSI in the high power range due tocommutation difficulties and reverse recovery highvoltages. Multilevel power converters are an interestingemerging technology for medium and high powerapplications including the fields of Renewable EnergySources and power quality issues as active filtering andreactive power compensators (static VAR compensators).The first multi level converter structures were the Hbridge connected inverters with several different DCsources, whose main drawback is the requirement of agalvanic insulation among the DC sources. Towards theeighties Akagi Nabae et alt. introduced the Neutral pointclamped (NPC) inverter [1]. This structure is able torealise an increased number of commutation states withthe possibility to achieve output voltage higher than thetraditional VSI, lower dv/dt, lower harmonic distortionand a reduced average switching frequency with aconsequent reduction of the switching losses. NPCinverters have also been generalised introducing thestructure of Multi Point Clamped Inverters (MPC) with ahigher voltage level numbers than the NPC.
Otherimportant advantage of the MPC inverters is the uniformvoltage sharing of the total DC Link voltage among thepower devices of each leg. This correct voltage sharing isnot fully assured in the traditional VSI inverter topologieswith more devices connected in series. Multilevelinverters have been extensively investigated in literatureshowing their main advantages and drawbacks in thevarious application fields. [2]-[6].In this paper, a novel digital modular PWM controlalgorithm, suitable for Multi Point Clamped inverters, isintroduced and compared with the most recent solutionproposed in literature [7], showing its greater flexibilityand its possible use without limitation on the number ofvoltage levels and for all the main carrier based PWMtechniques. This control algorithm has been verified andvalidated by means of simulations developed within theMATLAB-SIMULINK® programming environment. Forthe simulations, the mathematical model of a MPCinverter has been used, reformulating it into a particularlyadvantageous form suitable for the use within theMATLAB-SIMULINK® package. The inherentinstability of the DC Link Capacitor voltages and itscorrection have been considered. A particular emphasis isgiven to the Space Vector Modulation Control.


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