Space-vector pulsewidth modulation for a seven-level cascaded H-bridge inverter with the control of DC-link voltages

Abstract

Abstract.A control strategy of DC-link voltages for a seven-level cascaded H-bridge inverter is proposed in this paper. The DC-link voltage balancing is accomplished by an appropriate selection of H-bridges and control of their duty cycles in space-vector modulation (SVM) algorithm. The proposed SVM method allows to maintain the same voltage level on all inverter capacitors. Regardless of the balancing function, the SVM strategy makes it possible to generate the output voltage vector properly also in the case where the DC-link voltages are not balanced. The results of simulation and experimental investigations are presented in the paper.Key words: cascaded H-bridge inverter, DC-link voltage control, space vector modulation. Space-vector pulsewidth modulation for a seven-level cascaded H-bridge inverter with the control of DC-link voltagesA. LEWICKI * and M. MORAWIEC Faculty of Electrical and Control Engineering, Gdansk University of Technology, 11/12 Narutowicza St., 80-233 Gdansk, Poland ination pulse width modulation (SHE-PWM) [7]. The spacevector pulse width modulation (SVM) strategies for more than three-level inverters are more complex due to the large amount of space vectors [8,9]. Usually, the SVM strategies for CHB converters utilize three space vectors nearest to the sector in which the reference voltage vector is located [10,11]. Some of proposed SVM strategies concentrate on generating the output voltage vector in the CHB inverters, wherein the DC-link voltages are not equalized [12].The changes in DC-link voltages of CHB inverter are the results of current flow through the DC-link capacitors. The DC-link voltages can be balanced in several ways. The most popular methods are based on managing the H-bridges used to compose the inverter output voltage [7], on modifying the individual reference voltages as well as on changing modulation indexes of the H-bridges [8,13,14]. The DC-link voltages are usually balanced using controllers [8,15], but it is also possible to rotate the carrier signals for every modulation cycle between H-bridges of CHB converter in carrier-based modulation strategies [16].The CHB inverters are mainly applied in medium and highpower applications, where it is particularly important to convert electrical energy with maximum efficiency. In CHB inverters, it is possible to assume maximum value of the modulation indexes for selected H-bridges [17]. The output voltage is modulated in one H-bridge only in each of the inverter phases. The other H-bridges are positively or negatively connected or are bypassed [18]. Since their transistors do not switch, they do not generate commutation loses.In this paper, the DC-link balancing method and SVM strategy for seven-level CHB converters is proposed. Because the SVM strategies for more than three-level inverters are relatively complex [8,9], the topology of the seven-level CHB converter is analyzed as a set of 3 three-level CHB converters connected in series. Each of them is composed using three H-bridges (one H-bridge in each phase of the invert...

Structure and the space vector modulation for a medium‐voltage power‐electronic‐transformer based on two seven‐level cascade H‐bridge inverters

“…As a result, the DC-link voltages within each phase of the CHB inverter are equalised. The choice of H-bridges used to construct the threelevel inverters is realised at the beginning of each control period [17,53].…”

“…The research on PET control methods refers to the optimisation of the work and control methods of the DABs [15] as well as faulttolerant control [16]. The vast majority of papers show the methods for balancing the DC-link voltages [12,[17][18][19][20][21].…”

“…The DC-link voltages within each phase are equalised by an appropriate selection of negatively or positively connected H-bridges. The choice of H-bridges used to generate the output voltage depends on desired changes in DC-link voltages and the direction of the phase current [32] or power [17,52,53]. This is achieved by carrying out an ordering (via a sorting algorithm) of the capacitor voltages during each control period [17,53].…”

“…The DC-link voltages are equalised through the control of DABs and through the SVPWM algorithm applied to both CHB inverters. The applied SVPWM method is based on the solution presented in [17]. In this solution, the ML CHB inverter is considered as a set of three-level inverters connected in series and controlled using the same algorithm.…”

“…The solution presented in [17] utilises the classic method for selecting H‐bridges to generate the inverter output voltage, where the capacitor voltages are sorted at the beginning of each control period [53]. As a result, in the initial stages of the modulation algorithm and for some positions of output voltage vector, some H‐bridges are indicated as a first to be balanced and work with zero duty cycle (they remain bypassed).…”