A Novel SPWM Method with Voltage Balancing Capability for Multilevel Rectifier/Inverter Systems

Abstract

This paper presents a novel SPWM control method with voltage balancing capability for the diodeclamped multilevel rectifier/inverter system. A complete analysis of the voltage balance theory based on a 5-level back-to-back converter is given. The voltage balancing effects of the triplen harmonic offset voltage added to all three phase voltages are discussed. The proposed control utilizes the offset voltage to control the average currents flowing into and out form the inner junction. The voltage balancing was achieved by selecting proper offset voltages for both sides. A 5-level experimental system has built up and used to prove the theory.I. IWROD UFDNIn recent years, multilevel converters have begun to play a more and more important role in high-voltage, high-power applications. Compared with the traditional two-level voltage converter, the primary advantage of multilevel converters is their smaller output voltage step, which results in higher power quality, lower harmonic components, higher voltage capability, better electromagnetic compatibility, and lowerThe multilevel converter synthesizes the staircase output voltage which approaches the sinusoidal waveform with minimum harmonics. In order to satisfy the same harmonics requirement, the frequency needed by the multilevel converter is much lower than the conventional converter. Therefore the multilevel converter can achieve higher efficiency. The multilevel converters also have lower dV'dt. It has been found recently that the high dV/dt in the high-power PWM converter can induce corona discharge and lead to bearing or winding insulation failure.The multiple dc bus capacitors provide the capability of output multilevel voltage waveform. However, it also requires additional circuits and special control methods to keep the capacitor voltages well balanced. Dpt. (RrIhEgnhng / Stt4ksly Ei fshg, M1824 For the three-level diode-clamped converter, because there is only one additional voltage junction, the neutral point, and the symmetry of the upper and lower capacitors, it also has self voltage balancing potential. However the neutral point has a low frequency ripple at three times of the fundamental frequency. Some new researches have addressed on eliminating or attenuating the low frequency ripple [3-6].However, the dc bus voltage balancing for diodeclamped multilevel converters with the number of levels greater than three is more complicate. The multilevel converter capacitors tend to overcharge or completely discharge. Eventually, the converter converges to a three-level converter. Corzine proposed a dc-dc front end to regulate the center capacitor voltage of a four-level converter [7]. PWM hysteresis control method has been proposed to regulate the dc bus of a five-level rectifier [8]. Then the multi-band hysteresis comparators control strategy has been extended to a five-level back-to-back system. Although the technique is simple, the characteristics are not sufficient as a motor drive system. Thus an improved control strategy using the space vector PWM has al...

Reduction of total harmonic distortion of three-phase inverter using alternate switching strategy

“…This proposed research is mainly focused on open loop which is VSI. Open loop consists of PWM techniques such as sinusoidal pulse width modulation (SPWM) [10], space vector pulse width modulation (SVPWM) [11] and third harmonic pulse width modulation (THPWM) [12]. The modulation methods developed to control the multilevel inverters that are based on multi-carrier orders with PWM.…”