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Improved Asymmetric Space Vector Modulation for Voltage Source Converters with Low Carrier Ratio

Fig. 1. Single VSC system under study.
Symmetric space vector modulation (ASVM) is a popular pulse-width modulation (PWM) scheme used for voltage source converters (VSCs) operated with low carrier ratio (Rc), which is defined as the ratio between the switching frequency and operating ac frequency. Limited by the switching frequency of current power semiconductor devices, carrier ratio, in high power applications, cannot be high. Especially, for applications involving high speed generator or motor, carrier ratio will be extremely low such as around 9. In many cases, even if the devices can operate at higher switching frequency, it is also desirable to use lower switching frequency, in order to reduce loss and increase power capability. Low carrier ratio results in lower order voltage harmonics. To limit current harmonics, large ac line inductors are generally necessary, which can have cost and size penalty. Therefore, it is very desirable to improve the ASVM algorithm for better harmonic performance, so smaller or lighter passive components can be used to meet the total harmonic distortion (THD) requirement for ac currents.

This paper presents an improved asymmetric space vector modulation (ASVM) for two level voltage source converters (VSCs) when the switching frequency is only 9 times of line frequency. By adding two pulses in each line cycle when the fundamental voltage crosses zero, the total harmonic distortion (THD) of output current can be reduced significantly with very limited penalty. The applications of improved ASVM in a single VSC or two interleaved VSCs systems are shown separately in Fig. 1 and Fig. 2. With optimization, the ac current THD can be reduced to as low as 50% for single VSC and even lower to 20% for interleaved VSCs systems. Such THD reduction has close relationship with modulation index and interleaving angle. In addition, improved ASVM can also reduce the amplitude of circulating current which mainly determined the size of interphase inductors. Finally, the weights of total inductors needed to meet the same THD requirement are compared to demonstrate the benefits of improved ASVM when different PWM schemes are used. The analysis results are verified by experiments on a demo system. The example results are shown in Fig. 3 and Fig. 4. From Fig. 3, the THD of output current is 17.8% (100%) for single VSC system with traditional ASVM. From Fig. 4, the THD of output current is reduced to 5% (28%) for two paralleled VSCs system with 90° interleaving.

Fig. 2. Paralleled VSCs system under study.
Fig. 3. VAN, iA, iB, iC for traditional ASVM.
Fig. 4. VA1N, VA2N, iA1, iA2, iA for improved ASVM with interleaving.
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