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Hybrid Modulation for Neutral Point Voltage Reduction in DC-fed Three-Level Motor Drive Systems

Fig. 1. 3L VSI motor drive system with fan load
In a dc-fed motor drive system, a three level VSI, as shown in Fig. 1, can provide better performance on noise reduction and system efficiency. However, it also introduces neutral point voltage control problems. In the conventional nearest three space vector (NTSV) modulation method, there is fundamental frequency related harmonics on the dc voltage and large dc capacitor that is needed during the startup process when the motor speed is low. In order to reduce the dc link capacitors and improve system power density, a hybrid modulation method is proposed to limit the neutral point (NP) voltage ripple during the startup process for three level motor drive systems.

In NPC 3L inverters, since each phase leg has three output voltage levels: Vdc/2 (P), 0 (O), -Vdc/2 (N), there are a total of 27 switching states and 18 output voltage vectors for the three phases, as shown in Fig. 2. There have been many publications about the modulation methods for NPC 3L inverters. Different optimization goals, such as minimum loss or harmonic distortion can be achieved by using the redundant switching states. The neutral point voltage calculation model is presented to study the large NP voltage ripple for three level modulation during the startup process. Moreover, the possible overvoltage issue on the semiconductors considering the different switching speed of the devices for two level modulation in three level NPC converters is analyzed in detail. A zero neutral point voltage ripple modulation (ZNPVR) method is proposed to limit the NP voltage ripple and avoid overvoltage on the devices when the output fundamental frequency is low. When the output fundamental frequency is high, the modulation can smoothly transit to normal NTSV modulation for lower ripple and higher efficiency. Figure 3 shows the modulation method transition from the ZNPVR method to the NTSV method, which shows the effectiveness of ZNPVR modulation method.


Fig. 2. Switching states of an NPC inverter: red states the switching states in NTSV modualtion, and green states are used in ZNPVR Modulation
Fig. 3. Time domain simulation results: line to line voltage (Vab): orange; phase voltage (Van):blue; Output current (Iout):pink; NP voltage ripple (Vnp):green
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