PWM Methods

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    There are two major multilevel topologies: diode-clamped converter and flying-cap converter. First topology - diode clamped is especially popular in three-level application. The three-level version is called neutral-point-clamped converter (NPC) (Fig 1). The main advantage of this topology is relatively low number of capacitors used since diodes are preferred from economical point of view. Some disadvantages exist also, though, limiting diode-clamped topology only to three-level applications. Firstly, the high voltage stress on clamping diodes for converters with more than three levels. Secondly, the objective of charge balance of capacitors in DC-link is proved to be impossible for topologies with high number of levels (>3) under some operating conditions. Additionally, even though proper control of three-level topology solves voltage balance problem, a low frequency ripple in the neutral point potential appears when the system is working with large modulation i ndices and low power factors. As consequence voltage stress of semiconductor devices is higher than ½ UDC and low-frequency distortion may be observed in the AC voltage.
Fig.1 presents three-phase two-level NPC voltage source PWM converter. This topology can work in two modes:
- inverter - when energy, of adjusted amplitude and frequency, is converted from DC side to AC side. This mode is used in variable speed drives and AC power supply including uninterruptible power supply (UPS),
- rectifier - when energy of mains (50 Hz or 60Hz) is converted from AC side to DC side. This mode has application in power supply with Unity Power Factor (UPF).

Fig.1Three-phase three-level voltage source PWM converter.

Performance significantly depends on control methods and type of modulation.
The main difference between two level and three level converters is that in case of three lavel case three states are used in one phase (Fig 2), and in two level two states are performed. So every leg may achieve three different voltage levels in relation to neutral point: positive state (2-state), negative state (0-state) and zero state (1-state), what in combination with three legs allows for twenty seven different switching states. 2-state is set by switching on two top switches and turning off two lowest switches. Voltage level of +UDC/2 is attained. Switching off two top switches and switching on two lowest ones provides us with 0-state and voltage level of –UDC/2. State 1 is achieved through switching off two outer switches and switching on the two inner switches. So there are twenty-seven possible switching states in the three-level NPC converter.
Fig.2The states of three-phase three-level voltage source PWM converter.