NPC Inverter for Wind Turbine

The diode-clamped multilevel inverter employs clamping diodes and cascaded dc capacitors to produce ac voltage waveforms with multiple levels. The inverter can be generally configured as a three-, four-, or five-level topology, but only the three-level inverter, often known as neutral-point clamped (NPC) inverter, has found wide application in high-power medium-voltage (MV) drives [1-3]. The main features of the NPC inverter include reduced dv/dt and THD in its ac output voltages in comparison to the two-level inverter discussed earlier. More importantly, the inverter can be used in the MV drive to reach a certain voltage level without switching devices in series. For instance, the NPC inverter using 6000-V devices is suitable for the drives rated at 41-60 V.

In this chapter, various aspects of the three-level NPC inverter are discussed, including the inverter topology, operating principle and device commutation. A conventional Pulse width modulation (PWM) scheme for the NPC inverter is discussed in detail The dc input voltage of the inverter is normally split by two cascaded dc capacitors, providing a floating neutral point. The control of the neutral-point voltage deviation is also elaborated.

THREE-LEVEL INVERTER

Figure 1.1 shows the simplified circuit diagram of a three-level NPC inverter. The inverter leg A is composed of four active switches S1 to S4 with four anti-parallel diodes D1 to D4. In practice, either IGBT or GCT can be employed as a switching device.

Fig.1.1. Three-level NPC inverter.

On the dc side of the inverter, the dc bus capacitor is split into two, providing a neutral point Z. The diodes connected to the neutral point, DZ1 and DZ2, are the clamping diodes. When switches S2 and S3 are turned on, the inverter output termi- nal A is connected to the neutral point through one of the clamping diodes. The volt- age across each of the dc capacitors is E, which is normally equal to half of the total dc voltage Vd. With a finite value for Cd1 and Cd2, the capacitors can be charged or discharged by neutral current iZ, causing neutral-point voltage deviation. This issue will be further discussed in the later sections.