【正文】 point clamped(NPC)threelevel inverter using IGBTs Threelevel voltagefed inverters have recently bee more and more popular for higher power drive applications because of their easy voltage sharing features． 1ower dv/dt per switching for each of the devices， and superior harmonic quality at the output． The availability of HVIGBTs has led to the design of a new range of mediumhigh voltage inverter using threelevel NPC topology． This kind of inverter can realize a whole range with a voltage rating from 2． 3 kV to 4． 1 6 kV Series connection of HVIGBT modules is used in the 3． 3 kV and 4． 1 6 kV devices． The 2． 3 kV inverters need only one HVIGBT per switch[2,3]. 2． 1 Power section To meet the demands for medium voltage applications． a threelevel neutral point clamped inverter realizes the power section． In parison to a twolevel inverter． the NPC inverter offers the benefit that three voltage levels can be supplied to the output terminals， so for the same output current quality， only 1/4 of the switching frequency is necessary． Moreover the voltage ratings of the switches in NPC inverter topology will be reduced to 1/2． and the additional transient voltage stress on the motor can also be reduced to 1/2 pared to that of a twolevel inverter． The switching states of a threelevel inverter are summarized in Table 1． U． V and W denote each of the three phases respectively； P N and O are the dc bus points． The phase U， for example， is in state P(positive bus voltage)when the switches S1u and S2u are closed， whereas it is in state N (negative bus voltage) when the switches S3u and S4u are closed． At neutral point clamping， the phase is in O state when either S2u or S3u conducts depending on positive or negative phase current polarity， respectively． For neutral point voltage balancing， the average current injected at O should be zero． 2． 2 Line side converter For standard applications． a l2pulse diode rectifier feeds the divided DClink capacitor． This topology introduces low harmonics on the line side． For even higher requirements a 24pulse diode rectifier can be used as an input converter． For more advanced applications where regeneration capability is necessary, an active front． end converter can replace the diode rectifier, using the same structure as the inverter． 2． 3 Inverter control 第 6 頁 Motor Contro1． Motor control of induction machines is realized by using a rotor flux． oriented vector controller． Fig． 2 shows the block diagram of indirect vector controlled drive that incorporates both constant torque and high speed fieldweakening regions where the PW M modulator was used． In this figure， the mand flux is generated as function of speed． The feedback speed is added with the feed forward slip mand signal . the resulting frequency signal is integrated and then the unit vector signals(cos and sin )are generated． The vector rotator generates the voltage and angle mands for the PW M as shown． PWM Modulator． The demanded voltage vector is generated using an elaborate PWM modulator． The modulator extends the concepts of spacevector modulation to the threelevel inverter． The operation can be explained by starting from a regularly sampled sinetriangle parison from twolevel inverter． Instead of using one set of reference waveforms and one triangle defining the switching frequency， the threelevel modulator uses two sets of reference waveforms Ur1 and Ur2 and just one triangle． Thus, each switching transition is used in an optimal way so that several objectives are reached at the same time． Very low harmonics are generated． The switching frequency is low and thus switching losses are minimized． As in a twolevel inverter, a zerosequence ponent can be added to each set of reference waveform s in order to maximize the fundamental voltage ponent． As an additional degree of freedom， the position of the reference waveform s within the triangle can be changed． This can be used for current balance in the two halves of the DC1ink． 3 Testing results After Successful installation of three 750 kW /2． 3 kV threelevel inverters for one 2． 7 km