3-level inverter in NPC topology 1

3-level inverter in NPC topology for the medium-voltage grid with software-based DC link stabilization

Variable-speed drives are used in almost all application areas and power classes. The state of the art is to design the overall system consisting of motor and associated frequency inverter as compactly as possible in order to achieve a minimum installation volume. To avoid radiated interference, it is also necessary to arrange the motor and frequency inverter as close together as possible.

To achieve this, special designs are used in which the frequency inverter is integrated directly into the motor terminal box. These so-called inverter motors offer the additional advantage that the entire system can be replaced more quickly in the event of an inverter fault. Inverter motors are available on the market with outputs of less than 1 kW to 1,000 kW. However, operation is currently only possible on low-voltage networks with phase conductor voltages of 400 V or 690 V.

As part of a current research project, a 3-level inverter motor in NPC topology with a rated output of 1,500 kW at a mains voltage of 3,300 V was developed together with a manufacturer of motors and inverters for special applications, built as a laboratory sample (see Fig. 1) and initially put into operation at low voltage in the IALB test field.

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Multi-level topologies reduce the voltage change (dU/dt) on the machine, as well as the necessary blocking capability of the individual IGBT modules. In addition, the signal quality of the stator variables is improved by the different voltage levels that can be controlled.

While only one capacitance is required for the DC link in two-level converters, several capacitances are required for multi-level converters, depending on the topology and the number of levels. In order to prevent the DC link potentials from shifting during operation by recharging the capacitors, a DC link stabilization was designed and integrated by modifying the vector modulator. In addition, the switching behavior of the IGBT modules is taken into account when controlling the inverter.

A microcomputer card based on an F28M35x Concerto microcontroller from Texas Instruments was developed to control the inverter and put into operation with field-oriented speed control.

Fig. 2 shows the speed control of a drive. A field is set up at the start of the measurement and a setpoint jump to positive nominal speed is performed after 5 s of operation. Finally, operation is reversed after 35 s.

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The rated speed of the test machine is reached safely and without overshoot. The DC link voltages remain almost constant during the entire test.

At the end of this project, the inverter is to be put into operation with medium voltage at the project partner. The software developed can be used directly for this purpose. The next step is to integrate this inverter into the motor terminal box.