Roland Jakob

Multicell converters: basic concepts and industry applications

Multicell power converters were introduced ten years ago. This paper summarizes the main results obtained through the research done on these converters over this period: topology and derivation of multicell choppers and voltage-source inverters; harmonics cancellation; open-loop modulation techniques; self-balancing understanding; and optimization. During this same period, industrial products using this technique have been developed and the corresponding know-how has contributed to the field in various ways, such as modular design, low inductance busbars, specific drivers, etc.

3-Level high power converter with press pack IGBT

Converteam recently has introduced the MV7000 series of medium voltage drives, designed with Press Pack IGBT (PPI) in 3-level NPC topology in the power range from 4 MW to 32 MW [3]. PPI combines the benefits of disk type device technology with state-of-the-art semiconductor technology. This paper explains relevant technical features and design issues related to MV7000 including direct series connection of devices.

Dynamic self-clamping at short-circuit turn-off of high-voltage IGBTs

Measurements show that the IGBT is able to clamp the collector-emitter voltage to a certain value at short-circuit turn-off despite a very low gate turn-off resistor in combination with a high parasitic inductance is applied. The IGBT itself reduces the turn-off diC/dt by avalanche injection. However, device destructions during fast turn-off were observed which cannot be linked with an overvoltage failure mode. Measurements and semiconductor simulations of high-voltage IGBTs explain the self-clamping mechanism in detail. Possible failures which can be connected with filamentation processes are described. Options for improving the IGBT robustness during short-circuit turn-off are discussed.

New medium-voltage inverter design with very high power density

The three-level NPC VSC topology and press-pack IGBT (PPI) devices are the key technologies for high-power medium-voltage (MV) drives. In this paper, a novel design for a MV inverter based on these technologies is presented. The main feature of the new converter is the integration of the dc-capacitor bank into the converter cubicle. Any kind of clamp capacitors are omitted. This simplifies the converter structure and reduces the part count, thus increasing its reliability and power density. The design of the low-inductive bus bars between the semiconductor stacks and the dc capacitor bank, the mechanical design, and the controls are discussed in the paper. Measurement results are presented that prove the excellent performance of the inverter.

Measurement of a complete HV IGBT I-V-characteristic up to the breakdown point

This paper describes how to measure the complete output characteristic of a high-voltage IGBT non-destructively up to the breakdown point and beyond. Hereby, a deep knowledge of the IGBT behaviour at high voltages and saturation currents is gained. To construct the complete characteristic, short-circuit and curve-tracer measurements are combined. The results are compared and recapitulated with semiconductor simulations of IGBT models fitted to experimental characteristics.

Snubbered high-power press-pack IGBT converter

A medium-voltage IGBT converter with turn-on snubber is presented. The drastic reduction of turn-on losses allows the inverter to reach a peak output power well beyond the 12-MVA mark. The paper discusses the design challenges for the new inverter and presents test results.

Surge current capability of IGBTs

Commonly an IGBT cannot withstand a surge-current pulse (large overcurrent with a duration of some milliseconds) due to the current saturation characteristic of this device. In the case of a converters single device failure the IGBT switches are driven into pulse-blocking mode to prevent a DC-link short-circuit and subsequent IGBT failures. This strategy leads to an asymmetric short circuit of the load. The paper introduces the possibility to symmetrize the load short circuit with active turned on IGBTs and shows first measurements on high-voltage press-pack IGBT chips under heavy overcurrent condition with increased gate voltage.

IGBTs conducting diode-like surge currents

For special failure cases it would be of advantage, if the IGBT can conduct very high currents (surge currents) in the same way as bipolar diodes or thyristors. This paper shows that IGBTs can have a great surge-current capability at higher gate voltages. The limiting short-circuit current of the IGBT is shifted to high values to prevent desaturation. Measurements and simulations will show that the temperature rise during a typical 10 ms sine-shape surge-current pulse is still in controllable ranges, especially when double-side cooling is applied. It will be presented that at surge-current condition the internal IGBT behaviour changes to a diode-like one. Destructions and significant gateoxide ageing can be suppressed, if the gate voltage is adjusted to suitable values.