New generation high-efficiency bidirectional power converters
    for medium voltage drives

Project of the National Centre for Research and Development (NCBiR)
No INNOTECH-K1/HI1/3/159089/NCBR/12

The project includes conducting research on high-performance industrial medium-voltage converters built from isolated ac-dc/dc-ac modules containing high-frequency transformer based dual active bridge (DAB) dc/dc converter. The aim of the research is to gain knowledge of the bi-directional processing of electrical energy with high efficiency by means of inverters and high frequency transformers and connecting ac-dc/dc-ac modules and control them in a medium voltage (MV) multilevel power converters. The results of the research will be used to build a laboratory model of the MV multilevel cascaded H-bridge converter with a capacity of 600 kW, which will be used to assess the high-current systems and the verification methods and control algorithms.

Description of the results:

The developed MV multilevel power converter uses nine bidirectional high frequency isolated ac-dc/dc-ac power cells (three in series per phase) for 3.3kV adjustable speed induction motor. The core power cells are connected in a three-phase configuration both on the line side and the motor side.

Fig. 1. Circuit arrangement of MV drive using the topology of modular high frequency pulse power electronic transformer. The switching frequency of IGBTs above 5 kHz enables small dimensions of isolation transformers
and line side and machine side LC filters.

Fig. 2. The structure of 1kV ac-dc/dc-ac core power cell based on 1.7kV IGBTs: Cdc - dc link capacitor,
Cs - snubber capacitor protecting IGBTs from voltage spikes

Each power cell consists of digitally controlled active front-end 1700VIGBT-based H-bridge rectifier, an isolated dc/dc converter and a pwm 1700VIGBT-based H-bridge inverter. The dc-link voltage in the dc/dc converter is 1 kV. The isolated dc/dc converters use high-frequency transformers with unitary turns ratio characterizing high power density. Application of low loss ferrite cores and Litz-wire windings in high frequency transformers ensures efficient operation of the isolation stage of the MV power converter. The applied cores are resistant to thermal shock.

Fig. 3. Core ac-dc//dc-ac power cell easy for remove and maintenance: (1) - control card with digital signal processor ADSP-21363, (2) - line side 1700V IGBT two-level PWM rectifier, (3) - 1kV high frequency isolation transformer,
(4) - motor side 1700V IGBT two-level PWM inverter.

Fig. 4. MMB600IM3.3kV-01 power cell cabinet - perspective view showing one phase of the MV converter.

Fig. 5. Air flow inside the developed MV drive.

Fig. 6. Line-to-line output voltage (4.7kV peak voltage) before LC filter.

Fig. 7. Phase voltage (2 kV/div) and output current (50 A/div) waveforms.

Fig. 8. Measured efficiency of 3.3kV drive including high frequency isolation transformers and LC filters.

Table 1. Total harmonic distortion coefficient (THDI) as a function of MV drive power

Key Features of the Developed 3,3kV Power Converter:
  • Multilevel topology using cascade connection of 2-level low voltage PWM rectifiers provides almost harmonic-free input current with high power factor.
  • Since the converter is intended for use without an isolation transformer, it can maintain power conversion efficiency of above 95%.
  • Lowest cost of ownership when compared with line-frequency transformer equipped MV drive.
  • Galvanic isolation realized with high frequency transformers made from soft ferrites providing small dimensions and low losses.
  • Low torque ripple.
  • Sinusoidal wave for motors and supply currents (harmonic content less than 4%).
  • Minimum losses since the isolated dc/dc converters are controlled with advanced control algorithms of zero voltage switching (ZVS) and zero current switching (ZVS).
  • Modular design provides high reliability and low maintenance costs - each core power cell can be removed for maintenance.
  • High immunity of IGBT based power cells to external emc disturbances thanks to the use of fiber optic isolation of control signals.

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