Lars Helle

A novel loss reduced modulation strategy for matrix converters

This paper presents a new modulation strategy for three-phase to three-phase matrix power converters. The new modulation strategy is applicable whenever the output voltage reference is below half the input voltage. By applying this new modulation method, the switching losses are reduced by 15-35% compared to the conventional modulation scheme. The waveform quality of the new modulation strategy is evaluated with regard to both output flux harmonic distortion and input charge harmonic distortion.

Comparison of converter efficiency in large variable speed wind turbines

This paper presents a new and fast method for evaluating the efficiency of different power converter topologies in variable speed wind turbine applications. The method involves an accurate model of the considered generator while the converter models are based on ideal switches. The converter losses are modeled by analytical expressions of the switches, and the description of the losses incorporate both temperature, blocking voltage and switched current. The method is used to evaluate two converter topologies, a two-level back-to-back voltage source inverter (VSI), and a three-level back-to-back VSI for use in a 2 MW wind turbine system based on a doubly-fed induction generator (DFIG). From this evaluation, it appears that, with regards to the efficiency, the two-level VSI is the most suitable solution for the rotor side inverter while at the grid side, both inverter topologies show approximately the same efficiency. The evaluation method is validated by experimental results.

Analysis of symmetrical pulse width modulation strategies for matrix converters

This paper presents a new and easy way to understand the derivation of the modulation functions for matrix converters. It is shown how the duty cycles can be calculated using techniques known from the back-to-back (B2B) voltage source inverter (VSI), thus making the subject accessible to people with prior knowledge in standard drives. A new modulation strategy is presented minimizing harmonic distortion compared to the standard double-sided space vector modulation for matrix converters. An improvement to an existing modulator is also presented where the proposed modification improves the harmonic performance. The strategies are compared both analytically and by the use of both simulations and measurements on a 12 kW matrix converter prototype. It is found that the proposed modulator has superior performance compared to modulators with equal switching frequencies and an equal number of switchings.

Generalized discontinuous DC-link balancing modulation strategy for three-level inverters

This paper presents a new generalized discontinuous space-vector modulation approach applicable to neutral point clamped (NPC) inverters. By the proposed modulation scheme the DC-link voltage can be actively controlled to maintain a stable neutral point even if unbalanced loading of the DC-link capacitors occur due to e.g. nonlinear loads containing even harmonics. The maximum unbalance for which the modulation scheme are able to compensate is theoretically investigated and found to be a function of both load angle, modulation index and output power level. The proposed modulation method is compared to conventional modulation schemes with regard to both switching losses and wave form quality. For the same number of switchings, the proposed modulation scheme offers up to 25% lower switching losses than conventional modulation schemes while maintaining the same output wave form quality. The functionality of the modulation scheme is validated by simulation results.

Power density investigation on the press-pack IGBT 3L-HB-VSCs applied to large wind turbines

With three different DC-side and AC-side connections, the three-level H-bridge voltage source converters (3L-HB-VSCs) are alternatives to 3L neutral-point-clamped VSCs (3L-NPC-VSCs) for interfacing large wind turbines with electricity grids. In order to assess their feasibility for large wind turbines, they should be investigated in terms of power density, which is one of the most important design criteria for wind turbine converters due to turbine nacelle space limitation. In this study, by means of the converter electro-thermal models based on the converter characteristics, the power capabilities, DC capacitor sizes, converter cabinet volumes of the three 3L-HB-VSCs utilizing press-pack IGBTs are investigated in order to quantify and compare the power densities of the 3L-HB-VSCs employed as grid-side converters. Also, the suitable transformer types for the 3L-HB-VSCs are determined and comparatively studied in terms of volume and weight in order to estimate the size effects of the 3L-HB-VSC topology on the whole wind turbine connection system. Finally, based on the power density and transformer-size investigations, the feasibility of each 3L-HB-VSC is discussed.