Nye topologier for nettilslutning af effektkonvertere / New Topologies for Connecting Power Converters to the Utility Grid

The most common power converter is the diode rectifier, which rectifies the ac of the utility grid to dc. The diode rectifier generates harmonic currents on the utility grid. These harmonic currents are unwanted, since they cause extra power losses and disturbance to other loads. Harmonic currents also lead to a non-sinusoidal voltage on the utility grid. To limit harmonic distortion and protect the power system, standards and recommendations have been developed. This project is to provide models and tools that facilitate prediction of the harmonic distortion from the diode rectifier in a given system and to find reasonable (economical) solutions if the harmonic distortion exceeds acceptable levels. The focus is especially on systems like industrial plants with more than one rectifier. Models and calculation methodsare developed for predicting the harmonic distortion in a given application. A user-friendly calculation tool is developed for doing the required calculations. Four selected rectifier topologies with a high input power factor are analyzed. It is found that using ac- or dc-coils is a very effective method to reduce the harmonic currents. The 12-pulse topology is analysed showing that low total harmonic current distortion is possible with this topology. By analysing the basic control strategies of the active rectifier, it is shown capable of near sinusoidal line-current, bi-directional power flow, and that it is possibleto reduce the dc-link capacitor size and a control the dc-link voltage. Finally, a new integrated single-switch approach for the three-phase rectifier based of the third harmonic injection scheme is proposed, where a significant reduction of line-side harmonics is possible. Also different system level harmonic reduction techniques are analyzed. It is found that mixing single- and three-phase diode rectifier loads always reduces the total amount of 5th and often the 7th harmonic current in the system. The quasi 12-pulse topology is shown competitive or even superior to a true 12-pulse rectifier because of the low total harmonic distortion in most of the operating area. It is found that a reduction of the voltage distortion by a factor of two may be expected compared to a 6-pulse diode rectifier with dc- or ac-coils. Capacitor banks may be used for displacement power factor correction and for passive filtering in applications where displacement power factor correction is needed. However, these filters have some major drawbacks, such as large reactive power generation and resonance conditions. These may be avoided by using an active filter. The basic control strategies of the active filter are investigated. Finally a cost - benefit analysis is made based on available market information, and a general step-by-step approach proposed to find the cost-optimised rectifier topology that fulfills particular requirements. The applicability of the step-by-step method is demonstrated by a real application example. The project is sponsored by the Danish Academy of Technical Science and it is a part of the Danfoss Professor Programme. (Steffan Hansen, Frede Blaabjerg, John K. Pedersen; Paul Thøgersen, Danfoss Drives A/S)