## Abstract

This thesis deals with the harmonic distortion of the diode rectifier and a number of alternative rectifier topologies for adjustable speed drives. The main intention of this thesis is to provide models and tools that allow easy prediction of the harmonic distortion of ASD’s in a given system and to find reasonable (economical) solutions if the harmonic distortion is exceeding acceptable levels. To define some acceptable harmonic levels, the international standards IEC 61000-2-2, IEC 61000-2-4, the harmonic limiting standards EN 61000-3-2, EN 61000-3-12 (draft) and IEEE 519-1992 are reviewed.

Models and calculation methods for predicting the harmonic distortion in a given application are discussed. Simple voltage distortion calculation methods are shown and it is pointed out that exact calculation of the resulting harmonic voltage distortion takes the background distortion into account. It is recognized that the harmonic impedance in a given system, which must be known for calculation of the harmonic voltage distortion, can be difficult to determine exactly. This issue is therefore discussed.

Four levels of models for the harmonic current generation of both the single-phase and three-phase diode rectifier are presented. The first level is an ideal model where the diode rectifier basically is treated as an independent (harmonic) current source. The second level is an empirical model, where simulated (or measured) values of the harmonic currents of the diode rectifier for different parameters are stored in tables. The third level is found by analytical calculations and very precise results are obtained. The fourth level is the use of circuit-based simulators, which guarantees high precision even under non-ideal conditions. By use of the numerical circuit-based simulator SABER the phase-angle of the individual harmonic currents of different diode rectifier types is analyzed.

Four selected rectifier topologies with a high input power factor are presented. It is shown that using ac- or dc-coils is a very simple and efficient method to reduce the harmonic currents compared to the basic diode rectifier. Also the 12-pulse topology is analysed and a quite low total harmonic current distortion is shown possible with this topology. However, it is also shown that the 12-pulse topology is sensitive to unbalanced and pre-distorted grid. Furthermore, the basic control strategies of the active rectifier are discussed and the active rectifier is shown capable of near sinusoidal line-current, bi-directional power flow, the possibility to reduce the dc-link capacitor size and a controllable dc-link voltage. Finally, a new integrated single-switch approach for the three-phase rectifier based on the third harmonic injection scheme is proposed. The proposed scheme shows that significant reduction of line-side harmonics is possible.

Also different system level harmonic reduction techniques are presented. It is shown that mixing single- and three-phase diode rectifier loads always reduce 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 a low total harmonic distortion in most of the operating area. It is stated that a reduction of the voltage distortion by a factor of two can be expected compared to a 6-pulse diode rectifier with dc- or ac

Abstract

VI

coils. Capacitor banks for displacement power factor correction can be used for passive filtering in applications where displacement power factor correction is needed. Significant harmonic reduction can hereby be achieved. However, these filters have some major drawbacks that are discussed. A lot of the problems arising with the use of passive filters, such as large reactive power generation and resonance conditions can be avoided when using an active filter instead of passive filters. The basic control strategies of the active filter are discussed.

Finally a cost - benefit analysis is presented based on available market information and a general step-by-step approach is proposed to find the cost-optimal rectifier topology that fulfills individual requirements. The applicability of the stepwise method to find the costoptimal rectifier is demonstrated by a real application example. An easy-to-use calculation tool is developed for doing the required calculations.

Models and calculation methods for predicting the harmonic distortion in a given application are discussed. Simple voltage distortion calculation methods are shown and it is pointed out that exact calculation of the resulting harmonic voltage distortion takes the background distortion into account. It is recognized that the harmonic impedance in a given system, which must be known for calculation of the harmonic voltage distortion, can be difficult to determine exactly. This issue is therefore discussed.

Four levels of models for the harmonic current generation of both the single-phase and three-phase diode rectifier are presented. The first level is an ideal model where the diode rectifier basically is treated as an independent (harmonic) current source. The second level is an empirical model, where simulated (or measured) values of the harmonic currents of the diode rectifier for different parameters are stored in tables. The third level is found by analytical calculations and very precise results are obtained. The fourth level is the use of circuit-based simulators, which guarantees high precision even under non-ideal conditions. By use of the numerical circuit-based simulator SABER the phase-angle of the individual harmonic currents of different diode rectifier types is analyzed.

Four selected rectifier topologies with a high input power factor are presented. It is shown that using ac- or dc-coils is a very simple and efficient method to reduce the harmonic currents compared to the basic diode rectifier. Also the 12-pulse topology is analysed and a quite low total harmonic current distortion is shown possible with this topology. However, it is also shown that the 12-pulse topology is sensitive to unbalanced and pre-distorted grid. Furthermore, the basic control strategies of the active rectifier are discussed and the active rectifier is shown capable of near sinusoidal line-current, bi-directional power flow, the possibility to reduce the dc-link capacitor size and a controllable dc-link voltage. Finally, a new integrated single-switch approach for the three-phase rectifier based on the third harmonic injection scheme is proposed. The proposed scheme shows that significant reduction of line-side harmonics is possible.

Also different system level harmonic reduction techniques are presented. It is shown that mixing single- and three-phase diode rectifier loads always reduce 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 a low total harmonic distortion in most of the operating area. It is stated that a reduction of the voltage distortion by a factor of two can be expected compared to a 6-pulse diode rectifier with dc- or ac

Abstract

VI

coils. Capacitor banks for displacement power factor correction can be used for passive filtering in applications where displacement power factor correction is needed. Significant harmonic reduction can hereby be achieved. However, these filters have some major drawbacks that are discussed. A lot of the problems arising with the use of passive filters, such as large reactive power generation and resonance conditions can be avoided when using an active filter instead of passive filters. The basic control strategies of the active filter are discussed.

Finally a cost - benefit analysis is presented based on available market information and a general step-by-step approach is proposed to find the cost-optimal rectifier topology that fulfills individual requirements. The applicability of the stepwise method to find the costoptimal rectifier is demonstrated by a real application example. An easy-to-use calculation tool is developed for doing the required calculations.

Original language | English |
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Publisher | Institut for Energiteknik, Aalborg Universitet |
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Number of pages | 194 |

ISBN (Print) | 87-89179-37-4 |

Publication status | Published - 2000 |