Control of harmonics in 6-pulse rectifiers

Harmonics are always present in electrical power systems. Harmonic distortion is harmless as long as its level is within the limit. However, with the recent rapid advancement of power electronics, i.e. non linear loads, the use of the variable speed drives are increasing day by day. Harmonics produced by non-linear loads are a potential risk if they are not evaluated, predicted, and controlled. The power electronic switching devices like thyristor used in the rectifier circuits inject harmonic distortion to the utility grid in different applications. The harmonic distortion causes different problems in the power system. To minimize the unwanted effects of harmonic distortion, IEEE Std 519-1992 recommends the amount of harmonics that is acceptable in the power system. IEEE Std 519-1992 suggests that an individual harmonic distortion to be under 3% and the total harmonic distortion, THD, to be under 5% of the fundamental component. Harmonic distortion can be mitigated using different methods. Based on the system configuration either active filters, passive filters, or phase shifting methods are used. In medium voltage high power applications, generally, phase shifting method is better suited. In addition to harmonic distortion in AC side, AC-DC converter produce ripple in DC side. DC ripple can be mitigated by the use of filter circuits. However, when phase shifting method is used in AC side for harmonic mitigation, a method called pulse multiplication can be used in DC side to mitigate DC ripple. Phase shifting and pulse multiplication methods are investigated in detail in this research. A three-phase 6-pulse rectifier is modeled in Alternative Transients Program (ATP). Voltage and Current waveforms are obtained and the amount of harmonic distortion produced is calculated. It was found that the harmonic distortion produced by an ideal three-phase 6-pulse rectifier to be 31.1% and not under IEEE 519-1992 recommendation. Therefore a 12-pulse rectifier has been investigated. The analysis shows that a 12-pulse rectifier produces 15.3% THD and provides a window opportunity to be used in certain areas where the grid is comparatively stronger. For a rectifier to be able to be used without ac side filter and dc side filter, and to mitigate the THD under the IEEE std 519-1992 recommendation and the dc ripple under the specified value, a higher pulse rectifier will be needed. Further investigation is needed using 18-pulse and 24-pulse phase shifting rectifiers.