Elimination of Harmonic Disturbances in Softstarters

Sammanfattning: Power system infrastructure has been facing many challenges when it comes to harmonics distortion, and power electronic devices are no exception considering their wide usage in the system. In this thesis the focus is on the impact of these system harmonics on the electrical motors which are vastly used in industry for different applications. Motors usually come with a softstarter, and are connected to the grid. Softstarters are used to avoid mechanical stress and voltage drops in the network while starting up motors and being connected as a group with the motor to the grid means that they will also be affected by the system harmonics. Softstarter consists of various power electronics components, such as Switches in shape of thyristors, resistor and capacitor. An important part of a softstarter is the snubber circuits which is sensitive against high-frequency harmonics. The snubber circuit is mainly composed of a snubber resistor and a capacitor. In fact, presence of high-frequency harmonics in the system, would reduce the impedance of the snubber circuit’s capacitor. Thus the overall impedance of the snubber circuit would drop and accordingly current passing the snubber resistor would increase. The snubber resistor would become too hot and most probably melt. As a result, this phenomena in many cases, causes faults in the facility or even it causes a failure in the system. Considering the consequences of failure, it is necessary to cope with the destructive harmonics in an effective way. In this thesis, we are purposing a new strategy and a novel solution to maintain a reliable performance of the softstarter’s while facing high frequency harmonics of the system. As the source of the issue are the high frequency harmonics, the best solution would be to eliminate them when they show up. That can be done by a properly designed and adjusted high pass-filter. In this work we tried to consider a cost effective solution that can also eliminates the harmonics. We are proposing using an extra indicator that is fine tuned and is adjusted in a way that together with the rest of the components it shapes high-pass filter, which would block the high frequency harmonics as they appear. To be able to do that the existing circuit was deeply measured and analyzed and with the gathered information regarding the current behaviour of the system under high frequency harmonics, an inductor was adjusted and added to the circuit. Various tests to verify the performance are done and are presented in this report. From the result it can clearly be seen that the inductor together with the rest of the system could properly act as a high- pass filter and eliminate the destructive high frequency harmonics.