Power Electronic Based Active Protection for Worksite Accidental Energization

Abstract

Worksite safety is a major concern for utility companies maintaining distribution systems. Electric hazards often occur when worksite equipment accidentally contacts a live power line. This leads to dangerous ground potential rise, touch voltage or step voltage in surrounding areas. A number of approaches have been adopted by utility companies to ensure electrical safety on worksites. One of these involves trip grounding. Trip grounding is realized by connecting the equipment with a grounding rod. Once the equipment is energized, a relay at a substation will operate and trip the power supply due to the large fault current. Frequently however, when worksite grounding resistance or system impedance is high, the fault current will be low. This may result in a long fault clearing time or a tripping failure. In order to overcome the above limitations, this study proposes and tests a new approach to actively generate signals with specific patterns from an energized worksite using power electronic devices. Once these specific signals, transmitted through the power line itself, are recognized by a signal detector at the closest upstream recloser, the recloser trips the power supply and protects the worksite safety. The proposed method is first implemented using a diode-based signal generator scheme, which has a simple configuration but restricted application range. Then the approach is further refined using a thyristor-based signal generator scheme to fit a wide range of worksite conditions. As a result, worksite safety can be improved in a short time despite wide system parameter variations, such as high grounding resistance and long feeder distance. Practical considerations for prototype designs and parameter settings of the proposed active protection schemes are presented in this thesis. Simulation studies and sensitivity analysis are conducted to evaluate tripping performance. Comparative studies with conventional relay protection reveal that the proposed schemes offer better response speed and adapt to various worksite conditions.