Harmonics performance and system stability evaluation between 18-pulse and LCL filter based active front end converters under weak grid condition

Abstract

Adjustable speed drives (ASDs) equipped with 18-pulse converter front end provide input disturbance rejection and clean power quality in various practical applications. In recent years, active front end (AFE) converters with LCL filters have gained popularity because of their four quadrant operational capability and low input current harmonics performance. However, the caveat exists that there are challenges for AFEs in achieving low harmonics and system stability under generator or weak grid environment. A systematic and quantitative comparison on input current harmonics performance and system stability between 18-pulse and AFE converters cannot be found in published literatures. In this paper, a robust 75hp (55kW), 480V, 60Hz 18-pulse clean power converter with “windmill” autotransformer topology is presented and analyzed, along with a state-of-the-art equivalent power level AFE with LCL filter. Theoretical investigation on AFE system stability with LCL filter using pole-zero map and Bode diagram is presented, followed by simulation studies under weak grid condition. Under normal and weak grid conditions, experimental tests with a 75hp (55kW) “windmill” 18-pulse clean power ASD and a 10kW AFE system with LCL filter are used for validating the accuracy and effectiveness of the analytical findings.