Study on valve-side winding fault characteristics of converter transformer in LCC-HVDC system

Abstract

This paper studies the fault analysis method and fault characteristics of valve-side winding fault of converter transformer in line-commutated-converter high-voltage direct-current (LCC-HVDC) system. The time periods are divided according to the valve conduction state within one power frequency cycle, and these time periods are classified based on the common features. For each type of time period, the flow path of the fault current is analyzed, the circuit equation is established, and the current on both sides of the transformer is solved. By combining the various time periods together, the expressions of the currents on both sides of converter transformer in one power frequency cycle after fault are obtained. The fault characteristics of the valve-side winding fault are studied based on the expressions, and the differences between them and ordinary transformer are pointed out. The research results show that when a single-phase grounding fault occurs in the valve-side winding of the low-voltage (LV) bridge converter transformer in LCC-HVDC system, both the grid-side current and the valve-side measured current contain a lot of harmonics, which will affect the operating characteristics of differential protection. Finally, a simulation model is built using PSCAD to verify the fault characteristics obtained from analysis. These works provide a theoretical basis for analyzing the action characteristics of converter transformer protection when the valve-side winding fault occurs and studying the new protection principle.© 2017 Elsevier Inc. All rights reserved.