A converter transformer with series-connected line-side windings for a DC link using voltage source converters

A project is under way in Japan to develop a high-power voltage source converter system for future DC interconnection between AC networks and HVDC transmission. A 300 MW/AC 275 kV converter transformer with series-connected line-side windings for combination of multiple voltage source PWM converters in series on the AC side is being developed. Down-scaled transformer models were manufactured, and subjected to factory testing to verify the solutions for equalization of voltage sharing between the windings, prevention of DC magnetization in the core caused by the converter output voltages, and estimation of core losses when the core is magnetized by a PWM converter. Based on the design criteria confirmed by factory testing, 53 MVA GTO-based converters and converter transformers were installed for a prototype back-to-back link between actual 50 Hz/66 kV and 60 Hz/275 kV systems, and field testing is being conducted. The behavior of magnetic flux across the core during operation was monitored by embedded Hall-effect sensors. Satisfactory results were obtained.