Abstract: 300 Mvar large-scale synchronous condenser used as dynamic reactive power support in UHVDC project is the first application in DC convertor station. The requirements include good dynamic characteristics, high reliability and easy maintenance. There are no previous experience in design, manufacture, operation and maintenance for synchronous condenser at this capacity, both in China and aboard. The characteristics of large-scale synchronous condenser set are introduced first. Main contents and requirement for condenser starting and operation are also summarized. Key techniques are discussed including emergency power supply switchover, excitation system switchover, coasting synchronization, reactive power regulation and reactive load rejecting test. The discussion gives important reference for smooth operation of future large-scale synchronous condenser set in UHVDC projects.