HVDC Transmission Control Equipment with High Reliability

A triplicated digital control equipment has been developed to improve reliability and performance of HVDC transmission. For improved reliability, DDC is used which is best suited to triplicated systems. The triplicated control equipment made up of three digital control equipments utilizes the self-diagnostic function of microcomputer to the maximum extent. It also employs an intermediate value selection as a signal selection method. The computation in the three control systems is performed in synchronism with the ac system to minimize signal transfer among the three systems and thereby facilitate maintenance. A new PLL circuit for synchronism detection is used to protect the HVDC transmission against ac system disturbances. An Operating System (OS) specially programed for HVDC transmission control is incorporated in the microcomputer to perform ac system control. The developed digital control equipment was tested in the simulated transmission facility. The simulation test showed that the control system has improved performance and reliability.