Design and development of an improved calibrator for the DC electronic transformer in HVDC system

The direct current (DC) electronic transformer is widely used in high-voltage DC (HVDC) transmission systems as primary equipment, and its steady-state accuracy is necessary to be calibrated for safe and reliable operation of the electrical power system. Usually, the method of calibration is to compare the outputs of the standard transformer and the test object with the same source, and the high-precision digital multimeter (DMM) is used to convert the analog output into digital form with the synchronous pulse per second (PPS) triggering. The DMM acquires a series of data after each trigger firstly, then the sampling data are transmitted to the host computer, thus the sampling is noncontinuous. Besides, the DMM can’t ensure the precise synchronization of sampling only based on the PPS. These factors affect the calibration result. Therefore, a control scheme of continuous and synchronous sampling for the DMM is proposed, and an improved DC electronic transformer calibrator is designed and developed. The test results under laboratory conditions show that the DC steady-state accuracy of the calibrator can reach up to 0.05 accuracy class.