Analysis of DC voltage anomaly in HVDC transmission projects

Author： GOZ Electric Time：2024-10-29 09:32:40 Read：11

During the test of a UHV DC transmission system, the data of the DC voltage divider on the rectifier side was abnormal, especially when the power was reversed, which was inconsistent with the electrical principle. According to the corresponding AC voltage, DC voltage, current, tap switch position and trigger angle (arc extinction angle) under different power levels and power forward and reverse conditions during the system test, the possible causes of the problem were analyzed from the perspective of theoretical calculation, and corresponding simulation tests were carried out. The test results were basically consistent with the theoretical calculations. Finally, this paper uses the DC voltage divider on the inverter side for calibration compensation to make the actual voltage value of the DC system consistent with the complete set design, thereby ensuring the safe and stable operation of this UHV DC project.

Keywords: UHVDC; DC voltage divider; inverter side; calibration

The DC voltage divider measures the line voltage of the DC system in real time and is an indispensable device for the DC transmission system. The measurement results directly act on the closed-loop control and protection system of the DC control system. The accuracy of the measurement results directly affects the reliability of the operation of the DC control and protection system, and then affects the reliability and stability of the large power grid. Practice shows that it is not uncommon for abnormal DC voltage divider measurements to affect the normal operation of the DC system. In response to such problems, relevant literature analyzes and studies the structure of the DC voltage divider in the UHV converter station, explains the measurement principle of the DC voltage divider, and how to cooperate with the control and protection system. The relevant literature introduces a typical DC transmission system voltage stability control method, and analyzes in detail the factors affecting the response of the DC voltage stability control system to the voltage measurement fault in the high-voltage DC transmission system. The relevant literature studies in detail the situation in which the DC voltage divider abnormality causes the overvoltage protection device to operate when the UHV converter station is struck by lightning in rainy and foggy weather. The relevant literature analyzes the working principle of the DC voltage divider and its internal resistance heating, external electric field distortion and other problems, and proposes an optimization design method for the DC voltage divider equipment. The relevant literature analyzes the problem of abnormal fluctuation of a certain DC voltage, and gives the cause of the accident as the unstable operation of the DC voltage divider photoelectric conversion module, which in turn affects the modulation function of the DC system. This article describes in detail the abnormal DC voltage of a certain DC project, analyzes its cause, and provides a solution for compensation using the measurement data of the DC voltage divider on the inverter side.

Theoretical analysis

If the DC voltage divider is measured correctly and the DC voltage abnormality is caused by the control system, the S1 voltage will not be lower than the S2 voltage during reverse transmission, so the control system fault is excluded. There are two possible abnormalities:

When the S1 voltage measurement result is high and the S2 voltage measurement result is correct, the line voltage drop is added based on the S1 voltage measurement value: Although the S2 voltage measurement value is high, the line voltage drop has an upper limit. At this time, the S1 voltage obtained by the S2 controller is still in the normal range; ② Assuming that the S2 measurement value is low, the control system uses this low voltage value as a reference, plus the voltage difference, and the calculated S1 voltage is low, then S1 will adjust the trigger angle to increase the voltage, and the S1 voltage will be high. According to the above analysis, the low S2 voltage measurement or the high S1 voltage measurement will cause the system voltage to be high.

Perform steady-state parameter verification based on the actual operating state, that is, infer the abnormal measurement point of the voltage divider and the measurement deviation range based on the trigger angle, tap position, AC voltage, DC voltage, and DC current in the steady state.

After the precision calibration, the on-site DC voltage returned to normal, which was consistent with the theoretical analysis, and the system power and DC current were consistent with the design values, meeting the requirements of the complete set design. Through this analysis and processing of DC voltage anomalies, the following conclusions can be drawn:

1) The subsystem debugging should ensure detailed data records and clear conclusions, reduce the workload after commissioning, and reduce the risk of affecting the safe and stable operation of the system due to precision calibration and false data records during debugging.

2) The single variable steady-state verification can quickly and accurately locate the abnormal position and cause of the voltage, current, and angle of the DC transmission system.