Analysis of Common Fault Cases of DC Voltage Divider

作者： GOZ Electric 时间：2024-06-17 09:23:30 阅读：13

On March 16, 2022, during the normal inspection process, a UHV converter station was found to have a fault. When analyzing the fault, it was found that the SF6 pressure of the DC busbar DC divider of the back-end pole 1 protection system was locked due to low pressure. The operation mode of the DC divider is a bipolar four-converter, which adopts the earth return line 640kV step-down mode, and the line transmission power is 1000MW, which plays an important role in the local power grid system. During the fault diagnosis, the bipolar did not lock and there was no power loss.

2.1 Preliminary analysis of the fault

In order to troubleshoot the fault, the fault was preliminarily analyzed and checked, and it was found that there was no obvious abnormality in the relay. The A screen, B screen, and C screen of the pole 1 pole protection were checked, and it was found that the SF6 gas pressure of the pole 1 DC divider was low in the A set protection. Unlike the others, the B set and C set protection did not have input signals. During the fault diagnosis, in order to protect the entire system, a certain tripping mode was set. The low pressure of the pole busbar SF6 set a level 2 alarm and a level 1 trip. Setting a warning is conducive to timely detection of faults. The alarm signal in the line is connected to the pole control. When the signal transmission is completed, the trip protection is triggered. The OWS background alarm signal is reduced, and only "pole 1 pole protection system A" is reported. The cause of this fault may be that the voltage imbalance causes the external node to be connected. According to the weather conditions on the day of the inspection, the fault is easy to confuse, which may affect the inspection and judgment results, and certain treatment measures have been taken. After picking up the 1st level SF6 pressure protection trip, it was observed again that the power system OWS signal returned to normal. The DC divider low-voltage protection is in normal state, and the fault is preliminarily analyzed.

2.2 Fault handling

To handle the fault, first determine the location of the final converter station fault. Check the protection system and find that the relays in the pole B and C panels are working normally. The B and C sets are non-electric protection systems, and no fault signal is input. Check the DC fault recorder to handle the fault. The voltage of the circuit system affects the diagnosis of the fault. It was found that the DC voltage measurement values of unit B set (PMUB) and unit C set (PMUC) increased abnormally, and the voltage reached 535kV, which did not meet the maximum voltage limit requirements. The DC voltage was 500kV before the test. Assume that t1 and t2 represent the time when the fault occurs and the time when the converter is locked, respectively. The terminal voltage is measured, and the results are shown in Figure 4. (a) and (b) in the figure are the DC voltage measurement values Udc of PMUA and PMUB before and after the fault, respectively. (a) PMUA measurement voltage (b) PMUB measurement voltage Figure 4 Terminal voltage measurement As shown in Figure 4, when the fault is handled, the measurement cable is disconnected, the PMUB DC voltage measurement is abnormal, and the voltage rises to 535kV. After a period of time, it is found that the fault still exists. After analyzing the fault, it is confirmed that the relay terminal fault affects the line voltage fluctuation. According to the fault situation, it is decided to replace a new relay. After the relay is replaced, the fault is eliminated and the DC voltage divider returns to normal.

3 Conclusion.

Based on the structure and principle of the DC voltage divider, this paper studies the voltage divider capacitor waveform and harmonics in the normal state, and analyzes common faults according to the characteristics of the DC voltage divider in the converter station. According to the actual fault situation on site, the possible causes of the fault are analyzed. After preliminary analysis, a low signal of SF6 gas pressure in the DC voltage divider is found. After actual fault inspection, the relay terminal is judged to be faulty. After diagnosing and analyzing the fault, the fault was eliminated and the DC voltage divider was restored to normal state.