In mid-March 2001, during a pre-commissioning inspection of the newly installed automatic power transfer (BZT) device at the 110kV Jinyuan Substation, our bureau's relay protection personnel discovered no voltage on the standby power supply side. Since this voltage was introduced through a capacitive voltage transformer (CVT) installed on the standby power supply line side, relay protection and high-voltage testing personnel conducted a series of inspections and tests on the CVT and its secondary circuit. The results revealed a serious fault: the CVT's electromagnetic unit was burned out. Maintenance personnel promptly replaced it, preventing an equipment accident. [b]1 Equipment Fault Discovery Process[/b] Our bureau's Jinyuan Substation has two 110kV power supply lines. Under normal operation, one is the main power supply and the other is a CVT standby. To quickly close the standby line switch in the event of a permanent fault in the main power supply line, the 110kV system was equipped with a BZT device. As shown in Figure 1, the BZT device is connected to the voltages of the north and south sections of the Jinyuan 110kV bus and the voltages of the two line sides. Through the switching handle of the device, each line can be converted into a main power supply line or a backup line, and the corresponding bus voltage, line voltage, and secondary circuit can be switched accordingly. Under normal operating conditions, the Zijin line is the main power supply, and the Tjin line is the backup. At this time, the voltage of the Jinyuan 110kV north bus and the voltage of the Tjin 2 line side are switched to the BZT device. The north bus voltage reflects the operating status of the main power supply, and the Tjin 2 line side voltage reflects whether the backup power supply is normal and can play its backup role. [img=230,199]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dldrq/2001-4/35-1.jpg[/img] This BZT device was installed in December 2000. In mid-March of this year, relay protection personnel conducted a pre-commissioning inspection of the device. Workers measured the voltage of the two busbars and the voltage on the Zijin line side at the terminal block behind the device panel, and found them normal. However, when measuring the secondary voltage of the CVT on the backup power supply TJ2 line side, there was no voltage. At that time, the workers assumed the line was not energized and put the matter aside, only inspecting the device itself. Because the entire station's equipment was undergoing periodic high-voltage testing, the main power supply line equipment could only be shut down for testing after the backup line was put into operation. After contacting the dispatcher, the operators put the TJ2 line into operation to supply the entire station load, and the Zijin line was shut down. At this point, the relay protection personnel confirmed that the TJ2 line was energized, so they measured its line-side voltage again on the BZT device panel, and it was still not energized. The CVT secondary fuse was not blown; the circuit was disconnected and the wire cores were checked, and there were no problems; the secondary fuse was removed, and the voltage was measured directly at the secondary outgoing terminal, but there was still no voltage. The relay protection personnel then realized that there might be a fault inside the CVT. Therefore, after quickly completing the high-voltage test on the Zijin line equipment, the Zijin line was put into operation, the TJ2 line was de-energized, and the primary lead of its line-side CVT was disconnected for testing. This capacitive voltage transformer (CVT) was put into operation in December 2000. The model number is [img=162,25]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dldrq/2001-4/36-1.jpg[/img]. The electrical principle of the CVT is shown in Figure 2. High-voltage testing personnel first tested the high-voltage capacitor C1, the medium-voltage capacitor C2, and the total capacitance of the CVT. Then they tested the dielectric loss. The data showed little change compared to the factory test data and the test data before commissioning, indicating that the capacitor divider unit was functioning correctly. [img=277,173]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dldrq/2001-4/36-2.jpg[/img] To determine the problem with the electromagnetic unit of the CVT, the testers first used a multimeter in resistance mode to measure the resistance of the primary coil of the medium-voltage transformer, which was found to be over 500 ohms. Then, they applied an AC voltage to the primary coil of the medium-voltage transformer and measured the secondary voltage. When the primary voltage increased, the secondary voltage decreased instead of increasing. Finally, they applied an AC voltage to the da and dn coils on the secondary side of the medium-voltage transformer and measured the primary voltage with an electrostatic voltmeter; both voltages were zero. Based on these test results and data, the testers initially suspected a short circuit inside the electromagnetic unit. Because more detailed technical and test data for these CVT models were unavailable, the specific fault could not be determined at that time. Given the urgent need to put the equipment into operation (the carrier communication filter at this station is connected to this CVT), the maintenance personnel removed the CVT. Our bureau's materials company notified the equipment manufacturer's office in Zhengzhou, and a new capacitive voltage transformer was delivered the next day. The new CVT was the same model as the original, except for a slight difference in the structure of the electromagnetic unit. Using this new CVT as a reference, the staff retested the DC resistance of the coil of the removed CVT's electromagnetic unit and the voltage applied to the secondary side. The comparative test data is shown in Table 1. These data indicated that a short circuit had occurred in the primary winding of the medium-voltage transformer in the electromagnetic unit of the capacitive voltage transformer on the T-line side. Therefore, the staff quickly tested and installed the new CVT, promptly put it into operation, and transported the old CVT back to the bureau for dissection and inspection. [img=308,141]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dldrq/2001-4/36-3.jpg[/img] [b]2 CVT Disassembly Inspection and Fault Cause Analysis[/b] In April 2001, our bureau's professional technicians and CVT manufacturer personnel jointly conducted a disassembly inspection of the disassembled CVT. When the workers loosened several bolts on the electromagnetic unit's oil tank flange with a wrench, a pungent and eye-catching oil fumes sprayed out from the flange gaps, and a significant amount of internal pressure was clearly felt. After removing all the bolts, the capacitor unit was slightly lifted from the lower oil tank using an overhead crane. When removing the lead wire between the intermediate voltage terminal A′ and the lower terminal δ of the medium-voltage capacitor C2 and the electromagnetic unit, it was discovered that one of the four bolts securing the lower terminal δ of the medium-voltage capacitor C2 was missing. Because the oil tank was quite full, it was impossible to see where the missing bolt had fallen. Workers slowly drained the oil from the tank using tools. When the oil level was below the terminal block of the medium-voltage transformer, they finally saw that the fallen bolt had landed among several terminals of the primary winding tap of the medium-voltage transformer. Slight short-circuit welding marks were found at the point where the bolt contacted the terminal. The oil in the tank had lost its characteristic pale yellow color and turned a dark brown, like soy sauce. During the draining process, gas continuously escaped from the oil, and dark brown foam appeared on the surface. After all the oil was drained, it was observed that the iron core of the medium-voltage transformer had been burned, losing the characteristic luster of the silicon steel sheets. The outermost silicon steel sheets were deformed and bulged in the middle. The white cloth covering the windings of the medium-voltage transformer had been burned into black charcoal, and slag fell off when touched. The inner wall of the tank was covered with charred oil stains, leaving a black residue to the touch. To disconnect the leads of the compensating reactor, workers removed the cover plate above the terminal box and found that it was bulging due to excessive internal pressure. At this point, the CVT fault was clear: the primary coil of the medium-voltage transformer was burnt out. Even so, we still had oil personnel take oil samples for oil chromatography analysis. The analysis results showed that, except for acetylene (zero value), total hydrocarbons and hydrogen levels significantly exceeded warning levels; the calculated ratio was 0.3:20, indicating a fault type of low-temperature overheating (150–300℃), further confirming the fault. Based on the findings of the CVT disassembly inspection, our technical personnel and the equipment manufacturer's personnel unanimously agreed that the cause of the intermediate unit burnout was that a bolt securing the lower end of the medium-voltage capacitor C2 had fallen into the terminal block cluster of the primary winding of the medium-voltage transformer, short-circuiting part of the primary winding's turns. This reduced its AC impedance, causing the primary current to exceed the rated value, resulting in the primary winding burning out. However, the short circuit caused by the bolt was not too serious, or rather, the number of turns shorted by the bolt was not large. If the short circuit were severe, the heat generated by the short-circuit current would cause the transformer oil to decompose and release a large amount of gas in a short time, potentially causing an explosion of the downstream tank or causing the voltage across the high-voltage capacitor C1 to become too high and explode. As for why this bolt fell off during operation, we believe it was due to improper tightening during installation, which was not detected during inter-process inspections. After the equipment was put into operation, it was located in the alternating electromagnetic field of the medium-voltage transformer. Under the influence of the alternating electromagnetic field, it continuously vibrated, rotated, and shifted downwards, eventually falling off and causing a short circuit in the primary winding of the intermediate transformer. Fortunately, this problem was discovered during the inspection of the relay protection automatic device and was replaced in time, preventing a more serious equipment accident. 3 Lessons Learned Capacitive voltage transformers are widely used in power systems, but a failure caused by a loose bolt, as seen in this case, is extremely rare. For power equipment manufacturers, even a one in ten thousand defects in their products can represent a 100% potential malfunction for users. Fortunately, the CVT on the T-Jin line was discovered in time, preventing a larger equipment accident. Therefore, as power equipment manufacturers, installation personnel must strengthen their sense of responsibility, and quality inspectors must strictly control acceptance processes to ensure the quality of every product. On the other hand, the removed CVT's electromagnetic unit also has some structural flaws that need improvement from an operational perspective. The lower section oil tank lacks oil level and color indicators, making it impossible for operators to see the oil color and level; the tank also lacks an oil filling port and oil sampling valve, preventing maintenance personnel from periodically taking oil samples for analysis and refilling. Furthermore, the low-voltage terminal XL of the compensating reactor cannot be disconnected from ground, causing difficulties for testing. The damper is externally mounted. Because the CVT is manufactured with the capacitor divider, electromagnetic unit, and damper adjusted for errors, the dampers are not interchangeable. This causes inconvenience for the storage, transportation, installation, and commissioning of the CVT. Ideally, an internal damper should be used. Manufacturers of capacitor voltage transformers should consider these issues in future product designs. Improvements to the structure of the CVT electromagnetic unit by equipment manufacturers will facilitate equipment operation and maintenance. During inspections, operators can observe the oil status through the oil level indicator, allowing for early detection of problems. Testing personnel can periodically or after problems are discovered using the oil sampling valve to take oil samples for analysis, enabling early detection of latent faults within the CVT or determination of the fault's nature. Furthermore, if the BZT device at the Jinyuan substation can be put into operation sooner, connecting the backup line voltage to the device via a transformer, then when a transformer fails and the secondary voltage disappears, the BZT device will display a "Backup Voltage Disappeared" indicator, allowing for earlier fault detection.