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This post combines two cases of handling failures involving burned-out voltage transformers in high-voltage systems, to share experiences regarding the proper installation of voltage transformers as well as methods for dealing with similar faults. Case 1 of burnout caused by incorrect wiring of a voltage transformer 【Faulty equipment】10kV voltage transformer in the substation of a chemical manufacturing company 【Fault symptoms】This voltage transformer was put into operation during autumn, and no abnormalities occurred. But after a lightning strike in the spring of the following year, the voltage transformer was damaged. The maintenance crew believed that there was a problem with the quality of the PT, so they replaced the PT and the fuse and put the equipment back into operation. However, not long after, the TV was damaged again due to lightning strike. [Diagnostic Analysis] Original address: yunrun.com.cn/tech/1906.html ① Figure 1a shows the original wiring of this 10kV voltage transformer PT; the neutral point of the PT was grounded through a lightning arrester FUn. http://yunrun.com.cn/upload/201804/09/201804091618050911.png Figure 1a: Incorrect wiring of a 10kV voltage transformer. ② Electrical engineers from Changhui Instrument Manufacturing Co., Ltd. conducted a thorough investigation and found that when performing secondary wiring, the manufacturer producing this voltage transformer cabinet directly grounded point M on the secondary side of phase L2 of the PT, as shown by the dashed line in Figure 1a. In this way, when the FUn is broken down by lightning, both ends of the secondary coil of phase L2 are grounded. The coil short-circuited directly, causing the PT to burn out. 【Fault Handling】In accordance with relevant regulations, connect the wires as shown in Figure 1b, and move the grounding point M to the outlet end of fuse FUb. In this way, if FUn is struck by lightning, FUb will blow, thereby protecting the voltage transformer. http://yunrun.com.cn/upload/201804/09/201804091623465200.png Figure 1b: Wiring of a 10kV voltage transformer. Case study 2 on handling a voltage transformer burnout fault. [Faulty equipment] The voltage transformer cabinet in a certain high-voltage switchgear room. [Symptoms of the fault] During operation, a grounding signal was detected at the 35kV main substation. Upon inspection, it was found that the fault occurred in Cabinet No. 3 (the voltage transformer cabinet) within the lower-level 6kV switchgear room; one of the voltage transformers had burned out. [Diagnostic analysis] ① After replacing the voltage transformer and putting it back into service, it burned out again a few days later. ②The wiring of the voltage transformer is shown in Figure 2. Three JDZJ-6, 6/0.1kV voltage transformers are connected in an open delta configuration for use in monitoring the insulation of the power grid. Under normal circumstances, the voltage across the open delta is zero ； When a single-phase ground fault occurs in the power grid, a three-time zero-sequence voltage appears across the open delta of the voltage transformer, causing the voltage relay YJ to activate and send out a ground fault signal. http://yunrun.com.cn/upload/201804/09/201804091624098845.png Figure 2: Wiring of the voltage transformer. ③ During inspection, it was found that the coil of the voltage relay YJ had burned down and turned into a black, charred mass. Obviously, this coil is short-circuited, causing the open delta winding of the voltage transformer to become short-circuited as well; a large current flows through the transformer. ④Find the cause of the burned-out voltage relay. The feed cables of this substation have been in use for over a decade, and as the power load increases year by year, the cables heat up continuously, resulting in a severe deterioration of their insulation. Single-phase ground faults occur frequently, which causes the voltage relay YJ to activate. In the process of locating the fault point, the time required to isolate the fault was delayed, causing the YJ to operate frequently and thus burning out the coil. ⑤Except for YJ short circuit. Another factor that causes voltage transformers to burn out is a single-phase ground fault, which leads to ferroresonance in the transformer and results in resonant overvoltage. [Fault Handling] Replace the damaged voltage transformer and voltage relay, and gradually replace the feed cables with smaller diameters and lower insulation quality. [Experience Summary] Technical library: yunrun.com.cn/tech/ ① The secondary side of the voltage transformer coil must not be short-circuited. Its winding wire diameter is very thin; during normal operation, its secondary load consists generally of relay coils and instruments, which have high impedance and are essentially in an unloaded state, resulting in a very small current flowing through the transformer. When the secondary side is short-circuited, the secondary current is very high; without the protection of a fuse, the voltage transformer can easily be damaged. ②The feeder cables for substations and distribution rooms must be selected according to the load requirements. Cables with small diameters and poor insulation should be replaced promptly; otherwise, they may pose potential risks of faults.