Common reasons why voltage transformer fuses do not blow

Author： GOZ Electric Time：2024-09-11 09:33:40 Read：10

In the medium and low voltage distribution network system, a voltage transformer (VT) is installed on the busbar with monitoring, measurement and protection functions. Therefore, the safe and reliable operation of the voltage transformer directly affects the safety and stability of the distribution network. High-voltage fuses are generally connected in series in the voltage transformer circuit to protect the voltage transformer and prevent the voltage transformer failure from affecting the system. Fuses have the advantages of simple structure, safe and reliable operation, low price, and strong current limiting ability, and are widely used in power systems. After the fuse is blown, in order for the VT to continue to work normally, the operator must immediately replace the high-voltage fuse. If it is frequently blown, the reliability of the substation power supply is greatly reduced. In actual operation, the high-voltage fuse of the voltage transformer often blows abnormally, and sometimes the voltage transformer burns irregularly. The figure shows that the voltage transformer of a 35kV substation is burned because the fuse did not blow in time, and the winding scale is 700:1.

In recent years, people have made some progress in the research on fuses. In the past, people believed that ferromagnetic resonance was the main cause of the voltage transformer fuse blowing, but after in-depth research, people believed that ferromagnetic resonance was not the main cause. The main reasons why the fuse does not blow are as follows:

1) Ferromagnetic resonance produces saturation current. If ferromagnetic resonance is not suppressed, the long-term voltage transformer current will cause the voltage transformer fuse to overheat and blow, thereby protecting the distribution network.

2) Electromagnetic transient process. The capacitor discharge impact current after the fault is restored is the main cause of the fuse blowing. After the grounding fault is eliminated, the line voltage is restored from the line voltage to the normal phase voltage. Since the charge on the line-to-ground capacitor is cut off from the ground, the charge can only pass through the fuse and enter the ground through the neutral point of the primary side of the voltage transformer. When the line is long, the free charge is more, generating a larger impact current, causing the voltage transformer core to saturate. This saturation current may exceed the fuse's melting current and cause the fuse to blow. When single-phase instantaneous grounding occurs multiple times in the system, the above process occurs repeatedly, the impact current is large, and the fuse is more likely to blow.

3) Corona discharge: Since the effective part of the fuse is only 0.2~0.5mm, fuses with a rated voltage of 10kV or higher may blow due to the thermal effect related to corona discharge after oxidation.

4) Copper wire oxidation: After the copper wire is oxidized, its resistance increases and it is easier to blow out.

5) Fuse quality: At present, the structure of fuses varies greatly, and the manufacturing quality is often uneven. At present, after the fuse of the voltage transformer blows, the common method is to replace the fuse, but this does not prevent the fuse from blowing. Sometimes, the rated current of the fuse is increased to prevent blowing. This method can reduce the probability of blowing and reduce the protective effect of the fuse on the voltage transformer, resulting in the burning of the transformer. In order to further study the abnormal blowing phenomenon of the fuse and solve these problems in VTs, this paper tests VT fuses with different fuse materials and structures, including steady-state current characteristics, electromagnetic transient impact characteristics, breaking characteristics, X-ray characteristics, and fuse corona characteristics. On this basis, suggestions are made for the selection of VT fuses for distribution networks.

Through various test analyses of VT fuse performance, a basis is provided for the selection of VT fuses. The 2min steady-state current characteristics of electromagnetic voltage transformer fuses show that their steady-state current is too large to effectively isolate the transformer. The electromagnetic transient characteristics show that the steady-state current is too small, and the electromagnetic transient process is prone to cause frequent fuse blowing. The fuse corona discharge also affects the life of the fuse. The 2-minute steady-state current characteristics, fuse corona discharge level, and electromagnetic transient impact characteristics of the selected fuse should be tested. The selection method of the fuse is: when the distribution network system selects VT fuses, 35kV and 10kV systems should use fuses with a rated current of 1A. It is recommended to select single fuses, winding brackets, and fuses that are not easily oxidized. Fuses composed of copper melts should be avoided, and silver or nickel-based fuses are preferred.