Based on the specific problem studied in this article: a fuse of a certain voltage transformer exploded abnormally at the moment of closing, and it is known that another intact electromagnetic voltage transformer will be replaced, but the same explosion still occurs. In response to this issue, this chapter focuses on analyzing secondary side short-circuit faults, resonance caused by saturation of the intermediate transformer iron core, resonance when the power grid is connected to the primary side, unreasonable replacement of fuses, and high-frequency interference in the power system.
Analysis of the causes of the accident:
1. The situation of iron core saturation
In actual operation, closing the no-load circuit can cause overvoltage, leading to saturation of the iron core of the electromagnetic voltage transformer and a decrease in excitation reactance, which excites stable capacitor and inductor resonance, reduces the equivalent impedance of the system, brings large currents, and causes the high-voltage fuse of the electromagnetic voltage transformer to melt. The overvoltage caused by the system transition process causes the core of the nonlinear inductance element to saturate, triggering stable capacitance and inductance resonance, reducing the circuit impedance, forming a large resonance current, and causing the high-voltage current limiting fuse on the electromagnetic voltage transformer to burst. At this point, as the iron core of the electromagnetic voltage transformer has reached saturation and the excitation reactance is relatively small, the influence of the excitation current cannot be ignored. Due to the relatively small values of each resistor in the circuit, a large resonant current will be generated during the primary inversion of the electromagnetic voltage transformer.
2. Analysis of Short Circuit Fault on the Secondary Side of Voltage Transformer
In order to obtain the ideal voltage source during operation, an electromagnetic voltage transformer is connected in series with a compensating inductor coil in the network. At this point, the inductive reactance and the equivalent capacitance of the voltage divider form resonance to reduce the impact of the load on the output voltage error. Due to the low leakage impedance of transformers during normal operation, and the low load impedance when a short circuit fault occurs on the secondary side. Causing the overall calculation result to be biased, resulting in an excessive current value on the primary side of the electromagnetic voltage transformer. If the secondary side high-voltage fuse does not immediately disconnect this short-circuit current, the electromagnetic voltage transformer will be subjected to overcurrent in reverse. The experiment measured that its short-circuit current can reach dozens of times the rated current, causing its short-circuit capacity to exceed the fuse connected to the electromagnetic voltage transformer or the electric force of the short-circuit current to exceed the maximum electric force that the fuse can withstand, resulting in the secondary high voltage fuse of the voltage transformer melting.
3. Resonance occurs between the voltage transformer and the primary power grid connected to it
For the convenience of calculation, the following simplification has been made. As mentioned earlier, due to the high impedance of electromagnetic voltage transformers, they can be equivalent to a capacitor. Secondly, the circuit is mainly based on reactance, which is equivalent to reactance here. This is a simple circuit structure. When the line resonates with an electromagnetic voltage transformer to a certain harmonic, i.e. XL=XC, where xL is the equivalent reactance of the line and xC is the equivalent capacitance of the voltage transformer, an overvoltage will be generated on the electromagnetic voltage transformer, resulting in saturation of the transformer core. As analyzed earlier, it will cause significant resonant overcurrent, which can lead to the rupture of the connected electromagnetic voltage transformer.
4. Other reasons analysis
(1) Unreasonable replacement of high-voltage fuses and improper selection of high-voltage fuse models. Due to the low rated current of the high-voltage fuse of the electromagnetic voltage transformer, when one of the phase high-voltage fuses bursts. Due to mutual inductance between each phase, although the high-voltage fuses of the other two phases did not burst, they may have been severely damaged or the characteristics of the internal melt may have been compromised. If only one phase of the high-voltage fuse is replaced solely for economic considerations, the other two phases may still burst during the second insertion. During the replacement process, the equipment in the circuit, especially some capacitive devices, was not given sufficient discharge time. After being put back into operation, residual charges on the capacitors can also cause overvoltage, which in turn causes ferromagnetic resonance and overcurrent, causing the high-voltage fuse to melt.
(2) The impact of high-frequency interference. In the equivalent circuit of an electromagnetic voltage transformer, both the connected transmission line and its own equivalent circuit contain nonlinear inductance and capacitance components with high heat content, which will cause the invasion of high-order harmonics. Similarly, due to the carrier communication device connected to the secondary side, it will also bring high signal transmission. If the capacitance and inductance parameters of the electromagnetic voltage transformer and its connecting circuit are not set reasonably, resonance will occur between the terminal and a certain frequency of these high-frequency signals, which will also generate resonance overcurrent and cause the high-voltage fuse protecting the electromagnetic voltage transformer to burst.