Overvoltage phenomenon of power transformer and its protection measures

When the transformer is running, if the voltage exceeds its maximum allowable operating voltage, it is called the overvoltage of the transformer. Overvoltage often has a great hazard to the insulation of the transformer, and even breaks the insulation.

1, the problem is raised

When the transformer is running, if the voltage exceeds its maximum allowable operating voltage, it is called the overvoltage of the transformer. Overvoltage often has a great hazard to the insulation of the transformer, and even breaks the insulation. The overvoltage is divided into two types: operating overvoltage and atmospheric overvoltage. When the transmission line is directly struck by lightning or thundercloud discharge, the overvoltage caused by the drastic change of the electromagnetic field is called atmospheric overvoltage; when the switch on the transformer or line is closed or pulled, the electromagnetic energy oscillates and accumulates in the system. The overvoltage generated is called the operating overvoltage. Both of these overvoltages of the transformer are short-lived transients.

The operating overvoltage is generally 3.0 to 4.5 times the rated voltage, and the atmospheric overvoltage value is very high, up to 8 to 12 times the rated voltage, and the voltage distribution in the winding is extremely uneven, and the voltage at the end of the incoming line is 匝Very high. Therefore, necessary measures must be taken to prevent the occurrence of overvoltage and effective protection.

There are two cases where the overvoltage destroys the insulation in the transformer. One is to break the insulation between the winding and the core (or the fuel tank), the insulation between the high voltage winding and the low voltage winding (these insulation is called the main insulation); It is the insulation breakdown between the 匝 and 匝 or between one winding and the other winding in the same winding.

Since the overvoltage time is extremely short, the voltage rises from zero to the maximum value and then falls to zero in a very short time. Therefore, it has the characteristics of high frequency oscillation, and its frequency can reach above 100 kHz. In normal operation, the frequency of the grid is 50 Hz, the capacitive reactance of the transformer is large, and the inductive reactance ωL is small, so the influence of the capacitor can be neglected, and the current flows completely through the inside of the winding.

2, the cause analysis

The following is a brief description of the causes of two different types of overvoltages:

(1) Operating overvoltage

In the general power grid, the vast majority of the used transformers are step-down transformers. The following is an example of the operation of the overvoltage generated by the step-down transformer pull-down operation.

According to the folding algorithm of the transformer parameters, when the secondary side (low voltage side) capacitor is converted to the primary side (high voltage side), the capacitance conversion value is small, so the influence of the secondary side capacitance can be neglected. That is to say, the influence of the secondary side can be ignored at no load. In the case of primary windings, since the capacitance CFe' to ground capacitance per unit length is parallel, the total capacitance to ground is:

CFe=ΣCFe'

Since the inter-turn capacitance Ct' on the unit side length of the primary side is connected in series, the total capacitance value between turns is:

Ct=1/(Σ1/Ct')

In the transformer, usually CFe>>Ct, so in the qualitative analysis, the effect of the turn-to-turn capacitance can also be omitted.

When the no-load transformer is pulled from the grid, if the instantaneous value of the no-load current is not equal to zero but a certain value Ia, then the corresponding applied voltage instantaneous value is Ua. Then, at the moment of the pull operation, the magnetic field energy stored in the primary side inductance L1 is 1/2 (L1Ia2), and the electric field energy stored in the capacitance CFe is 1/2 (CFeUa2). Since the circuit of the transformer is a circuit in which the inductor L1 and the capacitor CFe are connected in parallel, an electromagnetic oscillation process will occur in the loop at the moment of the pull operation. During the oscillation process, when the current is equal to zero at a certain moment, the magnetic field energy is all converted into electric field energy, which is absorbed by the capacitor, and the voltage on the capacitor rises to the maximum value Ucmax.

When the operating current of the pull gate and the voltage on the capacitor are constant, the larger the inductance of the winding, the smaller the capacitance to the ground, the higher the overvoltage of the pull operation. In a power system, the operating overvoltage typically does not exceed 3.0 to 4.5 times the rated voltage.

(2) Atmospheric overvoltage

Atmospheric overvoltage is caused by the drastic changes in the electromagnetic field when the transmission line is directly subjected to lightning strikes or thundercloud discharges. When the transmission line is directly subjected to lightning strikes, a large amount of charge (set as a positive charge) carried by the thundercloud falls onto the transmission line through the discharge channel, and a large amount of free charge propagates to both ends of the transmission line, causing an impact on the transmission line. The voltage is called the lightning overvoltage. The section in which the lightning wave rises from zero to the maximum value is called the wave head, and the falling part is called the wave tail. If the time taken by the wave head is regarded as a quarter cycle of the periodic wave, the lightning wave can be regarded as a periodic wave with extremely high frequency. Thus, when an overvoltage wave reaches the transformer terminal, it is equivalent to adding a very high frequency to the transformer. This transient process is very fast. At the beginning, due to the high frequency, ωL is very large, 1/ωC ​​is small, and current flows only from the tantalum capacitor of the high voltage winding and the capacitance to the ground. Since the low voltage winding is close to the core, its capacitance to ground is large (ie, the capacitive reactance is small), and the low voltage winding can be approximately considered to be grounded. When a lightning wave strikes, the voltage distribution along the winding height depends on the ratio of the inter-turn capacitance Ct to the capacitance CFe. Under normal circumstances, since both capacitors exist, when an overvoltage occurs, part of the current is shunted by the capacitance to ground, so the current of each turn-to-turn capacitor current is not equal, and the current flowing through the turn-to-turn capacitance near the end of the power line is the largest. The smaller the back, the smaller the distribution of the voltage along the winding height becomes, the initial voltage distribution is more uneven, and there is a large voltage gradient between the first few turns near the end of the power line. Therefore, before the end of the power line, In several wire loops, the insulation between turns is greatly threatened, and the maximum turn-to-turn voltage may be as high as 50 to 200 times the rated voltage.

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