Overvoltage phenomenon is very common in power supply systems. If there are no preventive measures, it could happen at any time. There are many factors that can cause overvoltage in the power grid, mainly including resonant overvoltage, operational overvoltage, and lightning overvoltage. 

Resonant overvoltage occurs at a high frequency and poses significant harm during normal operation. Once overvoltage occurs, it often causes damage to electrical equipment and even leads to large-scale power outages. Usually, overvoltage accidents in medium and low voltage power grids are mostly caused by resonance phenomena.

In the medium and low voltage power grids for power production and operation, there are various forms and operation methods of faults, and the resonance properties are also different. Therefore, it is necessary to understand the properties and characteristics of various types of resonances, master the properties and characteristics of their oscillations, and formulate countermeasures and measures for vibration prevention and damping. At present, most of the 35kV and below distribution networks in China still operate in a neutral ungrounded manner, and some use old-fashioned arc (harmonic) suppression coils for grounding. Practice has proved that in neutral ungrounded systems, on the one hand, there are many ferromagnetic resonance overvoltages caused by the saturation of the voltage transformer core. Although many measures have been taken to limit resonance overvoltages, such as harmonic lamps, harmonic eliminators, adding resistors to the high voltage neutral point of TVs, or using a single TV, they have not been fundamentally solved. TV burnout and fuse melting continue to occur. On the other hand, the main characteristic of the neutral point ungrounded operation mode is that after single-phase grounding, it is allowed to maintain a certain period of time, usually 2 hours, without causing power outages for users. But with the expansion of the medium and low voltage power grid, the number of outgoing circuits increases and the number of lines grows. The capacitance current of the medium and low voltage power grid to ground also increases significantly. When the grounding arc cannot be automatically extinguished during single-phase grounding, it will inevitably generate arc overvoltage, which is generally 3-5 times or even higher than the phase voltage. This will cause electrical breakdown in weak insulation areas of the power grid and lead to equipment damage and power outages caused by phase to phase short circuits. Due to structural limitations, systems that use old-fashioned arc suppression coil grounding methods can only operate in an overcompensation state and cannot be in a fully compensated state. Therefore, the detuning setting is relatively large, about 20% -30%, and has no suppression effect on arc overvoltage. And manual adjustment of the tap is required, but at this time, the voltage cannot be adjusted to the optimal working position in a timely manner according to the changes in the ground capacitance current of the power grid, which affects the function and is not suitable for the needs of unmanned substations in the power grid.

Therefore, a grounding compensation device based on the principle of automatic tuning can be used to effectively solve such problems through operation modes of overcompensation, full compensation, and undercompensation. At present, the automatic tuning grounding compensation device mainly consists of five parts: grounding transformer, electric arc suppression coil, microcomputer control part, damping resistor part, neutral point dedicated transformer, and nonlinear resistor. The grounding transformer is connected to the arc suppression coil as an artificial neutral point. The current of the arc suppression coil is adjusted through the on load switch and remotely controlled automatically. The pre adjustment method is adopted, which means that under normal operating conditions, the tap of the arc suppression coil is adjusted to the optimal position at any time according to changes in the power grid parameters. Automatic tracking and automatic tuning are implemented using a microcomputer controller. By measuring the phase between the displacement voltage and the main and neutral point currents and voltages, it is possible to accurately calculate, judge, and issue instructions for automatic adjustment, displaying relevant parameters such as capacitor current, inductor current, residual current, and displacement voltage. It can also recall, alarm, automatically print and send signals remotely, meeting the needs of unmanned substations. 

The automatic tuning grounding compensation device can achieve full compensation operation or very small detuning, mainly because a high-power damping resistor is connected in series in the primary circuit of the arc suppression coil, reducing the amplitude of the neutral point resonant overvoltage to 5% -10% of the phase voltage. Because if the capacitance current of the system is equal to the working current of the arc suppression coil, that is, the neutral point voltage is limited below the allowable value during resonance, the full compensation method can be achieved, which is the optimal working mode with minimal residual current. When grounded, the residual current is very small and will not cause arc overvoltage. So, high-power damping resistors can be connected in series in the primary circuit of the arc suppression coil to increase the damping rate. The detuning rate of the arc suppression coil is related to the voltage and damping rate of the power grid. When the power grid is formed, its asymmetric voltage is basically a fixed value. To ensure effective suppression of arc overvoltage during single-phase grounding, the detuning rate of the arc suppression coil is required to be within ± 5%. Therefore, only by changing the damping rate can the displacement voltage be changed. Therefore, a resistor should be connected in series in the arc suppression coil circuit to ensure the damping rate and control the neutral point displacement voltage. In low-voltage power grids, due to the small asymmetric voltage at the neutral point, a specially designed neutral point dedicated transformer is used to improve measurement accuracy and enhance detection sensitivity; The use of nonlinear resistors has a significant suppression effect on the overvoltage caused by wire breakage and transmission under under overcompensation.