Ferromagnetic resonance over-voltage phenomenon often occurs in 35kV or 10kV distribution network due to electromagnetic voltage transformer saturation. The measures taken are to install primary harmonic eliminator and microcomputer harmonic elimination device.

When the grid resonance occurs, the zero sequence current flows through the high voltage winding of the voltage transformer, and the zero sequence voltage is induced at both ends of the open delta winding. When the resistance is connected, it will flow through the zero sequence current, and the resistance will consume energy. In principle, the smaller the resistance is, the more energy is consumed, and the more obvious the effect of limiting and damping resonance is.

The primary harmonic eliminator can not be grounded for a long time, because when the grid occurs continuous single-phase grounding, 100V power frequency zero sequence voltage will appear at both ends of the open delta winding of voltage transformer, which requires that the primary harmonic elimination should have sufficient capacity. At the same time, the primary side of the voltage transformer also flows a lot of current during the single-phase grounding period, so in the neutral point ungrounded power grid, the resistance with too small resistance value is not allowed to be connected to the open delta winding for a long time, so as to avoid the voltage transformer overheating and burning.

      The secondary side of voltage transformer is usually installed with microcomputer harmonic elimination device, which can detect the open triangle voltage of voltage transformer in real time and distinguish the fault information such as grounding, resonance and over-voltage. Thyristor is used to short-circuit the open triangle of voltage transformer. In order to protect the thyristor from burning, measures must be taken to limit the current in the circuit and the rising speed of current, which will lead to the fault information of grounding, resonance and overvoltage The impedance of each circuit is relatively large. Once the impedance increases, the effect of harmonic elimination will be affected.

Since primary harmonic elimination and microcomputer harmonic elimination have their own limitations when they operate separately, the method of combining primary harmonic elimination with intelligent microcomputer harmonic elimination device is usually adopted. In order to take harmonic elimination measures for distribution network, the harmonic elimination device should be selected according to the actual operation of the power grid to ensure the safety of the power grid and electrical equipment.