In the power system, there are three different types of resonance: low frequency resonance, power frequency resonance and high frequency resonance. The resonance circuit consists of inductance elements with iron cores (such as no-load transformer, voltage transformer) and capacitive elements in the system. Affected by the saturation of the iron core, the inductance parameter of the core inductance element is nonlinear. This circuit with nonlinear elements will generate ferromagnetic resonance overvoltage under certain resonance conditions. The matching of the nonlinear inductance of the voltage transformer and the line-to-ground capacitance leads to the ferromagnetic resonance overvoltage, which directly threatens the safe operation of the power system,and even causes the explosion of voltage transformer (PT) in severe cases. The traditional solution is to connect a resistor at both ends of the opening triangle of the voltage transformer, and theoretically, for the ferromagnetic resonance with lower frequency, the resistance should be smaller; while the too small resistance on the PT opening triangle will affect its normal operation, and even cause PT burnout in severe cases. In addition, the ferromagnetic resonance has different frequencies, so it is difficult to eliminate the resonance of all frequencies by this method.

Given the above situation, our company has independently developed the DWX-III computer harmonic elimination device by summarizing the shortcomings of other peers. The device adopts a two-way SCR at the PT opening triangle, and a high-performance microprocessor and a data acquisition system to analyze the zero sequence voltage signal of the power grid taken from the opening triangle, and then issues an instruction to the SCR after calculation. When the ferromagnetic resonance occurs in the system, the PT opening triangle has zero sequence voltage with different frequency components. The device automatically recognizes and distinguishes the ferromagnetic resonance (frequency dividing resonance, fundamental frequency resonance or high frequency resonance) and grounding according to different frequencies and different voltage values; if it is grounded, the grounding signal is sent by the signal relay. Only when the ferromagnetic resonance occurs in the power grid, the SCR will be turned on, the triangle winding suffers short circuit, and the ferromagnetic resonance disappears rapidly under strong damping. When the resonance disappears, the SCR returns to the blocking state.

The device applies to small current grounding systems of 66KV and below with neutral ungrounding, grounding by arc suppression coil and grounding through resistor. It can be widely applied to power supply systems in the fields of electric power, petroleum, chemical, coal, metallurgy, construction, etc.