In the distribution network with ungrounded neutral point, ferromagnetic resonance caused by voltage transformer often occurs, which seriously affects the safe and stable operation of the system. The installation of microcomputer harmonic elimination device is a common response measure at present. The microcomputer harmonic elimination device has the characteristics of low operation cost and easy maintenance, and only one microcomputer harmonic elimination device can be installed in a system.

In order to eliminate the ferromagnetic resonance, a new solution is adopted, that is, the microcomputer is used to monitor the working condition of the voltage transformer on-line. Once the resonance is found, the thyristor is triggered immediately and the open triangle of the voltage transformer is connected to eliminate the ferromagnetic resonance over-voltage fault. This method is suitable for 3 ~ 66kV neutral point ungrounded or neutral point grounded through resistance and arc suppression coil.

A bidirectional thyristor is connected in parallel at the output end of the open triangle of the voltage transformer, and the zero sequence voltage is input to the device. The microcomputer harmonic elimination device can automatically identify and distinguish system resonance and grounding according to different frequency and voltage value. If it is grounded, it will send out grounding alarm signal; if it is resonance, it will automatically trigger the instantaneous conduction of thyristor, destroy the condition of resonance between voltage transformer and line, absorb resonance energy, and dynamically eliminate ferromagnetic resonance of the system.