This paper analyzes the monitored harmonics data of a 110kV transformer substation, and finds that the insulation breakdown of the reactive compensation device’s series reactor is caused by the serious amplification of the 5th harmonic voltage. Replacing the 4.5% reactor with 6% reactor can lower the 5th harmonic voltage across the reactor, thus reducing the total harmonic distortion of the 10kV bus voltage and avoiding winding overheat and insulation aging of the reactor. Based on the findings, some suggestions on reducing harmonics’ impact on reactive compensation device are given.