This paper investigates a failure of a 35 kV neutral point unearthed system in a hilly territory at the altitude about 1 000 m.The system’s No.1 busbar electromagnetic voltage transformer（PT）had exploded with an open-circuit bus connected.Then the PT was replaced and a harmonic eliminating device was installed. However

the PT exploded again soon after the replacement.High voltage diagnostic tests and oil tests were conducted

and the changes of electric properties and insulation strength after the failure were obtained by comparing with historical data.Waveform recorded during the failure was also analyzed.Discharge to earth was found at the common terminal of the PT’s primary side

and apparent changes in the phase difference of the three-phase voltage were observed in the waveform.The cause of the failure is summarized to be improper insulation match between the PT and the harmonic eliminating device.As a result of the poor match

single-phase arc grounding occurred to cause over-voltage on the N-terminal and subsequent breakdown of its insulation to earth

which resulted in malfunction of the harmonic eliminating device

and subsequently resulted in rare fundamental frequency parallel resonance of single-phase（phase A） and 1/2 fundamental frequency resonance of phase B.The induced heavy current blew the primary side fuse to make the PT overheat and then explode. The investigation result suggests that this kind of failure can be avoided by proper selection of the device type.