The Research Of Flexible Measures To Limit The Neutral Point Over-voltage Of 110kV-220kV Transformer

There are currently three protection strategies in Neutral Point of Power Transformers: protective gap,MOA,and gap and MOA in parallel.But the operation experience shows that all the three measures have some defects.The flashover condition that happened in the protective gap of 110 k V transformer neutral point without grounding and the fault records are analyzed in this paper according to the 110 kV line tripping failure in Power Gird.Lightning,switching,single-phase ground and open-phase running over-voltage are also calculated in the operation process.The results show that the lightning overvoltage,single-phase ground overvoltage and the open-phase running overvoltage may higher than insulation levels of the transformer neutral point.Therefore a new protective method of controllable gap and arrester in parallel is given in this paper.When the system occurs single-phase ground fault but not losing ground,the overvoltage will not threaten the neutral point insulation,so the controllable gap and arrester should not operate.When the system occurs temporary overvoltage(Single phase ground fault and losing ground or open-phase running),the controllable gap operates to protect the neutral point insulation and arrester.When the system occurs lightning over-voltage or switching over-voltage,the controlled gap should not operate and the arrester will protect limit the over-voltage.And a series of tests like lightning impulse test,switching impulse test,frequency voltage test are done that verifies the rationality and credibility of the protection.