A new controllable gap in parallel with MOA at 110kV and 220kV transformer neutral-point without earthing

In order to protect insulation of 110kV and 220kV transformer neutral-points without earthing and overcome shortcomings in existing protective measurements such as gaps, metal oxide arresters (MOA, shortly), gaps in parallel with MOA, and so on, a new protective measurement was put forward. A controllable gap was used in parallel with MOA. The controllable gap was made up of a fixed gap in series with a controlled gap. The fixed gap was a composite goat horn gap. The controlled gap was a single-phase vacuum switch. Both the controllable gap and MOA cooperated to realize effective protection for transformer neutral-point. When there were accidents of single-phase-to-earth without earthing or open-phase operation in system, the control circuit provided action signals. The controlled gap was closed. Voltage on the fixed gap was higher and the gap was closed to protect insulation of transformer neutral-point. On lightning overvoltage, the controllable gap didn´t act. MOA acted to limit overvoltage of transformer neutral-point. On other overvoltage, both the controllable and MOA didn´t act. Insulation of transformer neutral-point could withstand such overvoltage. The new protective type could effectively protect insulation of transformer neutral-points and solve problems of existing measurements. It would have much engineering application value.