In order to improve the voltage protection level of the surge protection gap（SPG） and the coordination between the SPG and the surge protection devices（SPDs） in the secondary stage

we designed an SPG with an active field-breakdown trigger circuit

which was called the active SPG. The circuit model of the active trigger circuit was established and the mathematical expressions of the elements parameters in the circuit were obtained. In addition

the element parameters of the trigger circuit were selected to verify the performance of the active SPG. The results showed that the active field-breakdown trigger circuit effectively improved the voltage protection level of SPG. When the discharge gap distance d was in the range of 0.55 mm to 2.1 mm

the

voltage ratio K of the active SPG

which was defined as the specific value of the pulse breakdown voltage U

pulse

and the DC breakdown voltage UDC

was 50% less than the SPG without an active trigger circuit. In the condition that the discharge gap distance d was 0.95 mm

the peak value of the impulse voltage U

m

increased from 3.4 kV to 6.5 kV

and the U

pulse

was increased from 3.36 kV to 4.55 kV. It was obviously lower than 6.41 kV

which was the U

pulse

of the SPG without an active trigger circuit. The results of the study can provide a theoretical and experimental basis for the performance improvement of the active SPG and the accurate coordination of the SPDs with different characteristics. Besides

they had also laid the foundation for the research of the mechanism and application of overvoltage protection.