Rapid developments in EHV/UHV transmission systems require a deeper understanding of the mechanism of long air gap discharge. Leader propagation is one of the main processes in long gap breakdown. In this paper

the leader propagation characteristics of real size ±800 kV UHVDC transmission tower gaps under positive switching impulse voltages (185/ 2290 μs) are investigated. An integrated observation platform consisting of an impulse voltage divider

a coaxial shunt

a high-speed video camera

and a set of integrated optical electric field sensors (IOES)

is established. The waveforms of impulse voltage

discharge current

electric field variation at specific positions

and time-resolved photographs of discharge morphology are recorded. Axial leader velocity and the relationship between leader advancements and injected charge are obtained. The typical value of leader stable propagation velocity is 1.7-2.2 cm/μs

which varies slightly with the gap length and applied voltage amplitude. The leader velocity in the re-illumination process is much higher

and is seen as varying from 5 cm/μs to 30 cm/μs

with an average value around 10 cm/μs. The charge in leader channel per unit length is 20- 40 μC/m

which illustrates a near-direct proportion relationship between discharge current and leader velocity. The observed parameters are important for further simulation of the tower gap breakdown processes.