Influence of Air Density on the Impulse Strength of External Insulation

Positive Switching Impulse Discharge Performance and Voltage Correction of 1 meter Rod-plane Air Gap

“…References [1]- [3] recommend that if the relative humidity is less than 80%, the disruptive discharge voltage of external insulation is in direct proportion with the product of air density correction factor and humidity correction factor. It can be shown in (1), U= UO X8'M1+a(h -11)]w (1) where 3 is relative air density; h is absolute humidity (in g/m3); a is a constant depending on the type of test voltages, with a value of 0.010 (impulse), 0.012 (AC), or 0.014 (DC); m and w are characteristic exponents affected by 3 and h respectively, which are decided by parameter g, as shown in (2), 9~UB 2 500L8[1+ a(h -I1)] (2) where L represents minimum discharge path (in m) and UB is experimental voltage (in kV). Then the values of m and w corresponding to different values of g can be shown as follows: g<0.96, m=w<1.0; 0.96Xg<1.24, m=w=1.0; 2>g>1.24, w<1, m=1; g>2, m=1, w=0.…”

“…It is found in [4] that the discharge voltage U under the non-standard atmospheric condition can be expressed by (2), with Uo as the discharge voltage under the standard reference atmospheric condition. U = Uo x (a(1 -a(h _ 1)))n (3) where 3 is relative air density; h is absolute humidity (in g/m3); n is a characteristic exponent varying with 3 and h. As for a, it is a constant with a value of 0.009 (impulse), 0.0125 (AC).…”

Threshold curves of the various modes of corona discharge in atmospheric air

“…In agreement with [2] the propagation field of the streamers is given by: E, = 5006 k (10) k = 1 + (H -Ho)/100 (11) where H, is the standard value of 11 g/m3. For high altitudes the IEC-60 correction procedure is not adequate, because it implies that the field propagation streamers, that a sea level is about 500 kV/m, does not varies linearly with the altitude.…”

“…AbdelSalam and Allen for spherical point-plane gaps proposed the following relation [4]: being r the radius of the point electrode. For onset voltages Vo they propose: (2) [ ; ] G i p G is a gap factor defined as: G = d r where d is the size gap and:…”

“…Also this standard adopted for ambient air the point-point arrangements as a device for measuring DC high voltage. The revision of the IEC-60 was strongly based on the research carried out by Pigini et al [2] whom assumed that the streamers propagation field at sea level is about 500 kV/m, and they claimed that it varies linearly with 6 for higher altitudes. Recent researches have shown that the semi-empirical equations for correcting DC voltages are not appropriate at high altitudes above sea level, due to the combined effect of the relative air density and the humidity [3].…”