Paper Published on the16th International Symposium on High Voltage Engineering, Cape Town, South Africa, 2009
UNCERTAINTIES IN THE APPLICATION OF ATMOSPHERIC AND ALTITUDE CORRECTIONS AS RECOMMENDED IN IEC STANDARDS Dong Wu1*, Ming Li2 and Mats Kvarngren3 ABB HVDC, SE-771 80 Ludvika, Sweden, 2ABB Corporate Research, SE-721 78 Västerås, Sweden, 3STRI, SE-771 80 Ludvika, Sweden *Email:
[email protected] 1
Abstract: The dielectric strength of air is influenced by air density (temperature and pressure) and humidity. Such effects need to be taken into account when external insulation is designed and tested. Since the conditions of application and the conditions of the laboratory tests may be different, it is often necessary to make corrections between different atmospheric conditions. For engineers at manufacturers, utilities or high-voltage laboratories, they follow the relevant IEC standards. However, atmospheric conditions influence the dielectric strength of air in a complicated way. Simplified and generalized solutions may cause vacillations especially when different recommendations are given in different standards without sufficient clarifications. It is the intension of this paper to give an outline of such issues that may lead to uncertainties in the application of various IEC standards regarding the atmospheric correction. Some proposals are also given for discussion. 1.
INTRODUCTION
The dielectric strength of air is influenced by air density (temperature and pressure) and humidity. Such effects need to be taken into account when external insulation is designed and tested. Since the conditions at the application and the conditions at the laboratory tests may be different, it is often necessary to make corrections between different atmospheric conditions. Creditable studies and reviews have been published on the atmospheric corrections, e.g. [1, 2]. These studies were the base for the recommendations in IEC standards, e.g. [3-10]. For engineers at manufacturers, utilities or high-voltage laboratories, they follow the relevant IEC standards. However, atmospheric conditions influence the dielectric strength of air in a complicated way. Simplified and generalized solutions may cause vacillations especially when different recommendations are given in different standards without sufficient clarifications. Based on practical engineering experience, it is the intension of this paper to give an outline of such issues that may lead to uncertainties in the application of various IEC standards regarding the atmospheric correction. Some proposals are also given for discussion. 2. 2.1.
CORRECTION FOR AIR INSULATION Related parameters of air
The dielectric strength of air is influenced by the air density (temperature and pressure) and humidity. The influence of temperature and pressure can be taken into account simultaneously, at least as a first approximation, by the relative air density, δ, [2]:
δ=
p1 273 + t0 × p0 273 + t1
(1)
Where: p0 and t0 (in degree C) are the pressure and temperature at the standard reference conditions respectively; p1 and t1 are that at other air conditions. Thereafter the atmospheric conditions are converted into mainly two parameters, the relative air density and the absolute humidity. These two parameters have also certain co-effects. It should be noted that for outdoor conditions, it might be assumed that the effects of ambient temperature and humidity tend to cancel each other [4]. 2.2.
Influence of air density
The breakdown of a non-uniform long air gap takes often the processes as corona inception, streamer propagation, leader formation and propagation, and final jump. The streamer and leader processes are the decisive processes. It has been concluded in literature [1-2] that the influence of air density is most significant on the streamer formation and propagation. The air density has little influence on the leader process. Therefore, as an approximation, one may consider if the streamer dominates the breakdown processes in a gap, the dielectric strength of this air gap is proportional to relative air density. This is in principle the case for shorter gaps, shorter than 2 meters. For longer gaps, the breakdown will be resulted by both the streamer and the leader process. Therefore, the dielectric strength of a longer air gap is, in many cases, less than proportional to air density. According to above approximations, the change of the dielectric strength of air gaps with air density may be evaluated by the value of δm. For breakdown caused mainly by streamers, m is equal to one (m=1). Otherwise, for most of the non-uniform long gaps, m is smaller than one (m