Required Insulator Length
This process entails selection of dielectric strength of an insulator with respect to voltage and environmental stresses (stress/strength concept) in order to fulfil a specific availability requirement. This is done by evaluating risk of flashover of the various insulation options and selecting those yielding acceptable performance. In this context, insulator performance is typically quantified in terms of Mean Time Between Flashover (MTBF).
Practically, statistical dimensioning of insulators with respect to pollution can be performed by means of dedicated software such as the Insulator Selection Tool (IST). This program allows users to calculate e.g. required insulator length to obtain a given MTBF as a function of pollution level (see Fig. 2). Site severity distributions are normally characterized by 2% values of ESDD/NSDD distributions, while flashover strength of any specific type of insulator is derived from laboratory test results. The calculations take into account number of parallel insulators and number of pollution events.
Procedures for statistical dimensioning of AC and DC insulators are identical, however inputs are different since there is a difference in pollution catch between insulators energized by AC and DC voltage due to electrostatic charging. This effect can have a large impact on Site Pollution Severity (SPS), especially in areas of low wind. For coastal areas where pollution accumulates rather quickly and is deposited by strong winds, influence of electrostatic charging is lower. There is a procedure for estimating DC ESDD based on AC ESDD. Due to the fact that the normally hydrophobic properties of a composite insulator surface are intentionally suppressed during testing, results represent a ‘worst case scenario’. Nevertheless, it has be kept in mind that the pollution performance curve does not represent any specific or typical insulator profile. It is therefore highly recommended to verify the pollution performance of whatever DC insulators are selected by representative laboratory tests for the specific service conditions before final decisions are taken.
Required Clearance to Ground
Maximum attainable power transfer capacity of a converted OHL is given by two factors: maximum DC voltage and maximum conductor current. Maximum voltage is limited by corona and field effects and also by the required insulator length with regard to pollution performance. Maximum current, on the other hand, is limited by maximum allowed conductor temperature or maximum sag with respect to required ground clearance. Required conductor clearance to ground is further determined by the arcing distance of the insulators. Consequently, required insulator length and required clearance to ground are interrelated via DC voltage level.
Maximum Power Capacity
Maximum power capacity is determined by the recommended limits of corona and field effects as well as by requirements on insulator length and clearance to ground. Due to the coupling between required insulator length and required clearance to ground, maximum power capacity is found by applying a set of selected DC voltage levels in a step-by-step procedure comprising:
• Calculation of required insulation length based on pollution performance requirements;
• Determination of required clearance to ground based on the insulator arcing distance;
• Determination of maximum conductor temperature as limited by allowable conductor sag or other restrictions;
• Calculation of maximum power capacity for the selected DC voltage level.