Three particularly interesting papers were presented during the Technical Meeting of Study Committee D1 under Preferential Subject 1, Compact Insulation Systems (AC and DC). What made them so interesting was not only their content but also their relevance to today’s key issues in transmission. These include testing polymeric materials and insulators for DC or AC/ DC hybrid applications and reaching conclusions on how to optimize design.
As discussed in my first column of 2016, hydrophobicity retention as a material property is affected differently by DC stress when compared to the rms value of an equivalent AC stress. The higher resistance shown at DC means it takes longer to experience loss of hydrophobicity – especially for well-formulated grades of silicone rubber. This is a remarkable finding given the pollution behavior of hydrophilic surfaces such as glass or porcelain under DC and the correspondingly long creepage distances that require long conventional insulator strings. These papers dealt not with hydrophobic surfaces but rather with the so-called late ageing phase, i.e. characterized by a hydrophilic surface that allows initiation of dry-band arcing. During this phase, severe ageing such as erosion or tracking can occur. Moreover, risk of pollution flashover grows significantly.
Paper D1-109 presented comparative studies with the Inclined Plane Test as a material test and with the Salt Fog, 5000-h Multi-Stress Test and Rotating Wheel Test as insulator design tests. The key difference is that only a simple specimen shape is used in material tests so as to focus mainly on properties. Design tests, by contrast, evaluate both material properties and shed geometry as well as the housing to end fitting interface on short insulator specimens that are functionally complete. In both cases, results of material and design tests show that differences in behavior of AC and DC dry-band arcing can lead to different severity of damage. Typically, DC stress shows a trend to more damage because of the missing zero stage of the driving voltage. However, mobility of discharges can be different when they are subject to an ‘uplift’ by thermal and magnetic forces (see Fig. 1).
These differences between AC and DC ageing tests call for definition of corresponding test procedures. This is in line with earlier CIGRE publications such as TB 611 (Feasibility Study for a DC Tracking & Erosion Test: 2015) where ranking of material families in terms of erosion and tracking performance was shown to be much different in AC versus DC. For example, testing of complete short insulator designs during a DC ageing test also comprises evaluation of design aspects such as resistance of the seal to discharges and interaction with corrosion as well as overall impact of shed profile on the behavior and mobility of discharges. The paper concluded that the combination of both types of testing, i.e. DC material and insulator tests, formed the basis for successful service performance of 500 kV and 800 kV DC lines in China. It is also worth mentioning that erosion and tracking performance becomes important when loss of hydrophobicity has increased surface wettability such that discharges in the mA range can occur.
Paper D1-112 investigated the influence of a hybrid AC/DC voltage stress on pollution flashover performance (also under hydrophilic conditions) as well as on ageing. Here, a hybrid voltage directly galvanic superimposed on specimen electrodes was distinguished from a test set-up that uses field coupling to superimpose both voltages. The latter model is considered to be more realistic for all insulation between tower (ground) and high voltage. The former applies more for applications such as interphase spacers that are in direct contact with DC and AC systems. The impact of field coupling on pollution flashover voltage was practically negligible. However, it was found that just before full flashover occurred the arc stem moved towards the external field electrode (see Fig. 2, HV-FCE = High Voltage Field Coupling Electrode with DC). This effect was not seen under test conditions in the mA range, as typical for dry-band arcing. These results put into question the existing rule of thumb for composite insulator strings that for a hybrid field with less than 10% DC component, AC design principles apply whereas DC design principles apply when the DC component is more than 10%. If confirmed by further tests, this 10% threshold might need to be shifted to e.g. 50%.
A wheel test was also used as an ageing test (see Fig. 3). Field coupling was simulated by having specimens side-by-side, one with DC (-21 kV) stress and one with AC (15 kVrms) stress. Findings were that there was some surface erosion under DC but no erosion for AC specimens. On the DC specimen ground end fittings, which act as anodes, expected corrosion was found. Overall, however, no influence of the hybrid field could be found. Upcoming further research will focus on the influence of hybrid fields on hydrophobicity retention.
Paper D1-110 evaluated the current situation with regard to the 5000-h multi-stress test (IEC 62730). Up to now, this test applies only to AC stress but would have the potential to be used for DC as well (see Paper D1-109). When the current edition of IEC 62217 was prepared, a lot of care was taken to homogenize fog stress in the chamber and to prevent direct spraying toward insulator surfaces. In IEC 62730, however, direct spraying is still included and this causes a high and very specific stress on the housings – especially those with larger diameters. This creates risk either that erosion can be stimulated or that multiple flashovers cause significant extension of test time due to more frequent interruptions. Service experience has proven that these test conditions are in most cases not representative and particularly so for apparatus housed in silicone rubber. This paper summarizes recent results and proposes improving test parameters similar to what was introduced for IEC 62217. IEC TC 36 will take note of this input when next conducting maintenance of standard IEC 62730.
From the above review, it seems clear that increased DC as well as AC/DC transmission has raised interesting questions on how best to test material properties and insulator design. In fact, one of the ‘beauties’ of using composite insulators in these cases is that there are many technical opportunities to adjust material properties in order to make them more suitable for such applications.
Dr. Frank Schmuck