Power utilities and insulator manufacturers continue to express differing views as to the relative value of shielding electrodes in DC. Back in 1972, significant laboratory research had already been undertaken on the influence of shielding and grading electrodes on voltage distribution in AC as well as DC. Further investigations were later published.
There are a number of reasons why shielding electrodes of different design are felt necessary and specified for AC lines:
• to eliminate corona from insulator fittings (corona shielding function);
• to grade voltage potential along the insulator string in order to limit radio interference and, in the case of composite insulators, to reduce risk of ageing (grading function);
• to limit the effect of power arcs (protective function).
As is well known, grading electrodes are essential in AC to mitigate the voltage drops/electric field along insulators, especially on the line side. Fig 1, for example, compares the voltage drop, in p.u., measured across each cap & pin disc for a 19 unit 420 kV insulator string – with and without shielding electrodes. This trend can be reproduced from electrostatic field calculations (taking insulation geometry and permittivity of the different materials into account). While evaluated in clean conditions, it can also be assumed valid under pollution whenever capacitive currents prevail over conductive currents. It is immediately evident that, without grading electrodes, voltage drop on the first unit of the string can be very high, e.g. up to three times the average. Grading electrodes significantly reduce this stress and are therefore important to limit radio interference for cap & pin strings. They are even more important for composite insulators to avoid a critical level of localized electric field.
The situation, however, is different in DC, where voltage distribution is dominated by resistive currents – both in clean and contaminated service conditions. As example, the voltage distribution measured on a 32 unit cap & pin insulator string for 500 kV DC without grading electrodes is shown in Fig. 2. The string was uniformly contaminated with very low pollution (ESDD=0.01 mg/cm2) and exposed to a normal environment with humidity lower than 76%. The voltage on the first insulators in the string is only marginally higher than the average and therefore no additional field grading is needed. The situation is attenuated in real conditions, where higher pollution accumulation at string extremities can enhance lack of uniformity in voltage distribution.
It also has to be considered that shielding electrodes do not have the capability to control voltage distribution dominated by resistive current parameters. Confirmation of their negligible impact is given by RIV measurements performed on a 21-unit insulator string with and without shielding electrodes (see Fig. 3). Results confirm that the influence of shielding electrodes is negligible up to about 450 kV and then actually becomes detrimental once corona starts from the shielding electrodes themselves.
In conclusion, for both cap & pin and composite DC insulator strings:
• shielding electrodes with voltage grading function (unlike for AC) are not recommended;
• insulator fittings may nevertheless need to be locally shielded or suitably designed to avoid localized corona on the fittings themselves, especially for the highest system voltages. But if shielding electrodes are not suitably designed, their effect can even become detrimental (as in Fig. 3).
• shielding electrodes are also less necessary than in AC from the point of view of power arcs, given short circuit current and duration in DC.