Shielding electrodes areusually specified for AC lines to eliminate corona from insulator fittings, tograde the voltage potential along the string and limit radio interference frominsulators as well as the effect of power arcs. In particular, gradingelectrodes are essential in AC to mitigate voltage drops/electric field on insulators,especially on the line side. Grading electrodes significantly reduce the stressand limit radio interference for cap & pin insulator strings. This rolebecomes even more important for composite insulators to avoid localizedcritical electric field strength that may cause premature ageing. The situationis different in DC where voltage distribution is generally dominated byresistive currents, both under clean and contaminated conditions. However,design of shielding electrodes for DC line insulators in recent projects hasstill been similar to what is usually adopted in AC. This edited contributionto INMR by insulation expert, Alberto Pigini, reviews different shieldingrequirements for both AC and DC lines and presents results of past investigationsconducted on cap & pin insulator strings. The case for composite insulatorsis also analysed by ad hoc electric field calculations performed under AC andDC voltage.
Advertisement
Laboratory Experience with Cap & Pin Insulators
Grading electrodes are essential in AC to mitigate voltagedrop/electric field along insulators, especially on the line side. Fig. 1, forexample, reports voltage drop, in p.u., measured across each cap & pin discfor a 19 unit 420 kV insulator string – with and without shielding electrodes.
The trend can be reproduced by electrostatic field calculations,taking into account insulation geometry and permittivity of the differentmaterials. While evaluated under clean conditions, the voltage distribution canalso be assumed valid under contaminated conditions whenever capacitivecurrents prevail over conductive currents. Fig. 2 provides a direct comparisonbetween measured voltage drops, with and without shielding electrodes.
It is immediately evident that, without grading electrodes, thevoltage drop on the first unit of the string can be high, e.g. up to threetimes the average. Grading electrodes significantly reduce this stress and aretherefore important to limit radio interference for cap & pin insulatorstrings under AC voltage. Indications for shielding design to comply withcorona and radio interference design with special reference to UHV AC havealready been published.
The situation, however, isdifferent in DC where voltage distribution is dominated by resistive currents –both under clean and contaminated service conditions. Non-uniformdistributions, similar to those in AC, can be obtained for new cap & pininsulators, cleaned thoroughly under conditions of low humidity. Conductancevalues of 10-13 S with currents on the order of magnitude of nA have beenrecorded on new insulators cleaned using ultrasound. Conductance values of10-12 to 10-11 S have been recorded on new insulators cleaned carefully withalcohol, with surface currents of some tens of nA.
On the other hand, if an insulator string and fittings are welldesigned from the corona point of view, the configuration can be considered‘corona free’. In this case, the ion current in air is limited, i.e. on theorder of only a few nA. Under ideal laboratory conditions, with new andextremely clean insulators, ion currents in air can have influence voltagedistribution, being on the same order of magnitude as surface current. Thismakes voltage distribution close to that in AC, i.e. dominated by capacitances.
Under practical field conditions, however, surface conductance caneasily exceed 10-9 S for weathered or very lightly polluted insulators. Surfacecurrents are then on the order of hundreds of nA or higher, thereby making theeffect of ion currents in air negligible and with a tendency toward a linearvoltage distribution. Fig. 3 shows an example of voltage distribution measuredon a 32 unit cap & pin insulator string for 500 kV DC, without gradingelectrodes. The string was uniformly contaminated with low pollution (ESDD=0.01mg/cm2) and exposed to a normal environment with humidity below 76%.
Composite Insulators
Simplified Representation for Electric Field Calculation
As an example, reference is made to composite insulators for UHVapplications and design of suspension insulators adopted for the 1000 kV ACsystem in China.
Since the purpose of these calculations is to put into evidence thedifferent role of grading rings in AC and DC, a simplified schematization ofthe composite insulators is made. Total insulator lengths and size of theshielding electrodes were taken, as in Fig. 8, both for AC and DC. Thecomposite insulator is represented by a rod, without sheds, with a 90 mmdiameter of homogeneous material having a dielectric constant, ɛ, of 3 and a conductivity, σ, of 10-13 S/m. For the surrounding air,with ɛ of 1 and conductivity σ of 10-14 S/m were assumed in the simplifiedcase of absence of significant ionization by corona. Surface conductance wassimulated assuming a pollution layer of 1 mm having the same conductivity allalong the insulator. Different conductivities of the pollution layer wereconsidered as representing clean laboratory conditions (i.e. the ideal case),weathered and lightly polluted insulators that had lost part of their initialhydrophobicity. Voltage distribution was evaluated by ANSOFT under AC and DC atthe same voltage of 800 kV. To obtain an indication of field gradient along thesurface, reference was made to the voltage derivative along the surface.
Analysis
Behaviour of insulators under clean, ideal conditions is similar inAC and DC and shielding electrodes have basically the same role in terms ofmitigating electric field. However, as soon as insulators become ‘weathered’,with surface leakage current on the order of magnitude of 1 μA, distribution ofpotential for DC is dominated by surface conductance. As a result, in the caseof AC shielding electrodes, these have significant influence on electric fieldand may even be mandatory for composite insulators to avoid the possibility ofpremature ageing due to both dry and water-induced corona (i.e. max. allowableE-field of 0.42 kV/mm for more than 10 mm along the insulator surface and notexceeding 0.35 kV/mm on the end fitting seal have been recommended).
By contrast, under practical conditions, influence of shieldingelectrodes becomes negligible in DC. Shielding electrodes of reduced size maystill be useful in DC if there is a need to shield sharp metallic points on thefittings to avoid corona. But they may not be useful to control electric fieldalong the insulator, as necessary for AC applications. An additionalconsequence is that corona and RIV tests on DC insulators, with the insulatornew and performed under ideal clean conditions, may not be representative ofactual service conditions.
Actual insulator conditions may prove more complex than what hasbeen assumed in the above. As example, charges can accumulate in the space andalong the insulators as a consequence of ionization phenomena or conductivityalong the surface might not be uniform due to different levels ofcontamination, e.g. higher accumulation on the live side. However, in all casessurface condition will continue to dominate voltage distribution, makingnegligible the capacitive influence due to the shielding electrodes.
