Transpower, the TSO in New Zealand firstused pollution index measurements during uprating of its HVDC transmissionsystem. The country’s only HVDC system was originally installed in 1965, rated±250 kV, 600 MW, and pollution flashover performance has generally beensatisfactory over its first 24 years of operation, both in inland and coastalsections.

This HVDC link was subsequently uprated attwo stages. First, it was uprated to +250/-350 kV, 1240 MW by reconfiguring thevalve groups, installing new 350 kV valve groups and reinsulating the line for350 kV. In 1989, the insulation requirements for 350 kV equipment wereconsidered and a program was initiated to collect pollution data at variouslocations along the line and at the cable terminals. This program measured ESDDon energized transmission insulators at 8 line locations.

Pollution flashover tests were then carriedout on the type of insulators used on the 250 kV line (i.e. NGK CA808porcelain) as well as on the later more modern design (NGK CA745-EJ porcelainfog type) to establish relative performance. These tests were carried out attwo ESDD levels, corresponding to the inland and coastal areas. For the inlandsection, 14 new insulators at 350 kV did not fully equal the performance of 12of the old insulators at 250 kV. Also, a line overvoltage study showed that 14insulators would not meet the design criteria for switching surge withstandperformance. Hence, 15 insulators were used in that section

The line was reinsulated using porcelaininsulators with a creepage distance of 54 mm per disc and a disc spacing of 170mm, resulting in a creepage to string length ratio of 3.2. Coastal areas,having an ESDD level of 0.12 mg/cm2 required insulator strings consisting of 33discs while inland sections, with ESDD level of 0.01mg/cm2, required strings of15 discs.

Currently, Transpower has replaced many ofthese original porcelain insulators with glass. Also, during the past years,silicone composite and silicone-coated glass insulators have been considered toimprove pollution performance in coastal areas after an unsuccessful experiencewith EPDM material. For example, the EPDM insulators installed on Transpower’sHVDC line demonstrated minor erosion at the interface with the cold-endfittings, shed punctures along the insulator length and significant cracks onthe core rod housing close to the end fittings.

IEC 608154 presents a simplified methodfor determining the USCD required of DC insulators based on CIGRE TB 518Guidelines. According to this standard, the most accurate way to obtaininformation on site severity is to obtain data directly from the operationalexperience of DC lines. ESDD values measured on energized porcelain insulatorsmust then be corrected to determine site pollution severity if the candidateinsulator differs from the reference insulator. Therefore, Transpower currentlyuses the IEC correction guideline to determine the USCDdc for HTM and non-HTMinsulators other than the porcelain insulators used for ESDD measurements.

The reference dc UCSD (RUSCDdc) isdetermined and corrected for candidate glass (non-HTM) and silicone composite(HTM) insulators to obtain the required USCD for each candidate. The empiricalequation correlating the reference creepage distance to pollution severitygiven in IEC 60815-4 is in the form of the following equation:

Where B and α are empirical constants thatdiffer for each insulator type and γ is the pollution severity expressed interms of ESDD for type A pollution and site equivalent salinity, SES, for typeB pollution.

Using glass insulator units with 170 mmspacing and 550 mm creepage distance, a 44-disc insulator string is requiredfor coastal regions. Number of discs will decrease to 35 using insulator unitswith 190 mm spacing and 690 mm creepage distance. These numbers result ininsulator lengths of between 6.6 to 7.5 meters.

Since the transmission line structures hadnot been fully modified for the uprated system voltage (except for 24structures), such long insulators could not be fitted into the top geometry ofthe existing lattice towers without violating required electrical clearances.Currently, both glass insulator strings (33 units of 170 mm spacing) andcomposite insulators (NGK with 22664 mm creepage distance and 5.6 m length) areinstalled on different sections of the transmission line. The pollutionperformance of the silicone composite insulators has been satisfactory.Nevertheless, the long-term performance of these insulators has had to bemonitored.

The other challenge was the highcorrosivity index along the line’s route, which required a zinc collar at theend fitting of composite insulators. The poor pollution performance of previousinsulators (mainly porcelain and EPDM) in coastal regions combined withdifficulties associated with fitting a long glass insulator string intoexisting tower top geometry and uncertainties regarding pollution performanceof installed silicone composite insulators all led to the decision to monitorpollution performance of insulators installed. From May 2002 to June 2003,Transpower carried out a 12-month program at 15 AC substation sites to assessthe prevailing environmental factors at each. These tests included monthly dustdeposition measurements as well as actual monthly, 3, 6, and 12 monthequivalent salt deposit density measurements by measuring surface conductivity.

In 2019, Transpower started measuring theleakage current on DC energized silicone coated glass and composite insulators.Also, monthly DDDG and wind measurements started in September 2019.

Attend the 2022 INMR WORLD CONGRESS inBerlin where Transmission Design Engineer, Kamran Rezaei of Transpower, willreview service experience with different insulator designs on New Zealand’sHVDC transmission system. He will also explain how monitoring leakage currenton coated glass insulators has allowed assessing whether hydrophobicity canallow reduced creepage distance of strings by correlating data from fieldstudies with the approach of IEC 60815-4.

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