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Design of Insulator Strings for 600 kV DC Lines in Harsh Desert Environment

July 8, 2017 • ARTICLE ARCHIVE, Utility Practice & Experience
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A bulk power HVDC link between the Central and West Saudi power grids has been designed and is under construction. The purpose of the link is to provide economic dispatch for the generating units, flexibility to share capacity under normal conditions and provide back-up during emergency operating conditions. The scheme consists of two poles with a circa 770 km long overhead line between Bahrah Substation and Dhuruma Substation. The system is designed for nominal power carrying capacity: 3500 MW; and rated voltage: ±600 kV. This recent INMR article contributed by A. H. Al-Mubarak of the Saudi Electricity Company, A. Ardito of CESI, I. Banchi of PROJECT and INMR Columnist, Alberto Pigini, reviews the process of designing the line’s insulation for its harsh service environment, including seacoast and desert.


New 600 kV DC line in Saudi Arabia. insulator Design of Insulator Strings for 600 kV DC Lines  in Harsh Desert Environment fegeasefwa

Fig. 1: New 600 kV DC line in Saudi Arabia.
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The AC insulation design for EHV and UHV systems is in nearly all cases dominated by the performance requirements under switching overvoltages, which determine the arcing distance; the pollution performance requirement is then satisfied by selecting insulators with a suitable ratio between creepage distance and arcing distance (creepage factor CF).

In the case of DC systems, the insulation design is most often dominated by pollution requirements, especially in harsh environment as in Saudi Arabia. This is essentially due to the following three reasons:

• contamination deposited on insulators is generally higher in DC lines than in AC lines;
• for the same voltage level (reference to phase-to-phase rms value for AC), the creepage distance required for DC, at a given insulator contamination condition, is higher than for AC;
• magnitude of switching overvoltage in DC systems is generally lower in p.u. than in AC.

Therefore the pollution requirements are very critical for DC lines and have a determining influence on the design and cost of the overall system. In fact an over-dimensioning (leading to extremely long and costly insulators and huge towers to accommodate long insulator sets) may result in unacceptable investment costs. On the other side, an under-dimensioning may lead to unacceptable operating costs (e.g. with the need of costly palliative maintenance measures). It is therefore necessary, for the DC lines, to follow a detailed design approach, based on accurate estimate of the pollution severity and dielectric withstand characteristics of insulators, combined to a statistical evaluation of the insulator performance.

Estimates of Pollution Severity

The estimation of the site pollution severity requires the following basic information site related:

• ESDD: equivalent solid deposit density (mg/cm2) obtained by measurements on typical insulators
• NSDD: non soluble deposit density (mg/cm2) obtained by measurements on typical insulators
• Number of wetting events (events that, humidifying the pollution layer, makes the flashover possible)

Estimate of Pollution Severity for AC Systems

A wide and long duration research was carried out to map the contamination severity of Saudi Arabia by means of long duration measurements, as shown in the example of Fig. 2, in several test stations, located in different representative areas as shown in Fig. 3.

Example of pollution build-up dependence on time measured on different insulator types. insulator Design of Insulator Strings for 600 kV DC Lines  in Harsh Desert Environment Screen Shot 2016 02 11 at 1

Fig. 2: Example of pollution build-up dependence on time measured on different insulator types.
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Location of AC energized and non-energized test stations in Saudi Arabian territory. insulator Design of Insulator Strings for 600 kV DC Lines  in Harsh Desert Environment Screen Shot 2016 02 11 at 1

Fig. 3: Location of AC energized and non-energized test stations in Saudi Arabian territory.
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This research gave a quite unique and excellent information about the pollution conditions in harsh environments, such as Saudi Arabia.

The ratio NSDD to ESDD was found very spread. However the NSDD value is generally significantly higher than ESDD as expectable in marine/desert environment. Averaging the available information, a ratio NSDD/ESDD equal to 5 may be assumed. On the basis of the measurements, preliminary indications on the necessary unified specific creepage distance USCD were given, as reported in Table 1.

Table 1: Pollution Characteristics for AC Insulator Preliminary Design  insulator Design of Insulator Strings for 600 kV DC Lines  in Harsh Desert Environment Screen Shot 2016 10 21 at 15

Table 1: Pollution Characteristics for AC Insulator Preliminary Design
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The above information was the basis for SEC specifications of the necessary USCD for AC insulators. The following values of ESDD were finally assumed by SEC to standardize the insulation for AC lines, as shown in Table 2:

• ESDD=0.3 mg/cm2 in the inland areas of WOA (Western Operating Area), EOA (Eastern Operating Area), SOA (South Operating Area), while a lower value was assumed for COA (Central Operating Area).
• ESDD= 0.55 mg/cm2 in the coastal areas.

 

Table 2: Minimum Creepage Distance (referred to phase-to-ground voltage) Specified for AC Systems in SEC Specification for Glass/Porcelain Insulators *To be selected by the design engineer based on past experience in the project area ** Coastal area is defined as the area located within a distance of 100 km from the coastline for EOA and 50 km from the coast line for WOA and SOA  insulator Design of Insulator Strings for 600 kV DC Lines  in Harsh Desert Environment Screen Shot 2016 10 21 at 15

Table 2: Minimum Creepage Distance (referred to phase-to-ground voltage)
Specified for AC Systems in SEC Specification for
Glass/Porcelain Insulators
* To be selected by the design engineer based on past experience in the project area
** Coastal area is defined as the area located within a distance of 100 km from the coastline for EOA and 50 km from the coast line for WOA and SOA
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