36-WG11/Vienna/(36)39

Dong Wu 20/01/00 ABB Power Systems

1.

Scope and object

2. 2.1

General considerations Brief introduction of pollution flashover mechanism

2.2

Parameters and approaches for the insulator selection and dimensioning The selection and dimensioning of outdoor insulators is an involved process; a large number of parameters must be considered for a successful result to be obtained. For a given site, the required inputs are in three categories: system requirements, environmental conditions of the site, and insulator parameters from manufacture catalogues. Each of these three categories contains a number of parameters as indicated in the table below. These parameters are further discussed in later chapters. System requirements Withstand voltages Reliability, availability, maintainability. (R.A.M.) Costs Installation position, clearance, …

Environmental Conditions Altitude Pollution level and types Rain, fog, ice and snow, … Wind Temperature, humidity, UV Lightning Earthquake Vandalism

Insulator catalogue Type Material Profile Creepage Form factor (diameter) Arcing distance

To select suitable insulators from the catalogue based on the system requirements and the environmental conditions, three approaches (A, B, C, in the fig below) are recommended. Among them approach A gives the highest confidence in the results while C the lowest.

System requirements

Insulator parameters

Environmental conditions

Pollution level Existing operational experience or new field testing

Existing laboratory test results or new tests

M.S.C.D. Correction factors

A

C

B

Selection & dimensioning

In fig: M.S.C.D. is the minimum specific creepage distance. In reality, the dielectric strength of the insulator is determined by the complicate and dynamic interactions among the environmental and the insulator parameters. Such interactions are well represented in an operating line or station and can be represented in a test station. Such interactions can not be fully represented by laboratory tests, e.g. the tests specified in IEC507 and IEC1245. In approach C, such interaction can only be represented by the correction factors in a limited degree. Approach C is simple and cheap for the dimensioning process but the whole costs, including the R.A.M requirements have to be considered when choose among the three approaches. Whenever circumstances permit, the approach A should be adopted at the first hand. 2.3 Approach A To obtain the operational experience of the existing line or station, an example of a questionnaire is given in Appendix A (in old 815). To utilise obtained information the flowchart below may be followed.

Test staions of varied complecity can be used. A desciption of test stations of varied complecity are given in Appendix ?. 2.4

Approach B To utilise the existing test results or to specify new laboratory tests (methods and test severity), the pollution level and type of the site should be obtained first. This subject is presented in chapter 3. The information obtained from existing lines or test stations can also be used. For the laboratory test methods one can find them in corresponding IEC standard. Non-standard method may be used upon agreement.

2.5

Approach C To obtain the pollution level the method given in chapter 3 should be followed. The required minimum specific distance and correction factors are given in chapter 6.

3

Level Clean

Light

Medium

Heavy

Very heavy

Examples of typical environments At normal wind speed > 50 kmI from the sea, a desert, or open dry land > 10 km from man-made pollution sources II Within a shorter distance than mentioned above of pollution sources, but: • prevailing wind not directly from these pollution sources • and/or with regular monthly rain washing 10-50 kmIII from the sea, a desert, or open dry land 5-10 km from man-made pollution sources II Within a shorter distance than mentioned above of pollution sources, but: • prevailing wind not directly from these pollution sources • and/or with regular monthly rain washing 3-10 kmIV from the sea, a desert, or open dry land 1-5 km from man-made pollution sources II Within a shorter distance than mentioned above of pollution sources, but: • prevailing wind not directly from these pollution sources •and/or with regular monthly rain washing

ESDD mg/cm2 < 0.01

Further away from pollution sources than mentioned above (distance in the range specified for “Light” areas) but: • dense fog (or drizzle) often occurs after a long (several weeks or months) dry pollution accumulation season • and/or the present heavy rain with high conductivity • and/or there is a high NSDD level, between 5 and 10 times the ESDD

0.01-0.03

Within 3 kmIV of the sea, a desert, or open dry land Within 1 km of man-made pollution sources II

0.10-0.25

With a longer distance from pollution sources than mentioned above (distance in the range specified for “Medium” areas) but: • dense fog (or drizzle) often occurs after a long (several weeks or months) dry pollution accumulation season • and/or the present heavy rain with high conductivity • and/or there is a high NSDD level, between 5 and 10 times the ESDD Within the same distance of pollution sources as specified for “Heavy” areas and: • directly subjected to sea-spray or dense saline fog

0.03-0.10

0.01-0.03

0.03-0.10

>0.25

• or directly subjected to contaminants with high conductivity, or cement type dust with high density, and with frequent wetting by fog or drizzle Desert areas with fast accumulation of sand and salt, and regular condensation I. during a storm, the ESDD level at such a distance from the sea may reach a higher level, typically in the range 0.03-0.08 mg/cm2 . II. the presence of a major city will have an influence over a longer distance, i.e. the distance specified for sea, desert and dry land. III. during a storm, the ESDD level at such a distance from the sea may reach a higher level, typically in the range 0.1-0.2 mg/cm2 . IV. depending on the topography of the coastal area and the wind intensity

Description of test station Voltage Measurements used Operating • Flashover voltage I or higher • Leakage current voltage • Pollution levels on insulators • Weather A lower • Leakage current voltage • Pollution levels on insulators level • Weather

Results obtained Direct information about required insulator lengths, performance of different materials/shed profiles, critical weather conditions, and pollution levels.

Direct information about performance of different materials/ shed profiles, and critical weather conditions. Information about pollution levels after extrapolation to the operating voltage. A different • Leakage current Information about performance of different voltage • Pollution levels on insulators materials/ shed profiles, and pollution levels, type after AC/DC conversion of the results. • Weather Direct information about critical weather conditions. No voltage • Pollution levels on insulators Preliminary information about performance of different materials/shed profiles, and pollution • Weather levels, after conversion of measurements to energized conditions. No Rough estimate of the pollution level expected • Weather and air pollution II insulator on insulators after conversion to energized measurements conditions I. Flashover voltage may be obtained by using the insulators of several different lengths. Some of the insula tors of the shorter lengths are expected to flashover during critical weather conditions. Il. Instruments and methods are available for collecting and analysing airborne particles. One should note that it is not easy to predict the amount of pollution which will be accumulated on insulators from such measurements.