36-WG11/Stafford/120 IEC TC 36-WG11

F. Schmuck

Comments on the STRI-DSL-Method

Both, composite and glass cap and pin insulators have been tested in respect to their pollution performance. The selection of this test method was done by Statnett with the objective to introduce a short string length for 420 kV-insulators used for a voltage upgrade from 300 kV to 420 kV. The environment is know to be relatively uncritical but in some areas the line is very close to the sea. The results of testing are documented in the paper attached. The following excerpt can be given: -

due to the application of the pollution in the deposit phase, the aerodynamic behaviour of the insulator is considered as well the progressive method provides quantitative evaluation of the insulator performance the salt concentration can be adjusted corresponding to the in-situ conditions procedure can be used to simulate conditions for instantaneous flashovers

The definition of the variable test parameters to be correlated with the considered in-situ ones is the challenge for this procedure. The procedure is also suitable for large diameter apparatus.

Submitted to the NORD -IS 2001

Use of progressive stress in DSL-tests to compare the pollution performance of insulators S. Berlijn, C. Engelbrecht STRI, Sweden 1.

K. Halsan, T. Ohnstad Statnett, Norway

Abstract

The Dry-Salt-Layer (DSL) method has been developed at STRI as a service adopted pollution test method for ceramic and composite insulators. It is intended to simulate critical flashover conditions close to the coast that can be described as the accumulation of semi-dry salt particles on the insulator followed by wetting in the form of rain or fog. Normally pollution tests are performed as withstand test to qualify an insulator for use in a certain polluted environment. A withstand test is however not suited to compare the performance of various insulators with each other. The need for such a comparison arises when upgrading an HV-line or building a new line or replacing insulators. STRI has therefore developed a new test method, the DSL-progressive stress test. A picture of the set-up is shown in Figure 1. With the results of the DSL-progressive stress test it is not only possible to compare the pollution performance of different insulators, but it is Figure 1 Picture of the DSL test set-up also possible to estimate the flashover probability under certain pollution and network conditions. The successful application of the test and evaluation method is presented. 2.

Introduction

Composite insulators are finding increasing use in both overhead lines and substation equipment. This is especially true in polluted areas where it is believed that the hydrophobic properties, offered by some composite insulators, result in an improved flashover performance in comparison with glass and porcelain insulators.

Methods for the selection and specification of insulators have been the subject of many studies with a host of statistical and deterministic methods as described in [1]. The selection and dimensioning method recommended in [2], is based on the laboratory simulation of natural pollution. It comprises of three steps: 1. Evaluation of the type and severity of the pollution at the site where the insulators will be used 2. Specification of a laboratory test and its test parameters as representative as possible for the pollution at site 3. Selection of the insulators that show good behaviour under this test For composite insulators (i.e. hydrophobic insulators) no standardized pollution testing procedures are available. Indeed, a CIGRÉ taskforce concluded that for coastal environments, an alternative to the Salt-fog method is needed that will reproduce the same ranking of composite insulators as is determined under natural conditions [4]. Such a laboratory test method, called the “Dry-Salt-Layer method” (DSL) has been developed at STRI from the principles of the present Salt-Fog method. It is intended to simulate critical flashover conditions close to the coast that can be described as the accumulation of dry salt particles on the insulator followed by wetting in the form of rain or fog. [8] The DSL method can be used either to determine the flashover characteristics of an insulator or to qualify the insulator for a specific environment. The qualification of insulators is normally performed through a withstand test, of which the parameters, e.g. severity and number of tests, should be chosen with regard to the site severity and the number of insulators in parallel to ensure that the overhead line or substation will perform as expected. Besides the qualification of insulators in certain situations, e.g. a voltage upgrading of an existing transmission line, a more detailed analysis of the insulator’s flashover performance is sometimes needed. In these type of projects there is often only a limited space available to increase the length of the insulation. The flashover performance of the insulator also impacts the availability of that line. Methods are therefore required to facilitate the optimization of line and insulator performance versus costs. The progressive stress method is therefore introduced in the DSL test. With this method it is possible to compare the pollution performance of different insulators and to estimate the flashover performance under certain pollution conditions and at a certain voltage based on a required maximum outage probability of the line. In the following the DSL test method, the DSL withstand test, the DSL progressive stress tests as well as an application of the DSL progressives stress test will be explained. 3.

DSL test method

The withstand Dry-Salt-Layer test method (DSL) was developed to overcome the deficiencies identified [4] in the Salt-Fog method. It comprises a separate pollution deposit and wetting phase, as is shown in Figure 2. For the duration of the withstand test, that is, during both the pollution and the wetting phase, the test object is energized at the specified test voltage.

3.1

The deposit phase

The DSL utilizes wind as the driving force to deposit humid salt particles on to the test object. Salt Deposit Density (SDD) measurements at a coastal site have shown a strong correlation to wind speed, indicating wind as a dominating factor that determines the amount of pollution on insulators at coastal sites. In the laboratory, the humid salt particles are generated by a salt injection system, which is built up with standard IEC 60507 [3] nozzles. The salt spray is, however, not directed towards the test object as in the Salt-Fog test. Its purpose is rather to suspend humid salt particles in the airflow established in the laboratory chamber during the test.

Figure 2 Structure of the Dry-Salt-Layer withstand pollution test. The duration of the deposit phase depends on the selected severity and can vary between 20 and 60 minutes while the wetting phase is fixed at 100 minutes.

The test severity of the DSL is quantified by the time of exposure to the wind-borne salt particles. SDD measurements on a reference insulator, such as a string of IEEE type porcelain cap and pin insulators, or any other object specified by the customer, are used to calibrate the exposure time to the pollution severity specified 3.2

The wetting phase

The wetting phase utilizes a modified steam fog and lasts for 100 minutes. This newly developed wetting method has been developed to achieve optimal wetting conditions on composite insulators. In this method steam fog at the standard intensity is gently blown - at about 2 m/s - towards the test object. 4.

DSL withstand test

A withstand pollution test is used to qualify insulators for a certain environment. The test severity, named the withstand pollution severity, is selected based on actual pollution measurements at the site for which the insulators are intended, or from published guidelines or given by the customer. According to IEC 60507 [3] the pollution test is considered as a withstand, when a maximum of one flashover in four tests occurs. This principle is also applied to the DSL tests. A withstand test is aimed at verifying that the insulator has at most a 10% flashover probability at the specified pollution severity according to IEC 60071-1. The result of the test is either “passed” or “failed”, so no details are obtained about the actual flashover voltage. Due to the limited number of tests performed (i.e. a maximum of four tests), there is a certain risk that the insulator’s true flashover probability is higher than 10%. In other words, there is a risk that an insulator has passed the test when it actually should have failed.

The withstand test is not suited for a more accurate comparing of the pollution performance of different insulators, because if cannot provide the required details of the flashover performance of the insulators. Such a comparison is typically needed when the most cost-effective insulators are considered for the upgrading of an existing line to a higher operation voltage, or if a line needs to be re-insulated to improve its pollution performance. 5.

DSL progressive stress test

With the results of the DSL-progressive stress test, it is possible to estimate the U50 for each insulator at a specified pollution condition. This enables a more accurate comparison of the pollution performance of different insulators. Such results also allow the estimation the flashover probability under certain pollution and network conditions, which can be very valuable information whilst designing a grid. A progressive stress test, also called rising voltage test [5], is a test in which the voltage applied to the object is increased in a step-wise fashion, with the parameters selected in such a way that a statistical evaluation of the results is possible. This method has been successfully applied to normal AC flashover tests [5] and has now been applied to the DSL pollution test. From the statistical point of view [5] the parameters defining the progressive stress test are: the number of tests, the starting voltage, the step duration, the step amplitude, the number of steps and the rise time of the increase in voltage.

voltage in Volt

The use of a variable voltage in pollution tests requires careful consideration. This is especially so for pollution test methods that uses a pre-deposited pollution layer. It has been shown for such tests, that the flashover voltage varies during the test, decreasing at first due to the wetting of the pollution layer and then increasing because of the washing effect [7]. This problem was overcome by developing the wetting method to produce Main part of progressive stress an even wetting profile, and to aim the 400 progressive stress part to the time when 350 the insulator has its lowest flashover volt300 age. Previous tests have shown that the 250 time of lowest flashover voltage was in starting voltage the time interval 30 to 100 minutes [5] [7] 200 from the start of the test. An example of 150 the modified progressive stress test is pre100 sented in Figure 3. The voltage is ramped up from zero at the start of the wetting phase to reach the starting test voltage at the beginning of the test phase, i.e. the 30th minute of the wetting phase. The same step amplitude is used as during the test phase, but with a much shorter step duration. This is done to avoid a cold switch-on event [1], which may lead to a flashover at switch-on.

50

0 0

20

40

60

80

100

time in minutes

Figure 3 Example of the modified progressive stress procedure

Because the flashover characteristics are not exactly known beforehand, there is a risk that the insulator won’t flashover during the main part of the progressive stress test (that is from the 30th to 100th minute). For this reason the voltage is increased with shorter steps (e.g. three minutes), but still with the same step amplitude, from the 100th minute onwards to obtain a flashover as soon as possible. The step duration in the main part of the progressive stress test is chosen to be 10 minutes. With this step duration, 7 steps can be made in the allotted test time. The step duration is chosen so that the step is sufficiently long for static test conditions arise, that means that it should be long enough to enable the heating by the leakage current and to enable dry band arc phenomena, which are typical for pollution events. The steps should however not be so long that too few steps could be made in the time interval available so that a statistical evaluation of the result is no longer possible. This means that at least 5 steps should be made. The step amplitude together with the starting voltage is a function of the properties of the insulator under test, e.g. its expected U50 and standard deviation. More about how to select these values is explained in [5]. When the results of the DSL progressive stress test are obtained, this means the flashover voltages for each object tested, these results are evaluated using a statistical program written (and purchased) especially to evaluate results of progressive stress tests [6]. The result of this statistical evaluation is the U50 and the standard deviation in the results obtained. These two values are given for a 95% confidence level, but are for the purpose of progressive stress test in combination with DSL tests, converted to a 80% confidence level because this is the confidence level applicable to pollution withstand tests. Example of application of progressive stress in DSL tests

The DSL progressive stress tests has been used to compare one glass and two polymeric strings, which were considered for the upgrading of a 300 kV line to a 400 kV in a coastal environment in Norway. The applied voltage to the insulators during the test is presented in Figure 4. The evaluated results of one glass string and of one of the composite strings are presented in Figure 5 and Figure 6.

600 500 voltage in kV

6.

400 300 200 100 0 0

50

100

150

time in minutes

Figure 4 The applied voltage during the progressive stress test

Figure 5 Flashover probability function for the glass insulators tested

Figure 6 Flashover probability function for the composite insulators tested

From these results, the U50 can be determined as well as the estimation for the standard deviation. For the glass insulators the U50 equals 252 kV ± 15 kV and for the composite insulator it equals 468 kV ±15 kV for a confidence level of 80%. When comparing the U50 and the distribution functions, in principle three conclusions can be drawn; 1) there is a significant difference 2) there is a difference, but not significant 3) there is no difference. In this example there is a significant difference because the distribution functions do not overlap. The difference in the U50 is in the order of 200 kV. With the aid of the distribution functions presented in Figure 5 and Figure 6, i.e. the distribution function of the voltage for a certain ESDD level, the distribution function of the of the ESDD level for a certain voltage can be estimated. The latter distribution

function is the insulator’s performance function for different ESDD levels, or in other words it shows the flashover probability under certain pollution and network conditions. When knowing the distribution function of the environmental pollution the number of flashovers in a certain period can be estimated. The latter is an important design parameter for utilities. The leakage current measurements performed during the test showed that a step duration of 10 minutes is a good choice. It might be possible to shorten the step duration to 7 or 8 minutes, this is under investigation. When the results of the DSL-progressive stress tests performed are now compared to the results that would have been obtained when a DSL withstand test would have been performed the following remarks can be made. From Figure 5 it can be seen that the flashover probability for the glass insulator at the system voltage that is 243 kV is 25%. From Figure 6 it can be seen that the flashover probability for the composite insulator is around 3%. This means that in 97% (=100%-3%) of the cases the composite insulator would have passed the test and that in 63% of the cases the glass insulator would have passed the test. From the results obtained from the DSL progressive stress test it is clear that the pollution performance of the composite insulator tested is much better than the pollution performance of the glass insulators tested and that it is most likely that the U10 of the glass insulator (232 kV ± 15 kV) is lying below the required value (243 kV). Only in 37% (=100%-63%) of the DSL withstand tests performed one would have been able to detect a difference pollution performance of the two insulators tested. This means that only one out of the three DSL withstand tests performed would have given the same (correct) answer. 7.

Conclusions

Because there were no suitable pollution test methods available to test composite insulators, STRI developed the DSL method. Together with the DSL method two test methodologies can be used, the withstand test method and the progressive stress test method. The withstand method should be applied when insulators should be qualified for certain pollution and network conditions. The progressive stress method should be applied when one wants to compare the pollution performance of different insulator or when more detailed information about the flashover performance of the insulator under subject is needed. This is demonstrated by the test performed on two types of insulators. The results of the DSL progressive-stress test carried out showed that it is possible to use the progressive stress method while performing DSL tests. The obtained test results were as expected and showed a good repeatability. The obtained standard deviation was smaller than expected and illustrates the robustness of the DSL test procedure. However, it is recommended to perform four instead of three DSL-progressive stress tests on each object. This is due to the fact that the uncertainty in the measurement will decrease.

8.

Acknowledgements

The authors of the paper would like to thank prof. W. Hauschild for his contributions to this work. 9.

References

[1] CIGRÉ task force 33.04.01: “polluted insulators; a review of current knowledge”, CIGRÉ Technical Publication 158, June 2000 [2] IEC publication 60185: ”Guide for the selection of insulators in respect of polluted conditions”, 1986 [3] IEC publication 60507: “Artificial pollution tests on high voltage insulators to be used on a.c. systems”, second edition 1991 [4] CIGRÉ Study Committee 33 Working Group 04: “A critical comparison of artificial pollution test methods for HV insulators”, Electra No. 64, pp. 117-136 [5] W. Hauschild and W. Mosch: ”Statistical techniques for high-voltage engineering”, Book, Peter Peregrinus, 1992 [6] W. Hauschild: Statistical analysis program for the progressive stress method [7] Lambeth, P.J.: “Variable-voltage application for insulator pollution tests”, IEEE Trans. on Power Delivery, Vol.3, No.4, Oct. 1988, pp. 2103-2111. [8] Insulator News and Market Report: “New Dry Salt Layer test seen as more realistic method for maritime pollution testing”, Part 1 March/April 2000 and Part 2 May June 2000