IEEE 930:2004 pdf free download – IEEE Guide for the Statistical Analysis of Electrical lnsulation Breakdown Data
3.1.1 Commonly used testing techniques
There are two commonly used breakdown tests for clectrical insulation: constant stress tests and progressivestress tests. In these tests a number of identical specimens are subjected to identical test regimes intended tocause electrical breakdown. In constant stress tests the same voltage is applied to each specimen (they areoften tested in parallel) and the times to breakdown are measured.The times to breakdown may be widelydistributed with the longest time often being more than two orders of magnitude that of the shortest. In pro-gressive stress tests an increasing voltage is applied to each specimen，usually breakdown voltages aremeasured. The voltage may be increased continuously with time or in small steps.Other protocols,forexample impulse testing, may also be used.Breakdown voltages may be much less widely distributed withthe highest voltage sometimes only being 2% more than the lowest voltage.
Various international standards, e.g., BS 2918-2 and IEC 60243, give appropriate experimental proceduresfor constant and progressive stress tests. This guide is intended to provide a more rigorous treatment for thebreakdown data obtained in this way.
3.1.2 Other data
Breakdown data may also be available from other sources; for example, times to brcakdown of the insula-tion in service may be available.Such data is gencrally much more difficult to analyze since the history ofcach failed insulator may not be the same (sce 3.1.4), particularly as units that failed will have been replaced.lt may also be unclcar how many such insulation systems are in service and hence what proportion of themhave failed.The techniques described in this guide are, neverthcless, appropriate for such data provided suf-ficicnt care is exercised in their application.
3.1.3 Data requirements
The number of data points required depends upon the number of parameters that describes the distributionand the confidence demanded in the results. If possible,failure data on at least ten specimens should beobtained; serious errors may result with less than five specimens (see also 3.2.2).
lf all the specimens break down, the data is referred to as complete. In some cases, not all the specimensbreak down, the data is then referred to as censored.Censored data may be encountered in constant stresstests where the data are analyzed or the test is terminated before all the specimens break down.Censoreddata can also occur with progressive stress tests where the power supply has insufficient voltage capabilityto break down all the samples. In these cases, the data associated with a single group of specimens,thosewith the highest strength, are not known and the data set is said to be singly censored.’ Data may also beprogressively censored. In this case, specimens may be withdrawn (or their data discounted) at any time or voltage; such data are often referred to as “suspended.”This may be the case where specimen breakdown isdue to a spurious mechanism such as termination failure or flashover or where the specimen is deliberatelywithdrawn for alternative analysis. Censoring can occur by plan or by accident in many insulation tests andit is essential that this is taken into account in the data analysis. Less confidence can be placed in the analysisof a censored data set than in a complete set of data with the same number of specimens. If possible censoreddata sets should include at least ten (non-censored) data points and at least 30% of the specimens shouldhave broken down.
3.1.4 Practical precautions in data capture
Specimens should, as far as possiblc, be identical, have the same history prior to testing, and be tested underthe same conditions. In measuring the breakdown characteristics of materials it should be noted that thebreakdown field (kilovolt per millimeter)’is usually dependent upon the rate of voltage rise,specimenthickness, clectrode material, configuration and method of attachment, temperature, area, and frequency ifan alternating voltage is applied. Other factors such as humidity and specimen age may also be important.With insulating systcms such as cables and bushings, surface and interfacial partial discharges must be min-imized and stress enhanccments due to protrusions, contaminants and voids are likely to reduce breakdownstrengths considcrably.