ROLE AND SIGNIFICANCE OF UNCERTAINTY IN HV MEASUREMENT OF PORCELAIN INSULATORS A CASE STUDY

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1 International Conference on Ceramics, Bikaner, India International Journal of Modern Physics: Conference Series Vol. (01) 8 5 World Scientific Publishing Company DOI: 10.11/S ROLE AND SIGNIFICANCE OF UNCERTAINTY IN HV MEASUREMENT OF PORCELAIN INSULATORS A CASE STUDY RAHUL RAJ CHOUDHARY Department of EI&CE, Govt. Engineering College, Bikaner (Rajasthan) India rahulrajchoudhary@yahoo.co.uk POOJA BHARDWAJ Department of EI&CE, Govt. Engineering College, Bikaner (Rajasthan) India pooja.smec@gmail.com RAVINDRA DAYAMA Department of EI&CE, Govt. Engineering College, Bikaner (Rajasthan) India ravi_dayama@rediffmail.com The improved safety margins in complex systems have attained prime importance in the modern scientific environment. The analysis and implementation of complex systems demands the well quantified accuracy and capability of measurements. Careful measurement with properly identified and quantified uncertainties could lead to the actual discovery which further may contribute for social developments. Unfortunately most scientists and students are passively taught to ignore the possibility of definition problems in the field of measurement and are often source of great arguments. Identifying this issue, ISO has initiated the standardisation of methodologies but its Guide to the Expression of in Measurement (GUM) has yet to be adapted seriously in tertiary education institutions for understanding the concept of uncertainty. The paper has been focused for understanding the concepts of measurement and uncertainty. Further a case study for calculation and quantification of UOM for high voltage electrical testing of ceramic insulators has been explained. Keywords: of Measurement; Porcelain Insulator; Traceability. 1. Introduction The characterisation of any physical system provides the basis for betterment or new invention with enhanced attributes. The true characterisation of any physical process depends upon the level of accuracy and estimated deviation of used information from its true values. The information-rich use of a parameter to characterise the results of any analysis through measurement comes at a price. Since every measurement has a purpose which, especially outside the calibration laboratory, influences the design and outcome of the measurement. This provides the distinction between a meaningful measurement and 8

2 Role and Significance of in HV Measurement 9 meaningless quantification of any parameter. Consequently, only within the context of that purpose, the measurement results would have any meaningful information. This must be emphasised that the measurements made for one purpose may not be useful for another purpose. Even, the gathered results for one purpose, if used for other than the context of stated purposes, may lead to potentially risky and dangerous situations.[1] The branch of science concerned with maintaining and enhancing the accuracy of measurement, in any field, is known as metrology. It includes the identification, analysis and minimisation of errors, and the calculation and expression of the resulting uncertainties [1]. Though documentary standards are very useful guides for factors affecting a particular measurement but, blind applications of standards to any measurement for which they were not developed can lead to misleading results or dangerous situations. The accurate measurement has become a prime constraint for modern technocrats. Accurate measurement incorporates the utilisation of standards of measurement as well as the evaluation of uncertainties in a measurement process which has proved to be essential to all areas of science and technology. It is mandatory to consider how uncertain that the measured value is. The provisions and processes are mandatory to be adopted for quantification as well as expression of uncertainty in measurement. This highlights the need to know the uncertainty in a measurement, in order to assess the risk or for optimising the measurement. Further, the approaches, used for quantification and expression of uncertainty, must be consistent as well as conforming to the international standards [1]. Looking at such needs, ISO has come out with its Guide to the Expression of in Measurement (GUM) which bring clarity and consistency to the use of terms such as accuracy and uncertainty [1].. Measurement Any natural parameter or phenomenon can be quantified by the means of measurement. Measurement is the process of comparison between a given quantity and a known quantity of same phenomenon, chosen as a standard. Further, this results in quantitative representation of the parameter under measurement. Precisely, quantification of the value of measurand is termed as measurement, where measurand is the quantity that is subjected to measurement []. Initially, the appropriate specifications for the phenomenon of an unknown quantity must be identified. Further an appropriate standard method of measurement in the form of well-designed working model is identified. The related measurement procedure, in conformance with the chosen standard method is then followed up for the result []. Results of measurement are highly dependent upon the used method of measurement. Thus the method chosen must be in line with the purpose of measurement, otherwise this can lead to monetary losses and high risk. The method should also be accurate and simple.

3 50 R. R. Choudhary, P. Bhardwaj & R. Dayama. Error The hypothecated term error is defined as deviation in measured value from the hypothecated true value of the measurand. Practically, no measurement can be made with complete accuracy. The difference between measured and the true value (as per standard) is known as absolute error of measurement. The measurement error is a quantity which often can be evaluated and, from this knowledge, a correction to the measurement can be applied. The error attributed by inherent error in instrument or variation of environmental condition or loading error is termed as systematic error.such errors tend to have same magnitude and uniform trends for given set of conditions. The magnitude or trend of such error can be identified and correction factors may be applied accordingly. Random errors are caused due to random variations in the process of measurement, including the effects of man and the machine. These errors occur due to some uncontrollable disturbances like friction and backlash that change the system output. The effect of random error can be minimized by making the measurements as many times and using statistical methods like the mean of obtained values of measurement. Errors may also occur due to mistake in reading. These errors are usually because of human mistakes and these may be of any value so these cannot be corrected by any mathematical treatment. [, ]. of Measurement The uncertainty of measurement is defined as measure of limits within which the value of measurand is supposed to lie with stated level of confidence.it is important to distinguish between measurement uncertainty and error. reflects the capability of measurement, which is a measure of the bounds within which a value may be reasonably presumed to lie whereas measurement errors is the difference between an indicated value and the corresponding presumed true value. However, the identification of all of the errors and further estimation of their corrections are not possible all the times and this point towards in-exactness of itself and termed as uncertainty of measurement (UOM) [5, 6, 7, 8]. In general, the estimated overall uncertainty on a particular result indicates the capability of measurement process for reporting the results within the limits of accuracy. This requires the complete information about the limitations and associated uncertainty of system as well as method and associated references or standards. While estimating the uncertainty of measurement, all the sources must be taken into account. The probable sources of uncertainty, being vast in numbers may include the following factors [5, 6, 7, 8]. Random and systematic errors. Imperfect modelling of the definition of the measurand. Operator s limitations in reading analogue instruments. Measurand not defined with complete details.

4 Role and Significance of in HV Measurement 51 Inadequate sampling. Inadequate inclusion of environmental effects. Finite resolution of instrument. Inexact values of calibration standards and approximations or assumptions adopted in measurement. In the present framework of accreditation standard ISO/IEC 1705 [1], UOM is increasingly gaining attention and has been forced to be clearly estimated. This standard has clearly defined the concept for estimating UOM and when and how it should be expressed in test reports. 5. Calculation of UOM- Case study ISO/IEC 1705 requires UOM to be reported when required by the client and when relevant to the application and the interpretation of the measurement results, within the framework of certain specifications or decision limits. The UOM should be readily available and reported together with the result as X±U, where U is the uncertainty [1, 9, 10] calculation for high voltage power frequency transformer Product Test Standards used: IS: 71:1971 : High Voltage Power Frequency Transformer : Dry Power Frequency Voltage Flashover test on 11 KV5 KN Disc Porcelain Insulators Table 1: Capacitive Voltage Divider and peak voltmeter specifications Range used for testing 0 00 KV of capacitive voltage ± 1.0 % Accuracy from its calibration report ± 1.80% of reading Resolution KV Type A Evaluation Table : Observations of flash over test S.No Flashover voltage (KV) X i Mean Value (X 0 ) X i -X 0 (X i -Xo) TOTAL 60.0

5 5 R. R. Choudhary, P. Bhardwaj & R. Dayama Mean Reading = 7, Standard deviation (SD) = 0.71 Std. U r = SD 5 = 0.18 Degree of freedom V 1 = 5-1 = Std. uncertainty (% U r ) = ± 0.18*100/7 = ±.% Type B Evaluation 1. of Capacitive Voltage Divider and peak voltmeter (Expanded UOM ±1.0 %) A 1 = ± 1.0 % The distribution is normal and the coverage factor for approximately 95 % of confidence level is U 1 (%) = ± A 1 / =1.0/ = ± 0.65 % Degree of freedom V = Infinity. Accuracy of Capacitive Voltage Divider and peak voltmeter from its supplier s specifications A = ± 1.80% of RDG = ± 1.8*7*.01 For rectangular distribution U = U = 0.78 KV %U =U *100/Mean Reading= 1.0 % Degree of freedom V = Infinity. due to resolution of Capacitive Voltage Divider and peak voltmeter A = 0.1/=0.05 KV For rectangular distribution, the standard uncertainty U = A / () U = 0.05 =0.088KV % U = *100 =0.01 % Degree of freedom V = Infinity Combined Standard (U c ) A U C = U r + U1 + U + U = (. ) + (0.65) + (1.0) + (0.01) = = ±1.0 % V eff = ( U1) V ( U C ) ( U ) ( U ) + + V V ( U r ) + V 1 V eff = (0.65) (1.0) (1.0) (0.01) + + (0.) +

6 Role and Significance of in HV Measurement 5 Effective degree of freedom V eff = 05 Expanded for approximately 95 % of confidence level, the coverage factor K= Total expanded uncertainty for voltage U= ±1.0 = ±.60% Budget Symbol Source of (X i ) Table : Budget calculation Value (%) Probability distribution (Type A or B Factor) Divisor Standard Sensitivity coefficient (C i) U 1 Calibration 1.0 Normal Type A U Accuracy 1.80% of Rectangular () RDG distribution Type B U Resolution 0.1 Rectangular distribution Type B () U r Repeatability 0. Normal Type A U c % 1.0 Expanded 6. Conclusion Reporting of Results Normal (k= ) Applied Voltage = 7 KV±.60 % Contribution (U i %) The reporting of measurement results cannot be complete until uncertainties in the measurement as well as traceability to the standards are expressed. As the need to derive the decisions, based on the analytical results obtained, or to comply with regulatory limits (for quantitative determinations), the traceability and UOM has become essential features of measurement. UOM provides the idea about the range of results and about comparability of results received from different labs. Since uncertainty depends upon the used method, the reporting of UOM ensures the knowledge of details of the specific method being used. Traceability, in its true sense, can only be justified if there is documented evidence of the traceability chain and the clear estimation of its associated UOM. Further, removal of measurement biases and re-estimation of UOM are ongoing process and need to be carried out on periodic basis as time dependent components exist for the values of standards, their uncertainties, the corrections and uncertainties of associated measurement systems. The identification of calibration intervals may initially.60

7 5 R. R. Choudhary, P. Bhardwaj & R. Dayama be based upon the information provided by manufacturer. Since the required calibration interval depends upon the type of equipment and components of the measuring system, the calibration history may be utilized for estimating appropriate time interval for recalibration of the system. Acknowledgment Authors acknowledge the valuable guidance provided by Mr M.S.Fageria, Ex-Chief Engineer, JVVNL and Ex-Project Director, CERDC, Bikaner. Authors are also thankful for Ceramic Electrical Research and Development Centre, Bikaner for providing support for carrying out this work. References 1. General requirement for competence of testing and calibration laboratories(iso-iec 1705:005), May 005. Guide To The Expression Of In Measurement, Saudi Standard Draft No. 1 Saudi Arabian Standards Organization, (000). The Expression Of And Confidence In Measurement, M00 Edition, United Kingdom Accreditation Service, (007). Kirkup and R. B. Frenkel An Introduction to in Measurement Using the GUM Cambridge University Press 5. Keith Birch, Estimating Uncertainties in Testing - An Intermediate Guide to Estimating and Reporting of Measurement in Testing, British Measurement and Testing Association, ISSN M. Rosslein, Accred. Quality Assurance. 5 (000), p N. Mueller, Accred. Quality Assurance. 7 (00), p V.J. Barwick, S.L.R. Ellison, VAM Project..1, Development and Harmonization of Measurement Principles, Part d, Protocol for Evaluation from Validation Data, Version 5.1, January 000, p Policy on calibration and traceability of measurement,national accreditation board for testing and calibration of laboratories,issue no., E. So, R. Arseneau, Traceability of High Voltage Power and Energy Measurements for the Electrical Power Industry /1IEEE (01)