4.5.1 Structure of IEC 60060-2:2010
Requirements for measurement systems are specified in IEC 60060-2:2010. The main structure of IEC 60060-2:2010 is similar to that of the previous edition IEC 60060-2:1994, although there have been significant additions of contents to some clauses, for example, the additional requirements in Clause 5 on estimation of measurement uncertainties. Table 6 compares the main clauses of Edition 2.0 and Edition 3.0 of IEC 60060-2. Term “AMS” in Table 6 denotes “Approved Measuring System” and “RMS” denotes “Reference Measuring System”. Both terms are defined in IEC 60060-2.
Table 6: Comparison of the main clauses of editions 2.0 (1994) and 3.0 (2010)
IEC 60060-2:1994, Ed. 2.0 IEC 60060-2:2010, Ed. 3.0
3. Definitions and symbols
4. Qualification of AMS
3. Terms and definitions
4. Qualification of AMS
5. Acceptance tests on components
6. Performance tests on AMS
5. Tests and test requirements for AMS,
including the uncertainty estimation
7. Measurement of DC voltage 8. Measurement of AC voltage 9. Measurement of LI voltage 10. Measurement of SI voltage 6. Measurement of DC voltage 7. Measurement of AC voltage 8. Measurement of LI voltage 9. Measurement of SI voltage
11. Measurement of impulse currents Transferred to IEC 62475
12. Reference Measuring Systems (RMS) 10. Reference Measuring Systems (RMS)
Annex A: Accreditation systems
Annex B: Record of performance
Transferred to 4.1 and 4.4
Amendment 1/Annex H: Uncertainty estimation Annex A: Uncertainty estimation (GUM)
Annex B: Example for uncertainty calculation
Annex C: Step response measurement Annex C: Step response measurement
Annex D: Convolution
Annex D: Temperature rise of resistors
Annex E: RMS bibliography
Annex F: Summary of tests
Tables transferred to chapters 6 to 9
The two old clauses on acceptance and performance tests have been combined into the new Clause 5. This new clause gives detailed requirements on the determination of the measuring uncertainty. The summary tables for approving the HV measuring systems of different types of test voltages have been transferred from the old Annex F to the relevant clauses on individual voltage types. The old chapter on impulse current measurement has been transferred to the new standard IEC 62475:2010.
The content of six of the seven annexes of the old standard have been, where appropriate, transferred to the main text of the new standard. Only the annex on step response measurement (Annex F, now C) remains and is complemented by an informative annex (Annex D) on convolution.
4.5.2 Calibration and estimation of measuring uncertainty
It is strongly expressed in IEC 60060-2:2010 that the preferred calibration procedure is the comparison method using a Reference Measuring System to qualify an Approved Measuring System. This often means that a calibration is performed by a certified calibration laboratory.
The calibration procedure starts with the determination of the scale factor by which the recorded reading is multiplied to arrive at the actual test value. It also requires that the estimation of measurement uncertainty should follow ISO/IEC Guide 98-3, “Guide to the Expression of Uncertainty in Measurement” (also referred to
as GUM in literature). Two new Annexes, annex A and annex B are added to IEC 60060-2:2010 to provide additional practical information and examples of uncertainty estimation in high-voltage measurements. Required measurement uncertainty limits for all types of voltages, for the purposes of high-voltage tests as specified in IEC 60060-1:2010, remain unchanged. An expanded measuring uncertainty of UM ≤ 3 % is required for the test voltage, whereas for time parameters of the full lightning impulse and the switching
impulse, a value UM ≤ 10 % is required. For Reference Measuring Systems the values are UM ≤ 1 % for the
test voltage and UM ≤ 5 % for time parameters. Only the voltage measurement of front-chopped LI impulse
voltage is specified with higher uncertainties, with UM ≤ 5 % for an Approved Measuring System and for
UM ≤ 3 % for a Reference Measuring System.
Calibration by comparison should normally be performed at several voltage levels. When the rated voltage of the Reference Measuring System is sufficient to cover the assigned measurement range of the Approved Measuring System under calibration, then the number of voltage levels should be equal to or greater than 5 levels. In cases where a Reference Measuring System with sufficient operating voltage is not available, IEC 60060-2:2010, like the previous edition, allows the comparison calibration be performed with the highest calibration voltage being as low as 20 % of the assigned measurement range of the Approved Measuring System. In such cases, a voltage linearity test must be performed in addition to the comparison calibration or determination of scale factor. Several methods for determining linearity for various types of measurement systems are given for voltages up to the levels for 800 kV class equipment testing.
It should be noted that specific information on suitable linearity test for DC is not given and may pose a problem. Recent development has however extended the DC calibration voltage available in the world to 1000 kV [38, 39]. IEC 60060-2:2010 provides specific requirements for the calibration and these are given in Clause 5.2.1.3, “Comparison over limited voltage range” and Clause 5.3, “Linearity test”, of the standard. The graphical illustration of this approach is given in Clause 5.2.1.3 and is reproduced in Figure 10 below. As can be seen from Figure 10, the total number of test levels for checking scale factor and linearity, should be at least 6. The lowest voltage level of the linearity test should be performed at the scale factor calibration voltage, which should be at least 20 % of the highest voltage of use for the Approved Measuring System.
Figure 10: Calibration over a limited voltage range
4.5.3 System calibration by calibrations of components
IEC 60060-2:2010 still retains the approach of calibration of a measuring system by calibrations of its components, with the requirements given in Clause 5.2.2. This approach is provided as the alternative method to the method of comparison of the complete measuring system with a Reference Measuring System, which is specified as the preferred method. When planning the calibrations and combining the results for the complete system, the interactions between the components and the influence of the transmission system (measurement cables), have to be considered to arrive at the correct values.