Iso iec guide to the expression of uncertainty in measurement

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Got a question? Customer care. An IEC standard should be clear and easy to understand by all users in the industry. Background theory and material, whilst necessary for understanding, is not required for the day-to-day application of the standard. This paper presents a more detailed background and theory on the measurement of transformer losses and how to quantify precision.

The authors are all members of the IEC maintenance team working with the standard. The development of an uncertainty budget for a measurement starts from a model function of the measurement. The basic model function for power loss measurement will be developed for the case where instrument transformers are used to adapt the high voltages and currents to the inputs of a watt meter.

Input data will be errors and uncertainties of errors for the instrument transformers and of the watt meter. Ratio and magnitude errors are of course important, but in measurements at low power factor, phase displacement will be the dominant factors. In adherence to the Guide to the expression of uncertainty in measurement GUM , using the partial derivatives of the model function with respect to its parameters, an expression for the sensitivity to uncertainties of each parameter can be formally expressed and quantified.

For power transformers, there are two important loss measurements: no-load loss measurements, where voltage and current may be distorted, and load-loss measurements, where the power factor can be very small. For each of these two cases, slightly different modifications of the basic model function are needed. For no-load loss measurements a change in voltage level will have a nonlinear approximately quadratic influence on the measured loss. Although tests shall be performed as close as possible to nominal voltage, there will be an unavoidable uncertainty in the measurement of this voltage, and a corresponding term has to be added to the model function.

A second additional term relates to the fact that the no-load loss test signals are distorted, requiring a correction to the measured losses, as required by IEC standards, albeit with some uncertainty. A further correction is to be made to refer the result to nominal reference temperature, which can be appreciably higher than the temperature during test. Both corrections require modifications of the basic model function. The paper derives the expressions for uncertainty propagation from the modified model functions and sets out the resulting uncertainty budgets.

Furthermore, regional The losses of power transformers no-load and regulations, such as the European Union load losses are object of guarantee and directive for EcoDesign [1], may also pose penalty in many contracts and play an requirements on establishment of reliable important role in the evaluation of the total values for losses.