Uncertainty in measurements of power and energy on power networks

The problem on measurement of power and energy in single-phase and three-phase networks is always very important. The accuracy and precision of measurement is of equal importance for both manufacturers and consumers. The general problem is to determine the uncertainty of the energy produced in a specified power station and delivered to different consumers. Measuring instruments used for energy measurement are voltage and current instrument transformers and electricity meters. The approach proposed combines the information that can be derived from the nominal accuracy specifications of the instrument transformers with the information resulting from the calibration of the electricity meters. The combination allows achieving the evaluation of uncertainty in power and energy measurement. Both instrument transformers are subject to ratio error and phase error. In the paper an expression for maximum (positive and negative) errors as a function of the relative load current is derived. A method for calibration of electricity meters, measurement instruments used in the procedure, processing and presentation of the final results are presented as well. Having the information about the accuracy of both voltage and current instrument transformers and finding the uncertainty of electricity meter (according to the method outlined), it is possible to determine the total uncertainty of energy (power) measurement in conditions of load current variations and different modes of operation. The final accuracy of energy measurement may be presented by giving the specification limits in terms of measured value.