Electricity metering accuracy explained
Electricity metering accuracy is crucial in ensuring the integrity of a billing system.
Anomalies in measurements can, over a period of time, cost hundreds or thousands of dollars in errors. The accuracy of an energy meter is dependent on multiple factors such as the load of the network (full load conditions will be more accurate than partial load), power factor of the system, accuracy of the meter itself and other factors.
The accuracy depends on the design and build quality of the meter’s input channels - a higher quality measuring meter will provide better accuracy but will increase the price of the product. Some major parameters that affect the accuracy measurement of an energy meter are:
- Fluctuation of the reading value, represented in percentage % from the actual value (reading).
- A fixed error (noises) normally represented as percentage from full scale (FS) as its constant value.
- For power and energy measurements, the phase shift between the voltage and the current also affects the accuracy since the power equals voltage multiplied by current multiplied by the cosine of the phase angle.
- The phase angle accuracy is represented in degrees in current transformers creating additional errors to energy/power meters.
Accuracy metering standards
Since accuracy depends on the load of the system, IEC/as have developed different standards to define accuracy under different load conditions. This is known as ‘Accuracy Class’.
IEC/AS Standard 62053-11 covers Accuracy Class 0.5, 1.0 and 2 for electromechanical meters for active energy (watt-hours) - this means the accuracy as a percentage from reading under full load conditions and unity power factor. However, the accuracy deteriorates under lower load conditions when power factor is less than unity.
IEC/AS Standard 62053-21 covers Accuracy Class 1.0 and 2 for static/electronic meters for active energy (watt-hours), which means the accuracy as a percentage from reading based on full load conditions and unity power factor. However, the accuracy deteriorates under lower load conditions, power factor less than unity along with the presence of harmonics.
IEC/AS Standard 62053-22 covers a higher Accuracy Standard of 0.2S and 0.5S for static/electronic for active energy (watt-hours) providing a higher Accuracy Standard under full load conditions and unity power factor, in addition to better accuracy readings at much lower load currents, power factor conditions less than unity along with the presence of harmonics.
System accuracy vs meter accuracy
The accuracy of any energy measurement system is the summary of its components, ie, energy meter plus current transformer (CT). With the exception if a direct connected meter is utilised.
IEC/AS Standard 60044-1 defines the accuracy classes for CTs. Subject to the loading of the CT, accuracy variances will occur from the quoted accuracy class such as errors due to phase errors based on specified load impedance. Current transformers’ accuracy is defined as per IEC 60044-1, Classes 0.1, 0.2, 0.5, 1 and 3. In addition, Accuracy Class 0.2S and 0.5S standards for CTs apply for higher performance accuracy. The class designation is the measure of the CT’s accuracy. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% at rated current. Installing an energy meter with Accuracy Class 0.5S as a minimum requirement can assist in ensuring the energy monitoring application has a high degree of accuracy when taking into account the accuracy performance of the CTs involved.
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