Measurement accuracy - White paper

Measuring accuracy of energy meter

No measurement is 100% accurate, there is always a measurement uncertainty. The maximum measurement errors are laid down in a number of standards. We try to keep this measurement error as small as possible. How do we do this? We explain in this white paper.

White paper Measuring accuracy - Advice fortop

The measuring circuit and wiring diagram of energy meters

The figure below shows a typical connection diagram of an energy meter. This diagram shows the complete measuring circuit. In this example, three current transformers (CTs) are used. The measurement errors of these transformers must be added to the measurement error of the measuring instrument. Read more about current transformers.

fig. 1 energy meter connection diagram

If voltage transformers (VTs) are used (for medium voltage networks), the measurement error of the voltage transformers should also be added to the error of the energy meter. The following formula applies to three-phase measurement with symmetrical load:

Total measurement error = measurement error instrument + measurement error CT + measurement error VT

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Measurement accuracy of current transformers

The class accuracy of current transformers is specified in standard IEC60044-1. We distinguish the measurement errors of current transformers in overvoltage errors and angular errors.

Transposition errors

The percentage difference between the input current vector (I1) and the output current vector (I2) than one would expect based on the overshoot attitude.

Corner errors

The angular rotation between the input (I1) and output (I2) current vector is given in minutes. 1 degree (°) of angular rotation corresponds to 60 minutes (°°). Figure 2 shows the additional measurement error on the total measurement error due to the current transformers.

Additional measurement error in power measurement

Fig. 2 The additional measurement error in power measurement

The table below is derived from IEC60044-1. It shows the transfer error and the angle error as a function of the primary current. The errors indicated apply at the actual measured values and when the current transformer is loaded with a power between 25% and 100% of the value indicated on the type plate.

IEC60044-1	Transfer error (%)				Angle error (min)
Class	0,01ln	0,05ln	0,2ln	1 & 1,2ln
3	-	-	-	3*	-	-	-	-
1	-	3	1	1,0	-	180	90	60
0,5	-	1	0,75	0,5	-	90	45	30
0,5 (S)	1	0,75	0,5	0,5	90	45	30	30
0,2 (S)	0,75	0,35	0,2	0,2	30	15	10	10

Transfer error and angle error as a function of primary current

Using the vector diagram and the table, it is possible to determine exactly at what phase angle of load (cos-phi, and primary current (In)), the influence of a type of current transformer on the total measurement is. For a class 0.5 transformer this is shown in the following figure.

Additional measurement error CT class

Fig. 3 The additional measurement error CT class

We see in Figure 3 that no angular error is specified for class 3 measuring transformers. This means that these transformers are not suitable for energy measurements, because the phase angle is essential in their calculation.

The class accuracy of voltage transformers is defined in IEC60044-2.

Conclusion

The measuring transformers have a significant influence on the accuracy of the overall measurement. This error increases as the load becomes more inductive (φ) or the primary current becomes lower. It is therefore necessary to select the type of current transformer juist te selecteren.

Measuring accuracy of energy meters

Measurement accuracy of general electrical quantities
The maximum measuring error of the individual electrical quantities are measured according to IEC61557-12. This specifies the definition of the maximum measurement errors for electronic measuring devices. If it says class 0,2 according to IEC61557-12, it means that the maximum measuring error under reference conditions is 0,2% of the measured value.

Measuring accuracy of individual higher harmonics
Janitza measuring instruments distinguish themselves from the competition by the fact that all measuring instruments measure the individual higher harmonics for both voltage and current per phase. This makes it possible to assess whether connected electronic devices meet the emission requirements and to determine the apparent power (VA) with a higher measuring accuracy. As the IEC61557-12 does not stipulate any requirements for the measuring accuracy of the individual harmonics, the IEC61000-4-7 is used for this purpose

Flicker measurement accuracy
Very short voltage variations can be disturbing, especially for lighting. Users experience this as light flickering. For this reason, the voltage quality standard sets requirements for the maximum level of flicker. This level is determined using the flicker algorithm. The measurement method is laid down in IEC61000-4-15.

Measuring accuracy of energy consumption
IEC62053-22 (kWh) and IEC62053-23 (kVarh) are applied for measuring electrical energy. These standards are specifically aimed at electronic kWh meters and kVarh meters. Moreover, this standard describes extra high accuracies in the low current range (class 0.5 S and 0.2 S).

IEC62053-22	Total measurement error (%) at measured value cos-phi = 1			0,8 cap. < cos-phi > 0,5 ind.
Class	0,01ln	0,05ln	1ln	0,02ln	0,1ln	1ln
0,5 (S)	1	0,5	0,5	1	0,6	0,6
0,2 (S)	0,4	0,2	0,2	0,5	0,3	0,3

Measurement accuracy and comptable measurements (measurement code & MID)

Measurement accuracy and measurement code
Accountable measurements are consumption measurements that can be accounted for, i.e. an authorised metering company may use the measurement data from such measurements for "financial settlement". Accountable metering devices must comply with the measurement code.

The measurement company is responsible for recording the meter readings of your electricity meter and/or gas meter and passing these on to your network operator. The measuring companies often also sell, hire out and repair various meters. The measuring company is the owner of your gas and/or electricity meter. Only a certified metering company may read your meter and carry out repairs. TenneT, the national manager of the high-voltage grid, is responsible for certification. In contrast to many other countries, the measurement companies operate in the free market and are not part of the regulated grid manager.

Measuring code for measuring devices

The measurement code contains conditions for the design and operation of measuring devices. For metering devices for measuring electrical energy, this means that it describes the requirements that the measuring transformers and measuring instruments must meet in terms of accuracy and design. The energy meters and current transformers must be sealable and the measuring instruments must be provided with unique serial numbers, which make it possible to trace back the measuring accuracy. The measurement code at European level is called the MID (Measurement Instrument Directive) and describes the requirements at European level. It aims to create a single European market for accountable measuring instruments.

Read more about MID kWh meters

Measurement accuracy and underdetermination
After the connection of your grid operator, the electricity grid is your property. This is also called the "free domain". If you want to measure energy in this so-called free domain, you are not obliged to use measuring equipment that complies with the measurement code. This gives you the opportunity to select your own measurement equipment.

measurement accuracy under measurement

Accounting measurements and under-measurement

Measurement accuracy and measurement code

Measurement accuracy and measurement code are independent of each other. For example, an energy meter that is not authorised as a measuring instrument can be much more accurate than an authorised measuring instrument. In the free domain, it makes sense to purchase those meters that allow real cost savings.

These savings can consist of:

Identifying and limiting higher harmonics.
Identifying and limiting the generated reactive power.
Creating a clean, symmetrical load.
Preventing power consumption peaks.

View universal energy meter UMG 103 in the webshop

standards and references kWh meters

Standards for kWh meters
IEC62053-11	applies to electromechanical kWh meters of accuracy classes 0.5, 1 and 2, for the measurement of actual consumption in 50 and 60 Hz networks. It describes the general requirements and test procedures.
IEC62053-21	applies only to static (electronic) kWh meters of accuracy classes 1 and 2, for the measurement of actual consumption in 50 and 60 Hz networks. It describes the general requirements and test procedures.
IEC62053-22	applies only to static (electronic) kWh meters of accuracy classes 0.2S and 0.5S, for the measurement of actual consumption in 50 and 60 Hz networks. It describes the general requirements and the test procedures.
IEC62053-23	applies to static (electronic) kVarh meters of accuracy classes 2 and 3, for the measurement of reactive power in 50 and 60 Hz networks. For practical reasons, this standard is based on the conventional definition of reactive power for pure sinusoidal currents and voltages containing only the 50Hz component.

Standards on power analysers
IEC61557-12	specifies the requirements for measuring instruments (power analysers)in low-voltage grids up to 1000Vac and 1500Vdc. The standard shall be used in conjunction with IEC61557-1 describing the general requirements for measuring instruments. The specified measurement uncertainties with respect to real consumption (kWh) and reactive consumption (kVah) are derived from the IEC62053 series.
IEC61000-4-30	specifies how the various power quality aspects should be measured. It does not specify limit values for these, but describes the measuring intervals, accuracy, flagging and how to measure dips and overvoltages. A distinction is made between class A, B and S instruments.
IEC61000-4-7	general guidance for measuring instruments with regard to the measurement of harmonics and the individual harmonics in energy grids and connected devices.

White paper on voltage dips - Power Quality