The Act of 11 May 2001 — “Law on Measures” — imposed the obligation of legal metrological control on electricity meters and on current and voltage transformers of accuracy class 0.5 and better used in conjunction with meters (these devices were subject to official verification).
The detailed requirements were specified in the Regulation of the Minister of Economy of 19 April 2007 concerning the requirements to be met by instrument transformers of accuracy class 0.5 and better intended for use with active energy meters in AC systems, as well as the scope of tests and inspections performed during legal metrological control of these measuring instruments.
By regulation of 4 January 2008, the Minister of Economy repealed the above-mentioned regulation.
The fact that instrument transformers are currently not subject to legal metrological control only means that official verification by the Central Office of Measures is no longer required. However, this does not imply that installers or operators of metering and billing systems are released from the responsibility to verify the metrological properties of instrument transformers, which constitute an integral part of these systems and have a significant impact on the accuracy of electrical energy measurement.
Current Regulations
Currently in force is the Regulation of the Minister of Economy of 4 May 2007 (Journal of Laws No. 93, item 623) concerning the detailed operating conditions of the power system, which in Chapter II states:
1.
Devices forming part of any metering and billing system must have official verification or type approval in accordance with the requirements specified for the given device.
2.
For devices that are not subject to official verification or type approval, the device must have an appropriate certificate confirming measurement accuracy (calibration certificate).
Also legally binding is the Regulation of the Minister of Development of 2 June 2016 on requirements for measuring instruments, which states that:
§21. Additional devices connected directly or remotely to a measuring instrument must not unduly influence its metrological characteristics.
Practical Experience
The absolute necessity of calibrating instrument transformers before commissioning is primarily supported by our experience gained as an accredited calibration laboratory.
During the calibration of newly manufactured current and voltage transformers intended for operation in low-, medium-, and high-voltage networks, we have repeatedly encountered discrepancies between the accuracy class declared by manufacturers and the actual measurement performance.
Causes of Non-Conformity
Differences between the declared accuracy class and the actual error values of instrument transformers prepared for installation may be caused by, among others:
- material defects affecting transformer cores,
- assembly errors, e.g., in the ratio change system,
- unexpected events during transportation,
- the fact that manufacturers often perform final accuracy verification (calibration) within a limited scope compared to the full requirements of applicable standards.
Example from Practical Experience
In the case of multi-ratio current transformers, even those manufactured by well-established companies, it has occurred that during the manufacturer’s final inspection, accuracy was verified for only one rated ratio. For example, during on-site calibration it may be revealed that a dual-ratio current transformer meets the requirements of accuracy class 0.2 at a rated primary current of 2000 A, while at a rated primary current of 1000 A it barely meets the requirements of accuracy class 3.
Consequences
Therefore, commissioning instrument transformers whose metrological properties are accepted solely on the basis of incomplete final measurements performed by the manufacturer may lead to serious financial consequences in the future, including:
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the need to replace defective instrument transformer(s)
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the need to correct electricity meter readings
Such corrective actions require specialist diagnosis and verification and are both time-consuming and costly.
