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Low Power Instrument Transformers (LPIT) testing with new technologies

Low Power Instrument Transformers (LPIT) testing with new technologies
27 . Dec . 2021

In energy networks, energy exchange between different parties is settled based on energy metering. In medium and high voltage networks, the current and voltage is measured with instrument transformers and these are connected to an energy meter. The requirements for such equipment and systems international standardized. National regulators often set requirements regarding metrology. In the Netherlands these directives for metering installations are described in the electricity metering code.

Even though standards and guidelines follow new developments and technologies, the application of new technologies can sometimes be a challenging process. For instance, optical instrument transformers also called Low Power Instrument Transformers (LPITs) which transmits the measured currents and voltages in the form of sampled values by IEC 61850 protocol ​​to a fully digital energy meter. The new types of instrument transformers haves in terms of dimensions, weight, and safety in comparison the conventional instrument transformers.

Product standards are being developed for these new components within the international standardization organization IEC TC13, IEC TC38 and IEC TC57. For the instrument transformers, this has resulted in different parts of the IEC 61869 series. The IEC 61850 has been developed for the communication part. For energy meters developments within IEC TC13 are still ongoing. KEMA Labs (the CESI Testing, Inspection and Certification Division) actively participates in the developments within the international standardization organizations.

To test if the LPIT fulfill the requirements of the Dutch electricity metering code, KEMA Labs has been commissioned by the Dutch Transmission System Operator (TSO) in collaboration with the Dutch Metrology Institute (VSL) and several manufacturers. To research the interoperability and the system accuracy of an energy measurement system the setup contains a LPITwith IEC 61850-9-2 samples value output connected to an energy meter with digital input. These components communicate via a communication network equipped with an Ethernet Switch with added a time synchronization clock. This time synchronization is necessary to obtain the required accuracy of the measurement.

During the research, the following measurements were performed by KEMA Labs using traceable and calibrated measuring equipment:

  1. Accuracy of the complete measuring system, i.e. from primary components to pulse LED on the front of the energy meter. The accuracies are determined for different currents, with inductive as well as capacitive loads.
  2. Ratio error as well as phase displacement of the current transformers at different loads.
  3. Ratio error as well as phase displacement of the voltage measurement at different loads.

The above-mentioned measurements are regularly performed for traditional equipment, the challenge lies in dealing with communication aspects such as MAC addresses, IP addresses, VLANs and time synchronization with PPS and PTP.

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