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ON-SITE LIVE VERIFICATION OF HV INSTRUMENT TRANSFORMER ACCURACY

pubblicazioni - Articolo

ON-SITE LIVE VERIFICATION OF HV INSTRUMENT TRANSFORMER ACCURACY

Recently updated on Maggio 11th, 2021 at 08:57 am

GIORGIO DE DONÀ TERNA S.P.A. (Italy) GABRIELLA CROTTI ANGELO SARDI I.N.R.I.M. (Italy) UWE BRAND MICHEL GIRAUD SCHNIEWINDT GmbH (Germany) NIKOLA KULJACA PAOLO MAZZA1 CESI S.p.A. (Italy) ANDERS ANDERSSON STEPHAN WEISS INTEROPTIX INC. (U.S.A.) The availability of a tool aimed to instrument transformers accuracy on-site condition assessment is highly desirable, because reliable and precise energy metering is of essence to achieve electricity market correctness and transparency. Even tiny measurement errors, accumulated over a year long period, can lead to a significant financial unbalance in trading energy over transmission lines, and recent studies2 demonstrated that accuracy deteriorates over time both for capacitive coupled voltage transformers (CCVTs) and for inductive instrument transformers. Presently, all secondary metering equipment undergoes regular on-site calibration cycles whereas the metering instrument transformers have been neglected in this process. Verification of metering instrument transformer accuracy has been a major problem in the past years, since it was necessary to take current transformers (CTs) and in some cases also voltage transformers (VTs) out of service. Accuracy tests could only be performed at an accredited laboratory or in some rare instances on site to avoid long down times or the need for spare parts. Improvements in transducer technology, appropriate metrological procedures and live-line working procedures allow the verification of instrument transformer accuracy on site in Italy. These procedures can be performed without service outages and intervention on busbar configurations. CTs and VTs can be verified against a reference measuring system, which consists of current and voltage sensors, optical data link and data acquisition system. This new reference measurement system includes a split core high precision Rogowski coil as a current sensor, a low inductance resistive-capacitive precision voltage divider and a special high precision Optically Powered Data Link (OPDL). The mechanical design is relatively light and can easily be moved and repositioned. Optical fibres connect the sensors to a data acquisition and analysis system, which is specifically designed for this task. Special working procedures allow live line positioning of split-core Rogowski coil and the connection of the voltage divider. The Rogowski coil is positioned around live conductor using a “bare-hand” technique. Operation is made by a man bonded to the live conductor, on a tower insulated from earth. Live line connection of the voltage divider to the HV network is made possible utilizing a dual disconnect switch configuration. The power line is connected through a hand operated “distance” connecting technique to one side of the first open disconnect switch, which will be closed and produces only minor arcing limited by an inrush resistor connecting the other side of the disconnect switch to the voltage divider. A second disconnect switch parallel to the inrush resistor will be closed to eliminate the inrush resistor induced measurement error. 1 mazza@cesi.it 2 E. Anderson, A. Hyrczak, J. Karolak, A. Ratajczak, J. Wróblewski, R. Zajac “Metrological Properties of High-Voltage Instrument Transformers after many Years Service” – CIGRE 2004 General session PUBBLICATO A5059916 (PAD – 747237)PUBBLICATO A5059916 (PAD – 747237)

Metrological procedures include laboratory characterisation of the elements constituting the reference measurement system. These procedures take into account the different operating conditions, evaluation of the measurement uncertainty associated with the on-site operation and definition and adaption of suitable measurement procedures. Traceability of the measuring system to the national standards is assured through an unbroken chain of calibrations. This approach is a powerful tool for on-site assessment of the transformer accuracy deterioration, allowing identification of the voltage and current sensors, including burdens, which, due to ageing, no longer comply with the prescribed accuracy requirements and thus must undergo calibration. Finally, this new tool will benefit all parties involved in the free, deregulated electricity market granting them the right and the means to verify, at any moment, the accuracy of all instruments and sensors, also the brand-new ones, involved in the revenue metering process.

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