How Instrument Transformers Influence Power Quality Measurements: A Proposal of Accuracy Verification Tests

Abstract

The integration of renewable energy sources on a large scale in the electrical energy distribution systems, as well as the widespread of non-linear loads, has led to a significant increase in power quality (PQ) disturbances. For this reason, PQ monitoring is also becoming a key task in medium voltage (MV) grids. The measurement of PQ at MV levels can only be performed using instrument transformers (ITs) to scale down the level of voltage and current to levels suitable for the input stage of PQ instruments. However, no international standards currently require the verification of the errors introduced by ITs in the measurement of PQ phenomena. Moreover, this issue is only partially addressed in the scientific literature, where papers dealing with specific and limited aspects of the problem can be found. For this reason, this paper aims to comprehensively assess the issue, proposing IT accuracy verification tests for different PQ parameters. First, a set of PQ phenomena relevant for IT testing is chosen, as well as the associated ranges of variation, based on a review of the enforced standards and the scientific literature. For each selected PQ phenomenon, possible performance indices and test waveforms are proposed. Finally, the proposed procedure is validated by applying it to the characterization of two different types of commercial voltage transformers.

Characterization of Instrument Transformers under Realistic Conditions: Impact of Single and Combined Influence Quantities on Their Wideband Behavior

“… The definition of the accuracy and uncertainty limits of ITs in PQ measurements, functional to the establishment of “PQ Accuracy Classes” as an extension of the power frequency accuracy class concept of ITs to the frequency range up to 9 kHz. To this end, significant test waveforms and suitable performance indices for PQ parameters have been defined and experimented, as detailed in [ 35 ]. The development of missing reference measurement systems and related test procedures and methodologies to evaluate the relevant uncertainty contribution of ITs to PQ indices, to ensure the traceability and accuracy of the measurement results provided of ITs in the wideband measurement of PQ disturbances.…”

Sensor Effects in LCL-Type Grid-Connected Shunt Active Filters Control Using Higher-Order Sliding Mode Control Techniques

“…Moreover, the errors produced by sensors, affect the accuracy of the perturbation identification and significantly limit the power quality improvement in electric networks [ 22 , 23 , 24 ]. It is worth noting that this issue has not been fully addressed in the literature [ 25 ].…”

Current Ratio and Stability Issues of Electronically Enhanced Current Transformer Stimulated by Stray Inter-Winding Capacitance and Secondary-Side Disturbance Voltage

“…In recent years, the usage of precise instrument current transformers (CT) [ 1 ] is no longer confined merely to the top laboratory and calibration facilities. Instead, they are gaining momentum due to the widespread application of grid-tied converters, where precise metering of power, energy and power quality (PQ) assessment is essential [ 2 ]. To comply with accompanying standards, e.g., IEC 60044-1 and IEEE C57.13, the representative CT features, such as the nominal accuracy and frequency bandwidth, must be accomplished across a wide measuring range, frequently ranging between 0.1% and 120% of the nominal current.…”

“…The bandwidth in PQ assessment is usually limited to power grid frequency up to its 20th harmonic, although it can extend up to 9 kHz [ 2 , 3 ]. In general, the CT accuracy can be increased either by ferromagnetic core quality improvements or active techniques that modify the magnetic quantities in a compensating manner (denoted as electronically enhanced CT (EECT) or electronically assisted CT).…”