Voltage Dependence of the Reference System in Medium- and High-Voltage Current Transformer Calibrations

Abstract

A sampling current ratio measurement system has been developed to calibrate the voltage dependence of the ratio error and phase displacement of current transformers (CTs) used for medium-voltage (MV) and high-voltage (HV) applications. The influence of the voltage-induced leakage current on the apparent ratio error and phase displacement of the reference circuit was unambiguously demonstrated using a novel measurement method. Measurement results for voltages up to 24 kV show that the observed leakage current is purely capacitive and caused by the shielded cable capacitance of the HV cable used to feed the current. The maximum ratio error and phase displacement were found to be about 100 parts in 10 6 and 100 μrad for the smallest current of 100 A used for these investigations. Since these values scale linearly with the applied voltage and inversely proportional with current, the assumption of negligible voltage dependence used in other reference systems presented in literature is questionable. Using the corrected reference circuit, a commercial class 0,2S MV CT with 2500 A nominal current was calibrated at voltage levels up to 24 kV. The voltage dependence of the ratio error and phase displacement for primary currents up to 500 A was demonstrated to be capacitive as well, with a maximum impact on the current ratio of almost 0.9 % for a primary current of 100 A (corresponding to 3.6 % at 1 % of nominal current). This value is beyond the limits of in-force standards, which emphasizes the necessity of implementing voltage dependence testing in those standards.

Online Measurement Method and System of Excitation Impedance of Current Transformers Based on Norton’s Theorem and Differential Method to Measure Difference of Two Currents

“…It can not o the aforementioned shortcomings of the offline calibration but also obtain data during real-time operation. Research in this area can be classified into t One is based on the standard CT [9,10], which often involves manual op clamp CT with low security [11,12]. The other is based on a phasor meas (PMU) and complex algorithms [13,14].…”

“…As the change in circuit state caused by connecting a resistance primarily reflects R m , R m calculated by omitting X m is relatively close to its actual value. Similarly, (12) yields the solution for X m in (9) when assuming that R m is absent.…”

“…Research in this area can be classified into two categories. One is based on the standard CT [9,10], which often involves manual operation of the clamp CT with low security [11,12]. The other is based on a phasor measurement unit (PMU) and complex algorithms [13,14].…”

Transmission Line Protection Using Dynamic State Estimation and Advanced Sensors: Experimental Validation

“…The location of the smart code could be at one of the substations so that at least one end measurement is locally received. • CTs and VTs/CCVTs Calibration: For the application of AdvSens in high voltage transmission lines, metering transformers (such as CTs and VTs/CCVTs) are needed to be installed for conversion of the high voltages and currents to the measurement level [31]. A secondary stage calibration procedure could also be used for correcting the phase and voltage displacement of the signals provided that the calibration parameters are known within AdvSens.…”

Optimising Energy Management in Hybrid Microgrids

“…where S max , is the maximum apparent power in per unit (pu) that can circulate through each line, i max , is the maximum current that can pass through the conductor and v x is the voltage at bus n or m. The manufacturers provide the maximum current limit because the limiting factor is the temperature of the conductor due to the heat caused by the passage of the current. The model of the conductors is given by the standards and technical requirements that can be found in the technical literature [36], and the value of the maximum current in [37,38], which is 285 A for underground conductors (20 C ambient temperature). This is what is known as the thermal limit of the conductors and is a factor to be considered in lines shorter than 80 km, as is the case; while between 80 and 320 km the limiting factor is the voltage drop, in lines longer than 320 km it is the stability of the angle [34].…”