Karel Draxler

Calibration of Rogowski Coils With an Integrator at High Currents

This paper describes the calibration of Rogowski coils with an integrator for current measurement in the current range of up to 30 kA, at mains frequency. A current loop of ten turns with a current of 3 kA was used for calibration. The magnetic field generated in this way differs from the field of one turn placed at the center and passed with a current of 30 kA. The influence of the unevenness of winding and the cross section of the Rogowski coil may cause a calibration error when using a current loop. The difference in the results between using a current loop and one turn, and the measurement uncertainties are given here. Calculations of the mutual inductances of typical Rogowski coils and their uncertainties are presented in the Appendix.

Measurement of DC Currents in the Power Grid by Current Transformer

DC currents in power grids are mainly caused by geomagnetic activity especially during magnetic storms. It is desirable to monitor these currents to prevent saturation of transformers, which may cause blackout. However, adding DC current sensors to existing installations would be very costly. We suggest to convert some of the existing current transformers to fluxgate DC current sensors by injecting AC excitation current into their secondary winding. We successfully tested this concept on 500 A current transformer. The achievable accuracy is 10% for DC currents and 1.5% for AC currents, which is sufficient for protection and monitoring purposes. We analyze the DC current sensitivity dependence on the (changing) grid impedance, and we show that the sensitivity can be stabilized by controlling the secondary voltage component at the excitation frequency. Excitation current injected into the grid also depends on the grid impedance, but for realistic conditions it is below 2 A.

A combined angle of attack and angle of sideslip smart probe with twin differential sensor modules and doubled output signal

This article presents a combined system for an angle of attack (AOA) and an angle of sideslip (AOS) measurements that will be integrated into an existing air data computer system (ADC) due to an early warning against loss of air lift followed by uncontrolled fall of an airplane. We present a set of probes for AOA and AOS measurement whose parameters, advantages and disadvantages are compared. The results were acquired by direct measurement of sensors and through a newly developed smart probe that contains a microcontroller for basic signal processing and a sensor module for the probes connection. Within the project time span, some probe types were simulated in computational fluid dynamic (CFD) software and twelve probes were manufactured and tested. The most promising probe is described in details and compared with other types, its transfer characteristics depending on its orientation with respect to the airstream, velocity of the airstream and temperature. A unique sensor interconnection method resulting in double amplitude measurement that is based on asymmetric connection of differential pressure sensors is presented.

Influence of Instrument Transformers on Quality of Electrical Power and Energy Measurement

This paper describes a relation for resulting error by measurement of electric power or energy at high voltage networks using instrument transformers. A dependence of instrument transformer errors on their parameters, measured current or voltage and burden is theoretically derived. Theoretical results are completed with results of measurement on transformers for 22 kV network.

Magnetic Circuit of a High-Voltage Transformer up to 10 kHz

Power and energy measurements in smart grids require a measurement system capable of performing signal processing at the higher harmonic frequencies that are present in power grids. For the calibration process of instrument voltage transformers, or high-voltage dividers, it is necessary to have a high-voltage source with an appropriate frequency range. The fundamental element of such a source is the output high-voltage transformer operating at a nominal voltage of 10 kV and in the frequency range from 200 Hz up to 10 kHz. The output current is assumed to be lower than 20 mA. This paper focuses on the design and realization of the magnetic circuit of the transformer described above. Trafoperm, ferrite or nanocrystalline materials can be used for the frequency range considered here. Ferrite materials usually reach saturation at a magnetic flux density of 0.2 T with very low permeability values, while trafoperm material usually suffers from unacceptable power losses in higher frequency areas. This is the main reason why these materials are not suitable for use in a wide range of frequencies, and some combined magnetic cores must be used. The proposed solution is based on magnetic cut C type cores made from nanocrystalline alloy (VITROPERM 500), which behaves better in the frequency range under consideration. The magnetic parameters of this material were measured and compared with trafoperm, and then, the 10 kV high-voltage transformer was designed and manufactured.

AC/DC Current Transformer With Single Winding

Dc currents in power grids are created by induction from geomagnetic variations and newly also by transformerless power inverters installed in solar power stations. These currents may not only cause serious errors of current transformers (CTs) measuring the consumed energy but also cause overload of distribution transformers that may result in blackout. Installing dc current sensors into the grid would be very expensive. We have shown that the existing CTs in the grid can be used in the fluxgate mode to simultaneously measure dc current. Utilizing this mode, the CT ac error is inevitably increased. We analyze the sources of these errors and show techniques to reduce them. The experiments were made on the off-the-shelf 500 A/5 A CT. We show that the uncompensated 5 A dc current without compensation increases its ac amplitude error from 0.1% to 1%. We also found that the dominant additional error was caused by the increased burden. With the optimized design of the excitation circuit, it is possible to sense a dc current component with 10% accuracy and reduce the additional ac error from previously reported 1.5% to 0.1% when the dc current component is compensated. This technique can be widely used to improve the ac accuracy of the existing CTs.

A Fluxgate Current Sensor With an Amphitheater Busbar

Large dc and ac electric currents are often measured by open-loop sensors without a magnetic yoke. A widely used configuration uses a differential magnetic sensor inserted into a hole in a flat busbar. The use of a differential sensor offers the advantage of partial suppression of fields coming from external currents. Hall sensors and AMR sensors are currently used in this application. In this paper, we present a current sensor of this type that uses novel integrated fluxgate sensors, which offer a greater range than magnetoresistors and better stability than Hall sensors. The frequency response of this type of current sensor is limited due to the eddy currents in the solid busbar. We present a novel amphitheater geometry of the hole in the busbar of the sensor, which reduces the frequency dependence from 15% error at 1 kHz to 9%.

Determination of Rogowski Coil Constant

Method of determination of Rogowski coil (RC) constant at 50 Hz frequency is described in the article. The RC constant determining dependence between output voltage and measured current is checked and also a phase displacement of these two quantities is measured. Analysis of measurement uncertainties is also presented.

Use of a lock-in amplifier for calibrating an instrument current transformer

Errors of instrument current transformers (ICT) are mostly determined using a comparative method. The difference between the secondary currents (magnitude and phase displacement) of a standard and of the ICT under test is evaluated by means of special systems which display the errors of the ICT calibrated in relation to the standard. These systems can evaluate errors in a certain range of measured currents (mostly 5% - 120% of the rated value of the measured current) and only for certain transformation ratios of the standard and the ICTs under test. Measurements for smaller primary currents or for different ICT transformation ratios show a bigger error, or their accuracy is not generally determined. Due to increasing demands on measurement and energy saving, determining ICT errors at 1% of the primary current rated value or lower is now a topical issue. The use of a lock-in amplifier, as described in this paper, enables the evaluation of differences between a standard and an ICT calibrated at currents lower than 1% of the rated value, and also for atypical ICT transformation ratios.

DC-Compensated Current Transformer

Instrument current transformers (CTs) can be heavily influenced by DC component in the measured current. 50 A DC current in 500 A CT may cause 40% error in the current and power measurement. For small power factors the situation can be even worse due to 20 deg phase error. We use digital feedback compensation of the DC flux to suppress the overall error to 0.15%.