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Rogowski coil used with numerical integration of the measured data

  • Division 2
  • Metrology for Economy
  • Science
  • Economy
The apparatuses designated for the transmission of electrical energy from the power plant to the consumer are tested in a series of expensive acceptance tests before being put into operation. This includes short-circuit tests of high-voltage breakers with high alternating currents and tests of arresters with impulse currents. In particular, the measurement of short-circuit currents, superimposed by exponentially decaying DC currents with amplitudes of several 100 kA, is rather difficult. The measuring shunts, used traditionally for this purpose, have very large dimensions and must be water-cooled in extreme cases to draw off the heat of the power consumption, at least partially.
Rogowski coils are well suited for the almost wattless and unloaded measurement of high alternating and impulse currents. Because of their non-magnetic core, they have a negligible nonlinearity which is of advantage for reliable calibration at small currents. Despite these advantages, Rogowski coils are seldom used for the measurement of short-circuit currents with superimposed DC current, as their frequency response is limited at low frequencies. The output voltage of the Rogowski coil is proportional to the derivative of the measured current and must therefore be integrated for further data processing. In general, integration amplifiers are used for this purpose giving rise, however, to an additional contribution to the measurement uncertainty and increasing the costs considerably, especially when high accuracy at low frequencies is requested.
As an alternative, a numerical integration procedure on the basis of the simple trapezoidal rule was investigated at the PTB [1]. The calculations by simulation show that the integration error is less than 0.1 % for both the amplitude and the r.m.s. value of a sine wave, made for more than 1000 samples. For the experimental tests, a Rogowski coil was used, having an inner diameter of 26 cm and consisting of two halves that can be easily clapped around the current conductor. Both halves of the winding were arranged with utmost precision by the manufacturer. Detailed calibrations at AC currents of power frequency confirmed the excellent properties of the Rogowski coil. For example, the variation of the output voltage of the coil is less than 0.1 % for different positions of the current conductor and return conductor. The maximum nonlinearity is 0.1 % for currents up to 10 kA. The lower 3 dB limit frequency of the investigated Rogowski coil, designed in particular for the measurement of very high AC currents at power frequency and their harmonics, is significantly below 1 Hz, the upper limit frequency being much more than 100 kHz.
The numerical integration method was applied to three different current signals: asymmetrical short-circuit current with superimposed, exponentially decaying DC current, 8/20 impulse current (8 µs front time, 20 µs time to half-value) and step response of the Rogowski coil itself. The output voltage of the coil was first recorded with a digital recorder or a digital voltmeter, operated in the sampling mode, and then the digital data were numerically integrated by the PC. The control of the different current generators and measuring instruments as well as data processing, including numerical integration, was achieved by software. In order to check the numerical integration method, the current signals were also measured with a wide-band coaxial shunt.
The results of the investigation show that numerical integration of the output voltage of Rogowski coils is a simple, accurate and inexpensive method for current measurements and offers a very good alternative to the use of electronic integration amplifiers. The technical realisation of this method does not pose any problem because of the high sampling rate, bandwidth and amplitude resolution of the digital voltmeters and digital recorders available on the market, in combination with the excellent calculation capacity of today's PC. Even very low-frequency components of current typical for asymmetrical short-circuit currents can be well evaluated with the Rogowski coil, in combination with the numerical integration method. This offers a better alternative to the use of measuring shunts of large volume and, moreover, the advantage of potential-free measurements in the high-current circuit. The requirements for the measurement uncertainty, as will be specified in the future IEC Publication on high current measurements, can be easily met.
[1] Schon, K.; Schuppel, A.: Precision Rogowski coil used with numerical integration. Proc. 15th ISH in Ljubljana, 27.-31.08.2007, contribution T10-130


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