INFLUENCE OF COAXIAL CABLE ON RESPONSE OF HIGH- VOLTAGE RESISTIVE DIVIDERS

Transcription

1 The 2 th International Symposium on High Voltage Engineering, Buenos Aires, Argentina, August 27 Septemer 1, 217 INFLUENCE OF COAXIAL CABLE ON RESPONSE OF HIGH- VOLTAGE RESISTIVE DIVIDERS A. Bergman 1*, M. Nordlund 1, A-P Elg 1, J. Havunen 2, J. Hällström 2 and J. Meisner 3 1 SP Sveriges Tekniska Forskningsinstitut AB, Box 857, Borås, Sweden 2 VTT Technical Research Centre of Finland Ltd, Centre for Metrology MIKES Box 1, 244 VTT, Finland 3 PTB Physikalisch-Technische Bundesanstalt, Bundesallee 1, Braunschweig, Germany * anders.ergman@sp.se Astract: An effort is pursued y several European National Measurement Institutes to lower the uncertainties in caliration of UHV measuring systems for lightning impulse. To this end, several reference dividers are investigated as regards their accuracy oth for amplitude and for time parameters. During these investigations a deterioration of step response was identified when longer coaxial cales were inserted in the measuring circuit. The measured front time T 1 was also affected, in one oserved case y 2.5 % elongation of front time as another 25 m cale was inserted. Compared to the intention to calirate front time measurement to etter than 5 % uncertainty for front time, this contriution must e well known, or preferaly e eliminated. This paper presents the experimental findings from these investigations. The investigated cales included selected coaxial, tri-axial, and cales with a corrugated screen. The effect of cale length was also studied. The influence was first discovered when applying a very fast step (rise-time < 4 ns) to the high voltage arm of a resistive divider and convolution of this step with the time derivative of an ideal lightning impulse with.84/6 µs impulse. The calculated output was analysed with IEC 6183 compliant software to evaluate the front time. Susequently, these analyses have een augmented y additional comparative measurements where two reference dividers were connected to the same impulse generator, and varying the cale length of one of them. The summarized changes in front time calculated for different cominations of cale and impulse voltage dividers are shown and discussed. It is noted that a change in T 1 error depends oth on length of cale and its type. The results show that non-negligile front time errors may e introduced when the cale length is increased. To support these findings, further tests have een carried out with two reference impulse dividers connected in standard caliration configuration in accordance with IEC One divider was used as reference, while the cale for the other was varied. In this way, the change of error etween configurations could e measured. A theoretical study has also een performed, calculating the distortion of a lightning impulse on a coaxial cale. The results agree qualitatively with experiments, ut the detailed results show discrepancies that need further investigation. 1 INTRODUCTION An effort is pursued y several European National Measurement Institutes to lower the uncertainties in caliration of UHV measuring systems for lightning impulse. To this end, several reference dividers are investigated as regards their accuracy oth for amplitude and for time parameters. During these investigations a deterioration of step response was identified when longer coaxial cales were inserted in the measuring circuit. The measured impulse parameters were all affected, amplitude scale factor, front time T 1 and tail time T 2. The effect could e up to several percent, which is not acceptale for reference measuring systems. A literature study has revealed that the phenomenon has een known since a long time, the earliest reference found is from 1932 [1, 2]! A later study y Sato [3] has made a theoretical analysis of the effect, using the telegraph equation. The study shows that the effective resistance of the cale plays a central role. 2 EXPERIMENTAL INVESTIGATION 2.1 Method The influence of the coaxial cale on the front time was primarily investigated y comparative measurements etween reference measuring systems where the cale length was varied y inserting extra lengths of cale. As a supplementary check, a very fast step (risetime < 4 ns) was applied to the high voltage arm of a resistive divider. The step applied to the divider is generated y a mercury wetted relay ased step

2 generator with an output voltage of 2 V. Evaluation of the step response is performed y convolution in accordance with IEC66-2:21. The step is convolved with the time derivative of an ideal lightning impulse with.84/6 µs impulse (front/tail time). The calculated output is analysed with IEC 6183 compliant software to evaluate the front time. The front time of the convolved curve is compared with the front time of the ideal lightning impulse where the difference gives a measure of the error introduced y the cale. For one set of cales these analyses have een augmented y additional comparative measurements where two reference dividers were connected to the same impulse generator, and varying the cale length and type of one of them. Impulses of 4 kv with a front time of.84 µs were used during the high voltage comparison. A set of triaxial cales have een investigated with the same method as descried aove, convolving a step response with an ideal curve. In addition a lightning impulse generator, capale of producing.84/6 µs impulses with an amplitude of 2 kv, has een used as a supplementary method to investigate the front time error for different length of cales. Both coaxial and triaxial cales have een tested to clarify if the influence on high voltage impulse measurements is related to a specific cale or not. Tale 1. Cales investigated Name Cale type Z [Ω] C [pf/m] R [mω /m] Heliax ½ Coaxial Heliax 5± RG214 RG214 Coaxial Sucofeed Coaxial 5± /8 Sucofeed 3/8 high-flex Triaxial* Triaxial Camera Cale Hyperflex Triaxial 11mm 75± * Hyperflex 2 Nokia* Triaxial Belden 9888 Triaxial *Same type of cales ut different manufacturers. Heliax ½ cale is intended for applications up to 3 GHz and exhiits 4 mm solid inner conductor (copper-clad aluminium) and corrugated copper screen, providing excellent high frequency characteristics. RG214 is a ca. 1 mm diameter coaxial cale equipped with doule screens for optimum screening efficiency and intended for applications up to 1 GHz. The Sucofeed 3/8 cale is intended for applications up to 3 GHz and exhiits 2.8 mm solid inner conductor (copper-clad aluminium) and corrugated copper screen, providing excellent high frequency characteristics. 2.2 High voltage comparison Setup Two experiments were performed where the main difference was the length of the measuring cale for the reference measurement, i.e. the system that was unchanged while the other was equipped with different cale lengths. In one case the reference cale was 25 m and connecting to the recording device without use of shielded cainet. In the other case, the reference cale was only 5 m and oth cale screens were solidly connected to a high-quality shielded enclosure for the recording device. These two measurements give different results, leading to a surmise that the parallel connection of the cale shields affect the measurement. The same impulse voltage and waveshape were applied for each series of measurements m reference cale The test was conducted with a waveshape of.84/41 µs at -1 kv. The reference was VTT MIKES 4 kv reference divider and their reference recording device, a NI5124 digitizer and interconnected with 15 m Belden 9888 triaxial cale. The other ranch was a SMR7 reference divider equipped with successively longer RG214 cales /41 µs T1 error [%] T2 error [%] Figure 1. Errors oserved when changing length of signal cale. The reference cale is 15 m long. While 75 m cales will e excessively long for normal caliration work, it enales clear results to e otained, which can e used to evaluate impact also of shorter cales m reference cale The test was conducted with two waveshapes, one of.84/41 µs and one at 1.56/45 µs oth at -2 kv. The reference was SP-RISE 8 kv reference divider and their reference recording device, NI5124 digitizer and interconnected with 5 m triaxaxial cale. The other ranch was a SMR7 reference divider equipped with

3 successively longer RG214 cales. Some other cales were also tested /42 µs T1 error T2 error Figure 2. Errors oserved when changing length of signal cale. The reference cale is 5 m long. Common feature of oth cales is an apprecialy lower DC resistance. It remains to e proven that this is the reason for the lower errors otained with them. It should e noted that the zero-level of errors has not een firmly estalished, in fact, the authors elieve that the results with Heliax is close to zero errors, although reported as negative errors. In addition to these results, other cales such as the listed triaxial cales have een investigated. These cales have apprecialy higher resistance and errors are also large. Here is presented only the front time error versus increased cale length, as measured with a low voltage impulse of 2 V, and theoretically calculated..84/6 µs /45 µs T1 error T2 errror 2V LI Step Figure 4. Triaxial cale induced errors for front time. Figure 3 Errors oserved when changing length of signal cale. Reference cale is 5 m long Discussion The difference etween Figure 1 and Figure 2 is striking in that the induced changes of the errors are large, although, at first glance, they should e comparale. The main difference is however the arrangement of reference cale, which in the case of Figure 2 may act as a low-impedance path in parallel to the resistance of the shield of the test side. It is quite plausile that this will have an effect that at least qualitatively explains the oserved difference Tests with other cales It was deemed to e of interest to investigate the performance of high-quality GHz range cales. In this case two cales were availale for these tests, listed aove as Heliax and Sucofeed. Both are with solid center conductor emedded in foam dielectric. The shield is solid, corrugated copper. The findings are summarised in tale elow Tale 2. Experiment with radio-frequency cales T 1 error T 2 error error Heliax 1/2 -.6 % -.6 % -.2 % Sucofeed 3/8. %.3 %.4 % 3 SIMULATED RESPONSE 3.1 Background Although the transmission system, i.e. coaxial cale, should e included in every caliration of lightning impulse (LI) measuring systems, see [4], the importance of this requirement has often een neglected. In the course of measurements performed to characterise LI reference measuring systems, the authors oserved distortions in the measured front-time, which could e traced to the use of different lengths of coaxial cales. A literature search revealed that this had een oserved in 1932, in conjunction with design of delay lines for oscilloscopes [1, 2]. Later work y Sato [3] has discussed the phenomenon in relation to lightning impulse measurements, giving the theoretical ackground. His theory is summarised elow. 3.2 Theory The cale is assumed to e properly terminated at oth ends, so that no reflections appear. The complex impedance of the cale is approximated y a real numer to enale an analytical solution.

4 Figure 5. Cale with impedance Z as a real numer (from Sato [3]). If the line constants are given as series resistance R (Ω/m), inductance L (H/m), leakage conductance G (S/m) and capacitance C (F/m), then the characteristic impedance z and propagation constant γ are defined in the frequency domain as: R s L z, R s L G s C (1) G s C Figure 6. Definition of coaxial cale parameters Manipulating equation 1, it can e shown for perfect termination, relation etween the voltages E s at the sending end and E r at the receiving end is E r E s 1 Es e cosh( x) sinh( x) Using the following variales to simplify x (2) 2 4ca a LC, RC LG, c RG, d (3) For E s specified as a superposition of two t t exponential functions, g ( t) e e, we apply the convolution integral and for t > t we otain: E g( t t ) e r t d t t t g( t ) e 2 2 I1 d t d (4) 2 2 t Where I 1 is a modified Bessel function of the first kind, and t ax is the surge propagation time along the cale. 3.3 Simulation The solution of equation 4 has een implemented in software and used to investigate the cales used in the experimental work. The capacitance of the cale has een taken as the nominal value, whereas the series resistance value has een varied to simulate on one hand the result of assuming that the DC resistance is valid and on the other hand taking into account the skin effect on effective conductor resistance. For the purpose of this simulation, skin effect has een calculated for the inner conductor ased on its diameter and at a frequency of 1 MHz. The frequency has een chosen somewhat aritrarily, ut should e a reasonale estimate for work with lightning impulse. Tale 3. Calculated errors for different cales Name Cale length Scale factor error T 1 error T 2 error [m] [%] [%] [%] Triaxial Triaxial Triaxial RG RG RG Heliax Sucofeed As seen from Tale 3, results are not a very good approximation for the oserved experimental results. Especially tail time T 2 is far from experiments. The reason for this is not apparent and more research is needed to explain this phenomenon. Qualitatively, theory agrees with the experiments and lends some crediility to the surmise that the series resistance of the coaxial cale plays an important role 4 CONCLUSIONS The phenomenon that the coaxial cale length will impact on the performance of resistive reference voltage dividers for lightning impulse has een rediscovered. The effect can have a large effect on the achievale performance of reference measuring systems. The possile impact on international standards [4] should e carefully evaluated. This integral is not suitale for analytical solution, ut numerical methods are readily applicale.

5 ACKNOWLEDGMENTS The work reported here has received support from the EMPIR programme co-financed y the Participating States and from the European Union s Horizon 22 research and innovation programme. REFERENCES [1] F. P. Burch, "LXXI. On potential dividers for cathode ray oscillographs," The London, Edinurgh, and Dulin Philosophical Magazine and Journal of Science, vol. 13, pp , 1932/4/ [2] G. W. Bowdler, Measurements in high-voltage test circuits. Oxford: Pergamon, [3] S. Sato, T. Yamaguchi, S. Nishimura, and S. Nishimura, "Influence of measuring cale on lightning impulse parameters," Electronics and Communications in Japan, vol. 93, pp. 1-7, 21. [4] IEC 66-2: 21, High-Voltage Test Techniques - Part 2: Measuring systems.