Vlf sine 1 Hz – Universal voltage source for test- ing and diagnostics of medium-voltage cables
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9 2020 gt thi nghiem dien 1 tc ktd19md38 7318
| Requirement
VLF sine CLF Cos-Rect 50 Hz resonance systems DAC Withstand voltage test in compliance with IEC, VDE (CENELEC), IEEE yes yes yes yes, IEEE stand- ard in prepara- tion Load-dependent test signal yes Swing phase varies in the region of 30- 250 Hz acc. to IEEE400.2 [8], re- load phase varies depending on load Test frequency depends on cable length Test frequency depends on cable length tan delta measurement accuracy high (1*10 --4 ) unsuitable for tan delta high medium tan delta sensitivity / comparability high unsuitable for tan delta medium, sensi- tivity less than with VLF medium, load- dependent PD localisation possible yes yes yes yes PD level and PD pattern comparable with measurement at 50 Hz yes not yet studied in detail yes yes PD inception voltage comparable with measurement at 50 Hz yes not yet studied in detail yes yes Compact voltage source yes yes no yes Table 1: Comparison of various voltage shapes with regard to different practice-relevant requirements. Table 1 shows a comparison of the various voltage sources in relation to the above mentioned requirements. From the table it is apparent that the VLF 0.1 Hz sine volt- age as single voltage source meets all the stipulated requirements, i.e. is suitable even for measurement of partial discharges and the dissipation factor measurement (tan delta). With regard to the withstand voltage, almost all popular voltage sources on the market have been found suitable in theory and in practice. However, one must note that here it de- pends on the voltage shape: To achieve reliable results independent of the load (cable route), an ideal sinusoid is of ad- vantage. For this, it is important that the voltage source, as far as possible, always delivers the same signal shape and fre- quency to avoid any influence on the PD test or tan delta measurement, for exam- ple. The aim is to minimise the influence of the voltage source on the measurement result. This allows the user to compare the measurement results and thus to fine-tune decision-criteria for evaluating the condition of cable routes. The VLF sine voltage source is particularly suitable for this. With regard to the measurement of the dis- sipation factor, it is apparent that due to the high precision and sensitivity, a VLF sine measurement has an edge even over the 50 Hz measurement. At low frequency, the tan delta values for the PE-insulated cables are higher - with this, an increase in the tan delta can be detected better (more sensi- tive). It must be further mentioned that only sinusoidal voltage sources are suitable for a precise tan delta measurement. And from this, as mentioned above, the 0.1 Hz sine has been proved to the extent that stand- ards and limit values (IEEE 400.2-2013) for this measurement are available. Among other things, this is due to the fact that we have acquired more experience with the VLF sine in the meantime. Fig. 2: A higher tan delta value is measured at low frequencies (Graph source: [7]) Various scientific publications have already discussed the suitability of different voltage sources (VLF sine 0.1 Hz, 50 Hz, DAC, VLF Cos-Rect) for the partial discharge measurement. For example, the behaviour Page 4 of 9 of various voltage sources has been stud- ied on the following test samples: • Artificially created faults on cable routes and cable terminations • Dirty outdoor cable terminations, fault in outer duct / deflector • Cables in use • Defective joints • Aged and worn plastic-insulated cables and joints Depending on the publication, the behav- iour of two or more voltage sources was compared, particularly in view of the com- parability of the measurement results with those tested at operating frequency (50 and 60 Hz). To sum up, the following result can be derived from the publications (also see [1] to [6]): According to [6], the comparison of meas- urements with 2 x U 0 on 6 worn joints re- sulted in 5.5 times the PD level with ap- prox. 5,500 pC (as opposed to about 1,000 pC at 50 Hz and VLF sine) for a VLF Cos- Rect voltage source. Fig. 3: PD strengths of six joints (plug-in technology); [6] These higher measured values mean that a measurement with the Cos-Rect voltage source presents a higher load for worn joints. However, for proper diagnostics, it is important that the cable and accessories are not unduly loaded or even damaged by the measurement. In addition, in [6] it has been determined that the waveform of the test voltage has a greater impact than the increase in the level from 2 x U 0 to 3 x U 0 (compare sine with Cos-Rect). It is also apparent that sine 50 Hz and sine 0.1 Hz show almost identical levels. Until now, scientific studies have been conducted only on PD tests with a VLF Cos-Rect voltage source. Practical experi- ences on the suitability of the VLF Cos- Rect voltage shape for the PD test espe- cially with worn cable routes are still not available. With reference to the PD tests with VLF sine, all mentioned publications state that the PD inception voltage can be compared with the voltage of the 50 Hz measurement when tests were conducted on field objects (i.e. not artificially prepared objects). In artificially created faults, the inception volt- age during the VLF test and the 50 Hz test occasionally differed from each other, which is why [4] comes to the conclusion that faults and test bodies created artificial- ly in the laboratory are not suitable for se- lecting the optimum voltage source. With regard to the PD level and the PD pattern (distribution of measured values), the publications similarly showed that re- sults with VLF sine 0.1 Hz are comparable with the results of 50 Hz measurements. This likewise applies for worn joints in plug- in and heat shrinking technology. There were no relevant differences even in the location of partial discharges. With reference to a comparison (4 cable routes with total of 42 different faults) be- tween VLF sine 0.1 Hz, 50 Hz resonance system, 20-400 Hz resonance system and DAC (Cos-Rect was not used here), during the test on various medium-voltage cables we conclude for example [2], that no single technology seems better than the other (“Under the conditions in which this project was carried out, the experimental results show that no single partial discharge test- ing technology for installed MV cable sys- tems provides significantly better results than the others.”). No clear correlation was evident between the PD strength or the inception voltage and the voltage source in the [2] described study (“The possible relationship between the magnitude of the partial discharges and the type of high voltage source used was investigated, but no relationship was found between them due to the high dispersion of results. The same conclusion was reached on studying the possible relationship be- tween the PD inception voltage levels and Page 5 of 9 the type of voltage source used. It was not possible to establish any correlation”). While choosing from the [2] studied voltage sources (VLF sine 0.1 Hz, 50 Hz reso- nance system, 20-400 Hz resonance sys- tem and DAC), the user must preferably consider practical criteria such as possible accomplishment of tasks, weight, versatility and easy handling. Table 1 shows that of the four types of voltage sources – VLF sine, VLF Cos-Rect, 50 Hz and DAC – only the VLF sine voltage source satisfies all the requirements when considering the cable test as well as the tan delta and PD test. With regard to the tan delta measurement, the VLF sine has a favourable impact ow- ing to its higher sensitivity. tải về 225.12 Kb. Chia sẻ với bạn bè của bạn:
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