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
- Điều hướng trang này:
- Dissipation factor measurement (tan delta)
| 2. Diagnostic procedure
While the withstand voltage test basically only results in an “Ok” or “not Ok”, cable diagnostics deliver information on the cable condition and on the ageing behaviour. The dissipation factor measurement (also called tan delta measurement) and partial dis- charge measurement (PD test) have prov- en to be especially convincing cable diag- nostic procedures. Dissipation factor measurement (tan delta) The dissipation factor measurement is a non-destructive and integral procedure, and is hence useful for assessing the entire tested cable route. It can be applied for plastic-insulated cables, paper pulp insu- lated cables as well as for mixed lines. Pre- requisite for the interpretation of measure- ment results is to know the structure of the cable route. The dissipation factor meas- urement typically comprises several meas- urement cycles: As a rule, 6 to 10 meas- urements are performed at 0.5 x U 0 , 1.0 x U 0 , 1.5 x U 0 and 2 x U 0 . The following in- formation can be derived from the dissipa- tion factor measurement: • The tan delta mean value (MTD) of measurements in the individual voltage steps delivers information on water trees, i.e. damages caused by water in the insulation of plastic-insulated ca- bles. (These water trees can become electrical trees where partial discharges and breakdowns may occur). At the same time, the mean value gives infor- mation on the thermal or chemical age- ing behaviour of the cable route. • The tan delta standard deviation (STD) of measurements in the individual volt- Page 2 of 9 age steps can be used to collect indica- tors of partial discharges (PD) or to de- tect moist joints, for example. • The mean value difference (DTD) in the various voltage steps is helpful to de- tect water trees, partial discharges and vaporisation effects (e.g. at termina- tions). • Furthermore, inadequate insulation of paper pulp insulated cables due to wa- ter penetration can be detected with the dissipation factor measurement. The significance of the tan delta measure- ment depends on the measurement accu- racy and resolution. For example, we need a measurement accuracy of about 1*10E -4 to be able to detect an increase in the dis- sipation factor during measurement on a plastic-insulated cable (see Fig. 1). Fig. 1: The increase in the dissipation factor of phase 1 from 0.2 * 10 -3 at 1.5 x U 0 to 0.5 * 10 -3 at 2 x U 0 delivers information on a “hidden” fault. A subse- quent PD test indicates a defective cable termination. tải về 225.12 Kb. Chia sẻ với bạn bè của bạn:
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