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
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- 6. Effects of the voltage source – Ex- ample E-Werk Mittelbaden
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0.5U 0 (kV) U 0 (kV) 1.5U 0 (kV) 3.5 6.5 10 L1 0.068 0.036 0.060 L2 4.453 2.313 9.343 L3 0.063 0.004 0.050 Table 2: Measured values for Figure 4: Fig. 5: Drying effect of moist joints during a MWT The tan delta standard deviation on con- ductor 2, identified in Fig. 4, strengthens the suspicion of a moist joint, as the PD test cannot detect any PD activity (too much moisture). The MWT, i.e. the combi- nation of cable testing and tan delta meas- urement over 15 minutes, conveyed that the joint had dried up and that the tan delta had clearly dropped (Fig. 5). This rein- forced the suspicion of wet joints. 6. Effects of the voltage source – Ex- ample E-Werk Mittelbaden About ten years ago, the Elektrizitätswerk Mittelbaden Netzbetriebsgesellschaft mbH (short E-Werk Mittelbaden) carried out a comparison of VLF 0.1 Hz sine and a 50 Hz method for the PD test based on over 40 cable routes. As the 50 Hz method pro- duced too many highly varying evaluations and especially more negative prognosis at the time, which until today were not con- sidered as losses, the company decided on the VLF 0.1 Hz sine method. In the mean- while, the VLF measurement with sine volt- age has been established in hundreds of tests. This was evident in the diagnostic measurements, performed on 240 kilome- tres with 500 partial routes in the 20 kV network of the E-Werk Mittelbaden, on paper pulp insu- lated and mixed cable routes. At the E-Werk Mittelbaden, such cable routes with a VLF sine 0.1 Hz have been diagnosed by means of PD, and since about seven years, also by means of the tan delta measurement. According to Wer- ner Brucker, Director of network opera- tions, both these diagnostic procedures produced a good overall view of the age and condition of the network. Partial routes classified as dangerous will be replaced soon. The isolation of defective partial sec- tions resulted in huge savings, as the entire cable run didn’t have to be replaced. In practice, the VLF measurement has proven to be suitable during the commis- sioning tests of new or modified cable sys- tems for locating faults precisely, and in future for detecting faults in the accessories with a simultaneous PD test so that the work involved for troubleshooting, such as installation faults, or during maintenance, e.g. excavation work, can be kept to the minimum. Brucker emphasises the weight and suita- bility for daily use as an essential ad- vantage of the VLF sine voltage source. The 0.1 Hz technology can be transported and operated by any employee, which would not have been possible with a 50 Hz system. The use of a test car with two persons is required very rarely, as the portable meas- urement and testing device is enough for most cable lengths. A test car would be necessary only during the seventh meas- urement or so. E-Werk Mittelbaden saw a clear cost ad- vantage by applying the VLF 0.1 Hz tech- nology: measurements could be easily per- formed by a single employee in a short time. Due to the short connection time and test duration as well as the need for fewer people, several cable routes can easily be tested per year. Routes identified as critical during the measurements or partial sec- tions will be scheduled for repair or re- placement. Maintenance budgets can thus be deployed specifically. With the knowledge of weak points in the network and the condition-based maintenance, in spite of the growing cable stock, running the medium-voltage network with a low failure rate and in a cost-optimised manner has been successful. The maintenance plan at the E-Werk Mit- telbaden is approx. EUR 4 million, of which Page 8 of 9 EUR 2.5 million are relevant for the distri- bution network. The costs for cable diag- nostics at present are € 90,000/year. From the comparative tests conducted be- fore procuring the VLF equipment, Brucker knows that there were distinct differences between VLF 0.1 Hz sine and 50 Hz meas- urements for his tests. In practice, a changeover seemed no long- er relevant, as the E-Werk Mittelbaden be- came very familiar with the VLF 0.1 Hz measurements and its interpretation, and is able to use measured values with high reli- ability for classifying cable routes. Even the prognosis whether a cable route is in dan- ger in the short or medium term is possible with relative accuracy due to the wealth of experience, thus making it easier to priori- tise maintenance measures accordingly. tải về 225.12 Kb. Chia sẻ với bạn bè của bạn:
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