ON-SITE MEASUREMENTS OF SWITCHING OVERVOLTAGES IN DIFFERENT CONDITIONS OF CROATIAN 400 kV NETWORK

摘要

In the paper is presented historical review of resultsrnof on-site measurements of switching overvoltages inrnCroatian 400 kV network and its statistical analysis. Inrnthe first chapter are given few introductory remarks inrnwhich is stressed that all measuring projects have beenrnmade in international cooperation due to demands forrnvery expensive and rare measuring equipment. Also arernmentioned some historical occasions regarding buildingrn(and destroying) of Croatian 400 kV network in the lastrnfew decades. But study of overvoltages in real networkrnalways remains constant interest in this country.rnIn second chapter firstly is given comprehensive reportrnon methods of testing switching transients in real 400rnkV network. Described is the concept of s.c. testingrnconfiguration and few examples are given. A typicalrnmeasuring scheme is given in the example of 400 kVrnline between cities of Zagreb and Rijeka. Disposition ofrnvoltage dividers and main topology of network is givenrnthere. After that are given some details of differentrnmeasuring equipment with stress on fact that with timernthe characteristics of used equipment become better andrnmore convenient for work on-site. At the end of thisrnsubchapter are given all kind of switching operationsrnwhich have been used. It include energization of lines,rntheir switching off and experiments with single shortrncircuit on lines followed with fast automatic reclosure.rnIn the second subchapter is given short description ofrnapplied statistical procedure after collecting data fromrnthe field. A series of overvoltage collections have beenrnmade for each testing configuration and each measuringrnpoint. The principle was to collect samples for all threernphases for the same measuring point. So given empiricalrndistributions have been compared with severalrntheoretical distributions finding the best coincidence.rnIt is emphasized that procedure of finding the bestrntheoretical distribution is very delicate and importantrnphase of statistical analysis. It is found that the bestrntheoretical curves for majority of empirical distributionsrnare Gaussian and Weibull curves. Upon them arerncalculated all major characteristics of overvoltages, suchrnas: u50, u2, umax etc. All main results for phase-earthrnovervoltages on different 400 kV lines in Croatia arerngiven in Table 1. In the same table are given parametersrnof best theoretical functions and some results from riskrnof failure number of flashover calculations. Also arerngiven maximum values in kV of overvoltages measuredrnon lines end and beginning on all lines. Finally, somerncomparisons with other methods of study ofrnovervoltages may be seen in the same table. One case ofrnlong term registration of overvoltages and few resultsrnfrom EMTP and other calculations are given in thatrncolumn in the Table 1. In the text it is pointed out thatrnthe highest overvoltages are recordered in operation ofrnenergization of unloaded lines. The highest measuredrnvalue of phase-earth relative overvoltage is ue =2,18. Atrnthe same time calculated risks of failures are extremelyrnlow, and number of expected flashovers due tornswitching overvoltages is very low, too.rnIn conclusions it is pointed out that measurements andrnstatistical analysis showed very high reliability of usedrninsulation towards switching overvoltages. That was thernreason for recommendation for changes in clearancesrnand in tower reductions. Also as general result followsrnrecommendation for accepting lower insulation level inrnCroatian 400 kV network i.e. SIWV/LIWV = 950/1300.rnAfter conclusions are given two appendices with somerndetails of 400 kV voltage divider and with some basicrnequations for statistical computations. In last items arernmentioned with gratitude some institutions and personsrnwhich helped us and at end extensive list of references.