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Protection Characteristics of Spark Gaps

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Protection Characteristics of Spark Gaps fz <?rz>7orz> /fur-- *9- Protection characteristics of spark gaps Marja-Leena Pykala Veikko Palva 27 deceived ' ccq 2315 S3 OSTl Report Helsinki University of Technology High Voltage Institute Espoo, Finland 1997 DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. Report TWH ; Helsinki University of Technology High Voltage Institute ^*gh Voltage Itvs^ Protection characteristics of spark gaps Marja-Leena Pykala Veikko Palva ISSN 1237-895X ISBN 951-22-3438-6 January 27, 1997 Espoo, Finland 2 (24 ) Preface The High Voltage Institute of Helsinki University of Technology (HUT) has operated as the National Standards Laboratory of High Voltage Measurements since October 1995. Research is largely concentrated on high voltage measurement and metrology. This project concentrated on practical issues: distribution transformers and protective spark gaps. The project was supervised by the following expert group: Jarmo Elovaara from IVO Power Engineering Oy, Juha Sotikov from Finnish Electricity Association and Esa Virtanen from ABB Transmit Oy as well as Martti Aro, Matti Karttunen and Veikko Palva from Helsinki University of Technology. Abstract Distribution transformers in rural networks have to cope with transient overvoltages, even with those caused by the direct lightning strokes on the lines. In Finland, the 24 kV network conditions, such as wooden pole lines, high soil resistivity and isolated neutral network, lead into fast transient overvoltages. The distribution transformers (< 200 kVA) have been protected and a major part is still protected using protective spark gaps. The protection characteristics of different spark gap types were studied widely using improved measuring techniques. The main results are presented in this report. Results can be used as background information for national and international standardization work dealing with distribution transformers and protective devices. Contents 1. SUMMARY AND CONCLUSIONS 2. INTRODUCTION 3. DESCRIPTION OF TESTED SPARK GAPS 4. STANDARD LIGHTNING IMPULSE TESTS ON SPARK GAPS 5. TESTS ACCORDING TO FRENCH NATIONAL STANDARD C 52-192-1B 6. STEEP FRONT IMPULSE TESTS 7. WET POWER-FREQUENCY TESTS Acknowledgements References Helsinki University of Technology High Voltage Institute Otakaari 5L FIN-02150 Espoo, Finland Marja-Leena Pykala Phone +358 9 451 2404 Fax +358 9 451 2395 E-mail Marj a-Leena. Pykala® hut.fi 3 (24 ) 1. SUMMARY AND CONCLUSIONS The protection of distribution transformers against fast transient overvoltages by means of spark gaps requires careful consideration of the insulation co-ordination. Especially when .wooden pole lines are used and the specific soil resistivity is high (as e.g. in Finland), one has to take into account the direct strokes to the line, which results in high steep front voltages on the spark gap protected transformers. By taking all this into consideration in dimensioning and testing the transformers, very favourable results have been achieved in Finland as to the service statistics (interruptions, transformer failures etc.). The following statements and conclusions can be derived from the results of this report and its background material: a) Fast transient overvoltages in distribution networks stressing the spark gap protected transformers may have a steepness of the order of 1000 - 2000 kV/)is. They are caused by the direct strokes to the lines. b) Distribution transformers have to cope with these overvoltage stresses. Transformers have to be tested accordingly by applying a special steep front impulse test in addition to the standard lightning impulse 1,2/50 jis test. This testing practice has been used in Finland since the early 60’s. The service statistics have developed very positively, and now show only less than 0,5 % failures of distribution transformers yearly. c) 99 % protection levels of the spark gaps of different structure (and manufacturer) were first compared in the standard lightning impulse tests. Nowadays the predominantly used double spark gaps (2 x 40 mm) gave rather high dispersion in protection levels. The results were influenced by e.g. the structural details of the gaps and the mounting arrangements of the specimen. The middle electrode at free potential increases the dispersion. d) The results show that there is no margin or rather a small margin between the protection level and the test voltage of the transformer (125 kV, 1,2/50 ps). In service conditions the need for a margin may be still more pronounced. It is proposed that the use of double spark gaps with spacing 2 x 30 mm and single spark gaps of 80 mm should be considered in the future. The spacing of those spark gap structures with the highest measured protection levels should any way be reduced. e) As to the specification in the standards, it is proposed to state only the protection level of the spark gaps (with sufficient margin). The spark gap must be constructed and applied so that it will fulfil this requirement. f) The steep front impulse tests of the spark gaps demonstrate that the steepness range of 500 - 2000 kV/|is can be managed well. Generating and measuring techniques are feasible but, however, require certain skill and knowledge as well as equipment. g) At steep front impulses the sparkover voltages of the different spark gap structures show smaller dispersion and more similarity than for the standard lightning impulse. The voltage/time curve for a double spark gap is less steep than for a single spark gap. h) The steep front impulses are always chopped on the front. The sparkover voltage at the steepness of 2000 kV/ps for the double spark gap 2 x 40 mm is 220 - 260 kV, with the front time less than 0,15 jxs. This is the voltage stressing the transformer. i) In order to cope with these stresses, the voltage distribution of the transformer winding has to be sufficiently linear, and the insulation has to be dimensioned accordingly. 4 (24 ) j) One basic requirement in the application of the spark gaps for fast transient over ­ voltage protection is that the spark gaps must not operate at slow-front overvoltages (caused e.g. by switching operations). The same is valid for temporary overvoltages (caused e.g. by earth fault conditions). k) The double spark gaps were submitted to wet power-frequency tests. The results for 2 x 40 mm gaps show good consistency, and the sparkover voltage is high enough for the conditions in the 24 kV network. The double spark gap 2 x 20 mm, on the other hand, is too small to cope with the temporary and slow-front overvoltages in the 24 kV isolated neutral network (or with resonant earthed neutral). l) The proposed test C 52-192-IB 712/ does not correspond to the fast transient overvoltages existing in distribution networks with wooden pole lines. The double spark gap 2x20 mm with the given prospective peak value of 150 kV results in front steepnesses which are only slightly higher than in the case of 1,2/50 (is impulse. As pointed out earlier, the spacing 2 x 20 mm is too small with regard to the network requirements. m) The proposed test cannot substitute the conventional transformer test with lightning impulse chopped on the tail, having the same 100 % (or 115 %) test voltage level as the standard 1,2/50 (is test and time to chopping between 2 (is and 6 (is. 2. INTRODUCTION The basic reasons for the measurements of the spark gap characteristics were the needs to get additional information for transformer tests described in the current national standard and for new international standardization work on pole-mounted transformers protected with spark gaps. Another reason was to check the protection characteristics given in the current national standard. New structures of spark gaps on the market as well as improved impulse voltage measuring techniques also required supplementary and comparative studies. In Finland the distribution transformers (< 200 kVA, 24 kV) are commonly protected against fast transient overvoltages by means of spark gaps. This leads to increased stresses of the distribution transformers. Data of lightning parameters, wave shape characteristics, steepnesses of fast transient overvoltages and insulation co-ordination of distribution transformers are widely considered in references /!/, 121,131, 1161, /17/. Hence, the transformers with spark gap protection are subjected to a combination of standard lightning impulse and steep front impulse voltage tests based on the standards DEC 76-1151 and SFS 2646 161. In Finland the following test sequence is in use: 1) one or more 60 % standard lightning impulses 1,2/50 (is 2) one 100 % impulse 1,2/50 (is, Up = 125 kV for 24 kV equipment 3) reduced (calibration) steep front impulses 2000 kV/(is, spark gap 2x20 or 1x40 mm 4) five steep front impulses 2000 kV/(is, double spark gap 2x40 mm 5) two 100 % impulses 1,2/50 (is 6) one or more 60 % impulses 1,2/50 (is. Negative polarity is used. The steep front impulse of items 3) and 4) means a linearly rising front chopped impulse having a steepness of 2000 kV/jis. The impulse is chopped with a double spark gap installed at the distance of 2 m from transformer terminals. Each terminal 5 (24 ) is tested in one of the possible positions of the off-load tap-changer. The failure detection is based on possible deformations in voltage or current wave shapes. For the basis of the transformer testing, the characteristics of some protective spark gaps were tested at standard lightning impulses and steep front impulses. The detailed test results are presented in reference /4/. The sparkover voltage/time curve measurements were compared to earlier measurements /11/. An important reason for the spark gap measurements was the standardization work for pole mounted transformers (prHD 428.5) in CENELEC WG17. An English version of French National Standard C 52-192-1 Appendix B /12/ has been sent for consideration to the members of WG17 as a proposal for prHD 428.5.
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