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Protection characteristics of spark gaps
Marja-Leena Pykala Veikko Palva
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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. The protection characteristics of the spark gap with the transformer in the test circuit were measured with the C 52-192-1 circuit, in order to check the differences between the proposal for prHD 428.5 and the Finnish requirements.
The first results of double spark gaps at standard lightning impulses showed remarkable differences between different structures of protective spark gaps. So, the protection characteristics at standard lightning impulses were tested widely: four spark gap types in two basic mounting arrangements were measured in two laboratories. The protection characteristics at the steep front impulses were also measured. In addition, the wet power-frequency tests were performed.
3. DESCRIPTION OF TESTED SPARK GAPS
Three structures of double spark gaps were tested: A) C52, dimensions and material in Figure 1 B) TK 20830, see Figure 1 C) OJUP ZS1, see Figure 2 and one single spark gap: D) NLZWY 244, see Figure 3.
Spark gaps A) and B) conform to the requirement of C 52-192-IB /12/. Spark gap type C) has been used extensively in Finland for many years, and naturally also in the steep front impulse tests on distribution transformers. The spark gap spacing is 2 x 40 mm for SFS 2646 tests and 2 x 20 mm for tests according to standard C 52-192-IB.
Dimensions/mm A) C52 B) TK a 115 155 b 40 40 c 40 35 c' 75 55 d 80 175 h 340 315 1 255 125 s 200 200
a 45° cn X 012, stainless steel 012, zinc coated steel y 0 6, stainless steel 4,5 x 25, zinc coated st. z cast resin insulator glass insulator units
Figure 1. Structure of double spark gaps A and B 6 (24 )
Dimensions/mm C) OJUP a 75 a1 160 b 40 c 80 c’ 80 d 100 h 335 I 255 s 125 (%1 = lble spark gap C Dimensions/mm D) NLZWY at 120 a2 50 b 100 c1 140 c2 50 c3 110 d 140 e 1,5 r1 R20 r2 R30 a 60° P 40° 6 45° X 0 8 z bushing of porcelain Figure 3. Structure of single spark gap D 4. STANDARD LIGHTNING IMPULSE TESTS ON SPARK GAPS The protection characteristics of the spark gaps were measured as a basis for consideration of the transformer testing. The 99 % sparkover voltage was determined at the standard lightning impulse 1,2/50 ps. The first measurements showed some unexpected results. For instance, the protection level of the 2 x 40 mm double spark gap seemed to have changed from the results measured ten years ago. The small distance from the ceiling in the HUT laboratory also appeared to affect the results. So, three double spark gaps of different structure and material as well as one single spark gap were tested. Measurements were performed at HUT High Voltage Institute and ABB Corporate Research Finland in two mounting arrangements. 7 (24 ) 4.1 Mounting arrangements Two basic arrangements were used for the double spark gaps, see Figure 4: a) a double spark gap arranged as close to the normal position as possible on a top of a wooden pole, at a distance of 2 m from the supposed transformer terminals and conductors as in normal use b) a double spark gap 2 m above the laboratory floor. The distance between the ceiling and the spark gap was 1,9 m in HUT laboratory and over 10 m in the ABB laboratory. The capacitor in the Figure 4a) mounting was used to simulate the input capacitance of the transformer. The value 375 pF is approximately of the same size as of the distribution transformers in question. The capacitor was not used in other mounting arrangements. The single spark gap was mounted on a distribution transformer filled with oil but no active parts in it. The test voltage was applied and measured similarly to the mounting arrangement 2 m above the laboratory floor, see Figure 4b). Ceiling i Ceiling 1,9 m in HUT ' >10 min ABB , Al-tube ^SUPP* 3,4 m in HUT Double spark gap '1 * 25 mm > 10 m in ABB To voltage divider 100x0,5 mm Cu-foil Wooden pole Double spark gap Al-tube 2 m E 25 mm E ! Supply 8 To voltage divider 100x0,5 mm Cu-foil Support insulator 1,5 m C = 375 pF £ a) b) Figure 4. Mounting arrangements: a) top of the pole, b) height 2 m above laboratory floor 4.2 Test circuits In the HUT laboratory the test voltage was generated with an 800 kV impulse generator Haefely. The voltage was measured with shielded resistive voltage divider Haefely G600 and digital recorder TR-AS 100/10. The test circuit in the HUT laboratory is shown in Figure 5. The capacitor C was used only in measurements in arrangement 4a). In case 4a) the additional distance from the spark gap to the supposed transformer terminals was 2 m i.e totally 4 m from the spark gap to the voltage divider. In case 4b) the total length between the divider and the spark gap was 2 m. The impulse shape was 1,22/49 [is. The test circuit in the ABB laboratory was in principle close to that at HUT. The voltage was measured with a 600 kV resistive divider PYR600 constructed in the ABB laboratory. The impulse shape was 1,28/54 [is. 8 (24 ) The atmospheric corrections according to IEC 60-1 111 were applied in both laboratories. The value of the correction factor Kt 111 varied from 0,97 to 1,01 during the lightning impulse tests. Figure 5. Test circuit in HUT laboratory 4.3 Test methods and results The 50 % sparkover voltage U50 of the spark gaps was measured using both multiple-level and up-and-down methods described in EEC 60-1 111. In the multiple-level method 20 impulses were applied at each voltage level, and the number of voltage levels was five. So, totally 100 impulses per test series was used in the multiple-level method. In the up-and-down method the total number of impulses was 40. In both methods the voltage step between the levels was 3 kV (2,5 - 3,5 %). The 99 % sparkover voltage U99 and the standard deviation s were determined using statistical evaluation. The results of multiple-level as well as up-and-down methods were evaluated using the statistical analysis based on references 191 and 1101. All results 141 were evaluated in the HUT laboratory. 4.3.1 Results for the double spark gaps mounted on the pole Results for the double spark gaps are given in Tables 1 to 3. The numbering of the test series follows that used in the original results 141. Table 1. Test results of double spark gap A (C52) 2x40 mm, mounted on the pole, Figure 4a) fupd: up-and-down method, mlm: multiple-level method) Test Laboratory Method/ U50 95% confidence limits of s s U99 series Polarity kV Uso/kV kV % kV 1 HUT upd /+ 120 119...121 2,9 2,4 127 2 HUT mlm /+ 119 118...120 3,1 2,6 126 5 ABB upd /+ 111 110...113 3,5 3,2 119 6 ABB mlm /+ 111 109...114 6,0 5,4 125 3 HUT upd/- 108 106...110 4,9 4,6 119 4 HUT mlm /- 108 107...109 2,3 2,1 113 7 ABB upd /- 112 111...113 2,1 1,9 117 8 ABB mlm /- 114 110-116 8,6 7,5 134 9 (24 ) Table 2. Test results of double spark gap B (TK) 2x40 mm, mounted on the pole, Figure 4a) Test Laboratory Method/ U50 95% confidence limits of s s U99 series Polarity kV Uso/kV kV % kV 9 HUT upd/+ 124 120...127 8,4 6,8 143 10 HUT mlm /+ 120 117...122 5,9 4,9 133 13 ABB upd /+ 105 103...107 4,8 4,6 116 14 ABB mlm /+ 109 107...112 5,0 4,6 120 11 HUT upd /- 112 110...113 1,6 1,4 115 12 HUT mlm/- 113 112...114 2,9 2,5 120 15 ABB upd/- 115 113...116 3,4 3,0 123 16 ABB mlm /- 115 114...117 4,8 4,2 126 Table 3. Test results of double spark gap C (OJUP) 2x40 mm, mounted on the pole, Figure 4a) Test Laboratory Method/ U50 95% confidence limits of s s U99 series Polarity kV Uso/kV kV % kV i £ 17 ABB upd/+ 85 2,9 3,5 92 18 ABB mlm /+ 86 83...88 4,2 4,9 96 19 ABB upd/- 103 101...105 4,4 4,2 113 20 ABB mlm /- 104 103...105 2,5 2,4 110 For comparison purposes 99 % protection levels are shown in Figure 6. The test conditions are explained in Tables 1 to 3. The standard lightning impulse withstand voltage level 125 kV for 24 kV equipment is also drawn in Figure 6. Use/kV 125 kV Spark gap x A (C52) o B (TK) * C (OJUP) 125 kV Test series Figure 6. U99 results for double spark gaps mounted on the pole, test series in Tables 1 to 3 The measured 99 % protection levels of spark gap types A (C52) and B (TK) are higher than those of spark gap type C (OJUP): even over 20 % at positive polarity. The main reason for this is the different structure of the double spark gaps, especially the cross section. Also the surface 10 (24 ) quality of the arcing horns (stainless steel or hot dip galvanized) affects. The arcing horn diameter of types A and B is 12 mm and diameter of type C 8 mm. In HUT and ABB laboratories the results of spark gap types A and B differed about 10 kV at positive polarity, best seen in the U50 results. The reason for that was supposed to be the too short distance between the spark gap and the earthed ceiling in HUT laboratory (1,9 m). This distance should be fully sufficient according to standard IEC 60-1 111 when testing conventional specimens. In ABB laboratory the distance is over 10 m. In case of electrodes at free potential the situation is more sensitive. The short distance from any object can affect the electric field distribution of the double spark gap with the middle electrode at free potential. The middle electrode extends here about 200 mm above the spark gap level. In test series nos. 6, 8 and 9 the standard deviation is over 5 %, and it caused considerable differences between the U99 results (not seen in the U50 results) measured in the same laboratory. The main reason for the high standard deviation values and the differences between the successive test was the structure of the double spark gap having quite a short spark gap spacing and a middle electrode at free potential. Other reasons may be the small distances from generator or any earthed object. The support insulators of spark gap structures A (C52) and C (OJUP) were of cast resin, which may have different surface charges depending on the previous impulse (polarity, sparkover or withstand, amplitude) and the interval between the impulses. The effects of changing the mounting arrangement was also checked. For instance, the earth loop was changed by increasing the distance between the supply and earth conductors from 0,5 m to 0,8 m. No effect was seen. The circuit with or without capacitor simulating the input capacitance of the transformer did not have an effect on the results. Changing the measuring point from the support insulator to the double spark gap terminal in the mounting arrangement of Figure 4a) did not change the results. Differences between successive test series with the same adjustments should be small, but the spark gaps under test seemed to be rather sensitive to the most conditions during the tests. 4.3.2 Results for the double spark gaps at the height of 2 m above the laboratory floor One reason for the differences between the ABB and HUT results was supposed to be the small distance from the spark gap to the ceiling in HUT. In order to increase the distance to the ceiling, the measurements were repeated with the spark gap mounted at the height of 2 m above the laboratory floor. In the HUT laboratory the distance between the spark gap and the ceiling was now 3,4 m. Results for these measurements are given in Table 4. Table 4. Test results of double spark gap A (C52) 2 x 40 mm at the height of 2 m above the laboratory floor, Figure 4b) Test Laboratory Method/ U50 95% confidence limits of s s U99 series Polarity kV Usn/kV kV % kV 21 HUT upd /+ 108 107...110 4,2 3,9 118 22 ABB upd /+ 105 104...107 3,5 3,3 113 23 ABB upd /+ 113 111...114 2,8 2,5 119 24 HUT upd /- 112 110...113 2,9 2,6 118 25 ABB upd /- 110 108...113 4,8 4,4 122 26 ABB upd/- 112 110...113 3,6 3,2 120 11(24 ) The results at positive polarity measured in both HUT and ABB laboratories are close to those measured in ABB in Tables 1 to 3. In these tables the distance 1,9 m from the ceiling seems to result in 10 - 15 % higher U50 values in the HUT laboratory at positive impulses than in the ABB laboratory or with the spark gap mounted 2 m above the laboratory floor. The difference between the test series 22 and 23 was assumed to be caused by the polarisation of the cast resin insulator of the double spark gap A (C52). The measurements in ABB were made in the order 22, 25, 23 and 26. The lowest value was measured first. Some additional test series were measured applying about 75 % impulse at opposite polarity after each test impulse, but no differences between normal and these additional test series were seen. The effect of the earthed ceiling near (1,9 m) the protective spark gap was worked on by mounting the spark gap A (C52) first at the distance of 3,4 m from the ceiling. Then the spark gap was mountedupside down at the heights of 2 m and 1 m from the earthed laboratory floor simulating the influence of the ceiling. These mounting arrangements are shown in Figure 7 a), b) and c). These measurements were performed in the HUT laboratory only. Ceiling a) 3,4 m b) 3,4 m c) 4,4 m To voltage divider 100x0,5 mm Cu-foil Double spark gap 4 — Al-tube 25 mm Supply b) and c) Figure 7. Mounting arrangements of double spark gap A (C52) in HUT laboratory a) height 2 m, spark gap upwards b) height 2 m, spark gap downwards c) height 1 m, spark gap downwards Table 5. Test results of double spark gap A (C52) 2 x 40 mm at the height of 2 m and 1 m Test Arrangement Method/ U50 95% confidence limits of s s U99 series Figure 7 Polarity kV IWkV kV % kV 27 a) upd /+ 118 116...119 2,8 2,4 124 28 a) upd/+ 118 116...119 4,8 4,1 129 29 b) upd /+ 122 119...124 5,1 4,2 133 30 c) upd /+' 124 120...128 8,1 6,5 143 31 a) upd 1- 112 110...114 5,1 4,5 123 32 b) upd /- 111 109...114 5,5 5,0 124 33 c) upd /- 110 108-112 4,7 4,2 121 The order of the test series was: 27, 28, 31, 29, 32, 30 ja 33. 12 (24 ) The differences betweenthe mounting arrangements shown in Figures 7a) to c) were compared. The 99 % protection level for the tests series in Tables 4 and 5 are shown in Figure 8. Highest voltage level at positive polarity was measured with the double spark gap A 1 m above the laboratory floor with the spark gap downwards (test series 30). This result is of the same order of magnitude as the results obtained for spark gaps A (C52) and B (TK) in Tables 1 and 2 (test series 1,2,9 and 10). The free distance between the double spark gap (and especially the middle electrode) and the earthed metal plate seems to have an influence on the sparkover voltage at positive polarity. 1)99 / kV Mounting □ HUT 2 m a 2 m downwards x 1 m downwards 125 kV Test series Figure 8. Comparison of the results for double spark gap A (C52) mounted 2 m and 1 m above the laboratory floor, mounting arrangements shown in Figure 7, and test series in Tables4 and 5 4.3.3 Results for the single spark gaps mounted on the transformer bushing The single spark gap with the spacing 100 mm was measured only in the HUT laboratory for comparison with the double spark gap results. The single spark gap D (NLZWY) was mounted on the transformer filled with oil but no active parts in it. The results are given in Table 6. Table 6. Test results of single spark gap D (NLZWY) 100 mm on the transformer bushing Test Laboratory Method/ U50 95% confidence limits of s s U99 series Polarity kV Uso/kV kV % kV 34 HUT upd/+ 95 93...96 4,1 4,4 104 36 HUT upd/- 81 79-83 4,0 4,9 90 13 (24 ) 4.4 Summary of the standard lightning impulse tests The standard lightning impulse withstand voltage requirement is 125 kV for 24 kV equipment. The standard SFS 2646 76/ gives the specified 99 % protection level 120 kV for both 2 x 40 mm double spark gaps and 100 mm single spark gaps. A large number of tests have been performed because of some unexpected results. There were differences between the results measured in the HUT and ABB laboratories at the positive polarity and the mounting arrangement on the top of the pole. The reason was supposed to be the 1,9 m distance from the spark gap to the ceiling, which was further checked by measurements in other mounting arrangements. Standard EEC 60-1 111 requires usually a clearance of 1,5 times the length of the shortest possible discharge path from extraneous structures. In these spark gap tests with the middle electrode at free potential this requirement is not valid. The results in the HUT laboratory at positive polarity with the spark gap on the top of the pole are not taken into account in the next statements. The 99 % protection level of each spark gap type is below 125 kV, the standard lightning impulse withstand voltage for 24 kV transformers, except two test series with exceptionally high dispersion. The results of double spark gaps with 0 12 mm arcing horns, the one with cast resin insulator (type A) and the other with glass insulator (type B), are quite similar, with a protection level of 125 kV. The spark gap types A and B differ significantly from the spark gap structure with 0 8 mm arcing horns (type C) having the protection level of 115 kV. The spark gap type C (OJUP ZS1) has been used extensively in Finland. The protection level of the single spark gap was 105 kV. The value is lower than the corresponding value of the double spark gaps. In single spark gaps there is no middle electrode smoothing the electric field distribution as in double spark gaps. The conclusion is that not only the spark gap spacing itself but all details of the spark gap structure are important in determining the protection characteristics of the spark gaps at standard lightning impulses. The results show that a 99 % protection level for spark gaps with 0 12 mm arcing horns in some cases exceeds the standard lightning impulse withstand voltage 125 kV for 24 kV equipment. So, it is advisable to test and adjust the spark gap spacing for each new structure. The standard dealing with spark gap protection should specify the protection level requirement for spark gaps rather than the spark gap spacing. For instance in VDE-specification 714/ the following spark gap requirements are given for 24 kV equipment: 1) wet power-frequency withstand voltage 30 kV, 2) 90 % sparkover voltage 90 kV at standard lightning impulse (i.e. at least nine sparkovers of ten impulses, positive and negative polarity) and 3) highest sparkover voltage 125 kV at linear impulses with a steepness of 200 kV/ps (maximum of ten positive and ten negative impulses). For comparison similar requirements for gapped surge arresters are given in IEC standard 99-1 7157. In general the required margin between the protection level of the spark gaps and the withstand voltage requirement of equipment should be based on careful consideration. As the test results show, the dispersion in the sparkover voltage is relatively large, and it depends on many affecting factors. One additional factor in the service conditions is possible deformation on the electrode surfaces caused by the earth fault current. In spite of high speed automatic reclosing, 14 (24 ) about 0,3 s, marks due to arcing may affect the sparkover voltage of small spark gaps in question. 5. SPARK GAP TESTS ACCORDING TO STANDARD C 52-192-1B The transformer test sequence according to standard C 52-192-IB /12/ consists of one full standard lightning impulse at specified test voltage level with the spark gap disconnected. Then the spark gap is connected and adjusted to 1 x 20 mm, and chopped impulses with gradually increasing voltages are applied for reference. Five successive impulses are applied with the spark gap adjusted to 2 x 20 mm at prospective voltage levels of 110 and 150 kV at negative polarity. In this case prospective level means the value which would be reached without a spark gap at standard lightning impulse. After that the spark gap is disconnected and two 100 % standard lightning impulses are applied. The 100 kVA 24 kV distribution transformer, type KTMU 24 HC 100 associated with the double spark gap A (C52) was tested in the ABB laboratory, test report No. 9 AFX95-264 /13/. The test arrangement is shown in Figure 9. The test voltage was measured with a 600 kV resistive divider PYR600 and digital recorder RTD 710. Supply Double spark gap 2 x20 mm Earthing terminal of transformer Divider PYR600 L: DSO Ch2 /,; DSO Ch1 0,5 Q Figure 9. Test arrangement for C 52-192-1 test The impulse shape of the full impulse was 1,28/54 jis. The examples of chopped impulse voltages at prospective voltage levels 110 kV and 150 kV are shown in Figure 10. Fifteen impulses were applied, five to each transformer terminal. The average value of sparkover voltage, front time and steepness as well as standard deviation of measured sparkover voltage values were calculated according to IEC 60-1 111 , in the same way as in steep front impulse tests. The results of the chopped impulses with the spark gap adjusted to 2 x 20 mm are summarised in Table 7. 15 (24 ) Table 7. Protection characteristics of double spark gap A (C52) with spacing 2 x 20 mm, test arrangement according to standard C 52-192-IB Prospective U avg s s Ti S voltage level kV kV kV % gs kV/gs 110 94,3 1,8 1.9 0,59 161 150 109,4 2,4 2,2 0,44 239 These results are commented in Chapter 6 together with steep front impulse results. prosp 110 kV 97,0 kV 0,73 gs 159 kV/gs Ranges: -voltage -96,6 ... +33,6 kV -time 0,0025... 3,0 gs .9BJS .1.2015 1.5033 1.B010 2.1008 2.4005 2.7003 3.0( 48.7 32.9 ft u prosp 150 kV 17.2 A A u 109,0 kV /J Jl. ' Ti 0,43 gs 'VW Tc 0,44 gs -14.2 Vi 1 I 11 \f S=UZT1 254 kV/gs !! V -2a. 3 1 V Ranges: -45. S -voltage -108,4 ... +48,7 kV V -time 0,0025 ... 3,0 gs -61.3 \ -77.0 X\ -32.7 . IQS. 4 ti 0025 .3023 .E020 .303B 1.2015 3.5023 3.8010 2 3008 2.4005 2.7003 3.0f Figure 10. Impulse shapes in C 52-192-IB test, prospective peak values 110 kV and 150 kV 16 (24 ) 6. STEEP FRONT IMPULSE TESTS The sparkover voltage/time curve was determined at linearly rising steep front impulses with steepnesses of about 500,1000 and 2000 kV/ps. Impulse shapes are shown in Figure 11. U/kV so 0 -50 -100 -150 -200 c) 2000 kV/LLS •250 Figure 11. Linearly rising steep front impulses, nominal steepnesses 500, 1000 and 2000 kV/jis It is pointed out already here that the voltages having the impulse shapes shown above are the stresses affecting the transformer insulation, which has to be dimensioned accordingly. 17 (24 ) 6.1 Mounting arrangement The mounting arrangement is shown in Figure 12. It is close to Figure 4a), spark gap on top of the pole. The supply and earth conductors were inclined so that the distance between the terminal and spark gap remained in 2 m, but the distance between the spark gap and the laboratory ceiling was 2,6 m. No capacitor was used to simulate the transformer input capacitance. The tests were performed in HUT laboratory. Ceiling Supply Double spark gap Wooden pole Support To voltage divider insulator Figure 12. Mounting arrangement in steep front impulse tests 6.2 Test circuit The test voltage was generated with a 800 kV impulse generator adjusted for linearly rising steep front impulses. One effective way to achieve sufficient linearity is to use inductance in the main circuit. The test voltage was measured with fast resistive voltage divider, type MVJ 1500, made of ceramic resistors in the HUT laboratory. Digital storage oscilloscope Fluke 3392 A was used, bandwidth 200 MHz, sampling 200 MS/s and resolution 8 bits. The test circuit is shown in Figure 13. MVJ 1500 800 kV 62,5 nF 375 pF = = FL:560 O DSO Fluke Ch1 Figure 13. Test circuit for steep front impulse tests at HUT U = 230 gH (500 kV/gs), 150 gH (1000 kV/gs) and 10 gH (2000 kV/gs) 18 (24 ) The partial response time of divider MVJ 1500 defined according to the standard IRC 60-2 /8/ is Ta = 1 ns, which is a very good value. The resistance of ceramic resistors used in this fast divider depends on voltage consistently. The resistance and so the scale factor is decreasing when the voltage is increasing. The scale factor correction of divider MVJ 1500 was measured before the tests by comparing the peak voltage measured with the MVJ 1500 (U Mvj ) to laboratory reference divider Haefely G600 (Uref) at short 1,0/11 jis lightning impulses, the correction factor was calculated c = Uref/UMVj, see Figure 14. The correction needed is rather small and stable. The voltage values of the measured impulses given in the following tables have been corrected U = c U mvj - Factor c 0,99 0,98 0,97 0,96 0,95 0,94 0,93 0 50 100 150 200 250 300 350 400 U /kV Figure 14. Scale factor correction of divider MVJ 1500 vs. voltage Earlier spark gap results for steep front impulses have been measured in the ABB laboratory in 1981/11/. The 600 kV bifilar resistive divider and a surge oscilloscope Tektronix 507 were used. The front time evaluation was made according to EEC 60-2 (1973) using vol tage levels 50 % and 90 %, instead of 30 % and 90 % as defined in IEC 60-1 (1989) 111. A very fast measuring system is needed for steep front impulse measurements. The influence of the measuring system on the test results may be assessed using convolution evaluation, based on the unit step response measurements. The convolution evaluation was made with the HUT and ABB measuring systems. Estimated amplitude errors are below 3 % in both systems. Estimated steepness errors in the HUT system are below 1 %, and in earlier measurements /11/ less than 11 %. Up to 16 % corrections for response time have been applied for those earlier ABB measurements published in several contexts. Both corrected and uncorrected results are given in reference /11/. The convolution evaluation showed that the results with no correction are valid. The amplitude correction for response time appeared to be not useful in these front chopped impulses with a flat crest and large collapse time. 6.3 Test methods and results The measuring series at each steepness consisted of 20 impulses. The average value of sparkover voltage, front time and steepness as well as the standard deviation of sparkover voltage values were calculated from the measured values. The front time Ti and the steepness S = Up/Ti have been calculated according to EEC 60-1 111. Correction factors for ambient conditions were not applied. During these tests the value of the correction factor Kt 111 would have been from 1,00 to 1,02. 19(24 ) 6.3.1 Results for the double spark gaps Double spark gap A (C52) was tested with steepnesses of about 500, 1000 and 2000 kV/ps. Double spark gaps B (TK) and C (OJUP) were tested only with the steepness value 2000 kV/ps. Some test series were repeated. The results are shown in Tables 8 to 10. Table 8. Sparkover voltage/time measurements of a double spark gap A (C52) Nominal UAVG s s Ti S steepness Polarity kV kV % US kV/us 500 + 193 3,5 1,8 0,44 440 - 196 3,5 1,8 0,39 500 1000 + 215 5,3 2,5 0,27 808 + 215 5,5 2,6 0,27 807 - 210 4,3 2,1 0,24 868 - 210 4,6 2,2 0,24 873 2000 + 222 4,3 2,0 0,12 1830 + 228 4,4 1,9 0,12 1884 - 227 2,3 1,0 0,12 1925 - 228 1,8 0,8 0,12 1921 Table 9. Sparkover voltage/time measurements of a double spark gap B (TK) Nominal U avg s s Ti S steepness Polarity kV kV % gs kV/us 2000 + 223 3,1 1,4 0,13 1660 - 238 6,3 2,6 0,14 1715 - 244 4,5 1,9 0,12 1983 Table 10. Sparkover voltage/time measurements of a double spark gap C (OJUP) Nominal U avg s s Ti S steepness Polarity kV kV % US kV/us 2000 + 216 4,6 2,1 0,13 1614 - 226 2,5 1,1 0,12 1915 6.3.2 Results for the single spark gap Single spark gap D (NLZWY) on transformer bushing was tested with nominal steepness values 500,1000 and 2000 kV/ps. The results are shown in Table 11. 20 (24 ) Table 11. Sparkover voltage/time measurements of a single spark gap D (NLZWY) Nominal U a VG s s T, S steepness Polarity kV kV % US kV/|is 500 + 227 6,0 2,6 0,51 441 - 234 7,8 3,3 0,47 501 1000 + 253 6,7 2,7 0,28 912 - 257 5,0 2,0 0,27 957 2000 + 277 9,9 3,6 0,17 1642 - 274 2,8 1,0 0,13 2142 For comparison the earlier results /11/ of double spark gap OJUP (-81) and single spark gap NLZWY (-81) are shown in Table 12. The structure of spark gap OJUP was the same as in the new type. The single gap NLZWY (-81) differed slightly from the new type NLZWY 244. In the new structure the horizontal distance between upper and lower arcing horn ends is about 60 mm, and it was only 25 mm in the single gap used in 1981. The mounting arrangement was close to Figure 4b) with the spark gap 2 m above the laboratory floor. The correction factors for ambient conditions were applied to the voltage values, and the front time was evaluated using voltage levels 50 % and 90 % instead of 30 % and 90 % as defined in IEC 60-2 (1973). Table 12. Sparkover voltage/time measurements of double spark gap OJUP (-81) and single spark gap NLZWY (-81), results with no correction for response time /11Z Nominal UftVG s s Ti S steepness Polarity kV kV % ps kV/us Double spark gap OJUP (-81) 500 + 196 2,7 1,4 0,38 516 - 188 3,4 1,8 0,36 522 1000 + 216 3,7 1,7 0,30 720 - 215 3,7 1,7 0,21 1024 2000 + 230 5,6 2,4 0,11 2091 - 233 2,4 1,8 0,10 2330 Single spark gap NLZWY (-81) 500 + 205 1,4 0,7 0,41 500 - 204 1,5 0,7 0,40 505 1000 + 256 6,7 2,6 0,26 985 - 254 2,6 1,0 0,25 1033 2000 + 326 2,9 0,9 0,16 1988 - 320 7,5 2,3 0,16 2000 21(24 ) 6.4 Summary of the steep front impulse tests To begin with, it has to be noticed that the actual steepness of the different measuring series is not the same. For instance, impulses with a nominal steepness of 2000 kV/ps vary from 1600 to 2300 kV/ps. Generating and measuring steep front impulses is much more difficult than standard lightning impulses. The measured protection characteristics of three different spark gap types at steepness 2000 kV/ps did not differ as much as would be expected on the basis of the results at standard lightning impulses. The measured sparkover voltage values of spark gap types A (C52) and C (OJUP) are practically the same. The sparkover voltage/time curve of the single spark gap is steeper than that of the double spark gap. The sparkover voltage increases clearly with the steepness. At the steepness of 2000 kV/ps the sparkover voltage of the single spark gap is 20 - 40 % higher compared to the double spark gap. The main reason for this is the structure of the single spark gap: sharp arcing horns mounted on the transformer bushing and no middle electrode for smoothing the electric field distribution. The values measured now and earlier /11/ are close to each other, when the corrections for the response time used in 1980’s were not applied. The measured UAVG values from Tables 7 to 12 are also shown as a function of front time T% in Figure 15. The double spark gaps have almost a linear sparkover voltage/time characteristic. Straight lines corresponding to steepnesses values 500, 1000 and 2000 kV/ps are drawn in Figure 15. A negative 200 kV/ps line shows approximately the steepness of the impulses in the tests according to C 52-192-IB /12/, see Figure 10. U/kV 2000 kV/ps 125 kV 125 kV 500 kV/ps Uprosp Spark gap types 110 and 150 kV □ A (C52) o B (TK) a C (OJUP) x D (NLZWY) + NLZWY-81 • C52-1 92-1 test Figure 15. Sparkover voltage/time characteristics based on the results from Tables 7 to 12 22 (24 ) In addition to the results of steep front impulses, also the standard lightning impulse withstand voltage level 125 kV for 24 kV equipment is shown in Figure 15. Two points measured according to standard C 52-102-1B 712/ are shown for comparison, see results in Table 7. These measurements show that the voltage stresses of the C 52-102-lB-test are far below those of the steep front impulse test. The voltage value is even below the standard lightning impulse test requirement 125 kV. The proposed test 712/ means only a slight increase of the front steepness compared to the standard lightning impulse test. It also means a chopped impulse test which, however, differs from the chopped lightning impulse test stipulated in the transformer standard IEC 76-3 757. The test voltage requirement is 125 kV in a chopped impulse test for 24 kV transformers 757. 7. WET POWER-FREQUENCY TESTS The spark gap spacing must be adjusted so that no sparkovers occur at temporary or slow-front overvoltages, but protection against fast transient overvoltages must be adequate. The wet 1 % sparkover voltage values for protective spark gaps are given in standard SFS 2646 767, for instance 32 kV for spark gap spacing 1 x 80 mm and 42 kV for spark gap spacing 2 x40 mm. The wet power-frequency test was performed on double spark gaps A (C52), B (TK) and C (OJUP) according to publication EEC 60-1 111. The spark gap spacing was 2 x 40 mm for all spark gap types, and in addition 2 x 20 mm for spark gap type A (C52). The 1 % sparkover level was determined by increasing the voltage up to sparkover 20 times. 7.1 Test arrangement The double spark gap was mounted on the wooden pole at the height of 1,5 m above the laboratory floor. The test voltage was applied to the spark gap and it was earthed with a 13 mm2 Cu-cord. The test voltage was generated with a 300 kV, 300 kVA cascade transformer. The voltage was measured with a capacitive voltage divider and peak voltmeter SM-62, which is scaled to show the value u7\2. The air density correction factor was applied to the test results. The standard wet test procedure according to IEC 60-1 Clause 9.1 was used. The resisti vity of the water was 100 12m. The vertical/horizontal components of the rain were 1,771,3 mm/min. The double spark gaps were pre-wetted 15 min before the test. 7.2 Test results The test results are given in Table 13. The average value U AV g and the standard deviation from 20 successive sparkovers were calculated. The test voltage was increased at the rate of 1,5 kV/s, and the interval between the sparkovers was about 1 minute. 1 % sparkover voltage has been calculated from the measured values Ui = U A vg (1 - 2,3s). 23 (24 ) Table 13. Wet power-frequency test Double spark gap Oavg s s Ui type and spacing kV kV % kV A (C52) 2 x 20 mm 33 2,2 6,7 28 A (C52) 2 x 40 mm 44 2,3 5,1 39 B (TK) 2 x 40 mm 48 3,2 6,6 41 C (OJUP) 2 x 40 mm 46 2,9 6,4 39 The wet power frequency test results of double spark gap types A, B and C with spacing 2 x 40 mm are very close to each other. While the differences in structure resulted up to 20 % deviations at standard lightning impulses, no essential difference was seen in wet power frequency test results. Spark gap type A (C52) with spacing 2 x 20 mm was measured to check the protection arrangement of French National Standard C 52-192-IB /12/ in wet conditions. The measured 1 % sparkover voltage 28 kV is too low a value for 24 kV equipment. This voltage is too close to the stationary voltage under the earth fault condition in the rural 24 kV networks with the isolated neutral, and it does not leave a margin for slow-front transients. Acknowledgements The authors wish to thank the Foundation for Power Engineering Research and Development, ABB Transmit Oy and Flelsinki University of Technology for financing the project and especially ABB Transmit Oy and ABB Oy Corporate Research Finland for supplying the test objects and for performing some parallel tests. References: 1. CIGRE WG 33.01: Guide to procedures for estimating the lightning performance of transmission lines. CIGRE Brochure 63, 1991, 61 p. 2. V. Palva, M. Karttunen, O. Setala: Insulation co-ordination problems with special reference to overvoltages and their effect on insulation. CIGRE report 403, 1962,19 p. 3. M. Aro, J. Elovaara, M. Karttunen, K. Nousiainen, V. Palva: Suurjannitetekniikka (High-voltage techniques), 1996, Jyvaskyla, Otatieto Oy, 483 p. 4. T. Salmi: Jakelumuuntajan kipinavalisuojaus (Protection of distribution transformers by means of spark gaps), MSc thesis, Helsinki University of Technology, Finland, 1996,63 p. 5. IEC Standard 76-3: Power transformers, Part 3: Insulation levels and dielectric tests, 1980,71 p. 6. Finnish National Standard SFS 2646: Pylvasmuuntamot (Pole-mounted substations), 1987,22]?. 7. IEC Standard 60-1: High-voltage test techniques, Part 1: General definitions and test requirements, 1989, 131 p. 8. IEC Standard 60-2: High-voltage test techniques, Part 2: Measuring systems, 1994, 133]?. 9. Siemens MLM 53 , Program handling, 1996 24 (24 ) 10. W. Dixon, F. Massey: Introduction to Statistical Analysis, Los Angeles 1983, McGraw-Hill Inc, 678 p. 11. J. Karvinen: Suojakipinavalin suojausominaisuuksien tarkistaminen ja vertailu venttiilisuojien ominaisuuksiin (Protective characteristics of spark gaps, comparison to surge arresters), MSc thesis, Tampere University of Technology, Finland, 1981, 107 p. 12. prHD 428.5; French National Standard C52-192, Part 1: Appendix B: Lightning impulse withstand tests of transformers, associated with spark gaps, 4 p. 13. K. Niskanen, ABB Corporate Research, Test Report No. 9 AFX95-264: Jakelumuuntajan 100 kVA, 24 kV jannitekoestus (Testing of distribution transformer), 38 p. 14. DIN VDE 0675 Teil 3: Uberspannungsschutzgerate. Schutzfunkenstrecken fur Wechselspannungsnetze >1 kV. (Overvoltage protection equipment. Part 3:: Tests for protective spark gaps for a.c. networks.) VDE-Richtlinie, 1982, 10 p. 15. IEC Standard 99-1: Surge arresters. Part 1: Non-linear resistor type gapped surge arresters for a.c. systems, 1991,93 p. 16. IEC Standard 71-1: Insulation co-ordination. Part 1: Definitions, principles and rules, 1993,47 p. 17. E. Virtanen: On the practise of distribution transformer protection with spark gaps in Finland, 1997, 6 p. HELSINKI UNIVERSITY of TECHNOLOGY High Voltage Institute High Voltage Institute of Helsinki University of Technology is an independent Institute since October 1st, 1994. However, traditions on high voltage engineering in the University go back to early 1920’s. New premises for h.v. teaching, research and testing, were constructed and built in 1970. These and the facilities gave good tools for research of h.v. technology and h.v. measuring techniques which were then expanded and intensified. From October 1st, 1995 the High Voltage Institute acts as National Standards Laboratory of High Voltage Measurements. The quantities are maintained and calibration services are given for DC Voltage, AC Voltage, Capacitance, Disipation Factor, Capacitance Ratio, Inductance, Lightning Impulse Voltage, Switching Impulse Voltage, other impulse voltages, impulse current and energy of impulse. In addition to research on h.v. metrology and measurement techniques, other activities of the Institute include teaching of h.v. engineering, h.v. research in general with special emphasis on insulations of overhead lines, and testing services and calibration services. Research in the Institute is largely concentrated on high voltage measurements and metrology. Calibration systems for above mentioned quantities are under continuous research and development Much attention and work has been paid on research of lightning impulse voltage measuring techniques in its many aspects. The first impulse voltage calibrator in the world was presented in 1979. Reference voltage divider for 400 kV lightning impulse voltage has been developed and constructed. Special computer aided system for calibration of digital recorders was developed and is now in regular use. Research and development of software for curve fitting of impulses deal with parameter estimation of distorted and noisy impulses in practical h.v. tests and measurements. Wide international cooperation in the field of High Voltage Metrology include e.g. coordination of and participation in numerous intercomparisons of h.v. measuring systems worldwide and active participation in the work of CIGRE Working Group 33.03 (High Voltage Testing £ind Measuring Techniques). Staff Martti Aro, M.Sc Director Veikko Palva, Lis. Tech (PhD) Prof (Emmeritus) Jari Hallstrom, Lis.Tech (PhD) Manager, Calibrations Maija-Leena Pykala, M.Sc Manager, Tests Esa-Pekka Suomalainen, M.Sc Researcher Pekka Valve, M.Sc Researcher Jukka Piiroinen, M.Sc Researcher Hans Karlsson, B.Sc Associate Researcher Veli-Matti Niiranen Associate Researcher Jouni Makinen Technician Hannu Kokkola Technician Premises and Facilities Premises Test facilities Calibration Systems H.v. hall 31 m x 19 m x 16 m (h) AC voltage up to 900 kV Maintained quantities and best H.v. hall 13 m x 11 m x5,4 m (h) DC voltage up to 1200 kV uncertainties in calibrations 250 kV, 100 kV, 50 kV rooms LI voltage up to 2500 kV are given in ins ide page of Weather room 6mx3mx5m(h) SI voltage up to 1500 kV back cover. - 40 °C to + 60 °C AC current up to 5 kA (40 kA) Tensile load up to 200 kN