C I R E D 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003 NEW RESULTS IN MEDIUM VOLTAGE CABLE ASSESSMENT USING VERY LOW FREQUENCY WITH PARTIAL DISCHARGE AND DISSIPATION FACTOR MEASUREMENT Martin BAUR, Peter MOHAUPT, Timo SCHLICK BAUR Prüf- und Messtechnik GmbH - Austria [email protected] INTRODUCTION Figure 1 shows a typical electrical tree using power frequency 50 Hz. One can see a wide, bush like distributed tree at the source of the needle The paper presents actual field test results using a electrode. diagnostic system based on Very Low Frequency with 0.1 Hz sinusoidal waveform. The state of the art of a new diagnostic tool offers the user of underground cable systems and networks an efficient approach of maintenance support allowing highly efficient measurement procedures to judge the ageing characteristics, the rejuvenation requirements and a more precise investment planning forecast. On the other hand, the method offers just in time replacement and preliminary Figure 1: Electrical treeing on a XLPE cable detection of incipient faults, long before their sample with a needle defect at power breakdown. Using this diagnostic tool properly, the frequency 50 Hz cost of ownership, replacement, maintenance and “outage” costs can be reduced dramatically, at the Figure 2 shows the same test sample arrangement same time reaching far better system reliability and with an electrical tree at VLF test voltage at 0.1 Hz. power distribution redundancy. The tree structure is reduced to a single needle tree channel. The main reason lies in the much higher In the past, medium and high voltage cables were discharge capacity at power frequency compared to tested with DC. Advantages were given by low VLF. The number of discharges decreases per unit weight, portable, easy to use, economic, field of time proportionally to the frequency applied. proven test equipment. After introduction of Since the amount of discharges in one channel at synthetic HV insulation like PE, XLPE, EPR or power frequency is much higher, the material earlier PVC DC test voltages were also used. The destruction is higher; the gas pressure will increase test results with DC were restricted with limited the dielectric strength according to Paschens law. information or sometimes with destructive So, the tree growth is reduced. At VLF there is consequences [1]. Even more, the DC test may enough time for the gas to diffuse in the surrounding cause cable failures due to the low dissipation factor of the discharge channels, creating a lower pressure of PE or XLPE insulation and possible space within the tree, decreasing the breakdown level charges causing local field stress after reenergizing further. As a consequence, the tree growth at VLF is the AC power. This is the reason of introducing new much faster compared to power frequency. In fact, after laying test standards based on AC test voltage tree growth at power frequency might have 1.7 [2]. mm/h compared to VLF 12.3 mm/h. In the case of defects in the insulation, as proven in [3], the VLF voltage stress will lead much faster to a VLF-TESTING breakdown compared to power frequency. Therefore the VLF test voltage is a very good tool to compare between good or bad cables. It also shows During the commissioning of a cable or during a much better sensitivity compared to conventional maintenance it is beneficial to detect defects at an test methods: early stage before incipient failure of the insulation occurs. Furthermore the test should not reduce the cable lifetime. Investigations [3] have shown that by using a Very Low Frequency (VLF) test voltage better results, as opposed to other test voltage waveforms are possible. The following short scientific explanation will clarify the background of the VLF test method. BAU_Baur_A1 Session No 1 Paper No 74 - 1 - C I R E D 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003 Figure 2: Electrical treeing on a XLPE cable sample with a needle defect at VLF test voltage 0.1 Hz. Figure 4: Relative voltage on a model cable with different types of damages [3] Figure 3 shows the distribution of breakdowns over the testing time. These tests are results of practical field tests. The sensitivity in the case of XLPE is DISSIPATION FACTOR MEASUREMENT obvious. Almost 2/3 of cable failures are breaking (TAN DELTA TD) down after a VLF testing time of 10 min. Also The results on mass impregnated or oil filled cables are very good. Earlier PE or XLPE medium voltage cables now Different waveforms and test frequencies are not show extended water treeing (WT) phenomena’s. comparable in their breakdown mechanism. The lifetime of these cables is greatly reduced, [5,6,7]. A large amount of practical field experience has proven that there is a close relation between WT and VLF dissipation factor (TD). A very clear 60 assessment of degradation criteria can be measured [8]. The existing assessment criteria’s are 40 based on XLPE cable insulation compounds. Not 20 only the absolute value of TD measured with VLF at 0.1Hz at nominal rated cable voltage level 0.5Uo up 0 to 2Uo (phase to ground), but also the increase of 0-10 10- 20- 30- 40- 50- plastic TD over the rise of voltage are important factors. min 20 30 40 50 60 XLPE min min min min min Wrapped joint Fig. 6 represents the TD criteria for good, medium Paper DNAKBA aged or highly serviced aged, bad XLPE insulation Figure 3: Amount of breakdowns during the test time in min on different types of cables and accessories [4]. in the range of 0.5Uo up to 2.5Uo. TD diagnostic voltage levels are usually limited at 2Uo. The rate of In Figure 4 it seems that the sinusoidal VLF at 0.1Hz rise between Uo and and 2Uo is another important is the most sensitive in respect of dielectric strength. criteria [5,6,7]. Mechanical damages need only by 40% of the breakdown voltage compared to other methods One has to be aware of the damping characteristics TD at 2Uo when using oscillating wave (OSI), the dielectric 10-1 field stress over time is changing according to the > 2.2x10-3 insulation with length of the cable tested. Several standards such 10 -2 high degradation as VDE, CENELEC and IEEE have adopted the -3 > 1.2x10 service aged VLF site-test being adequate for medium cable after XLPE laying commissioning and maintenance test method 10-3 cable insulation [2,3]. < 1.2x10-3 new XLPE cable The future IEC 60060-3 standard on High Voltage insulation Test Techniques will recommend the alternative -4 10 methods voltage with sinusoidal very low frequency at 0.1 Hz as a voltage test and for diagnostic tests such as dissipation factor measurement and partial Figure 5: Absolute TD value assessment criteria on XLPE discharge measurement [3]. cables [8]. BAU_Baur_A1 Session No 1 Paper No 74 - 2 - C I R E D 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003 the location of PD sources. The question is e.g. the source type and the location at Uo or at 1.7Uo. -3 In most cases the PD does not result from the tan δ . 10 internal cable insulation, but from the accessories 7 like joints or terminations. Therefore the source location becomes more important. 6 An example is shown in the chart Figure 7. The different accessories, the cable insulation, 5 distinguishing the partial discharge inception voltage level (PDIV) and the distribution at practically tested 4 in mass impregnated cable insulation is shown [13]. 3 0,1 Hz dissipation factor 2 100% 20% 90% 1 80% 37% 70% 28% 0 60% 0 0,5 1 1,5 2 2,5 3 50% U / U 0 40% 24% 30% 52% reference cable (new) slightly service-aged (1) 20% 10% 12% moderately service-aged (2) moderately service-aged (3) 0% PDIV PD location strongly service-aged (4) cable joints cable terminations cable insulation Figure 6: Assessment criteria with Voltage TD Ratio on XLPE service aged cables [7,8]. PD activity > 1,3Uo PD activity up to 1,3Uo In fact the TD diagnosis on mass impregnated or oil PD activity up to Uo filled paper insulated cables is also a good Figure 7: Distribution and PDIV levels and location of assessment tool. Because of completely different PD in the case of paper insulated cables [13]. physical assembly of the cable insulation other more individual criteria’s have to be applied with VLF TD. Today, only limited experience is available. In the case of PE or XLPE cables only a few pC are The TD comparison has given useful results enough or only a short time (hours or days) is between phases or between similar types of the needed to break down the internal insulation. same lot of cable production. Whereas sometimes several years of several 100 In such a way different and critical cables can be pC or even nC are needed to initialize a complete identified. Many different authors worldwide have breakdown in case of paper mass impregnated approved the efficiency of the method. cables. Since practice shows that breakdowns are experienced in more than 50% of the accessories, the PD density and location measurement is of PARTIAL DISCHARGE MEASUREMENT AND greatest importance. A PD measuring level above LOCATION OF FAULTS (PD) 10 pC seems to be practical and more than enough for on site testing. Easy to use and portable test equipment are more important factors. Equipment For decades the PD measurement and the location with low level PD detection are available too; they based on time base propagation fault location has normally need a profound expert knowledge for field been the most important and efficient non- applications.