Aspects of PD interpretation in HV power cables. by Edward Gulski, Piotr Cichecki, Rogier Jongen

Transcription

1 Aspects of PD interpretation in HV power cables by Edward Gulski, Piotr Cichecki, Rogier Jongen General There are several aspects having influence on the diagnostic information and the condition judgment criteria for HV power cables: 1. Connection setting of the testing and diagnostic setup: earth connection configuration, where the earthing was connected? Close to sealing end or to main substation ground? 2. Background noise level: measurements in close substation and open substation in close distance to corona discharges sources will differ. In many cases background noise level can extend over 100pC so than PD pattern characteristics play major role in describing defect. Also in situation when high noise level is present, measurements are performed in comparative basis way (comparison of PD levels between cable phases). 3. Specific situation: e.g.: in some countries there is a lot of XLPE cables from 50 s and 60 s This cable suffer from water treeing and electrical treeing as the result they fail often. In other countries the XLPE cables were introduced in 80 and they have already improved water barrier protection so we do not face the problem of water treeing. Some countries have more problems e.g. with gas pressure and oil-filled cables instead of XLPE cables. 4. What type of cable is measured: mass impregnated/ oil filled (low or high pressure)/gas pressure cable. Each type of cable has own PD and tan delta characteristics. Moreover serviced aged cables may differ as the load and service condition are always different. Important is the age of the cable, operational history and failure history. Criteria of PD acceptance for HV power cables With regard to the criteria of PD acceptance for HV power cables the following has to be considered: 1. In general, due to significantly higher electric field strengths in the insulation and the accessories of HV power cables the degradation processes of discharging defects are much faster than those in MV power cables. 2. Therefore, at nominal voltage U 0 (during operation) the insulation of all HV XLPE cables should be PD free! Due to the fact that there are no international standards or guidelines for on-site PD detection and PD level acceptance for HV power cables the on the one hand and due to the fact that cable and accessories manufactures are delivering PD-free systems on the other hand the PD freedom of XLPE HV cables is important. 3. 1

2 4. Nevertheless it is possible that in certain type of insulation e.g. massimpregnated insulation or certain type of HV cables accessories the PD may occur during service life. But in general the HV cables are supposed to be PD free during operation [14]. 5. On the other hand we must be aware of local problems like water/electric treeing, background noise level. Important is to do analysis of VHF (TDR localization) record so there we can see real PD pulse characteristic. In case of PD we are not sure try to do TDR analysis and localize the problem. Practical aspects If PD are repetitive and it is confidence regarding PDIV level e.g: PDIV < U 0 (50kV) in next step must be distinguished if PD pulse is internal or external (noise) indicators: Increase of PD level in function of test voltage, PD pattern characteristics, TDR analysis. If PD is stable and possible localization than such PD level in [pc] must be recorded. Regarding interpretation some field experience values for HV power cables are shown in Table 1: Table 1: Cable type Safe Semi safe Dangerous XLPE new/old Localized PD in the cable insulation HV XLPE have to be PD free with a background noise level in the range of 25pC..30pC or lower. XLPE old Localized PD in accessories <25pC <100pC > 100pC Only as an Not localized PD <100pC <500pC >1000pC Only as an Oil-paper (oilfiled low pressure) old Oil-paper (oilfiled Localized PD in low cable insulation pressure) old and accessories Gas cable Not localized <25pC <300pC >500pC Gas cable Localized in cable <25pC insulation and accessories <50pC <200pC >200pC Only as an HV Gas pressure have to be PD free with a background noise level in the range of 25pC and lower The above table has relation to PD inception at test voltage > U 0. Moreover thie following has to be considered: - If the HV power cables is PD free up to 1.7 U 0 it can be concluded that no dangerous PD activity is active now and the quality insulation of this particular cable is high. - If the PDIV is below 1.7 U 0 and higher than 1.2 U 0 the cable can still be considered as PD-free and the quality insulation of this particular cable is good. - If the HV power cable has PDIV =< U 0 the cable is not PD free and extensive interpretation and application of cable typical knowledge is necessary to estimate the hazard of these PD s. 2

3 Power cable system: cable insulation and cable accessories (joints, terminations) are defect free. Power cable can withstand operational stresses without any impact to quality of the energy delivery (scenario for new and serviced aged circuits which were tested on-site and threshold limits for diagnostic parameters were not exceeded; breakdown did not occur during on-site voltage test or condition assessment test). Power cable fulfilled requirements for the factory tests but on-site after-laying test showed that there is defect present in cable system. In this situation two other scenarios are possible: a. Defect is present below nominal (U 0 ) voltage of the cable (U 0 ). Decision regarding repair, or maintenance (periodical monitoring measurements) has to be taken to avoid un-expected breakdown in service. b. Defect is present above nominal voltage (1.1U U 0 ) In this case (if defect has destructive character), defect prosperities described by diagnostic parameters may change in time. This can result in lowering dielectric strength of the cable accessory or cable insulation and lead to breakdown. Power cable failed (breakdown or weak-spots localized in the circuits) during afterlaying test, condition assessment or after repair test. Without maintenance decisions no further safe service operation is possible. Test Measurement Procedure e.g. OWTS HV150 The measurement is performed according to the scheme as shown in table 2. The test voltage is raised in steps of 10 kv and three shots at each voltage level are performed. During the damped AC voltage PD and dielectric losses is measured. Table 2: No. U rms [kv] U peak [kv] x U U peak [kv x Uo 3

4 During the test the following parameters will be estimated: PD inception voltage in kv PD level at PDIV in pc PD level at U0 in pc Depending on the PD level at Uo at each voltage higher than Uo a decision will be considered about the further increase of the test voltage. According to the IEEE400 documentation 50 damped AC excitations can be applied at maximum acceptable test voltage (equivalent of the voltage withstand test). According to the IEC60840 for new installed 110 kv power cables the test voltage after installation is 128 kv rms (2 U 0 ). For service aged cables the maximum test voltage has to be considered with the cable owner and is dependant on the history of the cable life (operational load, maintenance, failures, age). The following norms are applicable: IEC : High Voltage test techniques Part 3: Definitions and requirements for on-site testing; IEEE 400: Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems; IEEE 400.3: Guide for PD Testing of Shielded Power Cable Systems in a Field Environment; IEC 60270: Partial discharges measurements; IEC : Test methods for partial discharges measurements on lengths of extruded power cable; IEC 60840: Power cables with extruded insulation and the accessories for rated voltages above 30kV up to 150kV Test methods and requirements; IEC 62067: Power cables with extruded insulation and the accessories for rated voltages above 150kV; IEC 60141: Tests on oil-filled and gas-pressure cables and their accessories up to and including 400kV part 2; 4

5 References [1] Cigre Working Group B1.04, Brochure 279 Maintenance for HV Cables and Accessories [2] Cigre Working Group B1.09, Brochure 358 Remaining Life Management of Existing Underground Lines [3] Cigre Working Group D1.11, Brochure 228, Service Aged Insulation Materials [4] P.P. Seitz, B. Quak, E. Gulski, J.J. Smit, P. Cichecki, F. de Vries, F. Petzold, Novel Method for On-site Testing and Diagnosis of Transmission Cables up to 250kV, Proceedings JiCable '07. 7th Intern. Conf. Insulated Power Cables, Versailles, France, Paper 16, 2007 [5] F.J. Wester, E. Gulski and J.J. Smit, Detection of PD at Different AC Voltage Stresses in Power Cables, IEEE Electr. Insul. Mag., Vol. 23, No. 4, pp , 2007 [6] E. Gulski, E. Lemke, M. Gamlin, E. Gockenbach, W. Hauschild, E. Pultrum, Experiences in partial discharge detection of distribution power cable systems. Cigre, Vol 208 Electra, pp , 2003 [7] E. Gulski. P. Cichecki, E.R.S. Groot, J.J. Smit, F. de Vries, J. Slangen, Slangen. E.R.S.Groot, J. Pellis, D. van Houwelingen, T.J.W.H. Hermans, B. Wegbrands and L. Lamballais, Conditon Assessment of Service Aged HV Power Cables, Cigre, Paper D1-206, 2008 [8] J. Popma J. Pellis, Diagnostics for high voltage cable systems, proceedings ERA conference on HV plant life extension, Belgium, November, [9] J. Densley, Ageing Mechanisms and Diagnostics for Power Cables An Overview, IEEE Electrical Insulation Magazine, Vol. 17 Nr. 1 pp14-21, Jan/Feb 2001 [10] E. Gulski, E, F.J. Wester, Ph. Wester, E.R.S. Groot, J.W. van Doeland, Condition assessment of high voltage power cables. Proceedings CIGRE 2004 Session, paper D [11] E. Gulski, J.J. Smit, P. Cichecki, P.P. Seitz, B. Quak, F. de Vries, F. Petzold, Insulation Diagnosis of HV Power Cables, Proceedings Jicable '07. 7th International Conference on Insulated Power Cables, France, Versailles, June 2007, paper 51. [12] E. Gulski, P. Cichecki, J.J. Smit, F. de Vries, Vries. Bodega, Th. Hermans, P.P. Seitz, Dielectric loss diagnosis of service aged HV power cables, Proceedings of Cigre D1 Colloquium, Hungary Budapest, 2009 [13] E. Gulski, P.P. Seitz, B. Quak, F. Petzold, F. de Vries, (2008). Dedicated on-site condition monitoring of high voltage power cables up to 150kV. International journal of emerging electric power systems, 9(3), 1-15 [14] P. Cichecki, R.A. Jongen, E. Gulski, J.J. Smit, (2008). Statistical approach in power cables diagnostic data analysis. IEEE transactions on dielectrics and electrical insulation, 15(6), [15] E. Gulski, P. Cichecki, F.J. Wester, J.J. Smit, R. Bodega, T.J.W.H. Hermans, P.P. Seitz, B. Quak, F. de Vries, (2008). On-site testing and PD diagnosis of high voltage power cables. IEEE transactions on dielectrics and electrical insulation [16] IEC60502 Power cables with extruded insulation and their accessories for rated voltages from 1 kv (Um = 1,2 kv) up to 30 kv (Um = 36 kv) - Part 1: Cables for rated voltages of 1 kv (Um = 1,2 kv) and 3 kv (Um = 3,6 kv) [17] E. Gulski, P. Cichecki, J.J. Smit, F. de Vries, J. Pellis, D. van Houwelingen, F. Wester, R. Bodega, Th. Hermans, Condition Assessment of Transmission Power Cables, Cigre 2010, paper D1 205_2010 5