Condition assessment of synthetic insulated cables and accessories operating more than 20 years on the French network. M. MAMMERI SILEC France France

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1 21, rue d Artois, F PARIS CIGRE 2012 http: // B1-207 Condition assessment of synthetic insulated cables and accessories operating more than 20 years on the French network P. HONDAA RTE France L. BENARD PRYSMIAN France M. MAMMERI SILEC France France D. MEURICE EDF R&D France F. PEURTON NEXANS France SUMMARY The management of the cable assets in the field is now a main concern of Transmission System Operators (TSO). The French TSO has launched a program in partnership with his main suppliers and with the EDF R&D division to evaluate the ageing of synthetic insulated cables and accessories removed from the field. The cable system technologies and the cable design have evolved since the 60s, with improvements in their performance. The principle applied in France from the beginning of cable system development based on a comprehensive series of qualification tests including a long term test which ensures the performance quality of the system. However, the ageing mechanisms are often more complex than expected and an analysis on cables and accessories operating during more than 20 years was considered as an important step to evaluate the cable system design and its ageing behaviour. To obtain significant data, cable and accessory samples have been randomly picked out and their different components analysed. Physical and chemical properties and electrical performance have been investigated. Comparison to initial condition has been made. Similar characteristics have been globally found, however some singularities appeared and assumptions have been made trying to explain these phenomena. KEYWORDS Cable system - Ageing - Life expectancy Pierre.hondaa@rte-france.com

2 1. INTRODUCTION Underground links represent a significant and growing part of electricity transmission networks. They are constituted from cables whose technologies have changed over time. Paper impregnated insulation was firstly used before being replaced by a synthetic one, starting with thermoplastic Polyethylenes (LDPE - HDPE) and later cross-linked Polyethylene (XLPE). In the same way, metallic screens, which carry short-circuit currents and ensure a radial watertightness, were initially lead sheath before being progressively substituted by laminated aluminium bonded to the oversheath. Before installation, cable systems undergo several qualification tests. Especially the long term tests which have been carried out in France since the 60s to reproduce on about one year all the main stresses expected on the network. In these tests, a special attention is paid to thermo-mechanical aspects as the test loop is installed in real conditions. Nevertheless, such tests do not take into account all the stresses linked to laying or exploitation conditions. As older synthetic cables have now reached 50 years, a diagnostic survey has been launched to assess their conditions and ageing mechanisms. 2. EVOLUTION OF THE CABLE SYSTEM TECHNOLOGIES AND SPECIFIC TESTS FOR QUALIFICATION 2.1. Evolution of the system technologies As paper cables are not part of the study, the evolution of the different extruded synthetic insulation technologies used for the cables installed in the High and Extra High Voltage grids are summarised in the table 1. This table illustrates also the technology evolution of the associated acessories. Table 1: Evolution of the cable system technologies Voltage Cable Insulation Joint Termination LDPE: 1962 to 1988 Field moulded: 1962 to 1974 Field moulded: 1964 to kv HDPE: 1972 to 1987 Taped: since 1970 Prefabricated with oil: since 1970 XLPE: since 1987 Prefabricated: since 2005 Prefabricated with SF6: 1990 to kv 225 kv 400 kv LDPE: 1968 to 1991 HDPE: 1986 to 1988 XLPE: since 1987 LDPE: 1969 to 1995 HDPE: 1978 to 2001 XLPE: since 1994 LDPE: 1985 XLPE: since 1994 Field moulded: 1970 to 1974 Taped: 1972 to 2011 Prefabricated since 2005 Field moulded: 1969 à 1992 Taped baked: 1969 à 1989 Prefabricated: since 1989 * EHV terminations are now qualified with anti-explosion devices Dry Synthetic: since 1992 Field moulded: 1964 to 1971 Prefabricated with oil: since 1970 Dry Synthetic: since 2001 Field moulded with oil: 1969 to 1989 Prefabricated with SF6*: 1985 Prefabricated with oil*: since 2003 Prefabricated Prefabricated with SF6*: since 2003 In parallele to the cable system technology evolution, the cable design was also modified mainly on: The insulation thickness which decreased during the 80s from 14 to 11 mm for 63 kv cables and from 17 to 14 mm for 90 kv cables, to reach 8 mm for 63 kv cables and 11 mm for 90 kv cables at the end of the 90 s. For 225 kv cables the insulation thickness is 18 to 22 mm and around 27 mm for 400 kv cables. The replacement of the lead sheath by an aluminum foil with overlap, bonded to a polyethylene oversheath, at the end of the 90 s for 63 and 90 kv cables and, since 2006 for 225 and 400 kv cables, by a welded aluminium foil, bonded also to a polyethylene oversheath. 2

3 2.2. Main specific tests for cable qualification Long term test Since the beginning of the use of synthetic insulation for cables, a long term test is carried out to assess the system s behaviour during its working life, and in particular compatibility between the cable and the accessories. Such long term test is performed according to French standards not only to qualify 225 and 400 kv cable systems, as requested by IEC [1] since 2001, but also to qualify 63 and 90 kv cable systems for which no long term test was required by international standards Tests on components of cables with a longitudinally applied metal foil For all voltage levels on the French network, special tests have been developed to check the properties of the metal foil-oversheath combination, to guarantee it is fulfilling its functions properly. In particular, to verify the mechanical protection and the radial watertightness, the cable is submitted to a preconditioning test (mechanical or shrinkage test) followed by a significant ageing of the adhesive bonds of the components of the laminated covering (corrosion test), which is now part of the TB 446 [2]. 3. METHOD USED TO EVALUATE THE CABLE SYSTEM Investigations have been carried out on materials taken from the network when opportunities appeared. The constraints faced by the cable system including the method of installation, the load and the short circuits were taken into account to take benefit the test results and the examinations Cables The tests were carried out either on cable components or on complete cables as shown in table 2. Concerns Mechanical Characteristics Electrical Characteristics Chemical Characteristics Change in morphology Oxidation Presence of water & Water trees Ageing of the metallic screen Ageing of the complete cable Table 2: List of considered tests Tests Sheath Insulation Screen Complete SC PVCPELDPEHDPEXLPE Pb Al cable Tensile strength X X X X X X Elongation at break X X X X X X Pressure test at high temperature X X Resistivity X X Dielectric strength X X X Plasticizer and stabilizer contents DSC X X X FTIR analysis X X X Optical microscopy X X X X Loss of mass X FTIR analysis X X X OIT X X X Karl Fischer X X X X X Optical microscopy X X X Macroscopic observation Determination of alloy elements Metallurgical examination Corrosion ( SEM ) Loss angle Partial discharge test Impulse voltage and AC voltage tests X X X X 3

4 Mechanical characteristics Elongation at break, tensile strength and pressure test at high temperature on polymers are very useful to check that the robustness of these materials is sufficient to withstand thermo-mechanical constraints. Carrying out these tests on aged cable enables to assess potential decrease in the mechanical properties in the insulation or sheathing layers Electrical characteristics Electrical behaviour is evaluated by mean of a statistical approach on insulation films taken from the cable. Breakdown stresses obtained when applying a continuous voltage increase for several measurements are then treated through a Weibull statistic. Results are compared with typical values on current insulation materials. Electrical resistivity has been also investigated as a diagnosis parameter Chemical characteristics Loss of mass measurements on PVC sheaths are carried out to detect potential decrease of plasticizer or stabilizer contents due to cable environment (consumption or diffusion). If any, mechanical properties will be drastically affected Morphology changes Through Differential Scanning Calorimetry (DSC) DSC is a useful tool to follow structural changes in polymers. Each state change (crystallization, melting, etc) is associated with an enthalpy variation. In case of Polyethylene (LDPE or HDPE), secondary crystallizations (new crystallite formation or reorganization of main crystallites) are possible depending on the temperature. This secondary crystallization will give a shoulder on the main melting peak with a maximum temperature 5 to 10 C above the maximum temperature seen by the insulation. Its position then is a marker of the temperature of the cable in service. From a theoretical point of view, DSC measurements can also reveal large degradation of the insulation (for example scission of PE chains), which will give raise to a high decrease of the melting temperature (normal melting temperatures are in the range of 105 C for LDPE and 130 C for HDPE). Through oil bath observations and subsequent optical microscope observations Oil bath observations can be conducted on insulation at 115 C for LDPE (so also for XLPE) and 130 C for HDPE. When melted, the insulation becomes transparent, enabling to observe contaminants, screen interfaces and electrical trees formation. If detected, these defects can be observed through optical microscope and potential subsequent analysis such as Fourrier Transform Infra Red (FTIR) or Scanning Electron microscopy (SEM) coupled with Energy Dispersive X-Ray analysis (EDX) Potential oxidation evaluation Through Oxidation Induction Time (OIT) measurements This measurement reveals the presence and the efficiency of anti-oxidant. It consists in heating 10 mg of insulation up to 200 C under N 2 gas. After 5 minutes at this temperature, N 2 gas is replaced by O 2 and the heat flux is recorded. The OIT value corresponds to the onset time of the oxidation peak. This kind of measurement is applicable for all PE materials. However, the correlation between the OIT value and the remaining lifetime has to be evaluated. Through Infra-red spectra FTIR spectra give the absorbance as a function of the wave number expressed in cm -1. The wave number corresponds to vibration of chemical bounds (angular, stretch elongation, etc). In case of oxidation, main vibrations of C and O bounds are located at: Carbonyl C=O: cm -1 Ether C-O: cm -1 4

5 Water trees investigation One of the parameters leading to premature ageing of polymeric insulation of high voltage cables is the presence of water in the insulation. The prolonged action of the electrical field leads to the development of local microscopic defects called "water trees". Diffuse and elongated in shape, they are classified into two categories: "bow tie" trees originated from insulation impurities or micro-voids, bush trees initiated at the interface of the semiconductor screen / insulation. Microscope observations of stained slices of insulation can reveal the presence of such tree structures. Moisture content is evaluated through Karl Fisher titration method Tests on lead sheath From the 60s, pure lead sheath showing premature cracking and corrosion was replaced by lead alloys (addition of Sn, Sb, Te, Cu, Cd). The condition of the lead can be assessed by macroscopic observations. Chemical analysis enables to define the type of alloy and impurities. Scanning Electron Microscopy (SEM) examinations on cuts and surface characterize the material microstructure as well as the kind and the depth of the corrosion or the cracking. The Energy-Dispersive X-ray Spectroscopy (EDX) informs about the nature of the corrosion products Electrical tests on complete cables The sequence of tests applied allows firstly the verification of the behaviour of the cables facing the requirements of the technical specifications (partial discharge measurement, lightning impulse voltage test and power frequency voltage test) and secondly to detect possible changes (dielectric loss angle measurement as a function of voltage and temperature). Partial discharges are able to detect detachment of semi-conducting layers. The presence of small vacuoles in the insulation may only result in weak partial discharges which are difficult to detect. Lightning impulse voltage tests are able to reveal interface imperfections or a poor bonding between semi-conducting layers and insulation. The power frequency voltage test consists in applying 2 times the operating voltage U 0 for 24 hours. The conventional service time t of a cable kept under the voltage U is given by the inverse power law with the simplified formula: U n t = constant. By setting the coefficient n to the value 14 (knowing that values from 15 to 17 are commonly admitted), this test at 2.U 0 for 24h is equivalent to 45 years at the operating voltage. The success of this test allows verification of the good performance of the insulation on the long term if only pure electric ageing is considered. The increase in dielectric losses is likely to cause a thermal runaway especially in case of high voltage cables. Measured as a diagnosis, their enhancement highlights a local or a general ageing of the insulation. In the temperature range of use, the variation of the cable losses has to be slight Accessories The first phase of the study is limited to visual examinations and mechanical relaxation measurements of stress cones (terminations and joints). The evaluation will be completed at least by physical and chemical measurements similar to those proposed for the cables. 5

6 Method used to evaluate the evolution of the stress cone pressure in accessories The dielectric strength of the accessory depends on several parameters such as the surface conditions, the hardness of the stress cone material, the presence of lubricant and the most important, the interface pressure between the stress cone and the insulation of the cable. The stress relaxation of stress cone material causes a pressure decrease which can affect the life expectancy of accessories. The following model, when cable insulation diameter variation is not considered, is used to calculate the tightening pressure P at the initial state and after ageing. P = AE(1-C) / [C(1- )+1+ [3] with = 0.5 P= AE(1-C) / [0.5C+1.5] P: tightening pressure (Pa) D 0 : diameter over insulation (mm) D 1: inner diameter* of the stress cone (mm) D 2: outer diameter of the stress cone (mm) A: tightening = ( D 0 - D 1 ) / D 1 C: D 1 ²/D 2 ² : Poisson s ratio of the stress cone material. ( = 0.5 for elastomers) E: Young's modulus of the stress cone material (Pa) *: The inner diameter D 1 of the stress cone gives the interface pressure. The pressure at the initial state is calculated with the diameter before installation. The pressure after ageing is determined with the measured diameter of the stress cone after dismantling and stabilization. Ageing involves: - a dimensional relaxation which affects A and slightly C - a mechanical relaxation due to the decrease of the Young's modulus E. Dimensional relaxation The dimensional relaxation results in an increase of the inner diameter of the stress cone as compared to the initial one and thus a decrease of the tightening. The diameter of the non-aged stress cone is known. The stress cone is removed by sliding. Then the diameter D1 is measured after complete elastic recovery. Several similar stress cones of different ages are needed to plot the curve (tightening = f (time)). Mechanical relaxation The Young's modulus E(t) can be estimated from the model of Thirion and Chasset [4] as follows: F(,t) = F( ) x [1+( /t ) m ] and E (t) = F( t) / elongation applied during the test t : time (s) : time constant (s) m: constant F( ): stress after an infinite time (elastic response in steady state) Parameters m and are determined by mechanical relaxation tests which consist in measuring the mechanical stress as a function of time when applying a constant elongation of 25% for example. Tests at different temperatures provide information on the evolution of the Young's modulus in the range of operating temperatures of the stress cone. In practice, test durations of 7 to 10 days are sufficient to extrapolate longer time. In principle, measurements can be performed on unaged or aged stress cones. Nevertheless it is advisable to confirm the consistency of the results by testing both, whenever it is possible. 6

7 4. PRELIMINARY RESULTS 4.1. Cables Insulation OIT values of HDPE and LDPE tested insulations are comparable to the one usually obtained on unaged cable which means that the antioxidant protection is still effective. The relevance of this measurement for XLPE insulation is under evaluation. The mechanical properties of the tested insulations are similar to those obtained on unaged cables. This is consistent with the results of OIT measurements which have been made on HDPE and LDPE insulations. The nominal breakdown gradients obtained on insulating film are in accordance with the usual values. However, for one of the LDPE the breakdown gradient is lower by about 25% in the middle of insulation compared to the nominal value. Microscopic observations reveal some peculiarities of LDPE insulations on the oldest cables. On two of them, antioxidants aggregates appeared up to 700 m in the middle of the thickness. It is underlined that the electrical tests performed on samples with antioxidants aggregates did not affect the nominal breakdown gradient value. Two others have a few bow-tie type trees of small size (< 50 m). This kind of tree is known to have a weak evolution and not to be very dangerous. The measurements do not show any abnormality which would cause breakdowns of the tested insulations in the short and medium term. However, the investigation will be completed by electrical ageing tests under constant gradient where time to breakdown is considered. The combination of the two tests (under continuous voltage increase and under constant voltage) will give additional information on the residual life time expectancy Oversheath The obtained mechanical properties are typical for such materials. When values from the unaged state are available, a non-systematic trend is observed. Generally we obtain an increase of the tensile stress at break of 5 to 10% and a decrease of the elongation at break in the order of some %. For information, standards accept a variation of 30% for an accelerated ageing. The measured oxidation induction times at 200 C (OIT) are between 7 min and 160 min on 4 LDPE sheaths. A good coherence is observed between the OIT values and the age of the cables. 7 min found for a 37 year old cable indicates a decreased of the residual content of antioxidant and thus the possible initiation of degradation mechanisms on short term (oxidation, chain scission). The correspondence between the OIT value and life expectancy is still not well known. 7

8 Lead screen Microscopic examinations show quite heterogeneous structures probably resulting from the enlargement of the grains whose sizes are between 15 and 175 m. 4 of the 12 analyzed screens have fine inter-granular cracks on the outer skin. The other 8 screens do not have any cracks formation. The four cables exhibiting cracks between 5 and 33% of the lead thickness are not among the oldest investigated cables (ca. 20 years old). All these lead screens are Pb/Sn/Cd alloys where Cd is acting initially as hardener, but is known to precipitate within time. This results in partial inter-granular cracks as shown on figure 1. SEM examination and EDX measurements reveal the presence of oxides at grain boundaries, which might indicate sensitivity to corrosion. Taking into account the reduced number of samples, we are not able to determine more precisely the roots. Crack propagation within time is also not well known and it is difficult to extrapolate the life expectancy of these lead screens. This alloy has been forsaken in production and can be replaced for example by a Pb/Sb/Sn or Pb/Cu/Te alloys which are more resistant to cracks formation. Figure 1: Inter-granular cracks on the outer skin (15% of the lead thickness) Electrical tests on complete cable The results to the partial discharge test, to the impulse voltage test and to the power frequency voltage test, on the 3 tested cables (with HDPE or LDPE insulation) are satisfactory. The tangent of the loss angle is relatively constant and comparable between 0.5 and 2 U 0 to the losses of similar unaged cables in a temperature range up to the maximum temperature in normal service (70 C and 80 C for LDPE and HDPE, respectively). Results for a LDPE insulated 225 kv cable are given in figure 2. tangent of loss angle (x 10-4) 2,5 2 1,5 1 0, ,5 1 1,5 2 2,5 voltage ( x Uo ) 20 C 50 C 70 C Figure 2: Tangent of the loss angle on a 19 year old LDPE insulated 225 kv cable. 8

9 4.2. Tests on Accessories Dimensional relaxation The tightening of hypalon stress cones (3 samples considered) has decreased from 26.5% to 11% in 25 years and from 12.5% to 8% in 20 years for silicone (5 samples considered) stress cones. Only the initial tightening and the one measured at 20 or 25 years are available. The tightening curve is not determined. It is likely that the tightening curve has an asymptotic shape as shown in Figure 3. Figure 3: Dimensional relaxation of a hypalon stress cone Mechanical relaxation Measurements of the mechanical relaxation show that the Young's modulus of the hypalon has decreased by about 50% in 25 years. On the other hand, the measurements show relaxation of the silicone neither at 20 C nor at 60 C (figure 4). Elongation 25 % - 20 C Silicone Elongation 25 % - 20 C Hypalon Figure 4: Mechanical relaxation of the stresses Result analysis As mentioned in paragraph the pressure is proportional to the tightening and to the Young's modulus. Thus, measurements demonstrate that the stress relaxation leads to a significant pressure decrease and it must be considered as a way of ageing for accessories. Further investigations are needed to assess their impact on the life expectancy of accessories. 9

10 5. CONCLUSION Diagnosis method to assess the cable system behaviour has been developed. Globally, measurements on semi-conducting layers and insulation do not show any abnormalities which could lead to breakdowns in the short and medium term. This conclusion is consistent with the low failure rate observed on the synthetic insulated cables of the French network. The most noticeable facts revealed by the tests are: The residual amount of antioxidant (OIT measurement) of the tested LDPE sheaths is coherent with the age of cables. The nominal breakdown gradients obtained on insulating film are in accordance with the usual value. It is underlined that the electrical tests performed on samples with antioxidants aggregates did not affect the nominal breakdown gradient value. The presence of partial cracks for one of the lead alloys type of around 20 years old. The preliminary results show that the stress relaxation of the stress cones leads to a significant pressure decrease which could affect the life expectancy of accessories The current study requires further investigations for better interpretation and extrapolation of the test results: The OIT measures the time required to reach oxidation (when antioxidants are consumed) at 200 C. We do not have the equivalence between this time at 200 C and the time at the operating temperature of the sheaths. Further investigations are necessary to be able to interpret the OIT measurements. It would be also useful to determine the coefficient n of the Weibull law by additional breakdown tests on insulation samples in order to assess the residual life expectancy when cables are submitted to pure electrical ageing. The influence of the tightening pressure of the stress cone on the life expectancy needs additional investigations which will have to be conducted. Obviously, the samples were randomly chosen. It is remembered that the occurrence of breakdown is determined by the weakest point of the cable system. Then the results are able to identify homogeneous ageing. However we cannot determine easily any local singular issues. The insufficient number of samples and their diversity do not currently allow to enable a statistical trend. The study will be pursued on additional samples picked out from the network. Nevertheless, these tests were essential to assess the long-term reliability of components of underground power links. By taking benefit of a coherent qualification system, the combined results confirmed the very satisfactory return on experience in France. BIBLIOGRAPHY [1] IEC 62067: power cables with extruded insulation and their accessories for rated voltages above 150 kv (Um=170 kv) up to 500 kv (Um=550 kv) Tests methods and requirements [2] Cigré TB 446: Advanced design of metal laminated coverings: recommendation for tests, guide to use, operational feedback, WG B1.25, February [3] Numerical simulation of the pressure at the interfaces between the block of the joint and the cable insulation INSUCON 2006 JC. VERITE (EDF R&D), L. MOREAU (NEXANS), E. DORISON (EDF R&D) [4] Viscoelastic relaxation of rubber vulcanizates between the glass transition and equilibrium R. CHASSET, P. THIRION 10