DETERMINING HIGH VOLTAGE CABLE CONDUCTOR TEMPERATURES. Guy Van der Veken. Euromold, Belgium. INVESTIGATIONS. INTRODUCTION.

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1 DETERMINING HIGH VOLTAGE CABLE CONDUCTOR TEMPERATURES. Guy Van der Veken. Euromold, Belgium. INTRODUCTION. INVESTIGATIONS. Type tests on MV cable accessories are described in CENELEC HD68 and HD69 documents. Some of the tests described require elevated conductor temperatures within strict limits (e.g. 5K to 10K above the maximum permissible operating temperature of the extruded cable insulation). To accomplish this, over the allowed range of ambiant temperatures, the heating current is to be regulated. Due to the presence of high test-voltages across the cable s insulation, the on-line measurement of the conductor temperature on the tested cable is not possible using standard measuring techniques. 3 methods for determining the cable temperature are given in the document HD68: 1) Method 1 using the relationship between the conductor temperature, the heating current and the ambiant temperature. ) Method using the relationship between the conductor temperature, the heating current and the cable-jacket temperature. (These two methods require a preceding calibration of the cable, to establish these relationships.) 3) Method 3 using a parallel loop of same cable in the same environment that is heated with the same current, but is not carrying high voltage. Following points have been evaluated: 1) Validity of the methods as described. ) Uncertainty of the results obtained. 3) Comparison of the two methods. For this purpose, following variables have been examined: a) Thermocouple materials. b) Thermocouple execution. c) Thermocouple placement. d) Effect of conductor cross-section. e) Number of thermocouples used. Uncertainty factors evaluated include: a) Uncertainty of the measuring equipments. b) Uncertainty of the measurements. c) Uncertainty of the calculated temperature. RESULT Evaluation of the data leads to following conclusions: 1) The method, using jacket temperature measurement, results in the lowest deviation. ) Uncertainty of the temperature determined (± 3K for small crossections to ± 5K for large crosssections) is found to be high when compared to the temperature range given (5K).

2 DÉTERMINATION DE LA TEMPÉRATURE DE CÂBLES CONDUCTEURS À HAUTE TENSION Guy Van der Veken. Euromold, Belgique INTRODUCTION. Les tests de type sur les accessoires de câbles MV sont décrits dans les documents CENELEC HD68 et HD69. Certains tests décrits requièrent des températures élevées des conducteurs dans des limites strictes (p.e. 5K ou 10K au-dessus de la température de service maximum autorisée de l isolation extrudée du câble). Pour réaliser ceci dans les limites de température ambiante autorisées, le courant de chauffage doit être réglé. En raison de la présence de hautes tensions d essai sur l isolation du câble, la mesure en ligne de la température du conducteur du câble testé est impossible avec des techniques de mesure standard. Le document HD68 indique 3 méthodes pour déterminer la température du câble : 1) La méthode 1 utilise la relation entre la température du conducteur, le courant de chauffage et la température ambiante. ) La méthode utilise la relation entre la température du conducteur, le courant de chauffage et la température du manteau du câble. (Ces deux méthodes requièrent un calibrage préalable du câble pour établir ces relations.) 3) La méthode 3 utilise une boucle parallèle de câble identique dans le même environnement, qui est chauffé avec le même courant, mais ne transporte pas de haute tension. INVESTIGATIONS. Les points suivants ont été évalués : 1) La validité des méthodes décrites. ) L incertitude des résultats obtenus. 3) La comparaison des deux méthodes. Pour ce faire, les variables suivantes ont été examinées : a) Les matériaux des thermocouples. b) L exécution des thermocouples. c) L emplacement des thermocouples. d) L effet de la section du conducteur. e) Le nombre de thermocouples utilisé. Les facteurs d incertitude évalués comprennent : a) L incertitude des appareils de mesure. b) L incertitude des mesures. c) L incertitude de la température calculée. RÉSULTAT L évaluation des données mène aux conclusions suivantes : 1) La méthode, qui utilise la mesure de la température du manteau, produit le plus faible écart. ) L incertitude de la température déterminée (± 3K pour de petites sections à ± 5K pour de grandes sections) s est révélée élevée par rapport aux limites de température données (5K).

3 DETERMINING HIGH VOLTAGE CABLE CON- DUCTOR TEMPERATURES. Guy Van der Veken. Euromold, Belgium. PROBLEM DEFINITION General Temperature Heating Cooling The tests for checking high voltage products (joints, terminations, connectors, bushings) are described in standard HD68-S1. Some tests require an elevated temperature. For these tests, we must guarantee that the conductor temperature remains within strict limits (5K to 10K above the maximum conductor temperature). To achieve this in fluctuating ambient conditions, we must regulate the current. This means that we must measure the following values simultaneously: conductor temperature and current. However, this is not possible because the voltage in the cable is too high. This makes it impossible to measure the conductor temperature directly. We must determine the conductor temperature (which must be kept within strict limits) in another way. The answer can be found in Appendix A of this standard. We will carry out a no-voltage pre-test (calibration) i.e.: A 1 st method enables us to obtain the relationship between the conductor temperature, the ambient temperature and the current. With a nd method, we obtain the relationship between the conductor temperature, the cable-jacket temperature and the current. Specific test. The heat-cycle test is one of the tests in which we use an increased conductor temperature. This test consists of 18 cycles. Each cycle (see figure 1) lasts for 8 hours and consists of the following steps: * Heat the cable so that the conductor temperature is within the increased temperature zone for at least hours. * Leave the cable to cool down naturally for at least 3 hours, until the temperature difference between the conductor and the environment is maximum 10K. Increased temperature Environ- ment- 8 hrs Figure 1: Representation of the heat-cycle test. This test shows that it is also very important to know the relationship between the conductor temperature and the current. If the current estimate is wrong, there is a risk that a whole series of cycles of the heat-cycle test must be repeated until we have 18 good cycles. RESEARCH. In order to make a choice between the aforementioned methods, we investigated the following elements: 1. Checking the existing methods.. Which method gives the most accurate conductor temperature. 3. Is the 5K range feasible? In order to check this, we carried out an uncertainty study according to standard XP X07-00 of The tests. hrs 3 hrs 10K Time The test configuration. To carry out the novoltage pre-test (calibration of the cable), the test configuration in figure was used. At each measurement point, 1 thermocouple (calibrated) of each thermocouple group (see next paragraph) was attached. This enables us to establish which thermocouple group gives the most accurate temperature reading and whether there are major temperature differences between the different thermocouple groups.

4 MP1 MP make the conductor visible) is closed and the jacket is put back in its original position. current 70cm 90cm 70cm source Insulation MP4 Figure : Test configuration. MP3 uctor Thermocouple The thermocouple groups. Thermocouple group 1 (= Cenelec method): J-type, point soldered (figure 3). Thermocouple group (= point method): J- type, unsoldered (figure 4). Thermocouple group 3: (window mehtod): J-type, twisted, soldered (figure 5). Thermocouple group 4: strip thermocouple (this is a thermocouple that is attached onto a copper layer, making it possible to measure the jacket temperature with these thermocouples). junction Figure 3: Cenelec method. Figure 4: Point method. Figure 5: Window method. conductor junction Positioning of the thermocouples. The thermocouples in groups 1 and are connected to the conductor through a small hole, drilled in the cable (see figure 6). This figure also shows how the thermocouple must be positioned, i.e. where the thermocouple wire comes out of the cable, it will be bent. When taping the thermocouple, this bend will provide a pressure point. Thermocouple Insulation uctor Figure 6: Presentation of the thermocouple positioning with the drilled method. The thermocouples in group 3 are in contact with the conductor by inserting them between the different conductor wires (see figure 7). When the thermocouple is put in place, the window (rectangular cut-out in the cable-jacket and insulation to Figure 7: Presentation of the thermocouple positioning with the window method. Calibration. During calibration, the following measurement values are registered every 5 minutes: current, jacket-, environment- and conductortemperature. The thermocouples used are described in paragraph The thermocouple groups. The test configuration used is given in figure and the positioning of the thermocouples is described in the previous paragraph. Calibration was carried out for Al cables with a 50mm², 40mm² and 630mm² section. With this choice, we cover a wide range of high voltage cables regarding the cable section. In addition, it gives us a good idea of the better method for determining the conductor temperature and of the uncertainty about the conductor temperature we can expect when testing with an increased temperature. The uncertainty. To determine the uncertainty of the conductor temperature, we used the French standard XP X07-00 of This standard is based on establishing the variances on the variables needed for determining the conductor temperature, i.e. variance on current, ambient temperature and jacket temperature. A factor we must certainly take into account is the variance on the model. This is the difference between the conductor temperature measured during calibration and the calculated conductor temperature on the basis of the measured current and the measured jacket temperature (or ambient temperature according to the method used). Once these variances are known, we can establish the variance on the conductor temperature. Then, the uncertainty is indicated by: ϑ conductor = k * (V[ϑ conductor ]) 1/. (with ϑ conductor : uncertainty on the conductor; k: widening factor; V[ϑ conductor ]: variance on the conductor temperature.)

5 RESULTS. Cenelec Points Window. The results are discussed on the basis of figures. They are the result of the calculations based on the values measured during the tests. However, it is impossible to explain for each figure where all the values come from. Method 1 method. When looking at figure 8, we can clearly see that the variance on the model with method 1 (ϑ uctor based on ϑ Environment ) is always bigger than with method (ϑ uctor based on ϑ ). Variance on model ( C²) CENELEC POINTS WINDOW Method Method 1 Figure 8: The variances on the model. This greater variance on the model has a direct impact on the total uncertainty ϑ uctor (see figure 9). Here too, we see that method 1 always gives greater values ² 40² 630² 50² 40² 630² 50² 40² 630² In this paragraph, we will only consider the results obtained with method (ϑ uctor based on ϑ ). The reason for this is given in the previous paragraph. ϑ and ϑ 14 C 1 C 10 C 118 C 116 C 114 C 11 C 110 C 108 C 106 C 104 C Cenelec Points Window Cenelec Points Window Cenelec Points Window Cable 50mm² Cable 40mm² Cable 630mm² Temperature of conductor Uncertainty Figure 10: Representation of the average conductor temperatures and the uncertainties for the different cables and thermocouple positioning. When looking at this graph, we notice the following: * The conductor temperature is highest when the thermocouples are positioned according to the point method (for all three cables). * The point and window methods are two equivalent methods: they have nearly the same conductor temperature and uncertainty. * The conductor temperatures calculated on the basis of the Cenelec method are 6 to 10 C lower than the conductor temperature calculated on the basis of the point and window methods. * The uncertainty on the conductor temperature is approximately identical with the 3 methods (Cenelec, points, window). ϑuctor ( C) So we can conclude that the positioning of the thermocouples is better with the point or window method. Impact of the partial factors 0 50² 40² 630² 50² 40² 630² 50² 40² 630² CENELEC POINTS WINDOW Method 1 Method Figure 9: The uncertainties in graph. We can conclude from figures 8 and 9 that method (ϑ conductor based on ϑ ) should be preferred for determining ϑ uctor as this method gives us the smallest variance on the model and the smallest uncertainty. From paragraph "Method 1 method " we know that method is the most appropriate to determine ϑ uctor. Paragraph "Cenelec points window" gives us the positioning of the thermocouples (points or window). When discussing the results, we will only consider these methods. With method, V[ϑ uctor ] (= variance on the conductor temperature, whereby the conductor temperature is established on the basis of the jacket temperature) is determined as follows:

6 [ ] =. V[ ] +. V[] I V ϑ + ϑ. ϑ ϑ ϑ ϑ. I. ϑ [ ]. V[] I + V[ function] V ϑ ϑ I 50mm² - Window V[ϑ ] =.71 C² ϑ, = +/- 3.3 C 9% Whereby: ϑ ( ). V [ ϑ ] = factor 1 ϑ ( ). V [] I ϑ = factor I ϑ ϑ ( ).( ). V[ ]. V[] I. ϑ =factor 3 ϑ I V [ function] = factor 4 Factors 1,, 3, 4 are represented in fig. 11.a f. 50mm² - Points V[ϑ ] =.9 C² ϑ, = +/ C 11% 6% 3% 6% Figure 11b: Percentage of the partial factors for the 50mm²-Al cable (window method). 40mm² - Points V[ϑ ] = 3.6 C² ϑ, = +/- 3.6 C 6% 60% 35% 44% 3% Figure 11a: Percentage of the partial factors for the 50mm²-Al cable (point method). 19% % Figure 11c: Percentage of the partial factors for the 40mm²-Al cable (point method).

7 40mm² - Window V[ϑ ] = 3.13 C² ϑ, = +/ C 630mm² - Window V[ϑ ] = 5.46 C² ϑ, = +/ C 9% 9% 48% 45% 4% 1% % Figure 11d: Percentage of the partial factors for the 40mm²-Al cable (window method). % Figure 11f: Percentage of the partial factors for the 630mm²-Al cable (window method). 45% 630mm² - Points V[ϑ ] = 5.5 C² ϑ, = +/- 4.7 C 9% We can draw the following conclusions from figure 11: * is very small for all cables and positioning methods of the thermocouples. * The factors that contribute most to the uncertainty ϑ uctor are: * For cables with a small section: factors 1 and 3. * For cables with a medium section: factors 1 and 4. * For cables with a large section: factors 1 and 4. 4% % Figure 11e: Percentage of the partial factors for the 630mm²-Al cable (point method). This means we can make the uncertainty ϑ smaller by reducing factors 1, 3 and 4. We can do this as follows: * Use class 1 thermocouples instead of class. (This has an impact on factors 1 and 3.) * Adapting the current transformer class (this reduces factor 3): * the transformer ratio 400 class 0,5. * the transformation ratio > 400 class = 00/transformation ratio). * When calibrating the cable, we will: * for a cable section < 40mm² => set 6 current values instead of 5. * for a cable section 40mm² => set 7 current values instead of 5. (This will reduce V[function].)