1 Testing 320 kv HVDC XLPE Cable System H. He, W. Sloot DNV GL, KEMA Laboratories Arnhem, The Netherlands Abstract Two unique test requirements in testing of a high- voltage direct- current (HVDC) cable system compared to that of a high- voltage alternate- current (HVAC) cable system exist: a specified maximum temperature drop across the insulating layer of a cable ( θ!"# ) and a superimposed impulse withstand test. This work focuses on these two topics based on practical testing experience. The temperature difference θ!"# (excluding the semiconducting screens) is provided by a cable manufacturer depending on an insulating material (e.g. XLPE: cross- linked polyethylene) and a cable design. This temperature difference needs to be maintained in a specified limit during a stead state of a load cycle. In practice, it is not possible to measure θ!"# directly. By applying of heat transfer theory, a simple formula is introduced to calculate θ!"#. A superimposed impulse withstand test is essential for a HVDC cable system to be qualified for two different HVDC converter technologies, which are Line Commutated Converter (LCC) and Voltage Source Converter (VSC). In this paper, a laboratory test set- up is presented for switching/lightning impulse (SI/LI) voltage superimposed on a HVDC voltage. A universal divider is introduced to measure the combined stresses for a HVDC cable system. Keywords High- voltage; testing; HVDC cable system; thermal stress; impulse withstand; sphere gap Introduction In a HVAC cable, the electric field distribution depends on capacitances which are dominated by the permittivity of the insulation material. This permittivity is rather stable with temperature, therefore a specified θ!"# is not required for a HVAC cable testing. Contrary to a HVAC cable, the field distribution in a HVDC cable is determined by the resistivity of the insulation material. Furthermore, this resistivity varies greatly with temperature, which results in the need to control the temperature difference θ!"# during the thermal stresses represented by the load cycle in a test,. A HVDC cable in operation experiences SI/LI overvoltage as well, which is superimposed onto a DC service voltage. In order to simulate the in- service condition, there a switching surge and a lightning transient superimposed on an operating DC voltage is a standard requirement. This paper describes a test circuit with two separated test systems (a DC generator and an impulse generator), which were connected to each other with a test cable system located in between. The test cable system consists of three pieces of HVDC XLPE cable (50 m in total) assembled with two outdoor terminations and two joints. A major complexity is to prevent damage of each high voltage test system due to the presence of the other. Therefore, a sphere gap was installed to block DC voltage from the impulse generator; a damping impedance set was protecting the DC generator from very fast transients.
2 Thermal physical model List of symbols Q, heat transfer rate (heat flow), W; θ!, θ!", θ!,θ!", θ!, θ!"#, temperature of conductor, conductor screen, insulation, insulation screen, sheath and ambient, C; θ!", θ!, θ!", temperature drop across conductor screen, insulation and insulation screen respectively, K; θ!!!", temperature difference from conductor to insulation screen, K; T!", T!, T!", thermal resistance of conductor screen, insulation, insulation screen respectively, K m/w; T!, thermal resistance in total from metal sheath to over sheath, K m/w; T!", thermal resistance of external thermal insulation material, K m/w; Consequently, a temperature difference arises across the cable. The temperature distribution inside the cable can be either measured by thermocouples or calculated by a numerical model. Figure 1 is shown a diagram of an equivalent thermal circuit for a cable. Figure 1: Diagram of an equivalent thermal circuit for a cable Temperature difference across conductor screen, insulation and insulation screen is calculated as θ!" = θ! θ!" = QT!" (1) θ! = θ!" θ! = QT! (2) θ!" = θ! θ!" = QT!" (3) θ!" and θ! were measured directly by thermocouples. So overall temperature difference from conductor to insulation screen is θ!!!" = θ!" θ! = Q(T!" + T! + T!" ) (4) Based on Eq.(2) and Eq. (4), T! (5) θ! = θ!!!" T!" +T! +T!"
3 In conclude, Eq.(5) is the simple formula for the calculation of θ!"#. Oobviously, the surrounding plays also a role, thus the main temperature drop in a cable is between the cable surface (sheath) and environment (θ! θ!"# ). In order to control θ! within the limit of θ!"#, external thermal insulation material (e.g. plastic bubble) is applied to minimize the influence of environment. This thermal insulation material helps to adjust T!" based on different cable design and surrounding condition. Load cycle and temperature measurement In order to monitor the thermal environment of HVDC cable system, thermocouples were placed at more than 20 different locations on a reference cable and a test cable in total. The reference cable should be installed in close proximity with the test cable to ensure an equal thermal environment . The conductor temperature of the reference cable (based on rated maximum θ!, e.g. 70 C) was monitored to guide the same current applied to the test cable simultaneously. By applying Eq.(5), the specified Δθ!"# was calculated and monitored during the load cycle. A typical 24 hours load cycle is shown in Figure 2. Figure 2: Diagram of a typical 24 hours load cycle Figure 3 and Figure 4 schematically demonstrate the cable cross- section (e.g. 2500mm 2 ) and the locations of the measured temperature respectively. 60 insulation screen temperature θ is 2 3 conductor temperature θ c insulation screen temperature θ is Figure 3: A cross- section overview of cable construction
4 CABLE SYSTEM Termination A- side Termination B- side (thermally insulated) Pos. 1 Pos. 15 Pos. 2 Pos. 14 Pos. 3 Pos. 13 Pos. 12 Pos. 11 Pos. 4 Ambient 1 ( 0,2 m) Ambient 2 ( 2 m) Heating transformers Ambient 4 ( 0,2 m) Pos. 10 Pos. 5 Pos. 6 Pos. 7 Pos. 8 Pos. 9 Joint 2 Joint 1 REFERENCE CABLE (thermally insulated) Connection left, 1 Ambient 3 ( 0,2 m) Connection right, 1 Thermal insulation Connection right, 2 Connection left, 2 Sheath, bare Sheath, thermally insulated Conductor 1, 2 and 3 Insulation screen, positions 1-6 Figure 4: Locations of the measured temperature Experimental setup An experimental setup included a DC generator, an impulse generator and a test object, which was a 320kV DC cable system including HVDC XLPE cable (copper conductor and XLPE insulation, nominal capacitance of 0,26µF/km), two outdoor oil terminations and two premoulded joints with shield interruption. A schematic test circuit and a photo of a real test setup are shown in Figure 5 and Figure 6 respectively. Each test system was equipped its own divider in order to measure DC voltage (Divider 1 in Fig. 5) and SI/LI (Divider 3 in Fig. 5) respectively. A combined divider (Divider 2 in Fig. 5) was applied to measure the superimposed withstand voltage subjected directly to the test object. A pair of spheres (diameter of 0,5 m, SG2 in Fig. 5) was installed in vertical position, the lower sphere directly fixed to the outdoor terminations. The DC generator was also connected to the lower sphere with the protection water resistor (10 MΩ, R1 in Fig. 5) and the blocking impedance (35 mh, L1 in Fig. 5) in between. The upper sphere was separated from the lower sphere and the cable by an air gap and connected to the impulse generator. The surge arrestor (A1 in Fig. 5 ) was installed for extra protection of the DC generator.
5 Figure 5: Schematic view of the test circuit for superimposed impulse test of DC cable system Figure 6: Photograph of the test set- up of a 320kV DC cable system Test procedure Accurate measurement of voltage waveforms and self- ignition of sphere gap are the two essential aspects for a superimposed impulse test. By means of a sphere gap, this gap must ignite at a certain level and transfer the SI/LI to the cable system. The gap distance has to be considered and adjusted. Under the same polarity of two voltages (e.g. + UDC and + USI, a unipolar stress), the breakdown voltage of the sphere gap (Ubd unipolar = UDC USI UDC, thus USI 2 UDC), in contrary to a bipolar stress (e.g. + UDC and USI), the voltage (Ubd bipolar = UDC ( USI) UDC, thus absolute USI 0). Therefore under the bipolar stress, there is not critical for breakdown the air gap; however under the unipolar stress, it is a complex procedure due to the Ubd unipolar. Furthermore, this calibration steps are necessary to avoid overshot of the test object . The distance of sphere gap needs to be adjusted corresponding with the calibration of impulse voltages from low scales (50%, 65% and 80% of the test voltage) up to full scale (100% of the test voltage).
6 Analysis of measured results The three measured voltages were obtained with three dividers D1, D2, D3 (see Fig.5) during the testing. The DC voltage measured by the Divider 1 via a digital voltage meter and the full scale of DC voltage should be 320kV. The SI/LI voltages were overserved by the Divider 3 and the peak voltages should be 710kV and 740kV respectively (see Fig.7). DC voltage superimposes with same polarity of SI is shown in Figure 8. It is further found that DC voltage superimposes with opposite polarity SI and LI in Figure 9 and Figure 10 respectively. Figure 7: Representations of the switching impulses waveforms measured by Divider 3 Figure 8: The unipolar stress (U DC and U SI ) measured by Divider 2 Figure 9: The bipolar stress (U DC and U SI ) measured by Divider 2 Figure 10: The bipolar stress (U DC and U LI ) measured by Divider 2
7 In Figure 7, the Divider 3 measured a spike in front of the SI wave shape, which is the time delay of the protection sphere gap, it was until over the breakdown voltage, then sphere gap was conducting and the cable system was connected at that moment to the impulse generator. It is observed in Figure 8, a systematic DC offset error occurs during the tests. The Divider 2 was calibrated by DC voltage and Impulse voltages respectively. For combined voltages, a correction factor is needed (i.e. a factor of 0,78). This offset leads to a difference of peak voltages of impulses measured by Divider 3 and Divider 2. It is noticed that, once the arc between the spheres was ignited, the voltage on the cable system followed the voltage at the terminal of the impulse generator. Thus the voltage on the cable system in extreme short time (hundreds or thousands of µs) discharged via impulse generator to extinguish voltage of the arc (visible in graphs), then it would take short recover time (few hundreds of ms) to recharge to the DC voltage (invisible in graphs due to limitation of data collection). In the meantime, the protection resistors sustained the full difference of impulse voltage and DC test voltage. Conclusions The calculation of Δθ!"# is obtained based on heat transfer method. In order to maintain a constant surface temperature of the cable, needed to guarantee a specified Δθ!"#, the practice methods are to control load current through conductor, maintain constant ambient temperature or apply the external thermal insulation material. A proper test setup using a sphere gap has been developed for the application of SI/LI superimposed on a DC voltage. The correct gap adjustment is essential for unipolar stress condition, otherwise under certain circumstances, the high- frequency oscillations might occur. Further experimental work will focus on a method of controlling cable sheath temperature, and developing a new three- channel software with extended data capture time (up to recover time of DC voltage). Furthermore, a protection capacitor is another alternative of a sphere gap to realize the superimposed withstand test. References  Cigre 496: Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500kV, April  IEC Edition 1.0: : High voltage direct current(hvdc) power transmission Cables with extruded insulation and their accessories for rated voltages up to 320 kv for land applications Test methods and requirements.  E. Pultrum, W. Sloot, J. Fernandez, R. P. P. Smeets, High- voltage cable testing: type test experiences and new insights into pre- qualification, CEPSI,  IEC Edition3.0: : High- voltage test techniques Part 1: General definitions and test requirements.