21, rue d Artois, F-75008 PARIS B1-105 CIGRE 2014 http : //www.cigre.org The first HVAC and HVDC grid connection projects for wind power integration in German North Sea: experience, challenge and outlook Dr. Dongping Zhang, Volker Werle, Dr. Jochen Jung TenneT Offshore GmbH Germany SUMMARY Following the legal obligation, TenneT Offshore GmbH (earlier: E.ON Netz Offshore and transpower offshore gmbh) started to implement grid connections of offshore wind farms (OWF) in the North Sea since 2006. After the decision of German Federal Government, to shut down all nuclear power plants until 2022, the task turns more urgent. According to the expectation of the German government, the offshore installation capacity is estimated to 6.5 GW till 2020, which is adjusted to the actual situation of German offshore wind farms. As the first cross-border grid operator in Europe, TenneT is contributing to the Energiewende by connecting OWF in the German North Sea to the onshore grid. A lot of projects of HVAC and HVDC grid connection are now accomplished or under construction which have the transmission capacity of approx. 6.2 GW (as of January 2014). Further projects, based on the 320 kv-900 MW-class, are still under tender phase. It is planned to finalise the tenders in 2014 in order to meet the expectations and the ambition of the German government. With those projects, TenneT is now the largest utility operating DC VSC converters and DC XLPE cable technology worldwide. This publication describes the achieved experience, challenges and outlook of further development, especially for AC and DC submarine cable technology. Some considerations, e.g. the quality assurance of the DC XLPE cables, cable installation in environmentalsensitive area, offshore maintenance etc. will also be discussed in order to indicate the special problems to be faced by an offshore TSO. KEYWORDS XLPE cables, HVAC, HVDC, Wind integration, Germany North Sea, Energiewende, offshore TSO, offshore maintenance, Standardization dongping.zhang@tennet.eu
1. Introduction Following legal obligations based on the German Energy Laws, TenneT Offshore GmbH (former E.ON Netz Offshore GmbH and transpower offshore gmbh) started with its activities in offshore grid connection projects in December 2006. Concerning the transmission capacity, length of the grid transmission systems and other boundary conditions, there are two general technical variants to be chosen to connect the OWF to the onshore grid: HVAC and HVDC. For a small transmission capacity and a short transmission distance, an HVAC link system can generally be realized, i.e. a direct route between the grid connection point (NAP) of one wind farm and the onshore grid coupling point (NVP). Enlarging the transmission capacity and/or the transmission distance over an economical and technical threshold value, then the HVDC variant is preferred for the grid links [1]. However, the choice of the grid connection variants must be subject to a particular projectrelevant investigation. Especially the development scenario shall be applied as basis for this investigation. In the summation of the coetaneous demand of transmission capacity, there is a tendency of utilising high-capacitive HVDC connection systems in the German North Sea. Figure 1 shows the typical grid connection systems with the application of HVDC VSC technology. The electrical power generated by wind turbines is collected at the substation of the OWF and is transmitted to the offshore converter substation via HVAC submarine cables which have a typical length of up to 30 km. Due to the advantages such as the black-start capability etc., the HVDC VSC technology will be applied in the offshore converter station and the onshore converter station which are connected by HVDC XLPE submarine and land cables. Figure 1: Typical offshore grid connection with the application of HVDC VSC technology According to the geographical location of OWF, there are four clusters to be introduced for the grid integration of offshore wind energy in the German North Sea: BorWin, DolWin, HelWin and SylWin, which are defined with the allocated high-capacitive transmission routes and the grid coupling point (NVP) to the 400 kv transmission grids. According to the time sequence of the projects, the name of the projects will also have one identification number, e.g. BorWin1, DolWin1 etc. Figure 2 shows the geographical location of the projects and their onshore NVPs. 1
SylWin1 SylWin2 BorWin2 BorWin1 BorWin4 BorWin5 DolWin1 DolWin3 DolWin4 BorWin3 alpha ventus DolWin2 HelWin2 HelWin1 Nordergründe CS Büttel Riffgat UW Emden UW Hagermarsch UW Inhausen CS Emden/Ost CS Diele CS Dörpen West Figure 2: Geographical allocation of the cluster connection projects in German North Sea 2. The projects overview As of January 2014, there are already eleven grid connection projects in operation or under construction in German North Sea as followed: Table 1: Overview of the TenneT offshore projects of the grid connection XLPE Cable Projects MW KV length 1 alpha ventus 60 AC 110 78 km BorWin1 400 DC 150 400 km BorWin2 800 DC 300 400 km DolWin1 800 DC 320 343 km DolWin2 900 DC 320 268 km HelWin1 576 DC 250 260 km HelWin2 690 DC 320 260 km SylWin1 864 DC 320 410 km DolWin3 900 DC 320 322 km Riffgat 108 AC 155 138 km Nordergründe 111 AC 155 40 km Total 6209 2919 km 1 AC export cables between the AC substation platform and DC converter platform are not included. 2
Table 1 shows that more than 2600 km DC XLPE cables were installed, produced or in manufacturing process in the recent six years. Furthermore, there are two DC projects - BorWin3 and BorWin4 - of 320 kv and 900 MW class, which are in the tender phase as of January 2014. The huge demand has changed the market dramatically, especially for the cable technology of DC XLPE. 3. Technical Experience and Challenge For a TSO, reliability, availability, maintainability (RAM) and the total cost of ownership (TCO) are key issues for evaluation of the technical assets. Higher voltage level and larger transmission capacity require reliable cable design, more cable production facilities and superior testing methods for submarine cables, especially concerning their critical installation/repair conditions. Since there is no operation experience of DC XLPE cable above 300 kv worldwide so far, the quality assurance of those cables which have a typical production length of more than ten kilometres, even up to 30 km, must face a gigantic challenge. TenneT Offshore GmbH has standardized offshore DC cluster connections with the transmission capacity of 900 MW and the voltage level of 320 kv. Based on the cable technology, 320 kv is the realistic limit for such offshore grid connection projects in Germany, especially considering the absence of any operation/service experience of such cable systems. Already a miniature adjustment of the production parameter can result in a fatal change of the cable quality, especially by the introduction of new production facilities. It is recommended that the qualification of such new production facilities should not only be limited to CIGRE recommendation [2] [4]. A more detailed investigation shall be performed to ensure that the new production facility, e.g. the new extrusion line, can also fulfil the critical technical requirements to produce the HVDC cable, particularly for the know-how transfer between different locations and for the deviations between old and new production lines. The cleanliness of the insulation material, including the semi-conductive screens, and the production process are the essential factors affecting quality, reliability and availability of a cable system. Particularly for HVDC submarine cables, some special aspects shall be taken into account, e.g. achieving the maximum production length, minimizing the number of factory joints, which are not only time consuming in the manufacturing process but also could be technical weak points for HVDC cables, and moreover reducing the risk of scorch during the extrusion process, which forces a shutting down of the extrusion line for cleaning and therefore limits the maximal continuous manufacturing length. Furthermore, the interface between different materials in the accessories is also an essential challenge for DC XLPE cables. The testing of the cable system, especially of the accessories, shall be sufficient in order to identify the potential problems, especially for the total service life of next 30 to 40 years. The factory joint is widely applied for submarine cables. But there is no qualitative and satisfying test method for this component apart of the HV routine test. There is a strong need to develop a more sensitive test method, e.g. a PD (partial discharge) test for each factory joint even at very long cable length. Moreover, according to the experience of TenneT Offshore GmbH, a significant amount of mechanical damages occurred during the production, transportation or installation of submarine cables. These mechanical damages are mainly due to the length of the cable. During the cable production, a lot of improvements have been achieved in order to reduce this 3
risk, like optimizing the intermediate storage facility and tools. Normally the cable manufacturer and the installation company are two different entities. Therefore a clear definition and supervision of the interfaces between the different companies is very important in order to minimize the risk of mechanical damage during cable transportation and installation. Figure 3 shows an example where an HV cable is improperly installed on the offshore platform. Due to the unavoidable movement of the platform (up to ±1m), the cable was mechanically damaged and had to be replaced. Figure 3: Mechanical damage on the platform cable due to improper installation work Also for the cable design, there are some special requirements regarding the offshore implementation. For example, the cable installed on an offshore platform shall have a semiconductive layer for DC HV test after the installation as well as a fire retardant layer beside the normal PE-sheath. For this, some manufacturers offer so-called Sandwich -Oversheath. In order to minimize the risks of space charge effects in DC XLPE cables, TenneT Offshore GmbH decided to slightly modify the after-installation-test compared to the CIGRErecommendation [3]. The testing voltage is positive polarity on the positive pole and negative polarity on the negative pole of the cables. In order to determine the discharge on the cable, especially after the testing, the measurement of residual voltage has also been introduced. Figure 4: Vibration sword for the cable installation in the Waddensea area 4
In TenneT offshore GmbH s projects, the HDD (Horizontal Direct Drilling) is widely utilized for the land cables and landing point of submarine cables. Even some drillings with length of more than 1.500 m were realized, e.g. at the crossing of river Ems and at the landfall. The vibration sword (Fig. 4), which has less environmental impact, is applied in the German Waddensea area (World Natural Heritage by UNESCO). The selection of offshore laying and burial methods and the suitable equipment are also a challenge because the German authorities have strict requirements for cable burial depths. For this purpose an anchor penetration test has been performed successfully by TenneT Offshore GmbH and has been adapted by the Authorities. As remedial measurement of post-laying, the Mass Flow Excavator (MFE) has been used for some sections of the cable route. But it is questionable whether this method can also be applied for DC XLPE live cables. Until today there is no experience and no investigation of the cooling shock effect and the thereby caused overstress on the cable insulation and the semi-conductive screens. Different cable locating systems have been tested to ensure the horizontal and vertical position of the submarine cables to fulfil the requirement of the authorities. It is well-known that it is not realistic to dimension the HVAC export cables of OWF and HVDC cluster cables with a 100% load factor. Instead it is well accepted to calculate the current rating of those cables with load factors based on wind load statistics. However the 2Kcriteria [5] - the special requirement of the German authorities for minimizing the thermal environmental impact has also a significant influence on the cable dimensioning. The requirements introduced by TenneT Offshore GmbH for the cable current rating are: 1) The whole cable route must be capable to transmit 100% load without considering the 2K-criteria; 2) The wind load based on the wind statistics derived from the FINO 1 measurement platform in the German North Sea shall be applied for the calculation of the transmission capacity without violating the 2K-criteria. Figure 5: Temperature distribution of HVDC cables laying in Waddensea by FEM simulation To verify the compliance with the 2K-criteria, a temperature monitoring system has been 5
installed [6] in two morphologic different areas of the Waddensea near Hilgenriedersiel: Mischwatt (mixed sediment mudflat) and Sandwatt (sand flat) -. Temperature measurements and load data from alpha ventus are now available for more than 18 months of operation. Both, temperature measurements and calculations based on the load of alpha ventus, show that the 2K-criterion is never violated, and that there is a large safety margin even during strongest wind load phase. The calculations are based both on the IEC-standard - which is also part of the approval system of the German authorities [5] - and FEM-methods (Fig. 5). 0.8 0.6 Temperature increase in Mischwatt and Sandwatt, measurement depth: 15 cm calculation 24 h mean Mischwatt 24 h mean Sandwatt 0.4 Temperature [ C] 0.2 0 0.2 0.4 0.6 Dec 20 2011 Jul 07 2012 Jan 23 2013 Jun 26 2013 Figure 6: Temperature rise in Wattensea with the burial depth of 15 cm 3.5 Temperature increase in Mischwatt and Sandwatt, measurement depth: 100 cm calculation 24 h mean Mischwatt 24 h mean Sandwatt 3 2.5 Temperature [ C] 2 1.5 1 0.5 Dec 20 2011 Jul 07 2012 Jan 23 2013 Jun 26 2013 Figure 7: Temperature rise in Wattensea with the burial depth of 1 m The comparison of the calculation results based on IEC 60853-2 and the temperature measurements demonstrates the validity of the calculation method in burial depths of at least 1 m where the seasonal variation is slight (Fig. 7). At less laying depth the environmental 6
impact increases and dominates the temperature variation (Fig. 6). Figure 8 shows the typical forced energy unavailability of offshore grid connection. It is obvious that the offshore DC cables play the dominate role for the forced outage of an offshore DC grid connection system. Offshore DC Cable Main Circuit Onshore Onshore DC cables Air Cooling System Auxiliary Power Valve Cooling Control & Protection 0% 10% 20% 30% 40% Figure 8: Example of forced energy unavailability, contribution per system Therefore, the planning of the spare parts, particularly for the offshore DC cable, is crucial. Generally the spare parts for the offshore cables are planned for two repairs. Normally, the demand of spare cable length for one repair is about three times of the water depth of the cable location. For German North Sea with water depth less than 50 m, this would yield in a theoretical spare length of 150 m per cable failure. But due to different uncertainties, e.g. inaccuracy of fault location, layout of the applied repair ship, unexpected longitudinal water ingress etc., a longer spare cable should be planned and kept in whole length without any pre-cutting. 4. Outlook and Expectation for the further development It is well-known that lack of experience and lack of standards are the tremendous challenges for offshore grid connection systems. For the cable technology, especially the HVDC submarine cables, there is no international standard except the CIGRE-recommendations. It is estimated that an international standard will be introduced shortly with the substantially increasing number of projects and by means of the accordant experience. This standardisation should cover every component of grid connection systems from the planning to the operation. In order to optimize the measurements of quality assurance, the testing methods, particularly on the DC XLPE cables, will be further developed and verified, e.g. the routine-test method(s) on the factory joints of the submarine cables shall be amplified from the HV test. Furthermore the space charge measurement shall not be defined only as an item of development test by the manufacturers. Such measurement shall also be one of the criterions to identify the quality of DC cables. It is recommended to integrate this measurement into the type test or even into the routine test. Certainly there is the necessity to standardize the reliable method and the threshold value. 7
It is also estimated that the research work will be performed for the behaviour of water-tree under DC voltage, especially for the whole service life of 30-40 years. For example, the task could be to identify not only the qualitative but also the quantitative allowable volume of moisture in the insulation and the semi-conductive screens of DC XLPE cables. Moreover, the possibility of DC High Voltage tests combined with the partial discharge measurement shall be verified, especially for the FAT and the After-Installation-Test. Due to the difficulty of the transportation of the traditional resonance equipment to an offshore platform, the suitable test method for the long offshore cable linked between offshore platforms shall be further investigated, e.g. VLF test method or a shippable resonance test equipment. Till today, only the soak-test (24 hours at U 0 ) is technically and economically feasible for those HV submarine cables. As well known, the offshore bad weather impinges on the duration of submarine cable repair. The new repair method is estimated with the minimized influence by bad weather. Innovative solutions can be anticipated, such as cable temperature monitoring system, cable failure monitoring system, cable failure location system (online and offline) especially for submarine cable system with a cable route more than 100 km where it is not possible to integrate repeaters with external power supply. Of course, it is also to anticipate that new DC XLPE material will be introduced with the higher voltage, e.g. 500 kv. But as above mentioned reason, it is recommended to expand the operation and service experience firstly. It is also estimated to a market development of operation services of the offshore grid systems. More offers and service providers could be expected, such as the cable repair, cable route survey, platform maintenance etc., due to an increasing number of OWFs and grid connection systems in the German North Sea and in Europe. In order to enlarge the number of the suppliers, new TenneT projects, starting with BorWin3, have been split into different lots based on the experience of the previous projects. Therefore all project implementation phases (milestones of the project) of the grid connection systems and also the maintenance in the prospective operation must be further standardized, especially the interfaces between the system components, e.g. platforms, converter stations and cable systems etc. shall be further clearly defined and verified. 5. Summary As the only TSO, which has made huge investment in offshore grid connection infrastructure based on new technologies, such as HVDC VSC and DC XLPE cables, in the last six years, TenneT Offshore GmbH has gathered plenty of experience during the project execution and operation phase and will face enormous challenges still. It is necessary to extend and modify the international standards according to the special requirements of the offshore grid connection system, especially for DC XLPE cables. DC XLPE cables with a voltage level above 300 kv shouldn t be treated as an immature technology. However, in comparison with AC-technology, especially with regard to decades of experience in operation, this technology is still in development. The sharing of knowledge, know-how and experience between suppliers, research institutions and utilities will promote and accelerate this development. 8
BIBLIOGRAPHY [1] Dr. D. Zhang, Dr. T. J. Lebioda, Dr. Masoumeh Koochack-Zadeh and Dr. Jochen Jung: The First Three 800 MW Wind Park Grid Connection Projects with XLPE HVDC Cables, 10th International Workshop on Large Scale Integration of Wind Power and on Transmission Networks for offshore Wind Farms, Aarhus, 2011, Paper-No. WIW11-023 [2] CIGRE TB 490 Recommendations for Testing of Long AC Submarine Cables with Extruded Insulation for System Voltage above 30 (36) to 500 (550) kv, Feb. 2012 [3] CIGRE TB 496 Recommendations for Testing DC Extruded Cable Systems for Power Transmission at a Rated Voltage up to 500 kv, April 2012 [4] CIGRE TB 303 REVISION OF QUALIFICATION PROCEDURES FOR HV AND EHV AC EXTRUDED UNDERGROUND CABLE SYSTEMS, August 2006 [5] Bundesamt für Seeschifffahrt und Hydrographie (BSH), Standard Konstruktive Ausführung von Offshore-Windenergieanlagen, 2007; http://www.bsh.de/ [6] Bioconsult, Schuchardt & Scholle GbR, Temperaurmonitoring - Einbau der Sonden und erste Datenauslesung 9