Strategies for the Interconnection of Off-Shore Power Systems to Shore Using AC or DC

Transcription

1 2010 International Conference on Power System Technology 1 Strategies for the Interconnection of Off-Shore Power Systems to Shore Using AC or DC Piergiovanni La Seta, ember, IEEE, and Edwin Lerch Abstract This paper deals with the problem of choice whether the best solution for interconnecting off-shore power systems is via High Voltage Alternate Current (HVAC) or Direct Current (HVDC) transmission. The typical off-shore interconnection strategies based on HVAC and HVDC technologies are illustrated from the technical and economical point of view, with reference to some practical examples. T Index Terms interconnections, HVAC, HVDC. I. INTRODUCTION HE issue of interconnecting off-shore electrical power systems over long distances represents a challenge for manufacturers and operators. The design of off-shore interconnections has to take into consideration more severe design criteria and more restrictive constraints in order to guarantee feasibility under several operating conditions than for onshore, strongly connected systems. Examples of nowadays investment attracting off-shore electrical power systems are oil platforms, wind farms, and subsea pump stations. The electrical equipment, installed power and operational conditions of these systems are usually very different, and the interconnection method has to be correspondingly designed. The extraction of oil or gas at offshore locations is an economically relevant task to satisfy the growing primary energy request worldwide. Platforms at several off-shore sites in Norway, England, exico, Brazil etc., have been installed or planned in the last decades. Offshore platform systems can be operated as islanded networks, with local generation, or be alternatively electrified by means of onshore-to-offshore connections, through AC or DC links. The use of an AC or a DC interconnection is on a side related to cost minimization depending on the power to be transported and the distance to shore, on the other side to technical constraints such as voltage regulation or harmonic impact. The generation of electric energy from renewable sources, such as wind, has strongly increased in the last decades, but the installation of wind turbines on-shore has reached its profitability limit in many countries. Therefore the number of offshore wind farms installed or planned increases more and more. The connection of off-shore wind farms to transmission networks on the mainland via AC or DC links has to consider Authors are with Siemens AG, E D SE PTI NC, Erlangen, D Germany ( {piergiovanni.laseta, edwin.lerch}@siemens.com). in this case several factors related to the fluctuating active power generation as further constraints. Due to the complexity in posing cables on the seabed and the high costs related to their maintenance, accurate system studies for planning, design and operation of off-shore interconnection systems are required, including dynamic (start-up, component outages, faults) and harmonic studies (resonances, interferences, harmonic propagation). Aim of this paper is to illustrate the interconnection strategies based on HVAC and HVDC technologies for off-shore power systems from the technical and economical point of view, with reference to some practical examples. II. OFF-SHORE HVAC AND HVDC INTERCONNECTIONS Basically, in order to connect an electrical network, such as an industrial system, a power plant or an off-shore platform, to a grid over long distances there are mainly two possibilities. The first one consists in stepping up the voltage to a high or extra high level (e.g. 380 kv) on both grid and far network sides, and use a HVAC connection such as overhead lines or cables. The second one consists in installing AC/DC converters on both grid and far network sides and realizing the connection over a DC overhead line or cable. Usually, on-shore interconnections are realized by HVAC overhead lines. This is for relatively high distances and transmitted power, such as those typical in the most countries all over the world, the technically and economically most viable solution. Line commutated, thyristor based HVDC connections are usually installed only between strong grids, for very high distances (over 1000 km) and very high transmitted power (over 1000 VA), taking place e.g. in extended countries such as USA, China, and Brazil. When speaking about off-shore interconnections, instead, the boundary conditions are different. Oversee connections can be realized only by cables, which have different impedance and mostly capacitance values as overhead lines. For long distances the installation of shunt reactors to compensate the charging power may thus be necessary. Furthermore, component dimensions, volume and weight are important, not secondary issues for off-shore systems. Finally, the need of short circuit capacity or power/voltage regulation in case of disturbances may be decisive in order to choose HVAC or HVDC technology. In order to illustrate the issues regarding the planning of an off-shore interconnection, the electrification of an off-shore platform for oil or gas extraction is considered as an example /10/$ IEEE

2 2 The following sections try to answer the main questions. III. POWER AND DISTANCE In order to decide whether AC or DC is the best solution, it is possible to formulate the following rough estimation for the AC connection: The on-to-off-shore distance in km and the operating voltage in kv may not substantially differ, and the served load power in VA may not be higher than approx. 80% of the cable thermal limit in VA. The choice of the suitable section is usually an optimization problem between minimization of costs (including the possible installation of shunt reactors), minimization of losses, possibility of supplying more loads (e.g. in case future extensions are planned), etc. Anyway the optimal cable section is lower bounded by the minimal mechanical capability for seabed posing. For these reasons, often the HVAC cable sections are oversized. By a cable length approximately equal to the operating voltage (km = kv), the installation of shunt reactors for (capacitive) reactive power compensation is usually necessary in order to operate the AC cable under different load conditions. Fig. 3 reports the voltage profile along a 100 km long, 132 kv AC cable under no-load and full load condition, with and without shunt reactors in both cases. By means of a load flow calculation the feasibility can be exactly determined, however if the operating conditions differ too much from the mentioned rough rules, alternative solutions are requested. These can be briefly summarized as follows: The cable has to be operated with a voltage which is lower than the rated voltage (since the current thermal limit remains unchanged, the maximal allowed transmitted power decreases proportionally to the operating voltage); The operating frequency has to be lower than the usual frequencies 50 Hz or 60 Hz (this solution is suitable only if the off-shore system can operate with the same lower frequency); A DC connection instead of AC is realized. Fig. 1 and 2 show the basic structure of a HVAC and a HVDC interconnection to an off-shore oil platform, respectively. Fig. 1. Basic scheme of a HVAC interconnection to an off-shore platform. On-shore platform Fig. 2. Basic scheme of a HVDC interconnection to an off-shore platform. Voltage [%] Length [km] Full Load with Shunt Full Load w/o Shunt No load with Shunt No load w/o Shunt Fig. 3. Voltage profile across a 132 kv AC cable under no-load and full load operating conditions. The installation of a HVDC transmission system is therefore unavoidable, without considering financial aspects, i.e. only from the technical point of view, in the following cases: If the distance from shore is considerably higher than the maximal feasible distance covered by a HVAC cable; If active and particularly reactive power control on the off-shore system are primarily important/requested; If the off-shore system operating frequency is different as the grid frequency at shore (this situation can be found e.g. on some platforms in the North Sea). IV. COSTS Considerations about the costs are important, but often not exclusive in the decision process, whether AC or DC transmission solution is the best. In fact, the technical feasibility and the volume/weight factor can be more restrictive constraints as the costs. An estimation of the material and installation costs can be done with reference to the transmission power and voltage. Assuming as an example a transmission power of VA and a distance from shore of 100 km, the material and installation costs of a HVDC transmission system can be approximately estimated in 60% (cables) and 36% (converters) with respect to a comparable HVAC solution (assumed therefore as 100%). Assuming instead a distance from shore of 150 km, the converter cost incidence decreases to 24%. The costs for the switchgears and the transformers at both terminals have been assumed similar, so that these costs have not been considered in the estimation. Due to the controlled, multilevel Siemens HVDC PLUS solution for off-shore interconnection 100

3 3 (not line commutated, i.e. based on IGBTs instead of thyristors, and with very low harmonic content) components like filters can be avoided, with cost, volume and weight advantages. In general it can be said that the break even point between the HVAC and HVDC solutions can be determined in the range km, depending on the transmission power. the island the second solution had been found as the most suitable, however with increase in the total distance from shore to ca. 140 km. Fig. 5 reports the schematics of the chosen solution. V. VOLUE AND WEIGHT Both with HVAC and HVDC technologies for the interconnection the transformer(s) and switchgears for the offshore terminal have to be installed on the platform. Additionally in case of HVDC solution, the off-shore terminal equipment (e.g. DC reactors, converters) requires the installation on the platform as well. Problems concerning volume and weight can therefore take place because of the limited capacity on the platform. That is the reason why often the companies operating off-shore systems do use cranes with relatively limited capacity, such as max. 50 tons. Typical volume and weight of a 100 VA transformer are 7x3.5x6 m and 120 tons, while typical volume and weight of a gas insulated HV switchgear (e.g. 145 kv) are 8.5x3x6.5 m and 3 tons per field. Furthermore, approximate volume and weight of a 100 VA converter (off-shore terminal) are 150 tons and 30x25x20 m (volume and weight depend on the converter voltage level and design). The problems caused by limited place and weight capacity on the platform can represent sometimes heavier constraints as costs. The case of electrification from shore of a platform operated at 60 Hz (on-shore grid frequency is 50 Hz) is illustrated as an example. The situation before electrification is depicted in Fig. 4. The load on the platform at 100 km distance from shore, operating as an islanded network with 60 Hz frequency (2 generators on board), is planned to be increased by replacing the old ones with bigger motors and drives. Installing a further generator is not feasible, therefore the electrification option has to be studied. However this solution requires the installation of a converter, since this is the only possibility for two different operating frequencies. It is worth to mention that an island in an intermediate location, however not on the direct was, is present. Following solutions are technically possible: Interconnection via HVDC as depicted in Fig. 2; Interconnection via HVAC with converter on the island located at a certain distance from the direct way shore-platform, see Fig. 4; Interconnection via HVAC with converter on the platform. The first solution would be the most economical, however due to the limited available place and the weight constraints on the platform, the first solution could not be suitable. In fact, the place being available by removing the two generators is used for the step-down transformer, but there is no availability for the platform side converter. For the same reasons also the third solution could not be considered. Due to the presence of Fig. 4. Interconnection of an off-shore platform with two operating frequencies, before electrification. Grid (50 Hz) On-shore Each cable section (132/145 kv) approx. 70 km Frequency converter 50/60 Hz Island platform (60 Hz) Fig. 5. Interconnection of an off-shore platform with two operating frequencies and converter on an island due to volume/weight limitations on the platform. VI. DYNAIC OPERATIONS AND DISTURBANCES In a feasible and reliable electrical system the ability of the system to withstand dynamic phenomena, planned (e.g. startup of motors) as well as unplanned ones (e.g. short circuit followed by motor re-acceleration), has to be verified. This is a particularly important issue for off-shore systems. A disturbance may not lead to the black-out or cause damage to the equipment, with financial losses due to the operation interruption as well as to the replacement of components. By HVAC interconnection systems the main issues concerning the dynamic operations and disturbances are related to the short circuit capacity available at the off-shore terminal, and to the dynamic control of the shunt reactors depending on the loading condition of the off-shore system (no-load, half load, and full load conditions require different compensation degrees). Regarding the short circuit capacity at off-shore side, it is worth to mention that dynamic phenomena such as motor start-up or motor reacceleration after fault clearing have to be supported by the HVAC transmission system, i.e. a suitable system design has to consider and fulfill these requirements. If the interconnection is realized by HVDC transmission systems based on IGBT technology, many technical advantages compared to the HVAC solutions are given. From the operational point of view, the HVDC solution provides fast voltage/frequency control, i.e. active and reactive power. In particular, the off-shore side converter control can ensure a G G

4 4 fast reactive power supply up to about ±0.3 pu (referred to the HVDC nominal power) in case of disturbance, sudden load change, and motor start-up. The HVDC voltage control can operate on the AC side in a range of ±10%, which is added to the transformer voltage control (usually also ±10%) via OLTC. The Siemens HVDC PLUS solution can operate as an almost ideal fixed voltage source, provided that the output voltage and injected current are both in the designed operating range. Capabilities such as black start support or fault ridethrough are often required at the off-shore side. Fig. 6 and 7 show the exchanged power at the HVDC shore side and platform side, and the voltage at HVDC platform side and V bus on platform, respectively. Fig. 8 reports an example of single-line diagram for an offshore wind farm with HVDC interconnection to shore. The HVDC power and voltage curves assuming a single-line-toground fault on the wind farm side are shown in Fig. 9. As reported, the phase voltages at the on-shore terminals (converter station 1) are practically not affected by the fault. After fault inception the active power being transmitted by the HVDC is reduced from 0.75 pu to 0.50 pu, while the phase currents in the other un-faulty phases start to increase as a result of the power control. After fault clearing an increase in the active power takes place because the once faulted phase now contributes to the total active power transferred and the converter currents at the off-shore terminal (converter station 2) are in their peak value. The DC voltage in the first part of the transitory increases because more active power is entering the HVDC than the delivered. Similarly after fault clearing, the DC voltage suddenly decreases, anyway being in a range of less than ±2%. Usually the HVDC transmission system can be equipped with active/reactive power control, i.e. frequency/voltage and DC-link voltage/ac voltage control. On the other side, also wind turbines, being usually equipped with converters (DFIGs, synchronous or asynchronous machines interfaced by full converter), usually can provide active/reactive power control, also because of the ride-fault-through properties requested by the TSOs for wind farms. In the case of HVDC interconnection it is therefore necessary to coordinate the HVDC power/voltage control and the wind turbines power/voltage control via master controller, defining a priority table. Power HVDC (p.u.) 1,00 0,80 0,60 0,40 0,20 0,00-0,20-0,40-0, Time (s) P HVDC - Shore Q HVDC - Shore P HVDC - Platform Q HVDC - Platform Fig. 4. Start-up of a big motor on a platform connected via HVDC Power at the HVDC shore side and at the HVDC platform side. Voltage (p.u.) 1,05 1,00 0,95 0,90 0,85 0, Time (s) Voltage HVDC Platform Side Voltage Platform (V bus) Fig. 5. Start-up of a big motor on a platform connected via HVDC Voltage at the HVDC platform side, V and LV bus on platform. PCC platform On-shore Fig. 6. Example single-line diagram of an off-shore wind farm interconnected via HVDC.

5 5 U [pu] U [pu] I [pu] I [pu] P [pu] Q [pu] U dc [pu] U AC [pu] Fig. 7. HVDC voltage and power curves by off-shore wind farm interconnection with single-line-to-ground fault on wind farm side. VII. CONCLUSIONS Their economical importance justifies the increasing number of planned and installed off-shore power systems, for example for oil and gas platforms, and wind farms. The design of such systems, however, has to take into consideration more severe design criteria and more restrictive constraints in order to guarantee feasibility under several operating conditions than for on-shore, strongly connected systems. In this paper the main aspects concerning the interconnection strategies based on HVAC and HVDC technologies have been summarized. These include not only technical issues (basic design, dynamic operations and disturbances), but also important factors such as cost, volume and weight. The paper illustrates, with reference to some practical examples, the most important factors for the determination whether HVAC or HVDC is the best solution.

6 6 VIII. REFERENCES [1] [2] [3] P. Kundur, Power System Stability and Control, cgraw-hill, 1994 J. achowski, J. W. Bialek, and J. R. Bumby. Power System Dynamics and Stability, John Wiley & Sons, 1997 F. Schettler, H. Huang, N. Christl: HVDC Transmission Systems using Voltage-Sourced Converters Design and Applications, IEEE Power Engineering Society Summer eeting, July 2000 IX. BIOGRAPHIES Piergiovanni La Seta (S 03, 07) was born in Reggio Calabria, Italy, in He received the degree in anagement Engineering from University of Calabria, Italy in He has been scientific collaborator at Osnabrück University of Applied Sciences, Germany. He then moved to Technische Universität Dresden, Germany, where he received 2007 the Ph.D. Degree in Electrical Engineering. Since 2006 he is a Consultant for Electrical Networks at Siemens AG, E D SE PTI NC, in Erlangen, Germany. His research interests concern electric machine drives, power converters, HV transmission lines fault studies, and stability studies in industrial and transmission power systems. He is a member of IEEE and VDE in Germany. Edwin Lerch (1953) received his Dipl.-Eng. degrees from the University of Wuppertal/Germany in 1979, where he also completed his PhD (Dr.-Eng.) in electrical engineering in Since 1985 he has been a member of the systems planning department at Siemens in the industrial power system and machine group. He is currently working as a principal expert in the areas of power system stability, dynamics of multimachine systems, control, optimization and identification problems in electrical power systems. Since 1994 he is director of the department group Industrial Systems, Dynamic of Grid and achine. (Siemens AG, Sector Energy, Power Technologies International, Network Analysis and Consulting, Freyeslebenstr. 1, Erlangen/Germany, Phone: , Fax: , edwin.lerch@siemens.com)