Special Feature: AC/DC power transmission
HVDC transmission for large offshore wind farms Onshore and offshore wind farms are a rapidly growing worldwide industry. The development of larger, more efficient turbines is opening up new frontiers in wind energy generation in the form of large offshore wind farms. The use of high-voltage direct current (HVDC) technology can fully realise the potential of these developments, overcoming the major technical challenges facing traditional AC solutions and providing an efficient, economic and reliable solution.
by N. M. Kirby, Lie Xu, Martin Luckett and Werner Siepmann n recent years pressure has been applied to the international community to promote greater environmental responsibility. For the first time, in December 1997 at Kyoto, the industrialised countries agreed to reduce emissions of greenhouse gases. The German government has actively promoted the use of wind energy, as evidenced by the multitude of onshore wind farms, and recently introduced energy tariff guarantees are accelerating the development of the offshore wind farm market. Considering the durations of such projects—i.e. prototype, prototype field tests, pilot plant, field experience with the pilot project and start of the final extension stage—it can be seen that, to gain the maximum benefit from the tariff guarantees, the projects must be implemented as early as possible. With a total investment in the region of €2 billion for a large offshore wind farm, there will be a detailed assessment of the project risks and anticipated profit and payback periods, along with environmental issues. From the technical viewpoint the level of electrical impact of the project on the existing AC network, including voltage fluctuation levels, harmonic distortion and reactive power exchange limits, must also be the subject of detailed study. Project developers will therefore depend on reliable, existing technology such as that offered by conventional thyristor HVDC.
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Wind farm technology Electrical energy has been generated by windmills since the 1980s, but commercial use started in the early 1990s, the major players being in Denmark, USA, The Netherlands, Spain and Germany. Up to about 1MW unit power, most windmills utilised asynchronous generators, fixed speed, direct coupled to the grid, with stall control of the blades. This resulted in instability in the grid voltage due to the fixed speed, driven by the fixed grid frequency, causing variation of output power according to the wind speed. For this reason, and also for overload protection, windmill technology has recently moved from fixed speed to variable speed, and the unit size has been increased to several megawatts.
1 Onshore converter station layout
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Special Feature: AC/DC power transmission
offshore HVDC substation platform
30/145 kV
UW1 (350 MVA)
UW2 (350 MVA)
30/145 kV
HVDC converter substation
F F
F
F
F
F Pole 1 sea cable section
145 kV 50 Hz
F land cable section
integrated return cable
550 MW 400 kV 1375 A
F UW3 (250 MVA)
30/145 kV F F
three-phase two-winding converter transformers
single-phase three-winding converter transformers
F Pole 2 sea cable section
30/145 kV
HFF 380 kV 50 Hz F F F
land cable section
550 MW 400 kV 1375 A
F
HFF
UW4 (250 MVA)
integrated return cable
F
2 Single-line diagram
Windmills with variable speed require a converter between the generator and the grid, which is designed in most cases as an IGBT active front-end converter with voltage source. Two different solutions are available: 5 Fully fed synchronous generator, optionally with permanent magnet excitation—the converter rating is equal to that of the generator, and is connected at the stator windings. 5 Doubly fed asynchronous generator with slip ring rotor—the converter rating is only about 25% of that of generator, and is connected at the rotor windings via the slip rings. The main characteristics of variable speed windmills using converters and pitch control are: 5 The unit is designed for a power factor between 0·9 capacitive and 0·9 inductive, which can be dynamically controlled according to reference values supplied by an energy management control system. This provides a reactive power control range of ±50% of the full load active power, which may be taken into account during cable optimisation and grid voltage control/
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stabilisation. 5 The voltage and current waveforms are almost sinusoidal, requiring no additional filters. 5 The converter limits the output at the rated power. 5 Each windmill uses a transformer for connection to the local grid, so the existing grid voltage level can be adopted. 5 The pitch controller adjusts the active power of the windmill by varying the angle of the blades. The high cost of the foundations and offshore logistics forces wind farm developers to install the largest windmills possible, and blade material technology presently limits the power outputs of a single generator to 5MW. The environment is particularly onerous with indoor electrical equipment supplied to IP54 rating, outdoor up to IP66 in some cases, able to withstand salty atmosphere and ambient temperature between –20° and +50°C. A lifetime of 20 years is expected, and with high maintenance cost requirements, manned maintenance should be kept to the absolute minimum. In Germany there are planned projects POWER ENGINEERING JOURNAL JUNE 2002
Special Feature: AC/DC power transmission (although none is yet approved) for offshore wind farms in the North Sea and the Baltic Sea with capacities between 200 and 1500MW. The distances offshore vary up to about 150km. For offshore wind farms 3600 full load hours per year are expected. To minimise the impact of turbulence the distance between windmills should be about 700m, giving rise to a 500MW wind farm with 100 windmills of 5MW each, occupying an area of 7 × 7 km. All windmills are connected to a local 30kV AC grid in star, chain or combined configuration, and an offshore substation with transformer and switchgear will be the local point of common coupling for the energy transmission to the shore, which may be through AC or HVDC. As will be described later, at low power levels the simplest method may be AC due to the need for auxiliary power supplies in the offshore wind farm (including when the wind is not blowing). The present capacity of a three-core AC submarine cable is 200MW at 145kV, and therefore if a totally AC solution is implemented, the total number of cables will be determined by the overall wind farm power rating. In the case of the German offshore regions all cables would have to cross the German National Park around the North Sea coastline (Wattenmeer); therefore the total number of cables allowed will be restricted. Therefore, to facilitate the approvals process HVDC energy transmission technology should be used, with the maximum benefit being gained from conventional HVDC technology, which will be detailed later in this article.
many advantages over AC : 5 Sending and receiving end frequencies are independent. 5 Transmission distance using DC is not affected by cable charging current. 5 Offshore installation is isolated from mainland disturbances, and vice versa. 5 Power flow fully defined and controllable. 5 Low cable power losses. 5 Higher power transmission capability per cable. Although the HVDC link described is conventional in that it uses thyristor technology, in offshore situations there are many aspects that demand special attention, in order to achieve the necessary performance and reliability, such as provision of auxiliary supplies and a commutating voltage source. Offshore installation The main constraints that must be addressed in any offshore electrical installation include: 5 Limited space—equipment should be as compact as possible, to reduce the overall size and weight. 5 Extremely harsh and variable environment— constant exposure to the salt air, wind and water requires equipment to be either located indoors or in sealed enclosures. 5 Auxiliary supplies—loads are graded into essential and non-essential with some form of UPS, generator, battery etc. or a combination of these, to ensure supply availability
3 Offshore converter station platform
Use of HVDC with wind farms HVDC technology application As wind farms become larger and more distant from shore, the justification for using HVDC to transmit the power to the onshore network becomes easier, particularly at power levels of 500MW or more. The costs of the converter stations, offshore and onshore, are significant in themselves, but when put in the context of the complete project cost, including the cables and the wind turbine generators, they feature less prominently. Existing voltage-sourced converter (VSC) transmission technology cannot offer an economical solution at this power level due to the high cost of the multiple converters and cables that are required. The choice is therefore more logically between the use of AC or conventional HVDC. Conventional HVDC transmission offers POWER ENGINEERING JOURNAL JUNE 2002
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Special Feature: AC/DC power transmission control and monitoring of as much offshore plant as possible, with remote diagnostics.
600 550 500
The control system is designed with the ability to control the offshore grid voltage and frequency, and to handle disturbances in both the sending and receiving end systems. Auxiliary power is a problem for the offshore installation, and essential loads for the 1000MW link are expected to be in the order of 1MW for the converter station and 6MW for the windmills. The loads are categorised according to:
conventional HVDC
450
power, MW
400 350 300 250
VSC or conventional HVDC
200 150 100
AC
50 0 0
50
100
150
200
250
300
350
distance, km
4 Solutions with power range against distance
even when there is no wind. 5 Limited maintenance access—accessibility for maintenance is reduced, equipment is ideally maintenance-free with high reliability. These concerns lead to a set of design criteria for the offshore installation:
5 Single-line diagram of the simulated system (offshore)
synchronous compensator
5 Equipment should be as simple as possible with long maintenance intervals, or preferably no maintenance at all. 5 High reliability, which means that redundancy should be included in critical areas. 5 AC busbar voltage as low as possible to reduce AC harmonic filter and switchgear size. 5 Indoor equipment means that insulation levels and isolation clearances may be reduced. 5 Multi-storey structure to reduce the base area to a manageable size for platform legs and supports. 5 Extensive use of automation, with remote
145 kV AC
DC cable HVDC
wind farm phase-to-ground fault capacitor bank
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5 Uninterruptible—supplied via batteries, including control, protection, monitoring and communication. 5 Other essential loads—AC loads for cooling, heating, navigation lights, excitation, switchgear, emergency lighting. 5 Non-essential—heating, lighting and air conditioning for manned areas, cranes, lifts etc. Normally the auxiliary power is supplied from that generated by the wind farm; however, during conditions of low or no wind the auxiliary power may be taken from several sources: 5 A parallel AC cable from onshore (if installed as part of a trial). 5 Reverse power from onshore through the HVDC link—if necessary supported by a synchronous compensator. 5 From a diesel generator. Onshore installation The onshore HVDC converter station is a conventional installation with thyristor valves, converter transformers and AC filters, and will provide the extra facilities required for remotely controlling the offshore end of the link. A typical converter station layout is shown in Fig. 1. Some of the criteria that apply to the offshore installation also apply to that onshore, especially the need to minimise costs, but also if the substation is located near the shoreline or in a built-up area the environmental and size issues may be significant. Considering the interconnecting cable, the conflict between requirements of larger crosssectional area (CSA) to minimise losses, and smaller CSA to minimise charging current requires a compromise solution when using AC cable. For a DC cable the charging current may be removed from the calculations, so POWER ENGINEERING JOURNAL JUNE 2002
Special Feature: AC/DC power transmission 6 Operation with variation of wind speed
the cable may be optimised on the basis of conductor losses, copper cost and insulation ratings. Example scheme To illustrate these points, the example scheme shown in Fig. 2 is described in the following. The windmills are arranged in clusters of around 16 × 5 MW units (each of which is individually controllable in terms of real and reactive power). These are marshalled on to a common 30kV busbar, and this feeds to 30/145kV step-up transformers. The 145kV busbars are linked and cabled to the offshore converter station platform. The platform is illustrated in Fig. 3. The main points to note in the platform layout are: 5 Converter transformers located outdoors and accessible for maintenance, configured as three-phase, two winding units to minimise the overall size, with no exposed bushings. 5 Thyristor valves are arranged as bi-valves, which gives a good compromise between height and area. 5 Use of a relatively low AC voltage reduces the clearances for AC filters and switchgear, and therefore the overall volume of the indoor filter area Alternative solutions For large, offshore wind farms with power ratings above about 350MW it can be seen that thyristor-based conventional HVDC technology is the optimum solution. However, for smaller power ratings and offshore distances POWER ENGINEERING JOURNAL JUNE 2002
other viable solutions present themselves. AC transmission Present AC technology can be economically used for the lower rated schemes over short distances. Limits of AC cable power ratings over longer distances will probably not be improved enough to allow economical use of this technology on larger wind farms. Connection of wind farms via AC technology does not mitigate voltage and power variation effects due to the wind farms. Individual control of the voltage, power and reactive power of each windmill, combined with an overall energy management system to coordinate them, are essential. Voltage-sourced converters Voltage-sourced converter (VSC) based HVDC technology (VSC transmission) is a fairly new development that may be suitable for the lower power rated schemes and over smaller distances. VSC technology allows flexibility in reactive power control and alleviates some of the offshore low-voltage conditions. The reliability of this technology is yet to be fully proved by significant in-service time and can produce higher overall losses than conventional HVDC technology due to the nature of the switching devices. However, as this technology becomes more widely used for conventional applications and the maximum power rating of the technology improves, the high reliability and low loss requirements for wind farm projects should cease to be a barrier to this approach. 139
Special Feature: AC/DC power transmission 7 Single-phase to ground fault on the rectifier side (offshore)
Technology combinations Combinations of technology may produce useful hybrid solutions. For example, use of VSC offshore and conventional HVDC onshore may produce an efficient and economic solution. Conventional HVDC may be combined with a static var compensator (SVC) or STATCOM to provide effective var and voltage control where this is deemed necessary by AC network requirements. Comparison An estimated comparison of the different technologies is as shown in Fig. 4. The figures in the graph assume the use of a single threeconductor cable for an AC solution, and a single two-conductor cable for a DC solution. System studies and AC network interactions A simplified power circuit model was created,
as shown in Fig. 5, consisting of a 1000MW wind farm, a 100Mvar synchronous compensator, 850Mvar capacitor banks and a 12-pulse HVDC transmission system rated at 1000MW. The 1000MW wind farm model contains 200 induction generators with each rated at 5MW. The operational power factor for the induction generator is assumed to be 0·9 at rated power. The synchronous compensator controls the AC voltage on the common bus by providing or absorbing reactive power. The rectifier of the HVDC converter controls the DC current, with a frequency error measurement on the rectifier side generating the current order. The inverter uses gamma control to set the direct voltage. The relatively large synchronous compensator is used because there is no reactive power control by the HVDC converter for this preliminary study and no switched capacitor banks. In reality, with switched capacitor banks
8 Single-phase to ground fault on the inverter side (onshore)
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Special Feature: AC/DC power transmission and the reactive power control capability of the HVDC converter, a relatively small compensator can be used. It may also be possible to remove the synchronous compensator altogether through the use of a single AC cable suitably rated in parallel with the DC circuit. Furthermore, the use of doubly fed induction generators would remove the need for the synchronous compensator for providing reactive power support to the network. System studies using these technologies are currently being carried out in ALSTOM. Firstly, steady-state and variations of wind speed are studied and the results are shown in Fig. 6. From this it can be seen that the wind farms generate 0·65 pu of rated power and absorb approximately 0·35 pu reactive power. The synchronous compensator provides 0·67 pu (67Mvar) of reactive power to the network. When the wind speed drops at the time of 2s, the active power from the wind farm, and active power to the inverter AC side reduce accordingly. As the reactive power taken by the HVDC converter reduces, the synchronous compensator changes from generating to absorbing reactive power. Studies at fault conditions have also been carried out. Figs. 7 and 8 show the simulation results when single phase to ground faults were applied directly on the rectifier and inverter AC buses, respectively. As can be seen when the fault is applied on the rectifier side, because of the drop of the AC voltage, the synchronous compensator, which controls the AC voltage, provides reactive power support to the network. However, because of the DC connection, there is only a small impact on the AC network at the inverter side. After the fault is cleared the offshore AC system returns to normal in about 2s. When the fault is applied on the inverter side, the DC voltage reduces to zero and therefore, the transmitted active power goes down to zero. Consequently, the frequency on the rectifier increases rapidly. After the fault is cleared the offshore AC system returns to normal in about 1·5s. Conclusions Increasing use is being made of wind power generation through political and public pressure on a worldwide basis, and the limited availability of onshore sites is driving wind farms offshore. This inevitably means that the power generated has to be transmitted over longer distances in order to make a connection POWER ENGINEERING JOURNAL JUNE 2002
to an AC network for onward transmission and distribution to customers. The lower power and shorter distance schemes may be most economically provided through use of AC cable interconnections, and at the other extremes of high power and long distance there is little alternative but to use conventional HVDC. In between these extremes there is no simple rule to apply, and the result for each particular scheme will be different based on its own merits, taking into account the issues described in this article. In general, however, we expect that the well established and proven technology of conventional HVDC will be the optimal solution in cases where: 5 The distance offshore is greater than approximately 100km. 5 Connection into the AC network is at a weak point. 5 Wind farm size is greater than approximately 350MW and constraints are imposed on the quantity of cables laid. 5 Where multiple or long AC cables are necessary, equivalent transmission through a DC cable and converter equipment may be a more economical alternative. 5 To reach a suitable AC network connection point requires a significant length AC transmission line onshore. 5 AC network studies and simulations under different fault conditions show unstable behaviour, and HVDC control of power transmission may assist in recovery, limit the effect, or even correct the instability. In order to operate most efficiently the individual windmills should each have their own power, reactive power and voltage control, with an overall co-ordinating energy management system to control overall energy transmission. References 1 Kyoto Protocol to the United Nations Framework Convention on Climate Change, December 1997 2 BICC Cables: ‘Electric cables handbook’ (Blackwell Science, London, 1977, 3rd edn.) 3 JONES, L. E., and ACKERMANN, T.: ‘Transmission networks for offshore wind farms: special report’, European Wind Energy Conference, Denmark, July 2001 © IEE: 2002 Neil Kirby, Lie Xu and Martin Luckett are with ALSTOM T&D Power Electronic Systems, Stafford, UK. Werner Siepmann is with ALSTOM Power Conversion, Berlin, Germany. Contact:
[email protected]
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