Notes Topic 13

Notes Topic 13 - 1 2 3 4 5 6 7 8 9 10 11 12 13.4.2...

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Unformatted text preview: 1 2 3 4 5 6 7 8 9 10 11 12 13.4.2 Statutory approval work ............................................................................................................... 20 13.4.3 Major problems and failures......................................................................................................... 20 13.4.4 Wind farm life and decommissioning........................................................................................... 22 INTRODUCTION WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE WIND TURBINE MECHANICAL DESIGN GENERATORS GRID CONNECTION AND POWER CONDITIONING OPERATION CONTROL OF WIND TURBINES CONTROL FOR SAFETY INSTALLING SMALL TURBINES WIND FARM DESIGN 1 Introduction.................................................................................................................................................. 1 2 Wind Resources ........................................................................................................................................... 1 3 Wind Turbine Components and Concepts ................................................................................................... 1 4 Wind Turbine Aerodynamics....................................................................................................................... 1 5 Wind Turbine Blade Design and Blade Manufacture .................................................................................. 1 6 Wind Turbine Mechanical Design ............................................................................................................... 1 7 Generators .................................................................................................................................................... 1 8 Grid Connection and Power Conditioning................................................................................................... 1 9 Operation Control of Wind Turbines ........................................................................................................... 1 10 Control for Safety ..................................................................................................................................... 1 11 Installing Small Turbines.......................................................................................................................... 1 12 Wind Farm Design.................................................................................................................................... 1 13 WIND FARM INSTALLATION AND OPERATION............................................................................ 2 13.1 Wind Farm Electrical Design ................................................................................................................ 2 13.1.1 Circuit Components ........................................................................................................................ 3 13.1.1.1 LV cables ................................................................................................................................. 3 13.1.1.2 Step up Transformers ............................................................................................................... 3 13.1.2 Interconnection of one or more step up transformers using an MV cable network. ...................... 5 13.1.3 Grid Access..................................................................................................................................... 8 13.1.3.1 Effect on Network Voltages..................................................................................................... 8 13.1.3.2 Use of Multiple Wind Turbines to Minimise Voltage Fluctuations and Voltage Flicker........ 8 13.1.3.3 Selecting a Point of Connection to Minimise Voltage Variations ........................................... 9 13.1.3.4 Power Factor Correction .......................................................................................................... 9 13.1.3.5 Operation at Fixed Power Factor ............................................................................................. 9 13.1.3.6 Wind Turbine Voltage Control .............................................................................................. 10 13.1.3.7 Under voltage Ride Through.................................................................................................. 10 13.2 Wind Farm Civil Engineering ............................................................................................................. 11 13.2.1 Foundation design......................................................................................................................... 11 13.2.2 Road and Hard Stand design......................................................................................................... 12 13.3 Installation of Medium to Large Wind Turbines................................................................................. 13 13.3.1 Transportation to site .................................................................................................................... 13 13.3.2 Heavy haulage and craneage ........................................................................................................ 15 13.3.3 Turbine Commissioning ............................................................................................................... 17 13.4 Operating a wind farm......................................................................................................................... 19 13.4.1 General operations and maintenance ............................................................................................ 19 13-1 Figure 13-1 An example of a connection of a single WTG to a shared MVO distribution line......................... 7 Figure 13-2 Arrangement showing wind turbine hard-stand being used during turbine erection.................... 13 Figure 13-3 Wind turbine ocean freight for the Codrington wind farm in Victoria......................................... 14 Figure 13-4 Blade tip damaged during ocean freight. ...................................................................................... 14 Figure 13-5 Turbine component convoy en route to the Albany wind farm site. ............................................ 15 Figure 13-6 Cranes being used during wind turbine erection at the Codrington wind farm, Victoria............. 16 Figure 13-7 A helicopter being used to install small wind turbines in the United States................................. 17 Figure 13-8 Technicians in the hub of a 1.8MW wind turbine at Albany, Western Australia, during commissioning tests ................................................................................................................................... 18 Figure 13-9 Blade on fire at the Esperance Nine Mile Beach wind farm, Western Australia, following a bad lightning strike in 1999. ............................................................................................................................. 21 Figure 13-10 Result of a blade structural failure on a 1.5MW wind turbine in Germany in 2000. ................. 21 Figure 13-11 Decommissioning in January 2002 of the first wind farm built in Australia at Salmon Beach, Esperance, Western Australia. ................................................................................................................... 22 13 WIND FARM INSTALLATION AND OPERATION The highest ever recorded wind speed on mainland Australia was a gust of 267 km/hr (75 m/s), which occurred at Exmouth in Western Australia in March 1999 in the middle of cyclone Vance. At such wind speeds there is an enormous amount of force in the wind – enough to push trucks sideways along roads and to bend steel telegraph poles flat to the ground. Wind turbines operate in such an environment and it is only a matter of time before they experience extreme wind speed conditions, hopefully not as high. Similarly, wind farms can be built in places difficult to access and the road design required to get wind turbine components in can be quite complex. Installing a wind farm is not a simple task and this Topic also provides an overview of issues that have arisen in the actual installation exercise at Australian wind farms to date. This looks particularly at the reality of transporting, unloading and lifting some of the largest and most fragile engineering components in the world. Once the installation hype has died down and the wind farm is in and operating, it is then that the hard work begins in terms of operating the machines for their proposed lifetime. This is a very important aspect of the wind farm as it is the revenue you create which ensures the financial feasibility of the project. As has been said, wind turbines are subject to hostile environments and their operation and maintenance needs to be planned and thorough. 13.1 Wind Farm Electrical Design In the following discussions, low voltages (LV) will refer to three phase voltages less than 1kV. Medium voltages (MV) range up to 33kV. High voltages (HV) refer to voltages above 33kV. The three phase voltage levels commonly used in electrical networks within Australia are LV: 415V, MV: 3.3kV, 6.6kV, 11kV, 22kV, 33kV and HV: 66kV, 110kV, 132kV, 220kV, 275kV, 330kV and 500kV. 13-2 13.1.1 Circuit Components The main components that constitute the electrical interconnection of wind turbines to an electrical power system are low and medium voltage underground cables, transformers, switchgear and metering and often MV or HV overhead power lines. or, alternatively: 13.1.1.1 Cable voltage drop expressed in volts is reduced by one to two orders of magnitude for a given power level. Expressed as a percentage, cable voltage drop is dramatically reduced by two to four orders of magnitude for a given power level. LV cables Typically WTGs produce a low voltage three phase AC output. European WTGs such as Vestas produce electricity at an European standard of 690Vac. The low voltage, high current ratings of insulated gate bipolar transistors (IGBTs) requires that WTGs employing IGBT inverters operate at even lower voltages eg Enercon WTGs produce 400Vac at the inverter terminals. Where WTGs have a LV output, they are invariably connected by underground LV cables through to a stepup transformer. When the WTG has a three-phase output, there will be one or a number of cables used on each phase to provide the necessary high current rating. The cross-sectional area of the conductor (aluminium or copper) and the conductor material primarily determines the cable current rating. (Note: Copper is rarely used on overhead power lines because of its greater weight and cost, but is often used in underground cables where weight is not a significant factor and the higher cost of copper is partly offset by the lower insulation costs to encase the thinner conductor (copper has a higher conductivity than aluminium). To determine the extent of the de-rating of a particular cable type in situ, the soil thermal conductivity; the depth and spacing of the underground cables must all be taken into consideration. Depending on the wind resource and any extent of power limiting, the cable capacity will either be based on a cyclic or continuous rating. The three phase AC output of a WTG usually includes a neutral that is earthed to the WTG earthing system and connected via cable to a star point on the LV winding of the adjacent step-up transformer. For a balanced output of the WTG, the neutral current will be quite low. The neutral cable need not be so highly rated as the phase cables. However, it will need to be able to carry fault currents and lightning currents where earthing system of multiple WTGs are connected together via the neutral cable to obtain a sufficiently low earthing resistance. The increasing power ratings of grid connected WTGs necessitates that LV cables connecting a WTG to a LV/MV step-up transformer be made as short as practicable. Whereas several WTGs up to a 250kW rating may sometimes be connected to a shared step-up transformer by short LV cable runs (eg Ten Mile Lagoon Wind Farm near Esperance has three 225kV Vestas wind turbines connected via 690V cables to each 690/33kV step-up transformer), higher powered WTGs installed with greater physical separation need their own dedicated step-up transformer installed adjacent to or even inside the wind turbine tower. 13.1.1.2 Step up Transformers To efficiently transmit the power from each WTG or WTG cluster through to the point of connection of the wind farm to the grid, LV to MV step-up transformers are employed to raise the voltage level one to two orders of magnitude. This has effect of reducing MV cable currents by one to two orders of magnitude for a given power level. This follows from the basic equation P = V I, which when applied to a three-phase system AC becomes: Equation 13-1 P3− phase = 3 × VL − N × I L 13-3 Equation 13-2 P3− phase = 3 × VL − L × I L This follows from the basic equation V = I R, which when applied to a three-phase AC system becomes: Equation 13-3 V3− phase = I L × Z cable where Z cable is the single phase equivalent impedance of the cable and Equation 13-4 Z cable = Rcable + jX cable where R cable and X cable are the real and reactive components of the single phase equivalent cable impedance. Like percentage voltage drop, cable power losses are also dramatically reduced by two to four orders of magnitude, when the voltage level is raised one to two orders of magnitude by the step-up transformers. This follows from the basic equation P = I 2 R, which when applied to a three-phase AC system becomes: Equation 13-5 P3− phase cable 2 losses = 3 × I L × Rcable where R cable is defined above and is the actual physical resistance of each cable of the three phase cable. Step-up transformers are usually of a similar type to that used to connect LV (415V) consumer loads to the MV network. The MV side of the transformer has a delta winding which connects to the three MV phases without a neutral connection. The LV side has a star winding that provides a neutral connection in addition to the three LV phases. Inverter type wind turbines typically have a neutral connection, induction type wind turbines may or may not, however in both cases, earthing of the neutral keeps the phase voltages balanced with respect to earth. Wind turbine transformers often have a lower load factor (low average load with respect to its kVA rating) than equivalent transformers supplying LV loads. As a result, the I2R electrical losses associated with primary and secondary currents flowing through the resistive copper or aluminium windings (copper losses) are relatively smaller than the fixed energisation losses (iron core magnetisation and eddy current losses). The fixed losses typically increase and the I2R losses typically decrease (for a given value of I) as the size (kVA rating) of the transformer is increased. Unlike distribution transformers, there is no need to hold 13-4 transformer capacity in reserve for load increases. The maximum kVA output of a wind turbine is known and the step-up transformer only carries this load when there is a high level of passive air cooling. Thus to minimise the capital and operating costs of step-up transformers, they should not be unnecessarily oversized. The transformer design should be biased towards obtaining low fixed losses versus low I2R losses, if the load factor is expected to be low. Transformer designer/suppliers can assist with this optimisation, if told what will be the loading on the step-up transformers. The step-up transformer is normally equipped with a tap changer on the MV winding which can only be adjusted when the transformer is off-line, commonly called an off-line tap changer. The tap changer provides a means of adjusting the LV voltage so that it stays well within upper and lower limits over the full output range of the WTG and normal voltage variations on the MV system. MV systems typically are operated slightly above nominal voltage (0% - 5 % above nominal). This normally requires that the LV voltage is tapped down so that it close to nominal volts. Depending on power output of the WTG and the power factor, the LV may increase or decrease. (See Section 13.1.3.4). This may lead to a further adjustment of the tap position to centralise the LV voltage variations around nominal volts for all normal operating conditions. The WTG step-up transformers within a wind farm can be interconnected via cable in a string or radial pattern or a combination. The cable pattern is selected to minimise cable lengths; to make best use of access roads as cable routes (to minimise costs and environmental impact) and together with cable size selection, to minimise cable electrical losses. Several different cable sizes may be employed within a wind farm depending on the number of WTGs upstream. At the point of connection (point of common coupling) of the wind farm with the MV distribution network, one or more cables are connected via fused isolators to an overhead MV power line. If the MV line has been constructed or made available as a dedicated line for the wind farm, the point of connection is normally considered as being at the substation or power station busbar, to which the dedicated line is connected. At this point of connection, there is a circuit breaker, which is set to trip for over-currents and earth faults. Wind farms require a dedicated line, if either high wind farm reliability is required or because wind farm induced voltage variations or voltage flicker on the line fall outside the accepted standard for customer supply. Figure 13-1 shows the connection of a single WTG to a shared 22kV overhead distribution line. Separate MVO fuses or circuit breakers with earth switches are required on the MVO side of each step-up transformer, if more than one WT is connected to the underground cable. The transformers and LV cables are protected from overcurrent by typically having fuses or circuit breakers on each LV phase connection. In some cases, where there is more than one LV cable per phase, each cable may be separately fused. Temporary removal of these LV fuses provides a convenient means of electrically isolating the WTG, when necessary, for certain maintenance and repair procedures. Fuses or circuit breakers (in the case of higher rated wind turbines) are also employed on each phase on the MV side of the transformer. These protect the transformer from overcurrent flowing from the WTG through the transformer into a MV cable or grid fault. They also protect the grid from transformer faults. If LV fuses are not included, the MV fuses or circuit breakers protect the transformer from overcurrrent and provide means of isolating the transformer when required and a less convenient means of isolating the LV cables and WTG. MV or HV authorised switching operators must be mobilised to perform this function. 13.1.2 Interconnection of one or more step up transformers using an MV cable network. The choice of voltage level for the MV underground cable interconnection of all the step-up transformers depends on a number of factors. Firstly, the selected MV should be one of the preferred MV levels used in Australia eg 3.3kV, 6.6kV, 11kV, 22kV or 33kV. The local MV level is often the best choice, especially if the wind farm is to be cable connected to an existing MV distribution line or MV substation busbar. The differing costs of MV switchgear and cables and/or the power rating of the wind farm may lead to the selection of a different MV than that used on the local distribution network to which it is to be connected. For example, the cost of 33kV transformers, switchgear and cables is higher than for 11kV or 22kV equivalents, which are in more common usage. It may therefore be more economical to use 11kV or 22kV to interconnect all the WTGs and then interface with the local 33kV network via an additional step-up autotransformer. In an autotransformer, the primary and secondary windings share a common winding section and are therefore not electrically isolated. They are smaller and cheaper than an equivalently rated standard transformer. These benefits diminish as the voltage ratio increases. 13-5 13-6 13.1.3 Grid Access Figure 13-1 An example of a connection of a single WTG to a shared MVO distribution line Electricity utilities that own/operate the interconnected grid to which a wind farm is to be connected are interested in how the wind farm will affect to quality and reliability of supply on the grid. The wind farm designer will need to consider the following before and during negotiations for access to connect the wind farm to a MV or HV network. 13.1.3.1 Effect on Network Voltages The short-term power fluctuations associated with wind speed turbulence and in some cases with blades passing the WT tower can cause voltage flicker at the point of connection and nearby nodes (a general term for an electrical point on the network). Voltage flicker is a form of voltage variation, where the rate of change of voltage and the magnitude of the voltage change are sufficiently high to cause observable light intensity flicker from AC powered lighting. Slower real power variations from the wind farm flowing into the grid at the point of connection (or point of common coupling) may not cause observable voltage flicker, but will cause voltage fluctuations at that node and nearby nodes. The magnitude of the any wind farm induced voltage fluctuations at the point of connection depends on the network impedance at that node. The network impedance at a network node is the equivalent impedance between that node and the equivalent voltage source representing all the generators operating on the network. Equation 13-6 Z network impedance = Rnetwork resis tan ce + jX network reac tan ce The magnitude of voltage variations induced by wind farm real power fluctuations is much more sensitive to the magnitude of the resistive component R of the network impedance than to the magnitude of the reactive component X of the network impedance. The magnitude of voltage variations induced by wind farm reactive power fluctuations is much more sensitive to the magnitude of the reactive component X of the network impedance than to the magnitude of the resistive component R of the network impedance. Unacceptably high voltage fluctuations may result, if a high capacity wind farm is connected at a remote node with a high network impedance and where the resistive component R is a significant component of the impedance. (In this situation the network impedance is commonly referred to as having a low X/R ratio.) 13.1.3.2 Use of Multiple Wind Turbines to Minimise Voltage Fluctuations and Voltage Flicker Short-term power fluctuations are reduced as the number of well-spaced WTGs to supply a given power is increased. The physical spacing between WTGs leads to a very low correlation in the fluctuating component of the wind speeds experienced by all the WTGs within the wind farm. The power fluctuations from a wind (Source: C. Carter, Western Power) 13-7 13-8 farm with maximum output Pmax and comprising N WTGs has been found to be linearly dependent on the factor (Reference: 5): increasing real power is offset by the voltage decrease due to proportional increases in reactive power absorption. Equation 13-7 The optimum power factor will be close to unity (eg 0.98 pf absorbing) where the impedance of the point of wind farm connection has a high X/R ratio. As the X/R ratio reduces the optimum power factor becomes more absorbing (moves towards 0.9 pf absorbing). σP ∝ Pmax N Voltage fluctuations are approximately proportional to power fluctuations and so are similarly reduced by this factor. The use of multiple WTGs can lead to a significant reduction in short-term voltage fluctuations or voltage flicker. The use of multiple WTGs does not reduce the longer term voltage variations cause by a change in the wind farm’s power output from zero to rated power or vice versa (or some lesser swing). 13.1.3.3 Selecting a Point of Connection to Minimise Voltage Variations Normally the network impedance declines as the network operating voltage increases. For example, a connection at 132kV will lead to lower voltage fluctuations than connection to a MV network supplied from the same 132kV network via step-down transformer/s. Also, the voltage on a MV network is normally regulated by on-line tap changers, fitted to HV/MV step-down substation transformers, to directly supply customer MV loads (and LV loads via fixed-tap MV/LV distribution transformers). Thus allowable wind farm induced voltage variations at a MV point of connection will be less than where the wind farm is connected at the HV level. Connection at HV is normally much costlier, and may involve constructing longer HV overhead lines to the closest HV connection point. Just as with the size of consumer loads, the power output capacity and dynamic behaviour of the wind farm will lead to a technically preferred option – HV or MV connection. 13.1.3.4 Power Factor Correction Use switchable capacitors on induction type wind turbines to partially offset the reactive power absorbed by the induction generators. The power factor is thereby increased from around 0.8pf absorbing to be in the range from 0.9pf to 0.98pf absorbing. One or several capacitors are switched in and out at specified power output levels (There would be some hysteresis to avoid excessive capacitor switching.) The power factor of each induction type WTG fitted with capacitors will be closer to unity but will still vary as the real power changes. The resulting power factor of the output from a wind farm comprising many such units will also be closer to unit but vary much less due to the diversity of individual WTG power output. 13.1.3.5 Operation at Fixed Power Factor An inverter connected WTG usually has the ability to produce output at a specified fixed power factor within a defined range around unity. To offset the voltage rise as real power output increases, the power factor can be set to a negative value close to unity so that an increasing quantity of reactive power is absorbed as real power increases. At the optimum power factor for minimum voltage disturbance, the voltage rise due to 13-9 Fixed power factor operation where the power factor has been selected to minimise voltage variations can be thought of as simple open loop controller of voltage. As a consequence of its operation, the WTG cannot provide any voltage boost or stabilise externally induced voltage variations. Fixed power factor operation may not be an acceptable solution in circumstances where the wind farm is also expected to also contribute to the reactive power requirements of the grid (like synchronous generators are required to do). There will be situations where system operating configurations change to the extent that an optimal power factor for one system configuration (eg two transformers in service at a substation to which wind farm is connected) is not sufficiently optimal for a different normal configuration (eg only one transformer in service). 13.1.3.6 Wind Turbine Voltage Control In circumstances where fixed power factor operation is not acceptable for voltage control or boost, an active form of voltage control may be required. This can either be achieved by adding additional plant such as a static VAR compensator (SVC), which just controls the deliver and absorption of reactive power to stabilise voltages by switching capacitors in and out (course control) and by thyristor switching of reactors at varying angles to the system voltage waveform to provide fine reactive (and hence voltage) control. Inverter connected WTGs now provide the opportunity for voltage control by actively changing the power factor of the power output of the inverter/s. This has been long standing feature available with inverters used on stand alone power systems. WTG manufacturers that use inverters are currently working on providing this type of voltage control with their WTGs. Such as scheme has been installed on the Albany Wind Farm. The ultimate aim at Albany, is for the inverters to stay on-line and provide reactive power support even when there is no wind (no real power output). Unlike fixed power factor operation, active voltage control also controls externally induced voltage variations (eg caused by load changes, line trips etc) and can provide voltage boost, if and when required. 13.1.3.7 Under voltage Ride Through One of the more challenging technical requirements demanded by grid owner/operators is that major sources of generation capacity do not readily disconnect because of a system fault induced temporary under-voltage event. Loss of generation capacity due to a temporary under-voltage event may lead to an under-frequency event, where there is inadequate on-line generation to supply the system load (hence the overloaded generators start to slow down). The original fault that caused the under-voltage may be cleared by tripping a faulted line, for example, without necessarily losing load. In recent years, it has become a requirement of WTGs in Germany and other major European users that they stay connected during voltage depressions down to near zero volts (German utility ‘EON GmBh’ sets the 13-10 limit at 15% volts to stay on-line). The WTGs must stay connected long enough to allow fault detection and clearance via circuit breaker tripping. Clearance times vary according to whether the fault occurs at the HV, MV or LV level. At the HV level, faults produce wide-scale severely depressed voltages, but the fault clearance times on these important systems are short – typically less than 200 milliseconds. At the MV level, faults do not cause such widespread and severe voltage depressions (apart from the near vicinity of the fault) and fault clearance times are longer - up to more than one second. LV faults do not cause wide-scale low voltages; they just lead to a trip of the relevant LV circuit breaker or fuse. To align with these protection schemes, large WTGs and other generating plant are not required to stay connected as long during severe voltage depressions as they are for mild voltage depressions. The ‘EON’ requirement is to stay connected for voltages between 110% and 80% of nominal and disconnect only after three seconds, if voltage falls just below 80% and then linearly reduce the ride through time requirement down to 150 milliseconds at 15% of nominal voltages. EON does not require ride through, if voltages fall below 15% of nominal. During the ride through period, the WTG may be in pause mode (no power injection) or may still be injecting reduced at reduced power (due to depressed WTG terminal voltages). Following the fault, the WTG must re-establish pre-fault power levels within 200 milliseconds (changes in wind speed not withstanding). There is no standardisation on under-voltage ride through requirements within Australia. The relevant owner/operator of the grid to which a wind farm is to be connected will advise on their current technical requirements for under-voltage ride through, which many utilities are reviewing as the more wind farms are established in Australia. The technical requirements of protection schemes and under-voltage ride through have been developed to fit the expected behaviour of the traditional form of generation on large grids – synchronous generators. There is scope for discussion with grid owner/operators concerning technical variations to the under-voltage ride through requirements taking into account relevant requirements for the particular wind farm project and perhaps the characteristics of the WTG type. Induction or asynchronous generator type wind turbines can tolerate depressed voltages for short periods without damage (like synchronous generators). The problem during low voltages for induction wind turbines is the risk of over-speed as the real power output is reduced (due to the under-voltage). The WTG may trip on over-speed before the fault is cleared and voltages are restored. Fast acting blade pitching can control the rotor speed, but de-powers the WTG and thus extends the time to restore pre-fault power levels following the fault clearance. Inverter connected WTGs can stay connected during depressed voltages if the active devices (usually IGBTs) have voltage and current ratings and are protected by current limiting or fold back protection. If the inverter has the ability to either continue to supply a limited output current with a depressed terminal voltage or to cease switching the active devices but stay on-line (circuit breaker or disconnector on output remains closed), then maintaining rotor speed and power becomes the critical issue. Unlike directly connected induction generator type WTGs, the use of a variable AC-DC-fixed AC converter system on variable speed inverter connected WTGs provides the option of maintaining load on the rotor generator during the depressed terminal voltages by switching in a dump load onto the still healthy DC bus. At some point in the wind farm design it will be necessary to examine what is below the ground at the wind farm site. Sometimes it is not possible to build wind turbines because the ground structure is so poor that the foundation needs to be very large. Typically foundations are about 8% of a wind farm’s capital cost and if this size or complexity of the foundation rises the project can quickly become un-financial. To examine the ground structure it is necessary to undertake a geotechnical investigation. The details of conducting a geotechnical investigation were covered in Topic 6 along with many aspects relating to foundation design 13.2.2 Road and Hard Stand design Another task in the civil engineering design of a wind farm is to provide roads for the transport of wind turbine components and an area adjacent to each turbine location for construction purposes. This latter area is known as the turbine “hard-stand”, named as it usually consists of compacted or hard gravel or similar material which gives it a suitable bearing pressure for heavy transports and craneage. Figure 13-2 shows a typical hard-stand being used during wind turbine construction. The design of roads for wind farms is targeted at allowing adequate space and corner radiuses for transport movements, and providing adequate bearing pressures and minimal slopes so that heavy haulage equipment can traverse the site in all weather conditions. Typically roads built for this purpose in Australia are compacted gravel or limestone. Roads must also be designed with the environmental conditions of the project’s approval in mind. This means that drainage and cross fall must be adequate so that water erosion is prevented and if vegetation removal is required then this is managed appropriately. Often the road design requires attention in terms of visual amenity, and this can mean that the road alignment has to hug or follow land contours so that it is not entirely visible from certain view-sheds. If the road is to be built in a particularly environmentally sensitive area then sometimes a temporary facility using geotechnical matting or similar can be used. This has the advantage of being easily removed at project completion although some form of access to the turbines will still be needed for maintenance. Hard-stand designs vary enormously between projects but, in essence, a space of adequate resilience is required so that trucks and cranes can move about freely as necessary for construction. This prevents then from becoming bogged and provides a working space, and normally the hard-stand is reasonably flat to prevent loads on trucks becoming unstable. Sometimes crawler type cranes are used which move backwards and forwards on their tracks during construction and this means that the hard-stand has to be very level. A hard stand also has to be aligned such that if the rotor is put together on the ground and lifted as shown in Figure 13-2 that there is enough space around the area for the blades. Enercon GmBh (Germany), which supplied the twelve 1800kW variable speed WTGs for the Albany Wind Farm has developed such scheme to achieve the necessary under-voltage ride through performance. 13.2 Wind Farm Civil Engineering 13.2.1 Foundation design 13-11 13-12 Figure 13-2 Arrangement showing wind turbine hard-stand being used during turbine erection Figure 13-3 Wind turbine ocean freight for the Codrington wind farm in Victoria. (Source: Photo courtesy of AN WindEnergy) Figure 13-4 Blade tip damaged during ocean freight. (Source: Photo courtesy of Western Power) 13.3 Installation of Medium to Large Wind Turbines 13.3.1 Transportation to site Wind turbines are fragile and big and because of this their transportation to site requires careful planning. This is particularly the case with larger turbines for, as the machines get bigger, the problems associated with transportation get worse. In parts of Europe for example it is particularly difficult to transport blades and nacelles as bridges and fly-overs on freeways are not high enough to allow passage and the alternative routes are too convoluted. For this reason, many large turbines transported in Europe are done so in many smaller pieces and re-assembled on site. Turbine components to date have nearly all been imported into Australia. This has and will not always be the case – for example, most turbine towers are now manufactured in Australia – but it is possible that some degree of ocean freight for some of the equipment will be required. Fortunately, most wind turbine suppliers are very experienced with these matters and have special cradles and shipping containers specifically for wind turbine components. Figure 13-3 shows turbine components for the Codrington wind farm in Victoria at dock. 13-13 (Source: Photo courtesy of Western Power) Despite best intentions, transporting components by ocean freight around the world can lead to component damage and the wind farm developer must ensure adequate checks are made of equipment as they land in Australia. Typically it is the responsibility of the wind turbine supplier to rectify any damage. Figure 13-4 shows a blade tip damaged during ocean freight when a sea container shifted in heavy seas. 13-14 Australia has a relatively good road system although many rural roads were never designed for the transportation of 40m blades and 50 tonne nacelles. Figure 13-5 shows a convoy of nacelles passing along a small rural road in Albany, Western Australia. When undertaking such transport the following issues need to be considered: • • • • • Bearing pressures of roads and bridges crossed, Turning radiuses for trucks, especially with blades, Height of electrical transmissions lines, Police convoy assistance, and Public safety and disturbance. For the transportation movements shown in Figure 13-5 it was necessary to advertise in the local press a week before and to close down the road for about an hour. Fifty electrical cables also had to be raised and some domestic electricity supplies were disrupted, and numerous signs along the routes had to be removed to allow for the blade swing at corners. Such things can cause major disruption to the public and timing is critical so that this disturbance is kept to a minimum. transportation into the (sometimes complex) topography of a wind farm site, often multi-axle and height adjustable bogies are needed. Such sophisticated transport is costly and the wind farm designer must look at whether more should be spent on the road system to lower the transportation costs or vice versa. Cranes are an essential part of wind farm construction and the selection of the correct crane type is usually the domain of the wind turbine supplier. For very large turbines the crane can be a considerable size, as for example shown in Figure 13-2. Usually there are two types of cranes available; a crawler type as shown in Figure 13-2or a hydraulic type as used at the Codrington wind farm in Victoria in Figure 13-6 (the large crane in the Figure). The primary difference is that a crawler crane is designed specifically to move short distances with its jib fully extended, while the hydraulic crane relies on stabilising feet when the jib is up and hence cannot move without lowering the jib – however, it can take up to three days to disassemble and move a crawler crane while only a few hours for the hydraulic type. The choice is largely dictated by the costs involved. Figure 13-6 Cranes being used during wind turbine erection at the Codrington wind farm, Victoria. Figure 13-5 Turbine component convoy en route to the Albany wind farm site. (Source: Photo courtesy of AN WindEnergy) (Source: Photo courtesy of Western Power) Every turbine type has slightly different erection characteristics and it is beyond the scope of this Topic to even summarise these. Variations on the use of cranes have been tried, for example, building wind turbines using helicopters (see Figure 13-7) and even using the wind turbine’s tower as the cranes platform (known as a self erecting wind turbine). Few of these have proved beneficial and the construction method relying on a crane as shown in the proceeding figures has proved the most reliable and economic. 13.3.2 Heavy haulage and craneage Large trucks are needed for wind turbine construction and their size and number will depend on the wind turbine being used. For MW sized turbines loads of up to 50 tonne are not uncommon and to accomplish 13-15 13-16 their main control centre, and there are strict rules governing how and when this takes place. This is to ensure that such energisation does not put at risk electrical workers in the area and, as there is a considerable amount of energy involved energising transformers for a large wind farm, that it does not adversely effect the local electrical system’s stability. Figure 13-7 A helicopter being used to install small wind turbines in the United States Once the turbines have an energised supply there is usually a step-by-step turbine commissioning that is undertaken by technicians from the wind turbine supplier. This is usually overseen by the wind farm developer’s engineers and can take up to three days for a large turbine. While different for each turbine type the normal procedure in commissioning of the turbine sees the turbine’s circuit breakers closed and the electrical control of the turbine energised first. Once checks of this have been made the turbine can be started. Typically the very first test performed is to monitor the structures vibrations as the rotor speeds up and at this point, occasionally, blade out-of-balance can lead to problems. With the rotor operational the very first test is to ensure that the turbine’s safety mechanisms are operational with the most important being that for rotor over-speed. Turbines usually have a process by which the rotor is deliberately over-sped under a controlled condition to test for this. Other tests are also performed and Figure 13-8 shows technicians in the hub of a 1.8MW turbine at Albany, Western Australia, during commissioning testing of control equipment associated with blade pitching. Figure 13-8 Technicians in the hub of a 1.8MW wind turbine at Albany, Western Australia, during commissioning tests (Source: Photo courtesy of the American Wind Energy Association) Usually a secondary crane is also required on site, especially for very large turbines. This is so that the turbine components can be lifted off the transports safely and without damaging the components by letting them touch the ground. An example of a secondary crane being used is shown in Figure 13-6 Erecting wind turbines is a dangerous business and people have been killed when things have gone wrong. It only takes a few seconds for a load to shift or a crane outrigger to not be bedded correctly and a tragedy can result. It is always necessary to involve rigging and craneage experts for the construction of wind turbines and to properly plan this from a safety perspective. Two issues that have been found important in Australia regarding crane safety are wind speeds and public interest. Wind farms are obviously built on windy sites and yet it is difficult and unsafe to lift large wind turbine components when the wind speed is high. This is a matter for the crane operator and he/she alone must make the decision about when the wind speeds are safe to lift. The general public will want to see wind turbines being erected, especially when the blades are going on, and many hundreds can turn up to see this. It is up to the wind farm developer to allow not only public car parking space away from heavy transportation but also a viewing area that is close enough to see the action but far enough away in case something goes wrong. If such facilities cannot be made available, then a shuttle bus service provided by the developer is a good way of letting people in to see what is going on. 13.3.3 Turbine Commissioning The first moment that a wind turbine turns and generates its first kilowatt-hour is a very satisfying one for the wind farm designer and developer. Not only are years of work and planning finally coming to a conclusion but the project will be generating revenue and this is essential if the facility is to be viable. The first part of commissioning usually involves energising the wind farm electrical connection and step-up transformers. In Australia such switching is usually performed by the local utility, sometimes remotely from 13-17 (Source: Photo courtesy of Western Power) 13-18 Wind turbines are complex machines and invariably faults in the first few days of operation become evident. In Australia this is sometimes due to the length of time the equipment (especially control equipment) has been in storage since it was factory tested, with water condensation a primary cause of malfunctions. Because of this the capacity factor in the first few months of operation is usually lower than anticipated in modelling. This can sometimes concern a wind farm owner who is expecting the machines to operate flawlessly immediately they are turned on. 13.4 Operating a wind farm after ten years and are predicted to be up to five times higher at 20 years (Reference:1 ) than when first installed. Such increases are invariably due to the following issues; • • • Lowering of turbine efficiency due to dirty blades, A rise in unplanned maintenance, and Larger and more expensive components progressively requiring maintenance as the plant ages (such as gearboxes). When developing a project the developer must put in place what is known as an asset management plan which looks at the life-cycle of the plant to determine the best economic outcome in terms of maintenance. For example, it may be worth spending extra on gearbox maintenance, such as laboratory inspections of lubrication oil every month, rather than risk a failure and long term loss of production. 13.4.1 General operations and maintenance Operating wind plant is by electrical industry standards very reliable. While to many such machines can appear to be maintenance free illustrates the reality is that they are reasonably simple to maintain and operate. After all, in Australia large wind turbines are very successfully operated in some of the most remote communities in the world – for example, Thursday Island in Torres Straight, King Island in Bass Strait and Denham in outback Western Australia. Wind turbines operate automatically and independently with planned maintenance carried out to the manufacturers specifications by a team specific to this purpose. It is normal for wind turbines to occasionally trip off-line for minor faults or due to unstable wind conditions but normally the machine’s controller will reset and restart the turbine itself. Invariably, however, unplanned maintenance involving issues such as the failure of components, continual turbine tripping due to grid problems or problems due to corrosion or water condensation must be dealt with by maintenance staff. Usually in Australia there is a network controller who manages the electrical network and sometimes they have control over independent wind farms on their networks. This is particularly the case for large wind farm facilities connected to the major Australian grids. This is so that in the event of a system problem they can take the wind farm off line safely and know exactly what the plant is doing. Other than these events, the wind farms are usually allowed to operate to their maximum generation capability at all times. In most places in the world it is not normal to have an operator actually at a wind farm site, but it is normal practice for there to be some form of data logging at the wind turbine and a Supervisory Control and Data Acquisition (SCADA) link to an operations centre (see Topic 9). Such links can be up to hundreds of kilometres away – even the other side of the world if required – and these enable the operator to establish how the wind farm is running, its performance and to scrutinise faults as they occur. Often this SCADA link will allow a turbine to contact the operator to advise them that it has a problem. 13.4.2 Statutory approval work Statutory approvals for a wind farm usually have placed conditions on the operation of the plant, and these need to be maintained during its lifetime. Environmental conditions of approval often require that a report be delivered to the local authority every year to ensure that compliance is taking place. An example could be in terms of weed control and sometimes spot audits take place to verify independently that what is being stated in the report is actually taking place at the facility. Rarely do unforseen issues arise during the operations of wind plant but occasionally they do. This could be, for example, a noise issue at a local residence that, while meeting statutory constraints, is creating negative press about the facility or maybe television interference that was not picked up in modelling work. Often the wind farm developer can solve these problems simply and at low cost – by sound insulating the problem house or providing satellite dishes to those with poor TV reception. Very rarely are the problems that arise so controversial that the wind farm is closed down, but occasionally this does happen. This is a disaster for the wind developer (and the local wind industry) and can only be avoided by diligent planning during the project’s feasibility stage so that all likely issues are addressed. 13.4.3 Major problems and failures While wind turbines are reliable and safe they are not perfect and major catastrophic failure does sometimes occur. Figure 13-9 and Figure 13-10 show the results of the type of blade damage that can occur even on modern wind turbines (see also Figure 9.9 Reference: 4 ). Usually such failure is due to conditions outside of those considered normal for the turbine operation, with storms such as hurricanes and repeated lightning strikes being the predominant reason. Often particular characteristics of a wind farm site don’t become evident until the wind turbines have been operational for some time. For example, the turbulence from a particular direction, now that there are other turbines around, may be higher than anticipated or the wind from a certain direction may be particularly gusty at a frequency that was not measured during the wind monitoring. This can be remedied by variations to the default operational parameters of the control logic to cope with the unforseen conditions. A typical example of this in Australia is in relation to grid voltages which here are not as stable as in Europe and it is common for the control defaults in terms of voltage variations to be adjusted so that the turbine don’t continually trip due to voltage excursions. While the availability of modern wind turbines is high there are several things that can cause this to lower with age. For a 600kW wind turbine the costs associated with operations and maintenance usually doubles 13-19 13-20 13.4.4 Wind farm life and decommissioning Figure 13-9 Blade on fire at the Esperance Nine Mile Beach wind farm, Western Australia, following a bad lightning strike in 1999. Wind farms are usually designed for operational lifetimes of about 20 years and the experience is that, if well maintained, there is no reason why this should not be reached. After all, there are many aircraft operational in the skies that are over 50 years old. At some point in time though, the facility will need to be decommissioned and this will require cranes and trucks to be once again brought into the area. Allowance during the wind farm design has to be made for this to occur without damaging the environment and undoing any good rehabilitation work carried out when the facility was first constructed. Figure 13-11 shows a 60kW wind turbine just after decommissioning in 2002 from the very first wind farm built in Australia at Salmon Beach, Esperance, Western Australia. This facility was built in 1987 and consisted of six of these machines at a site adjacent to the Southern Ocean from which the turbines received ocean spray with very high concentrations of salt. Close inspection of the figure shows the wear and tear that this caused. However, despite this these machines were still in good working order and following refurbishment were put back into service in Queensland. The area at Salmon beach where the turbines were operated in now a community park and features one of the original turbines as a monument for the town. Such is the community feeling towards this sustainable form of electricity generation. (Source: Photo courtesy of Western Power) Figure 13-10 Result of a blade structural failure on a 1.5MW wind turbine in Germany in 2000. Figure 13-11 Decommissioning in January 2002 of the first wind farm built in Australia at Salmon Beach, Esperance, Western Australia. (Source: Photo courtesy of Western Power) When such a failure occurs it obviously damages the wind farm industry. However, it is necessary to reassure the public that such failure is extremely rare – after all, occasionally other engineering structures fail due to “acts of God” which could never be designed for and it is unfair to label wind turbines as dangerous due to such failures. Note that no member of the public has ever been injured or killed by an operational wind turbine and this is something the wind farm industry is justifiably very proud of. (Source: Photo courtesy of Western Power) 13-21 13-22 References: 1. Manwell, J.F., McGowan, J.G. & Rogers, A.L. (2002) “Wind Energy Explained – Theory, Design and Application”, John Wiley & Sons. 2. Burton, T., Sharpe, D., Jenkins, N. & Bossanyi, E. (2001) “Wind Energy Handbook”, John Wiley & Sons. 3. Hau, E. (2000) “Wind-turbines. Fundamentals, Technologies, Application, Economics”, Springer Verlag. 4. Pasqualetti, M.J., Gipe, P. & Righter, R. W. (2001) “Wind Power In View. Energy Landscapes in a Crowded World”, Academic Press. 13-23 ...
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