Wind power has been utilized by mankind since historical times. It has brought distantlands together, from the early days when the Greeks set sail on their ventures to laterwhen Colombus “discovered” America. The famous character “Sancho Panza” madewindmills immortal in literature.
Of late, windmills have been getting increasing attention on account of wind energybeing available “free” of cost and also on account of being the most nonpollutingsource of electricity. On the electricity front, it started with a wind turbine driving anelectricity generator, mainly an induction motor, which is universally available. Theoutput ratings were minuscule: a few hundred watts. Today, it is entering into the clubof megawatt-scale electricity producers. Kriegers Flak, an island located in the BalticSea between Sweden, Denmark, and Germany, is expected to fully commission itswind farm rated at 630 MW by 2010.
The latest rates of growth are striking. In Europe, wind farm installations grew at arate of 38% during 2007, compared to 19% during 2006. By 2007, total installed windpower capacity there stood at 67 GW. Germany, Denmark, and Portugal were promi-
nent. In the United States, total wind farm capacity stood at 11 GW in 2007. It is grow-ing fast in China, India, and many other countries [1].
Yet, total contribution by wind energy to production of world electricity energy atthe beginning of 2006 has been very minuscule—0.7% of the total of 17,500 TW-hr[10]. It has its drawbacks by its very nature.
9.1.1 Technology Growth in Wind Turbine Generators
Wind turbine generators (WTGs) started as fixed-speed wind turbines with conven-tional induction generators and capacitor banks as static reactive compensators. Capac-itors supplied reactive power for the air gap magnetic flux, which the induction gener-ators could not produce. Denmark initially standardized on this model, terming it theDanish Concept. These turbines contributed 71.6% of the total WTGs there by 2006[39].
Later, squirrel-cage rotors in induction generators were replaced with wound rotors.Variable rotor resistance, variable speed compatibility with gears, and capacitor banksbecame standard features.
Doubly fed induction generators (DFIGs) followed with partially rated power elec-tronic convertors. The converter helped to provide independent control of active andreactive power outputs of the WTGs. The PE converter rating was generally at 30% ofthe WTG rating.
Finally, TWGs with added functions in the PE convertors arrived. The PE portionincreased the costs but gave better control and helped in the fault-ride-through facility.This category constitutes just 0.2% of the total WTG population [2].
9.1.2 Nature of Wind
Wind may blow steadily during certain periods, varying by day, season, location, andso on. Let us say the velocities fall within some zones. The wind may die down, fallingalmost to nil. Then it may rise from a very low speed. There may be a wind lull, whenthe wind dies out and then rises in short bursts. A wind gust is the opposite phenome-non to a wind lull. A very strong wind is a storm. This nature of wind makes it an un-reliable source of power due to its variability and uncertainty.
9.1.3 Components of A Wind Turbine Generator
Figure 9-1 shows the components of a wind turbine generator. These are mainly:
� The rotor blades, whose pitch is adjustable as per wind velocity so as to catchmaximum wind energy.
� The gear box which adjusts the rpm of the rotor of the generator as closely aspossible to the grid synchronous frequency.
� The generator, which converts mechanical input into an electrical output.
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9.2 OPERATION OF WIND TURBINE GENERATORS
9.2.1 Output of a WTG
Power captured by a WTG is given by
Pm = ½ ��R2Cp(�)V3
where � = air density R = wind turbine radius = approximately rotor blade length Cp = power coefficient, which is a function of � = tip speed and wind velocity ratio� = pitch angle V = wind velocity � = R�/V that is, tip speed/wind velocity ratio (Figure 9-2 (a))� = angular speed of the blade × 2�
There are two operating conditions for a WTG:
1. For a given wind condition it should produce maximum possible power. Pleaserefer to Figure 9-2(a). This is possible when � stands at �opt.
2. There is a minimum wind condition below which the WTG becomes unstable.�is represents the crossover point for the stable condition limit. At �stall, theWTG will stall.
Note that the ratio of WTG blade tip speed to wind speed, �, plays an important part.The WTG control should perform in such a way that it is at �opt under different condi-tions of wind load.
The rate of change in � is given by another quotient �:
� = (dCp/d�)[�/Cp(�)]
Figure 9-2(b) shows the variations of � with �.A WTG is in an unstable condition when � becomes positive. The figure shows
power versus speed curves of a wind turbine with wind velocity as a parameter. Thedashed line is a boundary between high- and low-speed regions.
9.2.2 Performance Improvement through Blade Pitch Control
At low speeds, the pitch angle is almost zero. Maximum possible energy is scooped up(maximum power strategy). At high speeds, the pitch angle increases (Figure 9-3). Be-yond a certain wind speed, automatic mechanical brakes apply and electrical dumpingresistances are used as loads.
9.2.3 Efficiency of a WTG
Average efficiency of a WTG is defined as the ratio of energy delivered to grid to theenergy at the turbine rotor shaft. As the energy is transmitted from one member to the
next in the transmission system of a WTG, losses are incurred. Tables 9-1 to 9-3 give arelative idea of these losses.
9.2.3 Losses in a WTG
Average and rated efficiencies for the three different types of WTGs are 82–86% atlow wind speeds and 89.7–89.9% at high wind speeds. Thus, weather forecast and paststatistical data form important requirements for efficiency and reliability when inte-grating wind farm energies into today’s mega grids.
9.2.4 Flickers in the Output of a WTG
There are two main causes of flicker in the supply from a WTG:
Friction, windage, and oil churning losses 1.0%, including turbine bearing
Gear mesh losses 1.7%Total losses at rated load 2.7%
Source: [6].
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Mechanically Related Causes
� Motor turbine imbalance
� Rotor blades passing in front of the wind structure
� Structural modes due to mechanical eigenfrequencies (frequencies at which thereis mechanical resonance)
� Rotational sampling
The flickers caused by these mechanical causes have a regular pattern, low amplitude,and a low-frequency range of 0.65 to 0.71 Hz.
Wind Velocity Related Causes. Wind flow has regular bursts that can causeflicker. This flicker has a high amplitude and a range of 0.01 Hz–10 Hz. This flicker isobjectionable and has been investigated deeply.
Flickers Irritate. If Pst, the short-time flicker intensity, is greater than 6.35% in arange of about 9 Hz it becomes an irritation to the human eye. IEEE Specification 561lays down rules and limitations on these types of flickers. The flicker rules have their
Table 9-3. Losses in the frequency converter at rated load
Voltage drop losses of the diode rectifier 0.4%Resistive losses of rectifier and inductor 0.2%Step-up converter transistor losses (at rated current through the transistor) 0.75%Step-up converter diode losses (at rated current through the diode) 0.25%No load converter losses 0.1%Inverter load losses at cos(�) = 1 1.5%Inverter and inductor resistive losses 0.3%Total losses at rated generator voltage 2.75%
Source: [6].
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origin in the operation of industrial arc furnaces, which caused voltage dips at theselow frequencies.
If we consider SC, the capacity of a grid, as its inertia, then the larger the inertia, thelower the flicker and vice versa. If we consider �/R as grid impedance factor, then larg-er this factor, lower the flicker intensity and vice versa. Flicker emission also rises pro-portionately with the wind turbulence intensity.
Flicker intensity does not add up linearly as the number of WTGs in a cluster orwind farm, N, grows; it rises proportionately to ��N�2�.
Flickers produced by one turbine are not influenced by flickers produced by otherturbines. However, they are rather accentuated by flickers produced by the grid itself.
9.3 CONNECTION OF WIND ENERGY PLANTS TO THE GRID—THE GRID CODE
In the early days of wind electricity generation, the plant sizes were small. With an in-duction generator, there was no problem of synchronizing with grid frequency. Exter-nal capacitors took care of voltages; when there was a disturbance in the grid leadingto low voltages at the point of connection, the wind plants were disconnected andstayed disconnected until the grid disturbance was cleared. Today, wind plant sizeshave increased. Should a wind plant get disconnected due to a grid disturbance, itcould aggravate the situation. A grid code for interconnection has evolved.
The mean features of the grid codes are:
� A low-voltage ride-through (LVRT( is essential for getting into a grid.
� Accurate power control at a PF of ±0.95 has to be maintained at the point of con-nection.
� Accurate plant models must be submitted.
� SCADA data must be supplied as agreed with the system operator.
9.3.1 Low-Voltage Ride-through
Wind energy farms are now a sizeable constituent of the power supply. If a fault devel-ops in the grid, with voltages at the point of common coupling between the grid andthe wind farm, falling to low levels during the fault clearance time, the wind farmshould not disconnect. In other words, it must have a low-voltage ride-through capaci-ty.
If a fault occurs in the grid system to which a wind energy farm is connected, thevoltage at PCC dips to a low percentage, depending on the severity and location of thefault. The control system clears the fault within the time frame specified under griddiscipline. Figure 9-4 shows an envelope of the voltage profile under the GB GridCode (CC A 4.3) The right-hand side shows two illustrative faults: fault N, in whichPCC voltage dips to 25% for a brief time; and fault M, in which it dips to 62% for alonger time.
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Within the period during which the fault is being cleared and beyond to a safe limit,the wind energy system should hold on and ride through. At the instant of the start ofand during the duration of fault, the voltages on WTGs stay low. These WTGs dependon external capacitors for the air gap magnetic flux as well as reactive power compen-sation. The reactive power from the capacitors is proportional to the square of voltageand goes down significantly at low voltages. Besides, the grid fault, depending on itslocation, could also drain out some reactive power from the capacitors. All this tendsto speed up and destabilize the WTGs against a weakened flux in the air gap.
A sizeable correction to brake-up is anticipated through blade pitch angle control,which will try to slow the WTG down. However, the activating arms of the blade pitchcontrol have to be really strong to be effective in a very short time. This will also put astrong mechanical stress on the WTG system.
This is best supported with the help of dynamic reactive power control from anSVR on the grid or even synchronous generators at the interconnections of the windfarm system to the grid. Note that the Danish systems have two large synchronous gen-erators at their interconnection to the Nordal system in Sweden. These are rated at 160MVA, and 100 MVA respectively.
With PE converters on the WTGs, the converters can extend dynamic RP control.Figure 9-5 shows the effect on WTG power output of dynamic RPC.
9.4 AMERICAN GRID CODE
The grid code developed by the American Wind Energy Association (AWEA) requiresa low-voltage ride-through with normal fault clearing time of nine cycles for a three-phase fault. A single-phase fault to ground is allowed longer time to clear.
The WTG must stay online during these fault periods, with voltages dipping at PCCto 0.15 PU and later on cleared to 0.0 PU. The grid code also requires reactive powercontrol at the PCC at +0.95. It also requires that accurate plant models be supplied [1].
Putting a parallel link with a series bypass resistor in the WTG output helps to holddown the speedup and fallout of synchronization. After voltage restoration, the resis-tance link is shorted. This is shown in Figure 9-7.
9.5 A RESISTIVE BRAKING OF A WTG
Figure 9-8 below shows power-versus-speed characteristics of a WTG. The top curve rep-resents conditions with a series damping break resistor (SDBR). The bottom curve showsthe same without SDBR. A grid fault occurs at point 1 and the SBDR is switched in. Thecharacteristics change over to the bottom one at point 2. It travels along 2–3 within the du-ration of the fault. At point 3, the SDBR is shorted and the characteristics jump to point 4.However, actual WTG at this changeover is at a low speed, corresponding to point 3, andalso has a low power output. So the characteristics travel pack to point 5, which coincideswith the original point 1. The SDBR has prevented a runaway increase in WTG speed.
The ride-through periods for a WTG must be naturally much longer than those al-lowed under fault-clearance codes. Danish grid codes specify testing each WTG as perthe specified voltage profile shown in Figure 9-6.
9.6 POWER AND PF CONTROL
Accurate power control on both active and reactive power is now obtained on theDFIG machines with power electronic control, as explained earlier. This enables theWTGs to maintain the required PF automatically at the PCC.
9.7 MODELING OF A WIND TURBINE GENERATOR
It is desirable first to understand how a vastly spread electricity power system operatesphysically. The following gives a brief sketch.
A transmission system operator (TSO) looks after load following and power qualityon a very small scale time scale, say on a minute or 10 minute basis. For this, he has a
schedule of power offers from various generators. The TSO has also a schedule oftime-bound requirements from the customers. He matches these and balances the load.The TSO also has an updated chart of transmission facilities with all their characteris-tics in his computer. He selects an optimum route for a load dispatch. This route hasminimum operating losses and costs. Modern fast-operating computers and accuratedata are essential for his work. Supplying of accurate models of WTGs is compulsoryfor this reason.
9.7.1 Objectives
The TSO must simulate and forecast system stability problems with respect to frequen-cy deviations, voltage variations, transients caused by voltage dip faults, and powersystem oscillations, which generally occur between 0.1 Hz and 10 Hz.
9.7.2 Method
Electrical and mechanical parameters of a WTG are converted into algebraic quanti-ties. These algebraic notations are used to develop algorithms to arrive at characteristicfunctions. See a typical illustration in Figure 9-9.
Most of the grids in the world require a WTG dynamic model to be submitted to thetransmission system operator for permission to join the grid.
Typical Irish grid requirements are listed below [8]. Any WTG greater than 5 kWmust submit a model incorporating the following features:
1. Generator general characteristics
2. Turbine generator and drive train mechanical characteristics
3. Variation of power coefficients and pitch angle to tip speed ratio
4. Blade pitch control
5. Converter controls
6. Reactive components
7. Protection relays
Time per step for simulation should not to exceed 5 microseconds. Although models for simulations for thermal and hydro generators have long been
used and standardized, those for WTGs are still evolving and there are no standards.
9.7.3 Present Problem Areas in Modeling
1. Secrecy by WTG manufacturers who are developing the newest machines.
2. Nonstandardization, beginning with numbering and naming components.
3. Numerical instability arising out of rounding off and truncating practices, amongothers.
4. A large number of differing models.
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9.7.4 Model Validations
Models have to be validated for incorporation at the transmission system operator’soperating desk. This validation can be partly done at the manufacturers place or at adesignated laboratory test setup and, finally, in the field. Simulating a full 3 ph fault ofsufficient magnitude will be difficult do. The importance and employment of modelshas already been commented upon.
9.8 ECONOMICS OF WIND ENERGY
9.8.1 How Does a Modern Power System Operate on theMarketing Side?
Equivalent to a TSO on the physical operation side, there is a system operator on themarketing side. Basically, he matches the offers and bids closely and allots them at the
184 WIND POWER GENERATION
WT power
÷
Rotor speed ωR
pWT
0
0
0
pWT_refVT pTT
pTo
vW
pm
Imax
tm
1+pTv 1+pTw
1
pTH
WT ref. powerSpeed control thru torque
–1–11
1 +–
–
– –
Pitchcompensator
Wind speed
Turbine powerand torque
ρAmCp(λ,βW)
Power conversionPitch actuatorSpeed control thru Pitch
concluding time of day-ahead activities. These allotments are binding. A buyer cannotrefuse them or else he risks a penalty plus differentials. A supplier cannot refuse orelse the TSO makes the difference from other suppliers and debits the defaulter’s ac-count. The TSO also has a stock of his own previously contracted reserves that he sellsat market price.
It may be noted that actual transactions take place in the real-time market, whereprices and time slots can be negotiated and finalized over fixed time slots, say onehour. The prices at the end of this settlement period are the final prices at which ac-counts are settled.
Assume for a while that with all the available means in the hands of a TSO, the sup-ply falls short and some load goes without service. This leads to a loss-of-load possi-bility or LOLP. It also leads to hand an imbalance and a Cost of Imbalance.
How do the evolved marketing practices affect the working and future of wind en-ergy? Daily wind flow characteristics and daily load flow characteristics will vary.Universally, the late night and early periods show low loads. These have to be servedby the base-load-supplying thermal generators, which have to run at near uniform out-put on account of thermal considerations, cost considerations, and so on. The TSOdoes not take into account wind energy contribution in his daily scheduling over thisperiod. During peak load periods, when electricity gets the highest prices of the day,the cost of LOLP or the imbalance cost are the highest.
Smith [37] has worked out relations between capacity factor (CP) of wind energy,load factor (LF) of a system, and energy penetration (EP) for wind. Typically, for EPof 20%, an LF of 60% and wind capacity factor of 40% are required. Typical valuesfor penetration factors are given in Table 9-4.
Assume that the settlement period is 2 hours. The unit commitment by a WTG op-erator based on his judgment can go far wide due to uncertainty of wind flows. Hiscost of imbalance will go up. Should this period be reduced to 1 hour, he can commithis units in smaller quanta and reduce his cost of imbalance. Meteorological data, pastand current, over the next 24 hours can greatly help all the market players. This data ismade available commercially in the western countries.
9.8.2 Unit Commitment and Scheduling
With power marketing growing in scale, load balancing becomes important. For this,the system operator must know accurately how much unit commitment he can havefrom the wind farm and how much capacity he should hold in reserve. This is a de-manding task, considering the erratic nature of wind and the magnitude of loads to behandled. But the system operator has many tools in his hand, including dynamic sched-uling and accurate hourly weather forecasts.
Dynamic Scheduling. With accurate modeling of the system and computerizedsoftware he can find out what can happen to the system when an electric component isadded or subtracted or controlled in power output. All system components must be putin models for accurate simulation. Modeling right to the last details becomes impor-tant. Since the WTG technology is fast developing, standards for modeling these are
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not yet in place. Take the case of a 4.5 MW TWG weighing 450 t developed by Ener-cons in Germany. The very size contributes to LVRT on small duration faults. Thislarge piece of apparatus is not the last step in WTG development.
Accurate Hourly Weather Forecasts. Weather is not all that erratic. Weatherbehavior falls into a pattern—daily, seasonal, periodical, and geography specific. Ex-cursions out of this pattern might be in a band that can be anticipated. This, along withdaily weather forecast by meteorology departments, can help the system operator onunit commitments from the wind farm on the day-ahead schedule as well as on the dai-ly schedule, balancing them fairly closely. (Please see Chapter 13.) The system opera-tor need not commit too much capacity to reserves. In fact, although wind energy costsare next to nil, their operational costs go largely toward unit commitments, as Table 9-4 shows.
9.9 CAPACITY FACTOR OF A WTG
Effective load-carrying capacity (ELCC) is defined as the amount of additional loadthat can be reliably served. In determining this capacity, two factors enter into the pic-ture.
A load served during peak hours has the highest going market rate. If there is aLOLP during this period, cost of power is taken into for determining the capacity fac-tor of a WTG. This period will not necessarily always coincide with the period of de-livery at the rated output of a WTG. Thus, WTG capacity during the period of LOLPcounts against its rated capacity for planning purposes, unit commitments, and so on.
Capacity value of a WTG has been shown to range approximately from 10% to40% of the wind plant rated capacity (Table 9-4; [1]).
9.10 CAPACITY CREDIT CONSIDERATIONS
How do the capacity considerations affect wind energy installations? In many coun-tries today, major utilities, mostly based on thermal generation, have to buy “green”energy such as wind energy at prices stipulated by the energy regulator. Typically, thebasis for these prices is fixed on the following factors [10]:
� Capacity factor
� Operations and maintenance cost reductions for the utilities
� Electrical loss reduction for the utilities
� Environmental benefit
Hourly capacity factors are determined. The average of top 50% of the load hours(on both sides of the peak hours) is close to the estimation of the capacity factor forwind energy converters (WECs). In the deterministic method, historical data on windis collected and the capacity factor is calculated. In the probabilistic method, this is
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calculated from the wind data statistics and forecasts. This gives too high a capacity.Thus, the capacity factor is important for WEC’s since it directly affects their earnings.The capacity factor is also important from the grid operational point of view, for dailyplanning as well as for hourly operations. The capacity factor is important for futureplanners of the grid system. Probabilistic methods might be more useful here.
9.11 CAPACITY FACTOR FOR WECs IN A HYBRID SYSTEM
WECs are often combined with other distributed generators to ride over the unpre-dictability, again determined by the deterministic method. To that extent, the capacitycredit goes up, particularly when the hybrid contributes energy around the peak loadhours.
9.12 WIND PENETRATION LIMIT
The wind penetration is defined as the extent to which wind power can be added to apower system without compromising its operational reliability. Operational reliabilityin turn is defined in terms of a system’s dynamic response, requiring the system fre-quency to stay within the limits of protection for a set of wind-related events. Thismakes acceptable system dynamics the sole criteria for increasing wind penetration[11].
Wind penetration simply means the extent to which wind power can be added to apower system without affecting its operational reliability. This reliability depends onthe extent of reserves, external to the wind capacity, which are available to the system,internally or externally.
As per Mullane [12], the grid rule for Irish Electric Systems requires that the prima-ry reserve in a system should be 75% of the single largest feed of the system. Thisshould be available within 5 to 15 seconds subsequent to the event of the feed goingout.
Again, if the load factor of a system is 60% and the average wind plant capacity is40%, then the penetration has been worked out by Smith as 30% [1]. Average windpenetrations has been between 20% to 30%.
9.13 WIND POWER PROPORTION
Wind power proportion (WPP) is calculated by Soder [13] as a percentage as
WPP = maximal wind power/(lowest consumption + possible exchange)
Gotland, an island off Sweden, provides an example of a set minimum and maximumpower consumption, based on all types of generation, at 45 and 160 MW. During lean
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consumption periods, 180 MW of surplus power is exported. It has a maximum windpower capacity of 90 MW. Thus,
WPP = 90/(45 + 180) × 100= 40%
9.14 WIND INTEGRATION COST IN UNITED STATES
See Table 9-4 for this data.
9.15 WIND ENERGY FARMS
With rising contribution by wind power to electricity systems, individual WTGs orclusters thereof are giving up their place in the energy scenario to wind energy farms.These farms have a few noteworthy features. They are generally located away fromload centers. Connecting them into the grid system requires almost dedicated transmis-sion arms. Whereas WTGs can be planned, installed, and put in operation within ashort time span, connecting transmission links require long-time planning and execu-tion. Existing transmission lines might not have the capacity to carry the wind farmpower, as happened in the case of an East European customer who contracted for windenergy from a Dutch supplier [14]. An irregular flow on the direct line could also af-fect load flows and voltage regulation on the neighboring lines. This necessitates cen-tral control from the system operator over the operation and contribution by the windenergy farm.
9.16 PROMOTING GROWTH OF WIND ELECTRICITY
How we can increase wind energy penetration and at what level of operation can wemake it economical? The first issue involves operational reliability. The second issueinvolves dimensions and integration into the existing operations.
Operational reliability involves relativity between connected wind-energy-derivedcapacities to total system capacity before the wind connection. In a contingency, thelargest component of the wind capacity might trip. There must be sufficient reserve ca-pacity to cover this loss. Failure of a wind capacity member may happen very fast. Formaking up the deficit, the ramping-up rate of the balance system should be high.Ramping rate of the balance system is a critical factor in the allowable safe wind pene-tration. When a new WTG is added, some grids put a condition on its ramping rate.This ramping-up rate is also important.
Holding down captive reserves on account of wind energy in the system increasesthe cost of wind energy and absence of it retards its growth.
The solution to reducing these reserves lies in accessing a larger resource base andconsolidating balancing areas through use of dynamic scheduling. Accessing larger re-
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sources is achieved by interconnecting into neighboring resources, such as Denmark toNorth Germany and Denmark to Sweden, and into large grids, as in the proposed Euro-pean grid and interstate connection in the United States.
The case of Denmark is illustrative. With a 300 kV AC connection into the NorthGerman system, it could depend on reactive power supply in case of its grid fault. Italso exchanges power with Germany. It also has an HVDC connection to Sweden,which has high hydroelectric production. It could export wind energy whenever itsproduction was high and domestic consumption was low. Hydroelectric power is veryflexible.
Both these factors have helped the remarkable growth of wind power in Denmark. Strengthening transmission systems also helps. There is an execution gap between a
wind farm project and strengthening or adding a new transmission line. Transmissionsupport systems such as FACTs devices, capacitor banks, and so on can effectivelyhelp. The economics will have to be worked out.
Transmission tariffs and regulations need to be favorably biased. Thermal genera-tors connected to a transmission system are not allowed to feed in excess energy be-yond an allotted quota. This restriction needs be relaxed in the case of wind energy[1].
Transmission systems are allowed to sell fixed transmission rights (FTRs) overtheir lines. These are physical rights and can end in underutilization. Hunt has pro-posed financial transmission rights to correct this. Please see Chapter 15 of this bookon this subject. If this change is made, it will promote wind energy supply.
Developing day-ahead and hour-ahead marketing can cut down unit allocation costsand promote growth of wind energy. An example of NETA Systems in the UnitedKingdom can be found in Chapter 14.
Dynamic scheduling by the system operator helps. This has been discussed previ-ously.
Models are also important in today’s dynamic scheduling when incorporating windenergy.
9.16 PROMOTING GROWTH OF WIND ELECTRICITY 189
Table 9-4. Wind integration costs in the United States
Wind Load Unit Gas Total capacity Regulation following commitment supply operating
May 03 [9] Xcel-UWIG 3.5 0 0.41 1.44 na 1.85Sep 04 [7] Xcel-MNDOC 15 0.23 na 4.37 na 4.60June 06 [10] CA RPS Multi-year 40.45 na na na na
AnalysisJune 03 [11] We Energies 4 1.12 0.09 0.69 na 1.90June 03 [11] We Energies 29 1.02 0.15 1.75 na 2.92May 2005 [12] PacifiCorp 20 0 1.6 3.0 na 4.6April 06 [13] Xcel-PSCo 10 0.20 na 2.26 1.26 3.72April 06 [13] Xcel-PSCo 15 0.20 na 3.32 1.45 4.97
Source: [1].
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9.17 MAINTENANCE OF WTG
Problems applying to modeling also apply to standardizing maintenance. Manufacturerstreat failure data as proprietary and are not very cooperative in disclosing this. Ribrant[15] has surveyed failures in the Baltic area countries and finds that WTGs have the high-est exposure to mechanical stresses. Loads are also high and of a fluctuating nature. Sofar, maintenance is of a corrective type. Downtime, loss of production, and repair costsare high. Problems are aggravated in offshore installations. Preventive maintenance isdesirable. Geared WTGs operate through a weak shaft with torsion stresses. Quite often,there is mechanical resonance in the system, similar to electrical resonance. At this me-chanical resonance frequency (about 1–2 Hz), there is shaft relaxation at shaft faults,leading to failures [15]. Including mechanical characteristic in modeling is essential.
9.18 UNFCCC AND WIND ENERGY
There is continuous pressure from the environmental groups to reduce atmosphericpollution, of which the second largest offender is thermal electricity production. Origi-nally, it was intended that by a given target date electricity production from renewablesources would be 10% of total electricity production. This has since been revised.
Most countries are heading toward this 10% target. This is being helped by techni-cal improvements in WTGs, immense advances in weather forecasting, which helps indynamic scheduling, and last but not the least, by an abundant supply of wind energyat “nil” cost.
Wind generated energy has a drawback in its uncertain nature. Efforts are underwayto tie wind electricity generation with hydroelectric plants. Please refer to Chapter 2for details on this.
The uncertainty of wind in wind electricity generation prevents it from providingancillary services like load-following frequency control to the grid. We are talking of astage at which wind electricity is one of the major components of power electricitysystems. This is being aided by introducing FACTS devices between wind generationand a tie-up to a power grid [15].
Still, wind energy has a very bright future.
REFERENCES
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2. V. Akhmatov and P. B. Eriksen, A Large Wind Power System in Almost Island Opera-tion—A Danish Case Study, IEEE Transactions on Power Systems, Volume 22, Issue3,Aug. 2007, pp. 937–943.
3. P. Fairley, The Greening of GE [Alternative Energy], IEEE Spectrum, Volume 42, Issue 7,July 2005, pp. 28–33.
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