Suitable Wind Energy Conversion System for Bangladesh Sayed Zahed Abdullah Sohel1, Muhammad Kamar Uddin2 and Mohammad Amin3 Department of Electrical and Electronic Engineering International Islamic University Chittagong Chittagong, Bangladesh e-mail:
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AbstractThis paper presents the comparison of different wind energy conversion system (WECS) based on literatures and market data. Firstly an overview of these wind turbine generator topologies is studied and a comparison of these different types of generator is discussed. Finally an efficient WECS is proposed which is suitable for wind condition of Bangladesh. Keywords- Wind energy; WECS; WTS; 1G-PMSG; DD-PMSG; 3G-DFIG; Wind condition in Bangladesh
I.
R
INTRODUCTION
ENEWABLE energy has become an important energy source because of the increasing demand of fossil fuels, and pollution problems. Among the renewable energy sources wind energy is gaining particular attention day by day because of the technological advancement in Wind Energy Conversion system [1]. Wind turbine used in WECS are now capable of generating up to 7.5 MW which is almost three times greater than last decade [2]. At the end of 2006, the global wind electricity generating capacity increased to 74,223 MW from 59,091 MW in 2005. By the end of 2020, it is expected that this figure will have increased to well over 1,260,000 MW, which is 12% of the worlds electricity consumption [3], [4]. WECS with different generator systems have been developed to maximize the energy capture, to minimize costs, to improve power quality, and others [2]. Power can be extracted from wind turbine generator either at fixed speed or at variable speed mode. Variable speed Wind turbine system (VS-WTS) is gaining popularity compared with the fixed speed mode [5] because of its increased power extraction from wind, high efficiency, less mechanical stress, less noise and the ability to provide grid reactive power support [6]. Among the variable speed wind turbine generators, doubly fed induction generator (DFIG) and permanent magnet synchronous generator (PMSG) are two most popular wind generators [5]. In variable speed wind turbine configuration a DFIG is used with multi stage gearbox and a power electronic converter connected with the rotor, the stator winding is directly connected with the grid. Another variable speed configuration exists including a PMSG which is gearless and
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use a full scale converter and can be used in variable speed configuration with single stage gearbox [7], [8]. In Bangladesh, the total installed capacity of wind power generation is 1.9 MW in the year 2011. According to Bangladesh Power Development Board the total installed capacity is going to be reached to 200 MW after few years [9]. This paper focuses on to compare among 1G-PMSG, DDPMSG and 3G-DFIG WECS based on literatures and market data. Firstly an overview of these wind turbine generator topologies is studied and a comparison of these three types of generator is discussed. Finally an economical generator topology is proposed which will be efficient for wind condition of Bangladesh. II. OUTPUT POWER OF WIND TURBINE The kinetic energy of the wind can be evaluated by the following equation [10], [11]:
where m is the air mass in kg, v is the velocity of the wind in m/s. The air mass m can be determined from the air density and the air volume V according to Then, In a small time t, if air particles travel a distance , a volume of V results when the distance multiplied with the rotor area, A of wind turbine. Therefore, We know power is energy divided by time. Then the wind power is given by
The mechanical power that the turbine extracts from the wind, Pm, is inferior to Pwind. This is due to the fact that the wind speed after the turbine is not zero (the air needs to be carried off after the turbine). So, the power coefficient of the turbine Cp can be defined by:
The recuperated power is given by:
Here R is the radius of rotor. Cp depends on the tip speed ratio of the wind turbine and angle of the blade . Where conventionally a tip-speed ratio is defined as:
where, = rotational speed of rotor, R = radius to tip of rotor, A maximum value of is defined by the Betz limit, which states that a turbine can never extract more than 59.3% of power from an air stream. In reality, wind turbine rotors have maximum values in the range 25-45%. The wind turbine torque on the shaft can be calculated from the power:
By
The power output of a wind turbine at various wind speeds is conventionally described by its power curve shown in figure 1. The power curve gives the steady-state electrical power output as a function of the wind speed at the hub height and is generally measured using 10 min average data.
consisting of an aerodynamic rotor driving a low-speed shaft, a gearbox, a high-speed shaft and an induction (sometimes known as asynchronous) generator [2]. In case of variable speed wind turbine they are either geared driven or direct driven. In geared-drive wind turbines, one general configuration is multiple-stage gear with a high-speed generator; the other one is the multibrid concept which has a single stage gear and a low-speed generator. In direct driven concepts the turbine shaft is directly coupled (without gearbox) with generator rotor. Based on the rating of power converter related to the generator capacity, they can be further classified into wind generator systems with a partial-scale and a full-scale power electronic converter [8]. A. Fixed Speed WECS Fixed-speed wind turbine configuration consists of a squirrel-cage induction generator (SCIG) with a multiple-stage gearbox connected to the grid through a transformer as illustrated in Figure 2. As the generator operating slip changes slightly (less than 1%) with the operating power level changes, that is the SCIG operates only in a narrow range around the synchronous speed so the wind turbine equipped with this type of generator is often called the fixed-speed wind generator system [2] ,[3],[12]. Squirrel-cage induction machines consume reactive power and so it is needed to provide capacitor banks for power factor correction at each wind turbine. Soft-starter unit used to build up the magnetic flux slowly and so minimize transient currents when the generator energized. First network voltages are applied to the generator slowly to energize it, once done then it brings the drive train slowly to its operating rotational speed [13].
Figure 2 Fixed speed wind turbine with SCIG
Although SCIG are robust, easy and relatively cheap for mass production but lacking in optimization of aerodynamic efficiency, large mass of gearbox and problem with reactive power control makes it unsuitable for large amount of power generation. Figure 1 Power curve for a 2MW wind turbine
III.
DIFFERENT WECS
Based on the rotational speed wind turbine concepts can be classified into three categories that is fixed speed, variable speed with gearbox and variable speed without gearbox. Fixed-speed wind turbines are electrically simple devices
B. Variable speed with gear box Variable speed with gear box configuration uses two generators. One is DFIG with partial scale power converter and multi-stage gearbox and the other one is PMSG with full scale power converter and single stage gearbox [14]. The gearbox is designed so that maximum rotor speed corresponds
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to rated speed of the generator. A system is depicted in Figure 3 consists of a wind turbine with doubly-fed induction generator. Stator of the DFIG is directly connected to the grid while the rotor winding is connected via slip rings to a converter. The power converter controls the rotor frequency and thus the rotor speed. This concept supports a wide speed range operation, depending on the size of the frequency converter. Typically, the variable speed range is +30% around the synchronous speed [3], [13]. Figure 4 shows a single-stage gear drive Permanent magnet synchronous generator (PMSG) system. In this scheme, a variable speed wind turbine is connected to a single-stage planetary gearbox that increases the speed by a factor of roughly 10 and a low-speed permanent-magnet generator [15]. This concept has gained the attention because it has the advantages of a higher speed than the direct-drive concept and a lower mechanical component than the multiple-stage gearbox concept. A single-stage gearbox with multiple output shafts that drive a number of medium speed, medium-torque PMSGs, has also been introduced. Each of the generator outputs is connected to a dedicated power electronic converter [2], [16].
Figure 3 Variable-speed wind turbine with a DFIG
Figure 4 Variable-speed wind turbine with a single stage drive PMSG
C. Variable speed without gearbox Variable speed without gearbox configuration also known as direct driven configuration consists of a PMSG directly coupled with hub of the wind turbine as shown in Figure 5. Because of its gearless configuration it rotates at a low speed. To deliver a certain power at lower speed it has to produce a higher torque. A higher torque makes the generator larger in size. Therefore for direct-drive generators, the low speed and high torque operation require multi-poles. Directdrive generators are usually designed with a large diameter and small pole pitch to increase the efficiency, to reduce the
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weight of the active parts and to keep the end winding losses small [6], [17]. The advantages of direct-drive wind turbines are the simplified drive train, the high overall efficiency, the high reliability and availability by omitting the gearbox.
Figure 5 Variable-speed wind turbine with a direct drive PMSG
A most significant part of the wind energy conversion system is the generator. Generator converts the mechanical energy extracts from wind by the turbine to electrical energy. Different types of generator are being used with wind turbines. As the diameter of wind turbine is increasing, variable speed configuration has become the first choice for extracting maximum wind power. Main generators used in variable speed configuration are 3 types which are geared DFIG (3G-DFIG), single stage geared PMSG (1GPMSG), direct drive PMSG (DD-PMSG). D. Doubly fed induction generator Doubly fed induction generator (DFIG) is based on induction generator with a multi-phase wound rotor and a multi-phase slip ring assembly with brushes for access to the rotor winding [4]. The mechanical power generated by the wind turbine is transformed into electrical power by an induction generator and is fed into the main grid through the stator and the rotor windings. The rotor winding is connected to the main grid by self-commuted AC/DC converters allowing controlling the slip-ring voltage of the induction machine in magnitude and phase angle [18]. It is the advantage of a DFIG that the speed is variable within a sufficient range with limited converted cost. The stator active and reactive power can be controlled independently by controlling the rotor currents with the converter. Furthermore, the grid side converter can control its reactive power independently of the generator operation. This allows the performance of voltage support towards the grid. However, DFIG system has disadvantages as follows [19]: a. b. c. d.
Heat generates by friction of gear. Need regular maintenance of gearbox. Gearbox generates audible noise. Under grid fault condition machines torque overshoots and large stator peak currents. The brush slips ring needs maintenance which is set to bring power to the rotor.
e. f.
External synchronization circuit required between the stator and the grid to limit the start-up current. In case of grid disturbances, the ride-through capability of DFIG is required so that the control strategies may be very complex.
E. Permanent Magnet Synchronous Generator A permanent magnet synchronous generator is a synchronous generator where the excitation field is provided by a permanent magnet instead of a coil. In a permanent magnet generator the magnetic field of the rotor is produced by permanent magnets. Others types of generator use electromagnets to produce a magnetic field in a rotor winding. The direct current in the rotor field winding is fed through a slip-ring assembly or provided by a brushless exciter on the same shaft [2]. The direct driven permanent magnet synchronous generator system has high potential for the wind turbines because of its reduced failure, increased energy yield and reliability. The advantages of permanent magnet machines over electrically excited machines can be summarized as follows [18]: it has high efficiency and energy yield; magnetic field excitation needs no additional power supply; absence of field losses improve thermal characteristics of the machine; higher reliability due to the absence of mechanical components such as slip rings and light weight and therefore higher power to weight ratio. However, permanent magnet machines have disadvantages which can be summarized as follows: the cost of permanent magnet is high and it is difficult to handle demagnetization of permanent magnet at higher temperature [18]. IV.
assessment of wind energy in the coastal part of Bangladesh, Wind Atlas Analysis and Application Program (WAsP), micro-scale modeling software has been used. WAsP develops models for obstacles, roughness and terrain surrounding a measuring station and then generates a regional wind climate or a wind atlas for the region around 100 km2 in area. The developed wind atlas of four coastal locations of Bangladesh, Charfassan, Chittagong, Kutubdia and Cox`s Bazar using one year data of BCAS shows that at 50m height for the roughness value from 0 m (open sea, water areas) to 0.03m (farm land with very few buildings, trees, airport areas etc) the wind speed varies from 4.1 to 5.8 m/s with a power density of 100 250 W/m2. BCAS time series data on wind speed and direction at 25m height have been employed using WAsP techniques to obtain predictions of wind speed and power density at 50 m around the stations at Kuakata, Patenga, Charfassan, Kutubdia and CoxsBazar which is shown in figure 6. Monthly wind speed variation at 25m height is shown in the figure 7 from BCAS, GTZ and BCSIR data for three locations [20].
WIND CONDITION IN BANGLADESH
In Bangladesh, enough information on the wind speed over the country is not so much available and wind speed data at hub height of the wind turbine is rare. A survey was conducted at 1986 by Bangladesh Meteorological Department (BMD) which showed that at 10 meter height wind speed was very low. Better wind speeds investigated at Chittagong Coxs Bazar seacoast and coastal off-shore islands. Measurements at 20m and 30m heights have been made later on by Bangladesh Center for Advanced Studies (BCAS), Gesellschaft für Tchnische Zusammenarbeit (GTZ) and Bangladesh Council of Scientific and Industrial Research (BCSIR). A project entitled World Water and Environmental Resource Management (WERM) of Local Government Engineering Department (LGED) has been going on for measurements at the height of 20 and 30m for 20 locations all over Bangladesh. It is still not sufficient because maximum wind turbines are made to generate power stand above 50m height. This Solar and Wind Energy Resource Assessment (SWERA) Program provides predictions for wind speed and wind power density at different heights including 50m height to look for the possibility of wind power generation [20]. For prediction of wind speed at different heights and for
Figure 6 Power density for different coastal area [20]
Figure 7 Wind speed variation at 25m
Table 1 shows the wind speed and power density at 25, 30, 50 and 70m heights for two roughness conditions which have been taken from the developed wind atlas for the locations. It
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shows that wind speed at 50m height for all the locations varies from 4.0m/s to 5.8m/s for the coastal sea- shore areas (roughness in between 0.000.03m) [20]. TABLE I PREDICTED WIND SPEED (SP) AND POWER DENSITY (PD) IN THREE PROSPECTIVE COASTAL LOCATIONS [20]
Location
Kuakata
Charfassan
Kutubdia
Height (m) 25 30 50 70 25 30 50 70 25 30 50 70
Roughness 0.00m Sp Dp m/s w/m2 4.9 174 5.0 181 5.3 209 5.4 237 4.7 149 4.8 155 5.0 179 5.2 203 5.4 208 5.5 218 5.8 251 6.0 286
0.03m Sp m/s 3.7 3.8 4.3 4.6 3.5 3.7 4.1 4.4 4.1 4.2 4.7 5.1
Dp w/m2 90 97 119 136 76 82 101 117 105 113 141 165
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VI.
V.
REFERENCES [1]
[2]
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[4]
[5]
WIND GENERATOR FOR BANGLADESH
Highest average wind speed of Bangladesh recorded in Kuakata and it is 8m/s. So wind turbine generator configuration must be geared driven otherwise direct driven generator that is DD-PMSG will make the system bulky as the average speed is very low. With this speed large power generation is quite difficult if size and weight of the generator is considered. DD-PMSG will not be good choice for WECS in Bangladesh. Among the other two configurations 1GPMSG will be the effective wind turbine configuration because of the following reasons [21]a. Annual energy yield/total cost of 1G-PMSG is lower than 3G-DFIG. b. Full-scale power converter gives 1G-PMSG full control over reactive power, voltage and frequency, flickers and harmonics. c. Multistage gearbox of DFIG is to have some drawbacks, such as heat dissipation from friction, regular maintenance and audible noise. d. The slip ring is used to transfer the rotor power requires a regular maintenance, and maybe result in machine failures and electrical losses. e. Power electronics converter of the rotor side of 3GDFIG must be protected with high protection system because during fault condition high rotor current resulted by high stator current may destroy them.
CONCLUSION
This paper surveys literature on different wind turbine concepts as well as two main generator types. A comparison is shown between three mostly used wind turbine generator configurations based on literature and finally an effective wind turbine generator configuration is investigated for wind condition of Bangladesh. Single stage geared driven permanent magnet synchronous generator with full scale converter is chosen the suitable configuration for wind energy conversion system in Bangladesh.
[3]
Although wind speed of above mentioned coastal areas is very low for mass power generation but power density at the same height also shows that some of the locations are quite potential for wind generators.
Ride through capability is required in 3G-DFIG so corresponding control strategies become complicated.
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