An Energy Storage Technique for Gearless Wind Power Systems

1 An Energy Storage Technique for Gearless Wind Power Systes Sina Hazehlouia, Meber, IEEE, Afshin Izadian, Senior Meber, IEEE, and Sohel Anwar Abstract Hydraulic wind power transfer systes allow collecting of energy fro ultiple wind turbines into one generation unit. They bring the advantage of eliinating the gearbox as a heavy and costly coponent. The hydraulically connected wind turbines provide variety of energy storing capabilities to itigate the interittent nature of wind power. This paper introduces the hydraulic circuitry and control algorith for a novel wind energy electrical energy storage technique. The siulation results deonstrate successful operation of the storage to aintain the fluid in the syste and control the generator speed at a reference. I. INTRODUCTION HE hydraulic wind power transfer syste consists of a Tfixed displaceent pup driven by the prie over (wind turbine) and one or ore fixed displaceent hydraulic otors. The hydraulic transission uses the hydraulic pup to convert the echanical input energy into pressurized fluid. Hydraulic hoses and steel pipes are used to transfer the harvested energy to the hydraulic otors [1]. The proposed energy transfer syste offers several advantages ahead of their geared counterparts including the replaceent of gearbox with a hydraulic transission syste. Unlike traditional wind power generation, this syste offers lower operating and aintenance costs and allows for integration of ultiple wind turbines to one central generation unit. The new wind energy harvesting technique incorporates power generation equipent of individual towers in a central power generation unit. With the introduction of this new approach, the wind tower only accoodates a hydraulic pup, which passes the hydraulic fluid through high-pressure pipes attached to the hydraulic otor coupled to a generator at ground level. This will result in enhanced reliability, increased life span, and reduced aintenance cost of the wind turbine towers. Other benefits of this technique include highrate of energy transfer and size reduction of the power electronics [2-5]. The energy harvesting fro interittent sources, require energy storage units to sooth out the generation of power and frequency stability, which can easily deviate fro 60 Hz as the wind speed changes. High-pressure hydraulic systes provide an excellent platfor for incorporation of echanical and electrical energy storage units. This paper addresses the circuitry needed for energy storage and the control algorith that can enable it. In general, high wind speeds result in generation of excess flow in the syste. The energy of this flow is captured by an auxiliary generator and stored in a storage unit. The stored energy is released back to the syste to run the ain pup when the wind speed drops. In this case, the flow generated by the wind turbine is augented by the auxiliary pup flow to aintain the angular velocity deands of the loaded priary generator. A two-loop control syste with PI and a rate liit sliding control is designed to aintain the reference angular velocity [13-15], and control the storage charge and discharge power. II. HYDRAULIC WIND ENERGY TRANSFER SYSTEM The hydraulic wind power transfer syste consists of a fixed displaceent pup driven by the prie over (wind turbine) and one or ore fixed displaceent hydraulic otors. The hydraulic transission uses the hydraulic pup to convert the echanical input energy into pressurized fluid. Hydraulic hoses and steel pipes are used to transfer the harvested energy to the hydraulic otors [1]. Figure 1 displays a scheatic diagra of the wind energy transfer and the energy storage syste. As the figure deonstrates, a fixed displaceent pup is echanically coupled with the wind turbine and supplies pressurized hydraulic fluid to two fixed displaceent hydraulic otors. The hydraulic otors are coupled with electric generators to produce electric power in a central power generation unit. Since the wind turbine generates a large aount of torque at a relatively low angular velocity, a high displaceent hydraulic pup is required to flow high-pressure hydraulics to transfer the power to the generators. The pup ight also be equipped with a fixed internal speed-up echanis. Flexible highpressure pipes/hoses connect the pup to the piping toward the central generation unit. Manuscript received February 15 2013. This work was supported by IUPUI RSFG Funds and a grant fro the Solution Center, and was conducted at Energy Systes and Power Electronics Laboratory, Purdue School of Engineering and Technology, IUPUI. S. Hazelouia, A. Izadian, and S. Anwar are with the Purdue School of Engineering and Technology, IUPUI, Indianapolis, IN. 46202. Eail: izadian@ieee.org.. Fig. 1. Scheatic of the high-pressure hydraulic power transfer syste. The hydraulic pup is in a distance fro the central generation unit. 978-1-4673-5208-6/13/$31.00 2013 IEEE

2 The hydraulic circuit uses check valves to guarantee the unidirectional flow of the hydraulic flows. A pressure relief valve protects the syste coponents fro the destructive ipact of localized high-pressure fluids. These units also provide proper path for the energy storage to circulate the fluid in the syste without going through the hydraulic pup at the wind tower. The hydraulic circuit contains a specific volue of hydraulic fluid, which is distributed between hydraulic otors using a proportional valve. Since the electrical energy produced at the central generation unit could only be supplied to the grid at a specific frequency, a velocity control unit is required to aintain the constant angular velocity at the priary otor-generator. The speed regulation is accoplished by regulating the flow through a proportional valve and directing the excess fluid to the auxiliary otor. The operation of the hydraulic syste is split into two categories, naely syste operation at high wind and syste operation at low wind. A. Syste Operation at High Wind The wind speed fluctuates over tie. Therefore, utilization of fixed displaceent pups result in flow variation in the syste. If the wind speed is higher than the reference that generates 60 Hz voltage in the output, the condition is called high wind. If the excess flow of power and its energy is not captured, the generator s voltage frequency will deviate fro 60Hz. The proportional valve is regulated such that the required flow is delivered to the priary hydraulic otor, and the excess energy is captured by the auxiliary hydraulic otor. The auxiliary hydraulic otor is coupled with an electric otor/generator. At high wind, the auxiliary hydraulic otor runs the electric generator. The electric generator converts the echanical energy of the rotating shaft into electric energy and stores it in batteries. The priary otor is coupled to the ain generator and supplies electricity at a specific frequency to the load. Figure 2 illustrates the hydraulic circuit of the energy transfer syste at high wind. To regulate the aount of energy captured by auxiliary otor a rate liit controller is utilized. Fig. 2. Operating configurations of the syste at high wind. Fig. 3. Operating configurations of the syste at low wind. B. Syste Operation at Low Wind If the wind speed drops below a threshold speed, the condition is considered low wind. In this condition, the flow generated by hydraulic pup is not sufficient enough to aintain the reference angular velocity at the priary otor. In order to copensate for the flow deficiency, the energy stored in the storage should be released back to the syste. The storage in any for can run the auxiliary hydraulic pup to generate an augented pressurized fluid in the syste. Figure 3 illustrates the syste operation at low wind. In this configuration, a PI controller regulates the storage discharge rate such that the ain generator aintains the rated frequency. C. Hydraulic Circuit Dynaics The coplete atheatical odel of the hydraulic wind energy transfer syste is represented in [2-12]. Hydraulic pups deliver a constant flow deterined by Q, (1) p= Dpω p klp, Pp where Q is the pup flow delivery, D is the pup p p displaceent, k is the pup leakage coefficient, and is Lp, Pp the differential pressure. The flow and torque equations are derived for the hydraulic otor using the otor governing equations. The hydraulic flow supplied to the hydraulic otor can be obtained by Q = Dω+ kl, P, (2) where Q is the otor flow delivery, is the otor D displaceent, k is the otor leakage coefficient, and P is L, the differential pressure across the otor. Torque at the otor driving shaft is obtained by T, = DPη ech, (3) The total torque produced in the hydraulic otor is expressed as the su of the torques fro the otor loads and is given as T = TI + TB + TL, (4) where T is total torque in the otor and represent TI, TB, TL inertial torque, daping friction torque, and load torque, respectively. This equation can be rearranged as T TL = I( dω dt) + Bω, (5) where I is the otor inertia, ω is the otor angular velocity, and is the otor daping coefficient. B D. Hose Dynaics The fluid copressibility odel for a constant fluid bulk odulus is expressed in [9]. The copressibility equation represents the dynaics of the hydraulic hose and the hydraulic fluid. Based on the principles of ass conservation and the definition of bulk odulus, the fluid copressibility within the syste boundaries can be written as Qc = ( V β )( dp dt), (6) where V is the fluid volue subjected to pressure effect, β is the fixed fluid bulk odulus, P is the syste pressure, and

3 Q c is the flow rate of fluid copressibility, which is expressed as Qc = Qp Q. Hence, the pressure variation can be expressed as dp dt = ( Q Q ) β V. E. Storage Dynaic Without loss of generality, in this paper we consider the storage as a battery. The excess energy which is captured by the auxiliary otor is transfored to electrical energy through a generator. The charge current is calculated as I T ω η p / p / gen B = (9) VB where I B is the battery current T is the auxiliary p / pup/otor torque, ω is the auxiliary pup/otor p / angular velocity, and VB is the battery voltage. The battery state of charge (SOC) which is defined as the percentage of the initial battery capacity is calculated as C i SOC = (10) C 0 where C is the available charge of the battery, and i C 0 is the noinal capacity of the battery. The auxiliary otor/pup is coupled with the electric generator/otor. The dependency of the angular velocity of the auxiliary pup to the extracted battery current is expressed such that ki ω = (11) paux B p where k is the current coefficient of the electric generator which is coupled with the auxiliary pup. III. SYSTEM OPERATION AND DYNAMIC MODEL 1. Syste Operation at High Wind The overall hydraulic syste can be connected as odules to represent the dynaic behavior. Block diagras of the hydraulic transission syste using MATLAB Siulink are deonstrated in Figures 4. The odel incorporates the atheatical governing equations of individual hydraulic circuit coponents. The bulk odulus unit generates the operating pressure of the syste. Figure 4 shows a block diagra of the wind energy transfer syste in high wind. According to the figure, the wind turbine supplies power at a specific angular velocity to the ain hydraulic pup. The hydraulic pup supplies pressurized hydraulic fluid to the proportional valve which distributes the hydraulic fluid between the otors based on the reference priary otor angular velocity. The auxiliary otor captures the surplus energy of the flow and drives the electric generator to charge electrical storage. The generated electrical energy is stored in a battery through the power electronic converters. The priary otor is coupled with the ain generator and supplies electricity to the grid. (7) (8) Figure 5 displays the atheatical odel of every hydraulic coponent in the transission syste. The flows and pressures are calculated for all hydraulic coponents. The data fro the hydraulic circuit is utilized to calculate the flow of energy into the battery. Fig. 4. Hydraulic transission scheatic diagra in gasoline configuration. 2. Syste Operation at Low Wind The odel of the wind energy transfer at low wind is siilar to the high wind condition. However, in this configuration, the transfer syste is driven by the energy stored in the battery when released back to the syste. The current extracted fro the battery is regulated to accoodate the priary otor angular velocity deands. The auxiliary otor can be driven as a pup by the electric otor and flows pressurized fluid augented with the ain pup flow. The copressibility block calculates the gauge pressure along the pups and hydraulic otor terinals. Figures 5 show the block diagra of the atheatical odel of the hydraulic transfer syste at low wind conditions [10][11]. Fig. 5. Hydraulic transission scheatic diagra in electric configuration. IV. CONTROLLER DESIGN This section introduces the design of the controllers which are required to aintain the reference priary otor angular velocity at both high and low wind conditions. A rate liit controller regulates the position of the proportional valve at the high wind operation to aintain tracking of the reference speed. A PI controller is also utilized to regulate the battery discharge current at low wind operation. A. Rate liit Controller Design The rate liit controller directs the flow of the hydraulic fluid fro wind turbine at high wind, and fro wind turbine and auxiliary otor at low wind to the ain hydraulic otor. The controller adjusts the position of the valve towards the priary otor path to aintain tracking of the reference angular velocity. Figure 6 illustrates the diagra of the rate liit controller. At the high wind operation, the rate liit controller easures the error between the reference angular velocity and the priary pup angular velocity. If the error value is positive, then the controller sends a nuber of negative fixed

4 displaceent step signals to the valve, to regulate the flow and track the reference velocity. If the error value is negative, the controller opens the valve by sending a fixed positive step displaceent signal to the valve. Figure 7 shows the structure of the rate liit controller. The step values are designed to aintain syste stability while both fast response and error itigation criteria are fulfilled. Fig. 6. The diagra of the rate liit control closed loop syste Fig. 7. The rate liit controller structure B. PI Controller Design At low wind conditions and when the battery is being discharged, a PI controller is utilized. In this case, the PI controller regulates the angular velocity of the auxiliary pup to aintain velocity reference of the priary otor. The PI controller regulates the aount of battery discharge current to run the electric otor/generator coupled with the auxiliary pup. Figure 8 represents the closed-loop diagra of the PI control syste. proportional valve, and the PI controller for battery current controller. Figure 9 illustrates the angular velocity profile which is supplied to the hydraulic transission syste. The angular velocity profile is used to deterine the syste operating odes both at low wind and at high wind. Initially, a 400 rp step angular velocity is supplied to the hydraulic pup which siulates the high wind condition. The pup angular velocity is reduced to 100 rp after 5 seconds to siulate the low wind condition. Then, the angular velocity is increased to 600 rp to restore the high wind condition at t=10 sec. According to the hydraulic wind energy transfer configuration (High wind or Low wind), the associated controller generates a control coand to aintain the tracking of the reference angular velocity. The transission syste switches between these two configurations based on the wind speed. TABLE I SIMULATION PARAMETERS Quantity Sybol Value Unit k Current Coefficient 10 V B Battery Voltage 12 Volts C 0 Initial Battery Capacity 31.25 Ap.hr SOC 0 Initial State of Charge 50 % On/off Controller Step 0.0001 in Size k p Proportional Gain 0.001 k p Integral Gain 10 Fig. 9. Hydraulic pup angular velocity profile. Fig. 8. The diagra of the PI control closed loop syste The proportional gain adjusts the response tie characteristics such as settling tie and rise tie. At higher proportional gains (within the region of stability) a faster syste response is obtained. A proper integral gain itigates the steady state tracking error. V. SIMULATION RESULTS AND DISCUSSION In this section, the atheatical odel of the hydraulic wind energy transfer with the storage unit behavior is siulated and the perforance of the control syste to aintain the reference angular velocity is evaluated. The siulation paraeters are listed in Table 1. A constant priary otor angular velocity of 1000 rp is used as a reference for both the rate liit controller of the Fig. 10. Flow generated by the hydraulic pup. Figure 10 displays the flow passing through the ain pup when the angular velocity profile of Figure 10 is applied. According to this figure, the wind speed is initially high enough to aintain the reference priary otor angular velocity, and the pup flow is distributed between the ain and auxiliary otors through the proportional valve. The wind speed drops after 5 seconds and the syste switches to the low wind configuration, at which the ain pup flow is

5 augented with the auxiliary pup flow. The high wind conditions will occur in 5 seconds fro this event. the valve position at high wind with decreented rate to reduce the flow of ain otor and aintain the required velocity. The siulation results illustrate a rise tie of 0.135 sec and an overshoot percentage of 15.2%. Fig. 11. Priary otor flow. Fig. 14. Hydraulic Transission Auxiliary otor/pup angular velocity. Figure 15 illustrates the rate liit control effort to aintain the fluid in the syste by regulating the proportional valve position. The controller effort is zero while the syste runs in low wind condition between 5 to 10 seconds. The controller effort was either 0.0001 to open the valve or -0.0001 to close the valve. The siulation results deonstrate a high perforance syste operation. Fig. 12. Auxiliary otor/pup flow. Figures 11 and 12 show the priary otor and auxiliary otor flows. According to these figures, the rate liit controller initially controlled the syste flow distribution by regulating the proportional valve position. In this configuration, the auxiliary otor captured the excess flow energy and stored it in a battery though the electric generator. At low wind condition, the PI control regulated the current fro the battery and ran the electric otor to copensate for the ain pup flow and aintain the reference velocity. According to Figure 12, since the otor angular velocity was proportional to the flow, the controller aintained the fluid flow at a certain rate to aintain the reference velocity. The syste switching between these configurations resulted in an instantaneous variation in the syste shown as spikes at tie 5 and 10 seconds. Fig. 15. Control effort of the rate liit controller to regulate the proportional valve position. Figure 16 illustrates the position of the proportional valve which was regulated by the rate liit controller. According to the figure, the valve was iediately closed enough fro the initial position to reduce the flow of the priary otor at high wind. The valve was copletely opened in low wind to direct the entire flow towards the priary otor path and augent with the auxiliary otor flow. Fig. 13. Coparison of the priary otor angular velocity and the reference angular velocity. Figures 13 and 14 illustrate the angular velocities of the priary otor and the auxiliary otor/pup. As deonstrated in Figure 13, the controller successfully adjusted Fig. 16. Proportional valve position to distribute hydraulic flow between the otors to aintain the reference angular velocity.

6 Fig. 17. Control effort of the PI controller to regulate the discharge current. Figure 17 illustrates the effort of the PI controller in discharging the battery. The controller effort was zero when the syste was in high wind condition. As soon as the wind condition changed to low wind speed, the PI controller adjusted the battery discharge current to aintain the priary otor angular velocity. The charging process was deterined by the aount of fluid redirected fro the proportional valve to the auxiliary otor. Figure 18 displays the battery charge/discharge current. Fig. 18. Hydraulic transission battery charge/discharge Current. Fig. 19. Hydraulic transission syste battery state of charge. Figure 19 shows the battery state of charge variation as the syste operation ode changed. As the figure illustrates, the SOC increased in high wind condition. Figure 19 illustrates battery charge/discharge current controlled by the PI. The current is negative during charge cycles and positive when the battery is being discharged. The figure deonstrates a high perforance control of the battery charge and discharge process. dropped below a certain threshold. A atheatical odel of the storage syste was represented for both high wind and low wind operating conditions. A rate liit controller was designed to regulate the valve position opening to track a reference angular velocity at high wind. A PI current controller was utilized to regulate the plant operation at low wind condition. The siulation results deonstrated successful operation of the energy storage and release and its effects on the hydraulic wind energy syste operation. REFERENCES [1] K. Wu, et al., "Modelling and identification of a hydrostatic transission hardware-in-the-loop siulator," International Journal of Vehicle Design, vol. 34, pp. 52-64, 2004. [2] Sina Hazehlouia, Afshin Izadian, Ayana Pusha, Sohel Anwar, Controls of Hydraulic Wind Power Transfer., In Proceeding of IECON 2011 37th Annual Conference on IEEE Industrial Electronics Society, pp. 2475-2480, 2011. [3] Sina Hazehloui, and Afshin Izadian, State-Space Representation of a Hydraulic Wind Power Transfer., In Proceeding of International IEEE Conference in Electro/Inforation Technology (EIT), May 2012. [4] US Patent Application, US2010/061972 [5] Sina Hazehlouia, Afshin Izadian Modeling of Hydraulic Wind Power Transfer., In Proceeding of Power and Energy Conference at Illinois (PECI) 2012 IEEE, pp. 1-6, 2012. [6] M.V. Gorbeshko, Developent of Matheatical Models for The Hydraulic Machinery of Systes Controlling the Moving Coponents. Hydrotechnical Construction. Volue 3, No. 12. 1997 [7] http://www.athworks.co/help/toolbox/physod/hydro/ref/fixeddispla ceentpup.htl [8] http://www.athworks.co/help/toolbox/physod/hydro/ref/hydraulic otor.htl [9] A. V. Akkaya, Effect of Bulk Modulus on Perforance of a Hydrostatic Transission Control Syste Sadhana, vol. 31, Part. 5, October 2006, pp. 543-556. [10] A. Esposito, Fluid Power with Application, 7 th Edition, Prentice Hall, 2009. [11] G. Licsko, A. Chapneys, and C. Hos, Dynaical Analysis of a Hydraulic Pressure Relief Valve, Proceedings of the World Congress on Engineering 2011, vol. 2, July 1-3, 2009, London, U.K. [12] A. Pandula, and G. Halasz, Dynaic Model for Siulation of Check Valves in Pipe Systes. Periodica Polytechnica, Mech. Eng. Series, vol 46/2, pp. 91-100, 2002. [13] S. Gao, and N. Zhang A Review of Different Methodologies for Solving the Proble of Wind Power s Fluctuation, International Conference on Sustainable Power Generation and Supply (SUPERGEN 09), pp. 1-52009. [14] GC. Carrilo, A. E. Feijoo, J. Cidras, and J. Gonzalez, Power Fluctuation in an Isolated Wind Plant, IEEE Transaction on Energy Conversion, vol. 19, no. 1, March 2004. [15] P. Sorensen, N. A. Cutululis, A. V. Rodriguez, L. E. Jensen, J. Hjerrild, M. H. Donovan, and H. Madsen, Power Fluctuations fro Large Wind Fars, IEEE Transaction on Power Systes, vol. 22, no. 3, August 2007. VI. CONCLUSION This paper presented an energy storage technique to capture the excess energy of hydraulic wind transission syste. The stored energy was released to the plant when the wind speed