Vector Controlled Power Generation in a Point Absorber Based Wave Energy Conversion System

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1 Vector Controlled Power Generation in a Point Absorber Based Wave Energy Conversion System Jisha Thomas Chandy 1 and Mr. Vishnu J 2 1,2 Electrical & Electronics Dept of Engineering, Sree Buddha College Of Engineering Abstract Power Generation from renewable sources of energy has found a promising future and sustainability in the energy of waves. Wave Energy has a greater and more reliable potential compared to the other renewable sources of energy. Different Wave Energy Converters have been developed based on its location, power take off and the wave characteristics. The paper discusses on the software implementation of a point absorber based wave energy converter system, which utilizes Squirrel Cage Induction generator for power generation. The electrical machine is controlled by means of a simple vector control technique. The implementation is carried out in MATLAB Simulink platform and the wave forms are presented. Keywords Wave Energy, Vector Control, Point Absorber Buoy, Squirrel Cage Induction Generator, PI Controller. I. INTRODUCTION Wave energy was seen as a good potential of energy for power generation when there was an oil crisis in the 1970s [6]. Among the various other available renewable energy resources, energy in the form of waves possess greater predictability, higher energy density, and can travel longer distances with high energy. Different engineering concepts have given rise to different techniques of a wave energy converter. However there are still research works being done to design an optimum energy converter. Design challenges of a wave energy converter includes, the irregularity of the waves, loading on the WECs during extreme weather conditions, coupling of the WEC motion to drive a generator to produce a quality power to the utility grid, and finally, the requirement of vigorous testing which calls for expertise [3]. A point absorber buoy is a wave energy converter that is classified based on directional characteristics. It could absorb energy in all directions of the incident waves. It has smaller dimensions compared to the wavelengths, and float or are mounted on the sea bed. It usually produces a heave motion which is converted to linear or rotational motions to drive the generators [3]. Reference [2] suggests that the buoy can be coupled to the generator directly using a rack and pinion gear arrangement to produce a rotary motion and thus reduce the complexity and bulkiness of hydraulic systems, in high power point absorbers. Rotary generators are found to be more of an optimum solution to generating power, unlike the linear generators which have linear motion and suffer from underutilization. Squirrel Cage Induction Generators are more of a competitive solution compared other rotary generators like Permanent Magnet Synchronous Generators, in terms of its cost, ruggedness and reliability. The irregularity in the nature of the waves results in an oscillating motion of the buoy and hence an oscillation in the speed of the buoy and hence in the generated power. To produce an optimum power, a simplified vector control is adopted to control the SCIG. The paper focuses on modelling a 410KW point absorber based wave energy conversion system which is connected to a 25 kv distribution system and exports power to a 120 kv grid through a 30 km, 25 kv feeder. The SCIG transfers the generated power via the back to back converters that exhibit the bi directional power flow. The modelling is implemented in the MATLAB Simulink platform and it includes, the mechanical modelling of the All rights Reserved 311

2 modelling of the vector control technique, the back to back converters, and the connections to a grid. The description of the entire of wave energy conversion system, the modelling concepts and the results obtained will be presented in the following sections of this paper. II. WAVE ENERGY CONVERSION SYSTEM The block diagram for the point absorber based wave energy conversion system is shown in fig 1. When the sea bed is at a standstill, the buoy simply floats over it, and the weight of the buoy and the buoyancy force are balanced. When a sea wave hits the buoy it oscillates vertically. The rack and pinion gear arrangement converts this linear motion to a rotary motion and this runs the generator. The machine side converter provides the necessary excitation to the SCIG, while the grid side converter maintains the dc link voltage and dispatches power to the grid, when the power from the buoy falls to zero [2]. The power generation is controlled by the vector control technique. It takes the generator speed, reference speed and flux as its inputs and produces the gate pulses for the machine side converters. Fig 1. Block Diagram of the point absorber wave energy conversion system III. VECTOR CONTROL OF THE SCIG Vector Control of Induction Machines is carried out in order to control the variations in the air gap flux linkages, by controlling the magnitude and, frequency of the stator and rotor currents and their instantaneous phases. In vector control, the stator current commands ids* and iqs*, in the dq frames are derived from the required flux and torque values. Thus the torque producing component of the stator current (armature control) and the flux producing component of the stator current (field control), are obtained [4]. This control method is found most appropriate for the Wave energy conversion system, as the irregularities in the waves, can create oscillations in the air gap flux linkages, the speed and also the power. The indirect vector control method is proposed in this paper, where the generator speed, reference flux and reference torque are taken as inputs, and the stator current commands are generated. Figure 2 shows the phasor diagram which explains the indirect vector control scheme. Reference [2] [1] implements the vector control by using two current controllers, to produce the voltage command signals to be fed to the machine side voltage source All rights Reserved 312

3 Fig 2. Phasor diagram explaining the concept of Indirect Vector Control The proposed vector control is more simplified by utilizing, only one PI controller to produce the reference torque from the reference speed wref and the generator speed wr. The current commands iabc* and the instantaneous currents are taken as inputs to a current regulator to produce the required gate pulses for the machine side converters. 3.1 Implementation scheme of the Vector Control The schematic for implementing the vector control is shown in figure 3. The generator speed wr and a reference speed wref is fed to the PI controller to generate the reference torque Te*. The flux can be calculated as, Phi = Lm*id / (1+sTr) (1) where, id - d axis stator current, Lm - magnetizing inductance (H) and Tr = Lr/Rr; Lr rotor selfinductance, Rr rotor resistance; Fig 3. Block Diagram of the implementation of the Vector All rights Reserved 313

4 The stator current commands for the required torque and flux can be computed as, ids* = Phi* / Lm. (2) iqs* = (2/3)*(2/p)*(Lr/Lm)*(Te*/Phi) (3) The dq axis current commands are transformed to abc to obtain the current commands iabc*. The instantaneous stator current iabc is compared with the current commands and passed through a logic current regulator to produce the required gate pulses for the machine side converter. IV. MODELLING OF THE WAVE ENERGY CONVERSION SYSTEM 4.1 Mechanical Modeling The point absorber buoy is modeled as a mass spring damper system. Figure shows the buoy in the mass spring damper form. At stand still or equilibrium, both the sea wave height hw and displacement of the buoy from an equilibrium xb, are equal. During the incidence of the waves, the xb is forced to change due to the change in hw. The buoyancy force is proportional to (hw xb), and hence the spring force is represented as Kb(hw xb), where Kb is the spring constant that depends of the density of water ρ, and area of the buoy Ab. The water and surface buoy drag force is the damping force Db acting on the buoy [2]. The force applied on the buoy is the generator reaction force, Fm. So the mathematical equation that depicts the mechanical dynamics of the buoy is given by, Mb + Kb(xb-hw)+Db( ) = -Fm. (4) Kb = Ab ρg..(5) Mechanical Torque Tm is given by Tm = Kc Fm, where Kc is the gear ratio. Fig 4. Mass Spring Damper representation of the point absorber buoy. 4.2 Simulink Models and Parameters Fig 5. Simulink Model of the WEC All rights Reserved 314

5 Fig 6. Simulink Model of the Internal of the WEC System Table 1. Buoy Specifications Mass of the buoy (Mb) 18000kg Equivalent buoy constant (Kb) 960 kn/m Damping coefficient (Db) 300 kn-s/m Speed conversion ratio (Kc) 5/300 Table 2. Generator Parameters Nominal Power 410 KW Nominal Voltage 300V Stator resistance (Rs) p.u Rotor resistance (Rr) p.u Stator Leakage inductance (Lls) 0.18 pu Rotor Leakage inductance (Llr) 0.16 pu Magnetizing Inductance (Lm) 2.9 pu Number of poles (p) 4 Reference Speed (wref) 1.2 pu Reference Flux (Phi*) 0.96 DC Link Capacitor (C) 10000e-6 F Nominal dc bus Voltage 1150 V. RESULTS A 410 KW SCIG was chosen for the implementation. All the calculations and parametric distribution is carried out in per units. The obtained power, voltage and current waveforms are shown below. Universal Bridge converters are chosen for modelling the Machine and grid side All rights Reserved 315

6 Fig 7. B575 voltage in pu. Fig 8. Current Iabc_B575 in pu Fig 9. Generated Power in All rights Reserved 316

7 Fig 9. DC Link Voltage (volts) VI. CONCLUSION Wave Energy is a niche renewable energy type which has been seeing slow development for the past many years. The paper has discussed on the control method i.e. the vector control, of the electrical machine in a point absorber based WEC system. The oscillatory nature of the power waveforms can be further reduced, by designing suitable storage systems, or other means. Vector control has made it easier to control the field and the armature independently. The proposed vector control scheme follows its basic algorithm and is more simplified with reduced number of controllers, thus making the design approach simpler. REFERENCES [1] S. Hazra and P. Sensarma, Vector approach for self-excitation and control of induction machine in stand-alone wind power generation, IET Renewable Power Generation, vol. 5, no. 5, pp , [2] Samir Hazra, and Subhashish Bhattacharya, Control of Squirrel Cage Induction Generator in an Oscillating Point Absorber Based Wave Energy Conversion System IEEE, [3] B. Czech and P. Bauer, Wave energy converter concepts: design challenges and classification, IEEE Ind. Electron. Magazine, vol. 6, no. 2, pp. 4-16, Jun [4] R. Krishnan Electric Motors and Drives, Modelling, Analysis and Control, Upper Saddle River New Jersey Prentice Hall [5] Swagat Pati, Performance and Power Factor Improvement of Indirect Vector Controlled Cage Induction Generator in Wind Power Application Thesis Report in Mtech EEE, National Institute of Rourkela, July [6] Eiril Bjørnstad Control of Wave Energy Converter with constrained electric Power Take Off Thesis Report in Master of Science in Energy and Environment, Norwegian University of Science and Technology, January All rights Reserved 317