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Offshore-Wind Turbine and Tidal Turbine for. Power Fluctuation Compensation (HOT-PC). Mohammad Lutfur Rahman, Student Member, IEEE, Shunsuke Oka, ...
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Hybrid Power Generation System Using Offshore-Wind Turbine and Tidal Turbine for Power Fluctuation Compensation (HOT-PC) Mohammad Lutfur Rahman, Student Member, IEEE, Shunsuke Oka, and Yasuyuki Shirai, Member, IEEE Abstract—Hybrid power generation system using Offshore-wind turbine and Tidal turbine for Power fluctuation Compensation (HOT-PC) is an autonomous power system. Electric power is generated from both offshore wind and tidal and is distributed over the load system. Power quality problems such as frequency fluctuations and voltage sags, which arise due to a fault or a pulsed load, can cause interruptions of critical loads. This can be a serious concern for the survivability of the offshore-wind power system. In the proposed system HOT-PC, the induction generator for the tidal turbine can play also as a motor to store kinetic energy to mitigate (make stable) the frequency and voltage fluctuation. The tidal generator is controlled by six-pulse insulated gate bipolar transistor (IGBT) bidirectional converter system. When the generator/motor is driven by the converter to exceed the tidal turbine rotating speed, it is mechanically isolated from the turbine by a one-way clutch and can charge and discharge the kinetic energy like a fly-wheel. In order to study the feasibility of the system, the laboratory scale model was designed and made. This paper presents the fundamental configuration of the system, the experimental results, and discussions of basic operation performance of the proposed system. Index Terms—Energy conversion, fly-wheels, tidal power generation system, wind power generation system.

I. INTRODUCTION

O

NE important aspect of wind turbine applications, especially in an industrial environment, is that wind turbines generate electricity without creating pollution. Power systems based on renewable sources are affected by fluctuations on the generation side due to the seasonal and random nature of the energy resource [1], [2]. At the same time, the loads have also variable power demand. In this case, energy storage systems (batteries, fly-wheels, and so on) play an important role in matching up generation and demand. We have proposed the Hybrid power generation system using Offshore-wind turbine and Tidal turbine (HOTT) and have carried out the simulation studies using one of a real image of our proposed HOTT system [3]–[5]. The mathematical models of each component were modified from IEEE models available in PSCAD/EMTDC master library [6]. In the simulation model, the Manuscript received December 11, 2009; revised April 27, 2010; accepted May 05, 2010. Date of publication May 18, 2010; date of current version June 23, 2010. The authors are with the Graduate School of Energy Science, Department of Energy Science and Technology, Kyoto University, Kyoto 606-8501, Japan (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TSTE.2010.2050347

rated capacity and voltage of the wind turbine generator were 2.3 MW, 5.8 kV and those of the tidal turbine generator were 1.0 MW, 2.0 kV. The rotor radius of the wind turbine was 60 m and that of the tidal was 15 m. Each generator voltage was boosted up by the transformers to 12 kV and each generated ac power was converted to dc through a six-pulse GTO converter. The dc power was transmitted through the underwater dc transmission cable more than 10 km to the grid on the land. The system performances were successfully demonstrated by the simulation study based on the mathematical models in PSCAD. In this paper, we propose an improved system of HOTT to compensate generated power fluctuation and to stabilize frequency and voltage (HOT-PC). In the proposed system, a tidal generation system of smaller capacity than a wind generation system is installed parallel to the offshore-wind system. Generally, the tidal flow is more stable than wind flow. The output of the tidal generator (induction machine) is controlled by the use of an insulated gate bipolar transistor (IGBT) bidirectional inverter system in order to compensate the power fluctuation of offshore-wind turbine generator. Additionally, the tidal induction machine rotor can be mechanically isolated from the tidal turbine shaft by a one-way clutch, while the rotation speed of the turbine is lower than that of the rotating magnetic field given by the inverter. In other words, the induction machine can be operated as not only the tidal turbine generator but also a fly-wheel energy storage and motor/generator system by use of the IGBT inverter control. The basic control strategy and the feasibility of the proposed hybrid generation system were investigated by use of a small laboratory base hybrid system model according to a real system illustration. II. PROPOSED HOT-PC MODEL SYSTEM A. Model Setup Fig. 1(a) shows an image of the proposed system (HOT-PC) setup. A number of these make up an offshore-wind/tidal generation farm and the ac generation power is converted and gathered to dc power. The dc power is transmitted through a dc under-sea transmission cable to a grid dc/ac converter station. In order to investigate the system performances and the control strategies, a small laboratory base hybrid power system model, whose photograph and schematic view are shown in Fig. 1(b) and (c), were designed and made.The system has two types of generators, the tidal motor/generator and the offshore wind turbine generator. The tidal turbine (induction machine) can act as a motor or as a generator depending on the need. The

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Fig. 2. Offshore-wind turbine generator experimental model.

TABLE I RATING OF MAIN COMPONENTS (OFFSHORE-WIND SERVO MOTOR)

TABLE II RATING OF MAIN COMPONENTS (CORELESS SYNCHRONOUS GENERATOR)

Fig. 3. Tidal turbine generator/motor experimental model.

Fig. 1. (a) HOT-PC actual image. (b) Photography of laboratory scale prototype model of hybrid offshore-wind and tidal turbine with fly-wheel. (c) Schematic of prototype model of hybrid offshore-wind and tidal turbine with fly-wheel.

tidal generator provides smooth total output power, whereas the output power of a wind turbine depends on the wind velocity.

B. Offshore-Wind Turbine Generation System Fig. 2 shows an experimental model of the offshore-wind turbine generator system. It consists of a coreless synchronous generator and a servo-motor. The offshore-wind turbine is simulated by the servo-motor. To make the model system with the small servo-motor, the rated rotating speed is 2500 rpm and the gear ratio is 10.5 : 1. In a real system, the wind turbine would be

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Fig. 4. HOT-PC system configuration.

of lower rotating speed without the step-down gear. The rotating speed or the torque of the servo-motor is controlled by a computer. The electrical energy depends on the rounds per minute (rpm) of servo-motor which drives the coreless generator. Wind turbine generated ac power is converted to dc power through a six-pulse diode rectifier. The parameters of the servo-motor and the coreless synchronous generator are listed in Tables I and II, respectively. C. Tidal Turbine Generation/Storage (Fly-Wheel) System Fig. 3 shows the experimental model of a tidal turbine induction generator/motor. A rotating shaft of the servo-motor, which simulates the tidal turbine, is connected to that of the induction machine (generator/motor) through a one-way clutch. Normally, the servo-motor (tidal turbine) drives the induction generator to produce the electric energy. On the other hand, the induction machine is also driven by a bidirectional IGBT inverter/converter. The generation power can be controlled quickly by the frequency control of the inverter. Additionally, when the induction machine is accelerated by the inverter control up to higher rotation speed than that of the servo-motor (turbine), the servo-motor clutch turns to the OFFstate and the induction machine works as a motor. The rotational kinetic energy is stored as a function of the square of the rotation speed like a fly-wheel energy storage system. The stored energy can be extracted by decelerating with the inverter control. This generation system has the ability and flexibility to compensate the power fluctuations with proper control scheme. The design parameters of the servo-motor and the induction machine are listed in Tables III and IV, respectively. The rated speed of the induction machine was selected as 1110 rpm in order to store the rotating kinetic energy. To meet this speed,

TABLE III RATING OF MAIN COMPONENTS (TIDAL SERVO MOTOR)

TABLE IV RATING OF MAIN COMPONENTS (INDUCTION MACHINE)

the rated speed of the servo-motor (turbine) was 2500 rpm; however, the tidal turbine rotation speed would be much lower than that of the servo-motor and so the step-up gear would be necessary in the real system. D. Circuit Configuration Fig. 4 shows a schematic configuration of the proposed wind and tidal hybrid generation system (HOT-PC) model experimental setup. The offshore-wind coreless synchronous generator output is simply rectified by a six-pulse diode bridge to charge a dc capacitor. The tidal turbine induction generator/ motor output is connected to the dc capacitor through a six-pulse IGBT bidirectional inverter/converter. The dc link capacitor is connected to the commercial grid through a grid-connected inverter of single-phase and three-wire. The grid-connected inverter is of a transformer-less half-bridge type with a boost-up chopper circuit. The voltage-source inverter output current is

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Fig. 5. Induction machine operating mode change from motor to generator.

controlled by pulsewidth modulation (PWM) controller under the maximum power point tracking (MPPT) control. The MPPT control searches and keeps the dc link capacitor voltage which gives the maximum output power by controlling the output ac current. It gives the dc voltage perturbations of 4 V up and down

(2 V/s) every 4 s and checks how to change the output power due to them, and then decides the dc-voltage reference at the next stage to give more power. Several small controllers are implemented at both ends to provide the required performance to the system.

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Fig. 6. Induction machine operating mode change from generator to motor.

III. EXPERIMENTS AND DISCUSSIONS This section describes basic experiments to evaluate fundamental performances of the proposed hybrid model system.

A. Start Up First, the offshore-wind turbine servo-motor starts up with rotating speed control to build up the dc capacitor voltage around 110 V. Second, the grid connection inverter with MPPT control

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starts to convert dc power to ac (1-phase 3-wire) grid. Then, the IGBT bidirectional converter starts to drive the induction machine as a motor with the one-way clutch OFF (Fly-wheel mode). At last, the tidal turbine servo-motor starts up and when the rotating speed of it exceeds that of induction motor driven by the bidirectional inverter, the induction machine turns to the generator mode with the one-way clutch ON (Generator mode) and converts the tidal turbine energy to the grid power together with the wind turbine power through the dc link capacitor and the converters. B. Motor to Generator Fig. 5 shows one of the experimental results, that is, from top to bottom, the generator/motor voltages, currents and the instantaneous active and reactive powers of the tidal generation system, those of the offshore-wind generation system, the voltage, currents and the powers in dc link circuit, those of the load (ac grid) side and the rotating speed of the induction generator/motor, the servo-motors, and the coreless synchronous generator. The bottom left figure shows the load side ac current in the steady-state condition. Seen from the figure of the rotation speed, while the tidal turbine servo-motor worked at 875 rpm, the induction machine was driven in the motor mode at 1200 rpm (the one-way clutch was OFF). The wind turbine servo-motor and the generator were operated with a constant speed of 82 rpm throughout the test. The tidal turbine servo-motor was accelerated by 125 rpm every 2 s, and the rotating speed was 1250 rpm ( 1200 rpm) at a time of 6.16 s (the one-way clutch was ON). The tidal system (induction W to the machine) changed from the motor mode W. generator mode As shown in the dc side powers, the offshore-wind generated power Pdc2 was almost 190 W constant and the tidal generated power Pdc1 stepped up from 25 W (motor mode), to 90 W (at 6.16 s) and 200 W (at 8.45 s). The total dc power (Pdc3) was 160 W during motor mode (0.0–6.16 s), and stepped up to 270 W (6.17–8.45 s), 410 W (8.46–10 s). As is shown in the figure of the dc link voltage Vdc, the MPPT controller gave the dc voltage perturbations of 4 V up and down (2 V/s) every 4 s. The grid connection inverter with MPPT control worked according to the generation power changes at 6.16 and 8.45 s without any large disturbance in the dc link voltage. C. Generator to Motor Fig. 6 shows one of the experimental results when the HOT-PC model system successfully operated from the generator mode of the tidal induction machine to the motor mode. As shown in the image, the tidal system (induction machine) changed from the generator mode to the motor mode at 4.4 s smoothly. The induction machine was mechanically isolated by the clutch from the tidal turbine servo-motor after 4.4 s. D. Fly-Wheel Operation The fundamental test results showed that the proposed HOT-PC systems have an ability to cope with the power fluctuation compensation with a proper control strategy. Additionally, with proper frequency control of the bidirectional inverter, the kinetic energy of the tidal induction machine

Fig. 7. Powers in dc side in the fly-wheel operation of the induction machine. The induction machine was driven by the inverter whose voltage frequency was stepped down from 100 to 40 Hz. The kinetic (rotating) energy was extracted at each decelerating condition and converted to Pdc1 which is summed up to the total output power Pdc3.

can be used to assist in compensating fluctuation in the total generation power. Even when the tidal flow is too weak to produce any electric power, the induction machine plays as the fly-wheel energy storage system and compensates the power fluctuation to some extent. Fig. 7 shows the dc side powers of the tidal system Pdc1, the wind system Pdc2, and the total system Pdc3, while the rotating speed of the tidal induction motor was stepped down by 20 Hz from 100 Hz (2000 rpm) to 40 Hz (800 rpm) by changing the bidirectional inverter voltage frequency. As seen from Pdc1 in Fig. 7, the tidal induction machine worked as a motor to extract the kinetic energy. The extracted kinetic energy was summed up to Pdc3 together with the wind power Pdc2 and converted to the ac grid power through the grid connected inverter. The ability to compensate the wind power fluctuation by using the tidal induction machine as a fly-wheel was confirmed; however, the inertia of the machine rotor is not enough and will be increased in the next step. IV. CONCLUSION The fundamental experimental model results with the proposed HOT-PC test system demonstrate satisfactory operation for a range of wind and tidal speeds using a Diode rectifier, IGBT bidirectional converter, and MPPT grid inverter. Based upon the test results, it can be concluded that: 1) The wind and tidal hybrid generation system circuit model linked by use of dc link capacitor and the MPPT grid inverter showed good performances and stable operation. 2) The tidal induction generator/motor system with the bidirectional converter and the one-way clutch has an ability to compensate the power fluctuation due to the wind conditions. The induction machine can switch smoothly between generator and motor by controlling rotation speed of the tidal rotor and the inverter voltage frequency. 3) Additionally, in the motor mode of the induction machine, the rotating kinetic energy of it can be used for power conditioning as a fly-wheel system. 4) The HOT-PC technical challenge is to make a stable-power offshore wind turbine employing a tidal turbine motor/generator with negligible losses and quick response time. The

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key techniques of the offshore-wind and the tidal power hybrid system are a design, an electric transmission, a system and a stability operation, a system investigation, a reactive power and voltage frequency control strategy, and the interaction between offshore-wind and tidal generation systems. The advanced experimental study has to be carried out to ensure stability and also gives a better understanding of the control aspects required to make it more efficient.

Mohammad Lutfur Rahman (S’09) was born in Bangladesh. He received the B.S. degree computer engineering and the Masters degree of information technology in 2000 and 2003, respectively. He is working toward the Ph.D. degree in energy science at Kyoto University, Japan, since 2007. He was a lecturer with Eastern Asia University and Rajamangala University of Technology, Thanyaburi, Thailand. His research interests are offshore wind power system stability, tidal power generation system, and hybrid power system.

REFERENCES

Shunsuke Oka was born in Hyogo, Japan, on May 1, 1985. He graduated from the Department of Electrical and Electronic Engineering, Kyoto University, Kyoto, Japan, in 2009. His research interests are wind power generation stability and hybrid power generation systems.

[1] H. Akagi and H. Sato, “Control and performance of a doubly-fed induction machine intended for a flywheel energy storage system,” IEEE Trans. Power Electron., vol. 17, no. 1, pp. 109–116, Jan. 2002. [2] Wave Energy Conversion University of Michigan College of Engineering [Online]. Available: http://www.engin.umich.edu/dept/name/ research/projects/wave_device/wave_device.html [3] M. L. Rahman and Y. Shirai, “Hybrid offshore-wind and tidal turbine (HOTT) energy conversion I (6-pulse GTO rectifier and inverter),” in IEEE Int. Conf. Sustainable Energy Technologies, 2008 (ICSET 2008) , Singapore, Nov. 24–27, 2008, pp. 650–655. [4] M. L. Rahman and Y. Shirai, “DC connected hybrid offshore-wind and tidal turbine (HOTT) generation system,” in Zero-Carbon Energy Kyoto 2009. Kyoto, Japan: Springer, Feb. 2010, pp. 141–150. [5] M. L. Rahman and Y. Shirai, “Hybrid power system using offshorewind turbine and tidal turbine with flywheel (OTTF),” in Europe’s Offshore Wind 2009 (VIND 2009 eow2009), Stockholm, Sweden, Sep. 2009. [6] Wind Energy Associated Models PSCAD/ EMTDC Master Library, Oct. 2007 [Online]. Available: https://pscad.com

Yasuyuki Shirai (M’01) he was born in Kyoto Prefecture Japan. He received the B.E., M.E., and D.E. degrees in electrical engineering from Kyoto University, Kyoto, Japan, in 1980, 1982, and 1988, respectively. He became an Assistant Professor in 1985, an Associate Professor in 1996, and he is now a Professor in the Graduate School of Energy Science, Kyoto University. His areas of interest are applied superconductivity to power system apparatus, next-generation power system including renewable energy sources, and energy infrastructure.