CENTRE FOR ADVANCED POWER ELECTRONICS 117 CENTRE FOR ADVANCED POWER ELECTRONICS Introduction Power electronics is an enabling technology which integrates several areas of electrical engineering. These include semiconductor device technology, circuits and control system theory, electrical machine design as well as power system engineering. One finds application of the technology in electricity utilization systems, transportation, manufacturing, civil structures as well as in new areas pertaining to biomedical and bioscience engineering, control of larger capacity electrical networks and energy storage systems. As Singapore has a strong and vibrant electronic industry, the research work carried out in CAPE builds on this strength so as to fully exploit the potential of the technology. Three research programmes have been established in which active and intensive work is being pursued. The Power Quality Analysis and Enhancement Research Programme aspires to bring to electricity consumers improved power quality through application of power electronics. The research work involves in-depth analysis of the interaction between loads, power networks and various power quality improvement devices. It ultimately leads to better design of mitigation devices to alleviate various power-quality related problems. Mitigation methods currently investigated include voltage-source inverter-based series compensators, active series-shunt filters and devices using harmonic cancellation methods. The key research work under the Electrical Energy Conversion Systems Research Program includes data storage disk drive systems, multi-axis motion systems, self-bearing motor drive systems, new power electronic devices and circuits, and electric vehicle technologies. In carrying out the research, the strategies are the integration of unconventional geometries, new materials, numerical techniques and advance control to create new devices and systems. The Flexible Transmission-Distribution Systems Research Programme encompasses the analysis of the impact on network performance due to the applications of power electronic-based controllers. Considerable flexibility in the structure of the power systems can be achieved, such that the new control schemes can enhance the reliability as well as the quality of supply. With the de-regulation of the electricity market, a flexible supply system allows the supply of loads at tariff rates commensurate with the power quality level required. The use of power converters and incorporating distributed generation is also examined. At present, the Centre has 18 active staff members and 38 graduate students. Although this School-based research centre was only established in Aug 2002, research activities pertaining to power electronics in NTU have started since the establishment of the School of Electrical and Electronic Engineering. Some 28 MEng and 12 PhD students have graduated between 2001- 2003 while carrying out research work in the power electronics and related areas. Since 1997, 3 postdoctoral fellows, 2 research associates, 3 research fellows and one project officer have been engaged. A total of 108 refereed international journal papers were published in the last three years. The published works cover all aspects of power electronics, as well as those pertaining to condition monitoring and de-regulated markets. Members of the Centre have also established close collaboration with other research centres and commercial organizations. For example, there is active collaboration with Xian Jiaotong University on the design of a power conditioner suitable for use in isolated power networks. A study on large-scale battery energy storage system with DSTA is underway. A DSOfunded project on developing a pulse power supply for high-power laser is in progress. A 3-axis motion simulator was completed as part of inter-centre collaboration with Satellite Research Centre as well as the ongoing satellite power supply project. On disc-drive systems design, collaboration with Data Storage Institute has been ongoing for the past 5 years. Work on the de-regulated market is in conjunction with EMA and two power generating companies. The design of a conditioning monitoring system is now under field tests, again as a joint-project with another local SME. Other institutions and industry partners include University of Sydney (on large-scale network control), ESIEE-Amiens of France, Shanghai Jiaotong University, Tsinghua University and the Manitoba HVDC Research Center, Canada. 118 POWER QUALITY ANALYSIS AND ENHANCEMENT Objective The focus of the Power Quality Analysis and Enhancement Research Programme is to find ways and means to improve power quality through application of power electronics. A high power quality level is necessary for the proper operation of many modern power devices such as computers and electronic-based appliances. The research work involves in-depth analysis of the interaction between loads, power networks and various power quality improvement devices. A thorough understanding would ultimately lead to better design and utilization of mitigation devices to alleviate various power-quality related problems, particularly voltage dips and harmonic distortions. Mitigation methods currently investigated include voltagesource inverter-based series compensators, active seriesshunt filters and devices using harmonic cancellation methods. Highlights of Research Activities This programme brings together power quality research by individual professors and students in NTU. The research activities are wide ranging, from monitoring and data analysis to compensation solutions. Aspects of the compensators such as energy storage device, converter topology and interaction between compensator and protected loads are also actively investigated. The past year saw the beginning of an earnest effort to establish the group’s testing capability. Testing the tolerance of equipment towards certain power quality phenomena or gauging the performance of compensating devices forms an integral part in our quest for better understanding and better solution techniques. Hardware prototyping and testing are essential in verifying the methodology and ensuring that the ideas are practical and realistic. Power quality is a universal term encompassing many electromagnetic phenomena with distinct characteristics. Building a power quality corruptor that is able to generate wide-ranging kinds of disturbances and distortions is not easy. The system needs to be rated sufficiently to power loads that may be susceptible to any one of the phenomena. It must be equipped with adequate voltage support and current sourcing capabilities. In addition, some power quality disturbances exhibit fast changes that are difficult to mimic, especially those needing considerable power. A compromise is always necessary between the output bandwidth and the output power ratings. 119 Figure 1: Prototype power quality corruptor with three inverters in parallel To meet some of the aforementioned challenges, paralleling approach for building the power quality corruptor was adopted. Paralleling enables individual system to be kept small while maintaining the flexibility to expand to higher powers should the need arises. Figure 1 shows the prototype system involving three separately controlled invertors. Each inverter is rated at 1.2 kVA and multiple inverters can be paralleled to boost the output power to any required level. However, no two components are completely alike, and hence the unbalance in between the inverters causes circulating current to flow from one to another. This not only wastes energy but also limits the output capability of the entire system. However, a novel technique has been developed by Mr. Cheng Yeong Jia, a Ph.D student and his supervisor, Asst Prof Kenneth Sng to balance the current outputs of the inverters. The near-perfect equal current sharing in this approach of paralleled single-phase inverters is realized by implementing an instantaneous average current sharing scheme. This scheme requires an interconnected common cable to share information of the average current among all the inverters. The instantaneous average current is obtained by adding all the individual inductor current and dividing it with the total number of inverter modules. This information is then compared with respective inductor current of each module to obtain the current sharing error. The resultant current sharing error is fed into the controller of individual inverter to ensure that all inverters will produce almost equal inductor current. A prototype was constructed in the first quarter of this year and with its successful verification, several units were duplicated to allow the design to be used in various power quality studies. Figure 2 shows several sample power quality phenomena that were generated by the corruptor. It includes voltage interruption, which is a complete loss of voltage and harmonic distortions comprising of 3rd and 5th harmonic components. In actual fact, it has been proven that the inverter is able to mimic any conceivable wave-shape as long as its frequencies fall within the inverter output bandwidth of 10 kHz. pursued by Mr. Wang Tongxun, a Ph.D student under the guidance of Prof. Choi and Dr. Zhuo. Prof. Choi and several colleagues visited Xi’an Jiatong University in August to enhance the collaboration in this project and to strengthen the ties between the power engineering groups of the two universities. Figure 2: Sample power quality phenomena generated by the corruptor Figure 3: Estimating system voltage sag performance The Centre hosted a Tan Chin Tuan exchange fellow, Dr Zhuo Fang from Xi’an Jiaotong University, P.R.China, from January to April 2004. Dr Zhuo has vast experiences in harmonic active filtering and has commercialized some of his designs. He is very interested in tapping into the Centre’s knowledge in DVR (Dynamic Voltage Restorer) and has proposed a joint effort to develop a hybrid compensator combining the technologies in active filter and DVR. There is a great demand for such compensators from oil production companies as the power quality at their oil rigs are poor due to the types of electrical equipment used and the weaknesses of the onsite generators driven primarily by diesel combustion engines. This idea is currently being From the system configuration and the type of fault or equipment failure, the exposed areas are first determined to indicate the regions that will be affected when such saginducing event occurs. The protection system, which ultimately dictates the duration of the disturbance by the virtue of their reaction time, is evaluated to determine the expected durations of each of the disturbances. The information is then combined with the historical data of equipment failure rates and statistical distribution of fault types to arrive at a sag density table. This table contains the expected number of occurrence of voltage sags of various severity levels, as an estimation of the system voltage sag performance. The method also takes into 120 On the network analysis portion of the power quality research, substantial efforts were placed on automatic identification and classification of captured power quality phenomena. The analysis has since progressed from classifying according to the characteristics of the system responses to identifying the cause and source of the disturbance. At the same time, it is realized that using measurements to assess power quality level of a network can be complicated and lengthy. Many years of continuous measurements are necessary before a substantial number of samples can be gathered to generate a credible benchmarking level. Hence, analytical approaches were investigated with the aim of estimating these benchmarks from the knowledge of the network details and the historical data of the equipments used. Figure 4 below shows the estimation procedures. consideration unbalanced sags and the manner in which they are accounted for. This technique has been tested on a simplified reliability test system as shown in Figure 4. The results (Table 1) show the estimated sag performance of this test system if the unbalanced sags are accounted by the lowest of the three phase voltages. This table would be useful for three-phase loads that are sensitive to the lowest of the three phase voltages, or single phase devices spread over three phases where tripping of one of them interrupts the entire operation. With regard to the research activities in power quality enhancement, significant inroads were achieved in the adaptive control of shunt active filter undertaken by Mr. Tey Leong Hua and Dr. So Ping Lam. They proposed the use of artificial neural network (ANN) technique in the extraction circuit of the shunt active filter as shown in Figure 5. The ANN algorithm is used to compute the harmonic currents and reactive power drawn by the nonlinear load. These two signals are used as the reference signal for the hysteresis control of a three-phase IGBT voltage source inverter (VSI). With ANN, the shunt active filter is made adaptive to variations in nonlinear loads’ currents. It can also compensate for current unbalances and correct the power factor of the supply side to near unity. Furthermore, it also possesses the capability to regulate the dc capacitor voltage at the desired level. The application of ANN in active filters makes the extraction of harmonics faster resulting in faster adaptation of the active filters to any variation in the operating condition. It also makes it easier to alter the design of the control circuit and hence provide more flexibility to the active filter. Figure 4: Simplified Roy Billinton Test System Figure 5: System block diagram of single-phase adaptive shunt active filter Table 1: System voltage sag density table by the lowest phase voltage 121 Staff Members Assoc Prof Chen Shiun Program Director eschen@ntu.edu.sg Asst Prof R.M.A. Sumedha Rajakaruna Prof Choi San Shing Assoc Prof Don Mahinda Vilathgamuwa Asst Prof Sng Eng Kian, Kenneth Assoc Prof Ali Iftekhar Maswood Assoc Prof Luo Fang Lin Asst Prof Loh Poh Chiang, Andrew Asst Prof So Ping Lam Ongoing Research Projects Project Title Principal Investigator Funding A Self Tuning Unity Power Factor ASD For Power Quality Application Ali I Maswood AcRF ($22k) A DSP Based Controlled Rectifier Power Factor Correction Technique Ali I Maswood AcRF ($22k) STATCOM with Trinary Hybrid Multilevel Inverter Luo Fang Lin AcRF ($50k) MEng and PhD Theses Completed in 2004 Project Title Degree Student Supervisor(s) Voltage Restoration Systems and Control PhD Herath Mudiyanselage Wijekoon Banda Don Mahinda Vilathgamuwa Power Quality Improvement Using Unified Power Quality Conditioner PhD Tey Leong Hua So Ping Lam Seminars Name(s) of Speakers Title of Seminar Affiliation Zhou Feng Introduction to Active Power Filter (APF) Development Xian Jiao Tong University, P. R. China 122 Selected Publications in 2004 1. 2. 3. 4. 5. 6. 7. S. Chen, “Open Design of Networked Power Quality Monitoring Systems”, IEEE Trans. on Instrumentation and Measurement, Vol. 53, No. 2, pp. 597-601, April 2004. J.D. Li, S.S. Choi, and D.M. Vilathgamuwa, “Impacts of voltage phase shift on motor loads and series Custom Power devices including converter thermal effects”, accepted, IEEE Trans on Power Delivery. F.L. Luo, and Y. Liu “Voltage-Balancing Multilevel Inverters to Enhance Power Quality”, Electrical and Electronic Engineering Research Bulletin, NTU Singapore, Jan. 2004. A.I. Maswood, and E. Firmansiyah, “Simple Current Injection Technique for Power Factor Correction in Controlled Rectifier Applications”, in print, IEE Proceedings on Electric Power Applications, 2004. A.I. Maswood, and F. Liu, “A Novel Unity Power Factor Input Stage for AC Drive Application”, accepted, IEEE Transactions on Power Electronics, 2004. A.I. Maswood, and W. Shen, “A genetic algorithm based solution in PWM converter switching” accepted, IEE Proceedings on Electric Power Applications, 2004. E.K.K. Sng, S.S. Choi, and D.M. Vilathgamuwa, “Analysis of Series Compensation and DC-Link Voltage Controls of a Transformerless Self-Charging Dynamic Voltage Restorer”, IEEE Trans on Power Delivery, Vol. 19, No. 3, pp. 1511-1518, July 2004. 8. P.C. Tan, P.C. Loh, and D.G. Holmes, “A robust multilevel hybrid compensation system for 25-kV electrified railway applications”, IEEE Trans on Power Electronics, vol. 19, pp. 1043-1052, July 2004. 9. P.C. Tan, P.C. Loh, and D.G. Holmes, “Optimal impedance termination of 25kV electrified railway systems for improved power quality”, accepted, IEEE Trans on Power Delivery, 2004. 10. P. C. Tan, P.C. Loh, and D.G. Holmes, “High performance harmonic extraction algorithm for a 25kV traction power quality conditioner”, accepted, IEE Proceedings – Electric Power Applications, 2004. 11. L.H. Tey, P.L. So, and Y.C. Chu, “Improvement of Power Quality Using Adaptive Shunt Active Filter”, accepted, IEEE Trans. on Power Delivery, 2004. 12. L.H. Tey, P.L. So, and Y.C. Chu, “ANN with Hysteresis-Controlled Unified Power Quality Conditioner for Improving Power Quality”, Proc. POWERCON2004, Singapore, Nov. 2004. 13. D.M. Vilathgamuwa, H.M. Wijekoon, and S.S. Choi, “Interline Dynamic Voltage Restorer: A Novel and Economical Approach for Multi-Line PQ Compensation”, accepted, IEEE Trans. on Industrial Applications. 14. J. Wang, S. Chen, and T.T Lie, “System Voltage Sag Performance Estimation”, accepted, IEEE Trans. On Power Delivery. ELECTRICAL ENERGY CONVERSION SYSTEMS Objective The members of this research programme have wideranging interests in fundamental and applied research in the applications of power electronics and electromechanical devices for electrical energy conversion. This is a multi-disciplinary research area involving technologies from semiconductor, electronic circuit design, control and instrumentation, power engineering, magnetics, and mechatronics. The applications are for automation, motor drive systems, bio-medical systems, transportation, data storage, power supplies and satellite control systems. Our research strategies are the integration of unconventional electromechanical designs, new materials, advanced numerical techniques and non-linear control to create new energy conversion devices and systems. for electric vehicles, the control of induction and switched reluctance motors. Applied research in power applications for bio-medical engineering includes the development of rotary blood motor-pump drive system without mechanical bearings for implantable artificial heart. Current projects include the design of bearing-less motor drive system for orbiting satellite momentum/reaction wheels, the development of high power multi-output piezoelectric transformers, the development of high energy pulse power supply for laser applications, the study of new control algorithms for matrix converters, and the advanced control of drive systems for data storage. Future projects include development of silicon-carbide and carbon nanotube power devices, micro-generators and superconducting magnetics. Highlights of Research Activities The research activities in this research programme cover a wide range and are supported by the excellent facilities available in six laboratories. At the device and component level, there are projects on the modelling and characterisation of power semiconductor devices, passive components and energy storage devices, and the high frequency measurement, modelling and loss analysis of magnetic materials. At the circuit level, research activities include the study of power electronic circuits for power conversion and control of electromechanical systems, and new types of dc-to-dc converters. At the system and application level, research work is being done on the design and construction of permanent magnet wheel motor drives 123 Fig 1: Resonant Power Converter Research Members of the research group have strong collaborative ties with Xian University of Technology, Shanghai Jiaotong University, China, Ryerson University, Canada, Data Storage Institute, Defence Science and Technology Agency, and DSO Laboratories of Singapore. Among the recent research achievements produced by the group are new classes of dc-to-dc converters, a new converter topology for switched reluctance motor, a novel centrifugal blood pump drive with electromagnetic bearings, a three-axis rotary platform for hardware-in-the-loop testing of satellite sub-systems, and new control algorithms for hard disk drives. A patent for a novel characterization system for power diodes has been granted. Fig 2: Development of Matrix Converter Staff Members Assoc Prof Tseng King Jet Program Director ekjtseng@ntu.edu.sg Asst Prof Loh Poh Chiang, Andrew Assoc Prof Luo Fang Lin Asst Prof Hu Jinhui Asst Prof Sng Eng Kian Kenneth Assoc Prof Don Mahinda Vilathgamuwa Assoc Prof Wang Youyi Assoc Prof Tan Cher Ming 124 Ongoing Research Projects Project Title Principal Investigator Funding Design and Fabrication of Insulated Gate Bipolar Transistor using Wafer Bonding Technology Tan Cher Ming NTU ($333k) Development of High Power Multi-Output Piezoelectric Transformers Hu Jinhui NTU ($107k) Development of Bearingless Motor Drives Technology Tseng King Jet NTU ($238k) Advanced Nonlinear Control for Electric Drive Systems Wang Youyi NTU ($135k) Advanced Control Design for Hard Disk Drives Wang Youyi NTU ($75k) Nonlinear Control Designs for Hard Disk Drive Servos Wang Youyi DSI ($150k) Advanced Control of Data Storage Devices Wang Youyi CMMS ($121k) Study on Batteries and Battery Energy Storage Systems Tseng King Jet DSTA ($3.5k) High Power Pulse Power Supply Tseng King Jet DSO ($10k) Project Title Principal Investigator Funding Development of Advanced Motion Control Systems Luo Fang Lin NTU ($50k) Research Projects Completed in 2004 MEng and PhD Theses Completed in 2004 Project Title Degree Student Supervisor(s) Advanced Nonlinear Controllers for Digital Data Storage Systems MEng Dong Fengdan Wang Youyi Selected Publications in 2004 1. 2. 3. 4. 5. 6. 7. 8. 9. J. C. Zheng, G. Guo, and Y. Wang, “Feedforward Decoupling Control Design for Dual-Actuator Systems in Hard Disk Drives”, IEEE Trans. on Magnetics, Vol. 40, No. 4, pp. 2080 – 2082, 2004. Y. Li, G. Guo, and Y. Wang, “A Nonlinear Control Scheme for Fast Settling in HDDs”, IEEE Trans. on Magnetics, Vol. 40, No. 4, pp. 2086 – 2088, 2004. F. D. Dong, Y. Wang, and J. Y. Zhou, “A New Direct Seek Control Design of Optical Disk Drives”, accepted, Int. Journal of Microsystem Technologies, 2004. F. L. Luo, and H. Ye, “Negative Output Super-Lift Converters”, IEEETrans. on Power Electronics, Vol. 18, No. 5, Sep 2003, pp. 11131121. Z. Y. Pan, and F. L. Luo, “Novel Soft-Switching Inverter for Brushless DC Motor Variable Speed Drive System”, IEEE Trans. on Power Electronics, Vol. 19, No. 2, pp. 280 – 288, Mar 2004. F. L. Luo, and H. Ye, “Positive Output Multiple-Lift Push-Pull SwitchedCapacitor Luo-Converters”, IEEE Trans. on Industrial Electronics, Vol. 51, No. 3, pp. 594 – 602, Jun 2004. P. C. Loh, D. G. Holmes, Y. Fukuta, and T. A. Lipo, “A Reduced Common Mode Hysteresis Current Regulation Strategy for Multilevel Inverters”, IEEE Trans. Power Electronics, Vol. 19, pp. 192 - 200, Jan 2004. P. C. Loh, G. H. H. Pang, and D. G. Holmes, “Multi-level Discontinuous Pulsewidth Modulation: Common Mode Voltage Minimisation Analysis”, IEE Proc. Electric Power Application, Vol. 151, pp. 477 - 486, Jul 2004. P. C. Loh, D. G. Holmes, and T. A. Lipo, “Implementation and Control of Distributed PWM Cascaded Multilevel Inverters with Minimal Harmonic Distortion and Common-mode Voltage”, accepted, IEEE Trans. Power Electronics, 2004. 125 10. H. Chen, E. K. K. Sng, and K. J. Tseng, “Generalized Optimal Trajectory Control for Closed Loop Control of Series-parallel Resonant Converters”, Proc. IEEE-PESC ’04, Jun. 2004. 11. H. Chen, E. K. K. Sng and K. J. Tseng, “Optimum Trajectory Switching Control for Series Parallel Resonant Converter”, Proc. IECON ’03, Nov. 2003. 12. D. M. Vilathgamuwa, Y. Xie, and K. J. Tseng, “Development and Control of a 3-Axis Motion Simulator for Satellite ADCS Hardwarein-the-Loop Simulation”, Proc. IECON, 2004. 13. J. Du, J. Hu, and K. J. Tseng, “High-power, Multi-output Piezoelectric Transformers Operating at the Thickness Shear Vibration Mode”, IEEE Trans. on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 51, No. 5, pp. 502 - 509. 14. K.J. Tseng, J. Du, and J. Hu, “Piezoelectric Transformer with High Power Density and Multiple Outputs”, IEE Electronics Letters, Vol. 40, No. 12, pp. 786 - 788. 15. C. Zhang, K. J. Tseng, Y. Xiao, and K. Y. Zhu, “Model-based Predictive Control for A Compact and Efficient Flywheel Energy Storage System With Magnetically Assisted Bearings”, Proc. IEEE Power Electronics Specialists Conference, Germany, Jun 2004. 16. P. Wu, and K. J. Tseng, “Finite Element Analysis of Axial Gap Induction Motors”, Proc. Int. Conference on Power Electronics and Motion Control, China, Aug. 2004. 17. C. Zhang, and K. J. Tseng, “Design and FEM Analysis of Flywheel Energy Storage System Assisted by Integrated Magnetic Bearings”, Proc. IEEE Industrial Electronics Conference, South Korea, Nov. 2004. FLEXIBLE TRANSMISSION-DISTRIBUTION SYSTEMS Objective The Flexible Transmission-Distribution Systems Research Programme encompasses the analysis of the impacts on network performance due to the applications of power electronic-based controllers. Considerable flexibility into the structure of the power systems can be achieved, such that the new control schemes can enhance the reliability as well as the quality of supply. A flexible distribution system allows the supply of loads at tariff rates commensurate with the power quality level required. The use of power converters, incorporating distributed generation such as solid-oxide fuel cells, is also examined. Highlights of Research Activities The current research aims essentially at two areas: transmission systems and distribution networks. Currently, there are six graduate students affiliated with this research programme. Flexible Transmission Systems The rapid development of power electronics technology provides exciting opportunities to develop new power system equipment for better utilization of power systems. During the last two decades, a number of control devices under the term “Flexible AC Transmission Systems” (FACTS) technology have been proposed and implemented. FACTS technology is employed to extend the capacity of existing/ planned power transmission networks beyond their normal limits without the need to construct additional transmission lines. Applications of advanced control techniques to FACTS devices can expand a power system transmission capacity. This control-based expansion approach is appealing to power industries because of increasing difficulty to build new transmission lines due to environmental and economic constraints. One traditional way to increase the capacity of the available corridors is to use higher voltages on the transmission lines. The second way is to enlarge the total conductor cross-section. These methods are very expensive. A research team in CAPE, led by A/P Wang Youyi, is currently investigating a project on stability analysis and enhancement of large scale power systems incorporating FACTS devices. The project is a collaboration with Queensland University, Australia and is supported by Australia Research Council. The team also collaborates with the professors and researchers in City University of Hong Kong and Tsinghua University, China. 126 The research team led by A/P Haque is actively engaged in investigating the power flow control and improvement of dynamic performance and voltage stability of a power system using various FACTS devices. The results found by the team have been reported in 10 journal and conference papers in 2004. A project is also underway to re-examine the role of shunt reactive power compensator in long-distance power transmission. Unlike previous works in which transmission line is often represented by a simple series reactance, line losses are now included in the analysis and the more exact telegraph model is used instead. It is shown that for transmission level above the line surge impedance loading, the predicted steady-state stability limit using the exact line model is some 80% below that obtained using the simple model. Flexible Distribution Networks Current research on the distribution network areas aims at distributed generation (DG). DG is a promising technology that can be used to address some of the technical as well as environmental concerns in power systems. A typical DG has the capacity range from 10kW up to 10 MW. It can provide energy solutions to customers who require higher power quality or reliability than that offered by the conventional solutions. For example, developed electrical networks, such as that of Singapore, support a high proportion of sensitive loads. Power quality, including that of voltage sag, would be important technical consideration. Hence, in an effort to diversify the source of energy so as to enhance the supply system reliability, efforts have been made in Singapore to import natural gas as an alternative to fuel oil for power generation. To explore the naturalgas-fed DG generation technologies, new forms of power generation devices such as fuel cell (FC) which may help the grid to improve power quality and reliability are under active research. The DG research group in CAPE proposed the fastest and yet still prudent ways of changing the output power level of a Solid-Oxide Fuel Cell (SOFC) power plant connected to the ac grid through a conventional voltage source inverter. The objectives of this control strategy are to achieve the power change in minimum time while maintaining the fuel utilization factor in the allowable range and to operate the power plant with a constant power factor at all power levels. The analysis shows that power changes of up to a limited value can be safely achieved by a single step change of the stack current. The limit is dependent on the allowable range of utilization factor and the initial operating stack current. For a larger change of power, the maximum step change should be superimposed with a ramp change in current so that the utilization factor is maintained at the boundary of the feasible operating area until the desired power level is reached. For excessive power changes, the ramp rate needs to be varied by on-line control such that the utilization factor remains at the boundary. The proposed scheme of control can be verified through computer simulations as illustrated by Figure 1. Staff Members Figure 1: Response of SOFC under on-line power-flow control of fuel-cell current Assoc Prof Wang Youyi Program Director eyywang@ntu.edu.sg Prof Choi San Shing Asst Prof R M A Sumedha Rajakaruna Assoc Prof Govinda Bol Shrestha Assoc Prof Mohammed Hamidul Haque Assoc Prof Lie Tek Tjing Asst Prof So Ping Lam Ongoing Research Projects Project Title Principal Investigator Funding Stability Analysis and Control for Large Scale Power Systems Incorporating FACTS Devices Wang Youyi ARC (A$81k) Selected Publications in 2004 1. 2. 3. 4. L. Cong, Y. Wang, and D.J. Hill, “Coordinated Control Design of Generator Excitation and SVC for Transient Stability and Voltage Regulation Enhancement of Multi-machine Power Systems”, Int. Journal of Nonlinear and Robust Control, Vol. 14, No. 9, pp. 789– 805, 2004. Y.L. Tan, and Y. Wang, “A Reliable Nonlinear Excitation and SMES Controller for Transient Stabilization”, Int. J. Electrical Power & Energy Systems, Vol. 26, pp. 325–332, 2004. L. Cong, Y. Wang, and D.J. Hill, “Transient Stability and Voltage Regulation Enhancement via Coordinated Control of Generator Excitation and SVC”, accepted, Int. J. of Electric Power and Energy Systems, 2004. J.H. Chen, T.T. Lie, and D.M. Vilathgamuwa, “Damping of Power System Oscillations Using SSSC in Real Time Implementation”, Int. J. Electrical Power & Energy Systems, UK, Vol. 26, 2004, pp. 357– 364. 127 5. 6. 7. 8. 9. T.T. Lie, and H.L. Hui, “Optimal Dispatch in Pool Market with FACTS Devices”, Proc. 2004 IEEE Power Engineering Society General Meeting, June 2004, USA. Y.H. Li, S.S. Choi, and R.M.A.S. Rajakaruna, “An Analysis of the Control and Operation of a Solid Oxide Fuel Cell Power Plant in an Isolated System”, accepted, IEEE Trans. on Energy Conversion. M.H. Haque, “Power Flow Control and Voltage Stability Limit: Regulating Transformers versus UPFC”, IEE Proc. – Gener. Transm. Distrib., Vol. 151, No. 3, pp. 299-304, 2004. M.H. Haque, and P. Kumkratug, “Application of Lyapunov Stability Criterion to Determine the Control Strategy of a STATCOM”, IEE Proc. – Gener. Transm. Distrib., Vol. 151, No. 3, pp. 415-420, 2004. M.H. Haque, “Improvement of First Swing Stability Limit by Utilizing Full Benefit of Shunt FACTS Devices”, accepted, IEEE Trans. on Power Systems.