STABILITY ANALYSIS AND VOLTAGE SAG MITIGATION OF POWER SYSTEM IN OFFSHORE OIL RIG PLATFORM

STABILITY ANALYSIS AND VOLTAGE-SAG MITIGATION OF POWER SYSTEM IN OFFSHORE OIL RIG PLATFORM WU DI NATIONAL UNIVERSITY OF SINGAPORE 2011 STABILITY ANALYSIS AND VOLTAGE-SAG MITIGATION OF POWER SYSTEM IN OFFSHORE OIL RIG PLATFORM WU DI B Eng (Hons.), NUS A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2011 ACKNOWLEDGEMENTS I would like to express my deep and sincere gratitude to my supervisor, A/Prof Chang Che Sau His invaluable advice and guidance throughout my research, as well as his encouragement and trust at difficult times have been the “power and stability control” in this work I am grateful to research engineer, German Xavier, for passing me the knowledge and troubleshooting experience of Matlab Simulink, so that I had a fast start with this tool used in my research I thank research fellow, Dr Bai Hong, for her kind support and recommendations of readings to further my understanding in power system I wish to thank research engineer, Parikshit Yadav, for his many constructive discussions and generous help in my work I am also thankful to research fellow, Dr Wang Zhao Xia, for her advice and help in editing my technical papers for publication I would like to acknowledge the technologist-in-charge of the Power Systems Laboratory, Mr Seow Hung Cheng, for his assistance in providing supports and facilitating the meetings with our external collaborators Finally, I would like to thank my parents, my boyfriend, and my friends for their devoted love and constant support TABLE OF CONTENTS SUMMARY i LIST OF TABLES iii LIST OF FIGURES iv LIST OF SYMBOLS vii CHAPTER INTRODUCTION 1.1 1.2 1.3 Motivations and Objectives Earlier Work and Contributions of this Thesis Thesis Outline CHAPTER POWER SYSTEM MODELING 2.1 2.2 2.3 2.4 Overview of Power System in Offshore Oil Rig Platform Generation Side Modeling 2.2.1 Diesel Generator Model 2.2.2 Excitation System Model 2.2.3 Governor Model 2.2.4 Test of Model Load Side Modeling 2.3.1 Transformer and Converter Model 2.3.2 Induction Motor Drive Model Power System Model Test CHAPTER SMALL SIGNAL STABILITY OF INDUCTION MOTOR DRIVE 3.1 3.2 3.3 3.4 3.5 Stability of Drive Fed from Ideal Source 3.1.1 Small Signal Model 3.1.2 Genetic Algorithm 3.1.3 Operational Stability Prediction 3.1.4 Simulink Verification Stability of Drive Fed from Converter 3.2.1 Expansion of Small Signal Model 3.2.2 Eigenvalue Locus Analysis Closed-loop Design for Instability Elimination 3.3.1 Small Signal Model with Speed Feedback 3.3.2 Optimization of Proportional Regulator 3.3.3 Simulink Verification Application of Stability Analysis Methods to Keppel Induction Motor Discussion of Low-frequency Instability 1 9 11 11 12 15 16 19 19 22 25 29 29 29 31 33 34 36 36 39 42 42 43 46 49 52 CHAPTER VOLTAGE STABILITY OF POWER SYSTEM 4.1 4.2 4.3 4.4 Voltage Sag and Fluctuation Principle of Dynamic Voltage Restorer Conventional Dynamic Voltage Restorer 4.3.1 Conventional DVR Simulink Model 4.3.2 Simulation of Conventional DVR New Design of Dynamic Voltage Restorer 4.4.1 Increasing Voltage Compensation Accuracy 4.4.2 Reducing Voltage Total Harmonics Distortion 55 55 56 59 59 61 66 66 69 CHAPTER CONCLUSION 5.1 Project Outcomes 5.2 Future Work 74 74 77 REFERENCES 79 APPENDICES 83 SUMMARY Today’s semi-submersible offshore oil rig platform uses variable-speed induction motors for station—keeping during drilling and sailing During drilling, the platform needs to be maintained within its intended location to prevent interruption of operation and damage to the drills Thus, it is crucially important that the induction-motor drive and the power supply feeding the drive are stable This master thesis aims at analyzing and proposing analytical solutions to these two issues The main contributions of the thesis are: For the first time, a Matlab Simulink model is built to simulate the detailed dynamics of the induction-motor drive in offshore oil-rig platform The model and parameters used for the drive are verified by on-site tests of the Keppel FELS motors Genetic algorithm is developed to the small-signal model of the induction-motor drive to identify, within the full range of speed and load, the instability region of the induction motor fed from ideal variable-frequency supply or a converter The source of instability is also investigated from the two different supply configurations of the induction-motor drive It is shown by eigenvalue analysis that instability exists in the induction motor itself but not in the other parts of the drive i.e the converter supplying the motor Accordingly, speed feedback comprising a proportional regulator is added to the drive to eliminate the instability region intrinsic to the induction motor plotted using genetic algorithm Voltage sag is known to be the most common disturbance in offshore power system, which has led to blackout in severe cases A new design of dynamic voltage restorer (DVR) is modeled in Simulink, which deals with the influence of voltage sag on the induction-motor drive within strict harmonic limits imposed by marine standards The new i design outperforms the conventional DVR design by effectively increasing the accuracy of voltage compensation and significantly reducing the level of total voltage harmonic distortion ii LIST OF TABLES Table NO Table Caption/Title Page NO 2.2.3 Generation power and fuel consumption 15 2.3.1 Parameters for the DZZ single phase transformer 20 4.2.2 Voltage output states at different switching 71 A.1 Parameters of the unstable induction motor in SI unit 84 iii LIST OF FIGURES Fig NO Fig Caption/Title Page NO 1.1 Semi-submersible drilling unit 2.1 Main power system one-line diagram 10 2.2.1 a) Parameters of synchronous generator 11 2.2.1 b) Simulink model of one generation unit 12 2.2.2 a) Generation unit connection 13 2.2.2 b) Type AC5A-simplified rotating rectifier excitation system representation 14 2.2.2 c) Type AC5A with design parameters 14 2.2.3 a) Block diagram of governor control 15 2.2.3 b) Plot of generation power vs fuel consumption 16 2.2.4 a) Generator active power output due to load disturbance 17 2.2.4 b) Generator frequency variation due to load disturbance 18 2.2.4 c) Keppel load disturbance test part 18 2.2.5 d) Keppel load disturbance test part 18 2.3.1 a) Simulink model of transformer and converter 19 2.3.1 b) Equivalent electrical circuit of linear three winding transformer 20 2.3.1 c) 6-pulse rectifier configuration 21 2.3.1 d) Three-level neutral point clamped PWM inverter 21 2.3.2 a) V/f curve with boost voltage 23 2.3.2 b) Induction motor parameters 23 2.3.2 c) Torque-speed characteristics in operating range 24 2.3.2 d) PI-regulated speed feedback closed-loop V/f control 25 2.4 a) Simulink model of v/f controlled induction motor drive 26 2.4 b) Induction motor speed step change command and speed response 27 2.4 c) Induction motor load step change command and torque response 27 2.4 d) Diesel generator frequency variation in per unit 28 2.4 e) Diesel generator voltage variation in per unit 28 iv 3.1.3 Instability region for induction motor in per unit 34 3.1.4 a) Simulink model of open-loop V/f controlled induction motor drive 35 3.1.4 b) Rotor speed response to command at 500 RPM (at point A) 35 3.1.4 c) Rotor speed response to command at 600 RPM (at point B) 35 3.1.4 d) Rotor speed response to command at 700 RPM (at point C) 36 3.2.1 37 3.2.2 b) Open-loop induction motor drive fed by converter Eigenvalue locus comparison of induction motor fed from ideal supply and from converter Eigenvalue loci of system at RF and RF=0 40 3.2.2 c) Eigenvalue loci of system at CF and 2CF 41 3.2.2 d) Eigenvalue loci of system at LF and 0.3LF 41 3.3.1 Speed controller for closed-loop V/f controlled induction motor 42 3.3.2 a) Eigenvalue loci of closed-loop system at Kp=0.75 and Kp=1.3 44 3.3.2 b) Instability region for closed-loop system at Kp=1, 1.2, 1.3 45 3.3.2 c) Instability region for closed-loop system at Kp =0.75 45 3.3.3 a) Rotor speed response to command at 500 RPM when Kp =1.3 47 3.3.3 b) Rotor speed response to command at 600 RPM when Kp =1.3 47 3.3.3 c) Rotor speed response to command at 780 RPM when Kp =1.3 47 3.3.3 d) Rotor speed response to command at 500 RPM when Kp =0.75 48 3.3.3 e) Rotor speed response to command at 600 RPM when Kp =0.75 48 3.3.3 f) Rotor speed response to command at 700 RPM when Kp =0.75 48 3.4 a) Instability region for Keppel induction motor in per unit 50 3.4 b) Rotor speed response to command at frequency ratio=0.2, 0.3, 0.4, 0.5 51 3.5 a) Eigenvalue loci at H=0.1s and H=0.2s 52 3.5 b) Eigenvalue loci at Rr and 1.2 Rr 53 3.5 c) Eigenvalue loci at Lm and 0.5 Lm 53 4.2 a) Basic DVR topology 57 4.2 b) Control logic of DVR 58 4.3.1 a) Parameters of series transformer 59 4.3.1 b) Configuration of three-phase LC filter 60 3.2.2 a) v 39 Fig 4.4.2 b) Detailed structure of three-level NPC inverter Table 4.4.2 Voltage output states at different switching State S1 S2 S1’ S2’ Vout 0 1 -Vdc/2 1 0 1 0 +Vdc/2 71 phase to ground voltage (V) 6000 4000 2000 -2000 -4000 -6000 0.15 0.152 0.154 0.156 0.158 0.16 0.162 0.164 0.166 Time (s) Fig 4.4.2 c) Voltage output of three-level inverter To achieve a greater THDv reduction, a filter capacitance is placed at the line-side of the new DVR design instead of at the inverter-side In this case, the filtering requirement is achieved by utilizing the capacitor C and the impedance of the series transformer Each capacitance is set to 100 µ F as in the inverter-side design, and the system shows an optimal performance Simulation result favors this design since it provides strong harmonics suppression Fig 4.2.2 c) shows that after applying this new design feature, THDv is reduced to 0.52% at PCC In fact, the THDv not exceed 0.52% at all instants during voltage sag except at the switching transients [33] 72 Mag (% of Fundamental) Fundamental (60Hz) = 0.9968 , THD= 0.52% 0.3 0.25 0.2 0.15 0.1 0.05 0 10 20 30 40 Harmonic order Fig 4.2.2 d) Voltage harmonic spectrum 73 50 CHAPTER CONCLUSION 5.1 Outcomes The objective of this work is to study the Keppel offshore oil rig power system, analyze its stability in operation and propose a design solution to address the instability problem, if any A Simulink model of the main power system is firstly built to get an overview of the system dynamics and to serve as a basis for stability study By comparing the simulation results with Keppel test results, this model is proven to be representative of the actual study system By running another simulation under the same condition as in the Keppel tests, it is observed that the variations of AC busbar frequency and voltage are both within the required standard and there is no instability problem To have a comprehensive analysis of stability over the whole range of speed and load, a frequency domain approach is developed here, since it is faster, more extensive and less memory-consuming than time domain simulations The study investigates the phenomenon of low-frequency instability of induction-motor drive fed from ideal voltage supply and converter A small-signal model based on flux variables is used Another important feature of this work is the plots of the regions of instability of the motor drive using genetic algorithm in the torque vs speed plane Such plots give a visually clear impact of key design parameter over a wide operating region By adding two external equations representing the inclusion of the converter and using eigenvalue locus analysis, it is observed that the converter does not affect stability, and the 74 low-frequency range instability is intrinsic to the induction-motor itself To eliminate that instability, a speed-feedback closed-loop is designed and proportional regulator is optimized in the frequency domain using the extended small signal model and the corresponding region of instability The effectiveness of the above approach is verified by time-domain simulations of Simulink model The above proven small-signal stability analysis method is then applied to the Keppel induction-motor drive for stability investigation It is found that Keppel induction motor does not have any instability problem A comparative analysis of the Keppel motors and their unstable counterpart reveals the importance of parameter design of induction motor itself Compared with the unstable counterpart, the Keppel motor has larger inertia, higher rotor resistance, and lower magnetizing inductance which all favor stability Another focus on stability in this work is voltage stability at AC busbar where all induction motor loads are connected to via converters and transformers As variablefrequency induction-motor drives are sensitive to voltage sag and fluctuation, this disturbance commonly seen in offshore industry needs to be mitigated The thesis starts with modeling of the conventional dynamic voltage restorer in the offshore power system to test its performance Simulation results reveal two problems: (i) voltage sag mitigation is not very accurately compensated due to the influence of the induction motor dynamics on the voltage sag; and (ii) voltage harmonics level at point of common coupling is too high during voltage sag to be acceptable in offshore industry To address these problems, a new dynamic voltage restorer design is proposed It adds the feedback control with the existing feedfoward control as in the conventional design to increase accuracy of sag mitigation It 75 replaces the two-level inverter with a three-level and an inverter-side filter with line-side to minimize the voltage harmonics The New DVR Design is superior to the conventional DVR in two aspects: i) Unlike the conventional design, the new design is not adversely affected by the load dynamics on the shape of the voltage sag, thus when the conventional design can only achieve a mitigation accuracy of 82%, the new design shows a high accuracy of 99% Ii) Unlike the conventional design which has an exceedingly high harmonics level of 5.6% at point of common coupling, the new design minimizes harmonics level to 0.5%, greatly less than the required harmonic limits of 5% in offshore industry With the new design using simple control and realistic load modeling, voltage sag and fluctuation in AC busbar can be effectively mitigated and degradation in performance of induction-motor drives due to voltage sag can be avoided With both the stability of induction motor verified and the stability of voltage supply to induction-motor drive assured, induction motor drives can operate at its desired performance level in driving the propellers for station-keeping of the platform The overall stability of the offshore platform is ensured providing a secure and uninterrupted operation environment, which is crucial for oil and gas exploitation 76 5.2 Future Work Small-signal model of generator connecting to induction motor via transformer and converter can be developed to reflect the interaction of their dynamics and thus the overall stability of the generator-motor system The model will thus have the potential of being extended to multi-machine power systems The above multi-machine stability analysis will encompass the small-signal modeling of several generator units, 24-pulse transformer, uncontrollable 12-pulse rectifiers and threelevel NPC inverters, with generator units and the 24-pulse transformers connected directly to 11kV 60Hz common busbar in a ring configuration, three-level NPC inverter feeding variable frequency supply to several induction-motor loads having their own V/f and other controls It will definitely be a challenging work due to its complexity and novelty In voltage sag mitigation, three-phase programmable voltage source is used to simulate voltage sag at the supply due to its flexibility and simplicity in setting The voltage sag thus has a square shape of different magnitude and duration depending on the settings In practice, if one out of the two generators trips, the voltage at busbar will not drop instantly but will be affected by generator/ motor dynamics and other interactions Thus the actual shape of voltage sag at the supply could be a complex function An accurate modeling of all these will drastically increase the simulation time Nevertheless, work can be done to model a 50% voltage sag using tripping of generator model in a multiple machine system for comparing its simulation results with the one in this thesis To model the tripping, complex protection and tripping mechanism needs to be designed to prevent AVR and governor from going beyond control in response to the 77 feedback during tripping and stopping the simulation in Simulink Dynamic voltage restorer can also be incorporated in the small signal model of the power system and its impact on system stability can be investigated After it is done, it would be good to have a prototype to verify the performance in practice and fine-tune 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Appendices Fig A.1 Simulink Model for Load Disturbance Test 83 Table A.1 Parameters of the unstable induction motor in SI unit Stator resistance Rs = 0.216 Ω Rotor resistance Rr = 0.130 Ω Stator and rotor leakage inductance Lsl = Lrl = 2.29mH Mutual inductance Lm = 2.29mH Inertia J = 0.0315kgm2 Number of Poles P = (machine poles) Filter resistance RF = 0.216 Ω Filter capacitance CF = 324 µ F Filter inductance LF = 11.4mH Source inductance Lco = 0.367mH 84 Fig A.2 Proposed DVR Design Simulink Configuration in Power System 85 .. .STABILITY ANALYSIS AND VOLTAGE- SAG MITIGATION OF POWER SYSTEM IN OFFSHORE OIL RIG PLATFORM WU DI B Eng (Hons.), NUS A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF. .. semi-submersible offshore oil rig platform uses variable-speed induction motors for station—keeping during drilling and sailing During drilling, the platform needs to be maintained within its intended... understanding of the operation of power- system in offshore oil- rig platform Fig 1.1 shows the structure of a semi-submersible platform Dynamic positioning system sends signals to change the angle and