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Title: Investigations on Static Controllers for Self-Excited Induction Generators
Authors: Chauhan, Yogesh Kumar
Supervisor: Jain, Sanjay Kumar
Singh, Bhim
Keywords: Self Excited Induction Generator;STATCOM;SSSC;Static Controllers;Optimization;SEIG;Isolated Generator;Genetic Algorithm
Issue Date: 12-Oct-2010
Abstract: The fast depleting rate of fossils fuels such as coal, petroleum products, environment pollution and ecological concerns are drawing an attention to harness the renewable energy resources. Moreover, the challenges in extending the grid supply to far flung and hilly areas have forced the work on stand alone generating units. The self-excited induction generator (SEIG) is a strong candidate to extract electrical energy from renewable energy sources such as small hydro and wind due to various advantages over the conventional synchronous generator for small capacity applications. The SEIG is normally an externally driven three-phase squirrel cage induction machine with suitable capacitor bank at its stator terminals. The research on SEIG has got impetus after the energy crisis faced during seventies and various facets of SEIG have been investigated. The generated voltage of SEIG depends upon the speed of prime mover, shunt capacitance, magnetization characteristic of the machine and the nature of load. The SEIG terminal voltage droops with an increase in the load. Various conventional methods to improve the voltage regulation of SEIG include the use of series capacitors i.e. compensated SEIG configurations, constant voltage transformer (CVT), saturable core reactor etc. Although, the conventional methods are simple but their effectiveness, size, losses etc. have been the reasons to explore other methods. Moreover, the compensated SEIG configurations have resulted into unstable and oscillating behavior with the induction motor loads. The advancements in the characteristics of power electronic devices, hardware support for higher switching frequency operation etc. have motivated to use the switching devices based voltage regulators such as switching capacitors, continuously controlled capacitors etc. for improving the performance of SEIG. Moreover, the improvement in power converter topologies, digital signal processors, controllers and control techniques have asserted the voltage source converter applications for improving the SEIG performance by improving the voltage regulation. The static shunt compensator (STATCOM) is normally a pulse width modulation (PWM) based voltage source converter (VSC) with a capacitor on DC bus for self support. The STATCOM can emulate as a controlled inductive or capacitive element, which can absorb or supply reactive power with the proper switching arrangement. The static synchronous series compensator (SSSC), a synchronous voltage source converter connected in series with the line through a series transformer, has proven applicability in power system for power flow and damping control etc. The SSSC produces controllable compensating voltage, which can emulate a controlled capacitor or inductor independent to the line current. This research work is carried out with the broad objective to investigate the performance of SEIG with static controllers namely STATCOM and SSSC for feeding various loads. The following investigations are made to achieve the set objectives. Various regulating schemes for SEIG are discussed along with their relative suitability, and operational aspects. The voltage regulating schemes are categorized into shunt and series compensation schemes, which are sub classified into classical schemes, switching devices based schemes and converter based schemes. The frequency regulating schemes are based on arrangement for the active power control. The voltage-frequency regulating schemes, which are based on active and reactive power control arrangement, are also briefed for three-wire and four-wire systems. The steady state performance of SEIG is investigated for feeding various linear loads. The steady state performance is analyzed for voltage regulation optimization with shunt capacitance and speed as variables. The objective function is subjected to various equality and inequality constraints related to successful operation. The combination of shunt capacitance and speed for the formulated problem is calculated using genetic algorithm (GA), a global search and optimization approach. The transient performance of SEIG is studied for feeding linear and non-linear loads. The generalized induction machine model in q-d variables in stationary reference frame is developed and used for SEIG and an induction motor load, whereas, phase variables models are used for the non-linear loads. The balanced and unbalanced non-linear loads such as three-wire and four-wire uncontrolled rectifier fed resistive-capacitive loads are considered for the study. The effect of load harmonics on SEIG is estimated with total harmonic distortion (THD) and harmonic spectrum. For non-linear load, the THD in generator quantities are compared with permitted IEEE-519 standard. The performance analysis of SEIG-STATCOM system is analyzed for feeding static resistive-inductive load and dynamic (induction motor load and fully controlled rectifier fed DC motor) loads. The equations to model each system component are explained along with the equations for generating gating signals. A methodology is developed to decide the STATCOM rating and its associated parameters. Accordingly, the STATCOM is designed for voltage regulation of SEIG in two cases, namely full rating and reduced rating. A control technique is proposed for generating the gating signals for STATCOM devices. The control technique consists of two proportional-integral (PI) controllers, one for maintaining the DC bus voltage and other for maintaining the SEIG terminal voltage. The transient performance characteristics of the complete system are studied with both full rating and reduced rating STATCOM design parameters for voltage buildup, switching of STATCOM into the system, sudden application of the load and change of the loads. The power quality of the SEIG supply is estimated using harmonic spectrum and THD. The modeling and transient performance analysis of three-phase, four-wire isolated SEIG-STATCOM system are presented for feeding various types of balanced/unbalanced linear and non-linear loads. The employed STATCOM is an insulated gate bipolar transistor (IGBT) based pulse width modulation (PWM) operated four-leg voltage source converter (VSC) with self supporting DC bus. Various studies are carried out on three-phase, four-wire stand alone supply system to address the problems of voltage regulation, unbalanced operation, harmonics and neutral current for linear and non-linear loads. For non-linear loads, harmonic spectrum and total harmonic distortion (THD) are analyzed to assess the quality of the generator voltage and currents. The steady state performance analysis of series capacitor compensated SEIG, namely short shunt SEIG configuration is also studied for feeding static loads. The optimization problems, representing voltage regulation characteristic and performance characteristic of SEIG, as separate objective functions are formulated, subjected to various equality and inequality constraints arising due to operating conditions of SEIG. The objective function representing performance characteristic is taken as the weighted sum of voltage regulation characteristic and loadability of SEIG. The value of series and shunt capacitances at a specified speed are obtained for optimum voltage regulation and optimum performance using GA. The performance is studied for resistive and resistive-inductive loads. The transient performance of SEIG with static synchronous series compensator (SSSC) is carried out for resistive, resistive-inductive and resistive-capacitive loads. The performance has been investigated for SEIG-battery supported SSSC and SEIG-capacitor supported SSSC configurations. The employed SSSCs are IGBT based PWM operated current controlled-voltage source converters (CC-VSC). A methodology is proposed for the design of SSSC parameters and components for above mentioned SSSC configurations and static loads. The proposed control technique for battery supported SSSC consists of a proportional-integral (PI) controller that is employed to maintain the load voltage. The proposed control technique for capacitor supported SSSC consists of two PI controllers, one for regulating the DC bus voltage and other for regulating the SEIG terminal voltage. The transient performance of SEIG-SSSC system is investigated for feeding static loads. The design, modeling and transient performance of SEIG-SSSC are investigated to feed an induction motor load to obtain stable operation. The performance of short shunt SEIG feeding an induction motor load is simulated and the unstable behavior under different conditions like voltage collapse, sustained oscillations, high voltage and current are studied. The capability of SSSC is demonstrated in mitigating the unstable and oscillating behavior resulting during the loading of compensated SEIG with an induction motor load. The investigations summarized in this thesis on the regulating schemes for three-phase SEIGs, steady state and transient analysis of three-phase SEIG, SEIG operation with STATCOM feeding static and dynamic loads, SEIG-STATCOM operation as an isolated four-wire supply system, short shunt SEIG operation for optimum performance, SEIG operation with SSSC feeding static and induction motor load bring the objectives of thesis to a successful conclusion.
Appears in Collections:Doctoral Theses@EIED

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