ISOLATED AND GRID MODE OPERATION OF INDUCTION GENERATORS

Abstract

In recent times, the generating units utilizing renewable energy resources like small hydro and wind energy are becoming more popular due to increasing concern for environment, ever increasing energy cost and fast depleting fossil fuels. These resources can be harnessed more economically using induction generators because of their lower unit cost, inherent ruggedness, operational and maintenance simplicity. Moreover, induction generator's ability to generate power at varying speed facilitates its application in various modes such as stand-alone isolated mode; in parallel with synchronous generator for feeding local load; and grid mode. The stand-alone Self Excited Induction Generator (SEIG) is ideally suited for remote locations and far flung areas, where the extension of grid is economically prohibitive; while the grid connected induction generator can support the real generation to the existing grid. The induction and synchronous generators operating in parallel can supply varying local load with improved voltage regulation. A considerable information on the steady state and transient behaviour of SEIG is available in the literature. The SEIG suffers from poor voltage regulation due to the gap between VARs supplied by shunt capacitors and VARs demanded by load and machine. The voltage regulation can be improved by using SEIG with additional series capacitors in short shunt and long shunt configurations. However, to obtain the cost-effective voltage regulation while maintaining the simplicity and ruggedness of these configurations, the value of shunt and series capacitances should be optimized for optimum voltage regulation. The induction motors comprise a majority of practical loads due to their versatility, ease of operation, control and economy. The performance of short shunt and long shunt SEIG feeding induction motor (IM) loads will be considerably different than their performance with static loads because these loads cause voltage dip and inrush current when put into operation and their power factor does not remain constant throughout the loading range. Therefore, the performance of short shunt and long shunt SEIG feeding induction motor during steady state and transient conditions should be comprehensively studied to avoid unstable behaviour during start-up of a motor, sudden loading or steady state operation and thus to make them available for a large community of loads. The transient analysis of SEIG is important to find the transient values of voltage, current and torque for assessing the suitability of machine windings, capacitors and shaft. it Isolated and Grid Mode Operation of Induction Generators The knowledge of these values during unbalanced terminal conditions is further important because the unequal stator voltages and currents may lead to additional losses and shaft vibrations. Although, a considerable work on the transient performance of SEIG under balanced conditions has been reported in literature; the performance of SEIG, short shunt SEIG and long shunt SEIG during balanced and unbalanced terminal conditions including faults should be thoroughly investigated to assess the severity offaults in terms of excessive voltage, current, and torsional oscillations and to evolve a protection strategy. The poor frequency regulation of isolated generating units can be improved by using load controller. The operation of available load controllers results in either the reactive loading on the generator or the repeated switching of a binary weighted load. Consequently, a new cost-effective load controller scheme causing negligible reactive loading on the generator should be designed and control strategy eliminating the repeated switching should be formulated to dispense the costly governor. Therefore, the wider acceptance of the stand-alone self-excited induction generator will depend on the methodologies to be adopted for cost-effective voltage and frequency regulations, capability in handling dynamic loads and performance under balanced/unbalanced terminal conditions. The advantages of SEIG systems for isolated applications are well known, but they can not supply varying power factor load within its loading range with improved voltage regulation. Hence, the type of load restricts the SEIG application to a large community of loads. The load with varying power factor and magnitude make the use of synchronous generator advantageous for isolated applications. In small capacity applications the use of induction and synchronous generators operating in parallel may reduce the project cost. Simultaneously, the advantages of both induction and synchronous generators can be exploited i.e. the improved voltage regulation from synchronous generator and the low cost of power from induction generator. The fairly compensated induction generator can be used to avoid reactive loading on the synchronous generator. The dynamic performance of this configuration with load controller should be studied for assessing the feasibility of dispensing costly governors. The grid connected induction generator (GCIG) may contribute in supplementing the real power requirement of the grid by integrating power generated from resources located at different sites. Thus, the GCIG may avoid techno-economic difficulties to be faced in implementing large power projects. The GCIG results in substantial reactive load on power 7 Abstract /// system and therefore the load flow analysis of the system consisting of both induction and synchronous generators should be carried out to assess the reactive VAR requirement and possible compensation at the planning stage. Furthermore, the transient performance of GCIG under various faults should be investigated to emphasize the application of induction generator in a network. In view of the gaps and scopes outlined above, the investigations have been carried out in this thesis for strengthening the potential application of induction generator to harness small hydro potential to provide power supply of acceptable level of quality and reliability. To meet these objectives, steady state and dynamic studies have been carried out at various stages of the analysis for the following configurations - • Stand alone self-excited induction generator (SEIG). • Induction and synchronous generator operating in parallel and supplying local load. • Grid connected induction generator (GCIG). To start with, the cost effective voltage regulation of SEIG using additional series capacitors in short shunt and long shunt configurations has been discussed using steady state equivalent circuits. The voltage regulation problem is formulated as an optimization problem and an algorithm based on simulated annealing method has been developed to find the values of shunt and series capacitances and primemover speed for optimum voltage regulation throughout the loading range of the generator satisfying various operating constraints. The total loading range of the generator from no load to full load is divided into number of load points with uniform step size, and the objective function to be minimized is selected as the weighted sum of the square of the load voltage deviation from rated value at all these load points. The lower and upper limits on shunt and series capacitances and primemover speed are specified to cover possible solution space. The algorithm determines the optimum values of capacitances and primemover speed directly from all possible solution space subjected to equality constraints on loop impedance and inequality constraints on stator current, stator voltage, load voltage, magnetizing reactance and frequency. The effect of prime mover speed, weightage factor and the load power factor on optimum values of capacitors and primemover speed and correspondingly the change in the generator behaviour are studied to enable the selection of optimum size of capacitors under various operating conditions. Detailed simulated and experimental investigations have been carried out and the performance of both short shunt and long shunt SEIG configurations are IV Isolated and Grid Mode Operation of Induction Generators comprehensively compared for optimum voltage regulation. The strong agreement between simulated and experimental results is obtained. The steady state and transient performance of short shunt SEIG-IM and long shunt SEIG-IM configurations are studied to find suitability and feasibility of these configurations. The steady state equivalent circuit of induction motor load has been reduced into series R-L equivalent and the optimum voltage regulation problem is solved for selecting the values of capacitances and damping resistance resulting in optimum voltage regulation for various combinations of static and dynamic loads. The dynamic models of both these configurations are developed in stationary reference frame using d-q currents and voltages as state variables considering the effect of main and cross flux saturation for both machines operating as generator and motor. The unstable behaviour like sub-synchronous resonance is observed during start-up of induction motor from short shunt SEIG, which may result in excessive high motor current, pulsating torque of sustained nature or voltage collapse. The effectiveness of resistive load and damping resistance across the series capacitance in dampening the unstable behaviour has been studied and it is observed that damping resistance is required even for steady state operation. The appropriate values of damping resistance and shunt and series capacitances have been derived to enable successful starting, sudden loading and steady state operation. The start-up of induction motor load is made possible without the switching of any additional capacitors. The unstable behaviour in long shunt SEIG can be eliminated by using a resistive load or damping resistance, however, appropriate value of damping resistance provides better and successful operation y throughout the loading range. The values of shunt and series capacitances and damping resistance are selected for optimum voltage regulation. The simulated results for steady state and transient conditions are experimentally verified and both short shunt SEIG-IM and long shunt SEIG-IM configurations are compared on the basis of their steady state and transient performance. The generalised dynamic models of SEIG, short shunt SEIG and long shunt SEIG have been developed using d-q variables in stationary reference frame for studying their performances under various balanced and unbalanced terminal conditions. The analysis has been carried out considering the effect of main and cross flux saturation. These models are of handling any unbalanced configuration of load and/or capacitors, however, the performance of these configurations is discussed for various balanced and unbalanced faults, which include extreme cases of unbalancing. The faults are initiated when self-excited induction generators are supplying resistive loads under balanced steady state conditions. ** Abstract V The faults considered in this investigation include three-phase and line to line short circuit at load, shunt capacitors and machine terminals; shorting of three series capacitors and one series capacitor; single line opening at load and shunt capacitor bank; disconnection of one shunt capacitor and single phase load etc. The experimentation is carried out for these cases and a close match between simulated and experimental results is observed. The strategy for avoiding loss of residual magnetism and generator protection has been discussed. A new electronic load controller scheme has been developed for improving the poor frequency regulation of isolated generating unit. The developed scheme consists of Binary Weighted Load Control (BWLC) and Firing Angle Control (FAC) systems for coarse and fine control to regulate the generated output and the frequency. The configuration of the controller and its parameters are selected such that the controller cause a negligible reactive loading on the generator. A methodology is developed to eliminate the steady frequency error and the repeated switching of binary weighted load. The BWLC using Proportional (P) and Proportional Integral (PI) control, with single and multiple step correction capabilities are simulated, while FAC is simulated with PI control only. The controller and generator behaviours are studied for varying gain and inertia to obtain faster response with reduced transients. The controller performance in regulating frequency for isolated synchronous generator, short shunt SEIG and the configuration of induction and synchronous generators operating in parallel and feeding a local load is studied for the load variation in the entire loading range. The configuration of induction and synchronous generators operating in parallel is compensated by shunt capacitors for minimum reactive loading on the synchronous generator. The dynamic models are developed using d-q variables and considering the effect of saturation in the stationary reference frame for short shunt SEIG and in the reference frame fixed at the rotor of synchronous generator for other configurations. The performance of short shunt SEIG is studied for resistive load, while for other configurations, the performance is studied for both resistive and resistive-inductive loads. As a sample case, the performance of isolated synchronous generator with the governor control system is also studied for the load variation throughout the loading range of generator. The developed load controller scheme is found effective for all these configurations for the load variation in the entire loading range of generator/generators; thus the developed controller can dispense the costly governor needed for the isolated applications. vi Isolated and Grid Mode Operation of Induction Generators Following the study of isolated configurations, the steady state and dynamic performance of induction generator is studied to assess its suitability and limitations for grid mode application. The dynamic models of grid connected induction generator, grid 'connected induction generator with local load and shunt capacitors are derived. The dynamic models of these configurations for balanced conditions are developed in d-q variables while the models for unbalanced conditions are derived using hybrid variables (phase variables for stator and d-q variables for rotor). The rotor equations of induction machine are derived in the stationary reference frame, which eliminates the time varying inductance. The simulation is carried out incorporating the effect of saturation. The steady state performance of GCIG is studied for varying input power and grid voltage. The induction generator delivering rated output is found effective in grid mode application. Amethodology is evolved to carry out the load-flow analysis of the system having both induction and synchronous generators. The reactive VARs drawn by the induction generator delivering rated output is evaluated for the specified bus voltage to which it is connected. The procedure is repeated iteratively till the solution converges and load flow solution is obtained. The 7-Bus configuration of induction and synchronous generators, is considered to show the effectiveness of the developed methodology. Following the steady state and load flow study, the dynamic study is carried out under various balanced and unbalanced terminal conditions including faults occurring in the transmission line connecting the generators to the infinite bus. The performance of induction generator during connection to grid is studied to access the connection transients and disconnection from infinite bus is >- studied to assess the possible overvoltage. In addition to the perturbation in load, input power and capacitance, the faults such as three-phase short circuit (LLLG), line to line (LL), line to line to ground (LLG), line to ground (LG) and line opening (LO) are simulated. The investigations carried out in this thesis on optimum voltage regulation of SEIG; operation of short shunt and long shunt SEIG feeding induction motor load; performance of SEIG, short shunt SEIG and long shunt SEIG during unbalanced faults; frequency regulation of isolated units using a load controller; load flow of the system having both induction and synchronous generators, bring the basic objectives of this thesis to a successful conclusion.