ANALYSIS AND CONTROL OF STAND ALONE GENERATOR

Abstract

Electrification of un-electrified remote rural areas not connected to national grid is an important task for the sustainable development of country. Low electric load demand, economical and technical difficulties in extension of grid is main constraint for electrification of such areas. The un-electrified remote and far-flung areas have potential of sustainable renewable energy resources such as mini/ micro hydro, wind, bio-mass etc., which can be used to produce and supply electricity. The objective of the harnessing of such non-conventional energy sources could be achieved in a big way by the development of the suitable low cost generating systems. The electric power generation from these sources will not only supply the energy to the remote and isolated areas, but can also supplement the power requirements of the inter-connected systems. However, these systems could become more viable if their cost is reduced to the minimum. The self excited induction generator (SEIG) is a suitable power generation source utilizing renewable energy sources due to its advantages like simplicity, low cost, ruggedness, little maintenance, brushless construction, self-protection capability, ability to generate power while driven at variable speed etc. as compared to conventional synchronous generator. These advantages facilitate induction generator operation in stand alone/ isolated mode or in parallel with synchronous generator for supplying local load and in grid mode. The self excited induction generator has a major drawback of poor voltage regulation. The inherent poor voltage and frequency regulation of the SEIG is due to the difference between the reactive power supplied by the excitation capacitors and that demanded by the load and the machine. This is a major bottleneck for its application in isolated mode. The generated voltage ofthe SEIG depends upon the speed, excitation capacitance, load current and power factor ofthe load. Among various renewable energy based power systems, mini/micro hydro power scheme employs an uncontrolled turbine which maintains the constant input of hydro power. The use ofgovernor for input control is not an economical option due to its cost and operational maintenance. One way to regulate the voltage and frequency of the SEIG is to maintain a constant load at its terminals. Under such operation, SEIGrequires fixed capacitance for excitation resulting in a fixed-point of operation. For this purpose, a suitable control scheme is to be developed such that the load on the SEIG remains constant despite the change in the consumer load. Also, such control scheme should be simple, economical, rugged and reliable. The power output is kept constant by connecting a dump load in parallel with the consumer load such that the total generated power is held constant in order to regulate voltage and frequency of SEIG. The analog controllers are in use, need sensing, control and protection circuit which need too many electronic components. In event of failure of any component it becomes tedious and time consuming job to find the fault, repair and then put the system in order. However, a single chip like DSPs which gives cost effective solution and can eliminate the use of complicated control circuit. A single chip provides flexible solution, multiple features fast processing to implement advanced and complex algorithm. Accordingly, a DSP based load controller has been designed, developed and implemented through a new sample based controller. The proposed sample based controller is simple as compared to traditional PI controller as it requires only one parameter to be tuned for optimal operation. Experiments are carried out on developed prototype of DSP based load controller for SEIG system. The transient behavior of DSP TMS320F2812 based load controller for SEIG system at different operating conditions such as application and removal of static (resistive and reactive) and dynamic load is investigated to demonstrate the capabilities of the proposed load controller. The MATLAB based digital simulation results of the transient analysis have been compared with the experimental results to validate the developed model. Remote and isolated areas are characterized by sparsely distributed population with electric loads of single-phase. The single phase power supply is preferred over three phase in order to render the distribution system simple and cost effective. It is possible to use a three-phase induction motor as a single-phase SEIG. Beyond 5 kW load, the three phase induction machine, being inexpensive, readily available in the market with higher efficiency than equivalent rating of single phase induction machine, thus has become attractive proposition for supplying single phase power up to 20 kW. Three phase SEIG supplying single phase load is the case of unbalanced operation, it is required to operate the machine at de-rated power so that the temperature rise in the machine is restricted within permissible limits. A detailed study supported with MATLAB based digital simulation and application of load controller to regulate the voltage and frequency of single phase SEIG using three phase induction machine have been examined. A DSP based load controller is designed, developed and implemented for single phase SEIG using three phase induction motor. Further, different excitation capacitor configuration for delta and star connected machine as single phase SEIG have been analyzed. Single-phase induction motors can be used as single phase self excited induction generators for single phase power generation for the purpose of supplying smaller loads of less than 5kW. Single phase induction machine is an unsymmetrical machine having two phase windings. These two windings are normally unbalanced and classified as the main and auxiliary windings. When it is used as the SEIG, it has the flexibility of using the main and/or the auxiliary windings both for excitation and main winding for loading. The analysis of two winding SEIG for improved performance has been performed. Further, the same DSP based control algorithm as load controller has been tested and implemented with single phase induction motor working as single phase SEIG for its voltage and frequency regulation. The economical implementation of digital systems from a hardware complexity standpoint, with the goal of minimizing the computational work load have always been appealing research topic. The electrical load is maintained constant at SEIG terminals through a mark space ratio controlled load controller. The load controller keeps the total electrical power constant at variable consumer load though a dump load. The power dissipated in dump load is governed by the difference of generated power to consumer load. The load controller with uncontrolled rectifier and series connected chopper switch with mark space ratio chopper control gives unity power factor operation and it requires only one dump load. Such a load controller is nonlinear in nature and injects harmonics in the system. The harmonic generated are random in nature. The SEIG performance is severely affected with these harmonics. A dip in voltage is also observed as the harmonic content increases. An effort has been made to improve performance of load controller which injects minimum harmonics and work as a linear dump load. The AC chopper control gives wide control range. The harmonic pattern is symmetrical and only odd harmonics are present. The low order harmonics are eliminated and the order of dominant voltage harmonics can be controlled by adjusting the chopper frequency. It gives the linear control of the in fundamental component of the output voltage. Two different kind of AC chopper have been analyzed based on equal time ratio control (ETRC) AC chopper and sinusoidal PWM AC chopper control. The digital design and implementation of DSP based SEIG-load controller based on ETRC AC chopper controllable load controller for three phase SEIG and based on sinusoidal PWM AC chopper controllable load controller for single phase two winding SEIG has been examined. The developed sample based controller is simple as compared to traditional PI controller based application. The PI controller is to be tuned for proportional (Kp) and integral gain (Kj) for dynamic responses, where as in the developed sample based controller only gain (A) is to be adjusted for a given system to regulate overshoot. Further, a fuzzy logic based load controller have also been analyzed and implemented. A fuzzy logic based load controller gives nonlinear control with fast response and virtually no overshoot. The ETRC AC chopper load controller regulates the dump load as linear load with minimum harmonics and excellent dynamic response. A prototype SEIG-load controller system through fuzzy logic based controller with TMS320F2812 DSP has been developed, implemented and its transient behavior is investigated at different operating conditions such as application and removal of static (resistive and reactive) and dynamic loads. The application of load controller to SEIG system is a simple and cost effective approach to regulate the voltage and frequency but the load controller does not compensate for variable reactive power demand. The performance of SEIG is largely affected by power factor of the load as it draws a reactive current. Then, a part of the excitation capacitance is used to compensate for this reactive load current, so less leading VARs are available for the SEIG itself. On the other hand the load controller injects harmonics in SEIG system. The AC chopper based load controller reduces the harmonics in the system up to satisfactory level. In practice, with the increased used of electronic equipment, a large number of consumer loads are nonlinear in nature and therefore, they inject harmonics in the system. The SEIG's performance is also affected by these harmonics. Hence there is a need to develop a control scheme to regulate the voltage and frequency of SEIG with variable reactive power compensation and harmonic elimination. The static compensator (STATCOM) compensatqjfor reactive power with increase in load current. STATCOM comprises of a current controlled voltage source inverter with IV self sustained DC bus capacitor and coupling inductor. The DC bus capacitor along with coupling inductor together works as a second order filter and eliminates harmonics in the system. STATCOM balances the phase current and thus works as a load balancer. A digital control algorithm for STATCOM based SEIG system has been developed. The control algorithm has been first co-simulated with processor in the loop (PIL) using TMS320F2812 fixed point DSP and then experimentally validated. The Processor-in-theloop (PIL) provides one verification capability in development process. It is a cosimulation technique which helps to evaluate as how well a control algorithm operates on the fixed point digital signal processor selected for the application. The transient behaviour of SEIG-STATCOM system at different operating conditions such as application and removal of balanced/unbalanced, nonlinear and dynamic load have been investigated. The MATLAB based digital simulation of the transient response has been compared with the experimental results to validate the developed model. The stand alone operation of SEIG based fixed pitch wind energy conversion system (WECS) with regulated voltage and frequency has also been designed and simulated. The wind turbine is connected to the rotor ofthe SEIG through a step up gear box which gives a variable torque input with varying wind speed. The proposed controller consists ofIGBT based voltage source converter (VSC) and a battery bank in parallel with DC link capacitor. The proposed controller is having bidirectional flow capability ofactive and reactive power by which it controls the system voltage and frequency with variation of consumer load and the speed of the wind. The VSC functions as a voltage regulator, harmonic eliminator, load balancer for varying consumer load and varying wind speed. The feasibility of the proposed system is verified by simulations.