THREE PHASE SELF EXCITED INDUCTION GENERATOR FEEDING SINGLE PHASE LOADS

Abstract

The techno-economic difficulties in extending existing power grid to remote locations and the recent energy crises have led to increased use of locally available energy resources. With the growing demand of isolated power units, Self Excited Induction Generator (SEIG) is rapidly establishing itself in portable gensets and harnessing the non-conventional energy, due to some of its attractive features, as cheap, rugged construction, no need of synchronizing equipment, better overspeed capability and inherent protection against short circuit. The conventional alternator is found to have high cost, require frequent maintenance due to brushes and unsuitable for variable speed prime-mover. In remote and isolated regions, where three-phase transmission and distribution networks are difficult and expensive to provide, a need arises for the use of single-phase induction generators. However, the single-phase induction machines of capacity more than 5 kW are not readily available in the market and considerations such as cost, machine size, and delivery time all tend to favor the use of standard three-phase induction machines as single-phase generator. Since using a three-phase induction machine to feed single-phase load, is an extreme case of unbalancing, several excitation schemes have been tried for phase balancing. The first part of this thesis work deals with the performance analysis of three-phase SEIG, for perfect balanced condition. Closed loop impedance for of the machine for single-phase load has been formulated and the Newton-Raphson method has been used to obtain the per-phase reactance required for excitation. The investigations on reactance variation 111 (capacitor/inductor requirement) at various load points, in different phases of a 3.7 kW machine and a 7.5 kW machine have been presented. The second part deals with the partial balanced operation of the three-phase SEIG. C-2C capacitor excitation scheme has been used to evolve a simple and economically viable system. Complex loop impedance of the machine has been formulated using the positive and negative equivalent circuits. The absolute value of the complex loop impedance is considered as. the objective function. The unknown variables of the equivalent circuit (magnetizing reactance and generating frequency) of the system are solved by minimizing the objective function considering the bounds of the unknown variables. The constrained problem is converted into a series of unconstrained problem and then the Rosenbrock's rotating coordinate method of minimization is applied to find the minimum value of the objective function. Performance analysis of a 3.7 kW machine has been done with two different capacitor excitations. In addition, the loading limits of different rating machines, with various combinations of C and 2C capacitors is also presented. Further, the same analysis of computing the excitation requirement for perfect balanced condition, is extended to explore the possibility of using a three-phase induction motor on single-phase supply, in order to reduce the transmission and distribution cost in remote and rural areas. Iv