ANALYSIS OF SIX-PHASE SELF-EXCITED INDUCTION GENERATOR

Abstract

The increasing importance of fuel saving has been responsible for the revival of interest in so-called alternative source of energy. Thus the drive towards decentralization of power generation and increasing use of non-conventional energy sources such as wind energy, bio-gas, solar and hydro potential, etc. has become essential to adopt a low cost generating system, which is capable of operating in remote areas, and in conjunction with the variety of prime movers. With renewed interest in wind turbines and micro-hydro-generators as an alternate energy source, the induction generators are being considered as an alternative choice to the well-developed synchronous generators because of their lower unit cost, inherent ruggedness, operational and maintenance simplicity. The induction generator's ability to generate power at varying speed facilitates its application in various modes such as a self-excited stand-alone (isolated) mode; in parallel with synchronous generator to supplement the local load, and in grid-connected mode. Alternate energy sources are typically located in isolated areas where utility supply is not available or is frequently interrupted, thereby frequently placing the energy source in isolation to supply the load. Such situation requires that alternate energy power generating system perform the voltage regulation function by providing a source not only for real power to the load but also reactive power. It is well known that an induction machine may generate voltage if capacitor is connected to its stator terminals while its rotor is driven by a prime mover. In this case, the capacitor provides the lagging magnetizing reactive power, which is necessary to establish the air gap flux. This configuration is termed as self-excited induction generator (SEIG). However, one of the major drawbacks of stand-alone SEIG is its poor voltage regulation, which necessitates appropriate voltage regulating scheme. Use of additional passive elements (short shunt and long shunt) to provide selfregulating features was, therefore, considered worthy of exploration. The investigation spread over last two decades indicate the technical and economic viability of using number of phase higher than three in multi-phase ac machines in general for application in marine ship, thermal power plant to drive induced draft fans, electric vehicles, nuclear power plants etc, and induction machines in particular. The research in this area is still in its infancy, yet some extremely important findings have been reported in literature indicating the general feasibility of multi-phase systems. However, practical applications of multi-phase induction generator in renewable energy generation scheme such as wind energy and hydropower have not been reported so far. The literature regarding multi-phase induction generator is nearly nonexistent since it consists of only three studies before 2005. The generator scheme presented in one study is based on dual stator winding induction machine with displaced power and control three-phase winding, while other two studies deal with the double stator machine with extended rotor common to both the stators. In all the three cases, output is three-phase. The first paper on multi-phase induction generator appeared in 2005, followed by some more works on six-phase self-excited induction generator (SPSEIG). In these works, mathematical modeling and experimental performance analysis of SPSEIG is discussed showing its practical feasibility. There is no work available on steady state modeling and performance analysis of SPSEIG. This dissertation, therefore, presents a detailed steady state modeling and analysis of SPSEIG for thorough and systematic investigation of self-excited six-phase induction generator configured to operate as stand-alone electric energy source in conjunction with a hydro power plant. The machine under consideration has six stator phases divided into two wyeconnected three-phase sets, labeled abc and xyz, whose magnetic axis are displaced by an arbitrary angle. The windings of each three-phase set are uniformly distributed and have axes that are displaced by 120°. The basis of the analysis is nodal admittance method based on graph theory as applied to the equivalent circuit. The mathematical model developed is in matrix form, which is convenient for computer simulation. Advantages ofthe model are that the effect ofmutual coupling between the two three-phase stator set and the core loss component can be included or easily eliminated from the model. Also the leakage reactance of the stator and rotor may be handled separately if required by avoiding the assumptions ofstator and rotor leakage reactance being equal without any modifications in the model. Matrix equations developed by nodal admittance method are solved by Genetic Algorithm and Fminconoptimizer. The detailed steady state performance analysis of SPSEIG as simple shunt, short shunt and long shunt was carried out with delta connected capacitors banks connected to: (i) one three-phase set of winding and with separate loading on each three-phase set (symmetrical as wellas asymmetrical loading); (ii) both three-phase set of winding and with independent loading on each three-phase set (symmetrical as well as asymmetrical). Genetic Algorithm and Fmincon optimizer built-in MATLAB was utilized for the analysis purpose. For the purpose of experimental verification of the performance characteristics of SPSEIG, a three-phase, 6-pole, 36-slots, 1.0 kW, 400 V, 3.5 A, 50 Hz, 950 rpm squirrel cage induction machine was utilized as a basis. All the 72 stator terminals were taken out to the terminal box mounted on the top of the machine casing, so that various winding schemes for different number of poles and phases can be realized. The six-pole, six-phase connection was obtained by employing phase belt splitting. The six stator phases are divided into two star-connected three-phase sets (winding set abc and xyz), with magnetic axis of the two three-phase sets displaced by an angle of 30° electrical. Neutral point of the two three-phase sets are kept isolated in order to prevent physical fault propagation from one three-phase set to other one, and to prevent the flow of triplen harmonics. The test machine was coupled to a semi closed-loop small hydro power (SHP) test rig, installed at Alternate Hydro Energy Centre, Indian Institute of Technology Roorkee. The SHP test rig consists of two identical service pumps, each having 150 liter per second discharge capacity at the head of 10 meters, connected to a 5 kW cross-flow turbine of efficiency 56 % through pipe line networks. The turbine is fitted with 24 blades around its shaft and has the runner diameter of 300 mm. Control valves installed at the pipe line and the turbine are used to vary the head and discharge of water for controlling the speed of the turbine shaft. The head and discharge are measured with the help of a calibrated rectangular weir and a precise digital pressure gauge. This regulated hydraulic cross-flow turbine serves as prime mover through belt coupling in order to achieve desired speed with smooth control throughout the operation. A 5 kVA, 415-415/415V Y-A /Y six-phase to three-phase transformer was used in the experimental set up for combining the two three-phase outputs of the six-phase generator to supply a single three-phase load. Primary side star and delta input terminals of the transformer receive the two three-phase outputs of the SPSEIG and provide a three-phase output for the load through a star connected winding at secondary side. Experiments were carried out with the delta connected capacitors banks connected to: i. One three-phase set of winding; ii. Both three-phase set of winding. Resistive and resistive-inductive both types of loading were used during load test on SPSEIG. Experimental tests were conducted for the above two configuration of excitation capacitance with the loading subjected to: in i. One three-phase set of winding; ii. Both three-phase set of winding (symmetrical as well as asymmetrical loading), iii. Common three-phase loading through A-Y/Y transformer. Performance analysis was carried out for all the three configurations of SPSEIG i.e. simple shunt, short shunt and long shunt. Fluke 43-B Power Quality Analyzer was used for measurements and for recording of various waveforms. Experimental tests conducted on SPSEIG for various operating conditions clearly show the applicability of a six-phase capacitor excited induction generator for supplying two individual threephase loads. Self-excitation under no-load condition and loading performance under a typical resistive and resistive-inductive load are elaborated. The emphasis is placed on additional possibilities offered by using a six-phase SEIG which are not available with a three-phase SEIG. In particular, it is shown that the SPSEIG can be self-excited without any problems using a single threephase capacitor bank. This means that, in normal operation, loss of excitation at one of the threephase windings can be sustained and operation continued. Hence, SPSEIG offers an improved reliability, when compared to its three-phase counterpart. Further, it is also shown that the SPSEIG can be used to supply two different and independent three-phase loads. While the interaction between the two windings is inevitable and variation ofthe load at one winding changes operating conditions at the other winding, the situation is still satisfactory for a wide range of rural resistive loads. A further advantage of the SPSEIG with respect to a three-phase SEIG is the possibility of combining the outputs oftwo three-phase windings for the supply of a single three-phase load, by means of a three-winding transformer with dual star-delta connected primary. Failure of one three-phase generator winding in this case does not mean the shutdown ofthe system, since the load can still be supplied through the remaining healthy generator winding (with an appropriate reduction of the delivered power). Acomparative study conducted on six-phase SEIG has shown that: i. Variation in six-phase SEIG terminal voltage and current with the change in loading (for similar loading condition) is lower, ii. Range of operating frequency with sustained self excitation is broader for the six-phase SEIG, IV iii. Power delivered and overall efficiency of hydro power scheme employing six-phase SEIG is better than with three-phase SEIG, iv. A single generator is able to deliver power to two independent loads /customers and the generation process can continue even when only one capacitor bank at one of the three-phase windings is operational, v. Generation and customer supply can also continue when one three-phase winding fails if a single three-phase load is supplied through the interconnecting six-phase to three-phase transformer.