ABSTRACT
The importance of Self-Excited Induction Generator (SEIG) cannot be neglected when it comes to low cost of maintenance requirement, low cost of operation, low operational cost, high reliability and availability. This dissertation presents a dynamic analysis of an autonomous Self-Excited Induction Generator (SEIG) showing dynamic loss of load performance. Analysis is carried out based of the d-q axis model, while considering the performance characteristics of the machine at constant machine speed, capacitor value and for various power factor loads.
The transient surge, its decay and rise on the various machine parameters of phase voltage, phase current, capacitor current and load current are investigated for a gain and subsequent loss of load. The time it takes to excite the machine is also considered for the various power factors on an unloaded condition.
The effect of the loading on the frequency, as well as the drop in the value of the monitored parameter is also investigated.
CHAPTER ONE
INTRODUCTION
1.1: GENERAL
The high demand for renewal and a more environmental friendly source of energy has led to investigation on other energy sources like wind, solar, biomass, etc. The Induction generator (IG) has been found to be suitable for wind energy conversion and also in small hydro plants [1], [2]. This has also increased the level of research in Induction Generators (IGs) which was thought to have very less or no practical applications [3]. Researchers have found that a Self-Excited Induction Generator (SEIG) could be use with a conventional energy sources to generate power. An Induction machine (IM) has the ability to operate as a motor or a generator depending on the required mode of operation. The slip determines the mode of operation of the IM, either as a motor or a generator [4]-[8]. An Induction motor operates as an IG when it is driven slightly above it synchronous speed (full load slip about-0.05) [3]. As a generator it has numerous advantages over the conventional synchronous generators especially when operating under stand alone mode [9]. If the IM is driven by a prime mover above synchronous speed, electrical power is generated, thus the IM act as a generator [10]. In some cases, an Induction Generator is designed for usage instead of using an induction motor acting as a generator. The design for an induction generator has a reduced stator core width, therefore giving a voltage drop of 6% compared to 30% voltage drop from using an induction motor as an induction Generator [11]. The generating ability being sustained by enough reactive power gives rise to the Self Excited Induction Generator (SEIG).
When a SEIG is operating under an interconnected application, a synchronous grid supplies its reactive power. In stand-alone applications, the reactive power must be supplied by the load itself, or by a bank of capacitors. An electronic inverter can also assist to supply the reactive power [12], [13]. To boost up the excitation circuit,
thyristor-controlled inductors can be connected in parallel with the excitation capacitor [14].
A Self Excited Induction Generator (SEIG) is a machine resulting from driving an induction motor or asynchronous motor above it synchronous speed by it prime mover and drawing it magnetizing current from other synchronous machines connected into the system or capacitor load within the system. SEIG may be connected in parallel with other generator under certain conditions, on failure of that connection the machine will continue at a voltage and frequency determined by its control equipment [15].
When an Induction Generator (IG) is running in parallel with a large capacity supply system, the Induction Generator (IG) find it usage mainly as a co-generator on site with generators using by-products processing plant (ceramics and refuse derived fuel-RDF from landfill sites). It is also use to supply combined head and power locally and export any surplus electrical power to a utility grid [15]. According to Seyoum, D. and Wolf, P., a SEIG can also be said to be simply, an induction motor with sufficient capacitor connected across each of its terminals.
The Structure of SEIG is the same as that of the Induction motor. This basic structure involves the frame, stator and rotor core, stator and rotor winding, shaft and bearings [16]. The stator frame houses the stator windings and also encloses the rotor.
The equivalent circuit of induction machine has been reported [18]-[21].
A Self Excited Induction Generator (SEIG) has a peculiar characteristic, since there is a great necessity to sustain excitation even when the machine is loaded. SEIG also has a problem of voltage and frequency regulation, especially when using a variable speed prime-mover (example in wind power generation). To maintain a constant voltage and frequency, different models and method of analysis have been reported by different authors. The Control methods include a mechanical control involving stall control, pitch control and yaw control [11]. The other types of control involve the use of a load controller and a VAR compensator [11], [22]-[26].
The SEIG has an advantage of being rugged, very reliable, easily available, etc. It has numerous advantages over the synchronous generator when used with a variable prime mover, and also find its applications in the area of electric breaking [23], [27].
Delivering power to ordinary growing load for a long time might need the parallel combination of generators.
Reference [30] present the dynamic performance of SEIG under various loading conditions, while considering the behaviour of the terminal voltage for a unity, leading and lagging power factor conditions, and also the machine behaviour when the excitation capacitor is disconnected.
This dissertation investigate the dynamic loss of load performance of an autonomous SEIG for various power factor value, considering the surge experience by the machine characteristics of phase current, load current, capacitor current, phase voltage and the change in frequency during a load introduction and subsequent removal of the introduce load on the machine.
A relationship between the power factor value and the time taken for the surge experienced during loading and unloading to die down is also investigated, while considering the effect of the load on frequency for the various power factors. Chapter two present the modelling of induction machine, while chapter three gives a d-q axis modelling and analysis of the SEIG in the stationary reference frame. The machine parameters are also given in this chapter.
In chapter four, the simulated result is given and discussed while the last chapter summarises the work as conclusion and recommendation.
1.2 STATEMENT OF THE PROBLEM
Despite the advantages and usability of SEIG, it still has the problem of voltage and frequency stabilization, thus always requiring a good voltage and frequency stabilization scheme. Self-excited Induction Generator (SEIG) is speed, excitation source and load dependent. For a constant speed, non- variable capacitance operation, it is observed that the frequency varies owing to the introduction or removal of load. Since SEIG derives its excitation from a reactive source, the introduction of a reactive load changes the operating state of the generator.
The purpose of this study is focussed mainly on the performance of the machine during the loss/gain in load for various load power factors and its influence on the frequency of the generated voltage.
The investigation is concern with the performance of the phase voltage, phase current, capacitor current, load current, frequency and the voltage build up time, while monitoring the effect of the reactive nature of the load on;
The experienced surge during loading and subsequent unloading.
The experience changes in the state variables (voltage, currents, frequency).
1.2: PURPOSE OF STUDY
The purpose of this work is to investigate on the loss of load performance of Self- Excited Induction Generator, concerning its self with the effect of the reactive nature of the load on the machine, maximizing the effective and reliable use of SEIGs for a chosen load. A good build up time, accommodating surge and a minimal drop in circuit parameters at loading is desirable, and should be achieved for any chosen loading condition.
This material content is developed to serve as a GUIDE for students to conduct academic research
UNIVERSITY OF NIGERIA NSUKKA DEPARTMENT OF ELECTRICAL ENGINEERING
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