Effects of long-shunt and short-shunt connections on voltagevariations of a self-excited induction generator

This paper presents a comparative study of steady-state performances of both long-shunt and short-shunt configurations of an isolated self-excited induction generator under various loading conditions. Both simulated and experimental results are clearly compared to examine the effects of both connections on load voltage variation of the studied induction generator. The analyzed results show that the long-shunt configuration can maintain lower load voltages while the short-shunt connection has better voltage variation under higher load voltages     

Long-shunt and short-shunt connections on dynamic performance of aSEIG feeding an induction motor load

This paper presents a comparative study of long-shunt and short-shunt configurations on dynamic performance of an isolated self-excited induction generator (SEIG) feeding an induction motor (IM) load. The studied IM is suddenly connected to the output terminals of the studied configurations of the SEIG. Both simulated results and experimental results based on laboratory 1.1 kW induction machines are clearly compared to examine the effects of both connections on voltage variation of the studied SEIG. The analyzed results show that the long-shunt configuration may lead to unwanted oscillations while the short-shunt connection provides better voltage variation. The eigenvalue technique is also employed to examine the possible voltage collapse and unstable conditions in the studied SEIG-IM system     

Voltage regulation optimization of compensated self-excited induction generator with dynamic load

The configuration of short-shunt self-excited induction generator feeding induction motor loads (SEIG-IM) suffers from excessive transients during startup of motor load under no load and unstable operation. These problems may be due to subsynchronous resonance as obtained with series compensated transmission line or due to the connected load system. The use of damping resistors across series capacitors is proposed to damp out the starting transients and for the stable operation. The steady-state model of short shunt SEIG-IM with damping resistors and resistive and motor load is developed to obtain the values of shunt and series capacitances for optimum voltage regulation. The simulated annealing like approach is used to solve voltage regulation optimization problem. The values of shunt and series capacitances and damping resistance are obtained for optimum voltage regulation under entire loading range and stable operation during starting and loading. The results are experimentally verified, which establish the effectiveness of damping resistance and developed algorithm.     

Performance evaluation of series compensated self-excited six-phase induction generator for stand-alone renewable energy generation

This paper presents the steady-state behavior of a series compensated (short-shunt) self-excited six-phase induction generator (SPSEIG) configured to operate as stand-alone electric energy source in conjunction with a hydro power plant. A purely experimental treatment is provided with the emphasis placed on operating regimes that illustrate the advantages of using SPSEIG. In particular, it is shown that the SPSEIG can operate with a single three-phase capacitor bank, so that the loss of excitation or fault at one winding does not lead to the system shutdown. The generator can also supply two separate three-phase loads, which represent an additional advantage. Experimental results include loading transients with independent three-phase resistive and resistive–inductive load at each of the two three-phase winding sets, and measured steady-state characteristics for various load and/or capacitor bank configurations. Practical results for long-shunt configuration are also given for comparative performance evaluation of series compensated SPSEIG.     

Performance characteristics and optimum utilization of a cagemachine as capacitance excited induction generator

The performance characteristics of a cage induction machine operating as a self-excited induction generator (SEIG) in stand-alone mode are presented. A static capacitor bank is considered to self-excite the machine and to maintain its terminal voltage constant. The lagging reactive power requirement of self-excited induction generator is obtained for different load values. The effect of speed on the excitation requirement of the cage machine has also been studied. An algorithm is developed to achieve these characteristics using the Newton-Raphson method and a steady-state equivalent circuit of the machine. The developed analytical technique is extended to evaluate the number of steps of switching capacitors for loading the machine up to its full load rating while maintaining the terminal voltage within desired limits. The selection of an optimum terminal voltage corresponding to the maximum output of the machine for its optimum utilization is also made using single-variable optimization     

Capacitance requirements of a three-phase induction generatorself-excited with a single capacitance and supplying a single-phase load

This paper presents a practical method for computing the minimum capacitance required to initiate voltage build-up in a three-phase induction generator self-excited with a single capacitance and supplying a single-phase load. Attention is focused on the Steinmetz connection which gives superior performance over the plain single-phasing mode of operation. From a consideration of the input impedance of the induction generator and the self-excitation conditions, two nonlinear equations are obtained. Solution of one equation using the Secant method enables the excitation frequency to be determined, and the minimum excitation capacitance can be calculated from the second equation. This solution technique is subsequently employed in an iterative procedure for computing the capacitance required to maintain the terminal voltage at a preset value when the generator is supplying load. Experimental results obtained on a 2-kW induction machine are presented to verify the theoretical analysis where possible     

Steady state analysis and performance characteristics of athree-phase induction generator self excited with a singlecapacitor

Steady-state analysis and performance characteristics of a three-phase isolated star or delta connected induction generator self-excited with a single capacitor are discussed. Analytical expressions are derived to determine the no-load capacitance required to maintain self-excitation. The performance characteristics of a self-excited induction generator are affected by the terminal capacitance, C, machine speed, ν, and the load parameters. Generally, the value of C has a stronger influence on the performance characteristics and should be selected such that the terminal voltage, V_t, is near its rated value while keeping C close to its lower limiting value. The accuracy of the method was experimentally verified, and good agreement is obtained between the two sets of results. The performance characteristics of star and delta connected induction generators are compared. In the mode of operation discussed, the voltages and currents are unbalanced with relatively high losses and rather low efficiency. Among the two configurations discussed, the delta connected generator offers a higher current and lower terminal voltage and a wider range of excitation capacitance     

Application of Floquet's theory to the analysis of series-connectedwound-rotor self-excited synchronous generator

In this paper, Floquet's theory for solving differential equations with periodically varying coefficients has been utilized in evaluating the steady state performance of a three phase wound rotor series-connected self-excited synchronous generator SCSESG. This type of generator is practically realized by the series connection of stator and rotor windings of a conventional wound-rotor induction machine. Self excitation may occur when a suitable capacitor bank is connected across the machine terminals. The analysis gives the same results that are obtained when the d-q transformation model is utilized. Application of Floquet's theory has the advantage of reducing the mathematical manipulation needed. The results are checked experimentally. Saturation effects on each axis inductance as well as iron losses show satisfactory agreement. The generator acts as a hypothetical salient pole machine operating at half the rotor electrical angular frequency and is independent of load conditions provided that the prime mover speed is kept constant     

Unified power quality conditioner based on a three-level NPC inverter using fuzzy control techniques for all voltage disturbances compensation

This paper presents a novel and efficient control scheme for unified power quality conditioner (UPQC) based on three-level neutral point clamped (NPC) inverter using fuzzy logic techniques. The proposed UPQC is capable of mitigating source current harmonics and compensate all voltage disturbances such as voltage sags, swells, unbalances and harmonics. It is designed by the integration of series and shunt active filters (AFs) sharing a common DC bus capacitor. The DC voltage is maintained constant using proportional integral voltage controller. The synchronous reference frame (SRF) theory is used to get the reference signals for shunt active power filters (APFs) and the power reactive theory (p-q theory) for series APFs. The shunt and series APF reference signals derived from the control algorithm and sensed signals are injected in two controllers to generate switching signals. To improve the UPQC capability, fuzzy logic techniques are introduced to control the series APF. The performances of the proposed UPQC system are evaluated in terms of power factor correction, mitigation of voltage or current harmonics and all other voltage disturbances compensation using Matlab-Simulink software and SimPowerSystem toolbox. The simulation results illustrate the performance of the proposed UPQC at the common connection point of the nonlinear load to improve the power energy quality.     

1000MW级核电机组自并励静止励磁系统的性能参数探讨

上海发电机厂引进的西门子1 ooOMw级核电机组励磁方式为无刷励磁.由于无刷 励磁机及其整流盘的容量有限,励磁系统的强励能力受到限制,有时不能满足国内电网对于机组的强励要求.因而具有高强励顶值和电压上升率的自并励静止励磁系统成为1 000 MW级核电机组励磁方式的另一选择.就上海发电机厂的1000MW级核电机组静止励磁...     

Transient analysis of a PV power generator charging a capacitor for measurement of the I–V characteristics

Measuring the I–V characteristics is of high importance since it can be considered as a quality and performance certificate for each PV generator. The most precise and inexpensive measuring method is represented in capacitor charging by the PV generator. Using the equivalent circuit of the PV generator with a capacitor as load and applying transient analysis on the circuit, we obtain the capacitor charging voltage and current as a function of time, as well as their differentials as a function of short circuit current and capacitor size. The derived equations facilitate the calculation of proper capacitance size for measuring the I–V characteristics, and considers the acquisition speed of the measuring system as demonstrated through two measurement samples in this paper. The capacitor size is directly and indirectly proportional to the short circuit current and open circuit voltage of the PV generator, respectively. Accordingly, the paper presents a capacitance calculation chart, which enables selecting the correct capacitance for measuring the I–V characteristics by a computerized data acquisition system.     

Effect of excitation capacitors on transient performance ofreluctance generators

The transient responses of a reluctance generator connected to an infinite power system excited by a bank of terminal capacitances are compared to those when load excitation is used. A mathematical model is developed to simulate the machine with its terminal capacitor. With the aid of a least-square-error method, this model is used to optimize the machine parameters. The capacitance excitation requirements for different load conditions are then computed using a steady-state model. The comparison of the transient responses shows that the terminal-capacitor excitation method has several advantages over the load excitation method. It reduces the first rotor swing and gives more damping to the subsequent rotor oscillations. It also increases the critical fault-clearing time and hence the transient stability limits. In addition, it suppresses all power frequency torque oscillations, which are quite pronounced when load excitation is used     

Design and coordination of a capacitor and on‐load tap changer system for voltage control in a wind power plant of doubly fed induction generator wind turbines

Larger percentages of wind power penetration into the grid translate to more demanding requirements coming from grid codes; for example, voltage support at the point of connection has been introduced recently by several grid codes from around the world, thus making it important to analyse this control. Voltage control is actuated by reactive power injection, and for a wind power plant of doubly fed generator turbines, reactive power capability can be a challenge, which typically is overcome by installing reactive power compensators. The integration and the interaction between all these reactive power sources and the on‐load tap changer of the main substation transformer need to be analysed and taken into account in the control design. In this paper, a novel coordination and control strategy for capacitor banks and on‐load tap changer for a wind power plant is introduced. The capacitor banks are controlled in such way that the steady‐state usage of the converters for reactive power injection is driven below to a maximum desired value of 0.1 pu. Additionally, the control transients because of the capacitor bank switching are minimized by using a suitable control structure. The tap changer control is coordinated with the plant control to decrease the impact of the capacitors reactive power in the line drop calculation, thus reducing the amount of tap operations and improving the accuracy of the line drop voltage estimation. The coordination of the central controller with the plant components is analysed and tested through electromagnetic transient program simulations. Copyright © 2011 John Wiley & Sons, Ltd.     

Starting and Steady-State Characteristics of DC Motors Powered by Solar Cell Generators

The performance of dc motors (series, separately-excited, and shunt motors) powered by a solar cell generator and loaded by two different types of loads, one a constant load and one a ventilator load, were analyzed with respect to the transient (starting) and steady state operation. Direct current motors are employed in photovoltaic water pumping systems; therefore, the understanding of the system operation and the matching of the system components (solar cells, dc motor type, and load type) are important factors of the system design. Since the solar cell generator in a nonlinear and time-dependent power supply with an output that varies with the insolation (hourly and daily), the performance characteristics of the dc motor are different when supplied by a solar cell generator than when supplied by a conventional constant voltage source. The transient solution was obtained by using an available computer program - SUPER SCEPTRE. The separately - excited (or permanent magnet) motor with a ventilator load was found to be the most suitable for the solar cell generator. The series motor is quite acceptable, but the shunt motor gives poor performance. In all cases the ventilator load is more compatible with the solar cell generator than with the constant load.     

齿谐波励磁的混合励磁发电机带整流负载的定子铁耗计算

针对混合励磁发电机带整流负载时谐波含量增加对定子铁心损耗的影响,建立了齿谐波励磁的混合励磁发电机带整流负载的有限元场路耦合模型,分析了发电机带整流负载在齿谐波励磁系统断开和接通两种工况下的线电压谐波分量,借助传统定子铁心损耗计算模型计算出线电压谐波分量对定子铁心损耗的影响。同时利用有限元电磁仿真软件计算出电机空载及两种工况下的定子铁心损耗,仿真与计算结果比较表明,该计算方法在一定的误差内准确性较高。     

Induction generator controller based on the instantaneous reactive power theory

A novel control strategy for a stand-alone induction generator (IG), working with variable speed and load, is proposed. The IG is simultaneously excited by a capacitor and an inverter. The capacitor provides the rated reactive current needed to excite the IG while the inverter adds the reactive current needed to regulate the IG output voltage. The control strategy is based on the instantaneous reactive power theory. A 4 kW laboratory prototype has been built to validate the proposal and experimental results are presented.     

Optimisation of voltage and frequency regulation in an isolated wind-driven six-phase self-excited induction generator

This paper presents a constant voltage operation of a Six-Phase Self-Excited Induction Generator (SPSEIG) driven by a fixed speed wind turbine using an Ant colony optimisation (ACO) technique to predict the behaviour of a the machine. In this paper, an attempt has been made to estimate the excitation capacitance requirements of a SPSEIG for maintaining rated terminal voltage and frequency. The range of capacitance variation required for maintaining constant terminal voltage while supplying a load of variable magnitude is evaluated. Analytical approaches, suitable for all the configurations of shunt capacitances such as variable excitation capacitance connected across (i) single three-phase winding set only and (ii) both the three-phase winding sets of an SPSEIG for operation as a simple shunt on no load and pure resistive load, are presented. The mathematical model developed is based on loop impedance method using graph theory. It is shown that the proposed technique is very effective and useful for making the SPSEIG feasible for remote areas with wind potential. The proposed approach is tested and compared with Genetic Algorithm (GA) and Fmincon technique.     

Fuzzy multi-objective technique combined with VCS algorithm for unified power quality conditioner based on hybrid power source PEMFC/SC

This paper is proposed to establish an optimal control method for UPQC (Unified Power Quality Conditioner) to improve power quality and manage effectively equal power sharing between shunt and series inverter of UPQC under electrical faults condition. The UPQC is modeled to protect sensitive load from source side voltage disturbances under nonlinear load conditions. A hybrid power generator that integrates a proton exchange membrane fuel cell (PEMFC) as the main energy source and a super capacitor (SC) as secondary source is proposed to feed the FACT device. In this work, a new control strategy is presented for considering the voltage sag, power factor and total harmonic distortion (THD) as multi-objective of UPQC controller. For this purpose, a new powerful algorithm named virus colony search (VCS) is used for determining the coefficients of the PI controller of UPQC. By using the fuzzification process for the objectives function, a suitable fitness function is established for the optimization method. From the simulations, it can be seen that the results obtained by the proposed algorithm are best and attractive compared to other method. Consequently, the proposed strategy is effective and outstrips other strategies.     

Ramp-Rate Control of Photovoltaic Generator With Electric Double-Layer Capacitor

This paper describes ramp rate control of a photovoltaic (PV) generator with an electric double-layer capacitor. The capacitor absorbs rapid fluctuations of PV generation, and allows the generator to change its output at a limited ramp rate. The output is mainly determined by a moving average of the PV generation, but the capacitor voltage is also counted to keep the voltage at a specified value, and accordingly, to make the capacitor small. An expression for the capacitor size is derived. The feedback gain of the capacitor voltage is also examined to maintain enough energy storage in the capacitor. Some experiments are executed to demonstrate and validate the proposed method.      

基于可控两相零式电抗分流励磁系统解决农村小水电站稳定并网方法

针对农村小型水电站发电机普遍采用双绕组电抗分流自励恒压系统,在并网运行时易产生振荡或失磁甚至失步而解列现象.给并网运行带来困难的问题,提出可控两相零式电抗分流励磁系统。研究表明,该系统较原系统各项性能大为提高,能稳定并网运行,具有改造简单、成本低、并网运行可靠、安全性好等技术优势,是一种值得推荐的新励磁品种。