Dynamics and stability of a hybrid wind-diesel power system

Dynamic system analysis is carried out on an isolated electric power system consisting of a wind turbine generator (WTG) and a diesel engine generator (DG). The 150 kW wind turbine generator is operated in parallel with the diesel generator to serve an average load of 350 kW. A comprehensive digital computer model of a hybrid wind-diesel power generation system, including the diesel and wind power dynamics for stability evaluation, is developed. The dynamic performance of the power system and its control logic are studied, using the time domain solution approach. A systematic method of choosing the gain parameter of the wind turbine generator pitch control by the second method of Lyapunov that guarantees stability is presented. The response of the power system with the optimal gain setting to the random load changes has been studied. Analysis of stability has further been explored using the eigenvalue sensitivity technique.     

A smooth co-ordination control for a hybrid autonomous power system (HAPS) with battery energy storage (BES)

The standalone hybrid power system constitutes a synchronous generator driven by a diesel engine, renewable energy source (wind) apart from a battery energy storage system. A coherent control strategy to regulate the voltage and frequency of the standalone grid is proposed in this paper. The system is simulated using Matlab/Simulink for preliminary validation and further tested on a laboratory prototype which involves a TMS320LF2407A DSP controller to digitally implement the control strategy. The dynamic behavior of the system is perused through the direct connection of an induction machine. The control strategy is verified for step changes in load and variation in wind power.     

Neuro-fuzzy control for autonomous wind–diesel systems using biomass

This paper deals with the development of a neuro-fuzzy controller for a wind–diesel system composed of a stall regulated wind turbine with an induction generator connected to an ac bus-bar in parallel with a diesel generator set having a synchronous generator. A gasifier is capable of converting tons of wood chips per day into a gaseous fuel that is fed into a diesel engine. The controller inputs are the engine speed error and its derivative for the governor part of the controller, and the voltage error and its derivative for the automatic voltage regulator. These are readily measurable quantities leading to a simple controller which can be easily implemented. It is shown that by tuning the fuzzy logic controllers, optimal time domain performance of the autonomous wind–diesel system can be achieved in a wide range of operating conditions compared to fixed-parameter fuzzy logic controllers and PID controllers.     

Modular Autonomous Electrical Power Supply Systems Examples of Several Stand Alone Systems

A modular system for the supply of remote electrical consumers was developed, which makes possible a variable integration of wind energy and photovoltaic plants in connection with a diesel engine and a battery storage. The wind energy converters, equipped with asynchronous generators, and a fast pitch control, work parallel with a synchronous generator. The generator is driven by a diesel engine by means of an overrunning clutch, or started by a small DC-motor. If the diesel is off, or the starting process by the DC-motor is finished, the synchronous generator works as a rotating phase-shifter and takes over voltage control and supply of reactive power.

The speed versus power control of the wind energy converters does not only make possible an optimized parallel operation with the diesel-generator unit, but also directly takes over frequency control when operated singly without diesel engine. The location of the wind energy plants does not depend on the site of the diesel engine, because control cables are not necessary. To avoid too frequent starting of the diesel, a storage battery is installed. The report describes the electrical and control technical design on principle, as well as the experience with the following, already built plants:

-Coupling of two wind energy plants for the supply of water irrigation pumps.

-Combination of two WECs with a short-time battery storage and a diesel-set.

-Combination of two WECs with a photovoltaic generator and a battery storage.     

Dynamic characteristics of autonomous wind–diesel systems

In this paper the dynamics of a small autonomous system, comprising a diesel generator and a wind turbine, are investigated. The analysis is performed both in the frequency and time domain, using simplified models of the system components and taking into account the diesel engine speed governor and the wind turbine pitch controller (for pitch regulated machines). The investigation is extended to include different types of wind turbines, equipped with induction or synchronous generator and using pitch or stall regulation, as well as operation of the wind turbine in an autonomous or infinite system. The objective is to determine the main factors affecting the behaviour of the system and to illustrate the effect of the speed governor and pitch controller settings on the expected performance. Particular emphasis is placed on identifying the main modes of the system and determining their dependence on the controllers' parameters. A comparative assessment of the dynamic characteristics of different types of wind turbines is also included and the operation of the wind turbine in a small system and against an infinite bus is addressed and discussed.     

Stability simulation and parameter optimization of a hybrid wind-diesel power generation system

In this paper a systematic method of choosing the gain parameter of the wind turbine generator pitch control is presented using the Lyapunov technique that guarantees stability. A comprehensive digital computer model of a hybrid wind–diesel power generation system including the diesel and wind power dynamics with a superconducting magnetic energy storage (SMES) unit for stability evaluation is developed. The effect of introducing an SMES unit for improvement of stability and system dynamic response is studied. Analysis of stability has further been explored using an eigenvalue sensitivity technique. The eigenvalues of the system with and without an SMES unit are studied and the effect of variation of the SMES unit parameters on eigenvalue locations are plotted. The dynamic response of the power system to random load changes with optimal gain setting is also presented.     

An investigation of variable speed operation of diesel generators in hybrid energy systems

This paper provides a preliminary assessment of the performance and economic potential of a hybrid energy system (wind/diesel) power system which includes a variable speed diesel generator. Recent development in power electronics would be utilized to allow asynchronous operation of the diesel generator, while simultaneously delivering constant frequency electric power to the local electrical grid. In addition to the variable speed diesel, the system can include wind and/or solar electric sources. A hybrid energy system model recently developed at the University of Massachusetts is used to simulate this system configuration and other more conventional wind/diesel hybrid energy systems. Experimental data from a series of variable speed diesel generator tests were used to generate a series of fuel consumption curves used in the analytical model. In addition to performance (fuel savings) comparisons for fixed and variable speed systems, economic cost of energy calculations for the various system designs are presented. It is shown that the proposed system could offer both performance and economic advantages.     

An adaptive control scheme for biomass-based diesel–wind system

Control of autonomous diesel–wind systems is difficult because of time-varying dynamical properties, most of them as a result of plant non-stationary, non-linearity and random disturbances. In this paper, the system is composed of a pitch controlled wind turbine with an induction generator connected to an ac bus bar in parallel with a diesel generator set having a synchronous generator. A power plant can generate electric power using biomass from the olive tree in Spain. To have optimal response under disturbances, adaptive schemes appear to be a particularly attractive choice. The recursive least squares method and minimum-variance control algorithm is implemented to design the controller. This paper considers the application of an adaptive control methodology that allows the controller to automatically adjust to changing process variables and thereby provide uniform response over a wide range of operating conditions.     

Optimisation of real-time control for hybrid diesel–PV–battery systems

Hybrid diesel–PV–battery systems are one of the most cost effective options for off-grid power generation. A methodology for the optimal operation of such systems for an off-grid application is proposed in this paper. The methodology is based on the minimisation of an energy cost function. Based on this function, an optimal operating point for the diesel generator is identified, taking into account the characteristics of the diesel generator, battery bank and converter as well as the costs of fuel and battery usage. The operation of the diesel generator at this optimum operating point results in an overall energy cost reduction for the hybrid diesel–battery system. Simulation analysis shows that the proposed control strategy can achieve up to 4% reduction in the levelised cost of energy. This is mostly due to the savings made from the efficient usage of diesel generator and battery.     

Life cycle assessment of power generation alternatives for a stand-alone mobile house

This paper presents comparative life cycle assessment of nine different hybrid power generation solutions that meet the energy demand of a prototypical mobile home. In these nine solutions, photovoltaic panels and a wind turbine are used as the main energy source. Fuel cell and diesel generator are utilized as backup systems. Batteries, compressed H₂, and H₂ in metal hydrides are employed as backup energy storage. The findings of the study shows that renewable energy sources, although they are carbon-free, are not as environmentally friendly as may generally be thought. The comparative findings of this study indicate that a hybrid system with a wind turbine as a main power source and a diesel engine as backup power system is the most environmentally sound solution among the alternatives.     

A novel self-induced self-regulated single phase inductiongenerator. I. Basic system and theory

This paper describes a newly developed single phase capacitor self excited induction generator with self regulating features, suitable for oil engine driven portable gen-sets for autonomous/standby power generation. The system is also suitable for microhydel and wind energy systems. The generator has two specially designed stator windings in quadrature, connected externally to a shunt and a series capacitor respectively. It employs a standard die-cast squirrel cage rotor. Special features, advantages and theoretical concepts of the system are highlighted     

Torsional torque suppression of generator based on H∞ control theory in isolated power system

Renewable energy power plants, such as wind turbine generator and photovoltaic system, have been introduced in isolated power system recently. The output power fluctuations of wind turbine generator and load deviations result in frequency deviation and terminal voltage fluctuation. Furthermore, these power fluctuations also affect the turbine shafting of diesel generators and gas‐turbine generators, which are the main components of power generation systems in isolated islands. For stable operation of gas‐turbine generator, the torsional torque suppression as well as power system stabilization should be considered. In this paper, the control strategy that achieves torsional torque suppression and power system stabilization is presented based on H_∞ control theory. The effectiveness of the proposed control system is validated by numerical simulation results. Copyright © 2010 John Wiley & Sons, Ltd.     

An input‐output linearization–based control strategy for wind energy conversion system to enhance stability

This paper proposes a novel control strategy for doubly fed induction generator (DFIG)‐based wind energy conversion system to investigate the potential of enhancing the stability of wind energy transmission system, a synchronous generator weakly integrated to a power system with a DFIG‐based wind farm. The proposed approach uses state feedback to exactly linearize the nonlinear wind energy transmission system from control actions (active power and reactive power control order of DFIG) to selected outputs (power angle and voltage behind transient resistance of synchronous generator) at first. Then, on account of the linearized subsystem, the stability enhancement controller is designed based on linear quadratic regulator algorithm to contribute adequate damping characteristics to oscillations of the synchronous generator system under various operation points. The proposed control strategy successfully deals with the nonlinear behaviors exist from the inputs to outputs and improve the robustness with respect to the variation of system operation points. Furthermore, not only the rotor angle stability but also the voltage stability is enhanced by using the proposed control strategy. The simulation results carried on the studied system verify the effectiveness of the proposed control strategy of wind energy conversion system for system stability enhancement and the robustness against various system operation points.     

Analysis of the dynamic response improvement of a turbocharged diesel engine driven alternating current generating set

Reliability of electric supply systems is among the most required necessities of modern society. Turbocharged diesel engine driven alternating current generating sets are often used to prevent electric black outs and/or as prime electric energy suppliers. It is well known that turbocharged diesel engines suffer from an inadequate response to a sudden load increase, this being a consequence of the nature of the energy exchange between the engine and the turbocharger. The dynamic response of turbocharged diesel engines could be improved by electric assisting systems, either by direct energy supply with an integrated starter–generator–booster (ISG) mounted on the engine flywheel, or by an indirect energy supply with an electrically assisted turbocharger. An experimentally verified zero dimensional computer simulation method was used for the analysis of both types of electrical assistance. The paper offers an analysis of the interaction between a turbocharged diesel engine and different electric assisting systems, as well as the requirements for the supporting electric motors that could improve the dynamic response of a diesel engine while driving an AC generating set. When performance class compliance is a concern, it is evident that an integrated starter–generator–booster outperforms an electrically assisted turbocharger for the investigated generating set. However, the electric energy consumption and frequency recovery times are smaller when an electrically assisted turbocharger is applied.     

Load-frequency control of isolated wind-diesel-microhydro hybrid power systems (WDMHPS)

We examine load-frequency control of isolated WDMHPS provided with conventional proportional-plus-integral controllers. The parameters of the controller are optimised for system performance with step or realistic disturbances using an integral-square-error (ISE) criterion. Non-optimum gain settings may result if only step changes are assumed in input wind power or in load. The controller works for a continuous hybrid power system in either a continuous or a discrete mode. System performance deteriorates for discrete control. To evaluate the performance of the hybrid system producing electric power from wind and microhydro by operating with an induction generator and from diesel by using a synchronous alternator, we must consider for the state space model of the hybrid system the load-frequency and blade-pitch controllers in the continuous or discrete mode. A study of the transient responses of the system shows that transient changes in input wind power settle in 12 s while disturbances in load take only 4 s to stabilise.     

Operational behaviour of wind diesel systems incorporating short-term storage: An analysis via simulation calculations

The introduction of wind energy into autonomous electricity supplies based on a diesel generator aims at the reduction of the fuel consumption of the diesel. A storage unit integrated into the system may compensate for fluctuations in the power output of the wind turbine and raise the fuel saving potential. With the help of simulation calculations we investigate the long-term behaviour of wind diesel systems regarding different storage sizes for a site near the North Sea coast. Special emphasis is put on the stress the storage undergoes. The constraints on the application of batteries for use in wind diesel systems are highlighted. For this analysis highly resolved wind speed data are necessary. The generation of such data is described in the appendix.     

Dynamics and stability of wind and diesel turbine generators withsuperconducting magnetic energy storage unit on an isolated power system

Dynamic system analysis is carried out on an isolated electric power system consisting of a diesel generator and a wind-turbine generator. The 150 kW wind turbine is operated in parallel with a diesel generator to serve an average load of 350 kW. A comprehensive digital computer model of the interconnected power system including the diesel and wind-power dynamics with a superconducting magnetic energy storage (SMES) unit is developed. Time-domain solutions are used to study the performance of the power system and control logic. Based on a linear model of the system, it is shown that changes in control-system settings could be made to improve damping and optimization of gain parameters and stability studies are done using the Lyapunov technique and eigenvalue analysis. The effect of introducing the SMES unit for improvement of stability and system dynamic response is studied     

Comparative life-cycle assessment of a small wind turbine for residential off-grid use

As the popularity of renewable energy systems grows, small wind turbines are becoming a common choice for off-grid household power. However, the true benefits of such systems over the traditional internal combustion systems are unclear. This study employs a life-cycle assessment methodology in order to directly compare the environmental impacts, net-energy inputs, and life-cycle cost of two systems: a stand-alone small wind turbine system and a single-home diesel generator system. The primary focus for the investigation is the emission of greenhouse gases (GHG) including CO₂, CH₄, and N₂O. These emissions are calculated over the life-cycle of the two systems which provide the same amount of energy to a small off-grid home over a twenty-year period. The results show a considerable environmental benefit for small-scale wind power. The wind generator system offered a 93% reduction of GHG emissions when compared to the diesel system. Furthermore, the diesel generator net-energy input was over 200 MW, while the wind system produced an electrical energy output greater than its net-energy input. Economically, the conclusions were less clear. The assumption was made that diesel fuel cost over the next twenty years was based on May 2008 prices, increasing only in proportion to inflation. As such, the net-present cost of the wind turbine system was 14% greater than the diesel system. However, a larger model wind turbine would likely benefit from the effects of the ‘economy of scale,’ producing superior results both economically and environmentally.     

Dynamic participation of doubly fed induction generator in automatic generation control

Increasing levels of wind generation have resulted in an urgent need for the assessment of their impact on frequency control of power systems. The displacement of conventional generation with wind generation will result in erosion of system frequency. The paper analyzed the dynamic participation of doubly fed induction generator (DFIG) to system frequency responses of two-area interconnected power system having variety of conventional generating units. Frequency control support function responding proportionally to frequency deviation is proposed to take out the kinetic energy of turbine blades in order to improve the frequency response of the system. Impacts of different wind penetrations in the system and varying active power support from wind farm on frequency control have been investigated. Integral gains of AGC loop are optimized through craziness-based particle swarm optimization (CRPSO) in order to have optimal transient responses of area frequencies, tie-line power deviation and DFIG parameters.     

Network power flux control of a wind generator

In this paper, a network power flux control of a variable speed wind generator is investigated. The wind generator system consists of a doubly fed induction generator (DFIG) connected to the network associated to a flywheel energy storage system (FESS). The dynamic behaviour of a wind generator, including the models of the wind turbine, the doubly fed induction generator, the back-to-back AC/AC converter, the converter control and the power control of this system, is studied. Is also investigated a control method of the FESS system which consists of the classical squirrel-cage induction machine (IM) supplied off the variable speed wind generator (VSWG). In order to verify the validity of the proposed method, a dynamic model of the proposed system has been simulated, for different operating points, to demonstrate the performance of the system.