Smooth transition from wind only to wind diesel mode in an autonomous wind diesel system with a battery-based energy storage system

High wind penetration wind diesel hybrid systems (WDHS) have three modes of operation: Diesel Only (DO), Wind Diesel (WD) and Wind Only (WO). The WDHS presented in this article consists of a wind turbine generator (WTG), a diesel engine (DE), a synchronous machine (SM), the consumer load, a battery-based energy storage system (BESS), a discrete dump load (DL) and a distributed control system (DCS). The DE can be engaged (DO and WD modes)/disengaged (WO mode) from the SM by means of a clutch. The DCS consists of a sensor node, which measures the SM and DE speeds, calculates the reference active power P_REF necessary to balance the active power in the WDHS and communicates this P_REF value with a message to the BESS and DL actuator nodes. In the WO mode, the power sources are the WTG and the BESS (temporary) and if there is an active power shortfall, the DCS, to prevent a frequency collapse, must order to start the DE, wait until the DE reaches the SM speed and lock the clutch, changing to the WD mode. With the clutch locked, the combined actuation of the DE+BESS will raise the system frequency to the rated value. This WO to WD transition is simulated in this article showing graphs for frequency, voltage and active powers for the elements of the system. These graphs are compared with the ones obtained if the BESS does not actuate in WD mode. The comparison results show that with the BESS actuation in WD mode the settling time is reduced a 50%, the over and under shooting in the system frequency are eliminated and the system voltage variations are reduced a 40%.     

Effective active power control of a high penetration wind diesel system with a Ni–Cd battery energy storage

High penetration (HP) Wind Diesel Hybrid Systems (WDHS) have three modes of operation: Diesel Only (DO), Wind Diesel (WD) and Wind Only (WO). The HP-WDHS presented in this article consists of a Wind Turbine Generator (WTG), a Diesel Generator (DG), the consumer Load, a Ni–Cd Battery based Energy Storage System (BESS), a discrete Dump Load (DL) and a Distributed Control System (DCS). The DG includes a friction clutch which allows the Diesel Engine (DE) to be engaged (DO and WD modes)/disengaged (WO mode) to the Synchronous Machine (SM). The DCS consists of a sensor node which measures the SM speed and active power, calculates the reference active power P_REF necessary to balance the active power in the WDHS and communicates this P_REF value through a message to the BESS and DL actuator nodes. In the WD mode both the DG and WTG supply active power to the system and the DE speed governor regulates the system frequency. However in an HP-WDHS the power produced by the WTG (P_T) can be greater than the one consumed by the load (P_L). This situation means a negative power in the DG (power inversion) with its speed governor unable to regulate frequency. To avoid this situation, the DCS must order coordinated power consumption to the BESS and DL in order to keep the DG produced power positive. In this article it is shown by simulation how the DCS manages both a temporary power inversion and a permanent one with the mandatory transition from WD to WO mode. The presented graphs for frequency, voltage, active powers of the system elements and battery voltage/current show the effectiveness of the designed control.     

Impact of wind power in autonomous power systems—power fluctuations—modelling and control issues

This paper describes a detailed modelling approach to study the impact of wind power fluctuations on the frequency control in a non‐interconnected system with large‐scale wind power. The approach includes models for wind speed fluctuations, wind farm technologies, conventional generation technologies, power system protection and load. Analytical models for wind farms with three different wind turbine technologies, namely Doubly Fed Induction Generator, Permanent Magnet Synchronous Generator and Active Stall Induction Generator‐based wind turbines, are included. Likewise, analytical models for diesel and steam generation plants are applied. The power grid, including speed governors, automatic voltage regulators, protection system and loads is modelled in the same platform. Results for different load and wind profile cases are being presented for the case study of the island Rhodes, in Greece. The scenarios studied correspond to reference year of study 2012. The effect of wind fluctuations in the system frequency is studied for the different load cases, and comments on the penetration limits are being made based on the results. Copyright © 2010 John Wiley & Sons, Ltd.     

Flywheel energy storage systems: Review and simulation for an isolated wind power system

In flywheel based energy storage systems (FESSs), a flywheel stores mechanical energy that interchanges in form of electrical energy by means of an electrical machine with a bidirectional power converter. FESSs are suitable whenever numerous charge and discharge cycles (hundred of thousands) are needed with medium to high power (kW to MW) during short-time periods (seconds–minutes). Monitoring of the FESS state of charge is simple and reliable as only the spinning speed is needed. The materials for the flywheel, the type of electrical machine, the type of bearings and the confinement atmosphere which all together determine the FESSs energy efficiency (>85%) are reviewed. Main FESS applications: power quality, traction and aerospace are presented. Additionally in this paper it is presented the simulation of an isolated wind power system (IWPS) consisting of a wind turbine generator (WTG), a consumer load, a synchronous machine (SM) and a FESS. A low-speed iron flywheel driven by an asynchronous machine (ASM) is sized for the presented IWPS. The simulation results with graphs for system frequency, system voltage, active powers of the different elements, and FESS-ASM speed, direct and quadrature currents are presented showing that the FESS effectively smoothes the wind power and consumer load variations.     

New architecture and SCADA for stand‐alone hybrid (medium‐sized asynchronous wind turbine + UPS with battery + photovoltaic array) power system without diesel generator

In general, the commercialized medium‐sized asynchronous wind turbines are fully automated facilities designed to operate in parallel connection to the grid; in case of isolated operation, they need to be combined with diesel generator. This paper aims at studying the method of producing electricity of maximal quality with the wind, by constructing a new stand‐alone hybrid (medium‐sized asynchronous wind turbines, UPS with battery, and photovoltaic array) power system without diesel generator. This paper proposes a new architecture of stand‐alone hybrid power system that consists of medium‐sized asynchronous wind turbine, UPS, current limiter (reactor), photovoltaic array, and consumer and dump loads; accordingly, a supervisory control and data acquisition (SCADA) for this system is suggested along with the operation strategies depending on the output power of the UPS and wind turbine, consumer load, and the battery voltage of UPS. The case study was confirmed through the simulation results of the operation of a new stand‐alone hybrid (two 110 kW of asynchronous wind turbines, 250 kVA of UPS with battery, reactor, 36 kW of photovoltaic array, and consumer and dump loads) power system. The results of the simulation showed that the system frequency change of the new stand‐alone hybrid power system was 60 ± 0.5 Hz, and the one of the wind + diesel stand‐alone hybrid system was 60 ± 1 Hz, for the sudden change of consumer load and gust. This new system can be eligible as a standardizing option for the architecture of nondiesel stand‐alone hybrid system and its SCADA system.     

Voltage and Frequency Controller for a Three-Phase Four-Wire Autonomous Wind Energy Conversion System

This paper deals with control of voltage and frequency of an autonomous wind energy conversion system (AWECS) based on capacitor-excited asynchronous generator and feeding three-phase four-wire loads. The proposed controller consists of three single-phase insulated gate bipolar junction transistor (IGBT)-based voltage source converters (VSCs) and a battery at dc link. These three single-phase VSCs are connected to each phase of the generator through three single-phase transformers. The proposed controller is having bidirectional flow capability of active and reactive powers by which it controls the system voltage and frequency with variation of consumer loads and the speed of the wind. VSCs along with transformer function as a voltage regulator, a harmonic eliminator, a load balancer, and a neutral current compensator while the battery is used to control the active power flow which, in turn, maintains the constant system frequency. The complete electromechanical system is modeled and simulated in the MATLAB using the Simulink and the power system blockset (PSB) toolboxes. The simulated results are presented to demonstrate the capability of the proposed controller as a voltage and frequency regulator, harmonic eliminator, load balancer, and neutral current compensator for different electrical (varying consumer loads) and mechanical (varying wind speed) dynamic conditions in an autonomous wind energy conversion system.     

A review on the design and control strategy of energy storage system in wind power application

随着风力发电大规模入网,其随机性,波动性和间歇性特征对电力系统调频,调峰等有功平衡手段及电压稳定的影响越来越严重.储能系统能够在一定程度上控制风场的输出功率,平抑风电功率波动,改善风机低电压穿越能力,甚至为系统提供辅助服务,是从风场侧提高系统对风电的接纳能力的可行解决方案之一.作者在简要的介绍了风场储能技术应用现状的基础上,重点针对储能型风场内蓄电池储能系统的设计方案,容量优化及控制策略的研究现状及关键问题进行综述及探讨.     

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.     

Integrating renewable energy using a smart distribution system: Potential of self-regulating demand response

A self-regulating distribution system simulation platform is presented for a smart-grid with Distributed Energy Resource (DER) wind power injection in which load flow fluctuations are controlled via self-regulating air-source heat pump (HP) cycling. The grid power injection fluctuations are mitigated by bus-level HP control, while ensuring compliance to both consumer comfort constraints and distribution grid load flow requirements.The effects of applying a number of different bus-level HP control algorithms are evaluated. The results show that using building thermal mass in conjunction with simple control strategies can effectively accommodate large fluctuations associated with high penetration of wind energy. The number of HPs in each distribution phase significantly affects the load flow characteristics and the ability of the bus-level control to smooth the distribution grid regulator power.The bus-level control improves power and voltage ramping rates, reduces wind power injection fluctuations, and also reduces the energy reserve requirements.     

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.     

Load control of a wind-hydrogen stand-alone power system

A new generation of load controllers enable stand-alone power systems (SAPS) to use one or many standard (grid connected) wind turbines. The controllers use fuzzy logic software algorithms. The strategy is to use the control loads to balance the flow of active power in the system and hence control system frequency. The dynamic supply of reactive power by a synchronous compensator maintains the system voltage within the limits specified in EN50160. The resistive controller loads produce a certain amount of heat that is exchanged down to the end user (hot water). It was decided to investigate the implementation of a hydrogen subsystem into the SAPS that can work in parallel with the Distributed Intelligent Load Controller (DILC). The hydrogen subsystem can then function as energy storage on long-term basis and an active load controller on short-term basis.     

Dynamic contribution of variable-speed wind energy conversion system in system frequency regulation

Frequency regulation in a generation mix having large wind power penetration is a critical issue, as wind units isolate from the grid during disturbances with advanced power electronics controllers and reduce equivalent system inertia. Thus, it is important that wind turbines also contribute to system frequency control. This paper examines the dynamic contribution of doubly fed induction generator (DFIG)-based wind turbine in system frequency regulation. The modified inertial support scheme is proposed which helps the DFIG to provide the short term transient active power support to the grid during transients and arrests the fall in frequency. The frequency deviation is considered by the controller to provide the inertial control. An additional reference power output is used which helps the DFIG to release kinetic energy stored in rotating masses of the turbine. The optimal speed control parameters have been used for the DFIG to increases its participation in frequency control. The simulations carried out in a two-area interconnected power system demonstrate the contribution of the DFIG in load frequency control.     

Impact of wind power generating system integration on frequency stabilization in multi-area power system with fuzzy logic controller in deregulated environment

Among the available options for renewable energy integration in existing power system, wind power is being considered as one of the suited options for future electrical power generation. The major constraint of wind power generating system (WPGS) is that it does not provide inertial support because of power electronic converters between the grid and the WPGS to facilitate frequency stabilization. The proposed control strategy suggests a substantial contribution to system inertia in terms of short-term active power support in a two area restructured power system. The control scheme uses fuzzy logic based design and takes frequency deviation as input to provide quick active power support, which balances the drop in frequency and tie-line power during transient conditions. This paper presents a comprehensive study of the wind power impact with increasing wind power penetration on frequency stabilization in restructured power system scenario. Variation of load conditions are also analyzed in simulation studies for the same power system model with the proposed control scheme. Simulation results advocates the justification of control scheme over other schemes.     

Dynamic performance assessment of an isolated wind–diesel power system with superconducting magnetic energy storage unit under turbulent wind and load disturbances

This paper presents modelling and control aspects of an isolated wind–diesel system equipped with a superconducting magnetic energy storage (SMES) unit. The SMES unit is located at the induction generators' terminal bus, for exchanging real and reactive powers in four quadrants, with the wind–diesel system. The system components are modelled by non‐linear equations for accurate dynamic performance assessment and the SMES unit is modelled as a controllable current source. The control of the SMES unit is exercised through a multi‐input–multi‐output (MIMO) self‐tuning regulator (STR). The STR uses the local voltage and frequency measurements and generates appropriate signals for the control of the SMES unit. The SMES coil current deviation forms a part of one of the regulated variables of the STR for achieving a continuous control. The complete model of the hybrid system is developed and the parameters of the STR are adjusted for quality improvement of the power supply under turbulent wind. The scheme is then tested for load disturbances. The simulation results show the positive impact of the proposed scheme on the quality of the power supply both under turbulent wind as well as load disturbances. Copyright © 2002 John Wiley & Sons, Ltd.     

A new control methodology of wind turbine generators for frequency control of power system in isolated island

A wind turbine generator (WTG) system's output is not constant and fluctuates depending on wind conditions. Fluctuating power causes frequency deviations and adverse effects to an isolated power system when large output power from WTG systems is penetrated in the power system. This paper presents an output power control methodology of a WTG for frequency control using a load power estimator. The load power is estimated by a disturbance observer, and the output power command of the WTG is determined according to the estimated load. Besides, the WTG can also be controlled during wind turbulence since the output power command is determined by considering wind conditions. The reduction of the power system frequency deviation by using the WTG can be achieved by the proposed method. The effectiveness of the proposed method is validated by numerical simulations. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Contribution of Doubly Fed Wind Generators to Oscillation Damping

In this paper, the effects of increased wind power penetration by doubly fed asynchronous generators (DFAGs) on oscillation damping are investigated. With the help of an illustrative example, it is shown that the general trend for DFAGs is to increase interarea oscillation damping. However, there are exceptions for certain penetration levels (not necessarily large), for which the voltage control (VC) option of DFAGs can reduce damping. It is also shown that the modulation of active power generation of wind turbines is a powerful tool to introduce additional damping to interarea oscillations through a simple wind power system stabilizer design. The general trend for increased oscillation damping is verified in the case of a large interconnected system encompassing Southeastern Europe for a projected high level of wind penetration in Greece. For the same system, it is also shown that low-damping voltage oscillations possibly introduced by the VC mode of DFAGs can be adequately damped by properly adjusting control parameters.      

Impact of battery storage on economics of hybrid wind‐diesel power systems in commercial applications in hot regions

Integration of wind machines and battery storage with the diesel plants is pursued widely to reduce dependence on fossil fuels. The aim of this study is to assess the impact of battery storage on the economics of hybrid wind‐diesel power systems in commercial applications by analyzing wind‐speed data of Dhahran, East‐Coast, Kingdom of Saudi Arabia (K.S.A.). The annual load of a typical commercial building is 620,000 kWh. The monthly average wind speeds range from 3.3 to 5.6 m/s. The hybrid systems simulated consist of different combinations of 100‐kW commercial wind machines (CWMs) supplemented with battery storage and diesel generators. National Renewable Energy Laboratory's (NREL's) (HOMER Energy's) Hybrid Optimization Model for Electric Renewables (HOMER) software has been employed to perform the economic analysis. The simulation results indicate that for a hybrid system comprising of 100‐kW wind capacity together with 175‐kW diesel system and a battery storage of 4 h of autonomy (i.e. 4 h of average load), the wind penetration (at 37‐m hub height, with 0% annual capacity shortage) is 25%. The cost of generating energy (COE, $/kWh) from this hybrid wind–battery–diesel system has been found to be 0.139 $/kWh (assuming diesel fuel price of 0.1$/L). The investigation examines the effect of wind/battery penetration on: COE, operational hours of diesel gensets. Emphasis has also been placed on un‐met load, excess electricity, fuel savings and reduction in carbon emissions (for wind–diesel without battery storage, wind–diesel with storage, as compared to diesel‐only situation), cost of wind–battery–diesel systems, COE of different hybrid systems, etc. The study addresses benefits of incorporation of short‐term battery storage (in wind–diesel systems) in terms of fuel savings, diesel operation time, carbon emissions, and excess energy. The percentage fuel savings by using above hybrid system is 27% as compared to diesel‐only situation Copyright © 2012 John Wiley & Sons, Ltd.     

Coordinated control of TCPS and SMES for frequency regulation of interconnected restructured power systems with dynamic participation from DFIG based wind farm

Among the several wind generation technologies, variable-speed wind turbines utilizing doubly fed induction generators (DFIG) are gaining momentum in the power industry. Increased penetration of these wind turbine generators displaces conventional synchronous generators which results in erosion of system frequency. With this assertion, the paper analyzes the dynamic participation of DFIG for frequency control of an interconnected two-area power system in restructured competitive electricity market. Frequency control support function responding proportionally to frequency deviation is proposed to take out the kinetic energy of wind turbine for improving the frequency response of the system. Impacts of varying wind penetration in the system and varying active power support from DFIG on frequency control have been investigated. The presence of thyristor controlled phase shifter (TCPS) in series with the tie-line and Superconducting Magnetic Energy Storage (SMES) at the terminal of one area in conjunction with dynamic active power support from DFIG results in optimal transient performance for PoolCo transactions. Integral gains of AGC loop and parameters of TCPS and SMES 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.     

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.