Showing papers in "Iet Renewable Power Generation in 2011"

Smart energy management system for optimal microgrid economic operation

Abstract: This study presents a smart energy management system (SEMS) to optimise the operation of the microgrid. The SEMS consists of power forecasting module, energy storage system (ESS) management module and optimisation module. The characteristic of the photovoltaics (PV) output in different weather conditions has been studied and then a 1-day-ahead power forecasting module is presented. As energy storage needs to be optimised across multiple-time steps, considering the influence of energy price structures, their economics are particularly complex. Therefore the ESS module is applied to determine the optimal operation strategies. Accordingly, multiple-time set points of the storage device, and economic performance of ESS are also evaluated. Smart management of ESS, economic load dispatch and operation optimisation of distributed generation (DG) are simplified into a single-object optimisation problem in the SEMS. Finally, a matrix real-coded genetic algorithm (MRC-GA) optimisation module is described to achieve a practical method for load management, including three different operation policies and produces diagrams of the distributed generators and ESS.

Probabilistic approach for optimal allocation of wind-based distributed generation in distribution systems

Abstract: Recent development in small renewable/clean generation technologies such as wind turbines, photovoltaic, fuel cells, microturbines and so on has drawn distribution utilities' attention to possible changes in the distribution system infrastructure and policy by deploying distributed generation (DG) in distribution systems. In this study, a methodology has been proposed for optimally allocating wind-based DG units in the distribution system so as to minimise annual energy loss. The methodology is based on generating a probabilistic generation–load model that combines all possible operating conditions of the wind-based DG units and load levels with their probabilities, hence accommodating this model in a deterministic planning problem. The planning problem is formulated as mixed integer non-linear programming (MINLP), with an objective function for the system's annual energy losses minimise. The constraints include voltage limits at different buses (slack and load buses) of the system, feeder capacity, discrete size of the DG units, maximum investment on each bus, and maximum penetration limit of DG units. This proposed technique is applied to a typical rural distribution system and compared to the traditional planning technique (constant output power of DG units and constant peak load profile).

DC voltage control and power dispatch of a multi-terminal HVDC system for integrating large offshore wind farms

Abstract: Multi-terminal HVDC transmission technology using voltage source converters is proposed for integrating large offshore wind farms with transmission grids. Different DC voltage control and power dispatch strategies are proposed to demonstrate the flexibility and capability of such a transmission system in integrating large-scale variable wind generation. Various options for ensuring satisfactory ride through onshore grid faults are discussed. PSCAD/EMTDC simulations on a four-terminal high voltage direct current (HVDC) system with two offshore and two onshore converter stations during wind speed and power variations, and control mode switching are presented to show the robust performance and ability of the proposed system. Further studies during severe fault on one of the connected onshore AC networks are provided to validate the fault ride through capability of the multi-terminal HVDC system.

Energy storage system scheduling for an isolated microgrid

Abstract: A knowledge-based expert system (KBES) is proposed for the scheduling of an energy storage system (ESS) installed in a wind–diesel isolated power system. The program optimises the cost of operation by determining the diesel generation and the charging/discharging cycles of the storage system from the wind and load profiles one hour in advance. The rules created aim to minimise the use of the dump load normally associated with diesel operation. The results are compared to an offline optimisation algorithm applied to the same power system and ESS size that has a 24-h lookahead. The results obtained show that by minimising the energy wasted through the dump load with the use of the ESS and KBES controller, the required diesel generation is reduced, therefore reducing operation costs and emissions.

Frequency control in isolated island by using parallel operated battery systems applying H∞ control theory based on droop characteristics

Abstract: Stand-alone ac power supply system such as isolated islands is subject to large frequency and voltage fluctuations caused by power deviations of wind turbine generator and load demand. The autonomous decentralised frequency control system of parallel operated decentralised generators based on droop characteristic is presented in this study. The conventional droop control methods proposed in past researches show slow and oscillating dynamic responses. Moreover, the conventional droop control is affected by measurement noise when the fast controllability of the system is emphasised. This study proposes the improved droop control system for load sharing of multi-operated decentralised generators by applying H ∞ control theory, improving transient response of droop control and robustness against measurement noise and parameter variations. Simulation results validate the effectiveness of the proposed control system.

Probabilistic load flow computation of a power system containing wind farms using the method of combined cumulants and Gram-Charlier expansion

Abstract: Load flow is highly uncertain with the large-scale integration of wind power. It is unrealistic to adopt traditional deterministic load flow calculation for system planning and operation. A method is proposed in this study combining cumulants and Gram-Charlier expansion to calculate probabilistic load flow (PLF) of power system containing large-scale wind power. It has significantly reduced the computational time compared to Monte Carlo methods while maintaining a high degree of accuracy. The method was found quite suitable for the PLF calculation of power system with large-scale wind power injected.

Dynamic stability improvement of an integrated offshore wind and marine-current farm using a flywheel energy-storage system

Abstract: This study presents a control scheme using a flywheel energy-storage system (FESS) to simultaneously achieve power-fluctuation mitigation and dynamic-stability enhancement of an offshore wind farm (OWF) and marine-current farm (MCF) connected to a power grid. The performance of the studied OWF is simulated by an equivalent aggregated 80-MW doubly-fed induction generator (DFIG), while the characteristics of the studied MCF are simulated by an equivalent aggregated 40-MW squirrel-cage induction generator. A proportional–integral–derivative (PID) damping controller of the proposed FESS which is connected to the common AC bus of the OWF and MCF is designed by using modal control theory to contribute effective damping characteristics to the studied OWF and MCF under different operating conditions. A frequency-domain approach based on a linearised system model using eigenvalue analysis is performed. A time-domain scheme based on a non-linear system model subject to disturbance conditions is also carried out. It can be concluded from the simulation results that the proposed FESS combined with the designed PID damping controller can effectively stabilise the studied OWF and MCF under various disturbance conditions. The inherent power fluctuations injected to the power grid can also be effectively mitigated by the proposed control scheme.

Hybrid intelligent approach for short-term wind power forecasting in Portugal

Abstract: The increased integration of wind power into the electric grid, as nowadays occurs in Portugal, poses new challenges because of its intermittency and volatility. Hence, good forecasting tools play a key role in tackling these challenges. In this study, a hybrid intelligent approach is proposed for short-term wind power forecasting in Portugal. The proposed approach is based on the wavelet transform and a hybrid of neural networks and fuzzy logic. Results from a real-world case study are presented. A thorough comparison is carried out, taking into account the results obtained with other approaches. Conclusions are duly drawn.

Optimal operation strategy of energy storage unit in wind power integration based on stochastic programming

Abstract: In order to obtain maximum profits in trading of wind power for large-scale wind farms, energy storage unit (ESU) can be introduced to decrease the bid imbalance and to shift energy from the cheapest to the most expensive, so that the penalty can be reduced and energy can be traded with higher price for wind farm. As the forecast error of wind power output is a stochastic variable, stochastic programming is adopted to determine the optimal operation strategies of ESU. Mathematical model for seeking the maximum benefits of wind farm and ESU is also developed based on stochastic programming. Hybrid genetic algorithm and neural network methods are employed to solve the optimisation problem. Economic analysis is also included to illustrate the feasibility of the hybrid system. Results indicate that ESU can improve the profits of wind farm by decreasing the bid imbalance and shifting wind energy from low-price intervals to higher ones.

Aggregated wind power and flexible load offering strategy

Abstract: Intermittence is one the main obstacles in wind power penetration, due to imbalance charges set by market prices. To cope with this drawback, demand-side participation has been proposed as a good complement that can handle positive and negative imbalances. This study suggests an offering optimisation model for aggregated wind power and flexible loads in day-ahead electricity markets. Flexible load as a storage unit can either cover wind power imbalances or recover itself according to electricity price and load curve during various hours. The proposed offering model is compared with the disaggregated operations of wind and demand response with satisfactory results.

Study of Forecasting Renewable Energies in Smart Grids Using Linear predictive filters and Neural Networks

Abstract: Accurate forecasting of renewable energies such as wind and solar has become one of the most important issues in developing smart grids. Therefore introducing suitable means of weather forecasting with acceptable precision becomes a necessary task in today's changing power world. In this work, an intelligent way for hourly estimation of both wind speed and solar radiation in a typical smart grid has been proposed and its superior performance is compared to those of conventional methods and neural networks (NNs). The methodology is based on linear predictive coding and digital image processing principles using two dimensional (2-D) finite impulse response filters. Meteorological data have been collected during the period 1 January 2009 to 31 December 2009 from Casella automatic weather station (AWS) at Plymouth, UK. Numerical results indicate that a considerable improvement in forecasting process is achieved with 2-D predictive filtering compared to the conventional approaches.

Torque ripple analysis and reduction for wind energy conversion systems using uncontrolled rectifier and boost converter

Abstract: A conventional topology for a small-scale wind energy conversion system consists of a permanent magnet synchronous generator, a diode bridge rectifier, a boost converter and a grid-side inverter. Since generator phase currents contain low-order harmonics and cannot be controlled independently using a diode bridge rectifier, electromagnetic torque ripple is relatively large and may have a detrimental effect on the life of the turbine through fatigue induced by shaft torque ripple. This study investigates methods to reduce this electromagnetic torque ripple, from both the viewpoints of the circuit topology and the control strategy. The effect of the DC-side capacitor on torque ripple is investigated and different control strategies and their effect on torque ripple are compared and analysed. This shows that the torque ripple can be reduced by removing the DC-side capacitor and can be further reduced by controlling DC-side current to a constant value. These methods have been investigated theoretically and the validity of the results confirmed by both simulation and experiment.

Control strategies for a wave energy converter connected to a hydraulic power take-off

Abstract: Among the various types of wave energy converters currently being developed, heaving point absorbers are one of the simplest and most promising concepts. A typical efficient energy conversion system for point absorbers is based on hydraulic power take-off (PTO) systems, consisting in a double-acting cylinder, a hydraulic motor and two or more accumulators. This paper presents a simple model of a heaving oscillating buoy extracting power by means of a hydraulic system. The hydrodynamic behaviour of the absorber is modelled through application of the linear water wave theory. Apart from the basic elements listed above, the model of the hydraulic system includes leakages and pressure losses and takes into account the compressibility of the fluid. Also, possible control accumulators are considered in order to improve the performance of the hydraulic system by means of properly controlled valves. Different control variables are analysed depending on the wave inputs considered in order to improve the power extraction of the converter. The results prove that it is possible to achieve a great enhancement of the power extraction with the implementation of these control strategies and that a possible combination of some of them might be beneficial for improved efficiency of the components.

Electronic load controller with power quality improvement of isolated induction generator for small hydro power generation

Abstract: This study deals with an implementation of an instantaneous reactive power theory-based electronic load controller (ELC) for regulating the voltage and frequency of an isolated induction generator system (IG) that can supply electricity in remote areas. This ELC provides the fundamental reactive power and compensates harmonics of load currents. The proposed ELC is a combination of voltage source converter (VSC) with a dc link capacitor, a chopper and an auxiliary load at its dc bus. It controls active and reactive powers of the small hydro plant thus regulating the voltage and frequency of IG system. This ELC also balances the currents of the IG system under unbalanced load currents and eliminates the harmonics of the load currents thereby acting both as a load balancer and a harmonic eliminator. The zigzag/star transformer optimises the dc bus voltage of VSC and acts as a neutral current compensator. The proposed IG along with its ELC is implemented on a 3.7 kW IG system.

Sandia frequency shift parameter selection for multi-inverter systems to eliminate non-detection zone

Abstract: Among frequency drift islanding detection methods, Sandia frequency shift (SFS) is considered as one of the most effective methods in detecting islanding conditions for grid connected photovoltaic (PV) inverters. The performance of the SFS method during an islanding condition and its non-detection zone (NDZ) depends to a great extent on its parameters. Furthermore, the capability of the SFS method to detect an islanding condition deteriorates with multiple PV inverters. A mathematical formula is derived to aid protection engineers in determining the optimal setting of the SFS islanding detection parameters with multiple inverter-based distributed generation (DG), such as PV systems, to eliminate the NDZ. The derived formula is applied to multiple DG systems equipped with the over frequency/under frequency protection, active frequency drift and SFS islanding detection methods and is verified through NDZ analysis and simulation results on PSCAD/EMTDC. The derived formula provides an effective guideline for designing frequency drift methods in multi-inverter-based DG systems.

Comparison of direct maximum power point tracking algorithms using EN 50530 dynamic test procedure

Abstract: Three direct maximum power point tracking (MPPT) algorithms with non-adaptive voltage step are evaluated in the light of their performance for photovoltaic systems under dynamic conditions for a resistive load. A microcontroller-based buck–boost DC–DC converter platform is used to implement and compare the algorithms. Dynamic test procedures from a new standard for inverter efficiency determination EN 50530 were implemented to evaluate dynamic MPPT algorithms’ efficiency and their dependence.Experimental results show that MPPT algorithms’ regulating frequency and regulating voltage step play a crucial role in the dynamic performance of direct algorithms. A range of regulating frequencies and voltage steps were examined. If proper parameter values are chosen, all algorithms perform well and close to each other. Detailed evaluation was performed with determining partial MPPT efficiency under different irradiance slopes. Results show that at least 10 Hz should be used to satisfy the 99% tracking efficiency over all slopes in the range from 0.5 to 100 W/m2/s as specified by EN 50530 standard. Further increase of regulating frequency would result in higher slope efficiency only at dynamic conditions.

Multiple model multiple-input multiple-output predictive control for variable speed variable pitch wind energy conversion systems

Abstract: A multivariable control strategy based on model predictive control techniques for the control of variable-speed variable pitch wind energy conversion systems (WECSs) in the above-rated wind speed zone is proposed. Pitch angle and generator torque are controlled simultaneously to provide optimal regulation of the generated power and the generator speed while minimising torsional torque fluctuations in the drive train and pitch actuator activity. This has the effect of improving the power quality of the electrical power generated by the WECS and increasing the life time of the mechanical parts of the system. Furthermore, safe and acceptable operation of the system is guaranteed by incorporating most of the constraints on the physical variables of the WECS in the controller design. In order to cope with the non-linearity in the WECS and the continuous variation in the operating point, a multiple model predictive controller is suggested to provide near optimal performance within the whole operating region.

One-power-point operation for variable speed wind/tidal stream turbines with synchronous generators

Abstract: This study presents a new operating scheme for variable speed wind/tidal stream turbines employing synchronous generators. Current maximum power tracking schemes for variable speed wind energy conversion systems rely on periodic comparison of the output power for guiding the direction of maximum power tracking or an online algorithm which continuously provides a generator torque/speed reference corresponding to the maximum power status at different wind speed conditions. The proposed scheme utilises only one-power-point on the maximum power curve. Once the information for the maximum power status of a local wind speed is known, then the wind energy conversion system implements maximum power tracking and constant power control for different wind speed conditions using simple control. The operation strategy is applicable to tidal stream turbines. Simulation results demonstrate the concept.

Comparative study on impacts of wind profiles on thermal units scheduling costs

Abstract: Although wind power is a sustainable, environmentally friendly and a viable option for renewable energy in many places, the effects of its intermittent nature on power systems need to be carefully examined. This study investigates the effects of different wind profiles on the scheduling costs of thermal generation units. Two profiles are considered: synoptic-dominated and diurnal-dominated variations of aggregated wind power. To simulate the wind profile impacts, a linear mixed integer unit commitment problem is formulated in a general algebraic modelling system (GAMS) environment. The uncertainty associated with wind power is represented using a chance constrained formulation. The simulation results illustrate the significant impacts of different wind profiles on fuel saving benefits, startup costs and wind power curtailments. In addition, the results demonstrate the importance of the wide geographical dispersion of wind power production facilities to minimise the impacts of network constraints on the value of the harvested wind energy and the amount of curtailed energy.

Integral variable structure direct torque control of doubly fed induction generator

Abstract: This study proposes a novel integral variable structure direct torque control (IVS-DTC) scheme for a doubly fed induction generator (DFIG). The proposed scheme directly controls the torque and reactive power of the DFIG with rotor winding voltage, and hence no extra current control loops are required. Uncertainties in the parameters are included in the design procedure, which guarantees the robustness of the system. Compared to conventional direct torque control (DTC) scheme, the constant switching frequency in the proposed scheme does not introduce low-frequency sub-harmonics. Both computer simulation and hardware implementation results show that proposed scheme has satisfactory parametric robustness and generated power quality.

Fault ride-through capability enhancement of doubly-fed induction wind generators

Abstract: Recent fault ride-through (FRT) requirements have proven problematic for variable-speed wind generation systems. A particular problem regarding to doubly-fed induction generators (DFIGs) is that standard proportional integral (PI) current controllers, designed with very limited control bandwidth, cannot eliminate rotor current oscillations that occur during a grid fault. As a consequence, the current in the rotor-side converter can exceed the safety limits of semiconductor switches, which potentially leads to converter failure. This study introduces a hybrid current controller to enhance the FRT capability of DFIGs through keeping the rotor current below the safety limits. The proposed current controller includes two switching strategies: the standard PI current controller for normal operating conditions and a vector-based hysteresis current controller (with very fast transient response) for overcurrent protection during grid faults. Simulation studies are carried out to demonstrate the effectiveness of the proposed hybrid current controller under various symmetrical and asymmetrical voltage sag conditions.

Improved control strategy for stand-alone distributed generation system under unbalanced and non-linear loads

Abstract: This study presents an improved control strategy for a distributed generation system under stand-alone mode. Since there is no grid supply in a stand-alone system, the output voltage has to be controlled in the constant amplitude and frequency by the load-side inverter under the unbalanced and non-linear loads. The mathematic model of the load-side inverter is described. The dual-loop control method based on the stationary αβ reference coordinate is used for the load-side inverter so that the coordinate transformation is not needed. The outer-loop voltage controller consists of a proportional controller and a resonant regulator tuned at low-order harmonic frequency to compensate for the low-order harmonic components of the output voltage. A predictive current control scheme based on space vector modulation is adopted in the inner-loop current controller to improve the transient performance under the sudden load disturbances. Finally, experiment studies are carried out on a 5 kW stand-alone generation system. The experiment results show the excellent output voltage performance under both the steady-state and dynamic process with the load-adaptive ability.

Power conversion and control for a linear direct drive permanent magnet generator for wave energy

Abstract: This study describes the power conversion and control solution used in the electrical power take-off of a 35 kW test rig developed to investigate a linear, direct drive, air-cored, tubular, permanent magnet generator for an offshore wave energy device. The solution proposed is to collect the power extracted directly from individual coils of the generator, which have different induced voltages and cannot easily be connected into a small number of phases. Local energy storage is integrated into the system to smooth the electrical output power and reduce the rating of the downstream inverter for grid interfacing. The solution is demonstrated by analysis, backed up by simulation and test results. This shows the potential and limitations of the proposed conversion technology solution.

Dynamic model and control of a wind-turbine generator with energy storage

Abstract: This study presents a wind conversion system with integrated energy storage and dispatchable output power. The energy storage acts as an auxiliary source to mitigate the wind power fluctuations and to follow the load demand changes. A control strategy is developed which manages the flow of power among the wind-turbine generator, the energy storage and the grid. A dynamic model for the overall wind conversion system is developed. A controller is designed systematically for the wind conversion system based on the developed dynamic model. Performance of the system in various modes of operation is evaluated based on digital time-domain simulations in the power simulink (PSIM) software environment.

Operation analysis of a wave energy converter under different load conditions

Abstract: This study analyses the electrical behaviour of a direct-driven linear generator under different load conditions. The studied generator is used in a wave energy converter (WEC) that converts the energy in ocean waves into electric energy. To enable a grid connection of a WEC, the voltage must be converted, and thereby, the generator will be subjected to a non-linear damping. Depending on how the conversion system is designed, the damping will be different. In the case studied, the voltage is first rectified, and on the dc-side of the rectifier the voltage is kept constant by controlling the power through a converter. In order to study the electrical behaviour of the generator in this operation mode, a simulation model was made in MATLAB Simulink. The model of the generator was verified with experimental data from an offshore operating WEC. The result of the study shows that the model of the generator agrees with the real generator and can be used for analysing the electrical behaviour of the WEC. Moreover, the results show that the operation with a non-linear load will be different compared to a linear load case.

Time-frequency transform-based islanding detection in distributed generation

Abstract: This study presents a new technique for islanding detection in distributed generation (DG) using time-frequency transform such as S -transform. S -transform is an invertible time-frequency spectral localisation technique that provides the time-frequency contours of the voltage and current signals retrieved at the target DG location. The energy index (ratio of the spectral energy content of the voltage to current signals) is computed to track the islanding situation from non-islanding conditions such as sudden load change, tripping of other DG, etc. Further, to aid the islanding detection scheme, a cumulative sum detector is also computed based on the spectral energy content of the negative sequence components of the current and voltage signals. The results, based on extensive study, indicate that the proposed technique can reliably detect islanding in DGs connected to power distribution network.

Multi-objective daily operation management of distribution network considering fuel cell power plants

Abstract: Fuel cells are environmentally clean, have low emission of oxides of nitrogen and sulfur and, at the same time, they can operate with a very low level of noise. In addition, they can provide energy in a controlled way with higher efficiency compared to conventional power plants. This study presents an efficient multi-objective new fuzzy self adaptive particle swarm optimisation evolutionary algorithm to solve the multi-objective optimal operation management considering fuel cell power plants in the distribution network. The objective functions of the problem are to decrease the total electrical energy losses, the total electrical energy cost, the total pollutant emission and deviation of bus voltages. The proposed algorithm is tested on a real distribution test feeder and the results demonstrate the capabilities of the proposed approach to generate true and well-distributed Pareto-optimal non-dominated solutions.

Vector approach for self-excitation and control of induction machine in stand-alone wind power generation

Abstract: This study presents a detailed investigation on self-excitation of a squirrel-cage induction generator (SCIG) used in a wind energy conversion system. Air-gap flux of the SCIG is gradually built up through controlled current injection from a voltage source converter (VSC), connected directly across its stator terminals. Dc voltage of the VSC is ramped from a small initial value, which is the rectified output of the small terminal voltage developed because of remanent magnetism. Increase in air-gap flux increases generator terminal voltage and output power which further increases the dc bus voltage. The field-oriented control method is appropriately applied both for control of voltage build-up as well as dynamic transients. The critical factors deciding this collaborative excitation are analysed and sufficient conditions are derived analytically. System modelling and analytical results are validated through numerical simulation and verified on a 2.2 kW laboratory prototype.

Wind power optimal capacity allocation to remote areas taking into account transmission connection requirements

Abstract: Wind power is site dependent and is by nature partially dispatchable. Furthermore, good wind sites are far from grid. Owing to these problems, and along with the existing limitations in the transmission networks, a comprehensive analysis over an extended time is needed to properly explore all potential wind sites for wind capacity allocation. This problem is computationally expensive and decomposition methods are required to break down this problem. Here Benders decomposition approach is used, which is a popular technique for solving large-scale problems, to decompose the original problem into a master and a subproblem. The master problem is a linear problem, which allocates wind capacity to each site and determines the transmission line capacity for connection to the grid. The subproblem is a mixed-integer problem and performs a year-long unit commitment accompanied with DC optimal load flow. The subproblem uses the solution of the master problem to form the appropriate cut, representing operation cost, for the next iteration of the master problem. This procedure is iterated until the optimal solution is found. The IEEE 24-bus test system is used to demonstrate the proposed method. Simulation results show that the maximum profit is gained when there is a trade-off between transmission cost and wind curtailment. It is shown that by using a proper wind capacity allocation, wind penetration into the system will be maximised.

Lightning protection of wind turbines-a comparison of measured data with required protection levels

Abstract: Latest wind turbine lightning protection systems have been refined to the point where lightning damage is now relatively rare. This is increasingly important as wind turbines move offshore where access for maintenance is more difficult than for most land-based wind farms. Manufacturers have been trying to make sure that the lightning protection systems they install comply with the highest protection levels stipulated in the relevant IEC standards. In this study, data from the Nysted Offshore Wind farm in Denmark and that from a large number of other wind turbines worldwide is reviewed to show the range of lightning currents that have been measured on wind turbines currently in operation. These current values are compared with the required protection levels within the standards.