Output Power Smoothing Research Articles

The Design and Analysis of Large Solar PV Farm Configurations With DC-Connected Battery Systems

Typically, solar inverters curtail or “clip” the available power from the photovoltaic (PV) system when it exceeds the maximum ac capacity. This article discusses a battery system connected to the dc link of an inverter to recuperate this PV energy. Contrary to conventional approaches, which employ two dc–dc converters, one each for the battery and solar PV system, the proposed configuration utilizes a single dc–dc converter capable of simultaneously operating as a charge controller and a maximum power point tracking (MPPT) device. In addition to improving the overall system capacity factor, increasing the conversion efficiencies, and ensuring MPPT stability, the proposed configuration offers a simple solution for adding energy storage to existing PV installations. With this configuration, the excess power that will otherwise be curtailed due to inverter rating limitations is stored in the battery and supplied to the grid during periods of reduced irradiance. Moreover, a systematic methodology for sizing a dc-bus connected battery to minimize total PV energy curtailed was developed using an annual PV generation profile at the Louisville Gas and Electric and Kentucky Utilities E. W. Brown solar facility at Kentucky. The detailed behavior of the proposed system and its power electronics controls and operations were validated with case studies developed in PSCAD/EMTDC software for variable power generation and PV output power smoothing.

An Economic Model Predictive Control Approach for Wind Power Smoothing and Tower Load Mitigation

Abstract Wind power intermittency represents one of the major challenges facing the future growth of grid-connected wind energy projects. The integration of wind turbines and energy storage systems (ESS) provides a viable approach to mitigate the unfavorable impact on grid stability and power quality. In this study, an economic model predictive control (MPC) framework is presented for an integrated wind turbine and flywheel energy storage system (FESS). The control objective is to smooth wind power output and mitigate tower fatigue load. The optimal control problem within the model predictive control framework has been formulated as a convex optimal control problem with linear dynamics and convex constraints that can be solved globally. The performance of the proposed control algorithm is compared to that of a baseline wind turbine controller. The effect of the proposed control actions on the fatigue loads acting on the tower and blades is investigated. The simulation results, with various wind scenarios, showed the ability of the proposed control algorithm to achieve the aforementioned objectives in terms of smoothing output power and mitigating tower fatigue load with negligible effect on the wind energy harvested.

Power Output Smoothing for Renewable Energy System: Planning, Algorithms, and Analysis

The growing penetration of renewable energy sources in electricity generation will bring challenges to the power grid operations due to the intermittency and fluctuation of renewables. In this article, we employ an energy storage system (ESS) in a grid-connected renewable energy system (RES) to serve the electrical load from the power grid. We study the control algorithms of the ESS to smooth the renewable generation and analyze the tradeoff between the ESS size and the system performance, i.e., renewable utilization and operation cost. We provide performance guarantees of the proposed algorithms and propose an optimization framework to configure the ESS. To analyze the reliability of the RES, an analytical framework using the Markov modulated fluid queue is devised. It is shown in our simulation studies that the devised power output smoothing algorithm outperforms other existing algorithms in terms of operation cost under different sizes of ESSs. We also find that, to satisfy the renewable utilization requirement, the required ESS size blows up with both the amplitude of fluctuation and the intermittency level of the profile gap between the renewable generation and the load.

Collaborative Control and Allocation Method of RSC and GSC for DFIG System to Suppress High-Frequency Resonance and Harmonics

When doubly fed induction generator (DFIG) system connects to the parallel compensation grid with harmonics, high-frequency resonances (HFR) occur due to the insufficient phase margin, meanwhile the output power quality of the DFIG system is degraded due to the grid harmonics. For solving the two problems simultaneously, the collaborative control strategy which allots different targets to rotor side converter (RSC) and grid side converter (GSC) is proposed in this article, in which the HFR damping controller is planted at RSC or GSC, while the other converter will be responsible for improving the power quality of the DFIG system. The core of the collaborative control strategy is how to select RSC or GSC to achieve HFR damping, which is investigated by analyzing the impedance characteristic of the DFIG system after planting the HFR damping controller. With the collaborative control strategy, the excellent performance of HFR damping and harmonics suppression can be guaranteed for the DFIG system under grid harmonics, meanwhile the DFIG system can operate flexibly with different targets, i.e., sinusoidal output current, smooth output power. Finally, theoretical analysis and experimental results for different DFIG systems verify the correctness and availability of the proposed collaborative control strategy.

A Differential Evolution-Based Optimized Fuzzy Logic MPPT Method for Enhancing the Maximum Power Extraction of Proton Exchange Membrane Fuel Cells

Recently, fuel cells (FCs) have found vast employment in several applications. However, unique maximum power point tracking (MPPT) exists for each set of operating condition for the efficient operation of FCs. Therefore, this paper presents a differential evolution optimization algorithm (DEOA)-based optimized fuzzy-logic (OFLC) MPPT method for enhancing the maximum power extraction of FCs. The various settings for the membership functions (MFs) of the input and output variables are optimized in the proposed method. Thence, more degree-of-freedom can be employed for accurate and fast tracking of the optimal power point of the proton exchange membrane FCs (PEMFCs). Whereas, existing MPPT methods in the literature for FC applications suffer from decreased degree-of-freedom for optimizing their performance, and lack of adaptivity, which obstructs their suitability for the wide operating range of FCs. The superiority and performance effectiveness of the proposed OFLC MPPT method have been validated and compared with the most prevalent techniques in the literature. Moreover, the robustness and sensitivity of the proposed OFLC MPPT method have been tested at various step changes in the water content of membrane and various temperature changes. Moreover, the proposed design of the suggested OFLC MPPT is general and it can be implemented on low-cost microcontrollers. The results verify the superior performance of the proposed OFLC MPPT method from the accurate and fast MPPT extraction, smooth output power with low ripple, and simplicity of the design point of views.

Improving Power Output of Battery and Mode Switching Frequency Based on Real-Time Average Power Method for Multi-Mode Hybrid Energy Storage System in Electric Vehicles

In this paper, a power-split strategy based on a real-time average power method is developed for improving power output of battery and mode switching frequency of a multi-mode hybrid energy storage system (HESS) in electric vehicles. To achieve mode switching and power distribution for the multi-mode HESS, a rule-based strategy is designed based on the high-frequency power demand. Furthermore, a simple real-time average power method is adopted to process the high-frequency power demand. Then, the real-time average power is used as a variable logic threshold value for the power-split strategy. Since the power-split controller responds to the smooth average power rather than the high-frequency power demand, the high-frequency mode switching of the multi-mode HESS can be avoided. The ultra-capacitor works as an enhanced low-pass power filter and the battery can supply smooth and steady output power to the motor inverter. Comparative simulations between the developed power-split strategy and the rule-based strategy are performed. The advantages of the developed power-split strategy for improving the power output of the battery and the mode switching frequency of the multi-mode HESS in electric vehicles are indicated under three typical driving cycles. Moreover, the longer time duration of the real-time average power is designed, the smoother power output of the battery and the lower mode switching frequency of the multi-mode HESS can be achieved.

Coordinated control strategy of DC microgrid with hybrid energy storage system to smooth power output fluctuation

Abstract As the fossil energy crisis and environmental pollution become more and more serious, clean renewable energy becomes the inevitable choice of energy structure adjustment. The power system planning and operation has been greatly influenced by the instability of the power output of distributed renewable energy systems such as solar energy and wind energy. The hybrid energy storage system composed of accumulator and supercapacitor can solve the above problems. Based on the analysis of the energy storage requirements for the stable operation of the DC microgrid, battery–supercapacitor cascade approach is adopted to form hybrid energy storage system, in a single hybrid energy storage subsystem for battery and supercapacitor and in the microgrid system of different hybrid energy storage subsystem, respectively, and puts forward the corresponding power allocation method to realize the smooth control of the battery current, to reduce the battery charge and discharge times, to prolong the service life of battery and to improve the running stability of the microgrid.

Output power smoothing of wind power plants using self-tuned controlled SMES units

This paper exhibits a new application of the continuous mixed p-norm (CMPN) algorithm to self-tune all controlled superconducting magnetic energy storage (SMES) units to smooth wind power plants output power. The proposed algorithm is applied to self-tune all PI controllers of these storage units. In the present article, two wind farms are connected with the power grid. Each wind farm is equipped with a self-tuned (ST) controlled SMES unit. The control strategy of this unit is based on a voltage source converter (VSC) and a DC chopper. The VSC is implemented to control the reactive power and the DC chopper is utilized to control the real power exchange with the power grid. These aforementioned power electronic circuits are fully controlled by the proposed CMPN-ST-PI controllers. To achieve more realistic studies, measured values of wind speed that captured from Hokkaido wind power plants are implemented in system investigation. In addition, the turbine model depends on a two-mass structure, which highly affects system dynamics. Moreover, a practical SMES device of a rated capacity of 10 MVA, which is established in Kameyama, is joined the PCC of wind power plants. The proposed self-tuned controller is validated by using the simulation results, which are extensively performed on PSCAD program. The effectiveness of proposed adaptive controlled SMES devices is compared with that obtained by using optimal genetic algorithm-based PI controlled SMES devices under wind speed uncertainty conditions. With these CMPN-ST-PI controllers, the output power of wind power plants can be further smoothed and its fluctuations can be depressed.

Self-powered Flexible PDMS Channel Assisted Discrete Liquid Column Motion Based Triboelectric Nanogenerator (DLC-TENG) as Mechanical Transducer

Fluid flow in small scale channel as droplet, continuous or discrete volume of liquid has been used in the multidisciplinary research field to be utilized in wide variety of applications in chemistry, biology, medical technologies. Recently, energy scavenging technique that merges the microfluidics with liquid–solid contact electrification charging from the small flows has become a new area of investigation to be applied as sensory module. In this article, a flexible fluid based mechanical sensing element using the triboelectrification of the oscillatory discrete volume of liquid flowing inside the hydrophobic mini channel over PTFE polymeric film is described. To achieve the high output efficiency and to reduce the wettability of the channel, micropatterned PDMS surface was implied for smooth operation and constant power output of the device. Pressure gradient was applied for the forward–backward motion of the liquid column over the free-standing mode porous PTFE thin film-based electrode and a maximum of 10 nA of short circuit current and 20 mV of open circuit voltage was achieved at 40 mbar pressure and frequency of 1 Hz. Finally, the flexible DLC-TENG shows better performance as a transducer to be effectively used in measuring the frequency response to the angular displacement of the rotary actuator consequently pressure fluctuation sensing. The obtained results are promising whereas counting nominal deflection of pressure such as in the robotic manipulator or actuator is regarded as an essential factor.

Mileage-Responsive Wind Power Smoothing

This paper proposes a novel wind power smoothing control paradigm in context of performance-based regulation service. Conventional methods aim at adjusting wind power output using hard-coded filtering algorithms that can result in visually smoothed power output with unmeasurable impacts on system generation-demand balance. Distinguished from conventional methods, the newly proposed control method smooths wind power output from a power system perspective by using the regulation mileage as a key performance indicator. To simultaneously address the system needs and maximize wind energy harvesting, a mileage-responsive framework is developed to enable wind farms to optimally generate smoothing power. The effectiveness of the proposed method is well demonstrated through case studies, of which the simulation results shows a great potential for practical applications.

Frequency and voltage control of microgrid with high WECS penetration during wind gusts using superconducting magnetic energy storage

The intermittent behavior of wind power generation results in a fast variation of both frequency and voltage magnitudes of isolated microgrids with high wind power penetration. In this paper, a hybrid energy microgrid with wind energy conversion system (WECS) and diesel synchronous generators are analyzed during wind gust conditions. Superconducting magnetic energy storage (SMES) is connected at the same node of the WECS to mitigate the intermittent wind power generation. A developed fuzzy logic controller has been applied to achieve active power sharing. The damping of voltage fluctuation depends on SMES reactive power, which is controlled by a voltage source converter. The studied microgrid comprises 33 nodes, two diesel SGs, and two WECSs of squirrel cage induction generator type at 30% penetration level. The results demonstrate the effectiveness of the proposed control strategy to mitigate the frequency fluctuations during the wind speed gusts by smoothing power output from WECSs/SMES system. Moreover, voltage control is achieved by injecting reactive power from the SMES system. All simulations are performed by MATLAB/Simulink package.

Multi-objective electro-thermal coupling scheduling model for a hybrid energy system comprising wind power plant, conventional gas turbine, and regenerative electric boiler, considering uncertainty and demand response

Abstract Regenerative electric boiler (REB) can convert electricity into heating, improve the grid-connected space of wind power plant (WPP), and is an effective way of solving the severe problem of abandoned wind power in China. To achieve a synergistically coordinated optimization between the supply side and the user side, incentive-based demand response (IBDR) is implemented for decentralized thermal load while price-based demand response (PBDR) is implemented for the electrical load and centralized thermal load. The study then integrates WPP, REB, conventional gas turbine (CGT) and IBDR into a hybrid energy system (HES). Next, the uncertainty scenarios of wind speed are simulated using the Latin hypercube sampling method, and Kantorovich distance based on the values predicted using auto-regressive and moving average mode. Then a multi-objective optimization model for HES operation is proposed based on the objectives of maximizing economic revenue and minimizing the level of risk. Finally, an industrial park in northern China is selected for sample analysis. The results show: (1) if the power output of WPP is converted into heating, the economic benefits increase significantly, with certain risks attached. The REB can store energy during off-peak periods and dispense energy during peak periods. The smooth net power output curve is preferable for decreased operational risk. (2) PBDR can smooth the demand curves of electrical and thermal load. The peak-to-valley ratios of the electrical load and thermal load decrease by 1.207 and 1.500. (3) IBDR can provide more reserve service for a HES while simultaneously satisfying different load demands and yielding optimal operation results with the implementation of PBDR. The peak-to-valley ratios of the net power curves are 1.242 (IBDR alone) and 1.214 (IBDR and PBDR). Overall, the proposed optimization model could give full play to the coupling effect of centralized thermal load and decentralized thermal load, which would provide feasible tools for decision-makers to create an optimal operation plan.

Wave Power Output Smoothing through the Use of a High-Speed Kinetic Buffer

In this paper, a new control strategy for power output smoothing in a hybrid wave energy installation coupled to a flywheel energy storage system (FESS) is proposed. The control scheme is composed by three stages: a wave generator clustering process at the farm connection point; a power filtering process; and the control of the flywheel energy storage in order to improve the power output of the hybrid wave farm. The proposed control is validated at the existing Lysekil Wave Energy Site located in Sweden, by using real generator measurements. Results show that the application of the flywheel energy storage system reduces the maximum peak power output from the wave energy installation by 85% and the peak/average power ratio by 76%. It is shown that the proposed system can reduce grid losses by 51%, consequently improving the energy efficiency of the power network. The application of the proposed control strategy allows the hybrid wave power plant to follow a power reference signal that is imposed by the grid operator. In addition, the study demonstrates that the application of the proposed control allows the hybrid wave power plant to follow a power reference signal that is imposed by the grid operator. In addition, the study demonstrates that the application of the proposed control enables a wave farm with flywheel energy storage to be a controllable, flexible resource in order to fulfill future grid code requirements for marine energy installations.

Improved Operation Strategy with Alternative Control Targets for Voltage Source Converter under Harmonically Distorted Grid Considering Inter-Harmonics

This paper proposed an improved control method for grid-connected voltage source converter (VSC), when the grid voltage consisted of the integer harmonics and inter-harmonics. Control object of the proposed control can be alternated to achieve the sinusoidal current or smooth output power, which enhances the operation adaption of VSC under the harmonically distorted grid. On the basis of a PI regulator in the fundamental current control loop, the novel control strategy was proposed with a supplementary controller which consisted of a prepositive high-pass filter and a modified proportional-derivative controller. In the proposed control, the inter-harmonics could be suppressed without detecting frequency, while the traditional resonator was effective in the premise of knowing the harmonics frequency. Also, the influence of control gain on the steady performance and the stability of VSC was analyzed, and the influences on the fundamental control caused by the proposed controller were also analyzed to verify the practicability of the proposed control strategy. Finally, the effectiveness of the proposed strategy was verified by the experiments.

Master-slave wave farm systems based on energy filter with smoothed power output

Wave energy is an important renewable energy source. Previous studies of wave energy conversion (WEC) have focused on the maximum power take-off (PTO) techniques of a single machine. However, there is a lack of research on the energy and power quality of wave farm systems. Owing to the pulsating nature of ocean waves and popular PTO devices, the generated electrical power suffers from severe fluctuations. Existing solutions require extra energy storage and overrated power converters for wave power integration. In this study, we developed a master-slave wave farm system with rotor inertia energy storage; this system delivers self-smoothed power output to the grid and reduces the number of converters. Two control methods based on the moving average filter (MAF) and energy filter (EF) are proposed to smooth the output power of wave farms. RTDS simulations show that the proposed systems and control methods facilitate simple and smooth grid integration of wave energy.

Smoothing control strategy of wind and photovoltaic output power fluctuation by considering the state of health of battery energy storage system

The output power of wind and photovoltaic (PV) systems contain fluctuations due to the intermittent nature of solar irradiance and wind speed. It is required to smooth the fluctuated power before supplying it to the grid. Here, a fuzzy-based discrete Kalman filter approach is proposed for smoothing output power fluctuations of the wind and PV generation systems using a battery energy storage system. The proposed approach incorporates the state of health of the battery as a feedback to not only obtain smooth output power but also improve the battery health by adaptively regulating the battery power. The effectiveness of the proposed approach is verified by evaluating the power fluctuation rates and performance indices.

Enhancing FRT performance and smoothing output power of DFIG wind farm equipped by SFCL and SMES in a fuzzy framework

Wind farms equipped by Doubly-Fed Induction Generators (DFIG) should have two capabilities so that system operators can efficiently utilizes them. These capabilities include Fault-Ride-Through (FRT) and smoothing the output active power fluctuations, especially when these generators provide significant electrical power. The Superconducting Fault Current Limiter (SFCL) and also the Superconducting Magnetic Energy Storage (SMES) are supplementary devices which are used to enhance these capabilities and reduce electrical power swings. In the present work, improvement of FRT capability and smoothing the output power of wind farms are formulated as a multi-objective problem in a fuzzy framework. The variables of optimization are the Proportional-Integral (PI) gains of DFIG and SMES controllers and also SMES and SFCL parameters. Minimization of the initial energy stored in the SMES unit, the energy losses of SFCL, deviations of the DC-link voltage of DFIG, deviations of output active power of DFIG, deviations of output voltage of DFIG, and DFIG speed deviations are six objective functions of the problem. These objective functions are scaled by a fuzzy operator, then the scaled objective functions are aggregated by the “max-geometric mean” operator to obtain the multi-objective function. For optimizing this multi-objective function, the Hybrid Big Bang Big Crunch (HBB-BC) as a meta-heuristic optimization algorithm is used. The proposed algorithm is implemented on a case study to numerically evaluate its efficiency. Simulation results show that the proposed algorithm is more effective to enhance FRT capability and smooth output power of wind farm in comparison with the other algorithms.

An Improved Repetitive Control of DFIG-DC System for Torque Ripple Suppression

In order to obtain smooth torque and output power of the doubly fed induction generator (DFIG) dc system in which an uncontrollable rectifier is used to connect the stator winding and dc grid, an improved repetitive control (RC) regulator is proposed for the rotor-side converter to suppress the torque ripple in this paper. The torque control of the DFIG is divided into the average torque control and the torque ripple control. For the average torque control, a novel stator frequency estimation method is proposed to acquire the frequency and angle of the stator flux linkage. Then, the stator flux linkage orientation is applied for the decoupling control of the stator frequency and average torque. For the torque ripple control, an improved RC regulator is designed and directly employed for suppressing the torque ripple completely, which can avoid the sequence decomposition and rotor harmonic current calculation. The detailed design of the improved RC regulator is presented considering the control bandwidth, the fractional delay, and suppressing of the infinite dc gain. Finally, the proposed control method is validated by the experiment results.

Solar–hydro hybrid power station as a way to smooth power output and increase water retention

There is environmental, societal and also economic pressure to increase the share of renewable energy sources in covering energy demand. However, the stochastic and non-dispatchable nature of the two main such sources (photovoltaics and wind generation) complicates their integration into the power system. The operation mode of the run-of-river power plant with pondage that is considered here has potential to smooth electricity generation from photovoltaics, whilst also maintaining the hydropower capacity factor and increasing water retention – an important aspect when a decision-maker has to make a trade-off between power generation and, for example, irrigation. This paper presents the theoretical background to our research, introduces a discrete mathematical simulation and optimization model and provides a detailed analysis of the obtained results and the operation of the entire system. We also highlight the impact on the capacity factor of hydropower as well as addressing the potential need to install more than one water turbine to ensure greater flexibility of hydropower and efficient utilization of water resources. The optimization results of the analyzed case study show that for an observed average flow rate of 1.3 m3/s and annual irradiation of 1050 kWh/m2 a 176-kW water turbine and pondage capacity of 1.36 MWh is sufficient to smooth the power generation of a 687-kW PV installation while also satisfying constraints imposed on reliability and performance.

Energy storage system expansion planning in power systems: a review

In recent two decades, the power systems have confronted with considerable changes such as the power system restructuring, growth of distributed energy sources and renewable energy sources (RESs), and emergence of smart grid concept. One of the common challenges caused by these changes is flexibility necessity of energy resources. One of the best solutions to mitigate this challenge is energy storage systems (ESSs) utilisation. The main question is how to determine size, site, and type of ESSs to maximise their benefits. This study reviews the answers to this question according to the research studies. This study first classifies the studies related to ESS expansion planning into two main categories from the viewpoint of the power system operators and the investors. Next, the first main category is divided into three subcategories: ESS expansion planning in microgrids, distribution networks, and transmission networks. The second main category is classified into two subcategories: ESS expansion planning aim to smooth RESs output power and to maximise profit. In each subcategory the modelling approaches, solving methods, and the results are evaluated. Finally, based on the existing challenges, the future research directions of ESS expansion planning are outlined.