Showing papers on "Power optimizer published in 2006"

Control of permanent-magnet generators applied to variable-speed wind-energy systems connected to the grid

Abstract: Wind energy is a prominent area of application of variable-speed generators operating on the constant grid frequency. This paper describes the operation and control of one of these variable-speed wind generators: the direct driven permanent magnet synchronous generator (PMSG). This generator is connected to the power network by means of a fully controlled frequency converter, which consists of a pulsewidth-modulation (PWM) rectifier, an intermediate dc circuit, and a PWM inverter. The generator is controlled to obtain maximum power from the incident wind with maximum efficiency under different load conditions. Vector control of the grid-side inverter allows power factor regulation of the windmill. This paper shows the dynamic performance of the complete system. Different experimental tests in a 3-kW prototype have been carried out to verify the benefits of the proposed system.

Modeling of the wind turbine with a doubly fed induction generator for grid integration studies

Abstract: Due to its many advantages such as the improved power quality, high energy efficiency and controllability, etc. the variable speed wind turbine using a doubly fed induction generator (DFIG) is becoming a popular concept and thus the modeling of the DFIG based wind turbine becomes an interesting research topic. Fundamental frequency models have been presented but these models are often complex with significant numerical overhead as the power converter block consisting of power control, rotor side and grid side converter control and DC link are often simulated in detail. This paper develops a simple DFIG wind turbine model in which the power converter is simulated as a controlled voltage source, regulating the rotor current to meet the command of real and reactive power production. This model has the form of traditional generator model and hence is easy to integrate into the power system simulation tool such as PSS/E. As an example, the interaction between the Arklow Bank Wind Farm and the Irish National Grid was simulated using the proposed model. The model performance and accuracy was also compared with the detailed model developed by DIgSILENT. Considering the simplification adopted for the model development, the limitation and applicability of the model were also discussed in this paper.

Usefulness of DC power flow for active power flow analysis with flow controlling devices

Abstract: DC power flow is a commonly used tool for contingency analysis. Recently, due to its simplicity and robustness, it also becomes increasingly used for the real-time dispatch and techno-economic analysis of power systems. It is a simplification of a full power flow looking only at active power. Aspects such as voltage support and reactive power management are possible to analyse. However, such simplifications cannot always be justified and sometimes lead to unrealistic results. Especially the implementation of power flow controlling devices is not trivial since standard DC power flow fundamentally neglects their effects. Until recently, this was not an issue as the application of power flow controlling devices in the European grid was limited. However, with the liberalisation of European electricity market and the introduction of large wind energy systems, the need for real power flow control has emerged and therefore, the use of these devices has been reconsidered. Several phase shifting transformers (PST) are being installed or planned in order to control flows. Therefore, it is important to fundamentally re-validate the fast, but less accurate, DC power flow method. In this paper the assumptions of DC power flow are analysed, and its validity is assessed by comparing the results of power flow simulations using both the DC and AC approaches on a modified IEEE 300 bus system with PSTs.

Photovoltaic Power Conditioning System With Line Connection

Abstract: A photovoltaic (PV) power conditioning system (PCS) with line connection is proposed. Using the power slope versus voltage of the PV array, the maximum power point tracking (MPPT) controller that produces a smooth transition to the maximum power point is proposed. The dc current of the PV array is estimated without using a dc current sensor. A current controller is suggested to provide power to the line with an almost-unity power factor that is derived using the feedback linearization concept. The disturbance of the line voltage is detected using a fast sensing technique. All control functions are implemented in software with a single-chip microcontroller. Experimental results obtained on a 2-kW prototype show high performance such as an almost-unity power factor, a power efficiency of 94%, and a total harmonic distortion (THD) of 3.6%

Double-Input PWM DC/DC Converter for High-/Low-Voltage Sources

Abstract: A novel double-input pulsewidth-modulation (PWM) dc/dc converter for high-/low-voltage sources is proposed in this paper. With a PWM control scheme, the proposed double-input dc/dc converter can draw power from two different voltage sources simultaneously or individually. The operation modes and the steady-state analysis of the proposed double-input dc/dc converter are introduced in detail. The PWM control scheme for the power flow balancing is also presented. By using a single passive lossless soft-switching cell, switching losses of all power switches can be reduced significantly. Finally, experimental measurements are demonstrated to verify the performance of the proposed converter

Novel Solar-Cell Power Supply System Using a Multiple-Input DC–DC Converter

Abstract: Recently, the clean electric power generation systems have attracted a great deal of social attention to exploit the clean-energy resources such as solar arrays, wind generators, fuel cells, and so forth. In this case, a multiple-input dc–dc converter is useful to combine the several input power sources and to supply the regulated output voltage for the load from the power sources. The novel solar-cell power supply system using the buck–boost-type two-input dc–dc converter is proposed, in which a solar array and a commercial ac line are employed as power sources and are combined by two input windings of the energy-storage reactor. Also, its operation principle and performance characteristics are discussed. Furthermore, the solar-cell optimum-operating-point tracker is proposed and examined. It is confirmed by the experiment that the proposed solar-cell power supply system has excellent performance characteristics.

Equivalencing the collector system of a large wind power plant

Abstract: As the size and number of wind power plants (also called wind farms) increases, power system planners will need to study their impact on the power system in more detail. As the level of wind power penetration into the grid increases, the transmission system integration requirements becomes more critical. A very large wind power plant may contain hundreds of megawatt-size wind turbines. These turbines are interconnected by an intricate collector system. While the impact of individual turbines on the larger power system network is minimal, collectively, wind turbines can have a significant impact on the power systems during a severe disturbance such as a nearby fault. Since it is not practical to represent all individual wind turbines to conduct simulations, a simplified equivalent representation is required. This paper focuses on our effort to develop an equivalent representation of a wind power plant collector system for power system planning studies. The layout of the wind power plant, the size and type of conductors used, and the method of delivery (overhead or buried cables) all influence the performance of the collector system inside the wind power plant. Our effort to develop an equivalent representation of the collector system for wind power plants is an attempt to simplify power system modeling for future developments or planned expansions of wind power plants. Although we use a specific large wind power plant as a case study, the concept is applicable for any type of wind power plant.

A neutral-point clamped converter system for direct-drive variable-speed wind power unit

Abstract: Recent and ongoing developments in wind turbine technology indicate a trend towards utilization of high capacity (e.g., up to 5 MW) wind power units in large wind farms. Higher capacity of the wind turbine necessitates operation of the corresponding electric machine and the static converter system at higher voltages. This paper presents a neutral point diode clamped (NPC) converter system that inherently accommodates higher voltage and power ratings of a high capacity wind power unit. The overall control strategy of an NPC-based wind power unit and the details of the ac side and the dc side controls of the NPC converter system are also described. The generator-side NPC converter provides torque-speed control of the turbine-generator unit. The network-side NPC converter controls real and reactive power flow to the network and thus regulates the dc bus voltage and the ac side power-factor (or voltage) respectively. The paper also presents a new control approach to balance the dc capacitor voltages. The NPC converter system is augmented with a dc chopper that controls the synchronous generator field current. The NPC-based converter system is used to interface a 3 MW, direct-drive (gearless), synchronous machine based wind power unit to the utility grid. Performance of the overall NPC-based wind power unit, under the proposed controls, is evaluated based on time domain simulations in the power systems computer aided design (PSCAD) electromagnetic transient for DC (EMTDC) environment.

Nonlinear Control of Variable-Speed Wind Turbines for Generator Torque Limiting and Power Optimization

Abstract: To maximize wind power extraction, a variable-speed wind turbine (VSWT) should operate as close as possible to its optimal power coefficient. The generator torque is used as a control input to improve wind energy capture by forcing the wind turbine (WT) to stay close to the maximum energy point. In general, current control techniques do not take into account the dynamical and stochastic aspect of both turbine and wind, leading to significant power losses. In addition, they are not robust with respect to disturbances. In order to address these weaknesses, a nonlinear approach, without wind speed measurement for VSWT control, is proposed. Nonlinear static and dynamic state feedback controllers with wind speed estimator are then derived. The controllers were tested with a WT simple mathematical model and are validated with an aeroelastic wind turbine simulator in the presence of disturbances and measurement noise. The results have shown better performance in comparison with existing controllers.

A Bi-Directional Isolated DC/DC Converter as a Core Circuit of the Next-Generation Medium-Voltage Power Conversion System

Abstract: This paper describes a bi-directional isolated dc/dc converter considered as a core circuit of 3.3-kV/6.6-kV high-power-density power conversion systems in the next generation. The dc/dc converter is intended to use power switching devices based on SiC and/or GaN, which will be available on the market in the near future. A 350-V, 10-kW and 20-kHz dc/dc converter is designed, constructed and tested. It consists of two single-phase full-bridge converters with the latest trench-gate Si-IGBTs and a 20-kHz transformer with a nano-crystalline soft-magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full-bridge converters. The overall efficiency from the dc-input to dc-output terminals is accurately measured to be as high as 97%, excluding gate drive circuit and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. The loss analysis clarifies that the use of SiC-based power devices may bring a significant reduction in conducting and switching losses to the dc/dc converter. As a result, the overall efficiency may reach 99% or higher.

Voltage divider and its application in the two-stage power architecture

Abstract: An ultra-high power density DC/DC converter with fixed conversion ratio is proposed for the two-stage power architecture in the area of power management for the processor based power systems, including server, desktop, laptop, and other battery powered portable applications. Without the magnetic component inside, its power density has been scaled up by an order over the conventional approach. In addition, it also features the capability of holding ultra-high efficiency within whole load range, which is promising to the battery life extension in the laptop, 1 kw/inch/sup 3/ power density and 98% efficiency have been demonstrated in 120W and 60W DC/DC prototypes designed for two-stage VRs.

The Feasibility of Small-Scale Residential DC Distribution Systems

Abstract: The application of DC distribution of electrical power has been suggested as an efficient method of power delivery This concept is inspired by the absence of reactive power, the possibility of efficient integration of small distributed generation units and the fact that, internally, many appliances operate using a DC voltage A suitable choice of rectifier facilitates the improvement of the power quality as well as the power factor at the utility grid interface Stand-by losses can be largely reduced However, because of the inherent danger associated with DC voltages and currents, it is imperative that a considerable amount of design effort is allocated for risk analysis and the conception of protective devices and schemes, in order to guarantee personal and material (especially fire) safety This paper consists of the following topics: topological design, buffering of the DC bus, interfacing distributed generators, efficiency analysis and safety measures The conclusion of this work is that (at the moment) it is generally not efficient to implement a DC distribution system exclusively at the level of the end-user Rather, further research should focus on the extension of DC power delivery to higher levels of the electricity grid

PSIM circuit-oriented simulator model for the nonlinear photovoltaic sources

Abstract: The photovoltaic (PV) generator exhibits a nonlinear i-v characteristic and its operating point depends on the nature of the connected load and solar insolation In this article a simple and accurate PV model suitable for circuit-oriented simulator, PSIM, is developed Necessary theory and mathematical treatment for the solar cell array model is discussed To verify the model, i-v characteristics are generated through simulation for different solar insolations and compared with the experimentally obtained characteristics The developed circuit models are integrated into the PV system and as an example a maximum power (MP) point tracking of PV system with dc-dc converter is considered here Simulation and experimental result are in close agreement with each other This verifies the accuracy and suitability of the developed circuit-oriented models

Wind Farm Configuration and Energy Efficiency Studies - Series DC versus AC Layouts

Abstract: In this thesis, the design and control of a wind farm utilizing series-connected wind turbines with a DC output has been evaluated. The advantage is that a suitable DC voltage level, appropriate for transmission of the generated power directly, without using a large centralized DC/DC converter, can be obtained. This is achieved by series-connecting a number of wind turbines. In addition, the energy production using various wind turbines and wind park layouts have been investigated. Furthermore, the energy production costs have been determined. Finally, the limiting factors for the installation of smaller wind parks have been evaluated. For instance, when dynamic power pulsations have to be considered, from a power quality point of view, when a wind energy installation is to be connected to the grid. The results found are that the uneven power production from the individual wind turbines creates design as well as control difficulties for the wind farm with series-connected wind turbines. A control scheme for the wind turbines is proposed and investigated in this thesis. It is found that the proposed control scheme manages to safely operate the wind farm, even when large deviations in the individual power production of the turbines exist. A down-scaled prototype has been built, representing one wind turbine unit, and the base current control ability was verified experimentally. Moreover, it is found that it is necessary to design the individual wind turbine converters for a voltage level of about 35 % higher than the nominal voltage. Otherwise there will be a substantial energy loss due to the uneven power production from the individual wind turbines that occurs in actual installation. In addition, it is found that the series dc wind park has a good economical potential, since it eliminates the need for an offshore platform in the wind park. Finally, the electrical limiting factors for the installation of wind farms are determined using field experimental data. For instance, it is shown that for a wind park of about 10 wind turbines, the power pulsations are "smoothened" sufficiently so that the flicker emission never will be the limiting factor, even for fixed-speed turbines, when a wind energy installation is connected to a network. Worth mentioning is that it was found that the summation formula for flicker given in IEC 61400-21 can give a flicker prediction that is too low.

Stand-alone wind turbine system, apparatus, and method suitable for operating the same

Abstract: This invention provides a wind turbine-battery-dump load stand-alone renewable energy system and an optimal control of the same. The system may include both power conversion and control units. In one embodiment, the power conversion unit features a wind-turbine-driven three-phase induction generator, a diode rectifier, a battery charger, a boost dc/dc converter, a battery bank (48V), and a dc/ac inverter. A dump load is also used to dissipate excess power that is not required for either the battery charging or for the load. The integrated control unit may use the TMS320LF2407A DSP microcontroller from Texas Instruments, which allows operations of the wind power system and the battery storage system to be merged into a single package under a master controller. An embodiment of the control system features battery-charging control, battery voltage-boost control, dump load control, PWM inverter control, and system protection. It enables the use of renewable energy resources, while at the same time facilitating an efficient management of energy dispatch. This integrated control system offers remote villages the potential to fully supply their electrical power needs.

A Stand-Alone Hybrid Generation System Combining Solar Photovoltaic and Wind Turbine with Simple Maximum Power Point Tracking Control

Abstract: This paper proposes a hybrid energy system combing solar photovoltaic and wind turbine as a small-scale alternative source of electrical energy where conventional generation is not practical. A simple and cost effective control technique has been proposed for maximum power point tracking from the photovoltaic array and wind turbine under varying climatic conditions without measuring the irradiance of the photovoltaic or the wind speed. The proposed system is attractive owing to its simplicity, ease of control and low cost. A complete description of the proposed hybrid system along with detailed simulation results which ascertain its feasibility are given to demonstrate the availability of the proposed system in this paper. Simulation of the hybrid system under investigation was carried out using PSIM software.

Wind Power Impact on System Frequency Deviation and an ESS based Power Filtering Algorithm Solution

Abstract: Wind power is the fastest growing renewable energy. However due to its stochastic nature, fluctuating wind power results in adverse impacts on power systems, including system frequency deviations. Study on system frequency response in this paper shows power systems are more sensitive to the medium frequency power fluctuations (between 0.01 and 1 Hz), while the majority of wind power fluctuations are located in that regions and below. For small standalone power systems, even a modest wind penetration will lead to considerable system frequency deviation by the wind. To diminish the wind power impact on system frequency, an energy storage system (ESS) based wind power filtering algorithm is proposed in this paper, aimed at attenuation of those medium frequency fluctuations. Electromagnetic transient simulation results quantitatively demonstrate the effectiveness of this algorithm; the wind power is smoothed out and the system frequency deviations are limited to an acceptable level

Optimum configuration for renewable generating systems in residence using genetic algorithm

Abstract: Photovoltaic power generation and wind power generation have an advantage that energy is clean and infinite, but they have a disadvantage in that electrical output energy is unstable, and initial and maintenance costs are expensive. This paper presents an optimal configuration for renewable generating systems in residence using a genetic algorithm (GA). We define that the objective function is the total cost, where the total cost is the sum of initial cost, an operation cost, and a maintenance cost. We determine an optimal configuration of renewable generating systems, where total cost is more reasonable using GA. The simulation using this technique is shown.

Voltage Stability Analysis of Wind Farm Integration into Transmission Network

Abstract: In some regional grids in China, wind power penetration will increase rapidly because of the abundant wind resources in those areas and the government policy impetus. However, the power system security and stability may be affected due to the higher wind power penetration. Because majority of the wind farms with higher installed capacity intends to be connected into the transmission network of 220 kV voltage level, their impacts are becoming more widespread. In the grid impact studies of wind power integration, voltage stability is the mostly concerned problem that will affect the operation and security of wind farms and power grid. In this paper, the detailed wind turbines steady-state model and dynamic model are used to explore the wind power integration impact on voltage stability of the power system; the load flow calculation (P-V curve and V-Q curve) and dynamic contingency study are conducted; the different impacts on voltage stability of integrating wind farms based on different wind turbines technologies are illustrated and the following conclusions are presented: a. Wind turbines equipped with simple induction generator are not provided with reactive power regulation capability. Voltage stability deterioration is mainly due to the large amount of reactive power absorbed by the wind turbine generators during the continuous operation and system contingencies. b. Wind turbines equipped with doubly fed induction generator (DFIG) controlled by the PWM converters are provided with reactive power regulation capability; can absorb or supply reactive power during normal operation. The adverse affect on local network voltage stability is mitigated so that more wind power installed capacity can be incorporated into the grid. c. The transient voltage stability characteristics of wind turbines with DFIG are better than wind turbines with induction generator because of the voltage control capability of the DFIG based wind turbines; The DFIG based wind turbines have a better voltage recovery performance than the IG based wind turbines with same rating.

Wind farm and method for controlling the same

Abstract: A method for controlling dynamic power factor or the reactive power of a wind farm is provided. The wind farm comprises a number of wind turbines connected to a utility grid driven with a requested power factor or a requested reactive power. The wind turbine output voltage is controlled to a specific voltage set point. In the method, the wind farm power factor is measured and compared with the power factor requested for the utility grid, or the wind farm reactive power is measured and compared with the reactive power requested for the utility grid, respectively; the ratio of the wind farm voltage to the utility grid voltage is adjusted, and the output voltage of the individual wind turbines is regulated to correspond to the specific voltage set point; the steps are repeated until the power factor of the wind farm electricity corresponds to the requested reactive power.

Grid-side converter control of DFIG wind turbines to enhance power quality of distribution network

Abstract: A doubly fed induction generator wind turbine (DFIG-WT) with FACTS capabilities is presented. It is suggested to make use of the grid-side converter as a shunt active filter in order to support the grid with power factor correction as well as harmonic compensation. A vector control scheme is used to control the grid-side converter with a priority level feature to split extra room for compensation. It allows the grid-side converter to be remote-controlled and provide power quality support at a specific point away from the wind turbines plant. No harmonics are injected into the generator and the system is fully operational even at zero output power from the DFIG-WT. Simulation results illustrate good performance of the proposed system.

Grid-connected hybrid PV/wind power generation system with improved DC bus voltage regulation strategy

Abstract: The objective of this paper is to propose an improved dc bus voltage regulation strategy for the grid-connected PV/Wind power generation system. The proposed dc bus voltage regulation strategy can reduce the variation of the dc bus voltage and the size of the dc bus capacitor bank, significantly. Also, the change of the injected ac current amplitude will be moderate and the power quality of the utility can be improved. A 1.2 kW prototype system is built and tested to verify the feasibility of the proposed dc bus voltage regulation strategy. Hardware experimental results are shown to confirm the performance of the proposed dc bus voltage regulation strategy.

Power quality aspects in a wind power plant

Abstract: Like conventional power plants, wind power plants must provide the power quality required to ensure the stability and reliability of the power system it is connected to and to satisfy the customers connected to the same grid. When wind energy development began, wind power plants were very small, ranging in size from under one megawatt to tens megawatts with less than 100 turbines in each plant. Thus, the impact of wind power plant on the grid was very small, and any disturbance within or created by the plant was considered to be in the noise level. In the past 30 years, the size of wind turbines and the size of wind power plants have increased significantly. Notably, in Tehachapi, California, the amount of wind power generation has surpassed the infrastructure for which it was designed. At the same time, the lack of rules, standards, and regulations during early wind development has proven to be an increasing threat to the stability and power quality of the grid connected to a wind power plant. Fortunately, many new wind power plants are equipped with state of the art technology, which enables them to provide good service while producing clean power for the grid. The advances in power electronics have allowed many power system applications to become more flexible and to accomplish smoother regulation. Applications such as reactive power compensation, static transfer switches, energy storage, and variable-speed generations are commonly found in modern wind power plants. Although many operational aspects affect wind power plant operation, this paper, focuses on power quality. Because a wind power plant is connected to the grid, it is very important to understand the sources of disturbances that affect the power quality. In general, the voltage and frequency must be kept as stable as possible. The voltage and current distortions created by harmonics are also discussed in this paper as self-excitation, which may occur in a wind power plant due to loss of line.

Output Power Maximization of a Permanent Magnet Synchronous Generator Based Stand-alone Wind Turbine

Abstract: This paper proposes output power maximization control of wind energy system. A permanent magnet synchronous generator (PMSG) is used as a variable speed generator in the proposed wind energy system. In order to achieve the maximum power control of wind turbine across a wide range of wind speeds, a dynamic modeling and simulation of wind system with battery energy storage including wind disturbances is presented in this paper. This model should be able to support the analysis of the interaction between the mechanical structure of the wind turbine and the electrical load during different operational modes. These wind turbine concept models can be used and even extended for the study of different aspects, e.g. the assessment of power quality, control strategies, connection of the wind turbine at different types of grid and storage systems.

Maxium power point tracking method and tracking device thereof for a solar power system

Abstract: A maximum power point tracking method, applied to a tracking device, employs a DC/DC converter connecting with a solar cell array, and including a controller actuating the DC/DC converter to perform an active resistance characteristic; a maximum power point tracking circuit adjusting the active resistance of the DC/DC converter; monitoring a change of an output power of the solar cell array in determining a direction for adjusting the active resistance of the DC/DC converter; and the maximum power point tracking circuit repeatedly adjusting the active resistance of the DC/DC converter. If the change of the output power of the solar cell array is positive, the active resistance of the DC/DC converter is adjusted in the same direction; but, conversely, if the change of the output power of the solar cell array is negative, the active resistance of the DC/DC converter is adjusted in an opposite direction.

Method, apparatus and computer program product for wind turbine start-up and operation without grid power

Abstract: A wind turbine having features for black-starting includes an electric generation system for producing electricity by operation of the wind and comprising an interface for providing the electricity to an electric grid; a control system for controlling components of the wind turbine during start-up of the electric generation system, wherein start-up occurs during a deficient electric signal of the grid; and at least one energy providing element and at least one energy dissipative element for providing a balance between an output of the wind turbine and the electric signal of the grid. Methods and computer program products for operation of the wind turbine call for, among other things, synchronization of electric signals and control of components within the wind turbine.

System for precharging a dc link in a variable speed drive

Abstract: A variable speed drive (104) with a converter (202) that is controllable to precharge a DC link (204) is provided. The variable speed drive (104) also includes an inverter (206). The converter (202) converts a fixed line frequency, fixed line voltage AC power from an AC power source (102) into DC power. The DC link (204) filters the DC power from the converter (202). Finally, the inverter (206) is connected in parallel with the DC link (204) and converts the DC power from the DC link (204) into a variable frequency, variable voltage AC power. The converter (202) includes a plurality of pairs of power switches, wherein each pair of power switches includes a reverse blocking power switch arrangement (454, (650A, 652B)) connected in anti-parallel to another reverse blocking power switch arrangement (456, (652A, 650B)). Alternatively, each pair of power switches includes a reverse blocking power switch (454) connected in anti-parallel with a silicon carbide controlled rectifier (500).

Modeling and Control of a Grid-connected Wind/PV Hybrid Generation System

Abstract: This paper deals with power control of a wind and solar hybrid generation system for interconnection operation with electric distribution system. The proposed system consists of a variable-speed direct-drive wind generator, wind-side converter, solar array, dc-dc converter and grid interface inverter. Power control strategy is to extract the maximum energy available from varying condition of wind speed and solar irradiance while maintaining power quality at a satisfactory level. In order to capture the maximum power, variable speed control is employed for wind turbine and maximum power point tracking is applied for photovoltaic system. The grid interface inverter transfers the energy drawn from the wind turbine and PV array into the grid by keeping common dc voltage constant. Modeling and simulation study on the entire control scheme is carried out using a power system transient analysis tool, PSCAD/EMTDC. The simulation results show the control performance and dynamic behavior of the wind/PV system