Showing papers on "Power optimizer published in 2009"

A Review of the State of the Art of Power Electronics for Wind Turbines

Abstract: This paper reviews the power electronic applications for wind energy systems. Various wind turbine systems with different generators and power electronic converters are described, and different technical features are compared. The electrical topologies of wind farms with different wind turbines are summarized and the possible uses of power electronic converters with wind farms are shown. Finally, the possible methods of using the power electronic technology for improving wind turbine performance in power systems to meet the main grid connection requirements are discussed.

A review of grid code technical requirements for wind farms

Abstract: This paper provides an overview of grid code technical requirements regarding the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by transmission system operators (TSOs) of countries or regions with high wind penetration and therefore incorporate the accumulated experience after several years of system operation at significant wind penetration levels. The paper focuses on the most important technical requirements for wind farms, included in most grid codes, such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviour during grid disturbances. The paper also includes a review of modern wind turbine technologies, regarding their capability of satisfying the requirements set by the codes, demonstrating that recent developments in wind turbine technology provide wind farms with stability and regulation capabilities directly comparable to those of conventional generating plants.

Improved Energy Capture in Series String Photovoltaics via Smart Distributed Power Electronics

Abstract: This paper proposes an improved module integrated converter to increase energy capture in the photovoltaic (PV) series string. Prototypes for self-powered, high efficiency dc-dc converters that operate with autonomous control for tracking the maximum power of solar panels locally and on a fine scale are simulated, built and tested. The resulting module is a low-cost, reliable smart PV panel that operates independently of the geometry and complexity of the surrounding system. The controller maximizes energy capture by selection of one of three possible modes: buck, boost and pass-through. Autonomous controllers achieve noninteracting maximum power point tracking and a constant string voltage. The proposed module-integrated converters are verified in simulation. Experimental results show that the converter prototype achieves efficiencies of over 95% for most of its operating range. A 3-module PV series string was tested under mismatched solar irradiation conditions and increases of up to 38% power capture were measured.

Pitch angle control in wind turbines above the rated wind speed by multi-layer perceptron and radial basis function neural networks

Abstract: In wind energy conversion systems, one of the operational problems is the changeability and discontinuity of wind. In most cases, wind speed can fluctuate rapidly. Hence, quality of produced energy becomes an important problem in wind energy conversion plants. Several control techniques have been applied to improve the quality of power generated from wind turbines. Pitch control is the most efficient and popular power control method, especially for variable-speed wind turbines. It is a useful method for power regulation above the rated wind speed. This paper proposes an artificial neural network-based pitch angle controller for wind turbines. In the simulations, a variable-speed wind turbine is modeled, and its operation is observed by using two types of artificial neural network controllers. These are multi-layer perceptrons with back propagation learning algorithm and radial basis function network. It is shown that the power output was successfully regulated during high wind speed, and as a result overloading or outage of the wind turbine was prevented.

Maximum Power Tracking Control for a Wind Turbine System Including a Matrix Converter

Abstract: This paper focuses on maximum wind power extraction for a wind energy conversion system composed of a wind turbine, a squirrel-cage induction generator, and a matrix converter (MC). At a given wind velocity, the mechanical power available from a wind turbine is a function of its shaft speed. In order to track maximum power, the MC adjusts the induction generator terminal frequency, and thus, the turbine shaft speed. The MC also adjusts the reactive power transfer at the grid interface toward voltage regulation or power factor correction. A maximum power point tracking (MPPT) algorithm is included in the control system. Conclusions about the effectiveness of the proposed scheme are supported by analysis and simulation results.

DC-link Voltage Control of a Full Power Converter for Wind Generator Operating in Weak-Grid Systems

Abstract: When the wind power accounts for a large portion of the grid power, it may need to help the grid voltage and frequency regulation. This paper investigates a permanent-magnet wind generator with a full power voltage-source converter in weak-grid mode, where the DC-link voltage needs to be controlled from the generator side instead of the grid side. The energy relationship of the wind generator, DC-link energy storage, and load is established. An intrinsic right-half-plane zero, together with the wind power characteristics, the mechanical system inertia, and the DC-link energy storage, is identified as the physical limitations for the control. With the understanding of the system energy relationship and limitations, a hybrid adaptive control algorithm is proposed that searches for the optimal generator acceleration to achieve the maximum wind generator power change rate to match the load power variation. The proposed control scheme is verified through simulation of a 1.5-MW wind system as well as through the experiment of a scaled 1-kW, DSP-/field-programmable-gate-array-controlled, permanent-magnet-generator-based test bed. The results show that it is feasible to regulate DC link by the generator-side converter through the generator speed control. Some important applications issues are also investigated, including the DC-link energy storage requirement, wind speed change impact, and control transition between the weak-grid and strong-grid modes.

Photovoltaic DC/DC micro-converter

Abstract: A photo-voltaic (PV) power generating system and a control system for PV array string-level control and PV modules serially-connected into strings of PV modules. The system includes plural parallel strings of serially-connected power-generating photovoltaic modules that form a PV array, DC/DC micro-converters that are coupled to a DC voltage buss and to the output of a corresponding photovoltaic module or to the output of a string of photovoltaic modules; a gating or central inverter; and a control system. The control system is structured and arranged to control and manage each string of photovoltaic modules, to ensure that power delivered by the photovoltaic power generating system is not affected by photovoltaic modules or strings of photovoltaic modules that are not operating at maximum power transfer efficiency.

Static Var Compensator and Active Power Filter With Power Injection Capability, Using 27-Level Inverters and Photovoltaic Cells

Abstract: An active power filter and static var compensator with active power generation capability has been implemented using a 27-level inverter. Each phase of this inverter is composed of three ldquoHrdquo bridges, all of them connected to the same dc link and their outputs connected through output transformers scaled in the power of three. The filter can compensate load currents with a high harmonic content and a low power factor, resulting in sinusoidal currents from the source. To take advantage of this compensator, the dc link, instead of a capacitor, uses a battery pack, which is charged from a photovoltaic array connected to the batteries through a maximum power point tracker. This combined topology make it possible to produce active power and even to feed the loads during prolonged voltage outages. Simulation results for this application are shown, and some experiments with a 3-kVA device are displayed.

A Review of Non-Isolated High Step-Up DC/DC Converters in Renewable Energy Applications

Abstract: With the shortage of the energy and ever increasing of the oil price, research on the renewable and green energy sources, especially the solar arrays and the fuel cells, becomes more and more important. How to achieve high step-up and high efficiency DC/DC converters is the major consideration in the renewable grid-connected power applications due to the low voltage of PV arrays and fuel cells. The topology study with high step-up conversion is concentrated and most topologies recently proposed in these applications are covered and classified. The advantages and disadvantages of these converters are discussed and the major challenges of high step-up converters in renewable energy applications are summarized. This paper would like to make a clear picture on the general law and framework for the next generation non-isolated high step-up DC/DC converters.

Systems for highly efficient solar power conversion

Abstract: A high efficiency photovoltaic DC-DC converter achieves solar power conversion from high voltage, highly varying photovoltaic power sources to harvest maximum power from a solar source or strings of panels for DC or AC use, perhaps for transfer to a power grid at high power levels with coordinated control possible for various elements. Photovoltaic DC-DC converters can achieve efficiencies in conversion that are extraordinarily high compared to traditional through substantially power isomorphic photovoltaic DC-DC power conversion capability that can achieve 97%, 98%, 99.2% efficiency, or even only wire transmission losses. Switchmode impedance or voltage conversion circuit embodiments may have pairs of photovoltaic power interrupt switch elements and pairs of photovoltaic power shunt switch elements to first increase voltage and then decrease voltage as part of the desired photovoltaic DC-DC power conversion.

Method and apparatus for controlling a hybrid power system

Abstract: The present invention provides a simplified method of controlling power among the various sources and loads in a power system. Power generating sources are each connected to a common DC bus through a converter. The converter selectively transfers energy to the DC bus at a maximum rate or at a reduced rate according to the level of the DC voltage present on the DC bus. At least one storage device is preferably connected to the common DC bus through a power regulator. The power regulator selectively transfers energy to or from the DC bus as a function of DC voltage level present on the DC bus. Further, an inverter may be provided to bidirectionally convert between the DC voltage and an AC voltage for connection to a customer load or the utility grid. Each power conversion device is independently controlled to provide a modular and simplified power control system.

Characteristics of wind turbine generators for wind power plants

Abstract: This paper presents a summary of the most important characteristics of wind turbine generators applied in modern wind power plants. Various wind turbine generator designs, based on classification by machine type and speed control capabilities, are discussed along with their operational characteristics, voltage, reactive power, or power factor control capabilities, voltage ride-through characteristics, behavior during short circuits, and reactive power capabilities.

Contribution to frequency control through wind turbine inertial energy storage

Abstract: An innovative way for wind energy to participate in some sort of frequency control using kinetic energy stored in the rotor for a fast power reserve that could be delivered in a short period (from several seconds up to a few tens of seconds) is presented. This kinetic-energy-based fast reserve is ensured despite wind speed variations - a disoptimisation of the power coefficient through the modification of the rotor speed set point or through the pitch control. In case of a frequency drop, the power coefficient could then be increased to produce more energy. This approach could help to reduce the dynamics and the depth of the frequency drops and therefore allow wind farms to participate in frequency control. Basic economic aspects of the provision of fast power reserve by wind turbines as a function of the amount of reserve considered are also discussed.

A New Topology With High Efficiency Throughout All Load Range for Photovoltaic PCS

Abstract: In this paper, a new topology is proposed that can significantly reduce the converter rated power and increase the efficiency of total photovoltaic (PV) system. Since the output voltage of PV module has very wide operating range, in general, the DC/DC converter is used to produce constant high-DC-link voltage for DC/AC inverter. According to the analysis of the proposed topology, only 20% of total PV system power is processed by the DC/DC power conversion stage. The DC/DC power conversion stage used in proposed topology has flat efficiency curve throughout all load range and very high efficiency characteristics. In the proposed topology, because the converter efficiency curve is almost flat throughout all load range, the total system efficiency at light load is dramatically improved. The proposed topology is implemented for 250-kW power conditioning system. This system has only three DC/DC power conversion stage with 24-kW rated power. It is only one-third of total system power. The experimental results show that the proposed topology has good performance.

Adequacy assessment of generating systems containing wind power considering wind speed correlation

Abstract: Wind power is an important renewable energy resource. Electrical power generation from wind energy behaves quite differently from that of conventional sources, and maintaining a reliable power supply is an important issue in power systems containing wind energy. In these systems, the wind speeds at different wind sites are correlated to some degree if the distances between the sites are not very large. Genetic algorithm methods are applied here to adjust autoregressive moving-average time series models in order to simulate correlated hourly wind speeds with specified wind speed cross-correlation coefficients of two wind sites. Multi-state wind energy conversion system models are used to incorporate the correlated wind farms in reliability studies of generating systems. A method to generate random numbers with specified correlation coefficients for application in a state-sampling Monte Carlo simulation technique is introduced. It is shown that the proposed method can be used in the adequacy assessment of a generating system incorporating partially dependent wind farms.

Wind speed sensorless maximum power point tracking control of variable speed wind energy conversion systems

Abstract: A maximum power point tracking (MPPT) controller for variable speed wind energy conversion system (WECS) is proposed. The proposed method, without requiring the knowledge of wind speed, air density or turbine parameters, generates at its output the optimum speed command for speed control loop of rotor flux oriented vector controlled machine side converter control system using only the instantaneous active power as its input. The optimum speed commands which enable the WE to track peak power points are generated in accordance with the variation of the active power output due to the change in the command speed generated by the controller. The concept is analyzed in a direct drive variable speed permanent magnet synchronous generator (PMSG) WECS with back-to-back IGBT frequency converter. Vector control of the grid side converter is realized in the grid voltage vector reference frame. Simulation is carried out in order to verify the performance of the proposed controller.

Unit Commitment Risk Analysis of Wind Integrated Power Systems

Abstract: The utilization of wind power generation is increasing throughout the world and it is therefore important that these facilities be integrated in the existing generating capacity planning and operating protocols and procedures. This paper presents an approach to evaluate the contribution that wind power can make to the load carrying capability of a power generating system in an operating scenario. The basic concepts of unit commitment risk analysis are extended to include the inherent variability associated with wind power by developing short-term probability distributions of the wind speed and wind power output using auto-regressive moving average (ARMA) time series models. The operating capacity contributions attributable to wind power are illustrated by application to a small test system and are expressed in terms of the increased load carrying capability due to the wind power generating facilities.

Solar inverter and control method

Abstract: A power generation system including a photovoltaic (PV) module to generate direct current (DC) power is provided. The system includes a controller to determine a maximum power point for the power generation system and a boost converter for receiving control signals from the controller to boost the power from the PV module to a threshold voltage required to inject sinusoidal currents into the grid. A DC to alternating current (AC) multilevel inverter is provided in the system to supply the power from the PV module to a power grid. The system also includes a bypass circuit to bypass the boost converter when an input voltage of the DC to AC multilevel inverter is higher than or equal to the threshold voltage.

A wind turbine having a main power converter and an auxiliary power converter and a method for the control thereof

Abstract: A wind turbine comprising a main generator, a main power converter connected to the main generator and adapted to convert the electrical output power of the main generator to an electrical power to be fed into the utility grid up to a first rated converter output power and an auxiliary power converter connected to the main generator and adapted to convert the electrical output power of the main generator to an electrical power up to a second rated converter output power which is lower than the first rated converter output power is provided. Further, a control architecture for controlling the power balance of a wind turbine and a method for operating a wind turbine are provided.

Maximum power tracking control for a wind turbine system including a matrix converter

Abstract: This paper focuses on maximum wind power extraction for a wind energy conversion system composed of a wind turbine, a squirrel-cage induction generator, and a matrix converter. At a given wind velocity, the mechanical power available from a wind turbine is a function of its shaft speed. In order to track maximum power, the matrix converter (MC) adjusts the induction generator terminal frequency, and thus, the turbine shaft speed. The MC also adjusts the reactive power transfer at the grid interface towards voltage regulation or power factor correction. A Maximum Power Point Tracking (MPPT) algorithm is included in the control system. Conclusions about the effectiveness of the proposed scheme are supported by analysis and simulation results.

DC bus voltage control for two stage solar converter

Abstract: Systems, methods, and apparatus for supplying AC power to an AC power grid from a DC power source, such as a photovoltaic (PV) array are disclosed. The systems and methods can include a converter coupled to the DC power source that provides DC power to a DC bus at a DC bus voltage. The systems and methods can further include an inverter coupled to the DC bus for converting the DC power of the DC bus to an output AC power. The systems and methods can further include a control system configured to regulate the DC bus voltage of the DC bus to operate at a variable DC bus voltage setpoint. The control system can adjust the DC bus voltage setpoint based at least in part on the DC bus voltage and the output AC current of the inverter.

Modeling and control of an integrated wind power generation and energy storage system

Abstract: Wind energy is gaining the most interest among a variety of renewable energy resources, but the disadvantage is that wind power generation is intermittent, depending on weather conditions. Energy storage is necessary to get a smooth output from a wind turbine. This paper presents a new integrated power generation and energy storage system for doubly-fed induction generator based wind turbine systems. A battery energy storage system is connected to the DC link of the back-to-back power converters of the doubly-fed induction generator through a bi-directional DC/DC power converter. The energy storage device is controlled so as to smooth out the total output power as the wind speed varies. Control algorithms are developed for the grid-side converter, rotor-side converter and battery converter, and are tested on a simulation model developed in MATLAB/Simulink. The model contains a DFIG wind turbine, three PWM power converters and associated controllers, a DC-link capacitor, a battery, and an equivalent power grid. Simulation studies are carried out on a 2MW DFIG wind turbine and the results suggest that the integrated power generation and energy storage system can supply steady output power as the wind speed changes.

Forecasting electric power generation in a photovoltaic power system for an energy network

Abstract: Recently, there has been an increase in concern about the global environment Interest is growing in developing an energy network by which new energy systems such as photovoltaic and fuel cells generate power locally and electrical power and heat are controlled with a communication network We developed the power generation forecast method for photovoltaic power systems in an energy network The method makes use of weather information and regression analysis We carried out forecasting power output of the photovoltaic power system installed in Expo 2005, Aichi Japan As a result of comparing measurements with prediction values, the average prediction error per day was about 26% of the measured power © 2009 Wiley Periodicals, Inc Electr Eng Jpn, 167(4): 16–23, 2009; Published online in Wiley InterScience (wwwintersciencewileycom) DOI 101002/eej20755

Design and control of a DC collection grid for a wind farm

Abstract: In this thesis, the internal DC collection grid for a wind farm is investigated regarding the design, losses and dynamic operation for both normal operating conditions and for different fault conditions. The main advantage for the DC collection grid is the considerably lower weight of the 1 kHz transformers in the DC/DC converters compared to the equivalent 50 Hz transformers. For a wind farm with 48 2.3 MW wind turbines, and a DC/DC converter in each turbine as well as a main DC/DC converter for the whole wind farm, the losses for the DC system are 3 % of the transferred power, which is similar to the losses of a corresponding AC collection grid. For the dynamic control of the wind farm, the DC/DC converters control the power flow in the wind farm and thereby also the voltage levels for the 1.5 kV DC link in the turbine as well as for the 32 kV DC collection bus. For the limited bandwidth resulting from the switching frequency 1 kHz and the maximum voltage deviations of 5 % for the DC voltages, the required capacitances are 152 mF for the DC link in the wind turbine and 16 mF for the DC bus, both giving a stored energy corresponding to 74 ms transferred rated power. In the case of a fault in the connecting main grid, the output power from the wind farm must be decreased. Here, assuming that the excess power is dissipated in each turbine, the detection of the fault as well as the disconnection and the reconnection of the wind farm are investigated. The requirements for the HVDC link to avoid over voltages during a grid fault are stated and it is also shown that the reconnection can be done within 14 ms, which is well within the time specified in existing grid codes. Further, the behavior of the system during internal faults for the DC bus is investigated. Methods for detecting and finding the location of the faults are determined, and it is shown that a faulted part can be disconnected and the non-faulted parts reconnected within 300 ms without using fully rated DC breakers.