Grid Connection Point Research Articles

Simulation on Fault Detection Optimization Model of Large Intelligent Electronic Inverter

When a fault of power system occurs in grid-connected point, a serious impact is added to electronic inverter in photovoltaic micro-network internal including current, voltage and phase, the appropriate control strategies for completing fault detection of large-scale intelligent electronic inverter, is one of the currently hotspot difficulties. An engine fault diagnosis method based on PSO-SVM is proposed. Particle swarm method is used to search engine fault signals in designated space, so as to obtain optimal particle and provide the basis for an engine fault diagnosis. Support vector machine (SVM) method is applied to classify engine failure signal to complete the engine fault diagnosis. Experimental results show that the proposed algorithm for the engine fault diagnosis, can greatly improve the accuracy of diagnosis, so as to meet the needs of actual production, life, and achieve satisfactory results.

Modelling of a synchronous offshore pumping mode airborne wind energy farm

A wind farm for the deployment of pumping mode AWE (airborne wind energy) systems is presented in this paper. The topology presented is suitable for the deployment of such systems in a marine or similarly inaccessible environment. A brief technical description of AWE is provided, outlining the background, motivation and approaches taken by this emerging technology. A method of providing a continuous power supply from a cluster of AWE systems whose individual operation produces a periodic power supply is outlined. This method employs direct drive, directly interconnected permanent magnet synchronous generators on a local bus. A full-scale power converter is located at the point of grid connection, providing compliant power output for the remote cluster. In the case of a marine environment deployment, the power electronics are located onshore where maintenance and repair can be readily performed without the delays and costs associated with offshore maintenance and repair. The direct interconnection of synchronous generators introduces the requirement for a control system to control the connection of offline machines to the energised bus. A mathematical model of the system is outlined and the implementation of this model in Simulink is detailed. Simulation results under varied operating conditions are presented and discussed.

Evaluating reduced models of aggregated different doubly fed induction generator wind turbines for transient stabilities studies

ABSTRACTFor the representation of wind farms in transient stability studies of electrical power systems, reduced models based on aggregating identical wind turbines are commonly used. In the case of a wind farm with different wind turbines coupled to the same grid connection point, it is usual to aggregate identical wind turbines operating in similar conditions into an equivalent one. However, in the existing literature, there are not any references to the aggregation of different wind turbines (same wind turbine technology but different rated power or components) into a single one. This paper presents a comparative study of four reduced models for aggregating different DFIG wind turbines, experiencing different incoming winds, into an equivalent model. The first of them is the classical clustering model, in which each equivalent model experiences an equivalent wind. The other reduced models have the same equivalent generation system but different equivalent mechanical systems. Thus, the second and third ones are compound models with a clustering aggregated mechanical system and individual simplified models, respectively, to approximate the individual mechanical power according to the incoming wind speeds. The fourth is a mixed model that uses an equivalent wind speed, which is applied to an equivalent mechanical system (equivalent rotor and drive train) in order to approximate the mechanical power of the aggregated wind turbines. The equivalent models are validated by means of comparison with the complete model of the wind farm when simulated under wind fluctuations and grid disturbances. Finally, recommendations with regard to the applicability of models are established. Copyright © 2014 John Wiley & Sons, Ltd.

Reactive power management and control of distant large-scale grid-connected offshore wind power farms

Reactive power management and control of distant large-scale offshore wind power farms connected to the grid through high-voltage alternating current (HVAC) transmission cable are presented in this paper. The choice of the transmission option is based on the capacity of the considered wind farm (WF) and the distance to the onshore grid connection point. The WF is made up of identical doubly-fed induction generators (DFIGs). Modelling and improved analysis of the effective reactive power capability of DFIGs as affected by various operational constraints are provided. In addition, modelling and analysis of the reactive power demands, balance, and control are presented. The minimum capacity and reactive power settings for reactive power compensation required for the system are determined. Possibility of unity power factor operation suggested by the German electricity association (VDEW) is investigated. A summary of the main outcomes of the work presented in this paper is provided in the conclusions section.

The Study on Coordinated Control Strategy for Micro-Grid Containing Various Distributed Generators

According to external dual power line supply and the grid-connected and islanded operating modes of micro-grid and the changeover among them, a control strategy of micro-grid with different kinds of DG (Distributed Generation) is proposed. It monitors the operating states of both micro-grid and power network and provides the unified coordinating control by the controller of micro-grid. In grid-connected mode, all kinds of DG work in constant power mode (P-Q mode) to generate power output for load. In islanded mode, energy storage system works in constant voltage-frequency mode (V-f mode), which is selected as the master power for supporting the voltage and frequency in micro-grid. And the rest works in P-Q mode. The application at one smart community in Shanghai proved its feasibility. The proposed strategy keeps the stable operation of micro-grid and implements the seamless transition between two operating modes by synchronizing device in grid-connected point.

인버터 기반 신재생 에너지 발전 시스템의 계통 지원 운전을 위한 계통 전압 검출 방법

The Grid code is being strengthen as increase of renewable energy ratio. Especially, the grid connection regulations are continuously being updated for stable operation of power grids. Static grid support and Dynamic grid support must make an accurate measure at Grid connected point because they needs control algorithm individually. It has to exactly measure voltage including switching ripple at the output of the inverter generating system. In addition, it is necessary to have an accurate voltage measurement when the situation rapidly changing the grid impedance is caused by the input of serial impedance of transformer and line impedance as well as Grid Fault Device. In this paper, We propose a new detection method of grid voltage to calculate accurately the r.m.s voltage of the grid connection point along the standard required by the low voltage regulation. We verified performance through simulation grid fault device.

LVRT Capability of DFIG-Based WECS Under Asymmetrical Grid Fault Condition

In this paper, the low-voltage ride-through (LVRT) capability of the doubly fed induction generator (DFIG)-based wind energy conversion system in the asymmetrical grid fault situation is analyzed, and the control scheme for the system is proposed to follow the requirements defined by the grid codes. As analyzed in the paper, the control efforts of the negative-sequence current are much higher than that of the positive-sequence current for the DFIG. As a result, the control capability of the DFIG restrained by the dc-link voltage will degenerate for the fault type with higher negative-sequence voltage component and 2φ fault turns out to be the most serious scenario for the LVRT problem. When the fault location is close to the grid connection point, the DFIG may be out of control resulting in non-ride-through zones. In the worst circumstance when LVRT can succeed, the maximal positive-sequence reactive current supplied by the DFIG is around 0.4 pu, which coordinates with the present grid code. Increasing the power rating of the rotor-side converter can improve the LVRT capability of the DFIG but induce additional costs. Based on the analysis, an LVRT scheme for the DFIG is also proposed by taking account of the code requirements and the control capability of the converters. As verified by the simulation and experimental results, the scheme can promise the DFIG to supply the defined positive-sequence reactive current to support the power grid and mitigate the oscillations in the generator torque and dc-link voltage, which improves the reliability of the wind farm and the power system.

Simulation of grid connection and maximum power point tracking control of brushless doubly-fed generator in wind power system

In this paper, based on the analysis of the mathematical model in a common synchronous reference frame of the brushless doubly-fed generator (BDFG), the grid connection strategy and maximum energy extraction control were both analyzed. Besides, the transient simulation of no-load model and generation model of the BDFG have been developed on the MATLAB/Simulink platform. The test results during cutting-in grid confirmed the good dynamic performance of grid synchronization and effective power control approach for the BDFG-based variable speed wind turbines.

A Passive Islanding Detection Method for Grid-Connected Renewable Generation System

This study proposes an islanding detection method for use of grid-connected inverter of renewable generation system. The proposed method is based on an index: the change of power factor (COPF). When an inverter is grid-connected, the COPF is lower than the threshold values. In contrast, as an islanding occurs, the COPF becomes much higher than the threshold value. Detection systems monitor the terminal voltage and load current at the grid-connected point to calculate COPF, and issue an operating signal when the value of COPF exceeds a given threshold. In this study, experiments are conducted to illustrate the principles of the proposed technique for a prototype grid-connected inverter. The test results show that the proposed method is reliable, economical, and easy to implement for islanding detection of grid-connected inverter.

A statistical approach to electrical storage sizing with application to the recovery of braking energy

In the context of efficient energy use, electrical energy in electric drives plays a fundamental role. High efficiency energy storage systems permit energy recovery, peak shaving and power quality functions. Due to their cost and the importance of system integration, there is a need for a correct design based on technical-economical optimization. In this paper, a method to design a centralized storage system for the recovery of the power regenerated by a number of electric drives is presented. It is assumed that the drives follow deterministic power cycles, but shifted by an uncertain amount. Therefore the recoverable energy and, consequently, the storage size requires the optimization of a random cost function, embedding both the plant total cost and the saving due to the reduced energy consumption during the useful life of the storage. The underlying stochastic model for the power profile of the drives as a whole is built from a general Markov chain framework. A numerical example, based on Monte Carlo simulations, concerns the maximization of the recoverable potential energy of multiple bridge cranes, supplied by a unique grid connection point and a centralized supercapacitor storage system.

Coordinated Control of DFIG and FSIG-Based Wind Farms Under Unbalanced Grid Conditions

This paper investigates the control and operation of doubly-fed induction generator (DFIG) and fixed-speed induction generator (FSIG) based wind farms under unbalanced grid conditions. A DFIG system model suitable for analyzing unbalanced operation is developed, and used to assess the impact of an unbalanced supply on DFIG and FSIG operation. Unbalanced voltage at DFIG and FSIG terminals can cause unequal heating on the stator windings, extra mechanical stresses and output power fluctuations. These problems are particularly serious for the FSIG-based wind farm without a power electronic interface to the grid. To improve the stability of a wind energy system containing both DFIG and FSIG based wind farms during network unbalance, a control strategy of unbalanced voltage compensation by the DFIG systems is proposed. The DFIG system compensation ability and the impact of transmission network impedance are illustrated. The simulation results implemented in Matlab/Simulink show that the proposed DFIG control system improves not only its own performance, but also the stability of the FSIG system with the same grid connection point during network unbalance.

30分同時同量を制約とするマイクログリッドにおける太陽光発電システム導入量に応じたNaS電池kW容量

On constructing a microgrid, it is essential to design capacity of photovoltaic power generation (PV) systems and storage batteries in accordance with a control target. In this study, we constructed a simulation model of energy control system in the microgrid used in the demonstration project. By using this model, we investigated the minimum capacity of NaS battery for different PV system capacities for keeping the target power imbalance within ±3% over 30 min.The main results are as follows. The microgrid involving 330-kW PV systems (corresponding to the actual system) needs a NaS battery capacity of at least approximately ±20kW, and PV systems with a capacity up to about 890kW can be integrated in the microgrid with a NaS battery capacity of ±500kW (corresponding to the actual system). We estimated the minimum capacity of NaS battery for different PV system capacities and clarified that the output behavior of the NaS battery and PAFC when supply and demand power imbalance over 30 min. exceeds the ±3% limit. We suggested the improved control model and showed that it is effective in decreasing the minimum capacity of NaS battery, although it has negative effects on the reduction of short-period power flow fluctuation at the grid-connection point.

Application of Technology Development Index and Principal Component Analysis and Cluster Methods to Ocean Renewable Energy Facility Siting

AbstractTo assist in siting of offshore renewable energy facilities (wind, wave, and in-stream tidal, and ocean current), a marine spatial planning-based approach is proposed. The first level (Tier #1) screening determines the potential energy resource to be exploited and then identifies areas that are prohibited from siting because there is a direct, irreconcilable conflict, as determined by a stakeholder process and vetted by regulators. Areas that remain after these exclusions are implemented are candidates for facility siting. The next step involves considering technical (engineering and economic) attributes of the proposed energy development that further restricts the area under consideration. Finally, Tier #2 screening (not addressed here) evaluates other use conflicts such as recreational and commercial fishing areas, marine mammal feeding and breeding grounds and transit paths, bird migratory paths, feeding, and nesting areas, and similar issues that must be considered in facility siting.To facilitate the application of technology constraints on siting, two methods are proposed, a Technology Development Index (TDI) and a Principal Components - Cluster Analysis (PCCA). The TDI method, developed by the authors and presented in this paper, is the ratio of the Technical Challenge Index (TCI) to the Power Production Potential (PPP) of the energy extraction device. TCI is a measure of how difficult it is to site the device at a given location plus a measure of the distance to the closest electrical grid connection point. The PPP is an estimate of the annual power production of one of the devices. The site with the lowest TDI represents the optimum location. In practice, the study area is gridded and the TDI (TCI and PPP) is calculated for each grid. The method explicitly accounts for the spatial variability of all input data. Simulations can be performed either deterministically or stochastically, using a Monte Carlo method, so that uncertainties in the underlying input data are reflected in the estimated values of the TDI. The later approach allows detailed assessment of the sensitivity of the estimates to the input data and formulations of the TCI and PPP. The results are presented in the form of contours of TDI. The method can be applied to any offshore renewable energy type or extraction system once the technical attributes are specified.The PCCA approach uses several spatially varying variables that describe the key attributes of the siting decision (e.g., water depth, power production potential, distance to shore, and seabed conditions). The principal components are first determined from the gridded data and then clusters are identified. Finally, the clusters are mapped to the study area. The attributes and spatial distribution of clusters provide insight into the optimum locations for development.The two methods were employed in identifying potential areas for siting of a wind farm in coastal waters of Rhode Island, assuming lattice jacket support structures for the wind turbines. Both methods give consistent results and show locations where the ratio of technical challenge to power production is minimized.

A Study on Dumping Power Flow Fluctuation at Grid-Connection Point of Residential Micro-Grid with Clustered Photovoltaic Power Generation Systems Considering Difference in Solar Irradiance Patterns in Urban Districts

Power output fluctuation of photovoltaic power generation systems (PVSs) may cause negative impacts on the existing electric power system when the penetration of PVSs is quite large. A micro-grid consisting of clustered PVSs and a battery system would be one of the promising measures against negative impacts of clustered PVSs, while the capacity of battery system should be reduced as much as possible from the economic point of view. Because of the difference in output fluctuation among PVSs in the various locations, the total output fluctuations of PVSs would be relaxed due to the so-called “smoothing-effect”. By using data on solar irradiance simultaneously observed at five points, this study evaluates the total output fluctuation of several micro-grids and the required capacity of battery system, taking the smoothing effect into account. The main results are as follows. The balancing control is accomplished with the acceptable error by using the small capacity of battery system, while small output fluctuation still remains in each micro-grid. In such the situation, because the total fluctuation of five micro-grids is not so large, the acceptable error in balancing control can be increased by a few percentages, resulting in reduction in the required maximum power of battery system by a few ten percentages.

Operating capability as a PQ/PV node of a direct-drive wind turbine based on a permanent magnet synchronous generator

This paper describes the modelling and control system of a direct-drive PMSG wind turbine for effective active and reactive power generation control and voltage control at the grid connection point. This study focuses on the maximum power capability of the wind turbine, which is limited by its generator and power converter. The ability of this model and control strategy are assessed by means of simulations and discussed at length. The results of our study show that a PMSG wind turbine is able to actively participate in grid operation because it can independently control active and reactive power production (operating as a PQ node) or the active power and voltage at the connection node (operating as a PV node).

Supply and demand control of distributed generators in a microgrid for new energy

AbstractWe report the operational results of distributed generators (DGs) in a microgrid and present the effects after photovoltaic power generation (PV) systems were incorporated into the microgrid for an electric power system. The microgrid was constructed at the Expo 2005 Aichi site as part of a demonstration promoted by NEDO. A solution is needed for problems in which instability in DGs that utilize natural energy such as solar light and wind force negatively influences existing electric power systems. Consequently, we developed an energy‐control system and controlled the output of the DGs to reduce fluctuations at the grid‐connection point caused by unstable PV‐system output. Our microgrid consisted of DGs such as PV systems, fuel cells, and NaS batteries, and these DGs were controlled by an energy‐control system. We tested and then verified the practical effectiveness of installing the microgrid as follows. We found that (1) 99.5% of the power imbalance in supply and demand over a period of 30 min was within a range of ±3% under normal operating conditions, (2) the microgrid contributed to load leveling, (3) the energy‐control system smoothed the power‐flow fluctuations of PV‐system output at the grid connection point, and (4) installing microgrids should help reduce additional Load Frequency Control (LFC) capacity in the future. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 169(4): 1–8, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20872

制御特性の相違によるマイクログリッドにおける同時同量への影響

A clear criterion for evaluating the degree of contribution to a power system by microgrids is needed to facilitate the spread of microgrids. This requires clarifying the composition and scale of distributed generators (DGs) in the microgrid. Once the criterion has been identified, the control system characteristics needed to meet the criterion must be determined.To evaluate how differences in the control characteristics of the DGs in a microgrid affect the power flow at the grid-connection point, we conducted tests for the four cases. The goal was to control the energy so as to keep the difference between supply and demand within ±3% over 30 min. In our experiments, the goal was met for each case. However, there was a distinctive trend in the variation in power flow at the grid-connection point and in the imbalance between supply and demand over 30 min. There was superiority or inferiority from the viewpoint of control accuracy. Moreover, there were significant differences among the four cases in terms of the amount of the change and the cycle of power flow fluctuation at the grid-connection point that can be controlled. It is thus important to clarify how the composition and scale of the DGs in a microgrid and the characteristics of the control system affect the contribution of the microgrid to the power system.

Simulated Onshore-Fault Ride through of Offshore Wind Farms Connected through VSC HVDC

Effective Onshore-Fault Ride Through was demonstrated by simulation for a Fixed Speed Induction Generator (FSIG) offshore wind farm connected through a Voltage Source Converter HVDC link. When a terrestrial grid fault occurs, power through the onshore converter reduces and the DC link voltage increases. A control system was then used to block the offshore converter. The offshore AC network voltage was reduced to achieve rapid power rejection. Reactive power at the onshore converter was controlled to support the AC network voltage according to the GB Grid Code requirements. Two cases, a 200 ms terrestrial fault and a 50% retained voltage fault of duration 710 ms, at the grid connection point were studied. The simulation results show that power blocking at the offshore converter was effective and the DC link voltage was controlled.

新エネルギーの普及を目的としたマイクログリッドにおける分散型電源の需給制御

We report the operational results of distributed generators (DGs) in a microgrid and present the effects after photovoltaic power generation (PV) systems were incorporated into the microgrid for an electric power system. The microgrid was constructed at the Expo 2005 Aichi site as part of a demonstration promoted by NEDO. A solution is needed for problems in which instability in DGs that utilize natural energy such as solar light and wind force negatively influences existing electric power systems. Consequently, we developed an energy-control system and controlled the output of the DGs to reduce fluctuations at the grid-connection point caused by unstable PV-system output. Our microgrid consisted of DGs such as PV systems, fuel cells, and NaS batteries, and these DGs were controlled by an energy-control system. We tested and then verified the practical effectiveness of installing the microgrid as follows. We found that (1) 99.5% of the power imbalance in supply and demand over a period of 30 min was within a range of ±3% under normal operating conditions, (2) the microgrid contributed to load leveling, (3) the energy-control system smoothed the power-flow fluctuations of PV-system output at the grid connection point, and (4) installing microgrids should help reduce additional Load Frequency Control (LFC) capacity in the future. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 169(4): 1–8, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20872

How to Connect a Voltage Sag-Measuring Device: Phase to Phase or Phase to Neutral?

In contrast to most other power-quality phenomena, it requires several years of measurements to obtain a good insight of the voltage sag problems at a given grid connection point. In some countries, sag measurements at high- or medium-voltage level of several years are available. This paper analyzes how to calculate the sag characteristics at lower voltage levels if the sags measured at higher voltage levels are only characterized as the largest value of the voltage drop in each phase. Whether these values are based on phase-to-phase or phase-to-neutral voltages turns out to be an important parameter and is therefore analyzed in detail. Based on this analysis, this paper gives guidelines on how new sag meters should preferably be connected (measuring phase-to-phase or phase-to-neutral voltages).