Large-scale Wind Farm Integration Research Articles

Dynamic Operation Scenario Reactive Power Optimization Assessment With Large-Scale Wind Farm Integration

Abstract Dynamic reactive power optimization is important to ensure the security and economy for power system operation. With large-scale wind farm integration in power systems, dealing with the uncertainty of wind output power in dynamic reactive power optimization is a challenge. To address such issue, a Dynamic Operation Scenario (DOS) model is proposed considering the operation characteristic of the power system integrated with practical large-scale wind farm. Firstly, the optimal dynamic reactive power/voltage control variable vectors are obtained. Then, scenario reduction of the control variable vectors is performed. Finally, the proposed DOS model is verified with rationality and superiority.

Research of Wind Power Correlation With Three Different Data Types Based on Mixed Copula

With the increasing integration of large-scale wind farms, the stochastic characteristics of wind speed and the coupling relationship of geographically distributed wind farms become ineligible. Correlation investigation of wind farms based on copula theory is able to lay good foundation for further optimization and schedule in power systems. In this paper, three data types, including wind speed, calculated wind power, and actual wind power, are applied to explore the correlation of two geographically close wind farms. After graphical analysis and comparison of the time series, the structural and compositional characteristics of correlation by different data types are investigated based on mixed copula. Moreover, a two-stage filtration method is proposed to evaluate different types of copulas. Finally, taking into account the practical conditions after careful examination of actual wind power dataset, the practical conditions are applied to the calculated wind power dataset. Correlation research based on the adjusted calculated wind power dataset is further explored and revealed that it is more practical and prospective to provide reference for further power system operation with high penetration of wind farms.

Studies of Sub-Synchronous Oscillations in Large-Scale Wind Farm Integrated System

With the rapid development and construction of large-scale wind farms and grid-connected operation, the series compensation wind power AC transmission is gradually becoming the main way of power usage and improvement of wind power availability and grid stability, but the integration of wind farm will change the SSO (Sub-Synchronous oscillation) damping characteristics of synchronous generator system. Regarding the above SSO problem caused by integration of large-scale wind farms, this paper focusing on doubly fed induction generator (DFIG) based wind farms, aim to summarize the SSO mechanism in large-scale wind power integrated system with series compensation, which can be classified as three types: sub-synchronous control interaction (SSCI), sub-synchronous torsional interaction (SSTI), sub-synchronous resonance (SSR). Then, SSO modelling and analysis methods are categorized and compared by its applicable areas. Furthermore, this paper summarizes the suppression measures of actual SSO projects based on different control objectives. Finally, the research prospect on this field is explored.

Communication Architecture for Grid Integration of Cyber Physical Wind Energy Systems

As we move toward increasing the grid integration of large-scale wind farms (WFs), reliable monitoring, protection, and control are needed to ensure grid stability. WFs are considered to be large and complex cyber physical systems owing to coupling between the electric power system and information and communication technologies (ICT). In this study, we proposed a framework for a cyber physical wind energy system (CPWES), which consists of four layers: a WF power system layer, data acquisition and monitoring layer, communication network layer, and application layer. We performed detailed network modeling for the WF system, including the wind turbines, meteorological mast (met-mast), and substation based on IEC 61400-25 and IEC 61850 standards. Network parameters and configuration were based on a real WF (Korean Southwest offshore project). The simulation results of the end-to-end delay were obtained for different WF applications, and they were compared with the timing requirements of the IEC 1646 standard. The proposed architecture represents a reference model for WF systems, and it can be used to enable the design of future CPWESs.

Storage Sizing With Peak-Shaving Policy for Wind Farm Based on Cyclic Markov Chain Model

Due to the intermittent nature of wind power, large-scale wind farm integration creates technical challenges, such as increase of peak/valley net load difference and uncertainty of generation. Energy storage is essential in providing flexibility and ensuring system reliability. Storage sizing problem is widely studied for a given demand curve, and the needed storage capacity to achieve a certain level of peak-shaving performance is not analyzed. In this paper, a probabilistic model of storage sizing with peak-shaving policy optimization under required matching probability is established to minimize net cost considering time-variant energy price. Storage is used not only for reducing energy deficit and keeping generation reliable but also for energy shifting to obtain higher profit. A cyclic nonhomogeneous Markov chain (CNHMC) steady-state analysis method is proposed, serving as a more efficient way to test probability constraint than commonly used time-consuming sequential Monte-Carlo simulation. CNHMC is used in stored power modeling representing diurnal variation of wind power and load. Probability constraint is tested by obtained analytical expression of matching probability. Numerical test shows that reliable power supply is achieved with little profit sacrifice, peak/valley net load difference decreases with little increment on storage capacity, and the proposed solution method is fast and accurate.

Transient Stability Study of Power System with Large-Scale Wind Farm Integration

Large-scale wind power integration has become the current development trend of the power system. Large-scale wind power integration can change the original structure and characteristics of the system. Thus, it’s necessary to analyse the transient stability of power system which contains wind power, and to study the controlling strategy for improving the transient stability of power system. Based on EEAC, this paper studies the transient stability of the power system which contains wind power system theoretically, proposes the calculation method for accelerating area, decelerating area and margin, and illustrates the impact of wind power integration on the transient stability with power angle curve. Furthermore, this paper studies the modeling and simulation, and the experimental results prove the correctness of the theories.

Coordinated planning of large-scale wind farm integration system and regional transmission network considering static voltage stability constraints

The incoordination between centralized wind power development and transmission planning is the main cause of wind power curtailment. Moreover, engineering practice shows that voltage issue may trigger large-scale tripping-offline contingency of wind turbines. With the objective of optimum economy, a coordinated planning method of large-scale wind farm integration system and relevant regional transmission network was proposed considering static voltage stability in this paper. A practical criterion of static voltage stability was formulated as operational constraints in the proposed model to ensure the obtained scheme to be technically and economically optimal. Raster map and disjunctive inequalities were introduced to linearize the model such that the model can be solved by mixed integer linear programming solvers directly. Numerical simulations show the validity and rationality of the proposed coordinated planning method.

Toward Cost-Efficient and Reliable Unit Commitment Under Uncertainty

Large-scale integration of wind farms causes volatile bus net injections. Although these fluctuations are anticipated, their timing, magnitude and duration cannot be predicted accurately. In order to maintain the operational reliability of the system, this uncertainty must be adequately addressed at the day-ahead generation scheduling stage. The ad-hoc reserve rules incorporated in deterministic unit commitment formulations do not adequately account for this uncertainty. Scenario-based stochastic unit commitment formulations model this uncertainty more precisely, but require computationally demanding simulations. Interval and robust optimization techniques require less computing resources, but produce overly conservative and thus expensive generation schedules. This paper proposes a transmission-constrained unit commitment formulation that improves the performance of the interval unit commitment. The uncertainty is modeled using upper and lower bounds, as in the interval formulation, but inter-hour ramp requirements are based on net load scenarios. This improved interval formulation has been tested using the IEEE RTS-96 and compared with existing stochastic, interval and robust unit commitment techniques in terms of solution robustness and cost. These results show that the proposed method outperforms the existing interval technique both in terms of cost and computing time.

Coordinated planning of large-scale wind farm integration system and transmission network

Large-scale centralized exploitation of intermittent wind energy resources has become popular in many countries. However, as a result of the frequent occurrence of largescale wind curtailment, expansion of corresponding transmission projects has fallen behind the speed at which installed wind capacity can be developed. In this paper, a coordinated planning approach for a large-scale wind farm integration system and its related regional transmission network is proposed. A bilevel programming model is formulated with the objective of minimizing cost. To reach the global optimum of the bi-level model, this work proposes that the upper-level wind farm integration system planning problem needs to be solved jointly with the lower-level regional transmission planning problem. The bi-level model is expressed in terms of a linearized mathematical problem with equilibrium constraints (MPEC) by Karush-KuhnTucker conditions. It is then solved using mixed integer linear programming solvers. Numerical simulations are conducted to show the validity of the proposed coordinated planning method.

The study and application of a novel hybrid forecasting model – A case study of wind speed forecasting in China

Given the current increasingly serious energy crisis, the development and utilization of new energy resources are attracting increasing attention, and wind power is widely used among these renewable energy resources. However, the randomness of wind power can cause a series of problems in the power system. Furthermore, the integration of large-scale wind farms into the whole power grid can place a great burden on stability and security. Accurate wind speed forecasting would reduce the randomness of wind power, which could effectively alleviate the adverse effects on the power system. In this paper, a hybrid wind speed forecasting model is proposed with the hope of achieving better forecasting performance. Wavelet Packet Transform (WPT) was employed to decompose the wind speed series into several series with different frequencies. A Least Square Support Vector Machine (LSSVM), the parameters of which were tuned by a particle swarm optimization based on simulated annealing (PSOSA), was built to model those series. The optimal input form of the model was determined by Phase Space Reconstruction (PSR). To verify the effectiveness of the proposed model, the daily average wind speed series from four wind farms in Gansu Province, Northwest China, were used as a case study. The results of the simulation and Grey Relational Analysis indicate that the proposed model outperforms the comparison models, and the null hypothesis of the predicted series having the same mean of the real series was accepted.

Improvement on probabilistic small-signal stability of power system with large-scale wind farm integration

This paper studies probabilistic small-signal stability of power systems with wind farm integration, considering the stochastic uncertainty of system operating conditions. The distribution function of the real-part of system eigenvalue is computed by the method of probabilistic eigenvalue analysis. For improving probabilistic small-signal stability, PSS is adopted. A method for optimizing PSS based on participation factor and center frequency method is proposed. In order to evaluate the above proposed methods, the procedure is applied to a test system. The simulation results show that the stochastic variation of wind generation can induce a higher probability of system instability when compared with one that has no wind generation. With eigenvalues distributing in a wider range, it becomes difficult for PSS tuning. By applying the proposed optimized PSSs approach, probabilistic stability of system can be significantly improved.

Hybrid Secondary Voltage Control combined with Large-Scale Wind Farms and Synchronous Generators

For stable integration of large-scale wind farms, integration standards (Grid codes) have been proposed by the system operator. In particular, voltage control of large-scale wind farms is gradually becoming important because of the increasing size of individual wind farms. Among the various voltage control methods, Secondary Voltage Control (SVC) is a method that can control the reactive power reserve of a control area uniformly. This paper proposes hybrid SVC when a large-scale wind farm is integrated into the power grid. Using SVC, the burden of a wind turbine converter for generating reactive power can be reduced. To prove the effectiveness of the proposed strategy, a simulation study is carried out for the Jeju system. The proposed strategy can improve the voltage conditions and reactive power reserve with this hybrid SVC.

Optimization of Smoothing Control in Hybrid Wind and Battery Energy Storage System

In recent years, renewable energy technologies like wind power generation are developing very fast. However, due to the randomness and intermittency of wind, the integration of large-scale wind farm into grid will have negative impact on many aspects of the grid. An effective way to solve these problems is to install battery energy storage system (BESS) in a wind farm. In order to smooth the output power of wind farm, this paper established a mathematical model for solving optimization problems of hybrid wind and BESS system, solved it with modern interior point algorithm. Charge-discharge process of BESS is optimized using the result of wind power prediction. Sensitivity of constraints in the model is also analyzed and optimal design of output power and capacity of BESS can be obtained as a by-product.

Effect of Wind Farm on Transient and Small-Signal Stability Based on BPA

Large scale wind farm output variation always deteriorates the system stability. To study this problem, this paper builds the model of power system with the integration of large-scale wind farm based on BPA. The simulation results show that large oscillations of voltage and rotor angle of system will happen, when three-phase short circuit fault occurs on the main line for transmitting wind power. With wind farm output decreasing, the transient stability and small-signal stability of power system will be improved.

Transient stability risk assessment of power systems incorporating wind farms

Large-scale wind farm integration has brought several aspects of challenges to the transient stability of power systems. This paper focuses on the research of the transient stability of power systems incorporating with wind farms by utilizing risk assessment methods. The detailed model of double fed induction generator has been established. Wind penetration variation and multiple stochastic factors of power systems have been considered. The process of transient stability risk assessment based on the Monte Carlo method has been described and a comprehensive risk indicator has been proposed. An investigation has been conducted into an improved 10-generator 39-bus system with a wind farm incorporated to verify the validity and feasibility of the risk assessment method proposed.

Coordinated Control for Wind Turbine and VSC-HVDC Transmission to Enhance FRT Capability

The integration of large-scale offshore wind farms into electricity networks without causing stability hazards is one of the future challenges for interconnecting large offshore wind farms into national grids. This paper presents enhanced control strategies for offshore wind farm arrays interconnected via an HVDC link. To improve fault ride- through capabilities of the HVDC link and wind farms, a frequency controller is proposed and the third harmonic injection technique is applied as a means to improve the reliability of the offshore arrays. This new controller can easily coordinate the power flow from different sources (i.e. large-scale wind farms and conventional power stations). The power production from the offshore wind farms will depend upon the type of wind turbines installed. Thus, the performance of wind farms based on DFIG and FRC wint turbines is compared aiming to give a better understanding and to identify areas where control improvements can be introduced to optimise offshore wind power transmission.

A novel concept of offshore wind-power collection and transmission system based on cascaded converter topology

SUMMARY This paper presents a new concept for the grid integration of large-scale offshore wind farms. In the offshore wind farm, the wind turbines are integrated into the offshore DC collector grid by a cascaded converter. The high-voltage level for the high-voltage direct current (HVDC) transmission purpose is built by series connections of low voltage modules, without the use of an offshore step-up substation. The output power of each module is controlled by the switching of the related half-bridge, and a modified straightforward voltage balancing strategy is proposed to balance the storage capacitor voltages in the turbines. Supplementary balancing strategies are also presented to assist in balancing the capacitor voltages, when the differences in the wind velocities flowing to the turbines become larger. Because of the stable capacitor voltages, self-commutated voltage source converter (VSC) is adopted as the onshore inverter station connecting the HVDC link and the onshore AC grid. The VSC station not only regulates the DC-bus voltage of the HVDC link, but also provides reactive power compensation to the onshore grid. The time-domain simulation results show that the proposed offshore wind-power collection and transmission system provides a new option for integrating large-scale offshore wind farms. Copyright © 2012 John Wiley & Sons, Ltd.

Equivalencing of Large Wind Farms: Modelling, Simplification and Verification

This paper presents a technique for developing an equivalent model for a large-scale wind farms that captures the important electrical transients of interest that arise during power system disturbances. The layout of the wind farm, the size and type of conductors used, and the diversity in wind speed all influence the performance of the equivalent model during disturbances. The grouping algorithm focuses on the transient interaction of individual wind turbines and represents the detailed model by a multi-machine equivalent. The equivalencing procedure is illustrated through case studies that are modelled and verified using DIgSILENT PowerFactory simulations. A good comparison is achieved for all simulation scenarios while comparing the equivalent and detailed models. Equivalent models derived on the basis of this proposed approach reduce the computation effort without significant loss in accuracy, which will be especially useful while assessing large-scale wind farm integration.