Grid-connected Wind Farm Research Articles

Overcurrent protection scheme for collector lines in wind farm based on fault component current correlation analysis and multi‐agent system

Collector line relay failures of large-scale grid-connected wind farms have not received much attention so far. In this study, the adaptability of conventional overcurrent setting method is analysed when used in doubly fed induction generator-based wind farms, revealing the coordination difficulties between collector line relay and fuse protection of wind turbine (WT) branch as well as the potential maloperation causes of backup overcurrent relays (BOCRs). Then, a novel three-stage setting method of overcurrent relays is proposed. In view of time-bound instantaneous overcurrent relays, a time delay setting approach is put forward considering the time–current characteristic of fuses. Based on the multi-agent system, which is both flexible and effective, comprehensive criteria containing fault component current correlation analysis and differential current criterion are applied to BOCRs. The proposed scheme is validated by the simulation of 46-WT wind farm on DIgSILENT/Powerfactory platform. Simulation results show that the relay is able to trip within 0.32 s and the BOCRs perform well when confronting single fault or even multiple faults.

Torsional subsynchronous oscillations caused by grid-connected wind farms in a complex multi-machine power system under the condition of near strong modal resonance

Dynamic interactions introduced by grid-connected wind farms are multivariable. This is why wind farms’ models need to be simplified when the impedance-model-based analysis is applied to examine whether negative resistance is supplied by the wind farms to cause subsynchronous oscillations (SSOs). In this paper, we examine and reveal the mechanism of torsional SSOs caused by grid-connected wind farms from the standpoint of near strong modal resonance (NESMOR). The examination and revelation consider fully the multivariable dynamic interactions introduced by wind farms in a complex multi-machine power system without simplifying the models of wind farms. We carry out the analysis based on the multi-input multi-output (MIMO) closed-loop model with two interconnected subsystems. The analysis indicates that the open-loop modal proximity (OLMP) of interconnected subsystems is a special case of NESMOR. When the NESMOR happens, growing torsional SSOs may be excited by the grid-connected wind farms. Hence, the analysis unveils the connection between the modal condition of OLMP and NESMOR, shedding a deeper insight into the mechanism of torsional SSOs caused by wind farms. In addition, we propose a method to detect the NESMOR. An example multi-machine power system with two grid-connected wind farms is presented to demonstrate the analysis and method.

Dynamic Aggregation of Same Wind Turbine Generators in Parallel Connection for Studying Oscillation Stability of a Wind Farm

This paper examines the dynamic aggregation of a group of M same wind turbine generators (WTGs) in a parallel connection. The group of WTGs could either be a part of a grid-connected wind farm or constitute the entire wind farm. Analytical derivation in this paper indicates that the group of M same WTGs is equivalent to M dynamically independent subsystems. Each of the subsystems is comprised of a single WTG. Hence, the dynamic aggregation of a grid-connected group of same WTGs in a parallel connection can be represented by grid-connected single-WTG subsystems for studying the oscillation stability. Further analysis in this paper reveals that when the number of WTGs in a parallel connection increases, it is possible that oscillation modes of the group of WTGs may move into the right half of a complex plane. Thus, the dynamic coupling of the increasing number of WTGs in a parallel connection may likely cause growing oscillations in the wind farm. An example grid-connected DFIG wind farm is used to demonstrate and evaluate the analysis and conclusions made in this paper. The oscillation stability of the example wind farm was examined when the dynamic models of a group of DFIGs in a parallel connection in the example wind farm were identical and different.

Improved Control of Grid-connected DFIG-based Wind Turbine using Proportional-Resonant Regulators during Unbalanced Grid

The quality of power and current control are the greatest challenges of grid-connected wind farms during abnormal conditions. The negative- and positive-sequence components of the grid currents may be injected into a wind generation system during grid faults, which can affect the power stability and damage the wind system. The proposed work assures a low-voltage ride through capability of doubly-fed induction generator- based wind turbines under the grid voltage sag. A new technique to protect the wind system and to recompense the reactive power during failures of the utility grid according to the Spanish grid code is proposed. The control design is implemented to the power converters, and the grid current regulation is developed by using proportional-resonant regulators in a stationary two-phase (αβ) reference frame. The control performance is significantly validated by applying the real-time simulation for the rotor-side converter and the hardware in the loop simulation technique for the experiment of the generator’s grid-side converter control.

Small-Signal Stability Limit of a Grid-Connected PMSG Wind Farm Dominated by the Dynamics of PLLs

This paper presents a derivation of the small-signal stability limit of a grid-connected wind farm with M permanent magnet synchronous generators (PMSGs) as determined by the dynamics of phased locked loops (PLLs). The stability limit reveals the mechanism whereby the PLLs can destabilize the wind farm in the case of a weak grid connection. Computation of the derived stability limit is simple and obviates the need to establish a parametric model of the wind farm. The stability limit can approximately indicate the potential instability risk collectively posed by the PLLs in the wind farm. Case studies are presented to evaluate the analytical conclusions obtained and the derived small-signal stability limit. Application of the stability limit during the planning and operating stages of a grid-connected wind farm is also demonstrated. Various factors that can affect the stability of the wind farm, and that must be considered during planning and operation, are examined and useful conclusions are obtained. These factors include the length of the transmission line connecting the wind farm to the AC grid, the topology of the wind farm, the wind speed, and its distribution over the wind farm, as well as the parameters of the PLLs.

Balancing wind-power fluctuation via onsite storage under uncertainty: Power-to-hydrogen-to-power versus lithium battery

Imbalance costs caused by forecasting errors are considerable for grid-connected wind farms. In order to reduce such costs, two onsite storage technologies, i.e., power-to-hydrogen-to-power and lithium battery, are investigated considering 14 uncertain technological and economic parameters. Probability density distributions of wind forecasting errors and power level are first considered to quantify the imbalance and excess wind power. Then, robust optimal sizing of the onsite storage is performed under uncertainty to maximize wind-farm profit (the net present value). Global sensitivity analysis is further carried out for parameters prioritization to highlight the key influential parameters. The results show that the profit of power-to-hydrogen-to-power case is sensitive to the hydrogen price, wind forecasting accuracy and hydrogen storage price. When hydrogen price ranges in (2, 6) €/kg, installing only electrolyzer can earn profits over 100 k€/MWWP in 9% scenarios with capacity below 250 kW/MWWP, under high hydrogen price (over 4 €/kg); while installing only fuel cell can achieve such high profits only in 1.3% scenarios with capacity below 180 kW/MWWP. Installing both electrolyzer and fuel cell (only suggested in 22% scenarios) results in profits below 160 k€/MWWP, and particularly 20% scenarios allow for a profit below 50 k€/MWWP due to the contradictory effects of wind forecasting error, hydrogen and electricity price. For lithium battery, investment cost is the single highly influential factor, which should be reduced to 760 €/kWh. The battery capacity is limited to 88 kW h/MWWP. For profits over 100 k€/MWWP (in 3% scenarios), the battery should be with an investment cost below 510 €/kWh and a depth of discharge over 63%. The power-to-hydrogen-to-power case is more advantageous in terms of profitability, reliability and utilization factor (full-load operating hours), while lithium battery is more helpful to reduce the lost wind and has less environmental impact considering current hydrogen market.

Study on influence of various wind farm correlations on voltage stability with multidimensional data streams method

ABSTRACTRecently, the size of wind farms has been increasing. Large-scale grid-connection of wind farms has a great influence on power system voltage stability. Most of the current research methods consider the correlation of only a single variable, which is not accurate enough for the description of actual production, where there are many variables. This paper presents a multidimensional data stream method to analyze correlations. This method can analyze the correlation degree between wind speed, fan fault, and other variables. It needs only real-time data collected by wind farm, and does not need to construct complicated correlation functions. It overcomes the disadvantage of the state model of traditional wind farms which is not accurate enough, and brings large errors to the analysis results. The method is totally based on measured data so that it is relatively quick and has the potential to be implemented in real-world applications. Then, this method is used to study the influence of correlations on power system voltage stability. A case study based on the standard IEEE 24-bus power system validates the proposed method. Under various working conditions, the numerical simulation results also show that the proposed method has good robustness and applicability.

Impedance Modeling and Stability Analysis of Grid-Connected DFIG-Based Wind Farm With a VSC-HVDC

A new type of subsynchronous oscillation (SSO) has been observed recently in double-fed induction generator (DFIG)-based wind farm integrated via voltage source converter-based HVdc (VSC-HVdc) system. However, the mechanism of this emerging oscillation is not entirely understood. In this paper, the impedance models of DFIG with and without considering the phase-locked loop (PLL) dynamics are both derived. Then, the impedance-based simplified equivalent circuit of the multiple DFIGs interfaced with VSC-HVdc system is established. This model can be further represented as the RLC series resonance circuit to quantify the start-oscillating condition intuitively. The theoretical analysis results show that DFIGs behave as an inductance in series with a negative resistance at the resonance point, whose interaction with wind farm side VSC (WFVSC) (regard as a resistance–capacitance) constitutes an equivalent RLC resonance circuit with negative resistance. Therefore, the oscillation tends to occur due to the negative damping. In addition, the impact of various factors including number of grid-connected DFIG-wind turbines (WTs), wind speed, and parameters of PI controllers and PLL on the SSO characteristics is analyzed based on the proposed simplified model. Finally, the correctness of the theoretical analysis is validated by both the time-domain simulation and hardware-in-loop experiments.

Impact of Power Grid Strength and PLL Parameters on Stability of Grid-Connected DFIG Wind Farm

This paper investigates the impact of powergrid strength and phase-locked loop (PLL) parameters on small signal stability of grid-connected doubly fed induction generator (DFIG)based wind farm. Modal analysis of the grid-connected DFIG wind turbine under different operating conditions and various power grid strengths are investigated at first. Modal analysis results reveal that the DFIG connected to a weak grid may easily lose stability under the heavy-duty operating conditions due to PLL oscillation. The object of this paper is to identify the PLL oscillation mechanism as well as influence factors and propose a damping solution for this oscillation mode. A simplified linear system model of the grid-connected DFIG wind turbine is proposed for analyzing the PLL oscillation. Through the complex torque coefficients method and using this model, the oscillation mechanism and influence factors including the power grid strength and the PLL parameters are identified. To suppress this PLL oscillation, a mixed H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> robust damping controller is proposed and designed for the DFIG. Electromagnetic transient simulation results of both singleDFIG system and multiply-DFIG system verify the correctness of the analysis results and effectiveness of the proposed damping controller.

Accurate aggregated modelling of wind farm systems in modified sequence domain for stability analysis

With the wind power penetration rate continuously rising, a suitable wind farm model considering both accuracy and simplicity is required for power system stability analysis. To accomplish this goal, an accurate aggregated modified sequence impedance model of the wind farm is established in this paper. The modified sequence impedance model of wind turbine generator is established, which considers the coupling between the positive- and negative-sequence impedances. The modified sequence impedances of the ac collection system, transformers, and ac grid are also derived. In this way, the impedance network of the entire wind farm system can be formed and simplified to acquire the accurate aggregated modified sequence impedance. Furthermore, the proposed model is compared with the traditional single-machine aggregated model which shows the necessity of establishing the accurate aggregated model. It is worth noting that the differences among each wind turbine generator’s operation points can be included in the proposed model. Compared with the results of impedance measurements of a target wind farm, the accuracy of the proposed aggregated sequence impedance model of the wind farm is verified in frequency domain. The application of the proposed model in the stability analysis of a grid-connected wind farm is also presented with consideration of the ac grid strength. What is more, the effect of the inertial controller on the wind farm system stability is also analyzed to make the impact of this study much more appealing and up to date.

Stability Analysis and Optimization of Grid Connected Wind Farm

With the rapid growth of economy, the demand for electricity is increasing. For the purpose of environmental protection, the development of new energy has become an important way. Wind power generation is an important part of the development of new energy power, wind power development has occupied a significant percentage of the grid. This requires full study of the connecting technology of large-scale wind farm to the power grid to ensure the safe and stable operation of the grid. This paper mainly studies the stability and optimization of grid-connected wind farm operation, hoping to help the relevant people.

Capacity Optimization Based on Energy Storage to Restrain Severe Fluctuation of Wind Power

In order to solve the coordination problem between the economy and the stabilization effect of energy storage system, a capacity optimization model based on energy storage to suppress the violent fluctuation of wind power is proposed, and a multi-objective function with the maximum wind power dissipation capacity and the minimum operating cost of energy storage system is established. Considering the average annual cost and penalty cost of the whole life cycle, an evaluation index with correlation coefficient as the fitting degree is proposed, and the reference power of wind farm grid connection is optimized by particle swarm algorithm. Using the operation data of Qidong Wind Farm in Jiangsu Province, the theoretical validity is verified, the coordination is improved to the greatest extent, and the capacity demand for energy storage system is reduced.

A Method of Reduced-Order Modal Computation for Planning Grid Connection of a Large-Scale Wind Farm

For planning the grid connection of a wind farm, small-signal stability needs to be examined to ensure the stability of wind farm as a standalone system. However, for a large-scale wind farm with m wind turbine generators (WTGs), the dimension of state matrix is nm if on average the order of dynamic models of the WTGs is n. Thus, when the number of the WTGs in the wind farm is over several hundred or more, high-dimensional numerical difficulty of modal computation may occur. Therefore, in this paper, we propose a method of reduced-order modal computation for the application in planning the grid connection of the large-scale wind farm. Dimensions of matrices involved in the reduced-order modal computation are m and n such that the numerical difficulty of modal computation is effectively avoided. In the paper, the effectiveness of proposed method of reduced-order modal computation is demonstrated and evaluated by an illustrative simple example wind farm and an example large-scale wind farm with 200 WTGs.

Harmonic resonance overvoltage due to main transformer energization in large wind farms, Zafarana, Egypt

Abstract When a large power transformer is energized, a high magnitude inrush current may result in harmonic resonance overvoltage (HRO), if the system has resonance phenomenon. Such overvoltages may cause large stresses on the system. This paper investigates the energization of two winding 220/22 kV main power transformer in a weak system of a grid-connected wind farm. To achieve this aim, the produced HRO during the energization of the main power transformers and the affecting factors are investigated. In addition, the effect of the sympathetic inrush phenomena is studied. Finally, the sequential energization of the three-phase transformer and the pre-insertion resistor method are discussed as mitigation methods. The investigated scenarios and results will contribute in developing the power system restoration procedures for existent wind farm, its extension, and future-planned wind farms.

Small-Signal Instability of PMSG Wind Farm Caused by the PLL Under the Condition of Near Strong Open-Loop Modal Resonance

This paper investigates the small-signal instability risk brought about by a phase locked loop (PLL) in a grid-connected PMSG wind farm. The investigation is based on a closed-loop interconnected model, wherein, the PLL and the remainder of power system (ROPS) are modeled as two separate open-loop subsystems. Analysis indicates that the closeness of two open-loop oscillation modes separately from the PLL and ROPS subsystems on the complex plane is the modal condition of near strong open-loop modal resonance (NESMOR). When the bandwidth of a dynamic component of the wind farm is close to the bandwidth of the PLL, it is possible that the NESMOR occurs between the dynamic component and the PLL. Under the condition of NESMOR, dynamic interactions introduced by the PLL degrade the small-signal stability of the grid-connected wind farm. Under the condition of weaker grid connection, the impact of NESMOR becomes stronger to lead to more degradation of system small-signal stability. Study cases are presented to demonstrate and validate the analysis and conclusions made in the paper.

Cooperative Rotor-Side SMES and Transient Control for Improving the LVRT Capability of Grid-Connected DFIG-Based Wind Farm

This paper presents a novel scheme to enhance low voltage ride through (LVRT) capacity of a 5 × 1.5 MW doubly-fed-induction-generator-based (DFIG-based) wind farm. One superconducting magnetic energy storage (SMES) unit is introduced to connect in parallel with the rotor side of each DFIG. Two LVRT strategies of inductance emulating control and transient voltage control are used in the SMES energy storage side converter and rotor side converter, respectively. Comprehensive simulations and performance evaluations are carried out in both the symmetrical and asymmetrical grid fault cases. The results show that the proposed scheme can not only protect the key parameters of each DFIG unit, but also enhance the grid voltage effectively under different fault initial angles.

Research on nodal wind power values and optimal accommodation based on locational marginal price

Under the current grid operation mode, reasonable integration of wind power is conducive to the exertion of its value. In this paper, the line flow transmission margin value considering wind power integration is assimilated into the locational marginal price (LMP). Accordingly, the LMP includes four components: energy consumption, network loss, congestion and line flow transmission margin. Based on the LMP difference of load nodes before and after wind power integration and the idea of power flow tracing, the specific values of different grid-connected wind farms at each node are evaluated, namely, the energy value and the network value of wind power. The load nodes that have a critical impact on wind power accommodation for each wind farm are identified. Moreover, the reserve cost caused by the uncertainty of wind power is considered, in which the maximum total value of all wind farms is set as the optimal objective. Finally, this paper analyses the LMP, value and accommodation of a single wind farm in detail through a microgrid system and an IEEE 118-bus system with multiple wind farms to verify the proposed method.

Concept of Modal Repulsion for Examining the Subsynchronous Oscillations Caused by Wind Farms in Power Systems

This paper investigates the torsional subsynchronous oscillations (SSOs) of synchronous generators caused by the grid-connected wind farms. The investigation does not rely on the representation of a wind farm by an aggregated wind generator and is based on a multi-input and multi-output closed-loop model. The model is comprised of the wind farm subsystem, which includes all the wind farms, and the subsystem of rest of power system (ROPS). Theoretical analysis proves that the open-loop SSO modal coupling of wind farm subsystem and ROPS subsystem results in the closed-loop SSO modal repulsion on the complex plane. Subsequently, damping of torsional SSOs decreases and growing SSOs may occur when the closed-loop modal repulsion is strong. Hence, closed-loop modal repulsion explains and reveals the mechanism about why the wind farms may cause the torsional SSOs in the power system. In addition, a method of open-loop modal analysis is proposed to detect the SSO instability risk brought about by the wind farms. The detection is based on the estimation of degree of closed-loop modal repulsion and only needs the modal information of open-loop subsystems. An example power system with wind farms is presented to demonstrate analysis and conclusions made in the paper.

Theoretical Study on Control Strategy of Grid-Connected High Voltage Ride Through in Doubly-Fed Wind Farm

Aiming at the problem of high voltage disconnection when doubly-fed wind farms are merged into weak power grids, this paper researches the theory of rotor excitation control and proposed an additional control model of DFIG grid-connected rotor flux based on structure decentralization theory. In this model, the high voltage ride-through capability for grid-connected wind farms is enhanced by designing the d-axis and q-axis adaptive terminal sliding mode controllers of the synchronous rotating coordinate system. Using the simulation software of MATLAB/Simulink, this paper established a simulation model of wind farm high voltage ride through composed of 2MW doubly fed wind turbines and carried out the off-line simulation of the whole process of high voltage traversing. Real-time simulation experiment of high voltage ride-through was also conducted on a self-developed real-time simulation platform for grid-connected doubly-fed wind farms. Real-time simulation results prove the accuracy of theoretical and offline simulation analysis and the feasibility of control strategy.

Study on Voltage Stability and Control Strategy of Grid-Connected Wind Farm

Aiming at the Low voltage ride through capability of doubly-fed wind farm when it is merged into a weak grid, this paper researches the theory of rotor excitation control and proposed an additional control model of DFIG grid-connected rotor flux based on structure decentralization theory. In this model, the Low voltage ride through capability of the doubly-fed wind farm is enhanced by designing the d-axis and q-axis adaptive terminal sliding mode controllers for the synchronous rotating coordinate system. Simulation results show that the proposed control model not only can better realize the rapid regulation of reactive power of doubly-fed wind farms, but also improve the ability of the system to resist grid voltage fluctuations, and improve the transient stability of power systems. It has certain theoretical significance and practical application value.