Abstract:This study analyzes the stability and reactive characteristics of the hybrid offshore wind farm that includes grid-forming (GFM) and grid-following (GFL) wind turbines (WTs) integrated with a diode rectifier unit (DRU) based high-voltage direct current (HVDC) system. The determination method for the proportion of GFM WTs is proposed while considering system stability and optimal offshore reactive power constraints. First, the small-signal stability is studied based on the developed linear model, and crucial factors that affect the stability are captured by eigenvalue analysis. The reactive power-frequency compensation control of GFM WTs is then proposed to improve the reactive power and frequency dynamics. Second, the relationship between offshore reactive power imbalance and the effectiveness of GFM capability is analyzed. Offshore reactive power optimization methods are next proposed to diminish offshore reactive load. These methods include the optimal design for the reactive capacity of the AC filter and the reactive power compensation control of GFL WTs. Third, in terms of stability and optimal offshore reactive power constraints, the principle and calculation method for determining the proportion of GFM WTs are proposed, and the critical proportion of GFM WTs is determined over the full active power range. Finally, case studies using a detailed model are conducted by time-domain simulations in PSCAD/EMTDC. The simulations verify the theoretical analysis results and the effectiveness of the proposed determination method for the proportion of GFM WTs and reactive power optimization methods.

Abstract:With good adaptability to weak power grids, the grid-forming inverter becomes the foundation of future power grids with high-proportion renewable energy. Moreover, the virtual synchronous generator (VSG) control is recognized as the mainstream control strategy for grid-forming inverters. For permanent magnet synchronous generator (PMSG) based wind generation systems connected to power grid via VSG-controlled grid-forming inverters, some novel impacts on the low-frequency oscillations (LFOs) emerge in power grids. The first impact involves the negative/positive damping effect on LFOs. In this paper, the small-signal torque model of VSG-controlled PMSG-based wind generation systems is established based on the damping torque analysis method, revealing the influence mechanism of machine-side dynamics on LFOs and proving the necessity of the double-mass model for accurate stability analysis. The second impact is the resonance effect between torsional oscillation and LFOs. Subsequently, this paper uses the open-loop resonance analysis method to study the resonance mechanism and to predict the root trajectory. Then, a damping enhancement strategy is proposed to weaken and eliminate the negative damping effect of machine-side dynamics on LFOs and the resonance effect between torsional oscillation and LFOs. Finally, the analysis result is validated through a case study involving the connection of the VSG-controlled PMSG-based wind generation system to the IEEE 39-bus AC grid, supporting the industrial application and stable operation of VSG-controlled PMSG-based wind generation systems.

Abstract:The transient synchronization characteristics and instability mechanism of the permanent magnet synchronous generator (PMSG)-based grid-forming wind energy conversion system (GFM-WECS) under symmetrical grid fault have received little attention to date. In this paper, considering the dynamics of DC-link voltage, the transient stability and an improved control strategy of PMSG-based GFM-WECS are studied in detail. Firstly, considering the dynamic interactions between the machine-side converter and the grid-side converter, the large-signal equivalent model of GFM-WECS is established. Furthermore, a novel Lyapunov function is derived to evaluate the transient stability margin and instability boundary of GFM-WECS during grid voltage sag. Additionally, the impacts of current-limitation control on the transient stability of GFM-WECS are revealed. Then, a stability evaluation index is proposed to evaluate the transient stability margin of GFM-WECS. Moreover, an improved control strategy is proposed to enhance the transient response characteristics and low voltage ride-through (LVRT) capability of GFM-WECS under symmetrical grid fault. Finally, simulations and experimental results are conducted to verify the effectiveness of the proposed control strategy.

Abstract:Conventional offshore wind farm (OWF) integration systems typically employ AC cables to gather power to a modular multilevel converter (MMC) platform, subsequently delivering it to onshore grids through high-voltage direct current (HVDC) transmission. However, scaling up the capacity of OWFs introduces significant challenges due to the high costs associated with AC collection cables and offshore MMC platforms. This paper proposes a diode rectifier (DR)-MMC hub based hybrid AC/DC collection and HVDC transmission system for large-scale offshore wind farms. The wind farms in proximity to the offshore converter platform utilize AC collection, while distant wind farms connect to the platform using DC collection. The combined AC/DC power is then transmitted to the offshore DR-MMC hub platform. The topology and operation principle of the DR-MMC hub as well as the integration system are presented. Based on the operational characteristics, the capacity design method for DR-MMC hub is proposed. And the control and startup strategies of the integration system are designed. Furthermore, an economic comparison with the conventional MMC-HVDC based offshore wind power integration system is conducted. Finally, the technical feasibility of the proposed integration scheme is verified through PSCAD/EMTDC simulation with the integration scale of 2 GW.

Abstract:In this paper, a dynamic equivalent method applicable to the direct-drive permanent magnet synchronous generator (PMSG) based wind farms under asymmetrical faults is proposed. Firstly, PMSGs are clustered based on their different active power characteristics under asymmetrical faults. Further, single-machine equivalent models (SMEMs) are constructed for different clusters of PMSGs. In particular, an SMEM with multi-segmented slope recovery is introduced for PMSGs with ramp recovery characteristics. Further, a collector network equivalent method for wind farms applicable to both symmetrical and asymmetrical faults is presented. Moreover, an iterative simulation method is used to gain the required clustering indicators before the fault actually occurs. Eventually, the effectiveness of the proposed dynamic equivalent method is verified on a modified IEEE 39-bus system.

Abstract:The increasing penetration of renewable energy resources degrades the frequency stability of power systems. The present work addresses this issue by proposing a look-ahead dispatch model of power systems based on a linear alternating current optimal power flow framework with nonlinear frequency constraints. Meanwhile, the poor efficiency for solving this formulation is addressed by introducing a physics-informed neural network (PINN) to predict key frequency-control parameter values accurately. The PINN ensures that the learned results are applicable to the original physical frequency dynamics model, and applying the predicted parameter values enables the resulting dispatch model to be solved quickly and efficiently using readily available commercial solvers. The feasibility and advantages of the proposed model are demonstrated by the results of numerical computations applied to a modified IEEE 118-bus test system.

Abstract:Most permanent magnet synchronous generator (PMSG) based wind generation systems currently employ grid-following control, relying on a phase-locked loop (PLL) for grid connection. However, it leads to a lack of inertia support in the system. To address this, the virtual inertia control (VIC) is crucial for improvement, yet it introduces potential instability due to torsional oscillation interaction with PLL and low-frequency oscillations, which is an underexplored area. This paper presents a comprehensive analysis of the grid-connected PMSG-based wind generation system. It confirms the necessity of employing a full-order model for studying stability on the quasi-electromechanical timescale (QET) by a comparison with the reduced-order model. Then, a comprehensive modal analysis is conducted to analyze the effect of VIC parameters, shaft inertia time constant, PLL parameters, and torsional oscillation damping (TOD) controller gain on the interaction of QET oscillations under two typical control strategies. The occurrence of interaction and mode conversion is observed when the oscillation frequency and root loci of the torsional, PLL, and low-frequency oscillations are close. Finally, a theoretical analysis is validated via simulation verification in Simulink. These findings offer a valuable guidance for industrial PMSG applications considering VIC.

Abstract:The optimal torque (OT) method, which is preferred for its simplicity, is widely employed in maximum power point tracking (MPPT) control strategies for wind energy capture in wind turbine generators (WTGs). Based on the OT method, the decreased torque gain (DTG) method is developed to improve turbine acceleration through a reduction of the torque gain coefficient. However, the DTG method does not fully align with the acceleration performance required by wind turbines, which subsequently limits improvements in wind energy capture efficiency. To address these concerns, a novel MPPT control strategy is proposed, which introduces redefined torque curve and torque command conceptualized based on a higher-order function relative to rotor speed. Additionally, an adaptive algorithm for the periodic update of the torque command is suggested to better accommodate the variability of turbulent wind speeds, thus aiming to improve the wind energy capture efficiency. The effectiveness of the proposed MPPT control strategy is substantiated through the wind turbine simulator (WTS)-based experiments.

Abstract:As the penetration of intermittent renewable energy resources in microgrids (MGs) continues to grow globally, optimal operation management becomes increasingly crucial due to the variability of these sources. One potential solution to this challenge is the use of demand response (DR) programs, which are practical and relatively low-cost options. However, ensuring the security of MG operation also requires evaluating its flexibility by determining the acceptable boundaries of uncertain variables. Additionally, in real-world operational decision-making problems, there is a simultaneous optimization of multiple objectives, including the maximization of system flexibility and the minimization of system cost. This paper presents a methodology for developing a cost-aware flexibility evaluation method for MGs connected to the upstream grid, which are subject to volatile market prices. The model is based on the feasibility analysis of the uncertain space of wind power generation and load, and it also investigates the level of inflexibility present in the system. The impact of the DR program on the flexibility of MGs is quantified through a case study. The case study confirms the success of the proposed method and underscores the significance of cost modeling in flexibility evaluation problems.

Abstract:The power loss minimization and DC voltage stability of the multi-terminal direct current (MTDC) system with large-scale wind farm (WF) cluster affect the stability and power quality of the interconnected power grid. This paper proposes a distributed optimal voltage control (DOVC) strategy, which aims to optimize voltage distribution in MTDC and WF systems, reduce system power losses, and track power dispatch commands. The proposed DOVC strategy employs a bi-level distributed control architecture. At the upper level, the MTDC controller coordinates power flow, DC-side voltage of grid-side voltage source converters (GSVSCs), and WF-side voltage source converters (WFVSCs) for power loss minimization and DC voltage stabilization of the MTDC system. At the lower level, the WF controller coordinates the controlled bus voltage of WFVSC and the active and reactive power of wind turbines (WTs) to maintain WT terminal voltages within feasible range. Then, the WF controller minimizes the power loss of the WF system, while tracking the optimal command from the upper-level control strategy. Considering the computational tasks of multi-objective optimization with large-scale WF cluster, the proposed DOVC strategy is executed in a distributed manner based on the alternating direction method of multipliers (ADMM). An MTDC system with large-scale WF cluster is established in MATLAB to validate the effectiveness of the proposed DOVC strategy.

Abstract:Multiterminal low-frequency transmission system (LFTS) has promising potential for large-scale offshore wind power integration. Nevertheless, the existing protection suffers from low sensitivity, and even operates incorrectly because the converters connected to both ends of cables change fault characteristics substantially. To address this problem, this paper firstly inspects the adaptability of current differential protection, revealing the manner in which control strategies after fault impact the sensitivity of the existing protection. Then, based on the characteristics of armored three-core cable, phase-mode transformation is utilized to decouple the fault information and the specific moduli are selected to reflect all kinds of fault types. The expression of backward traveling-wave (BTW) voltage based on interpolation is derived under the condition of low sampling frequency. Finally, a pilot protection based on BTW voltage difference for submarine cables of LFTS with integrated offshore wind power is proposed, which has higher sensitivity because the difference between BTW calculated from local information and the one from remote information is considerable during fault transient period. Simulation tests compare the performance of the existing protection with that of the proposed protection. Extensive simulations corroborate that the proposed protection reliably identifies the fault cable in various fault scenarios.

Abstract:The development of low-carbon energy systems and renewable energy sources (RESs) are critical to solving the energy crisis around the world. However, renewable energy generation control strategies lead to fault characteristics such as fault current amplitude limitation and phase angle distortion. Focusing on large-scale renewable energy transmission lines, the sensitivity of traditional current differential protection and distance protection may be reduced, and there is even the risk of maloperation. Therefore, a suitable transmission line model is established, which considers the distributed capacitance. Afterward, a novel dynamic state estimation based protection (DSEBP) for large-scale renewable energy transmission lines is proposed. The proposed DSEBP adopts instantaneous measurements and additional protection criteria to ensure the quick action and reliability. Finally, faults are identified by checking the matching degree between the actual measurements and the established transmission line model. The performance of the proposed DSEBP is verified through PSCAD/EMTDC and real-time digital simulator (RTDS) hardware-in-loop tests. The results demonstrate that the proposed DSEBP can identify various types of faults quickly and reliably. Meanwhile, the proposed DSEBP has a better capability to withstand fault resistance and disturbance.

Abstract:The phenomenon of sub-synchronous oscillation (SSO) poses significant threats to the stability of power systems. The advent of artificial intelligence (AI) has revolutionized SSO research through data-driven methodologies, which necessitates a substantial collection of data for effective training, a requirement frequently unfulfilled in practical power systems due to limited data availability. To address the critical issue of data scarcity in training AI models, this paper proposes a novel transfer-learning-based (TL-based) Wasserstein generative adversarial network (WGAN) approach for synthetic data generation of SSO in wind farms. To improve the capability of WGAN to capture the bidirectional temporal features inherent in oscillation data, a bidirectional long short-term memory (BiLSTM) layer is introduced. Additionally, to address the training instability caused by few-shot learning scenarios, the discriminator is augmented with mini-batch discrimination (MBD) layers and gradient penalty (GP) terms. Finally, TL is leveraged to fine-tune the model, effectively bridging the gap between the training data and real-world system data. To evaluate the quality of the synthetic data, two indexes are proposed based on dynamic time warping (DTW) and frequency domain analysis, followed by a classification task. Case studies demonstrate the effectiveness of the proposed approach in swiftly generating a large volume of synthetic SSO data, thereby significantly mitigating the issue of data scarcity prevalent in SSO research.

Abstract:The increasing penetration of wind power poses challenges to the power grid operation and scheduling. Yet, if the uncertainty of wind power can be economically and effectively managed on the source side, it can drive the power grids towards renewable-dominant future. In this paper, an enhanced scheduling strategy for wind farm-flexible load joint operation system (WF-FLJOS) is proposed. The proposed strategy is designed to manage the uncertainty of wind power on the generation side when integrated into a large-scale power grid. Moreover, it can contribute to saving energy costs on the load side. Compared with the current wind farm operation rules, more stringent assessment requirements are put forward for wind power output accuracy, and the internal organization framework of WF-FLJOS is designed. For potential power violations of wind farms and flexible loads, the violation penalty mechanisms are developed to regulate the behavior of the participants. The joint operation model of the WF-FLJOS is proposed and the submission and tracking approach of the generation schedule for the wind farm is investigated. Numerical results indicate that the proposed strategy can not only improve the ability of the wind farm to track the generation schedule, but also consider the benefits of both the farm side and the load side. Meanwhile, the proposed strategy effectively reduces the schedule adjustment pressure on the main grid caused by the rolling correction mode of the intraday schedule for wind farms.

Abstract:Adequacy is a key concern of power system planning, which refers to the availability of sufficient facilities to meet demand. The capacity value (CV) of variable renewable energy (VRE) generation represents its equivalent contribution to system adequacy, in comparison to conventional generators. While VRE continues to grow and increasingly dominates the generation portfolio, its CV is becoming non-negligible, with the corresponding impact mechanisms becoming more complicated and nuanced. In this paper, the concept of CV is revisited by analyzing how VRE contributes to power system balancing at a high renewable energy penetration level. A generalized loss function is incorporated into the CV evaluation framework considering the adequacy of the power system. An analytical method for the CV evaluation of VRE is then derived using the statistical properties of both hourly load and VRE generation. Through the explicit CV expression, several critical impact factors, including the VRE generation variance, source-load correlation, and system adequacy level, are identified and discussed. Case studies demonstrate the accuracy and effectiveness of the proposed method in comparison to the traditional capacity factor-based methods and convolution-based methods. In the IEEE-RTS79 test system, the CV of a 2500 MW wind farm (with 40% renewable energy penetration level) is found to be 6.8% of its nameplate capacity. Additionally, the sensitivity of CV to various impact factors in power systems with high renewable energy penetration is analyzed.

Abstract:The reduced frequency regulation capability in low-inertia power systems necessitates enhanced frequency support from photovoltaic (PV) systems. However, the regulation capability of PV system under conventional control scheme is limited, which requires flexible power control and support from battery energy storage systems (BESSs). This paper proposes an energy management strategy of PV-BESS to provide stable frequency support to the grid. The proposed strategy initially develops a maximum power point tracking (MPPT)-based power reserve control (PRC) for PV power reserve. At this stage, the BESS is manipulated to buffer the power fluctuations caused by MPPT execution and environmental variability. To enhance the regulation capability of BESS and mitigate battery aging, the proposed strategy incorporates a flexible PRC for state of charge (SOC) recovery. Subsequently, the battery can be rationally charged or discharged during the PRC process to maintain SOC. Simulation results obtained from MATLAB/Simulink and experimental results from the StarSim platform validate that the proposed strategy achieves stable PV power reserve, mitigates fluctuations induced by environmental uncertainties, enables efficient SOC recovery, and improves grid frequency quality. Overall, the proposed strategy ensures stable operation of PV-BESS both under steady-state conditions and during frequency events.

Abstract:The integrated energy cyber-physical system (IECPS), a typical cyber-physical system (CPS), demonstrates tight interaction between cyber and physical spaces across time and space, exhibiting inherent spatial-temporal properties. Effective IECPS modeling requires the simultaneous consideration of both temporal and spatial properties, which remains a significant challenge. This paper proposes a hierarchical spatial-temporal event modeling method for IECPS based on hybrid automata (HA). Different event model layers are defined, representing events as functions of attributes, time, and space, with attributes characterized by system states. An integration method for multi-layer events is introduced, enabling accurate reflection of the system spatial-temporal characteristics and the current operating state of energy units. The modeling is applied to the optimal regulation of IECPS. Numerical simulations demonstrate that the proposed HA-based modeling method achieves precise regulation of IECPS while reducing system operating costs.

Abstract:Wind power prediction interval (WPPI) models in the literature have predominantly been developed for and tested on specific case studies. However, wind behavior and characteristics can vary significantly across regions. Thus, a prediction model that performs well in one case might underperform in another. To address this shortcoming, this paper proposes an ensemble WPPI framework that integrates multiple WPPI models with distinct characteristics to improve robustness. Another important and often overlooked factor is the role of probabilistic wind power prediction (WPP) in quantifying wind power uncertainty, which should be handled by operating reserve. Operating reserve in WPPI frameworks enhances the efficacy of WPP. In this regard, the proposed framework employs a novel bi-layer optimization approach that takes both WPPI quality and reserve requirements into account. Comprehensive analysis with different real-world datasets and various benchmark models validates the quality of the obtained WPPIs while resulting in more optimal reserve requirements.

Abstract:Frequency regulation of voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) system with offshore wind farms enhances the frequency stability by compensating the power for a disturbed AC system. However, it is difficult to reasonably allocate frequency-regulation resources due to a lack of coordination mechanisms between wind farms and the MTDC system. Moreover, it is difficult for the frequency control of the wind farms to manage changes in wind speed; and the risk of wind-turbine stalls is high. Thus, based on the kinetic energy of wind turbines and the power margin of the converters, the frequency-regulation capability of wind turbines is evaluated, and a dynamic frequency-support scheme considering the real-time frequency-support capability of the wind turbines and system frequency evolution is proposed to improve the frequency-support performance. A power adaptation technique at variable wind speeds is developed; the active power in the frequency-support stage and restoration stage is switched according to the wind speed. A hierarchical zoning frequency-regulation scheme is designed to use the frequency-regulation resources of different links in the MTDC system with wind farms. The simulation results show that the novel frequency-regulation strategy maintains frequency stability with wind-speed changes and avoids multiple frequency dips.

Abstract:With the rapid development of renewable energy, wind-thermal-bundled power transmission by line-commutated converter based high-voltage direct current (LCC-HVDC) systems has been widely developed. The dynamic interaction mechanisms among permanent magnet synchronous generators (PMSGs), synchronous generators (SGs), and LCC-HVDC system become complex. To deal with this issue, a path analysis method (PAM) is proposed to study the dynamic interaction mechanism, and the damping reconstruction is used to analyze the damping characteristic of the system. First, based on the modular modeling, linearized models for the PMSG subsystem, the LCC-HVDC subsystem, and the SG subsystem are established. Second, based on the closed-loop transfer function diagram of the system, the disturbance transfer path and coupling relationship among subsystems are analyzed by the PAM, and the damping characteristic analysis of the SG-dominated oscillation mode is studied based on the damping reconstruction. Compared with the PAM, the small-signal model of the system is obtained and eigenvalue analysis results are presented. Then, the effect of the control parameters on the damping characteristic is analyzed and the conclusions are verified by time-domain simulations. Finally, the penalty functions of the oscillation modes and decay modes are taken as the objective function, and an optimization strategy based on the Monte Carlo method is proposed to solve the parameter optimization problem. Numerical simulation results are presented to validate the effectiveness of the proposed strategy.

Abstract:Wind energy systems (WESs) based on doubly-fed induction generators (DFIGs) have enormous potential for meeting the future demands related to clean energy. Due to the low inertia and intermittency of power injection, a WES is equipped with a virtual inertial controller (VIC) to support the system during a frequency deviation event. The frequency deviation measured by a phase locked loop (PLL) installed on a point of common coupling (PCC) bus is the input signal to the VIC. However, a VIC with an improper inertial gain could deteriorate the damping of the power system, which may lead to instability. To address this issue, a mathematical formulation for calculating the synchronizing and damping torque coefficients of a WES-integrated single-machine infinite bus (SMIB) system while considering PLL and VIC dynamics is proposed in this paper. In addition, a power system stabilizer (PSS) is designed for wind energy integrated power systems to enhance electromechanical oscillation damping. A small-signal stability assessment is performed using the infinite bus connected to a synchronous generator of higher-order dynamics integrated with a VIC-equipped WES. Finally, the performance and robustness of the proposed PSS is demonstrated through time-domain simulation in SMIB and nine-bus test systems integrated with WES under several case studies.

Abstract:To tackle the energy crisis and climate change, wind farms are being heavily invested in across the world. In China’s coastal areas, there are abundant wind resources and numerous offshore wind farms are being constructed. The secure operation of these wind farms may suffer from typhoons, and researchers have studied power system operation and resilience enhancement in typhoon scenarios. However, the intricate movement of a typhoon makes it challenging to evaluate its spatial-temporal impacts. Most published papers only consider predefined typhoon trajectories neglecting uncertainties. To address this challenge, this study proposes a stochastic unit commitment model that incorporates high-penetration offshore wind power generation in typhoon scenarios. It adopts a data-driven method to describe the uncertainties of typhoon trajectories and considers the realistic anti-typhoon mode in offshore wind farms. A two-stage stochastic unit commitment model is designed to enhance power system resilience in typhoon scenarios. We formulate the model into a mixed-integer linear programming problem and then solve it based on the computationally-efficient progressive hedging algorithm (PHA). Finally, numerical experiments validate the effectiveness of the proposed method.

Abstract:To avoid the secondary frequency dip caused by the steep drop of the electrical power of wind turbines (WTs) at the end of frequency support stage, the torque limit-based inertial control (TLIC) method sets the power reference as a linear function of rotor speed, rather than the step form for the stepwise inertial control. However, the compensation effect on the frequency nadir (FN) caused by the load surge is weakened as the TLIC power is no longer in the step form. Specifically, the maximum point of the frequency response component (FRC) contributed by TLIC occurs earlier than the minimum point of FRC corresponding to the load surge, so that the FN cannot be adequately raised. Therefore, this paper first investigates the relation between the peak and nadir times of FRCs stimulated by the TLIC and load power. On this basis, with the compensation principle of matching the peak and nadir times of FRCs, the improved TLIC method based on delayed support is proposed. Finally, the effectiveness of the proposed method is validated via the experiments on the test bench of wind-integrated power system.

Abstract:In recent years, sub-synchronous oscillation accidents caused by wind power integration have received extensive attention. The recorded constant-amplitude waveforms can be induced by either linear or nonlinear oscillation mechanisms. Hence, the nonlinear behavior needs to be distinguished prior to choosing the analysis method. Since the 1960s, the higher-order statistics (HOS) theory has become a powerful tool for the detection of nonlinear behavior (DNB) in production quality control wherein it has mainly been applied to mechanical condition monitoring and fault diagnosis. This study focuses on the hard limiters of the voltage source converter (VSC) control systems in the wind farms and attempts to detect the nonlinear behavior caused by bi- or uni-lateral saturation hard limiting using the HOS analysis. First, the conventional describing function is extended to obtain the detailed frequency domain information on the bi- and uni-lateral saturation hard limiting. Furthermore, the bi- and tri-spectra are introduced as the HOS, which are extended into bi- and tri-coherence spectra to eliminate the effects of the linear parts on the harmonic characteristics of hard limiting in the VSC control system, respectively. The effectiveness of the HOS in the DNB and the classification of the hard-limiting types is proven, and its detailed derivation and estimation procedure is presented. Finally, the quadratic and cubic phase coupling in the signals is illustrated, and the performance of the proposed method is evaluated and discussed.

Abstract:As the proportion of converter-interfaced renewable energy resources in the power system is increasing, the strength of the power grid at the connection point of wind turbine generators (WTGs) is gradually weakening. Existing research has shown that when connected with the weak grid, the stability of the traditional grid-following controlled converters will deteriorate, and unstable phenomena such as oscillation are prone to arise. Due to the limitations of linear analysis that cannot sufficiently capture the stability phenomena, transient stability must be investigated. So far, standalone time-domain simulations or analytical Lyapunov stability criteria have been used to investigate transient stability. However, the time-domain simulations have proven to be computationally too heavy, while analytical methods are difficult to formulate for larger systems, require many modelling assumptions, and are often conservative in estimating the stability boundary. This paper proposes and demonstrates an innovative approach to estimating the transient stability boundary via combining the linear Lyapunov function and the reverse-time trajectory technique. The proposed methodology eliminates the need of time-consuming simulations and the conservative nature of Lyapunov functions. This study brings out the clear distinction between the stability boundaries with different post-fault active current ramp rate controls. At the same time, it provides a new perspective on critical clearing time for wind turbine systems. The stability boundary is verified using time-domain simulation studies.

Abstract:With the increasing wind power penetration in the power system, the auxiliary frequency control (AFC) of wind farm (WF) has been widely used. The traditional system frequency response (SFR) model is not suitable for the wind power generation system due to its poor accuracy and applicability. In this paper, a piecewise reduced-order frequency response (P-ROFR) model is proposed, and an optimized auxiliary frequency control (O-AFC) scheme of WF based on the P-ROFR model is proposed. Firstly, a full-order frequency response model considering the change in operating point of wind turbine is established to improve the applicability. In order to simplify the full-order model, a P-ROFR model with second-order structure and high accuracy at each frequency response stage is proposed. Based on the proposed P-ROFR model, the relationship between the frequency response indexes and the auxiliary frequency controller coefficients is expressed explicitly. Then, an O-AFC scheme with the derived explicit expression as the optimization objective is proposed in order to improve the frequency support capability on the premise of ensuring the full release of the rotor kinetic energy and the full use of the effect of time delay on frequency regulation. Finally, the effectiveness of the proposed P-ROFR model and the performance of the proposed O-AFC scheme are verified by simulation studies.

Abstract:In recent years, with the growth of wind energy resources, the capability of wind farms to damp low-frequency oscillations (LFOs) has provided a notable advantage for the stability enhancement of the modern power grid. Meanwhile, owing to variations in the power system operating point (OP), the damping characteristics of LFOs may be affected adversely. In this respect, this paper presents a coordinated robust proportional-integral-derivative (PID) based damping control approach for permanent magnet synchronous generators (PMSGs) to effectively stabilize LFOs, while considering power system operational uncertainties in the form of a polytopic model constructed by linearizing the power system under a given set of OPs. The proposed approach works by modulating the DC-link voltage control loop of the grid-side converter (GSC) via a supplementary PID controller, which is synthesized by transforming the design problem into H-infinity static output feedback (SOF) control methodology. The solution of H-infinity SOF control problem involves satisfying linear matrix inequality (LMI) constraints based on the parameter-dependent Lyapunov function to ensure asymptotic stability such that the minimal H-infinity performance objective is simultaneously accomplished for the entire polytope. The coordinated damping controllers for the multiple wind farms are then designed sequentially by using the proposed approach. Eigenvalue analysis confirms the improved damping characteristics of the closed-loop system for several representative OPs. Afterward, the simulation results, including the performance comparison with existing approaches, validate the higher robustness of the proposed approach for a wide range of operating scenarios.

Abstract:The oscillation phenomena associated with the control of voltage source converters (VSCs) are concerning, making it crucial to locate the sources of such oscillations and suppress the oscillations. Therefore, this paper presents a location scheme based on the energy structure and nonlinearity detection. The energy structure, which conforms to the principle of the energy-based method and dissipativity theory, is developed to describe the transient energy flow for VSCs, based on which a defined characteristic quantity is implemented to narrow the scope for locating the sources of oscillations. Moreover, based on the self-sustained oscillation characteristics of VSCs, an index for nonlinearity detection is applied to locate the VSCs that produce the oscillation energy. The combination of the energy structure and nonlinearity detection distinguishes the contributions of different VSCs to the oscillation. The results of a case study implemented by the PSCAD/EMTDC simulation validate the proposed scheme.

Abstract:Wind-thermal-bundled system has emerged as the predominant type of power system, incorporating a significant proportion of renewable energy. The dynamic interaction mechanism of the system is complex, and the issue of oscillation stability is significant. In this paper, the damping characteristics of the direct current (DC) capacitance oscillation mode are analyzed using the path analysis method (PAM). This method combines the transfer-function block diagram with the damping torque analysis (DTA). Firstly, the linear models of the permanent magnet synchronous generator (PMSG), the synchronous generator (SG), and the alternating current (AC) grid are established based on the transfer functions. The closed-loop transfer-function block diagram of the wind-thermal-bundled systems is derived. Secondly, the block diagram reveals the damping path and the dynamic interaction mechanism of the system. According to the superposition principle, the transfer-function block diagram is reconstructed to achieve the damping separation. The damping coefficient of the DTA is used to quantify the effect of the interaction between the subsystems on the damping characteristics of the oscillation mode. Then, the eigenvalue analysis is used to analyze the system stability. Finally, the damping characteristic analysis is validated by time-domain simulations.

Abstract:The brushless doubly-fed induction generator (BDFIG) presents significant potential for application in wind power systems, primarily due to the elimination of slip rings and brushes. The application of virtual synchronous control (VSynC) has been demonstrated to effectively augment the inertia of BDFIG systems. However, the dynamic characteristics and stability of BDFIG under weak grid conditions remain largely unexplored. The critical stabilizing factors for BDFIG-based wind turbines (WTs) are methodically investigated, and an enhanced VSynC method based on linear active disturbance rejection control (LADRC) is proposed. The stability analysis reveals that the proposed method can virtually enhance the stability of the grid-connected system under weak grid conditions. The accuracy of the theoretical analysis and the effectiveness of the proposed method are affirmed through extensive simulations and detailed experiments.

Abstract:The widespread adoption of renewable energy sources presents significant challenges for power system dispatching. This paper proposes a dynamic optimal power flow (DOPF) method based on reinforcement learning (RL) to address the dispatching challenges. The proposed method considers a scenario where large-scale offshore wind farms are interconnected and have access to an onshore power grid through multiple points of common coupling (PCCs). First, the operational area model of the offshore power grid at the PCCs is established by combining the prediction results and the transmission capacity limit of the offshore power grid. Built upon this, a dynamic optimization model of the power system and its RL environment are constructed with the consideration of offshore power dispatching constraints. Then, an improved algorithm based on the conditional generative adversarial network (CGAN) and the soft actor-critic (SAC) algorithm is proposed. By analyzing an improved IEEE 118-node example, the proposed method proves to have the advantage of economy over a longer timescale. The resulting strategy satisfies power system operation constraints, effectively addressing the constraint problem of action space of RL, and it has the added benefit of faster solution speeds.