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The work reported in this paper aims at developing an algorithm for system operators to solve a robust security-constrained AC optimal power flow (R-SC-ACOPF) problem, which also optimizes the settings of discrete controllers including load tap changers (LTCs) and shunt capacitor banks (SCBs). An iterative algorithm involving a master problem and a number of sub-problems is proposed. The master problem selects the optimal discrete controllers’ settings by taking into account primal SC-ACOPF constraints based on the single-component worst contingencies identified by the sub-problems. Each sub-problem is a bi-level max-min problem used to find the current worst contingency. The overall objective is to minimize the total cost, including generation and load shedding cost, while satisfying all relevant constraints. The master problem and sub-problems allow representing LTC tap positions, SCB status, and contingencies using binary variables. Two case studies are presented as applications of this novel technique. [ABSTRACT FROM AUTHOR]

Microgrid's voltage regulation is of particular importance during both grid-connected and islanded modes of operation. Especially, during the islanded mode, when the support from the upstream grid is lost, stable voltage regulation is vital for the reliable operation of critical loads. This paper proposes a robust and data-driven control approach for secondary voltage control of AC microgrids in the presence of uncertainties. To this end, unfalsified adaptive control (UAC) is utilized to select the best stabilizing controller from a set of pre-designed controllers with the minimum knowledge required from the microgrid. Two microgrid test systems are simulated in MATLAB to verify the effectiveness of the proposed method under different scenarios like load change and communication link failure. [ABSTRACT FROM AUTHOR]

This paper presents a method for controlling the voltage of inverter-based Microgrids by proposing a new scale-free distributed cooperative controller. The main contribution of this paper is that the proposed distributed cooperative controller is scale-free where is independent of any information about the communication system and the number of distributed generators, as such it works for any Microgrids with any size. Moreover, the communication network is modeled by a general time-varying graph which enhances the resilience of the proposed protocol against communication link failure, data packet loss, and arbitrarily fast plug and play operation in the presence of arbitrarily finite communication delays as the protocol does not require the knowledge of the upper bound on the delay. The stability analysis of the proposed protocol is provided. The proposed method is simulated on the CIGRE medium voltage Microgrid test system. The simulation results demonstrate the feasibility of the proposed scale-free distributed nonlinear protocol for regulating voltage of Microgrids in the presence of communication failures, data packet loss, noise, and degradation. [ABSTRACT FROM AUTHOR]

This paper proposes a direct damping feedback control method against power system oscillations under small or large disturbances. For a targeted oscillation mode, this control method continuously minimizes the difference between the real-time estimated damping ratio and a desired value by changing power outputs of selected inverter-based resources. The method adopts a proportional-integral controller whose parameters are tuned using a nonlinear single-input-single-output model on damping estimation and control with a single-oscillator equivalent regarding the targeted power system mode. Also, optimization of these controller parameters considers both robustness and time performance in damping control. The paper also proposes utilizing a “zero-th order” parametric resonance phenomenon to simplify the controller design. Tests on a small power system and a 140-bus 48-machine Northeast Power Coordinating Council system validate the effectiveness of the proposed damping controller utilizing battery-based energy storage systems. [ABSTRACT FROM AUTHOR]

With the large-scale wind power integration into the power system, the inertia and damping characteristics of the system have been weakened, which is not conducive to fulfill the grid-friendly operation of wind power. Aiming at this, an adaptive control scheme combining virtual inertia control (VIC) and supplementary damping controller (SDC) is proposed in this paper. Firstly, the mathematical model of wind grid-connected power system with VIC is established. Secondly, the polytopic linear parameter varying (LPV) system is built to describe the uncertainty of system operating points, and the gap metric-based nonlinear evaluation method is used for polytope vertex configuration. Next, Youla-Kucera parametrization are extended to polytopic LPV system to design an adaptive damping control, in which VIC and SDC are coordinated to provide damping and virtual inertia simultaneously. Finally, the proposed adaptive control scheme is compared with several existing control techniques to validate its effectiveness and advantages in suppressing system oscillation in the modified IEEE 39-bus system integrated with 2 wind turbines. [ABSTRACT FROM AUTHOR]

This paper investigates a procedure of coordinated tuning of power system decentralized damping controllers based on reduced-order model (ROM). In the paper, the cross-gramian technique is applied to obtain an equivalent ROM for power systems. Once the controllers' locations are selected appropriately, the critical modes are approximated well in the reduced model. The controllers are designed on ROM using search algorithm and then tested on the unreduced system to demonstrate their availability. The benefit of adopting ROM is to decrease the complexity of objective function computation. The proposed method is tested on power systems with 4-machine and 156-machine to validate its efficiency. [ABSTRACT FROM AUTHOR]

In this paper, a novel distributed control strategy addressing a (feasible) psycho-social-physical welfare problem in islanded Direct Current (DC) smart grids is proposed. Firstly, we formulate a (convex) optimization problem that allows prosumers to share current with each other, taking into account the technical and physical aspects and constraints of the grid (e.g., stability, safety), as well as psycho-social factors (i.e., prosumers’ personal values). Secondly, we design a controller whose (unforced) dynamics represent the continuous time primal-dual dynamics of the considered optimization problem. Thirdly, a passive interconnection between the physical grid and the controller is presented. Global asymptotic convergence of the closed-loop system to the desired steady-state is proved and simulations based on collected data on psycho-social aspects illustrate and confirm the theoretical results. [ABSTRACT FROM AUTHOR]

Power system control and transient stability analysis play essential roles in secure system operation. Control of power systems typically involves highly nonlinear and complex dynamics. Most of the existing works address such problems with additional assumptions in system dynamics, leading to a requirement for a complete and general solution. This paper, therefore, proposes a novel control framework for various power system control and stability problems leveraging a learning-based approach. The proposed framework includes a two-module structure that iteratively and jointly learns the candidate Lyapunov function and control law via deep neural networks in a learning module. Meanwhile, it guides the learning procedure towards valid results satisfying Lyapunov conditions in a falsification module. The introduced termination criteria ensure provable system stability. This control framework is verified through several studies handling different types of power system control problems. The results show that the proposed framework is generalizable and can simplify the control design for complex power systems with the stability guarantee and enlarged region of attraction. [ABSTRACT FROM AUTHOR]

The paper presents the theoretical foundation and practical implementation aspects of Fractional Order Controllers (FOCs) for power system applications. With this aim, the paper provides a comprehensive mathematical background on the stability analysis of dynamic systems with inclusion of fractional order derivatives and discusses their software implementation based on the Oustaloup's Recursive Approximation (ORA) method. Then the paper illustrates a variety of examples of ORA-based FOCs, namely, automatic generation control of synchronous machines; frequency control of a converter-interfaced energy storage system; and voltage control through a static synchronous compensator. The WSCC 9-bus test system and a realistic 1,479-bus model of the Irish transmission system are employed to test and compare the examined FOCs with their integer-order versions. [ABSTRACT FROM AUTHOR]

This is the second part of a two-part paper on a new adaptive coordinated voltage control (ACVC) strategy. The overall basic scheme and the online control for prepared faults have been presented in Part I. In this paper, learning control is explored to accumulate knowledge, so that the online control performances can be improved for unknown emergencies. For unknown situations where no past experiences can be exploited, a learning scheme is used by which the control knowledge can be acquired gradually. The learnt knowledge goes into the database and is improved the next time a related situation happens. After full knowledge is acquired, the unknown fault becomes a prepared fault with prepared knowledge. With the learning scheme, control for any emergency can be realized in a rapid and effective response. The learning process is demonstrated by providing control for an unknown emergency in the New England 39-bus power system. The ACVC performance for a sequence of randomly generated fault and load event emergencies is presented at the end of this paper. [ABSTRACT FROM PUBLISHER]

Based on the knowledge accumulated off-line and feature extractions, a novel data-based learning and control method is proposed for the long-term voltage stability problem in this paper. All the spatial-temporal data is considered and the features of different emergency events are extracted by principle component analysis which can reduce the dimension and reveal the significant internal structure of the data. An artificial neural network is used to build a classifier to reinforce the relationship directly between the system dynamics and optimal control actions. With the prepared control knowledge, it is faster to find an optimal control action online with a good system performance. Simulation results on the 6-bus system, New England 39-bus system and Iceland 189-bus system are given to show the potential of this method for on-line control. [ABSTRACT FROM AUTHOR]

Eigen-analysis is widely used in the studies of power system oscillation and small-signal stability. However, it may give inaccurate analyses on subsynchronous oscillation (SSO) when nonlinearity is not negligible. In this paper, a nonlinear analytical approach based on the describing function and generalized Nyquist criterion is proposed to analyze the characteristics of SSO with wind farms. The paper first presents describing function-based model reduction considering key nonlinear elements involved in SSO, and then uses a generalized Nyquist criterion for accurate estimation of SSO amplitude and frequency. The results are verified by time-domain simulations on a detailed model with different scenarios considering variations of the system condition and controller parameters. [ABSTRACT FROM AUTHOR]

This paper describes the design and implementation of a proof-of-concept Pacific dc Intertie (PDCI) wide area damping controller and includes system test results on the North American Western Interconnection (WI). To damp inter-area oscillations, the controller modulates the power transfer of the PDCI, a ±500 kV dc transmission line in the WI. The control system utilizes real-time phasor measurement unit (PMU) feedback to construct a commanded power signal which is added to the scheduled power flow for the PDCI. After years of design, simulations, and development, this controller has been implemented in hardware and successfully tested in both open and closed-loop operation. The most important design specifications were safe, reliable performance, no degradation of any system modes in any circumstances, and improve damping to the controllable modes in the WI. The main finding is that the controller adds significant damping to the modes of the WI and does not adversely affect the system response in any of the test cases. The primary contribution of this paper, to the state of the art research, is the design methods and test results of the first North American real-time control system that uses wide area PMU feedback. [ABSTRACT FROM AUTHOR]

Interarea oscillation is one of the main challenges for the secure and stable operation of a large-scale interconnected power system. To address this problem, this paper presents an adaptive supplementary damping controller (SDC) for voltage source converter based high voltage direct current (VSC-HVDC) by using the goal representation heuristic dynamic programming (GrHDP). Compared with conventional heuristic dynamic programming (HDP), GrHDP adds a new goal representation network to generate an adaptive reward signal to achieve better mapping relationship between the system states and the control action. To compensate phase shift between the output and input signals of a large-scale interconnected AC/DC power system, this paper proposes a parallel phase shift channel for the GrHDP. Moreover, the proposed GrHDP-SDC is a model-free control scheme, which does not need the mathematic model of the power system and has quick online learning ability to adapt to the variation of system operating conditions. Case studies are carried out on the 10-machine 39-bus system and 16-machine 68-bus system, respectively. Compared with the conventional lead-lag SDC (LL-SDC) and HDP-SDC, simulation results show that the proposed GrHDP-SDC can always provide satisfactory damping performance over a wide range of system operating conditions and disturbances. [ABSTRACT FROM PUBLISHER]

In multi-infeed hybrid AC-DC (MIDC) systems, the asynchronous interconnection between regional grids, complicated system dynamics and possible emergency faults have enormous effects on the frequency stability. To address the frequency instability problems in emergency situations, this paper proposes a decentralized emergency frequency control (EFC) strategy based on coordinated droop for the MIDC system. First, a P-f droop control for LCC-HVDC systems is introduced, and the coordinated droop mechanism among LCC-HVDC systems and generators is designed. Then, to appropriately allocate the power imbalance among the LCC-HVDC systems and generators, an optimal emergency frequency control (OEFC) problem is formulated, and the optimal droop coefficients are selected in a decentralized way, which can deal with various control objectives. A Lyapunov stability analysis shows that the closed-loop equilibrium is locally asymptotically stable considering the LCC-HVDC dynamics. The effectiveness of the proposed emergency control strategy is verified through simulations. [ABSTRACT FROM AUTHOR]

The integration of photovoltaics (PVs) in low-voltage (LV) grids is expected to rise within the following years posing technical challenges to the reliable operation of the electrical system. To tackle these challenges, distributed energy storage systems (ESSs) coupled with PVs at prosumer side arise as a promising solution. Therefore, during the last years several control schemes have been developed to manage ESSs. To overcome disadvantages of conventional control strategies, a new localized control strategy is proposed in this paper. The proposed control aims to improve voltage profile along distribution feeders, by mitigating the peaks of the net injected/absorbed power at prosumers’ point of common coupling. Additionally, the new control aims to maximize prosumers’ self-consumption. To achieve peak mitigation at the net power profile, an exponential droop is introduced that charges/discharges ESSs with different rates based on the residual power between PV generation and load demand. Using this droop, ESS charging/discharging power is exponentially increased as the residual power becomes greater. To maximize the self-consumption of the installation, an optimization procedure is developed that properly adjusts the exponential droop parameters to prosumer's generation and consumption profile. The effectiveness of the control strategy is validated by both simulation and experimental results. [ABSTRACT FROM AUTHOR]

Since the publication of the original paper on power system stability definitions in 2004, the dynamic behavior of power systems has gradually changed due to the increasing penetration of converter interfaced generation technologies, loads, and transmission devices. In recognition of this change, a Task Force was established in 2016 to re-examine and extend, where appropriate, the classic definitions and classifications of the basic stability terms to incorporate the effects of fast-response power electronic devices. This paper based on an IEEE PES report summarizes the major results of the work of the Task Force and presents extended definitions and classification of power system stability. [ABSTRACT FROM AUTHOR]

Energy storage enables modification of the customer load profile from the grid perspective without leading to a decrease in comfort level. To meet the future challenges of the energy sector, distribution system operators (DSOs) in Finland have recently discussed power-based distribution tariffs (PBDTs) for small customers. The current distribution tariffs of small customers do not respond well to current challenges, and they require reform. The peak powers of the customers can be decreased with energy storage, and via incentives included in the PBDTs, energy storage can prove to be profitable for household customers. In this paper, we study the profitability of electrical energy storage under different pricing structures. The study simulates changes in customer consumption with a modeled storage system. Simulations consider the requirement of load forecasting in the storage control system. The results of the study show that energy storage can be profitable if the consumption includes only a small number of high peak loads during the pricing period when applying new distribution tariff structures. However, the price level of the tariffs and the tariff mechanisms affect the results. [ABSTRACT FROM AUTHOR]

Soft normally open points (SNOPs) connected to distribution networks are instrumental in maintaining uninterrupted power supply and improving power quality during the fault period. In order to achieve the optimal service restoration scheme, a novel bi-level service restoration model based on the robust optimization method is proposed in this paper. The upper level of the proposed model aims at minimizing the risk of load loss. The switch statuses and range of power transmitted by SNOP terminals can be obtained. In the lower level of the model, the interval robust optimization method is adopted to cope with the uncertainties of DGs and loads. If there are no optimal solutions, the upper level model needs to be solved once more to adjust the network topology. Finally, a test system is established to verify the effectiveness of the proposed model and solution method. The effects of forecasting errors, the DG penetration, and the ratio of single terminal capacity of SNOP to line capacity on the optimization results are analyzed. [ABSTRACT FROM AUTHOR]

Using VSC-MTDC to provide frequency support for interconnected power systems is an attractive solution. Considering the disadvantages of traditional methods in power shortage or surplus absorption into the VSC-MTDC from AC systems requiring active power support (RAPS system) as well as flexible and effective power balance in AC systems providing active power support (PAPS system), this paper proposes a novel frequency support control method for interconnected power systems using VSC-MTDC. The main principle of the proposed frequency support control method is firstly to make the active power shortage or surplus of the RAPS system be fully and naturally absorbed into the DC grid with the AC slack bus mode control proposed for RAPS converters; in addition, the smooth switching control and the power limiting control are further proposed to realize smooth and safe power shortage or surplus absorption. Secondly, according to the proposed flexible PAPS systems matching strategy, one or more appropriate PAPS systems with adequate capacity are selected to effectively and accurately balance the unbalanced power via the corresponding PAPS converters using the active balancing power control. The theoretical analysis and simulation results using PSCAD/EMTDC have verified the validity, flexibility and universality of the proposed frequency support control method. [ABSTRACT FROM AUTHOR]

This paper presents a wide-area control approach to improve the transient stability of the power systems while also damping the post-fault inter-area oscillations. The proposed approach employs a nonlinear Kalman filter to estimate the inter-area modes using phasor measurement units (PMUs). The wide-area control system then uses the estimated inter-area dynamics through the developed control algorithms to improve the first swing and damping stability of the system. The performance of the proposed approach is evaluated on the simplified Australian test system with different load models including induction motors and the results show significant improvement in the stability of the system by simply adding the wide-area control signals to the available controllers in the system. The application of the proposed wide-area control system is shown to be feasible on realistic systems by improving the system stability and efficiency. [ABSTRACT FROM PUBLISHER]

This paper models and analyzes a power distribution system with sources connected through inverters. The stabilizing effect of generator rotational inertia is emulated through inverter droop control logic. We model the nonlinear dynamics of the system with multiple distributed droop inverters using dynamical phasors. Linearizing the system around the steady-state, we construct closed-loop transfer functions from exogenous disturbances to system states. This allows to evaluate the load disturbance impact on the inverter power and line voltage profile. As an example, the distribution system with 150 inverters is used to illustrate the approach. [ABSTRACT FROM PUBLISHER]

High-voltage direct-current (HVDC) systems for constant or intermittent power delivery have recently been developed further to support grid frequency regulation (GFR). This paper proposes a new control strategy for a line-commutated converter-based (LCC) HVDC system, wherein the dc-link voltage and current are optimally regulated to improve real-time GFR in both rectifier- and inverter-side grids. A dynamic model of an LCC HVDC system is developed using the dc voltage and current as input variables, and is integrated with feedback loops for inertia emulation and droop control. A linear quadratic Gaussian (LQG) controller is also designed for optimal secondary frequency control, while mitigating conflict between the droop controllers of HVDC converters. An eigenvalue analysis is then conducted, focusing on the effects of model parameters and controller gains on the proposed strategy. Simulation case studies are performed using the models of a real HVDC system and a CIGRE benchmark system. The case study results confirm that the proposed strategy enables the HVDC system to improve GFR, in coordination with generators in both-side grids, by exploiting the fast dynamics of HVDC converters. The proposed strategy is also effective under various conditions for the LQG parameters, inertia emulation, and droop control. [ABSTRACT FROM AUTHOR]

With the growing study on the active distribution network, an objective and efficient reliability evaluation method for such cyber-physical systems (CPS) is urgently needed. In this paper, first considering the cyber transmission performance, a new method is proposed for efficient evaluation of cyber link validity based on frequency-time domain transformation. In order to quantify the indirect interdependencies of cyber failures, an unresponsiveness probability model for switches is established. Then, an innovative method is proposed by combining the equivalent method and the minimal path method. Finally, a test system is established to verify the effectiveness of the proposed method. This study is instrumental in informing the efficient planning of contemporary CPS-based distribution networks. [ABSTRACT FROM AUTHOR]

As the power system evolves to better address environmental and reliability concerns, its overall dynamics changes and consequently challenges the adequacy of established operating procedures. This evolution brings a significant increase in the number of integrated renewable energy sources (RES) and HVDC connections. RES are normally connected to the grid through power electronic converters, which reduces or completely eliminates their electrical coupling to the power grid. As a result, such units are unable to contribute to the inertia of the system, posing a potential threat to the grid's physical security. More specifically, postfault frequency characteristics, such as rate of change of frequency (RoCoF), frequency nadir, and quasi-steady-state frequency, depend on the system inertia and their respective limits may potentially be violated. One way to address this problem is adding constraints to the unit commitment formulation so that operations maintain their frequency characteristics within acceptable limits. This paper studies how such changes affect the system frequency deviation from its rated value. The power system is modeled as an enterprise control that combines multiple layers of control operations into one package. Such integration allows capturing how the effects of wind integration and the addition of unit commitment inertia constraints propagate through control layers to impact the system frequency. [ABSTRACT FROM AUTHOR]

This paper proposes a novel control approach to reduce the rated power of energy storage system (ESS) in the smoothing of wind power output. Wind power generation causes frequency fluctuations in power systems. Therefore, ESSs are used to absorb the fluctuating power of wind generators. Generally, the controller in an ESS is composed of a first-order low-pass filter (FLF) with a large time constant. However, the FLF incurs a high cost because its phase-lag induces an unnecessary increase in the rated power of ESS. This study addresses the reduction in the rated power of ESS without using a complex controller. First, the requirement of the controller is analyzed and its structure is proposed. Then, the absorption of the short-period wind power output is reduced by tuning a gain in the controller until the grid code is violated. The effectiveness of the proposed controller is verified by comparing it to the conventional controllers via scenario simulations using real wind data. [ABSTRACT FROM AUTHOR]

In this paper, a distributed economic model predictive control (EMPC) method is proposed for economic operation and frequency regulation in isolated AC subsystems interconnected through multi-terminal HVDC (MTDC) transmission system. Economic-driven frequency regulation, including economic operation and frequency regulation, normally implemented in a separate and hierarchical way, can be integrated through the proposed EMPC. Instead of competing with neighboring subsystems, the proposed distributed EMPC encourages cooperation among the interconnected subsystems, improving system-wide control performance. A suitable Lyapunov function is utilized to prove the stability of the proposed method. Simulations are used to demonstrate the performance of the proposed EMPC method. [ABSTRACT FROM AUTHOR]

The synchronization requirement is a bottleneck of many distributed optimization algorithms, particularly for solving problems with computationally heterogeneous subproblems and during the occurrence of communication failure/delay. This paper presents a double-loop learning-aided asynchronous alternating direction method of multipliers (LA-ADMM) that has information prediction capability and handles a considerable level of asynchrony between subproblems. A momentum-extrapolation prediction-correction technique is developed to enable subproblems to predict their neighbors missing shared variable information instead of using the latest received values. An online streaming-based anomaly classification is designed to observe the performance of predicted data and control Lagrange multipliers update over the course of iterations. The proposed LA-ADMM reduces under-utilization of computation resources, especially if subproblems are computationally heterogeneous. This algorithm also enhances distributed optimization robustness against communication failure/delay that may result in a considerable level of asynchrony between subproblems. LA-ADMM is applied to solve the optimal power flow problem for several test systems. Promising results are obtained as compared to the classical synchronous ADMM and asynchronous ADMM without the anomaly switch control. [ABSTRACT FROM AUTHOR]

This paper deals with the transient stability of a grid-forming converter while embedding a current reference saturation strategy. The novelty of this work consists in investigating the impact of the current reference angle on the transient stability. In case of a balanced voltage sag, analytical formulas to estimate the critical clearing angle (CCA) and critical clearing time (CCT) while considering different values of the current reference angle are derived. It is demonstrated that the choice of this angle is constrained by the ability of the power converter to switch back to the voltage control mode. Based on that, its optimal value that enhances the transient stability and allows a switching from the saturated current control mode to the voltage control mode is calculated. Thereafter, the effectiveness of this optimal choice to guarantee the stability in case of a phase shift caused by a line re-closing event is verified. Time-domain simulations and experimental tests validate the correctness of the presented theoretical approaches. [ABSTRACT FROM AUTHOR]

This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG. [ABSTRACT FROM PUBLISHER]

To isolate the negative impacts of pulsed power loads (PPL) in a shipboard power system (SPS), energy storage systems (ESS) are usually equipped for online ESS charging and offline ESS discharging for PPL deployment. In order to achieve fast and smooth ESS charging, the generation control and ESS charging control need to be well coordinated. In addition, control constraints, such as generation and charging current bounds and ramp rates, and changes of operating conditions during charging, should also be properly considered. To better solve this important but insufficiently investigated problem, two control algorithms are presented in this paper. The first PI-based control algorithm is designed based on analysis of the desired performance and rules for coordination. The second feedback linearization based control algorithm is designed based on a simplified SPS model and ramp rates difference of supply and demand. The algorithms are simple to implement and can effectively reduce the negative impacts during supercapacitor charging. The algorithms are tested with detailed single-generator and multiple-generator SPS models. Power hardware-in-the-loop and real-time digital simulation studies are performed to demonstrate the effectiveness of the proposed algorithms. [ABSTRACT FROM PUBLISHER]

This paper proposes a multifunctional control strategy and associated control algorithms for distributed wind-turbine (WT) based nano-grids connected to a distorted utility-grid. The contribution is on a new strategy with innovative control algorithms to coordinate multiple converters for a multitasking operation of the nano-grids. The novelty is on a unique control design with feasibilities: maximizing the generated power from WT, maintaining power quality in both ac- and dc-sides under critical conditions of the power grid, and improving power quality against distortion from local nonlinear loads under a reduced switching frequency. A robust fast-dynamic predictive control method is developed for current controllers to fulfil the multifunction. Unconstrained deadbeat control inputs are derived in twofold targets: ensuring fast dynamic response and significantly reducing both the computation and switching frequency for finite predictive control. The control system is applied on a permanent-magnet synchronous generator (PMSG) WT-based nano-grid connected to a distorted utility-grid. An OPAL-RT-based real-time platform is used for comparative studies among the proportional integration (PI) control, finite predictive control (FS-MPC), and proposed control method. The performance verification exhibits the power quality improvement in both the nano- and utility-grids under critical conditions via high-performed regulation of currents, voltages, reactive power, and rotor-speed of the PMSG-WT. [ABSTRACT FROM AUTHOR]

This paper proposes a method for the model order reduction (MOR) of large scale power system models that produces reduced order models (ROM) which preserve the access to selected parameters exactly like the original full order model (FOM). The preserved parameters are related to decentralized power system devices, including, but not limited to, power system stabilizers that are used for the damping control of system electromechanical oscillations. The problem formulation of this parametric MOR (PMOR) is described, as well as the implementation details regarding efficiency and flexibility. Test results are described for large practical power system models used in small-signal electromechanical stability studies. The ROM produced sufficiently accurate responses following parameters changes, when equivalently compared to the original FOM results. The proposed method allows preserving specified nonlinearities in chosen devices of the parameterized ROMs. It can also be applied to the reduction of differential algebraic equation models related to other areas of engineering. [ABSTRACT FROM AUTHOR]

The increasing penetration of Renewable Energy Sources (RES) and, more in general, of converter connected power generation is progressively reducing the overall inertia of the electricity system introducing new challenges in the network Frequency Support (FS). Therefore, FS strategies for RES are becoming an important aspect to be developed in order to en-sure the secure operation of the electricity power grid. The aim of the present paper is to propose an innovative approach for the inertial emulation of Wind Turbine Generators (WTGs) in order to ensure a positive effect on the system frequency avoiding unstable operations of the WTG and reducing the negative impact of the rotor speed recovery on the secondary frequency drop. Moreover, the paper provides a detailed definition of the triggering logics adopted to activate and deactivate the FS making it suitable for implementation in available industrial controllers. The proposed controller is firstly tested on a simplified configuration to show its enhanced performances with respect to previously developed approaches. Then, simulations are carried out in a more realistic network to assess the positive impact of the controller on the system frequency. Results highlighted a 40% reduction of the system Rate of Change of Frequency while the frequency deviation at the transient nadir is improved by 35%. [ABSTRACT FROM AUTHOR]

As Type-3 wind turbine (WT) has already become a typical type of highly penetrated power sources in modern power systems, characterizing its properties during grid fault is a basic requirement for system analysis. However, many significant factors have not been considered in the existing analytical methods and results, such as various switching of internal control and protection circuits in low voltage ride through (LVRT) solutions. In this paper, performance of Type-3 WT during LVRT is first illustrated and summarized as the sequential switching characteristic in multi time scales, viz., instant control, ac control, dc voltage control, and rotor speed control time scales. Thus, for magnitude drops in stator voltage, a mathematical model is proposed to understand this characteristic and fault current analysis's features. Then, an analytical method, based on the operational inductance, is proposed so that fault current components, with the sequential switching considered, can be depicted with a unified and simple analytical expression. By identifying magnitude of symmetrical current, the transient and steady-state equivalent circuits of Type-3 WT are proposed to estimate momentary current and electrical power approximately. Finally, analytical results are verified by comparisons with real-time simulation results of a detailed 1.5 MW WT model in RT-LAB. [ABSTRACT FROM AUTHOR]

Frequency control of isolated power systems with high penetration of wind power is a challenging task for system operators. The difficulty results from the limited frequency response capability of thermal power units to keep up with the fluctuations of wind power. This paper develops a field verification architecture to leverage aluminum smelting loads (ASLs) to participate in primary frequency control. A new method is proposed to coordinate ASLs with thermal power units to quickly stabilize the isolated power system with significant wind power fluctuations or curtailments. Moreover, a closed-loop control system for the ASLs controller is developed and tested on a real-world isolated power system in Inner Mongolia, China. Hardware-in-the-loop simulations, as well as field tests, are conducted under various scenarios of wind power fluctuations or curtailments to verify the proposed architecture for frequency control. The highly consistent simulation results and field tests demonstrate the good frequency control capability of the ASL. The regulated power from the ASL is demonstrated to be able to reach 6% of its rated power within 4 s. Working jointly with thermal power units, the ASL helps maintain the stable operation of the isolated power system, significantly improving the frequency response characteristic. [ABSTRACT FROM AUTHOR]

Line-switching operations introduce several concerns to microgrid operators since they trigger reliability and power quality issues in addition to increasing the wear and tear cost of switching devices. This paper proposes a novel methodology for achieving seamless line-switching in low-inertia islanded microgrids. The proposed methodology aims at mitigating the detrimental line-switching impacts by minimizing the power flow in the switched lines at the moments of executing the switching operations. The minimization of the power flow is realized by optimizing the droop control parameters of the DGs during the transition from one network topology to another. Besides, during executing a series of line-switching operations, the sequence of switching is also optimized to minimize the total switched power. While optimizing the line-switching execution sequence, the transitional droops are optimized for each line-switching operation within that sequence. A dynamic mixed-integer nonlinear programming problem is formulated to find both the optimized dynamic transitional droops and line-switching sequence simultaneously. Simulation results demonstrate the effectiveness of the proposed seamless line-switching methodology where the switched power can be reduced by more than 80% for a single line-switching operation and 54% considering a series of four switch exchanges in addition to achieving several instances of zero-current switching. As a result, a significant decrease in the peak of voltage transients has been recorded for some cases. [ABSTRACT FROM AUTHOR]

In this paper, we introduce an approach for a fast assessment of the dynamic characteristics of system frequency during the period of cold load pickup following a large-scale blackout. The approach is developed based on a load model and a model of generation control system: the load model considers the unique time-dependent characteristic of inrush power surges during the period of cold load pickup, while the model of generation control system mimics the characteristics of various governors that are crucial to the determination of the key variables including initial rate of frequency change, frequency nadir and settling frequency. With the proposed linearization and estimation methods, the model of generation control system can be quickly constructed to offer a great amount of flexibility and simplicity for the study of the impact of cold load on frequency response. The proposed approach allows system operators to quickly evaluate many different options for system restoration. The application of the proposed approach is demonstrated in a case study based on the IEEE 39-bus system. The differences between the results obtained using the proposed approach and those obtained from complete PSS/E simulation are very small, which suggests that the linearized model of generation control system is fast, and accurate enough to address the major concerns of cold load pickup in system restoration. [ABSTRACT FROM PUBLISHER]

This paper proposes dynamic energy level balancing between distributed storage devices as a strategy to improve frequency regulation and reliability in droop controlled microgrids. This has been achieved with a distributed multi-agent cooperative control system which modifies the output power of droop controlled storage devices so that they reach a balanced energy state. As the storage devices approach a common energy level they are able to contribute their full power capacity to deal with generation and demand fluctuations in the microgrid. The cooperative control system also provides secondary frequency control, restoring the microgrid to the reference frequency. Simulations have been completed showing that the cooperative control system improves frequency regulation compared to traditional droop control strategies when the storage devices begin at different energy levels and the microgrid experiences generation or demand variability. A control input saturation constraint has been developed which ensures that the cooperative control system will not overload the storage devices. [ABSTRACT FROM AUTHOR]

This paper establishes a closed-loop interconnected dynamic model of a power system with a doubly fed induction generator (DFIG) for wind power generation, wherein, the DFIG and power system are modeled as two open-loop interconnected subsystems. The established model indicates that the impact of the dynamic interactions introduced by the DFIG on the power system's small-signal angular stability is the difference between the closed-loop and open-loop electromechanical oscillation modes (EOMs). The analysis conducted in this study reveals that when an open-loop DFIG oscillation mode (DOM) is close to an open-loop EOM on the complex plane, the modal coupling of the open-loop EOM and DOM can cause strong dynamic interactions between the DFIG and the power system; thus, the DFIG has a significant impact on the power system's small-signal angular stability. An example multimachine power system with grid-connected DFIGs is presented to demonstrate and validate the analysis and conclusions made in the paper. [ABSTRACT FROM PUBLISHER]

This paper develops a novel framework to design an optimal robust excitation system controller considering the uncertainties in the parameters of the model of the excitation system. The uncertainties may cause the parameter values to vary from their nominal values within a specified upper and lower limit. These uncertainties can have a significant influence on the dynamic characteristics of the power system, that is, the variations in the parameters of the excitation controller model due to the uncertainties in the parameters can cause the system to change from being stable to unstable. It is, therefore, important to design a robust excitation system controller that can ensure that irrespective of the values of the parameters within the boundary of the uncertainties, the power system will not have any variation from its stability. The proposed framework decomposes the uncertainties in the parameters of the excitation system model into two components: matched and unmatched. To eliminate the uncertainties from both components, a linear quadratic regulator problem is constructed to deal with the matched component, while an augmented control is used to cope with the unmatched component. The robustness of the resulting controller is verified using time-domain dynamic stability simulations of a single-machine test system and the IEEE 39-bus New England system. [ABSTRACT FROM PUBLISHER]

This paper proposes a new computational intel ligence-based control strategy, to enhance the low voltage ride-through capability of grid-connected wind turbines (WTs) with doubly fed induction generators (DFIGs). Grid codes world-wide require that WTs should supply reactive power to the grid during and after the fault, in order to support the grid voltage. The conventional crowbar-based systems that were initially applied in order to protect the rotor-side converter at the occurrence of grid faults, do not fulfill this requirement, as during the connection of the crowbar, the DFIG behaves as a squirrel cage machine, absorbing reactive power from the grid. This drawback led to the design of control systems that eliminate or even avoid the use of the crowbar. In order to conform to the above-mentioned requirement, this paper proposes a coordinated control strategy of the DFIG converters during a grid fault, managing to ride-through the fault without the use of any auxiliary hardware. The coordination of the two controllers is achieved via a fuzzy controller which is properly tuned using genetic algorithms. To validate the proposed control strategy, a case study of a 1.5-MW DFIG supplying a relatively weak electrical system is carried out by simulation. [ABSTRACT FROM PUBLISHER]

An algorithm facilitating automated purchase of energy on real-time balancing market is proposed in this paper. The main inputs of the algorithm are a prediction of the area control error, its uncertainty in the form of a standard deviation and a set of previously procured ancillary services (contracted power reserves) available for power balance control. The algorithm evaluates all possible power balance scenarios for the closest hour in a probabilistic manner and gives a cost-optimal recommendation for the real-time balancing market demand. A case study on realistic data for the Czech Republic control area was performed by simulation of the power balance control with a Monte Carlo simulator of the Czech transmission system. The results showed potential for savings on the regulation energy costs. [ABSTRACT FROM PUBLISHER]

The paper introduces a new flexible alternating current transmission systems (FACTS) controller based on pulse width modulated series compensator (PWMSC) for damping inter-area oscillations in power systems. The PWMSC can modulate the impedance of a transmission line through the variation of the duty cycle of AC link. The main purpose of this work is to verify the capability of the PWMSC to mitigate low frequency oscillations. For this reason, a novel current injection model of PWMSC is proposed and incorporated in the system model. This study proposes a novel tuning strategy for robust PWMSC damping controller based on the augmented Lagrangian particle swarm optimization (ALPSO) satisfying the multiple H\infty performance criteria. Proposed method does not need any conservative assumption unlike the conventional techniques, and often enables us to find the desired parameters of controller just by solving the constrained optimization problem in a straightforward way. In the paper, a verification of the designed controller by means of digital simulations of single machine and four-machine power systems is presented. The net results show that the proposed controller improves the stability of both test systems significantly in a number of fault cases at different levels of power flow in tie-line. [ABSTRACT FROM PUBLISHER]

This paper presents an approach to deal with the uncertainty of generation and load in the optimal island partitioning of the automated radial distribution networks. The proposed approach is based on the concept of interval analysis, Relative Distance Measure (RDM) interval arithmetic, mixed integer linear programming with interval coefficients, and risk management tools. It defines a number of different island partitioning scenarios taking into account dependence (correlation) between loads and DGs. The best island partitioning scenario is obtained by employing the Minimal risk (Minimax) criterion for risk measurement. In this way, the presented approach provides a tool for a decision-maker to determine the island partitioning scenario that minimizes the risk of unsuccessful island operations in the uncertain environment. [ABSTRACT FROM AUTHOR]

Power system controllers are designed and tuned to achieve the desired dynamic performance under prespecified system conditions. However, changes in the system, e.g., load variation, generation changes, and faults, can cause departure of the system from previously assumed parameters. This in turn can change and reduce damping, which may enable large transients to violate the operational limits of a controlled apparatus. Set point automatic adjustment with correction enabled (SPAACE) is a strategy that modulates the set point of an apparatus to mitigate such transients. This paper 1) proposes a quadratic prediction algorithm for SPAACE to enhance its performance, 2) presents a supplemental strategy to improve set point tracking even when limits are not violated, 3) introduces an approach to implement SPAACE in unbalanced systems, and 4) analyzes the stability of SPAACE. Several case studies are presented to demonstrate the superior ability of this strategy for mitigating transients in different scenarios compared to conventional PI-based control strategies. [ABSTRACT FROM AUTHOR]

It is well known that traditional line-commutated converter (LCC) based high voltage direct current (HVDC) system is not able to control its reactive power and terminal AC voltages. This paper investigates the reactive power and AC voltage control at the inverter side of the LCC HVDC system with controllable capacitors. The system's ability of operating under negative extinction angle is utilized to achieve a wide range of reactive power control and, in particular, the ability of exporting reactive power. In connection with the inverter AC terminal voltage or reactive power control, among different control possibilities at the rectifier side, active power control is desirable since large variations of active power transfer is very unfavorable. Detailed theoretical analysis is carried out first to show the reactive power controllability, and the capacitor voltage level is selected based on the desired control range. In addition, a new extinction angle measurement approach is proposed for negative extinction angle measurements. The effectiveness of the reactive power/voltage control capability for the proposed system is validated through simulation results using Real-Time Digital Simulator (RTDS). To verify the effectiveness of the reactive power and voltage control, CCC HVDC and LCC HVDC with SVC are also set up in RTDS, and simulation comparisons are made. Furthermore, contribution to AC voltage control in power system using the proposed method is demonstrated through simulation results of the modified two-area four-machine AC power system. [ABSTRACT FROM AUTHOR]

This paper summarizes a set of six benchmark systems for the analysis and control of electromechanical oscillations in power systems, recommended by the IEEE Task Force on Benchmark Systems for Stability Controls of the Power System Dynamic Performance Committee. The benchmark systems were chosen for their tutorial value and particular characteristics leading to control the system design problems relevant to the research community. For each benchmark, the modeling guidelines are provided, along with eigenvalues and time-domain results produced with at least two simulation softwares, and one possible control approach is provided for each system as well. Researchers and practicing engineers are encouraged to use these benchmark systems when assessing new oscillation damping control strategies. [ABSTRACT FROM AUTHOR]

An improved approach based on sequential method for the AC-DC power flow calculation is proposed in this paper. This approach solves the convergence problem caused by voltage violations at AC buses during the power flow calculation for the DC subsystems. The convergence property can be significantly improved by adjusting the converter transformer tap position flexibly. In order to adjust the tap position of the converter transformer flexibly, three mainly modifications are proposed. Firstly, the equations for whole DC systems are decoupled into individual DC systems so as to easily figure out which DC system's tap position needs adjustment. Secondly, the tap ratio of a converter transformer is selected as an alternative state variable to replace the cosine of the control angle when necessary. Thirdly, the Newton-Raphson method is utilized to solve DC subsystems instead of the method using the linear equations. Furthermore, a theoretical analysis of the advantages of the proposed approach is also presented. Numerical simulations and practical applications show that the proposed approach meet the requirement of different system operating conditions and has advantages in terms of convergence and speed. The proposed approach has been successfully integrated into the Energy Management System (EMS) for China Southern Power Grid. [ABSTRACT FROM PUBLISHER]

This paper focuses on the education aspect of potential functions and presents the concept as a general approach for formulation of dynamic processes in different areas of engineering, e.g., dynamics of motion, electrical fields, power systems, control, and robotics. In power systems, the potential functions have been used in transient stability analysis in capacities such as 1) transient energy function and 2) the theoretical backbone of the equal-area criterion. The unified representation introduced in this paper is to help students (especially global learners by providing them with the big picture of the subject) discover similarities between different disciplines, which facilitates inter-application of the analytical methods. Recognizing the importance of digital computer based simulation techniques in education, the paper also introduces a software tool for assisting students/researchers in experimenting with different forms of potential functions and their application to a dynamic environment consisting of mobile robots, targets, and obstacles. [ABSTRACT FROM PUBLISHER]