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An efficient power battery thermal management strategy under low-temperature conditions can effectively alleviate mileage anxiety associated with electric vehicles and improve environmental adaptability. This paper aims to propose a novel coupled thermal management strategy for range extended electric vehicles (REEVs) power systems containing a range extender and a power battery. First, a novel coupled thermal management structure for REEVs power system is designed. Sequentially, a fuzzy controller is developed based on analysis of control rules for different working conditions on the basis of the structure. Furthermore, the stability of the proposed strategy is strictly proven by Lyapunov’s stability theory and phase plane analysis. Finally, a coupled thermal management model is built in AMEsim, and co-simulation between MATLAB/Simulink and AMEsim is developed. The results show that the proposed coupled thermal management strategy efficiently reduces the warm-up time and the steady-state temperature error of the engine and power battery and improves the energy utilization efficiency. [ABSTRACT FROM AUTHOR]

Power system stability is widely assessed based on tools that rely on the representation of voltages and currents through quasi-static phasor calculus, implying that the network itself and the synchronous machine stators are modeled by algebraic equations. Accordingly, the associated fast transients are neglected, assuming those decay rapidly. However, with the increasing penetration of converter interfaced generation in power systems, the reliance on quasi-static phasor calculus is to be questioned. In this paper, the validity of quasi-static phasor calculus models is verified, and dynamic phasor calculus is considered as the alternative. A methodology to systematically compare quasi-static and dynamic phasor calculus is developed. It includes frequency response, modal, and sensitivity analyses. The methodology is applied to an IEEE test network considering penetrations of converter interfaced generation of up to 100%. The models are implemented in MATLAB. The H-infinity norm is proposed as an indicator to identify differences in the applicability of the models. The results show that the quasi-static phasor calculus is suitable for stability analysis only if low bandwidths of converter controls are given. Dynamic phasor calculus instead is suitable and applicable to generic stability studies of integrated power electric and electronic systems with high penetration of renewables. [ABSTRACT FROM AUTHOR]

This paper summarizes the technical activities of the Task Force on Power System Dynamic State and Parameter Estimation. This Task Force was established by the IEEE Working Group on State Estimation Algorithms to investigate the added benefits of dynamic state and parameter estimation for the enhancement of the reliability, security, and resilience of electric power systems. The motivations and engineering values of dynamic state estimation (DSE) are discussed in detail. Then, a set of potential applications that will rely on DSE is presented and discussed. Furthermore, a unified framework is proposed to clarify the important concepts related to DSE, forecasting-aided state estimation, tracking state estimation, and static state estimation. An overview of the current progress in DSE and dynamic parameter estimation is provided. The paper also provides future research needs and directions for the power engineering community. [ABSTRACT FROM AUTHOR]

This paper deals with the innovative “variable-voltage” bus concept for future more-electric aircraft platforms. Using new functionalities and opportunities offered by innovative sources actively controlled by power electronics, there is an opportunity for a significant increase of their output power (within powertrain capabilities, machine thermal limits, and power converter maximum current) to supply increased load demands under certain conditions. This paper investigates applications of this concept to satisfy power demands for the wing ice-protection system for business jet. The study includes a detailed study into the control challenges of the proposed approach and suggests corresponding theoretical solutions. Furthermore, the controller design aspects are explored with considerations to achieve stable operation. The concept is tested in simulation environment and validated through experimental studies. This paper might be of interest for the researchers and engineers focused on innovative solutions for future aircraft platforms. [ABSTRACT FROM AUTHOR]

This paper provides the concept of design, implementation, and system integration of energy efficient power management solution for a fault-tolerant more electric engine/aircraft (MEE/MEA). The power management solution consists of three components, i.e., the power management system (PMS), the stability analysis module, and the condition-based control (CBC) module. The PMS is to optimize the electrical power system (EPS) level efficiency based on constraints with consideration of power limits of the dc system in the MEE/MEA architecture, which are calculated by the stability analysis module, and system reconfiguration, which is provided by the CBC module. This paper focuses on the design and implementation of PMS algorithm for EPS of a fault-tolerant MEE/MEA architecture. A PMS solution to optimize EPS level efficiency optimization is proposed, which is based on multiple objective optimization problem (MOOP). The proposed MOOP is solved with a modified nondominated genetic algorithm with local search mechanisms. The proposed power management solution are implemented in the MEE/MEA simulation model as well as hardware-in-loop virtual testbed and results show the effectiveness of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Electromechanical modes are inherent to any interconnected power systems which provide a measure of the small-signal stability margin of the system. A number of algorithms have been developed for the estimation of these modes using synchrophasor measurements. However, most of these algorithms are not designed to operate in the presence of forced oscillations (FO). These FOs are results of periodic rogue input driving the system. When FOs are present, estimates of system modes can be biased depending on the frequency and the amplitude of the FOs. To tackle this problem, a new algorithm is proposed in this paper to estimate system modes in the presence of FOs. In the proposed method, the over-determined modified Yule–Walker method which is used to estimate autoregressive coefficients of an autoregressive moving average (ARMA) signal model is extended to an ARMA with exogenous input (ARMAX) model that incorporates the presence of FOs. Two versions of the proposed method are included in this paper based on the requirement of the information of the duration of FOs in the signal. Results obtained by implementing the proposed algorithm on simulated data and real-world data validate the effectiveness of both versions of the proposed method. [ABSTRACT FROM AUTHOR]

A continuous Online Monitoring System (OMS) for power system stability based on Phasor Measurements (PMU measurements) at all the generator buses is proposed in this paper. Unlike the state-of-the-art methods, the proposed OMS does not require information about fault clearance. This paper proposes a convolutional neural network, whose input is the heatmap representation of the measurements, for instability prediction. Through extensive simulations on standard IEEE 118-bus and IEEE 145-bus systems, the effectiveness of the proposed OMS is demonstrated under varying loading conditions, fault scenarios, topology changes, and generator parameter variations. Two different methods are also proposed to identify the set of critical generators that are most impacted in the unstable cases. [ABSTRACT FROM AUTHOR]

Load characteristics have substantial influence on the voltage stability of power systems. The self-restorative characteristic and stalling of induction motor loads can deteriorate the voltage stability, so it is necessary to develop accurate and efficient dynamic analysis methods for voltage stability analysis of systems with induction motor loads. In this paper, a set of methods based on holomorphic embedding is proposed, which is able to solve steady states and dynamics of a power system with induction motors. The paper is the first work that applies holomorphic embedding to dynamic problems of power systems. The test cases on the IEEE 14-bus system, the NPCC 140-bus system, and the Polish 2383-bus system verify both accuracy and efficiency of the proposed methods. [ABSTRACT FROM AUTHOR]

High intermittency in today's renewable-rich distribution systems, and the prohibitive cost of upgrading the system infrastructure, has rendered voltage instability a real concern for heavily loaded systems. The voltage instability of distribution systems can spread to the corresponding transmission grid, as proven by real-life incidents. Recent advances in energy storage systems (ESSs), however, have offered a viable means of enhancing the reliability and robustness of power systems. In this paper, the optimal placement, sizing, and operation of ESS devices in wind-intensive distribution power systems are studied. The objectives of this paper are to minimize the ESS size (power and energy), required to ensure a desired voltage stability margin (VSM), and to minimize the reactive power loss and reactive power import from the upstream network. Wind uncertainty has been accounted for through a risk-based stochastic optimization framework. Besides, active network management (ANM) tools, such as the tap position of on-load tap changers, modelled by using a new method, and reactive power capabilities of both ESS devices and wind farms, are used as additional means to reduce the required ESS size. The results show that the required ESS significantly increases with increase in the desired VSM. Nevertheless, the increase can be considerably curtailed by using risk-based VS constraints and ANM tools. [ABSTRACT FROM AUTHOR]

In this paper, a new systematic method is proposed to analyze the system frequency in power systems under uncertain variability (UV). UV gradually increases in power systems, which originates from the renewable energy generation, random loads, frequency measurement, and communication channels, exerting noteworthy impacts on dynamic system frequency. The UV is modeled as a stochastic process in this paper. Then, the stochastic differential equations are used to describe the dynamic system frequency response (SFR) of a power system under UV, which is based on the SFR model. To assess system frequency dynamics under UV, an index of intra-range probability is put forward. Based on stochastic analysis theory, an analytic method is proposed to analyze the system frequency under UV, by which the intra-range probability can be analytically solved. Compared with Monte Carlo simulation in a typical SFR case and the Iceland electricity transmission network, the proposed method shows almost the same results using much less computation resource. Finally, insights for improving the SFR under UV are provided. The proposed method can be used to quickly verify whether the system frequency could withstand the UV and stay in the security range. [ABSTRACT FROM AUTHOR]

This paper develops a comprehensive dynamic-phasor-based small-signal model for the modular multilevel converter (MMC) system under unbalanced grid conditions, which takes into consideration the dynamics of the MMC system under a balanced grid condition as well as the other emerging internal harmonics produced by the unbalanced grid components and more complex controllers. The small-signal model is validated by comparing the time-domain responses from a detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC. Based on the eigen-analysis and PSCAD-based time-domain simulation studies, this paper reveals the poorly damped modes that cause unstable oscillations in MMC system under the unbalanced grid conditions. From the results, more poorly damped modes involved in the internal harmonics are emerging under unbalanced grid conditions compared with the modes under the balanced grid conditions, and can highly impact the system dynamic behaviors by the selection of the control parameters. It is also shown that, under the unbalanced grid conditions, the feasible region of the control parameters of the vector current control gets narrowed indicating the reduction of the stability margin, smaller (larger) value of proportional (integral) gain of the circulating current suppressing controller can enhance the system stability, and larger values of the proportional and integral gains of the negative-sequence current suppressing control can result in the harmonic instability of the MMC system. Besides, the parameters of multiple complex-coefficient filters for positive- and negative-sequence components extraction also have a big impact on the small-signal stability of the MMC system and are required to be properly selected. [ABSTRACT FROM AUTHOR]

Most techniques for power system analysis model the grid by exact electrical circuits. For instance, in power flow study, state estimation, and voltage stability assessment, the use of admittance parameters (i.e., the nodal admittance matrix) and hybrid parameters is common. Moreover, network reduction techniques (e.g., Kron reduction) are often applied to decrease the size of large grid models (i.e., with hundreds or thousands of state variables), thereby alleviating the computational burden. However, researchers normally disregard the fact that the applicability of these methods is not generally guaranteed. In reality, the nodal admittance must satisfy certain properties in order for hybrid parameters to exist and Kron reduction to be feasible. Recently, this problem was solved for particular cases of monophase and balanced triphase grids. This paper investigates the general case of unbalanced polyphase grids. First, conditions determining the rank of the so-called compound nodal admittance matrix and its diagonal subblocks are deduced from the characteristics of the electrical components and the network graph. Second, the implications of these findings concerning the feasibility of Kron reduction and the existence of hybrid parameters are discussed. In this regard, this paper provides a rigorous theoretical foundation for various applications in power system analysis. [ABSTRACT FROM AUTHOR]

Battery energy storage system (BESS) services will be of great help when operating power systems at high renewable energy penetrations. This paper demonstrates the requirements for effective frequency and inertia services. Some system operators currently apply scalars to financially reward improved performance. This paper demonstrates a method of validating and improving these scalars by considering the delay time and ramp time of the BESS. The importance of the method used to measure the rate of change of frequency is discussed in the paper. While the rate of change of frequency services improves with reduced delivery times, limits are identified where further reductions produce a negligible impact. This demonstrates the tangible benefits of a rapid response, and that current technology could meet the criteria. A variety of BESS capacities are trialed, which demonstrate consistency in performance and the occurrence of diminishing returns [ABSTRACT FROM AUTHOR]

This paper examines the small-signal stability of an integrated multiterminal dc (MTDC)/ac power system. The examination is based on a closed-loop multi-input multi-output interconnected model of the MTDC/ac power system, which comprises an MTDC subsystem and an ac subsystem. The focus of the examination is the condition of open-loop modal coupling when an open-loop oscillation mode of the MTDC subsystem is close to an open-loop oscillation mode of the ac subsystem on the complex plane. Analysis in the paper indicates that when the open-loop modal coupling happens, the multivariable ac–dc dynamic interactions between the MTDC and ac subsystems degrade the small-signal stability of the MTDC/ac power system. Estimation on the degree of stability degradation is derived. Hence, the paper reveals the mechanism about why the integration of the MTDC network may bring about the small-signal instability risk. In addition, a method of open-loop modal analysis to detect the risk is proposed. Example multimachine power systems with an MTDC network are presented to validate the analysis and conclusions made in the paper. Application of the proposed open-loop modal coupling analysis for parameter tuning of converter control systems of the MTDC network is demonstrated. [ABSTRACT FROM AUTHOR]

Due to the anticipated proliferation of HVDC links up to and beyond 2020 into transmission systems, it is expected that in some locations, two or more HVDC links will be installed with a short electrical distance between them and will form a multiinfeed (MI)-HVDC scenario for the system. The control and operation of MI-HVDC systems are of particular concern for weak grids such as that likely to be found in the North of Scotland network. It is shown that the ac voltage regulation in the VSC-HVDC link may adversely affect the dynamic performance of the host weak ac system under the MI-HVDC scenario. To overcome this identified limitation associated with the ac voltage controller in the VSC-HVDC link, a new controller modification conforming to the grid code is proposed in this paper. Furthermore, this paper presents a robust control design method for ac voltage control using the fractional order (FO) control design theory. The influences of the proposed FO-based ac voltage controller are assessed alongside various voltage controllers for the representative North Scotland system. It is shown that the FO-based ac voltage controller has favorable effects on the dynamic behavior of weak ac systems with MI-HVDC. [ABSTRACT FROM AUTHOR]

The paper proposes a new adaptive approach to power system model reduction for fast and accurate time-domain simulation. This new approach is a compromise between linear model reduction for faster simulation and nonlinear model reduction for better accuracy. During the simulation period, the approach adaptively switches among detailed and linearly or nonlinearly reduced models based on variations of the system state: it employs unreduced models for the fault-on period, uses weighted column norms of the admittance matrix to decide which functions to be linearized in power system differential-algebraic equations for large changes of the state, and adopts a linearly reduced model for small changes of the state. Two versions of the adaptive model reduction approach are introduced. The first version uses traditional power system partitioning where the model reduction is applied to a defined large external area in a power system and the other area defined as the study area keeps full detailed models. The second version applies the adaptive model reduction to the whole system. The paper also conducts comprehensive case studies comparing simulation results using the proposed adaptively reduced models with the linearly reduced model and coherency-based reduced model on the Northeast Power Coordinating Council 140-bus 48-machine system. [ABSTRACT FROM AUTHOR]

This paper presents an online event location estimating process in a wide-area power system using the phasor measurement unit (PMU). For real-world applications of PMU-based event location estimation (ELE), it is necessary to consider the insufficiency of PMU data caused by a lack of PMUs, communication failure, PMU malfunction, etc., because insufficient PMU data cannot guarantee event observability over the entire network. To ensure possible event locations in a real-world system, an estimation method is developed by combining offline zonal analysis and an online ELE algorithm. The K-mean clustering method is utilized for electrical-bus clustering, which enables us to recognize separate electrical zones (EZs). Then, we propose a two-stage ELE algorithm by estimating the event EZ and localizing the specified event section within the estimated EZ. Proposed ELE algorithm is applied to a real-world PMU data collected from southwestern part of the United States. Finally, successful results from a real-world PMU data application are presented at the end of this paper. [ABSTRACT FROM AUTHOR]

This paper comprehensively analyzes the stability of a grid-interfacing inverter with the LCL -filter in the discrete domain, where the LCL-filter, along with the controller, is modeled in a polar coordinate. System open-loop and closed-loop poles are analytically studied and expressed in the z -domain. Through the poles movement and distribution analysis, the relationship between system stability and the ratio of resonance frequency over sampling frequency is mathematically revealed and calculated as well as the system control gain limit. Moreover, this paper demonstrates that a grid-voltage feedforward regulator would significantly alter the inverter stability in a weak power system. By means of Jury stability criterion, the stability status under different filter resonance frequency is given. The selection of resonance frequency and filter parameters makes a considerable difference on system behavior. Finally, to improve the robustness against grid inductance variation, a conservative design recommendation of filter parameters and control gain is given. Through the tests on a laboratory-scale prototype, the theoretical analysis is validated by experimental results. [ABSTRACT FROM AUTHOR]

Time delay is prejudicial to power system stability and generally worsens the performance of a control system. In this paper, the approaches for stability analysis and stabilization of a power system consisting of time-varying delay are established. The paper starts with the modeling of the power system according to time-delay differential algebraic equations. An integral quadratic constraints (IQC) method is constructed to guarantee the stability of the proposed feedback controller. The frequency-domain IQC and time-domain dissipation inequality are effectively combined to perform stability analysis and stabilization of the power system considering time delays, without complex numerical computation. A two-area four-machine system, an IEEE 10-machine 39-bus system, and a WECC 179-bus system are employed to support the stability criteria and the IQC method. The simulation results indicate that the combined methods effectively damp the oscillation and improve the performance. [ABSTRACT FROM AUTHOR]

This paper extends the dynamic coherency determination (DCD) method by including Type-3 wind power plants (WPPs) models in the coherency evaluation process and quantifies impacts of the high depth of wind power penetration on the coherency phenomena of interconnected power systems. The method is based on frequency-deviation signals, measured at nongenerator buses and terminal buses of WPPs and synchronous generators. Salient features of the presented method are its ability to account for the hybrid nature of enhanced generic models of Type-3 WPPs and expanding notion of the power system coherency beyond the classical definition by including models of Type-3 WPPs. This paper also investigates impacts of the high depth of wind power penetration; i.e., up to 30%, on the coherency structure of the 16-machine/68-bus NPCC equivalent system. The studies are based on time-domain simulations in PSS/E software. The investigations reveal that Type-3 WPPs can introduce new coherent areas, significantly change areas’ boundaries and number of areas, and alter frequencies/dampings of interarea modes. [ABSTRACT FROM AUTHOR]

Event-triggered control (ETC) is becoming increasingly popular in load frequency control (LFC) operation in power systems. So far, the existing literature has primarily focused on load disturbance rejection in face of communication delay without considering much on networked-induced measurement disturbances and messages sent. In this paper, we study the LFC operation as a problem of networked control system and propose, for the first time, an ETC based switching approach for frequency regulation. We derive delay-dependent conditions to obtain the lower bound on inter-event time during event-trigger mechanism which assures the robust stability under networked induced time-varying delays and measurement disturbance and reduction in the amount of sent measurements. The effectiveness of the proposed approach is verified via numerical examples based on single-, two-area power systems and single-area with electric vehicle (EV) participation. [ABSTRACT FROM AUTHOR]

This paper investigates the modal repulsion happened in the sub-synchronous resonance between a synchronous generator and series-compensated transmission line. The investigation is based on the closed-loop model of the series-compensated power system consisted of the torsional and electrical open-loop subsystems. Analytical explanation is presented to show how the SSO modal repulsion is formed as the result of moving close and apart between two open-loop SSO modes. Analysis is carried out to indicate that the peak of the SSO modal repulsion occurs when the two open-loop SSO modes from the torsional and electrical subsystem are closest to each other on the complex plane, which is referred to as the open-loop modal coupling. Estimation on the peak, i.e., the maximum damping degradation of the SSO modes, is derived in the paper. Subsequently, a method of open-loop modal analysis is proposed. Applications of the proposed method of open-loop modal analysis to study the SSOs caused by the DFIGs under the condition of the modal repulsion in the power system are demonstrated and evaluated by example power systems. The concept of modal repulsion explains why the grid connection of the DFIGs may bring about the SSO instability risk in the power system. [ABSTRACT FROM AUTHOR]

This paper proposes a fast and robust unscented Kalman filter based decentralized dynamic state estimator (DSE) for power system online monitoring and control. The proposed robust DSE is able to detect, identify, and suppress three types of outliers, namely the observation, innovation, and structural outliers. Observation outliers refer to the received PMU measurements providing unreliable metered values due to gross errors or cyber attacks; innovation outliers are typically caused by impulsive system process noise, whereas structural outliers are induced by incorrect parameters of the generators or its associated controllers, such as exciters and speed governors. To enable the fast estimation of generator states of large-scale power systems in a decentralized manner, two model decoupling approaches are presented and compared. It is shown that the generator decoupling approach presented in this paper achieves higher statistical efficiency than the ones proposed in the literature in the presence of both small and large measurement noise. To detect and distinguish three types of outliers, projection statistics based multiple hypothesis testing approach is proposed. Specifically, three hypotheses corresponding to the occurrence of three types of outliers are assumed by constructing three innovation matrices; these matrices are made up by time-correlated innovation vectors, and/or predicted states, and/or measurements; then projection statistics are applied to each of the innovation matrix and its calculated projection values are checked by a statistical test to validate the assumed hypothesis. The identified outliers are further suppressed by a generalized maximum-likelihood-type estimator. Numerical results carried out on the IEEE 39-bus system demonstrate the effectiveness and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

Wide-area damping controller (WADC) is a very effective tool to damp out the low-frequency oscillation in wide-area power systems. However, the WADC needs communication networks to transmit remote signals; hence, the network-induced delay and packet dropout are inevitably introduced in the communication of wide-area measurement systems (WAMS). In order to effectively compensate delay and packet dropout, in this paper, a new wide-area dynamic damping controller (WADDC) is designed. Firstl the discrete-time model of power system with random delay, packet dropout, and external disturbance is established. Then, the WADDC based on robust H∞ control is designed and the sufficient condition about stability of the closed-loop system is given. Finally, the controller designed in this paper is tested on the two-area four-machine power system and the 16-machine 68-bus system. Simulation results demonstrate the effectiveness the proposed methods for damping out the low-frequency oscillation and compensating the network-induced delay and packet dropout. [ABSTRACT FROM AUTHOR]

This paper proposes a novel method to analyze impacts of uncertain variability on power system dynamics. There is considerable interest in integrating intermittent renewable energy and plug-in electric vehicles into power systems. Therefore, power systems are in an environment with increasing uncertain variability. In this paper, the uncertain variability is described as a continuous-time stochastic process, and thus, the power system under uncertain variability is modeled by stochastic differential equations. To quantify impacts of uncertain variability on power system dynamics, an intra-region probability index is presented. Based on the stochastic averaging method, an analytical method is proposed to calculate the intra-region probability. Compared to Monte Carlo simulation, the proposed method is many orders of magnitude faster without sacrificing the result accuracy. Furthermore, several insights are given to improve the power system dynamics under uncertain variability. [ABSTRACT FROM AUTHOR]

Large-scale wind farms (WFs) are gradually required to participate in power system frequency regulation, especially in a weak grid. Corresponding ancillary frequency control functionalities and the contribution of the WF to system frequency stability are well studied. However, the impact of WF ancillary frequency control on wind turbine generators (WTGs) is rarely considered. It is found in this paper that the ancillary frequency control deteriorates drive-train torsional oscillation of the WTG, which restricts application of the ancillary control. To overcome this problem, this paper proposes an adaptive damping control scheme based on reactive power modulation of the WTG to suppress the drive-train torsional oscillation, which can assist the WF to meet the grid code requirements. Superior to the traditional active power modulation method, the proposed adaptive control scheme can achieve satisfactory damping improvement with less control effort. Furthermore, the adaptive scheme with a torsional frequency updating mechanism can effectively counteract torsional damping deterioration caused by parameter uncertainties, operating condition or control mode variations. Feasibility and effectiveness of the proposed damping control are validated by simulations and real-time experiments. [ABSTRACT FROM AUTHOR]

The uncertainties of renewable energy may bring impacts to power system reliability. This paper proposes a novel generation rescheduling algorithm, which adjusts generation outputs to mitigate the variations of branch power flows and relieve the overload probability. This algorithm helps prevent power congestion and balance the aggregate renewable generations and loads in power systems. Besides, the paper also proposes a probabilistic method evaluating the system uncertainty issue in the power system considering generation rescheduling operation. The proposed generation rescheduling algorithm has been tested on the Arizona transmission system. The generation rescheduling method enhances power system reliability analysis with a high penetration of renewable energy resources. [ABSTRACT FROM PUBLISHER]

In this paper, a line modal potential energy (LMPE) method is proposed for the interarea oscillation analysis and damping control of doubly fed induction generators (DFIGs). This paper presents a novel attempt to design the damping control loop using the LMPE approach from the network viewpoint. First, an LMPE function containing a DFIG output line is constructed. The distribution characteristics of the LMPE in a network are analyzed in different modes, and the dominant mode of the system is identified. In addition, this approach provides the basis for damping regulation. Second, the wide-area control signals and control locations are selected. Third, a damping controller for DFIGs based on LMPE and extended state observer-\rmH{\rm{\infty }} control is designed, and this design is shown to have a certain anti-interference ability. The proposed method can be used for submode analysis and oscillation suppression. Finally, the correctness of the LMPE method is verified through the analysis and simulation of a four-generator, two-area system and an actual power system in China . [ABSTRACT FROM AUTHOR]

Power system operation will encounter numerous variabilities as the proliferation of renewable energy continues. Such random events will require the evaluation of corresponding variables and the assessment of their impacts on the monitoring and the control of stochastic power systems. In this paper, a global sensitivity analysis (GSA) method is proposed to perform a priority ranking of renewable energy variabilities that will affect the voltage stability of power systems. First, a probabilistic model for the load margin calculation is presented considering the renewable power generation. Then, GSA is applied to models with correlated random input variables and the importance index is designed for evaluating the impacts of such variables. Next, the stochastic response surface method is adopted in GSA to improve the computation efficiency of the proposed evaluation method. Finally, the overall procedure is presented to identify critical variables that can affect the variability of load margins in voltage stability analyses. The proposed method is applied to power systems with a large penetration of renewable power and the influences of critical variabilities on voltage stability are analyzed. In this paper, the proposed method is tested using the IEEE 9-bus and the IEEE 118-bus systems, and its accuracy is compared with those of the local sensitivity analysis method and the commonly used GSA methods. [ABSTRACT FROM PUBLISHER]

This paper examines the impact of dynamic interactions between the voltage source converters (VSCs) on the small-signal stability of an AC/MTDC (multiterminal dc) power system. The examination is carried out based on a closed-loop interconnected model consisted of an open-loop VSC subsystem and an open-loop subsystem of the rest of MTDC network (RDCN). Open-loop modal coupling is the modal condition that an oscillation mode of open-loop VSC subsystem is close to an oscillation mode of open-loop subsystem of the RDCN on the complex plane. Analysis indicates that the open-loop modal coupling may very likely degrade the small-signal stability of the AC/MTDC power system. If a VSC uses the active power control, the open-loop VSC subsystem is of no oscillation mode. Hence, open-loop modal coupling between the VSCs cannot occur in the MTDC network implementing the master–slave control. Whilst the open-loop modal coupling may introduce instability risk to the MTDC network using the dc voltage droop control. Therefore, the paper reveals the mechanism about how and why the dynamic interactions between the VSCs may degrade the small-signal stability of the AC/MTDC power system from the perspective of the system modal condition—the open-loop modal coupling. [ABSTRACT FROM PUBLISHER]

The main framework of a new adaptive coordinated voltage control (ACVC) has been presented and published in our previous paper. ACVC is realized based on knowledge exploration from nondominated solution sets of multiobjective optimization. Knowledge is prepared and accumulated from offline training and online learning processes. This facilitates fast responses. An important basic work is to further study knowledge sensitivity to power system characteristics such that the efficiency of ACVC for various dynamics of power systems can be improved. A comparative study and discussion of power system scale, allocation of control devices, operational point variations, and severity of emergencies are presented in this paper. An improved form of knowledge is also proposed for ACVC based on these discussions. Numerical simulations on the IEEE 39-bus, 118-bus, and 300-bus power systems are performed for comparison and discussion. [ABSTRACT FROM PUBLISHER]

This paper investigates the small-signal stability of power electronics-based power systems in frequency domain. A comparison between the impedance-based and the eigenvalue-based stability analysis methods is presented. A relation between the characteristics equation of the eigenvalues and poles and zeros of the minor-loop gain from the impedance-based analysis have been derived analytically. It is shown that both stability analysis methods can effectively determine the stability of the system. In the case of the impedance-based method, a low phase-margin in the Nyquist plot of the minor-loop gain indicates that the system can exhibit harmonic oscillations. A weakness of the impedance method is the limited observability of certain states given its dependence on the definition of local source-load subsystems, which makes it necessary to investigate the stability at different subsystems. To address this limitation, the paper discusses critical locations where the application of the method can reveal the impact of a passive component or a controller gain on the stability. On the other hand, the eigenvalue-based method, being global, can determine the stability of the entire system; however, it cannot unambiguously predict sustained harmonic oscillations in voltage source converter (VSC) based high voltage dc (HVdc) systems caused by pulse-width modulation (PWM) switching. To generalize the observations, the two methods have been applied to dc–dc converters. To illustrate the difference and the relation between the two-methods, the two stability analysis methods are then applied to a two-terminal VSC-based HVdc system as an example of power electronics-based power systems, and the theoretical analysis has been further validated by simulation and experiments. [ABSTRACT FROM AUTHOR]

This paper presents a sufficient and necessary condition for transient stability of multimachine power systems based on the passivity formalism. The condition derived using the extended classical model (ECM) with zero transfer conductance (TC). The ECM of power systems relaxes slightly the assumption of “voltage-behind-transient-reactance” from “constant”, which is used in the conventional classical model to “positive, bounded and independent of machine rotor angles”. The results of time domain simulation of a full model of Western Interconnection used for regular operation planning study are presented for verifying the model assumptions and the storage functions of the system in four different scenarios (well stable, marginally stable, marginally unstable and very unstable), thus illustrating the sufficiency and necessity of stability criterion based on passivity. [ABSTRACT FROM AUTHOR]

Electromechanical wave propagation is present in power systems as a transient spatial delay of generator rotor angle variations when a disturbance occurs. The control and mitigation of the this wave phenomenon, which spreads out to different locations at a comparably slow speed, is important for enhancing power system transient stability, as shown in part two of this two-paper series. In an idealized power system consisting of infinitesimal generators and line impedances, this traveling wave can exist to infinite frequencies, its speed can be derived from a wave equation and its reflection can be eliminated by a characteristic termination. Part one of this paper series examines realistic power systems with finite size generators and line impedances. It shows that there is a finite approximation of the traveling wave in real power systems as the wave can exist only within a finite frequency range. This paper also examines aspects such as reflections due to discrete elements, controller design based on the load modulation effect, and the existence of traveling wave and its attenuation in the Australian power system. [ABSTRACT FROM AUTHOR]

This second part of a two-paper series gives an overview of key insights and control implications arising from the wave characteristics of power system transient stability. These insights, which are unique to traveling wave interpretation, are obtained through time-domain analysis of artificial and real test systems: “critical region for a stressed link near the end of a longitudinal system,” “dispersed control in a path,” “placement of controllers considering structure and load distribution in a system,” and “localized transient stability enhancement.” Good transient stability enhancement is successfully achieved by controllers in all test systems in terms of larger critical clearing time, which directly translates to increased maximum safe power transfer. The controller design is based on the transmission line analogy of traveling waves and load modulation effect. The concepts in this paper series can assist in improving the design of control strategies to enhance power system transient stability by considering the wave aspects. [ABSTRACT FROM AUTHOR]

Wind power generators will become increasingly useful in modern power systems; therefore, they should be able to support the system frequency in a way similar to that of conventional generators. Droop and virtual inertia are the most reported methods in the literature. However, their impact on the generator and system frequency stability, when the double-mass mechanical dynamics of a wind power generator are considered, has not been addressed thoroughly. In this paper, small-signal modeling, analysis, and eigenvalues studies are used to show that incorporating a wind power generator in the frequency regulation can expose its shaft to forces stimulating its natural resonance frequency dynamics and lead to instability. This paper shows that the mechanical resonance of frequency-regulating wind generators must be studied and enhanced not individually but as a part of the whole power system stability analyses. To overcome the stability problem caused by implementing frequency regulation in wind generators, this paper investigates different alternatives to stabilize the generator dynamics and, at the same time, minimize undesirable interference in the conventional wind power generator controllers. Time-domain simulation results validate the analytical results and discussions. [ABSTRACT FROM AUTHOR]

This paper critically evaluates a number of sensitivity analysis (SA) techniques to identify the most influential parameters affecting power system small-disturbance stability. SA of uncertain parameters has attracted increased attention with the adoption of deregulated market structure, intermittent energy resources, and new types of loads. Identification of the most influential parameters affecting system stability using SA techniques will facilitate better operation and control with reduced monitoring (only of the parameters of interest) by system operators and stakeholders. In total, nine SA techniques have been described, implemented, and compared in this paper. These can be categorized into three different types: local, screening, and global SA. This comparative analysis highlights their computational complexity and simulation time. The methods have been illustrated using a two-area power system and 68 bus NETS-NYPS test system. The priority ranking of all uncertain parameters has been evaluated, identifying the most critical parameters with respect to the small-signal stability of the test systems. It is shown that for many applications, the Morris screening approach is most suitable, providing a good balance between accuracy and efficiency. [ABSTRACT FROM AUTHOR]

This paper proposes the exploitation of multiterminal HVDC grids to improve transfer capability in power systems. Multiterminal HVDC systems based on voltage source converters (VSC-MTDC) have been recognized as a promising alternative for the wind power integration. Under low wind scenarios, these grids originally dedicated for wind power transmission can be exploited as an additional interarea transmission path, providing extra dynamic security. The paper focuses on small-signal stability assessment, especially in poor damped oscillations associated with interarea modes. Simulations performed through a generic computational framework have shown that the high level of flexibility and controllability provided by voltage source converters can considerably improve the transfer capacity, while preserving adequate dynamic performance. [ABSTRACT FROM AUTHOR]

Due to the increasing penetration level of microgrids (MGs), it becomes a critical issue for MGs to help sustaining power system stability. Therefore, ancillary services, such as the low-voltage ride-through (LVRT) capability should be incorporated in MGs in order to guarantee stable operation of the utility grid during grid faults. In this paper, a LVRT control strategy based on positive/negative sequence droop control is proposed for grid-interactive MGs to ride-through voltage sags with not only inductive/resistive, but also complex line impedance. By using the proposed control strategy, MGs can support the grid voltage, make profits, and also ride-through the voltage dip during the whole fault period. A two-layer hierarchical control strategy is proposed in this paper. The primary controller consists of voltage and current inner loops, a conventional droop control and a virtual impedance loop, while the secondary controller is based on a positive/negative sequence droop scheme which is able to coordinate the power injection during voltage sags. Experimental results are obtained to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

This paper presents analytical and simulation studies of wind farms (WFs) connected to power system through a unified interphase power controller (UIPC), which should control power flow and the short-circuit contribution of WF. The UIPC is based on an interphase power controller (IPC) in which the phase-shifting transforms are replaced by two series converters (SECs). In this paper, the UIPC model is developed based on phase angles of injected SEC voltage and a control scheme is proposed to control power flow and limit the short-circuit contribution of WF. The WF model is based on an equivalent aggregated variable-speed double-fed induction generator. The PSCAD/EMTDC simulation results show that the UIPC is an effective and novel solution for power-flow control and limitation of short-circuit contribution of WF in power systems. [ABSTRACT FROM AUTHOR]

Special protection schemes (SPSs) are an efficient alternative to increase the flexibility in the operation of power systems. The implementation of SPSs is often motivated by technical rather than economic reasons. The aim of this paper is to propose a novel framework for the analysis and development of SPSs in a context from the Chilean experience that comprises both technical and economic incentives to enable high penetration of solar energy. With this purpose, this paper explores the new capabilities of SPSs based on maximizing the cost-effective integration of solar power into a power system, given a set of technical challenges (congestion and dynamic response in case of contingencies) that must be considered in their design. In this framework, three main drivers were identified and discussed. The discussion includes the mechanisms associated with each driver that promote the massive integration of solar power. Moreover, within the proposed technical-economic framework, a methodology for driver selection and SPS design is proposed. The methodology and the drivers identified are evaluated in the Chilean power system. [ABSTRACT FROM AUTHOR]

This paper proposes a new approach for power system transient stability simulation, which is based on a semi-analytical solution (SAS) of power system differential-algebraic equations. In this paper, an SAS is derived using the Adomian decomposition method as a closed-form explicit function of symbolic variables such as time, the initial state, and other variables on system conditions and; hence, it can directly give a power system's dynamic trajectory being accurate for a certain time window. Unlike a traditional numerical integration based simulation approach, the proposed new approach offline derives an SAS and online evaluates the SAS by plugging in values of symbolic variables for a series of time windows making up the desired simulation period. This paper further studies the maximum length of the time window for an SAS being accurate and proposes a divergence indicator for simulation using adaptive time windows. Implementation of this new approach on parallel computers is also studied. The new approach is validated through contingency simulation of the IEEE 10-generator 39-bus system with detailed generator models. [ABSTRACT FROM AUTHOR]

In a recent paper, we proposed a system identification method for constructing reduced-order models for the electro-mechanical dynamics of large power systems, divided into multiple coherent clusters, using Synchrophasors. Every cluster in the actual model was represented as an aggregate generator in the reduced-order model. An aggregate network graph connected one aggregate generator to another. In this paper, we extend this identification approach to differential-algebraic (DAE) models. First, every cluster is associated with a unique terminal bus, referred to as the pilot bus, that couples its internal network to the rest of the system. The proposed algorithm uses Synchrophasor measurements from the pilot buses to identify the dynamic model of the aggregate generator for each cluster using nonlinear least squares while retaining the identity of all the pilot buses. The resulting reduced-order model is in the form of a nonlinear electric circuit described by aggregate differential and algebraic equations. We illustrate our results using two case studies, one for the IEEE 9-bus power system and another for the IEEE 39-bus power system. We also discuss how these reduced-order DAE models may be useful for designing shunt controllers at the pilot buses by using Synchrophasor feedback. [ABSTRACT FROM AUTHOR]

This two-part paper presents a framework for the characterization and minimal representation of the feasibility regions of power systems in the demand space. These feasibility regions are called loadability sets, and they represent the projection of generation-demand-network space onto the demand space only. Loadability sets have been characterized previously for power systems with either no or a single active line flow constraint. In Part I of this two-part paper, we generalize this characterization to power systems with any number of active line flow constraints. The proposed characterization approach makes use of the Fourier–Motzkin elimination method which leads to the generation of a large number of constraints. We set course to address this shortcoming in the second part of this paper. The notion of umbrella set is revisited in Part II to identify and remove the redundant constraints produced in the loadability set characterization process. The outcome of the proposed framework is the minimal representation of the power system feasibility region in the demand space. We envision multiple applications for the proposed framework in power system planning, operations planning, and real-time operation. [ABSTRACT FROM AUTHOR]

When generators of a power system are subject to a large disturbance, their electromechanical oscillations (EOs) are essentially nonlinear as observed from their rotor angle waveforms due to the nonlinear power network. However, traditional modal analysis has not given enough considerations to nonlinearities in EOs. This paper analytically formulates the accurate oscillation frequency (OF) of an EO mode addressing nonlinearities. It is revealed that when the system model and condition are fixed, the OF of any EO mode is energy-dependent as a function of the oscillation amplitude (OA). That is characterized by a new tool named Frequency-Amplitude (F-A) curve regarding each mode, from which the frequency decreases from the natural frequency toward zero when the amplitude grows to a critical threshold. The paper proves that an F-A curve is actually a projection of the system trajectory between the stable equilibrium and the stability boundary onto the OF-OA plane regarding one mode. Based on the concept of F-A curve, a stability index is defined and estimated from measurements for online angle stability analysis. The proposed methodology is presented in detail using an SMIB system and then demonstrated on the IEEE 9-bus system and WECC 179-bus system having multiple EO modes. [ABSTRACT FROM PUBLISHER]

This paper presents a new method for studying electromechanical transients in power systems using three phase, combined transmission and distribution models (hybrid models). The methodology models individual phases of an electric network and associated unbalance in load and generation. Therefore, the impacts of load unbalance, single phase distributed generation and line impedance unbalance on electromechanical transients can be studied without using electromagnetic transient simulation (EMTP) programs. The implementation of this methodology in software is called the Three Phase Dynamics Analyzer (TPDA). Case studies included in the paper demonstrate the accuracy of TPDA and its ability to simulate electromechanical transients in hybrid models. TPDA has the potential for providing electric utilities and power system planners with more accurate assessment of system stability than traditional dynamic simulation software that assume balanced network topology. [ABSTRACT FROM PUBLISHER]

This paper proposes an improved voltage regulation method in a multisource-based dc electrical power system in the more electric aircraft. The proposed approach, which can be used in terrestrial dc microgrids as well, effectively improves the load sharing accuracy under high droop gain circumstance with consideration of cable impedance. Since no extra communication line and controllers are required, it is easily implemented and also increases the system modularity and reliability. By using the proposed approach, the dc transmission losses can be reduced and the system stability is not deteriorated for normal and fault scenarios. In this paper, optimal droop gain settings are investigated and the selection of individual droop gains as well as the proportional power sharing ratio has been described. Simulation and experimental results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

The electric power system is witnessing a shift in the technology of generation. Conventional thermal generation based on synchronous machines is gradually being replaced by power electronics interfaced renewable generation. This new mode of generation, however, lacks the natural inertia and governor damping, which are quintessential features of synchronous machines. The loss of these features results in increasing frequency excursions and, ultimately, system instability. Among the numerous studies on mitigating these undesirable effects, the main approach involves virtual inertia (VI) emulation to mimic the behavior of synchronous machines. In this paper, explicit models of grid-following and grid-forming VI devices are developed for inertia emulation and fast frequency response in low-inertia systems. An optimization problem is formulated to optimize the parameters and location of these devices in a power system to increase its resilience. Finally, a case study based on a high-fidelity model of the South-East Australian system is used to illustrate the effectiveness of such devices. [ABSTRACT FROM AUTHOR]

More electric aircrafts (MEAs) comprise a vast amount of power electronic loads, which usually behave as constant power loads (CPLs). The incremental negative impedance of CPLs threatens system stability. To ensure an effective control of power flow in MEAs, eliminating the undesired behavior of CPLs is a necessity. This aim requires spontaneous power estimation of the time-varying uncertain loads. In this paper, an adaptive backstepping controller, which is interconnected to a third-degree cubature Kalman filter, is developed for a dc microgrid (MG) feeding nonideal CPLs. At first, the load power is considered as an artificial state and augmented into the system states, which enables estimation of not only the dc MG states but also the unknown value of the load power. The estimated load power is then forwarded to a backstepping controller. The systematic approach of this controller allows obtaining the control signal, which is the duty ratio of the switch, to not only system stabilizing but also tracking a desired voltage of the dc bus under the load power variations. The proposed adaptive controller is tested on a dc MG that has one CPL. The conducted experimental results verify the proposed nonlinear control in tracking the desired voltage of the dc bus under slow and fast variations of the load power. [ABSTRACT FROM AUTHOR]

AC cascaded system is one of the basic components of the power conversion system in the modern more-electric aircraft (MEA). However, the ac cascaded system suffers from an instability problem, i.e., even if each subsystem is stable individually, the formed cascaded system may become unstable. Most of the existing stabilization methods are a linear control method, which is based on a small signal model and can only ensure small signal stability near the operation point in theory. Therefore, these existing linear stabilization methods may be ineffective to fix large-signal stability issues. In this paper, a large signal stability constraining dichotomy solution (SCDS)-based model predictive control (MPC) is proposed to improve the stability of the ac cascaded system in the MEA by the Lyapunov criteria. This approach first theoretically presents how to add the system stability item to the MPC cost function in an analytic way instead of using the traditional “experience-based” weight method. Besides, the SCDS-MPC itself is also an improvement to the existing finite control set MPC. It can realize a constant switching frequency and achieve better steady-state/dynamic current control performance. Finally, a 1.5 kW ac cascaded system is fabricated in the laboratory to verify the proposed SCDS-MPC method. [ABSTRACT FROM AUTHOR]