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This paper develops a modeling method for robust stability analysis of nonlinear electrical power systems over a range of operating points and under parameter uncertainties. Standard methods can guarantee stability under nominal conditions, but do not take into account any uncertainties of the model. In this study, stability is assessed by using structured singular value (SSV) analysis, also known as $\mu$ analysis. This method provides a measure of stability robustness of linear systems against all considered sources of structured uncertainties. The aim of this study is to apply the SSV method for robust small-signal analysis of nonlinear systems over a range of operating points and parameter variations. To that end, a modeling methodology is developed to represent any such system with an equivalent linear model that contains all system variability, in addition to being suitable for $\mu$ analysis. The method employs symbolic linearization around an arbitrary operating point. Furthermore, in order to reduce conservativeness in the stability assessment of the nonlinear system, the approach takes into account dependences of operating points on parameter variations. The methodology is verified through $\mu$ analysis of the equivalent linear model of a 4-kW permanent magnet machine drive, which successfully predicts the destabilizing torque over a range of different operating points and under parameter variations. Further, the predictions from $\mu$ analysis are validated against experimental results. [ABSTRACT FROM PUBLISHER]

The improper operation of the third-zone distance protection has been attributed as one of the factors for power system blackouts. The third-zone protection could incorrectly operate during power-swing scenarios. This research proposes a fast and practical methodology using local measurements for blocking the third-zone distance relays during stable power swings. The proposed scheme is referred to as the BTZ scheme. The proposed method calculates the relative speed of a fictitious equivalent machine from the local relay measurements. If the relative speed goes through a zero crossing, the swing is classified as a stable power swing; whereas if the speed does not go through a zero value, then the swing is classified as an unstable power swing. The benefit of the proposed method is that it analyzes the power swings from a system stability point of view, and does not need rigorous offline simulation studies to determine the relay settings. The performance of the proposed method in this paper is compared with an industry-standard method (conventional double blinder method). As is well known, the settings of the blinder-based methods are system specific and need several simulations to arrive at the blinder values. Transient simulation studies on a three-bus system and a modified Western Coordination Council 9-bus system are used to test the performance of the proposed approach. [ABSTRACT FROM PUBLISHER]

Series resonance between capacitance and line inductance may magnify background harmonic voltage and worsen the harmonic voltage distortion in power systems. To solve this problem, in this paper, the transmission line theory is used to set up the distributed parameter model of power system feeders, and analyze the harmonic voltage propagation caused by the background harmonic voltage in power systems. Then, a new strategy is proposed for the site selection of resistive active power filter to damp the background harmonic voltage in power systems. Experiments have been performed to verify the theoretical analysis and demonstrate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

This paper presents a method of estimating the electromechanical damping and frequency coefficients of interconnected power systems. The method combines AESOPS approach and eigenvalue analysis. Computation time is reduced by applying an approximation based on an eigenvalue sensitivity formula, which evaluates eigenvectors of the 2ng\times 2ng [A(\omegaK)] and [A(\omegaK)]^\rm T matrix and thereafter use them to obtain improved estimates of the eigenvalues. Numerical results from a 16-generator system demonstrate the feasibility of the proposed method. [ABSTRACT FROM PUBLISHER]

In this paper, a modeling technique for small-signal stability assessment of unbalanced power systems is presented. Since power distribution systems are inherently unbalanced, due to its lines and loads characteristics, and the penetration of distributed generation into these systems is increasing nowadays, such a tool is needed in order to ensure a secure and reliable operation of these systems. The main contribution of this paper is the development of a phasor-based model for the study of dynamic phenomena in unbalanced power systems. Using an assumption on the net torque of the generator, it is possible to precisely define an equilibrium point for the phasor model of the system, thus enabling its linearization around this point, and, consequently, its eigenvalue/eigenvector analysis for small-signal stability assessment. The modeling technique presented here was compared to the dynamic behavior observed in ATP simulations and the results show that, for the generator and controller models used, the proposed modeling approach is adequate and yields reliable and precise results. [ABSTRACT FROM PUBLISHER]

The objective of this paper is to propose a modularized bidirectional single-phase grid-connected inverter (BGI) with constant-frequency asynchronous sigma–delta modulation (CF-ASDM) control for the renewable energy (RE)-supplied dc power system. The proposed BGI can be operated either in the inverter mode (IM) or rectified mode (RM); both are necessary features for the RE-supplied dc power system. By comparing to the conventional ASDM with variable switching frequency, the proposed CF-ASDM can simplify the filter design and can reduce the computational burden of the digital signal processor while maintaining the merit of quick dynamic response. Moreover, the proposed BGI has a redundant central controller which can increase the system reliability. The major contribution of this paper includes developing the modularized BGI with CF-ASDM control scheme for IM and RM operations thoroughly and applying it to the RE-supplied dc power system successfully. In this paper, the mathematical equations to achieve the constant switching frequency operation for the CF-ASDM will be derived. Computer simulations and hardware experiments are shown to verify the performance of the proposed modularized BGI with CF-ASDM control. [ABSTRACT FROM PUBLISHER]

For the operators of a power system, underfrequency load shedding is important for maintaining the system in a stable and reliable state whenever it suffers from a severe disturbance. The underfrequency relay protects the power system against a blackout when the system frequency declines to predetermined settings. This paper determines the shed load at each stage for the underfrequency relay (81L). The genetic algorithm (GA) is employed to minimize the shed load and maximize the lowest swing frequency. Penalty functions and chromosomes with varying lengths are utilized in the GA to determine the optimal shed loads at all stages. The uncertainty in bus loads is modeled by a fuzzy set. A stand-alone power system with diesel and wind-power generators is adopted to demonstrate the applicability of the proposed method. [ABSTRACT FROM PUBLISHER]

With the increasing use of renewable energy resources and energy storage devices, inverter-based distributed energy resources (DERs) become the important components in microgrids. As diesel generators with direct ac connections are the current most cost effective and reliable power sources, the stability investigation of microgrids should include both types of DERs. In this paper, dynamics of diesel generation will be included and the interaction of the diesel generators and the inverter-based DERs will be investigated using eigenvalue analysis and time-domain simulations. The significant contributions of this paper include: 1) identification of the stability problem in microgrids with inverter-based DERs and conventional generators and 2) investigation of the interaction problem of inverter-based DERs and conventional generators in islanded microgrids. [ABSTRACT FROM AUTHOR]

This paper addresses stability problems in power systems with loads that exhibit constant-power behavior. Instability may occur in such systems due to the negative incremental impedance of constant-power loads (CPLs). Existing approaches to stabilizing such systems require modification of the source and/or the load control characteristics, or isolating the CPL from the rest of the system by additional active devices, which are difficult to implement and often conflict with other system requirements such as control bandwidth, size, weight, and cost. In this paper, we investigate passive damping as a general method to stabilize power systems with CPL. Using a representative system model consisting of a voltage source, an LC filter, and an ideal CPL, we demonstrate that a CPL system can be stabilized by a simple passive damping circuit added to one of the filter elements. Three different damping methods are considered and analytical models are developed for each method to define damping parameters required for stabilizing the system. Time- and frequency-domain measurements from an experimental system are presented to validate the methods. [ABSTRACT FROM AUTHOR]

This paper discusses a novel approach for damping interarea oscillations in a bulk power network using multiple urnfled power flow controllers (UPFCs) utilizing ultracapacitors, also known more generally as electrochemical capacitors (ECs). In this paper, a new control is introduced to mitigate interarea oscillations by directly controlling the UPFCs' sending and receiving bus voltages that better utilizes the stored energy in the ECs. The results of this controller are compared with and without ECs. The proposed control provides better interarea oscillation mitigation when applied to multiple UPFCs in the 118-bus IEEE test system. [ABSTRACT FROM AUTHOR]

A load model is one of the most important elements in power system simulation and control. Recently, the constant impedance, constant current, and constant power load in combination with the induction motor load have been widely used as the composite load model, whose parameters are all identified from the field measurements in measurement-based load modeling practices so far. However, there is virtually no research conducted on whether all these parameters could really be identified. This paper investigates the possibility on reducing the number of composite load model parameters to be identified from field measurements. This paper first shows that direct application of the IEEE load motor parameters in the composite load model may be inadequate on describing the load dynamics over different operating status. Then the perturbation method is used to derive the trajectory sensitivities of the equivalent motor parameters, based on which the reduction on the identified parameters of the composite load model is presented. Two cases of measurement-based load modeling in North China and Northeast China are studied to illustrate the validity of the reduction method. It is shown that the reduction does not lose the model's capability on describing the load dynamics. The reduction on the number of identified parameters not only provides a possible way to solve the multi-valued load model problem based on the current practices on measurement-based load modeling, but it also facilitates building of the load model with more components included in it. Meanwhile, reducing the identified parameters reduces the identification time; thus, the proposed strategy significantly enhances the efficiency of the load modeling work. [ABSTRACT FROM AUTHOR]

In isolated power systems, frequency stability is traditionally considered to be assured by maintaining sufficient spinning reserve that is usually equal to the largest generation output of all the units in operation. However, the system security depends on the spinning reserve allocation to the units in operation and the provision of adequate emergency or fast reserve that need be available in the first crucial seconds after a loss of generation event. The system emergency reserve is usually much lower than that expected if only the unit droops are taken into account. Also, the generating units can be overloaded for a short time period and this capability can contribute to a faster frequency restoration. This paper presents an improved speed governor model that utilizes the short time overloading capability of the generating units. The efficient computational method described in the paper for the optimal spinning reserve allocation in isolated power systems was developed by taking into account a composite security criterion in the most economic way. Furthermore, a statistical analysis is presented in order to investigate the contribution of spinning reserve to the short-term system security. In the paper the developed method is applied to a typical isolated power system and it demonstrates its advantages. [ABSTRACT FROM AUTHOR]

An advanced high side voltage control (HSVC) that can improve power system angle and voltage stability by adding supplemental control to conventional generator excitation systems has been developed. An earlier version of this advanced HSVC was presented at the IEEE PES 2000 Summer Meeting. This paper describes the principles, characteristics, and advantages of applying HSVC as well as highlights of recent improvements for estimating the high side voltage by incorporating active current into the control algorithm. We confirmed by simulation analysis that the accuracy of the estimated high side voltage and the resultant high side voltage control are improved. Moreover, the improved method was verified at a hydro power plant, and it was shown that the advanced HSVC was accurate and the control was stable, as reported in a companion paper. [ABSTRACT FROM AUTHOR]

The accuracy of the load model has great effects on power system stability analysis and control. Based on our practice in China on modeling load from field measurements, this paper systematically develops a measurement-based composite load model. Principles guiding the load modeling practice are discussed based on detailed analysis on stochastic characteristics of the modeling procedure. The structure of the measurement-based composite load model is presented. A multicurve identification technique is described to derive parameters. The generalization capability of this built load model is also investigated in this paper. Two cases are studied to illustrate the accuracy of the developed load model on describing the load dynamic characteristics in the actual power system. [ABSTRACT FROM AUTHOR]

The paper is of tutorial nature, covering a wide range of the spectral and singular analyses of the electric power systems' (EPS) nodal admittance matrix and Jacobian matrix. Results of the analyses are applied to detect sensors and weak places in El'S and to visualize them. This allows for the solution to many important problems related to power system operation like estimation of net- work reinforcement and determination of sites for power quality metering. In this paper the theoretical background of the approach and its above mentioned applications are presented and discussed in the scope of their usefulness for the power system operation, planning and control. [ABSTRACT FROM AUTHOR]

This paper presents a Simulink-based educational tool developed for the purpose of illustrating power system control and stability notions as well introducing students to realistic, though tractable in size, design problems. Relevant courses are taught to last-year undergraduate as well as graduate students at the University of Liège, Belgium, and the National Technical University of Athens, Greece. After' a brief description of the corresponding curricula, the paper describes the simulation tool and gives examples of problems and assignments given to the students. [ABSTRACT FROM AUTHOR]

Offshore oil and gas vessels operating in deep and ultra-deep waters demand larger and more sophisticated power and control systems. This tendency brings new challenges in integrated power system’s design, especially for platforms/vessels requiring both dynamic positioning and high levels of redundancy. Voltage stability is essential in such systems, being in islanded operation (with related limited power generation) and with continuously changing load demands. In particular, voltage stability issues can arise due to the increasing amount of power electronic converters installed onboard, used to feed variable frequency drives and other electronic loads. Indeed, most of these have a controlled front-end, whose control can affect network voltage with a destabilizing effect named constant power loads (CPLs) instability. Such a behavior deserves special attention in islanded power systems, mostly if the quota of power electronics loads on the total installed power reaches very high values (up to the 85% for new large all electric ships). This paper initially focuses on the CPL voltage instability. Two different models to assess voltage stability in marine systems with high penetration of electronic power conversion are given, focusing on a design-stage assessment. Using the conditions obtained by such models, a practical stability analysis methodology is proposed, to help assessing voltage stability already at design stage, to avoid equipment retrofits during vessel building or commissioning. Finally, some practical case studies are discussed, and solutions to overcome the CPL instability are suggested. [ABSTRACT FROM PUBLISHER]

This paper proposes a tractable and scalable algorithm to identify and analyze bifurcation points of a large-scale power system model, which are directly related to dynamic voltage instability problems. Different types of bifurcations are analyzed, including: saddle-node (fold), Hopf, singularity-induced and limit-induced. An algorithm that combines optimization and predictor-corrector procedure is proposed for equilibrium tracing. The algorithm is based on calculation of only critical (rightmost and closest-to-zero) eigenvalues. The proposed algorithm is extended to the case of dynamic voltage stability assessment for power systems with optimized topology (simultaneously subjected to the topology control changes and generation re-dispatch). The proposed approach is illustrated on two (medium- and large-scale real-world) test power systems. [ABSTRACT FROM PUBLISHER]

This paper focuses on the use of quadratic time-frequency distributions (QTFDs) to characterize the temporal variation of low-frequency electromechanical oscillations (LFEOs) using ringdown data. A refined Margenau-Hill distribution (MHD) and a procedure to extract the instantaneous frequencies and damping factors as well as initial phase angles of system low-frequency modes when the analyzed signal is multi-component are proposed. The refinement process enjoys a fast-converging iterative algorithm and without loss of generality it can be applied to other QTFDs for using in other applications. A non-stationary synthetic signal as well as simulated ringdown data of a 5-area, 16-machine power system and real measured data of Western Systems Coordinating Council (WSCC) system are used to demonstrate the efficiency of the proposed method in the presence of measurement noise. [ABSTRACT FROM AUTHOR]

This paper describes the control of an isolated multiphase high-frequency-link (HFL) matrix inverter by controlling the time evolution of its switching states (or, switching sequences). It is referred to as the optimal-switching-sequence-based control (OSBC). Unlike several conventional control schemes, where, typically the control is based on averaged model of the inverter and the modulation scheme sets a predetermined switching sequence, OSBC synthesizes the fundamental switching sequence depending on the control needs on the fly. For instance, OSBC can seamlessly play with the inverter switching sequences if the input voltage of the inverter changes as evident in solar or wind-based energy systems. This necessitates that OSBC use a switching-sequence-based discontinuous (instead of averaged) modeling approach. Further, the stability of the closed-loop inverter is determined in OSBC using an advanced composite Lyapunov-function based approach, which also enables one to predetermine the reachability of the inverter dynamics for a given switching sequence. Thus, optimal control in OSBC is ensured under stability bound of the switching sequence. This provides a fundamental difference between OSBC and model predictive control for inverters. This also implies that in a multiobjective OSBC, one can shift the weights of the individual cost functions to yield better inverter performance without compromising the stability. [ABSTRACT FROM PUBLISHER]

The extensive usage of open communication networks in power system control causes inevitable time delays. This paper studies impacts of such delays on the stability of one-area and two-area load frequency control (LFC) systems and proposes an analytical method to determine delay margins, the upper bound on the delay for stability. The proposed method first eliminates transcendental terms in characteristic equation of LFC systems without making any approximation and transforms the transcendental characteristic equation into a regular polynomial. The key result of the elimination process is that real roots of the new polynomial correspond to imaginary roots of the transcendental characteristic equation. With the help of new polynomial, it is also possible to determine the delay-dependency of system stability and root tendency with respect to the time delay. An analytical formula is then developed to compute delay margins in terms of system parameters. For a large set of controller gains, delay margins of LFC systems are computed to investigate the qualitative effect of controller gains on the delay margin. Finally, simulations studies are carried out to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper proposes a fuzzy logic controlled bridge type fault current limiter (FCL) to enhance the transient stability of multi-machine power systems. The transient stability performance of the fuzzy logic controlled bridge type FCL is compared with that of another static nonlinear controlled bridge type FCL. The total kinetic energy (TKE) of the generators in the system is used to determine the transient stability enhancement index. Also, the critical clearing time has been presented as a stability limit. Instead of conventional reclosing, the optimal reclosing of circuit breakers is considered. Simulations are performed by using the Matlab/Simulink software. Simulation results of both permanent and temporary faults at different points of the IEEE 30-bus power system indicate that the fuzzy logic controlled bridge type FCL can enhance the transient stability of the system well. Also, the performance of the proposed fuzzy logic controller is better than that of the static nonlinear controller. [ABSTRACT FROM AUTHOR]

Accurate measurement of the rotor angle and speed of synchronous generators is instrumental in developing powerful local or wide-area control and monitoring systems to enhance power grid stability and reliability. Exogenous input signals such as field voltage and mechanical torque are critical information in this context, but obtaining them raises significant logistical challenges, which in turn complicates the estimation of the generator dynamic states from easily available terminal phasor measurement unit (PMU) signals only. To overcome these issues, the authors of this paper employ the extended Kalman filter with unknown inputs, referred to as the EKF-UI technique, for decentralized dynamic state estimation of a synchronous machine states using terminal active and reactive powers, voltage phasor and frequency measurements. The formulation is fully decentralized without single-machine infinite bus (SMIB) or excitation model assumption so that only local information is required. It is demonstrated that using the decentralized EKF-UI scheme, synchronous machine states can be estimated accurately enough to enable wide-area power system stabilizers (WA-PSS) and system integrity protection schemes (SIPS). Simulation results on New-England test system, Hydro-Québec simplified system, and Kundur network highlight the efficiency of the proposed method under fault conditions with electromagnetic transients and full-order generator models in realistic multi-machine setups. [ABSTRACT FROM AUTHOR]

Many interconnected power systems are constructed to meet the ever-increasing electric power transfer demand. A disturbance anywhere in such an interconnected system will directly influence the power transfer capability of major transfer interfaces, as well as the transient stability of the system. Instead of the current practice of monitoring the power transfer limit on a given interface, this paper proposes a real-time synchrophasor method to monitor the stability margin using generator phase angle differences and alert operators of the potential risk to the system at the steady state operation. This is accomplished by developing an algorithm to directly correlate the phase angle separation of critical generator bus pairs to power transfer on major interfaces. These critical generator bus pairs are selected systematically through transient stability simulation. The Eastern North American power system is used as a platform to illustrate the utility of the proposed method. [ABSTRACT FROM AUTHOR]

The subsynchronous resonance (SSR) problem may become more serious with the continuous evolvement of a series-compensated power system. Consequently, the existing SSR mitigation scheme might no longer keep the system stable. This issue has been encountered by Shangdu Power Plant, where the deployed supplementary excitation damping controllers (SEDCs) that well stabilized the system before cannot maintain torsional stability after the recent change occurred in the power system. Therefore, a combined mitigation scheme of SEDC and generator terminal subsynchronous damping controller (GTSDC) is proposed in this paper to regain system stability. SEDC provides electrical damping by modulating the excitation voltage at the rotor side. Meanwhile, GTSDC can damp SSR via injecting super-synchronous and subsynchronous currents into the generator stator. Field tests have proven the effectiveness of this combined scheme in addressing the deteriorated SSR issue. Eigenvalue analysis indicates that the combined scheme can provide enough positive damping for the system under any operating condition. Thanks to the low investment of SEDC and the flexibility of GTSDC, this combined scheme provides an economical and scalable solution for SSR mitigation, especially when the target system changes constantly. [ABSTRACT FROM AUTHOR]

This paper presents a generic rms model of voltage source converter (VSC) in modular multilevel converter (MMC) topology for high-voltage direct current (HVDC) application. The generic rms model is suitable for stability analysis of a MMC-VSC-HVDC interconnected to ac power system. Based on the rms model, a linearized model for analytical model analysis is also developed and described. The parameters of the generic rms model are identified via an established averaged EMT model. The dynamic performance of the proposed generic model is tested in both time and frequency domain analysis using a typical high-voltage transmission system. [ABSTRACT FROM AUTHOR]

This paper proposes a wavelet-based power system stabilizer (WPSS) by using both the scaling coefficient energy and the wavelet coefficient energy with border distortions of a one-cycle sliding window. The boundary scaling coefficient energy is used for real-time extraction and compensation of electromechanical oscillations in the synchronous generator, whereas the boundary wavelet coefficient energy is used for the real-time detection of electrical oscillations and for tripping the WPSS at the fault inception time. The performance of the proposed WPSS was assessed in an experimental power system laboratory setup and compared with a conventional power system stabilizer. [ABSTRACT FROM PUBLISHER]

This paper proposes a decentralized adaptive emergency control scheme against power system voltage instability. Decentralized control architecture is proposed by segregating the system into several local areas or zones based on the concept of electrical distance. Intelligent agents are assigned in each area to monitor the bus voltages and generator reactive powers to detect any threat of voltage collapse and to actuate countermeasures. A novel performance index has been formulated based on the load voltage and generator reactive power violations to identify the severity of disturbance and the risk of system emergency in each area. The coordination of the timing of the countermeasures among the agents is achieved through the formulation of the integral of the performance index. The simplicity and the adaptive nature of the proposed control scheme to provide countermeasures against any disturbance make it useful for real-time application. The robustness of the proposed approach has been validated through several case studies using the New England 39-bus test system and a more realistic Nordic32 test system. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of joint optimal allocation and sizing of dynamic Var sources (ODynVarS) for short-term dynamic performance enhancement of power systems. A heuristic optimization based approach is proposed with the aim of achieving a desirable performance under multiple contingencies with respect to a reference short-term voltage response. Based on a pre-selected set of credible contingencies and potential candidate locations for device additions, the approach employs the mean-variance mapping optimization (MVMO) in combination with an integrated mixed-integer search strategy (IMIS) and two intervention schemes, to accelerate and guide the search process towards a techno-economically optimum solution. The first intervention is performed at fitness evaluation stage to screen economic solutions which entail technical feasibility and to reduce computational effort. IMIS and the second intervention is concerned with MVMO's knowledge management, which facilitates the search for solutions comprising a small number of devices with relatively large VAr capacity. The approach is tested using the IEEE 39-bus New England system. [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]

In this paper, feedback linearization (FBL) technique together with disturbance observer (DO) approach is proposed to mitigate the effect of the resonant frequency of grid-tied LCL filter systems. The state-feedback control law is employed to achieve stabilization of the LCL filter system under a wide range of resonant frequency variation. The DO is designed to counteract the effect of model uncertainty and unknown disturbance aiming to achieve asymptotic stability under FBL control. Specifically, the observer is designed to estimate an additional input, representing uncertainty and unknown disturbance, from measurable variables. Then, the FBL control utilizes the disturbance estimate to compensate for its effect. An interesting feature of the composite controller is its ability to meet the transient response specifications even in the presence of model uncertainty and external disturbance. The composite controller was implemented for simulation and experimental evaluation, and performance testing. High performance with respect to disturbance rejection and parameter variation has been demonstrated. [ABSTRACT FROM AUTHOR]

This paper develops a computationally efficient method to bound the impact of multiple uncertain parameters in a dynamic load model. Load model trajectory sensitivity is first conducted on regional dynamics only (e.g., a large industrial load bus and its model for adequately representing the bus voltage dynamics) to identify critical and correlated load parameters. Systemwide trajectory sensitivity on the entire power system model is then evaluated for this reduced set of parameters, and finally, the impact of multiple uncertain parameters on the representation of power system dynamics is bounded. To reinforce this reasoning, we elaborate on the conceptual meaning of the load model trajectory sensitivity, its implication, and its applicability to the entire power grid analysis. This research also develops a fluctuation index of trajectory sensitivity to effectively rank and select the model parameters based on the impact of their perturbations on the system's dynamic performance. Case studies for the Korean power system demonstrate the validity and efficacy of the developed methods for adequately bounding the uncertainty impacts with reference to the comprehensive time-domain dynamic simulation approach. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized control based on static var compensator (SVC) devices to improve the frequency control of high voltage transmission systems. For a realistic structure preserving power system with voltage dependent loads, we propose a sufficient condition based on a strict Lyapunov energy function, which is utilized to design a nonlinear adaptive SVC controller. This controller only requires local bus measures and parameters. Furthermore, we also present an SVC with a conventional frequency droop controller. The performance of the two considered SVC control systems are evaluated on the well known IEEE New England 39-bus 10-machine power system and compared with conventional SVC controllers. [ABSTRACT FROM AUTHOR]

Virtual inertia has been extensively implemented in power systems with distributed energy resources, especially wind energy conversion systems. This paper proposes an optimal virtual inertia planning strategy for stability enhancement of power system, which is penetrated with large volume of renewable resources provided by doubly fed induction generators (DFIGs). The stability of DFIG is first analyzed and the concept of critical frequency drop is proposed. Moreover, the intersection area between the two predefined operation curves of DFIG is defined as the stability margin and it is calculated in the form of a function of local virtual inertia. Subsequently, a virtual inertia planning strategy is proposed, wherein a coherency index that considers the homogeneity of the stability margin at each node is presented, followed by a nonconvex optimization problem. The problem is simplified with the Lyapunov function method and resolved. Two case studies are conducted to verify the effectiveness and performance of the proposed approach: a 12-bus system and the IEEE-118 bus system. Both cases show that with the proposed approach, the system stability margin coherency level is enhanced and the general stability is promoted. [ABSTRACT FROM AUTHOR]

In this paper, an adaptive method for tuning a proportional-resonance controller for synchronization of the grid-connected inverters is presented. In the proposed approach, the grid frequency is obtained by minimizing the error signal using a frequency-locked loop mechanism that consists of a resonant adaptive filter and a perturbation-based extremum seeking algorithm. Simulations and experimental studies are presented to demonstrate performance of the proposed controller in face of the frequency variations of an emulated grid voltage waveform. The results are compared with conventional nonadaptive methods, which indicate that performance characteristics for voltage/frequency tracking and power factor can be achieved based on the IEEE 1547 standard. [ABSTRACT FROM AUTHOR]

During the out-of-step oscillation process, the frequency of voltage at every point in power system changes continuously. In this paper, the voltage frequency variation regulation is analyzed in an equivalent two-terminal system whose two-terminal potential amplitudes are not equal and all electrical component impedance angles are different. It finds the bus voltage frequencies at different sides of out-of-step center own different characteristics. In order to use the frequency variation characteristics to construct the criterion of power system out-of-step protection, a fast frequency calculation algorithm is proposed, in which the frequency is calculated by toggling different formulas at different sampling points and the calculation accuracy is improved by the aid of coordinate rotation digital computer (CORDIC) algorithm. The algorithm is easy to implement and measurement errors can keep under a low level. Finally, a novel frequency based out-of-step protection with wide area information is described. It can judge the occurrence of out-of-step oscillation in the first out-of-step oscillation cycle and determine the electrical component where the out-of-step center locates, which lays the foundation for the determination of out-of-step splitting section for complex power systems. The test results with IEEE39 system in PSCAD verify the correctness of the proposed protection. [ABSTRACT FROM AUTHOR]

This paper presents a quantitative method to determine delay requirements of the information and communication technology (ICT) system supporting wide-area power oscillation damping (WAPOD) controllers. An allowable time delay for the ICT infrastructure named “equivalent time delay (ETD)” is defined. The ETD is calculated by numerically comparing the damping behavior of the system when local input signals (LI) and remote input signals (RI) are used in the damping controller. The use of a WAPOD is only justified when its response outperforms that of a controller using local inputs. Therefore, the total time delay in the control loop must be below the calculated ETD. As such, the ETD serves as a design criteria to determine ICT latency requirements. The selection of an effective RI signal can be carried out by considering the maximum delays (ETDs) of different wide-area measurements. A damping improvement ETD^x~\% has been proposed using the same methodology indicating a minimum outperformance of the remote signals. The proposed method is demonstrated using the well-known Klein-Rogers-Kundur multi-machine power system and the Vietnamese power system model. [ABSTRACT FROM PUBLISHER]

This paper presents a novel approach to enable frequent computational cycles in online dynamic security assessment by using the terms of the transient energy function (TEF) as input features to a machine learning algorithm. The aim is to train a single classifier that is capable of classifying stable and unstable operating points independent of the contingency. The network is trained based on the current system topology and the loading conditions. The potential of the proposed approach is demonstrated with the New England 39-bus test power system model using the support vector machine as the machine learning technique. It is shown that the classifier can be trained using a small set of data when the terms of the TEF are used as input features. The prediction accuracy of the proposed scheme was tested under the balanced and unbalanced faults with the presence of voltage sensitive and dynamic loads for different operating points. [ABSTRACT FROM PUBLISHER]

Wind power has had a significant impact on power system stability due to its stochastic nature. In this paper, the impact of stochastic excitation on power-system small-signal stability is investigated based on stochastic differential equation (SDE) theory. The mechanical power input of an asynchronous wind turbine is considered as a stochastic excitation to the system and the system state equation is formulated based on SDE. The mean stability and mean square stability of such a system are demonstrated, and the steady-state expectation and covariance of the system state variables are obtained by mathematical deduction. The numerical simulations are presented on a two-machine infinite bus system and the IEEE 145-bus test system. [ABSTRACT FROM PUBLISHER]

The penetration level of plug-in electric vehicles (PEVs) has the potential to be notably increased in the near future, and as a consequence, power systems face new challenges and opportunities. In particular, PEVs are able to provide different types of power system ancillary services. The capability of storing energy and the instantaneous active power control of the fast-switching converters of PEVs are two attractive features that enable PEVs to provide various ancillary services, e.g., primary frequency control (PFC). However, concurrently, PEVs are obliged to be operated and controlled within limits, which curbs the grid support from PEVs. This paper proposes a new model for PEV using a participation factor, which facilitates the incorporation of several PEV fleets characteristics such as minimum desired state of charge (SOC) of the PEV owners, drive train power limitations, constant current and constant voltage charging modes of PEVs. In order to reduce computational complexity, an aggregate model of PEVs is provided using statistical data. In the end, the performance of PEVs for the provision of PFC is evaluated in a power system. Results show that PEV fleets can successfully improve frequency response, once all the operating constraints are respected. [ABSTRACT FROM PUBLISHER]

This paper presents a risk-based probabilistic small-disturbance security analysis (PSSA) methodology for use with power systems with uncertainties. This novel addition to existing dynamic security assessment (DSA) techniques can be used to quantify the small-disturbance stability risks associated with forecasted operating conditions. This approach first establishes the probability density functions (PDFs) for the damping of the critical oscillatory electromechanical modes by modeling the stochastic variation of system uncertainties, such as loading levels, intermittent generation sources, and power flows through voltage-source converter–high-voltage direct current (VSC–HVDC) lines. The produced PDFs are then combined with severity measures (either simple risk matrices or continuous functions) in order to quantify the risk of stability issues for the system associated with the forecasted operating scenario. In addition, the PSSA is used to establish risk-based operational limits and the concept of a probabilistic security margin is introduced to more accurately represent the probabilistic operation of uncertain power systems. The proposed techniques are demonstrated using a multiarea meshed power system incorporating two VSC-HVDC systems, one of which is connected to a large wind farm. [ABSTRACT FROM AUTHOR]

Stability is an important constraint in power system operation and the transient stability constrained optimal power flow (OPF) has always received considerable attention in recent years. In this paper, the defects of the existing models and algorithms around this topic are firstly analyzed, on the basis of which, a multi-objective optimization method is proposed. The basic idea of the proposed method is to model transient stability as an objective function rather than an inequality constraint and consider classic transient stability constrained OPF (TSCOPF) as a tradeoff procedure using Pareto ideology. Second, a master-slave parallel elitist non-dominated sorting genetic algorithm II is used to solve the proposed multi-objective optimization problem, the parallel algorithm shows an excellent acceleration effect and provides a set of Pareto optimal solutions for decision makers to select. An innovative weight assigning technique based on fuzzy membership variance is also introduced for a more scientific and objective optimal solution decision. Case study results demonstrate the proposed multi-objective method has many advantages, compared with traditional TSCOPF methods. [ABSTRACT FROM PUBLISHER]

Due to the uneven geographical distribution between load and wind power, currently it is difficult to integrate the large-scale wind power into the State Grid in China. There are significant wind curtailments for wind farms due to system restrictions. This paper proposes a new way of utilizing wind power, i.e., in-situ consumption, as an alternative to the costly development of long-distance transmission of large-scale wind power. Electrolytic aluminum load, which is one of the most typical high energy consuming loads, is employed to absorb the excess wind power in western China. An actual isolated industrial power system for aluminum production driven by wind power and coal-fired power is described. The penetration level of wind power in the isolated power system is up to 48.8%. The power imbalance between generation and load demand caused by wind power fluctuation or the tripping of coal-fired generators can dramatically impact the frequency stability of the isolated power system, which is of small inertia. An online identification method of power imbalance based on Wide Area Measurement System (WAMS) is presented. According to the characteristics of the electrolytic aluminum load, a system frequency control method by regulating the bus voltages of aluminum loads to eliminate the power imbalance is introduced. The simulation is done in Real Time Digital Simulator (RTDS) and the results verify the validity of the proposed frequency control method. [ABSTRACT FROM PUBLISHER]

The paper demonstrates a power swing blocking and unblocking technique for a distance relay in a hybrid transmission line consisting of an overhead line and an underground cable using the Lissajous figure of the voltage and current signals. The tangents perpendicular to the axis of the current signal (denoted as TPI) on the Lissajous figure are found at exactly half cycle spacing during normal condition, while their spacings deviate from half cycle and are unequal during the power swing and a large number of TPIs are found within a small duration after fault initiation. This feature of the Lissajous figure is utilized to detect the power swing and the fault during the power swing. The proposed method is tested on a 5-machine 14-bus power system, simulated in electromagnetic transient program for symmetrical, asymmetrical, and nonideal faults, and found accurate. The faults during the power swing are detected within few milliseconds after fault initiation for different fault locations, fault resistances, load angles, fault inception times, and swing frequencies. The comparative analysis with two conventional and two novel methods shows the potential of the proposed method. [ABSTRACT FROM AUTHOR]

Unstable subsynchronous control interaction (SSCI) between wind farms (WFs) and series-compensated grid has posed a big threat to the safety of equipment and the stability of system. Many studies have been conducted on its analysis and control. However, very little research has been done on the protection. To fill the gap, this paper proposes a system-wide protection against unstable SSCI in wind power systems with fixed series compensations. The protection is composed of a centralized protection coordinator (CPC) and several distributed protection relays (DPRs). Local DPRs measure the subsynchronous impedances of WFs in real time. These impedances are next collected by the CPC to calculate the sensitivities of tripping for each WF. The sensitivities are then used to determine the candidate WFs and the number of WTGs to trip at those WFs, so as to stabilize the SSCI. The effectiveness of the proposed protection is verified with detailed electromagnetic simulations on a wind power system that had experienced actual SSCI incidents. [ABSTRACT FROM AUTHOR]

It is commonly assumed that power swings are symmetrical phenomena that impose almost identical effects on the three-phase voltages and currents of the power system. However, there is another type of power swings which is asymmetrical and occurs mainly during single-pole tripping. In the nowadays heavily loaded power systems, the likelihood of occurrence of this phenomenon is increased considerably. Although the effects of symmetrical power swings on distance protection are widely analyzed in the literature, asymmetrical power swings have been less studied so far. This paper focuses on this type of power swings and their effects on distance protection. Besides, two new techniques are proposed to detect faults during asymmetrical power swings and enhance performance of power swing blocking schemes. The first technique is based on the rate of change of zero sequence current, while the second one is based on the magnitudes of voltage and current in each phase, as well as their relative phase angle. The presented analyses and proposed methods are validated using two benchmark test systems and promising results are obtained. [ABSTRACT FROM AUTHOR]

In this paper, the impact of load dynamics on the oscillatory stability of power systems is studied. The well-known composite load model is linearized to yield third order transfer functions for active and reactive power dynamics of loads. It is shown that the third order model exhibits a recovery response that is accompanied by damped oscillations. In studying the impact of loads’ transfer function parameters on the small signal stability of power systems, a resonance is observed between the load and the electromechanical modes of the system that significantly alters the electromechanical modes’ damping. The role of the active and reactive power dynamics, as well as the operating condition of the system on the impact from a dynamic load on electromechanical modes are investigated. For preliminary insights, a single machine infinite bus system (SMIB) is used initially to avoid the unnecessary complexities associated with large grids. The general conclusions drawn from the SMIB studies are then confirmed on a 14 generator model of the Australian grid. [ABSTRACT FROM AUTHOR]

Application of data mining based methods in online voltage stability assessment has attracted vast attentions in recent years. To account for significant system changes, most of the data mining based methods reconstruct an entire model based on the updated training database. Instead of entirely rebuilding a model in offline mode, this paper presents a novel online learning framework for monitoring the voltage stability of a transmission grid using wide area measurements. A new enhanced online random forest model based on the drift detection and online bagging techniques is proposed. It enables to online update the trees involving tree growth and/or tree replacement. The trees in the forest are then combined via a weighted majority voting, which makes the decision model better adapted to system changes. The framework was first demonstrated on the IEEE 57-bus system, and then applied to a practical power system, the Taiwan power (Taipower) system composed of 1821 buses. In addition to accuracy-based measures, robustness and speed of the proposed framework were also validated. Extensive studies demonstrate that the proposed framework is able to provide reliable and accurate online voltage stability assessment. [ABSTRACT FROM AUTHOR]

Generator-tripping and load-shedding are important emergency control measures to maintain the transient stability of the power system. In this paper, emergency control is modeled as a large-scale optimal control problem. An efficient parallel sequential approach, which consists of preprocessing layer, simulation layer, and optimization layer, is proposed to solve the problem. In the preprocessing layer, power system partitioning is performed to construct a Jacobian matrix of double-layered bordered block diagonal (BBD) structure, control variable selection and initialization are designed to reduce computation cost and improve convergence. In the simulation layer, the differential algebraic equations are solved in parallel with the sensitivity analysis by reusing the LU-factorizations. Based on the BBD structure of Jacobian matrix, a parallel block algorithm is proposed to further accelerate computation speed. In the optimization layer, the resulting nonlinear programming problem is efficiently solved by predictor–corrector interior point method. The efficiency and practicality of the proposed approach are verified by numerical experiments on three test systems. [ABSTRACT FROM AUTHOR]

As the operational conditions of power systems change, the operating point (i.e., the equilibrium point) may approach the small-signal security boundary. Given the current operating condition as well as the load and generation variations, it is meaningful to compute the small-signal security margin (SSSM). In this paper, an effective method that consists of three stages to compute the SSSM is proposed and examined. In the first stage, which is an estimation stage, the critical mode crossing the damping ratio line is estimated. The SSSM is then computed by a test function or by a direct method in the second stage, which is a correction stage. The computational results are then verified in the third stage, which is a verification stage. Numerical results on a 10-machine and a 160-machine system (with the state variables being 1600) show that the proposed three-stage method is efficient and promising. [ABSTRACT FROM AUTHOR]