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This paper discusses a recent event in the western American power system when a forced oscillation was observed at a frequency that was close to a well-known 0.38-Hz inter-area electromechanical mode frequency of the western system. The event motivates a systematic investigation in this paper on the possibility of resonant interactions between forced oscillations and electromechanical inter-area oscillatory modes in power systems. When the natural oscillatory mode of a power system is poorly damped, and the forced oscillation occurs at a frequency close to system mode frequency at critical locations for the mode, resonance is observed in simulation test cases of the paper. It is shown that the MW oscillations on tie-lines can be as high as 477 MW from a 10-MW forced oscillation in Kundur test system because of resonance. This paper discusses the underlying system conditions and effects as related to resonance in power systems caused by forced oscillations and discusses ways to detect such scenarios using synchrophasors. Simulated data from Kundur two-area test power system as well as measurement data from western American power system are used to study the effect of forced oscillations in power systems. [ABSTRACT FROM AUTHOR]

Small isolated power systems often experience generator outages, which are responsible for the activation of the under-frequency load shedding scheme with the corresponding negative impact on electricity consumers and, hence, market loss. There are three main causes of this problem: the power system's low inertia, the speed governors' low capacity, and a poor size-ratio between generator and system. The most extensive research line in this area is focused on the optimization of the load shedding scheme, which is a partial solution. Another research line is presented to solve the problem from the point of view of the system operator. This paper proposes an online method to predict and correct possible load shedding by redistributing load dispatching. This proposal uses artificial intelligence techniques, in particular neural networks, and a special-purpose power system simulator. In order to evaluate the proposal, the achieved solution is applied to a real case study: the island of Gran Canaria. This application shows the improvement that might be achieved by implementing this simple method. The method proposed in this paper is strongly recommended for regions that have suitable geographical sites as well as energy problems similar to those of the Canary Islands (see tech. rep. “Map of the Canary Islands Power Systems” by Red Electrica de Espana). [ABSTRACT FROM AUTHOR]

In this paper, we propose a novel control algorithm for improving steady-state (quasi-static) voltage stability by use of an embedded voltage source converter (VSC) based high voltage direct current (HVDC) system. In this paper, the terms “steady-state” and “quasi-static” are used interchangeably. By embedded HVDC, we refer to a meshed AC system with all HVDC terminals connected within the same AC grid. The sensitivity between VSC control input and the voltage stability margin is introduced. Based on this sensitivity, the proposed control algorithm jointly satisfies system-wide voltage stability margin as well as local voltage magnitude requirements. The proposed approach is to first migrate the entire system to have sufficient voltage stability margin, and then to correct any voltage magnitude violation while keeping that stability margin. A contour-based visualization of the VSC capability space for maintaining system voltage stability is introduced, which can effectively illustrate how the singular value sensitivity (SVS) based control achieves both the local and global voltage stability requirements. The efficacy of the proposed algorithm is shown via case studies on a 6-bus and 118-bus system with and without static VAR compensation. [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]

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]

Identification of optimal locations to deploy the phasor measurement units (PMUs) is a preliminary task in wide area monitoring and control of power systems. Given the cost and other associated restrictions, it is highly unlikely for any utility to install all the required number of PMUs to ensure the complete observability of the network. This necessitates judicious identification of locations in the network to deploy the PMUs. Ideally, the choice of location must depend on the intended purpose. In this paper, an approach for placement of PMUs considering both the stability and topological aspects is presented. First, a probabilistic framework based on proximity to instability is proposed to rank the generators/load buses in the order of vulnerability. Subsequently, a binary integer linear programming formulation is proposed to identify the optimal locations considering both stability and topological aspects. The optimal locations obtained using the proposed approach ensures a priori observability of most vulnerable buses in addition to enhancing the network observability. Results on IEEE 39-bus system and 246-bus equivalent system of Indian southern grid are presented to illustrate the proposed approach. [ABSTRACT FROM PUBLISHER]

Several transient stability software packages are widely used for power grid planning and operations. Prior research and software documentations have shown that packages can vary in the implementations of dynamic models, and hence could potentially yield different results for the same system simulation. This paper presents a systematic methodology to determine the sources of the discrepancies seen in results obtained from different transient stability packages. This methodology can be applied to various types of dynamic models and is illustrated with generator-associated dynamic models in this paper. Consistent metrics, data mining techniques, and engineering heuristics are used to determine the discrepancy sources. This hybrid top-down+bottom-up method has been implemented on a practical, large-system case to demonstrate its scalability and transferability to real-world system studies. [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]

All-electric ship power systems include less generation capacity and smaller rotating inertia compared with large power systems. The systems include large portions of nonlinear loads and dynamic loads, which may reduce the stability margin. Moreover, various operational constraints, such as system frequency constraint, motor voltage constraint and dynamic cable constraint, need to be satisfied in operational real time. Further, pulse loads draw very high short-time power in an intermittent way, which may significantly deteriorate the power quality of the system. In this paper, a novel multi-agent system cooperative controller for a medium voltage AC (MVAC) system of all-electric ship power systems is developed to balance load and generation in real time while satisfying system's operational constraints and considering load priorities. The new method coordinates the pulse load and the propulsion load to reduce the impact of pulse load changes on the power quality of all-electric ship power systems. The dynamic behavior of the new method is evaluated using case studies in PSCAD software. [ABSTRACT FROM PUBLISHER]

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]

This paper addresses the distributed optimal frequency control of power systems considering a network-preserving model with nonlinear power flows and excitation voltage dynamics. Salient features of the proposed distributed control strategy are fourfold, first, nonlinearity is considered to cope with large disturbances, second, only a part of generators are controllable, third, no load measurement is required, fourth, communication connectivity is required only for the controllable generators. To this end, benefiting from the concept of “virtual load demand,” we first design the distributed controller for the controllable generators by leveraging the primal-dual decomposition technique. We then propose a method to estimate the virtual load demand of each controllable generator based on local frequencies. We derive incremental passivity conditions for the uncontrollable generators. Finally, we prove that the closed-loop system is asymptotically stable and its equilibrium attains the optimal solution to the associated economic dispatch problem. Simulations, including small and large-disturbance scenarios, are carried on the New England system, demonstrating the effectiveness of our design. [ABSTRACT FROM AUTHOR]

Single-phase autoreclosing (SPAR) is among the alternatives for mitigating the many harmful effects that a line-to-ground fault in a transmission line (TL) may cause. This procedure basically consists of opening the compromised phase, waiting a preset time span (dead time), and recomposing the previously isolated phase. In the vast majority of SPAR implementations, the open phase switching proceeds with no guarantee of whether the TL is still faulted. Switching the open phase into a fault might represent a more severe threat to the system than simply disconnecting the TL in the first place. Hence, in an effort to ensure higher levels of operational reliability to SPAR, the present paper offers a new methodology to check the soundness of the open phase during the dead time before its reclosing. First, the method's fundamental concepts are exposed. Second, a huge number of steady-state tests are presented, for which both the system topology and its operative condition were varied. To conclude, the SPAR is simulated in the time-domain with the most critical condition found in the steady-state analysis. The proposed strategy was capable of correctly identifying whether the system was faulted in every performed simulation. [ABSTRACT FROM AUTHOR]

The transient responses and stability followed by grid faults of the modern power systems experience diverse changes and have created concerns, especially for those dominated by doubly fed induction generator (DFIG) based wind turbines (WTs), since the transient characteristics of the DFIG-based WT are totally different from those of a synchronous generator (SG). To physically understand the transient response and theoretically analyze the transient stability of DFIG-based WT, this paper proposes a magnitude/phase dynamical model with synthetical consideration on the controllers in the rotor speed control timescale (around seconds). The proposed model physically explains the relationship between the imbalanced active power as well as the output active/reactive powers themselves and the DFIG-based WT's internal voltage vector, whose form is similar to that of the SG. By comparing with the SG, the distinctive transient phase characteristics of the DFIG-based WT are illustrated. An additional phase limitation is found, exceeding which the operation point does not exist. Based on the proposed model, the transient stability of a simple DFIG-based WT-dominated system is theoretically analyzed, and a new instability phenomenon different from that in an SG-dominated system is identified. Furthermore, some key factors influencing the transient stability are discussed. [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]

Over the last decade, many power systems have significantly changed with the proliferation of renewable generation sources, such as wind and solar photovoltaic. Due to their variability and nonsynchronous nature, new challenges and complexities have emerged regarding operational security of modern power systems. The 2016 South Australia (SA) blackout was the first known blackout due to such a high renewable situation. An official report has recently been published to review the causes and provide the corresponding recommendations for improvement of network operation, control, and security. However, there are still a number of critical issues and debates which remain unsolved, such as network bottleneck identification, overvoltage explanation, pole slip concern, frequency dip mystery, and frequency/voltage instability debate. In this paper, based on the reconstruction of the event, these unsettled issues are prudently analyzed to unveil their root causes. In addition, an innovative scheme is proposed to prevent the blackout by identifying the network separation at an early stage. This research will not only further advance the understanding of the 2016 SA blackout, but also will provide valuable guidelines for the management of future renewable-rich networks. [ABSTRACT FROM AUTHOR]

Direct-current microgrids (DC-MGs) can operate in either grid-connected or stand-alone mode. In particular, stand-alone DC-MG has many distinct applications. However, the optimal power flow problem (OPF) of a stand-alone DC-MG is inherently non-convex. In this paper, the OPF of DC-MG is investigated considering convex relaxation based on second-order cone programming. Mild assumptions are proposed to guarantee the exactness of relaxation, which only require uniform nodal voltage upper bounds and positive network loss. It is revealed that the exactness of second-order conic (SOC) relaxation of DC networks does not rely on topology or operating mode of DC networks, and an optimal solution must be unique if it exists. If line constraints are considered, the exactness of SOC relaxation may not hold. In this regard, two heuristic methods are proposed to give approximate solutions. Numerical experiments confirm the theoretic results. [ABSTRACT FROM AUTHOR]

This paper proposes a novel voltage stability-constrained optimal power-flow (VSC-OPF) model utilizing a recently proposed sufficient condition on power-flow Jacobian nonsingularity. We show that this condition is second-order conic representable when load powers are fixed. Through the incorporation of the convex sufficient condition and thanks to the recent development of convex relaxation of OPF models, we cast a VSC-OPF formulation as a second-order cone program. An approximate model is introduced to improve the scalability of the formulation to larger systems. Extensive computation results on Matpower and NESTA instances confirm the effectiveness and efficiency of the formulation. [ABSTRACT FROM AUTHOR]

The transition to a deregulated electric power system industry has created a highly competitive electricity market with immense power transfer activity performed between the market participants. A poorly-damped oscillation of small-signal instability is usually imminent during a stress power system condition attributed by the base case power transfer or available transfer capability (ATC) compounded with an outage of critical line. The contingency ranking by means of lowest damping ratio is introduced so that execution of the most critical line outage will incite to a poorly-damped oscillation during the base case ATC. The rescheduling of few critical generators determined by using the normalized participation factor is presented in this paper as a novel, non-intricate and cost-effective solution for estimating a new value of ATC while ensuring a secure power system operating condition. Sturdiness of the proposed method is verified through comparison with the weighted-average sensitivity of stability index required for rescheduling the critical generators performed in a case study of 39-bus New England system. Comparative study on the results of cost minimization in generators rescheduling has also been done based on the critical generators selected by the two methods. [ABSTRACT FROM AUTHOR]

Frequency deviation of power systems caused by grid-connected wind power fluctuations is one of the key factors limiting the level of wind penetration. Applying an energy storage system (ESS) to overcome the constraint of frequency deviation can increase the level of permitted wind penetration. This paper proposes a method to evaluate the wind penetration as limited by grid frequency deviation and to size the ESS to increase the permitted wind penetration to an expected level. The proposed method treats wind power fluctuations as a stochastic process rather than deterministic signal and takes the frequency response characteristics of power grid into full consideration. Application of the proposed method is based on theoretical derivations and does not require conducting dynamic simulation. It can catch the stochastic characteristic of wind power fluctuations. [ABSTRACT FROM AUTHOR]

This paper proposes a method to control the output power of a wind farm with the aim of improving the transient stability of a multi-machine power system. Doubly fed induction generators (DFIG) are considered for the variable speed wind farms. The variation of the frequency of the DFIG terminal bus is used to modulate the torque reference and thus the output power of the DFIG in the post-disturbance condition. This in turn modifies the electrical power of the nearby alternators and causes improvement of stability. The proposed control technique is validated in WSCC 3-machine 9-bus system and IEEE 16-machine 68-bus system. The study is carried out in PSCAD/EMTDC as well as in MATLAB platforms. [ABSTRACT FROM AUTHOR]

This paper presents a decentralized wide-area coordinated supplementary control of multiple line-commutated converter (LCC)-HVDC links to 1) prevent interactions among the HVDC links and 2) enhance the damping of the inter-area oscillatory modes. The proposed approach is based on the sparsity-promoting optimal control. The main features of the proposed approach are 1) it requires minimal communication infrastructure to achieve the control objectives and thus reduces the impacts of communication delays and noise, 2) it entails in an optimal gain which preserves the closed-loop stability and 3) it does not require the estimates of the system states. The performance of the proposed controller is evaluated based on eigen analysis and time-domain simulation of an interconnected AC system that includes five LCC-HVDC links. Performance of the proposed controller is also compared with those of fully centralized and conventional local supplementary controllers and its merits are highlighted. The studies indicate the proposed controller, based on 13 remotely communicated signals, provides similar performance as that of a fully centralized optimal controller using 2050 communicated signals and is far superior to the conventional local supplementary controllers. [ABSTRACT FROM AUTHOR]

A stability study of distribution networks with a high penetration of distributed generators (DGs) actively supporting the network is presented in this paper. A possible way of mitigating the voltage rise caused by DGs is the local control of reactive power. Among the different possible options, the Q(U) control (reactive power as a function of the voltage) is one of the commonly suggested solutions. However, the Q(U) control can, under certain conditions, lead to instability. This paper summarizes the results of a stability study conducted on a single-inverter system and on a multiinverter system. It shows that the requirements for reaching a stable operation can easily be met for integrated systems but could be a significant constraint for systems relying on communication. [ABSTRACT FROM PUBLISHER]

A closed-loop interconnected model of a power system with a voltage source converter (VSC)-based HVDC line (VSC-HVDC line) is established, wherein, a selected control system of the VSC-HVDC line and rest of the power system are modeled as two separate open-loop subsystems. Based on the established model, a special condition of open-loop modal coupling is studied, when a complex pole of the open-loop selected control subsystem of the VSC-HVDC line is close to a subsynchronous oscillation (SSO) mode of open-loop power subsystem on the complex plane. Theoretical analysis indicates that when the open-loop modal coupling occurs, subsynchronous interactions (SSIs) between the VSC-HVDC line and the rest of power system may become strong. It is very likely that the open-loop modal coupling may degrade the damping of power system SSOs. Hence, the mechanism that the VSC-HVDC line may cause the SSOs in the power system is revealed from the perspective of open-loop modal coupling. In this paper, study cases of example power systems with the VSC-HVDC line are presented to demonstrate and evaluate the analysis and conclusions made. [ABSTRACT FROM AUTHOR]

Emergency control is an important measure for prevention of power system blackouts. This paper presents a new event-driven emergency load-shedding scheme that minimizes load shedding while meeting constraints on transient angle stability, voltage/frequency deviation security, and availability of load to be shed at any bus. The evaluation of these constraints is based on time domain trajectories provided by numerical simulations. The optimal event-driven load-shedding (OEDLS) model was created to minimize the total load-shedding cost. By analyzing the characteristics of the optimal solution of this model, a trajectory sensitivity based solution approach is presented. It linearizes the nonlinear OEDLS model through trajectory sensitivity and iteratively converges to the optimal solution. To accelerate the problem solving process, a parallel computation scheme is implemented for obtaining the trajectory sensitivities. The validity of the proposed algorithm and parallel computation scheme was tested and verified using two different power systems. [ABSTRACT FROM AUTHOR]

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]

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]

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 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]

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]

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]

Power system static voltage stability region boundary (SVSRB) is a complicated hypersurface in parameter space. Due to inequality constraints such as generator reactive power limits, different critical points on the SVSRB may be related to different groups of active constraints and can be associated with different types of loadability limit, which makes it difficult to approximate the whole SVSRB at once or assemble many local approximations of the SVSRB. To overcome the difficulty, this paper proposes an approach for globally approximating the whole SVSRB considering generator reactive power limits at one go. First, a parametric optimization model incorporating all inequality constraints is formulated to describe the SVSRB, where the optimal solutions are the critical points on the SVSRB. The optimality condition, i.e., parametric Karush–Kuhn–Tucker conditions (KKT conditions), is also derived. Then, by applying the Galerkin method to the parametric KKT conditions in an interior-point-method-like manner, the whole SVSRB is approximated and represented using a single polynomial expression. Case studies in a 10-bus system and IEEE 300-bus system verify the effectiveness and accuracy of the proposed method. [ABSTRACT FROM AUTHOR]

Modern power system suffers voltage instability more frequently due to the increasing load. Sensitivity analysis based preventive control is widely recognized as an effective method for preventing voltage instability. However, the well-known load margin based sensitivity methods are not suitable for real-time application since they must solve the left eigenvector of zero eigenvalue of the Jacobi matrix at the critical point and require the system-wide information. Therefore, this paper proposes a fast sensitivity-based method for selecting the most effective preventive controls. The sensitivity analysis uses a local voltage stability index, i.e., load impedance modulus margin, which is derived from the local measurements (nodal voltage and current), and the sensitivity ranking can be obtained more accurately and faster than traditional methods based on load margin. Such computational advantages make it suitable for online application. The effectiveness of the proposed method is successfully validated by both small and large-scale power systems. [ABSTRACT FROM AUTHOR]

For economic reasons, expansions of transmission networks are based on a compromise between capital cost and security of power system operation. At the same time, the power system must withstand typical highly likely contingencies. Various emergency state control systems, also called special protections, prevent emergencies following less likely contingencies. This paper describes overloads emergency state control (OLEC) used for the alleviation of overload in transmission networks near a power plant. Such control uses the functions of automated power reduction of generating units and has been implemented for a large steam power plant. The OLEC presented here uses different methods of generation curtailment for steam generation units. A case study illustrates the efficiency of the proposed control method. [ABSTRACT FROM AUTHOR]

A simplified voltage stability analysis method is to use a static Thévenin equivalent to represent the electrical connection to a load center. Assuming fixed values of the Thévenin voltage and reactance, a power-voltage ( $PV$ ) curve analysis can be performed to find the voltage collapse point and stability margin. This paper proposes a method to compute the static Thévenin equivalent voltage and reactance of a power system using measured data. The method is validated with simulated PMU data and with 24-hour SCADA data at a wind hub on a medium voltage transmission system in the US western system. For the wind hub, the Thévenin equivalent parameters are used to compute the maximum power transfer capability of the wind hub as constrained by voltage stability. Then the method is extended to online application by using short data windows and forgetting factors. [ABSTRACT FROM AUTHOR]

The majority of the fixed-speed wind turbines are equipped with squirrel-cage induction generators (IG) and ac-connected to the power system. During a fault in power system, voltage and rotor speed instability may occur, especially when the strength of the system compared to the power delivered by the IG is relatively low. This paper investigates a new method for improving the transient stability of power systems equipped with squirrel cage IGs. For stability improvement, the unique property of the induction machines which is the possibility of reversing the direction of the rotating flux is employed. In this method, after clearing the fault, operating mode of the induction machine is changed from the generating to the plugging mode for a short time. This leads to opposition between the rotating flux and mechanical torques, thus preventing the generator from further acceleration. Simulation results show that the proposed method can be effective in enhancing the transient stability. Since in this method, there is no need for accessory equipment, the proposed method is more attractive than the previous ones, from economic point of view. [ABSTRACT FROM AUTHOR]

This paper investigates interconverter interactions in multi-infeed systems that include both rectifiers and inverters. Performance indices used earlier for multi-inverter systems are extended to include the rectifier-inverter mix. These indices can be used to predict the maximum available power (MAP) and power stability limits of the converters as well as transient phenomena, such as transient overvoltages (TOV) and inverter commutation failures (CF ).The results show that the presence of a rectifier in the neighborhood of inverters is effectively similar to an increase in the inverters' short-circuit ratio in that the inverters achieve a higher MAP, are less susceptible to commutation failure, and have smaller TOV. [ABSTRACT FROM AUTHOR]

It is well known that the Superconducting Magnetic Energy Storage (SMES) is effective to damp the power swing after the occurrence of faults. Nevertheless, if the SMES is also applied for transient stability improvement, a large power capacity of SMES is required. Additionally, the SMES is not able to absorb enough energy during faults since the bus voltage where the SMES is installed, drops considerably. To enhance the SMES control effect and transient stability, this paper proposes the coordinated control of the optimized resistive type superconducting fault current limiter (SFCL) and SMES. When the fault occurs, the SFCL rapidly suppresses the transient power swing by limiting the fault current. Subsequently, the SMES damps out the remaining power swing. The optimization problem of SFCL resistance and power controller parameters of SMES with optimal coil size is formulated based on an augmentation of transient stability margin and damping performance. Solving the problem by the particle swarm optimization, the optimal parameters of SFCL and SMES can be automatically obtained. Simulation study confirms the superior stabilizing effect of the coordinated SFCL and SMES over the individual device. The SFCL not only solves the voltage drop problem at the SMES bus, but also assists the SMES to stabilize the system. Besides, the MW and MJ capacities of the SMES operated with SFCL are significantly reduced. [ABSTRACT FROM PUBLISHER]

The large integration of photovoltaic (PV) power generation systems into power systems causes deterioration in power system stability. In our previous work, we showed that reactive power control using the inverters of PV systems, known as dynamic voltage support (DVS) capability, is a promising approach to improve the short-term voltage stability in power systems. In this paper, we propose a novel DVS capability as a function of PV inverters. In contrast to the conventional DVS capability, the proposed method uses both active and reactive power injection to improve the short-term voltage stability. Numerical examples show that the proposed DVS capability further improves the short-term voltage stability compared with the conventional DVS capability. Furthermore, the proposed method can alleviate a frequency drop after a fault caused by interruption in PV systems. [ABSTRACT FROM AUTHOR]

Load shedding (LS) is the last firewall and the most expensive control action against power system blackout. In the conventional under frequency LS (UFLS) schemes, the load drop locations are already determined independently of the event location. Furthermore, the frequency thresholds of LS relays are prespecified and constant values which may not be a comprehensive solution for widespread range of possible events. This paper addresses the decentralized LS in which the instantaneous voltage deviation of load buses is used to determine the frequency thresholds of LS relays. The higher frequency thresholds are assigned to the loads with larger voltage decay which are often located in the vicinity of disturbance location. The proposed method simultaneously benefits from individual UFLS and under voltage LS (UVLS) features which operate in the power system without coordination. Numerical simulations in DigSilent PowerFactory software confirm the efficiency of proposed methodology in the stabilization of the power system after various severe contingencies. [ABSTRACT FROM PUBLISHER]

Stochastic optimal power flow can provide the system operator with adequate strategies for controlling the power flow to maintain secure operation under stochastic parameter variations. One limitation of stochastic optimal power flow has been that only limits on line flows have been used as stability constraints. In many systems voltage stability and small-signal stability also play an important role in constraining the operation. [ABSTRACT FROM PUBLISHER]

Distributed energy resources and new types of electrical loads are morphologically changing the signals of power grids. Grasping the wideband information of field data will play an important role in helping smart grids to operate in a safe manner. A wideband signal model composed of deterministic and stochastic components is proposed to enhance the wideband measurement. Based on the model, an adaptive decomposition scheme is designed for the offline analysis of wideband signals. In the scheme, a modified robust local regression smoothing method is presented to denoise the wideband signal. A totally data-dependent threshold is established to extract adaptively interharmonic components. Further, the Taylor–Fourier transform algorithm is used to estimate the parameters of deterministic components based on the subsignals extracted by the Chebyshev-II infinite impulse response filter bank. Finally, the stochastic components are extracted using the adaptive threshold based on the signal that the deterministic components are removed. The performances of the decomposition scheme under different conditions have been validated. In the end, the proposed decomposition scheme is carried out and verified with the field data of the 24-h current signals at one residence community. [ABSTRACT FROM AUTHOR]