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This paper presents a novel protection strategy for assuring power-system frequency stability. After many years of use, the existing (static) under-frequency load-shedding concept has proven to be reliable and fast. However, it does not adapt well to volatile operating conditions, which might often lead to a system collapse. This developed into a major obstacle, since operational conditions of modern power systems are persistently subjected to fast and unpredictable changes. Therefore, there is a need to introduce a new capability in under-frequency load-shedding that would be able to fine-tune the mismatch in active power in any conditions. From the practical aspect, a systematic implementation of improvements should not threaten system stability. In terms of cost, a minimum intervention in the existing concept is desirable. This is why we developed an effective upgrade of the existing concept by introducing a specialized group of intelligent electronic devices supported by a principal component analysis technique. This group should be operated alongside the existing solution to monitor the actual frequency conditions and to take over power-balancing when fine-tuning is required. This supports the existing load-shedding and improves the process of frequency stabilization. [ABSTRACT FROM AUTHOR]

Abstract As a result of the global decarbonisation policy, the increased penetration of RES (Renewable Energy Sources) into the electrical power system is introducing some serious technical challenges, one of the most serious being to maintain voltage profiles within acceptable/allowable limits. The unpredictable generation of a considerable share of the renewable energy can result in voltage deviations that system-network operators are not able to control effectively. At the present stage of the technology, the power electronic converters that are applied for most of the RES are used as a kind of adapter that ensures optimal RES operation and transmits the active power to the grid. But each of these converters, together with an energy source (or sink), inherently also exhibits the characteristics of a FACTS device. As a result, there should be no hindrance to applying the theories for enhanced EPS (Electric Power System) stability limits, developed for FACTS devices, also for RES converters. Many of the problems that are caused in an EPS by the large share of RES could be solved by the RES itself, also at the system level, if properly controlled and IT connected to each other and to the system operator. In this context, the present paper reviews the impact of FACTS devices on the voltage stability with methods based on the calculation of PV curves. An analytical, in-depth approach can clearly demonstrate the theoretical feasibility of FACTS devices for voltage-stability enhancement. The paper presents such an approach for CSC, SSSC, SVC and STATCOM, based on an analytical calculation of the area between the PV curves and the voltage margins in a PV diagram. The basic contribution of the paper is a theoretical explanation of the suitability of various FACTS devices for maintaining an optimal voltage profile, taking into account any variation of a device parameter. Highlights • Growth of renewable energy sources can significantly influence voltage stability. • Influence of FACTS devices on voltage stability in electric power system is shown. • A new idea named PV area criteria for voltage stability is presented. • PV area criterion estimates suitability of FACTS for voltage-stability enhancement. [ABSTRACT FROM AUTHOR]

This paper presents a novel and comprehensive method of analyzing transformer taps and phases in power flow (PF) studies. The method is based on constructing a criterion function, which encapsulates physical constraints for the analyzed taps and phases. The root of the criterion function corresponds to the correct solution. To find the root, the Newton method is used. The criterion function is external to the PF routine, and so, the root-finding (and with it, the tap and phase analysis) is done without altering the underlying PF routine. The method is developed for quadrature-boosting-transformers, as well as for regular phase shifting and tap changing transformers. A possibility of accounting for changing impedance is developed and incorporated in the procedure, using real-world phase-impedance relation data of a phase shifting transformer. Generalizing control modes to different types of variables and to non-local variables is illustrated. The method is tested on large benchmark systems: the IEEE118, IEEE300, and Pegase13 659 bus system. Tests show that (since the method is Newtonian) the convergence is normally achieved within 10 iterations. [ABSTRACT FROM AUTHOR]

The aim of this work is to include the distance-protection behavior in direct methods for transient stability assessments. In this way the direct methods assess an electric-power system’s behavior more realistically and hence more reliably. In order to consider the delayed tripping zones of distance-protection relays, the time component was added to the Lyapunov energy function. The main innovation is that the dwell time of the post-fault impedance trajectory inside various tripping zones is calculated based on the speed of the transformation between the kinetic and the potential energy parts of the Lyapunov function. It enables the identification of unwanted trips during power swings. The tripping-zone settings can be revised accordingly. The method was verified by a comparison between the direct method and the well-known, time-domain, numerical-simulation method on a single-machine, infinite-bus test system where the results have to be identical. The application of the proposed method on a multi-machine power system gives good results. [ABSTRACT FROM AUTHOR]

In recent years, the doubly-fed induction machine (DFIM) has become one of the most commonly used generators, especially for wind-power plants. The second field where DFIMs have a clear advantage over "classic" generators is their application in hydro pumped-storage plants, where larger units are employed. This paper addresses the transient-stability problem associated with DFIMs. In order to be able to understand the machine's behavior at an engineering level, the currents and magnetic fluxes during the transients, as well as the oscillations of the rotor speed and the power after the fault, are analyzed. The results of the analysis show that the resistance of the so-called "crowbar protection" is of vital importance for power- and speed-oscillation damping as well as for maintaining the system's transient stability. Therefore, optimization of the crowbar-protection resistance should be performed prior to the installation of a large DFIM in a relatively weak grid. [ABSTRACT FROM AUTHOR]

The magnetization of a transformer is usually associated with the well-known phenomenon of inrush current. However, inrush current does not only affect the transformer being switched on. Instead, it has a significant impact also on all parallel connected transformers, which most certainly includes measurement transformers. This is known as a sympathetic inrush phenomenon. While the transformer being switched on might be a subject to a sudden high saturation level, the parallel transformers are gradually drawn to saturation as well, but of opposite polarity. The sympathetic inrush is expected in situations where the ohmic part represents a significant portion of the total system impedance. In contrast to other available literature on this subject, in this paper a modal approach to solving equivalent circuit by expressing differential equations in the state-equation form is selected. From the derivation of eigenvalues and eigenvectors, the phenomenon can be systematically investigated. A special attention will be given to circumstances, when the already operational transformer is fully loaded as within substations two or more transformers often operate in parallel, among which at least one of them is usually loaded. Finally, simulation results are compared to captured Wide Area Monitoring System (WAMS) measurements of the phenomenon and reasons for discrepancies are discussed. [ABSTRACT FROM AUTHOR]

The importance of the wide-area monitoring system is rapidly increasing and many transmission system operators have already made several steps toward implementing wide-area monitoring, protection, and control. Underfrequency load shedding (UFLS) is undoubtedly one of areas that can be significantly improved within such an upgrade. However, in order to pursue finding a concept for practical implementation, as little measurements as possible should be used for the purpose. In this paper, an approach for obtaining a few-seconds-in-advance frequency prediction is described. It enables making decisions about the amount of load to be shed and, at the same time, being aware that inaccuracies in the procedure are inevitable. An extensive testing of the approach was performed on the 110 kV part of the Slovenian power-system model and the predictive UFLS appears to be highly superior to traditional UFLS on evoked frequency conditions as well as the total amount of disconnected loads. Consideration of practical limitations, such as all major time delays, unveiled a limitation for using the procedure at extreme values of system-wide frequency gradients. Consequently, the proposed algorithm is to be considered a “smart” supplement to already operational traditional UFLS. [ABSTRACT FROM PUBLISHER]

• Energy functions of a STATCOM that reflect its real control are derived. • Consideration of the STATCOM's real control in direct methods is enabled. • Globally-optimal control strategy for a STATCOM and RESs is proposed. • Globally-optimal reference in Lyapunov sense for the simpler strategies is available. The inclusion of large numbers of renewable energy sources (RESs) can lead to the operation of electric-power systems close to their natural stability limits. To improve stability, the flexibility and fast control of RESs can be exploited. However, RESs must be "told" what to do for the sake of the "common good". One possibility is to apply global control based on energy functions, but this requires the appropriate mathematical tools and models. In addition to the "real power feed", the RESs' power converters, among others, offer the functionality of a static synchronous compensator (STATCOM). Therefore, the aim of this paper was to develop energy functions for a STATCOM—the ones that consider its control strategies and limitations—that can be applied in energy-function-based methods for a transient-stability assessment and improvement. The two main contributions of this paper derive from the developed energy function (EF). The first contribution is the ability to use a direct method for the transient-stability assessment of an EPS that includes STATCOMs. The second contribution is the application of the developed EF in the control strategies of STATCOM and RES for a globally optimal, transient-stability improvement that can be used as a reference for other, simpler and more applicable control strategies. Such a reference has not existed until now. The STATCOM's energy functions were developed as additional expressions that can be added to the already-known energy function of the electric-power system in the structure-preserving frame. The correctness of the developed energy functions was proved by applying them in a direct method for the transient-stability assessment and a comparison of these results with those obtained with a numerical simulation. Examples of the use of energy functions for the control of a STATCOM's reactive current show how the STATCOM can improve the transient stability in a globally optimal way and can be used as a reference for a STATCOM's simpler control strategies based on local parameters. An example of a RES's reactive current control that is based on the developed energy functions shows that the RES can be successfully applied for a transient-stability enhancement. The paper can be seen as the first step towards a globally optimal control of the RES during transients. [ABSTRACT FROM AUTHOR]

Underfrequency load shedding is one of the most important protection mechanisms in a power system. However, in the majority of power systems it has remained unchanged for decades, despite the advances in computer and communication technologies. Wide Area Monitoring System proved to be very useful and reliable in the last few years and this is why much effort is given into development of Wide Area Monitoring, Protection and Control concepts. Adaptive underfrequency load shedding is certainly a very suitable candidate for the task. In order to accelerate changes in this area, a proposal for an adaptive scheme has been developed, which is a good option from both the technical and economic points of view. In contrast to majority of adaptive schemes, it handles problems of active-power deficit estimation and its variations during the transient due to voltage dependent loads differently, i.e., by on-line forecasting the operating point trajectory in a phase space. The scheme has been tested on previously proven (by comparison to WAMS measurements) dynamic model of a part of a Slovenian power system and the results indicate a large improvement compared to the traditional scheme. [ABSTRACT FROM AUTHOR]

Serial controllable FACTS devices, such as a Controllable Series Compensator (CSC) and a Static Synchronous Series Compensator (SSSC), can have a significant impact on the voltage stability of an electric power system. This paper presents a mathematical derivation for the voltage dependency of a CSC and a SSSC in a single-load infinitive-bus model. The comparison of the impact of a CSC and a SSSC on the voltage stability in a five-bus system confirmed the new analytical equations. According to theoretical considerations, from the point of view of voltage controllability, each device has its own advantages, but in the case of low voltages or low loads a SSSC is superior to a CSC. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a universal approach to the modeling of the 2nd generation FACTS devices for power-flow calculations by applying the Newton–Raphson method for current-based models is discussed. First, the basic principles of modeling 2nd generation FACTS devices using current-based models and the current-based models themselves are presented. This is followed by a discussion about the universality of the proposed model. Next, a definition of the FACTS devices’ free parameters, a mathematical description of their type, the problem of the initial conditions and the handling of constraints are discussed. The derived model was then tested on the IEEE 57, IEEE 118 and IEEE 300 bus systems. Finally, the main achievements are summarized. [Copyright &y& Elsevier]

The actual activation of underfrequency load shedding is rarely needed in a power system. However, this does not diminish its significance, as it is the last barrier before a system blackout if underfrequency conditions occur. Like in all other areas of application, a tendency also exists to maximize the efficiency of the underfrequency load-shedding scheme and to bring its operation to the limits. By default, the scheme should be able to stop a frequency fall under any circumstances. However, trying to lower the quantity of the disconnected load in the same process is the purpose of this paper. After the theoretical background is summarized, in terms of the reaction of individual generators in the islanded system to a certain power imbalance, a procedure is described showing how it is possible to adjust any of the adaptive load-shedding schemes to utilize as much primary frequency control as possible. As a reference, a theoretically optimal underfrequency load-shedding scheme is presented. [ABSTRACT FROM PUBLISHER]

Underfrequency load shedding is one of the most important protection systems, as in many cases it represents the last chance to prevent a system blackout after a serious disturbance occurs in a power system. In order to improve traditional schemes, many efforts have been concentrated on attempts to use the frequency gradient as an indicator for determining the lack of active power in a system. This paper concentrates on analyzing the factors that influence the gradient. Analyses have shown that the gradient can give misleading information about the active-power deficit, as long as certain factors are ignored or assumed to be constant. A draft of a gradient-based underfrequency load-shedding scheme is presented, which also highlights the problems associated with the use of gradient. On the one hand, a frequency gradient alone does not appear to be sufficient for the active-power deficit estimation. Nevertheless, for the actual load-shedding procedure it is found to be very useful. [ABSTRACT FROM AUTHOR]

Abstract: The research and development of underfrequency load-shedding protection tends to rely on an adaptive approach. Unfortunately, adaptive schemes are very dependent on an active power imbalance determination, based on a measurement of the frequency first derivative. Therefore, their effectiveness might be seriously reduced by a lack of data or the wrong estimation of certain parameters that are needed to accurately define the relation between the frequency first derivative and the active power imbalance. Consequently, it is reasonable to attempt to avoid having to calculate the imbalance. A novel approach to underfrequency load shedding is presented in this paper, using the frequency second derivative as a source of information for a forecast of the frequency trajectory. A Newton-method-based approximation and the interpolation of the frequency second derivative are continuously performed in order to forecast the minimum frequency value using a numerical integration. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a new approach to modeling an interline power flow controller (IPFC) for power flow calculations by applying the Newton–Raphson method is presented as a continuation of the ideas already applied to static synchronous series compensator (SSSC). The presented approach differs from the methods proposed by other authors in terms of the interpretation of the device''s branches. They are considered on the basis of their currents, and so it can, therefore, be denoted as a current-based model of an IPFC. First, the basic features of the IPFC are presented; as this is the basis for the current-based model that is incorporated into the Newton–Raphson load-flow model. Next, the basic equations are derived from the generic IPFC model. Further handling of the constraints is then discussed. The derived model was tested on the IEEE 57, IEEE 118 and IEEE 300 bus systems. The effectiveness of the proposed model''s iteration procedure is compared with other models. The comparison is based on data available in the references. We have shown that the proposed model exhibits a faster convergence than other models, and in addition, the number of iterations needed to achieve the prescribed accuracy is almost constant, regardless of the location, a consideration of the constraints or the selection of the controlled variable. [Copyright &y& Elsevier]

In order to be able to successfully apply interline power flow controllers (IPFCs) for power-system transient-stability improvement and for the damping of oscillations, proper control strategies should be applied to these devices. The basis for the implementation of such a strategy that is proposed in this paper is to know the energy function of a power system that includes IPFCs. Already-known energy functions that proved to be suitable for an electric-power system do not include such a device. There- fore, in this paper an energy function that considers the IPFC's action in the form of a supplement to the already-known structure-preserving energy functions was constructed. Furthermore, the strategy for implementing the newly developed IPFC energy functions in order to ensure a globally optimum control strategy in the Lyapunov sense is presented. The results of the numerical examples show that the magnitudes of the IPFC's series-injected voltages remain set to their maximum values and that only the angles of these voltages change, similar to the "bang-bang" control strategies proposed for some other FACTS devices. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a new approach to modeling a static synchronous series compensator (SSSC) for power-flow calculations by applying the Newton–Raphson method is presented. This new approach differs from known methods in terms of the interpretation of the device''s branch. It is considered on the basis of its current and is therefore denoted as a current-based model of an SSSC. This approach might in principle be applicable also for other FACTS devices (i.e., UPFC, IPFC, GUPFC). In the paper, the current-based model of an SSSC is presented as the models of this device have difficulties with convergence in power-flow calculations and there are very few references covering these topics. First, the basic features of an SSSC are presented, as it is the basis for the current-based model that is incorporated into the Newton–Raphson load-flow model. The results of the tests at the IEEE 57-bus system are discussed in detail and compared with the existing injection SSSC load-flow model [X.P. Zhang, Advanced modeling of the multicontrol functional static synchronous series compensator (SSSC) in Newton power flow, IEEE Trans. Power Syst. 18 (November (4)) 2003]. [Copyright &y& Elsevier]

Presents energy functions that incorporate the action of controllable series devices (CDS). Function of flexible alternating current transmission services devices; Objective of transient stability evaluation; Categories of CDS; Application of CDS on the transient stability of electric power system.

It is expected that a coordinated operation of several system integrity protection schemes will become a necessity in the future. This research represents an innovative strategy for coordinating under-frequency load shedding and intentional controlled islanding schemes for improving electric power system stability and resilience. In the great majority of real-world cases, both approaches follow conventional tactics, i.e., disconnecting a fixed number of feeders at predefined frequency thresholds and isolating a predefined area of a power system regardless of the actual conditions. Under the newly arisen network conditions in which weather-dependent distributed energy sources introduce a significant level of intermittency, conventional approaches need to be upgraded in order to retain a high level of power system operation security. In this paper, a mixed-integer linear programming approach is used to adjust the island size, including/excluding additional substations according to the available amount of generation in the region. The fine-tuning of the power rebalancing is achieved by potentially blocking selected load shedding stages. This minimizes the power imbalance of the newly formed islands, which helps to reduce the number of partial or even total blackouts and also accelerates the power system's restoration process. The optimization approach was tested on a generic IEEE 39-bus network and shows promising results along with the capability of coping with real-world applications using wide-area monitoring systems as a source of real-time measurements. The results also indicated the importance of appropriate load modelling since both voltage and frequency dependence are recognized to have a significant effect on intentional controlled islanding. [ABSTRACT FROM AUTHOR]

This research represents a conceptual shift in the process of introducing flexibility into power system frequency stability-related protection. The existing underfrequency load shedding (UFLS) solution, although robust and fast, has often proved to be incapable of adjusting to different operating conditions. It triggers upon detection of frequency threshold violations, and functions by interrupting the electricity supply to a certain number of consumers, both of which values are decided upon beforehand. Consequently, it often does not comply with its main purpose, i.e., bringing frequency decay to a halt. Instead, the power imbalance is often reversed, resulting in equally undesirable frequency overshoots. Researchers have sought a solution to this shortcoming either by increasing the amount of available information (by means of wide-area communication) or through complex changes to all involved protection relays. In this research, we retain the existing concept of UFLS that performs so well for fast-occurring frequency events. The flexible rebalancing of power is achieved by a small and specialized group of intelligent electronic devices (IEDs) with machine learning functionalities. These IEDs interrupt consumers only when the need to do so is detected with a high degree of certainty. Their small number assures the fine-tuning of power rebalancing and, at the same time, poses no serious threat to system stability in cases of malfunction. [ABSTRACT FROM AUTHOR]

This research represents a conceptual shift in the process of introducing flexibility into power system frequency stability-related protection. The existing underfrequency load shedding (UFLS) solution, although robust and fast, has often proved to be incapable of adjusting to different operating conditions. It triggers upon detection of frequency threshold violations, and functions by interrupting the electricity supply to a certain number of consumers, both of which values are decided upon beforehand. Consequently, it often does not comply with its main purpose, i.e., bringing frequency decay to a halt. Instead, the power imbalance is often reversed, resulting in equally undesirable frequency overshoots. Researchers have sought a solution to this shortcoming either by increasing the amount of available information (by means of wide-area communication) or through complex changes to all involved protection relays. In this research, we retain the existing concept of UFLS that performs so well for fast-occurring frequency events. The flexible rebalancing of power is achieved by a small and specialized group of intelligent electronic devices (IEDs) with machine learning functionalities. These IEDs interrupt consumers only when the need to do so is detected with a high degree of certainty. Their small number assures the fine-tuning of power rebalancing and, at the same time, poses no serious threat to system stability in cases of malfunction. [ABSTRACT FROM AUTHOR]

This paper presents a real-time application of a data-mining method with the purpose of indirect contingency screening, by rapid recognition of hazardous, reoccurring power-system operating conditions. The method, suitable for real-time applications, is demonstrated using the Slovenian power-system model for first-swing stability issues. The presented demonstration is segmented into a few parts. First, a database containing a set of pre-fault operating conditions (represented by a measurement matrix) and critical clearing times corresponding to several contingencies is constructed. Measurements matrix from the database is decomposed using the principal component analysis method and represented in a coordinate system defined by the principal components as dense clusters of points. Consequentially, monitoring real-time steady-state conditions in terms of seeking similarity to existing database clusters is facilitated (global screening). This is established by identifying the shortest Euclidean distance metric in the mentioned coordinate system. Finally, a second-stage (local) screening is applied in order to adaptively account for the fluctuating sensitivity of contingencies to variations in pre-fault operating conditions. In this manner, an accurate indication of each contingency impact is provided rapidly as long as a similar operating state exists within the database. Otherwise, the case is thoroughly investigated and added to the database as a new entry. This approach is applicable to a wide spectrum of dynamic problems, considering that problem-relevant sets of input data are available. [ABSTRACT FROM AUTHOR]

This paper proposes a Newtonian method for power flow (PF) calculations that incorporates Flexible Alternating Current Transmission System (FACTS) devices, without altering the standard PF routines. The method constructs a criterion function that is external to the PF routine, and which has the property of becoming zero when the FACTS devices are correctly modeled. The external character of the criterion function and the property of becoming zero at the correct solution, are used to form the FACTS-device modeling problem as an external root search, i.e., it occurs externally to the PF. The Newton method is used for the root search, which provides for a Newtonian convergence character of the proposed method, and so, convergence is achieved within a few iterations. By using numerically obtained sensitivities, it is shown that the method easily incorporates different control modes into the modeled FACTS devices—a property that will prove beneficial with the development of smart grids. The method was tested by modeling configurations with single and multiple FACTS devices on the IEEE 14-, the IEEE 300- and the Pegase 13659-bus systems. [ABSTRACT FROM AUTHOR]

The letter proposes a new method which analyses the states of transformers in power flow studies, by directly considering the limits of both the transformer state (e.g., minimum and maximum tap) and the control (e.g., minimum and maximum voltage). The method uses an empirically developed cost function, which rapidly increases where the transformer limits are exceeded, and decreases where they are not. Unconstrained optimization techniques are used to minimize the cost function. Since the structural limits (e.g., maximum or minimum tap) are stricter than the control limits (e.g., minimum and maximum voltage), the cost function is designed so that its increase rate for exceeding control limits is lower if structural limits are exceeded. The method is developed for phase-shifting, tap-changing, and quadrature-boosting transformers, and is tested on the IEEE 300 bus system with six devices operating simultaneously. [ABSTRACT FROM AUTHOR]

Underfrequency load shedding (UFLS) is one of the measures performed when an emergency situation occurs in the power system in order to prevent system collapse. Owing to its importance, a lot of effort is dedicated to research and development of the optimal UFLS. Therefore, several different approaches are available to improve the so-called traditional UFLS schemes into more sophisticated adaptive schemes. On the other hand, the literature lacks a systematic analysis of traditional UFLS schemes, which still present the majority within all UFLS schemes actually used in real power systems. For that reason, the probabilistic power flow is used to quantify the level of performance of many different configurations of traditional UFLS schemes (varied both in the number of shedding steps as well as their size) in the whole variety of operating conditions. With the use of probability power flow the probability density function of active-power imbalance is calculated. The obtained results enable to extract those configurations that yield the best results in most probable events and at the same time avoid giving too much attention on poor performance during less probable system conditions. The results were obtained by performing dynamic simulations on the north-western part of a Slovenian power system. [ABSTRACT FROM AUTHOR]

Underfrequency load shedding (UFLS) is one of the measures performed when an emergency situation occurs in the power system in order to prevent system collapse. Owing to its importance, a lot of effort is dedicated to research and development of the optimal UFLS. Therefore, several different approaches are available to improve the so‐called traditional UFLS schemes into more sophisticated adaptive schemes. On the other hand, the literature lacks a systematic analysis of traditional UFLS schemes, which still present the majority within all UFLS schemes actually used in real power systems. For that reason, the probabilistic power flow is used to quantify the level of performance of many different configurations of traditional UFLS schemes (varied both in the number of shedding steps as well as their size) in the whole variety of operating conditions. With the use of probability power flow the probability density function of active‐power imbalance is calculated. The obtained results enable to extract those configurations that yield the best results in most probable events and at the same time avoid giving too much attention on poor performance during less probable system conditions. The results were obtained by performing dynamic simulations on the north‐western part of a Slovenian power system. [ABSTRACT FROM AUTHOR]

Abstract: By applying second-generation FACTS devices, it is possible to improve the quality of the electric power supply, including the balancing of potentially unbalanced voltages. However, in order to be able to analyse such conditions, the procedures for a three-phase, load-flow calculation have to be applied. Therefore, in this article the three-phase, current-based models of FACTS devices for load-flow calculations using the Newton–Raphson method are described. The derived procedures are based on already-developed models for one-phase calculations. Although the principles are the same, there are still some details that had to be investigated, and some additional testing was necessary. The robustness of the method with respect to the initial values is of crucial significance; therefore, special attention was paid to this issue. Finally, two typical tests on IEEE 57 and IEEE 300 bus electrical power systems are presented. [Copyright &y& Elsevier]

Highlights • A practical approach for determining control parameters is presented. • A Unified Power-Flow Controller is considered. • The strategy to reaches a global optimum in the Lyapunov sense. • Optimality of the strategy was demonstrated on two power-system models. • The strategy is easy to implement within commercial dynamic analysis software tools. Abstract In the paper, a practical and innovative approach for determining control parameters of multiple and/or multi-parametric controllable elements with the aim of handling electromechanical dynamic phenomena in a power system, is presented. For the sake of a clear representation, a Unified Power-Flow Controller (as some kind of a synonym for multi-parametric devices) is provided, even though the suggested idea may be easily extended to coordinated control of arbitrary number of other devices as well. Application of a power-system transient energy function enables the strategy to reach a global optimum in the Lyapunov sense. Setting of controlled damping-device parameters pursues minimization of the time derivative of the transient energy function, which corresponds to the fastest possible dissipation of the energy surplus, gained during a disturbance. This is achieved by putting into use its newly presented predictable relationship with the damping-device parameter setting. Optimality of the strategy was demonstrated on both the single-machine infinite-bus model as well as on the Slovenian power-system model for damping of reconstructed power oscillations. An additional important contribution of the strategy is its easy-to-implement feature within existing commercial power-system dynamic analysis software tools, as transient energy function approach normally requires special software solutions. [ABSTRACT FROM AUTHOR]

This letter introduces the application of a data mining method with the purpose of contingency screening, by rapid recognition of hazardous, reoccurring power-system operating conditions. The method, suitable for real-time applications, is demonstrated on the north-western part of the Slovenian power-system, for first-swing stability issues. The presented demonstration consists of two steps: First, a database containing a set of prefault operating states and the corresponding critical clearing times of several contingencies is constructed. Second, the prefault measurements matrix is decomposed using the principal component analysis method and represented in a coordinate system, defined by the principal components. Since operating states form dense clusters of points in this coordinate system, the similarity between current and past conditions is established by identifying the shortest Euclidean distance metric. In this manner, the indication of each contingency impact is provided rapidly as long as a similar operating state exists within the database. Otherwise, the case is thoroughly investigated and included in the database. This approach is applicable to a wide spectrum of dynamic problems, providing that problem-relevant sets of input data are considered. [ABSTRACT FROM PUBLISHER]

In order to be able to successfully apply direct methods or other energy-function-based calculations in power systems, which include flexible AC transmission systems (FACTS) devices, the influence of those devices should be properly considered. This is currently not always possible in power systems incorporating unified power flow controllers (UPFCs) because the present energy functions for electric power systems do not involve proper UPFC actions. This paper presents a way of incorporating the transient-stability augmentation action of the most versatile FACTS device, i.e., the UPFC, into an energy function for multi- machine systems. After making some assumptions, a new term of the structure-preserving energy function (SPEF) that represents a UPFC's energy function was constructed. This term can simply be added to any existing SPEE The extended SPEF was tested for one UPFC in a longitudinal test system and for one and two UPFCs in an IEEE nine-bus machine test system. A comparison between the critical clearing times (CCTs) acquired directly with the use of the newly constructed SPEF and those obtained with time-simulation results shows that the proposed UPFC's energy function is suitable because the correct CCTs were obtained. [ABSTRACT FROM AUTHOR]