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This paper investigates the dynamic patterns in the small-signal behavior of power systems with wind power generation. The interactions between synchronous generators and wind generators are investigated. In addition, the impact of increased wind generation penetration on the damping and frequency of the synchronous generator's electromechanical oscillations is addressed. Wind generators of three different technologies are considered throughout this study. Very detailed dynamic models of wind generators are used and detailed. [ABSTRACT FROM AUTHOR]

Multi‐terminal high‐voltage direct current technology based on voltage‐source converter stations (VSC‐MTDC) is expected to be one of the most important contributors to the future of electric power systems. In fact, among other features, it has already been shown how this technology can contribute to improve transient stability in power systems by the use of supplementary controllers. Along this line, this paper will investigate in detail how these supplementary controllers affect electromechanical oscillations, by means of small‐signal stability analysis. The paper analyses two control strategies based on the modulation of active‐power injections (P‐WAF) and reactive‐power injections (Q‐WAF) in the VSC stations which were presented in previous work. Both control strategies use global signals of the frequencies of the VSC‐MTDC system and they presented significant improvements on transient stability. The paper will provide guidelines for the design of these type of controllers to improve both large‐ and small‐disturbance angle stability. Small‐signal stability analysis (in Matlab) has been compared with non‐linear time domain simulation (in PSS/E) to confirm the results using CIGRE Nordic32A benchmark test system with a VSC‐MTDC system. The paper analyses the impact of the controller gains and communication latency on electromechanical‐oscillation damping. The main conclusion of the paper is that transient‐stability‐tailored supplementary controllers in VSC‐MTDC systems can be tuned to damp inter‐area oscillations too, maintaining their effectiveness. [ABSTRACT FROM AUTHOR]

This paper describes and analyzes the Spanish AGC system. The differences with respect to a standard hierarchical structure are explained. A simple model useful for simulation is proposed. Using this model, the performance of the system and the influence of several parameters in system response is evaluated. The paper also suggests some changes in the configuration and parameter settings of the system to improve its performance. Index Terms--Automatic generation control (AGC), frequency domain analysis, transfer function identification.

The security criteria of a power system require that branch power flows and bus voltages are within their limits, not only in normal operating conditions but also when any credible contingency occurs. In the Spanish electricity market, voltage constraints resolved by connecting a set of offline generators located in the areas where they occur. Thus, for a market participant it is necessary to predict approximately when its generating units are connected in order to prepare the annual budget and/or decide the time and location of new plants. This paper proposes a methodology based on decision trees to estimate the daily load pattern of units that have not been cleared in the daily energy market and can be connected to alleviate the network constraints in the Spanish power system. The method explains the behavior of the daily load patterns of a nonconnected unit (obtained through clustering techniques) with a set of explanatory variables. The explanatory variables consist of the demand-generation imbalance in the electrical area of the generating unit and the maintenance scheduling of the transmission lines that feed the area. The method proposed is illustrated with a case study. Index Terms--Clustering, competitive electricity market, congestion management, decision trees, power system dispatch, security assessment.

In this work, the optimal participation of heterogeneous, Renewable Energy Source (RES)-based Virtual Power Plant (VPP) in Day-Ahead Market (DAM) and Intra-Day Market (IDM) is studied. For this purpose, a detailed model of the RES-based VPP and of the market operation is needed. The VPP includes both dispatchable and non-dispatchable RESs and flexible demand assets. This paper presents an improved, linear solar thermal plant model to consider its non-linear efficiency curve. A novel demand model with two flexibility levels that are associated with the different market sessions is also proposed. The market operation allows for updates of energy offers and this is used by the VPP to submit DAM auctions and to participate subsequently in IDM to correct for deviations. Finally, the optimal participation of the VPP in energy markets is assessed under different weather conditions. [ABSTRACT FROM AUTHOR]

Wind farms are increasingly interested in participating in the secondary frequency control, especially in power systems where AGC is organized by regulation zones comprising generation units of different technologies. In order to estimate the wind farms behaviour and overall impact on automatic generation control (AGC), a reasonably simple wind farm model is needed. First‐order and second‐order linear models of wind farms are proposed in the literature in AGC‐related studies. However, these simple models neglect key dynamic features for AGC integration of wind farms, such as artificial ramp limitations, dead bands, communication delays and start‐up delays caused by turbine orientation. This paper presents a still simple wind farm model, however representing all relevant dynamics and availability constraints at power system control level. Applying an illustrative case study, model parameters are tuned and validated by means of field measurements, recorded during a response trail run of a real wind farm of 30 MW installed power operating within the Spanish power system. [ABSTRACT FROM AUTHOR]

A novel framework for dynamic equiv-alencing that we recently introduced in the context of classical swing-equation models is extended in this paper to detailed models in structure-preserving, differential/algebraic-equation form. The system is partitioned into a study area and one or more external areas on the basis of synchrony, a generalization of slow-coherency that forms one leg of our framework. Retaining a detailed model for a single reference generator from each external area, the dynamics of the remaining external generators are then modally equivalenced in the style of Selective Modal Analysis; this modal equivalencing is the other leg of our framework. The equivalenced external generators are thereby collectively replaced by a linear multi-port 'admittance', which is easily represented using controlled current injectors at the buses of the replaced generators. The rest of the system model can be retained in its original nonlinear dynamic form. The approach is tested - with encouraging results - on the familiar 93rd-order, 10-machine, 39-bus New England model, using an implementation in the EUROSTAG simulation package. Key Words - Dynamic Equivalents, Differential-Algebraic Equations, Synchrony, Selective Modal Analysis, Slow Coherency.

This paper explores synchrony, a recently introduced generalization of the concept of slow-coherency, and outlines how it can form the basis for efficient construction of dynamic equivalents by aggregation. We describe a novel approach for selecting the inter-area modes that are to be represented by the aggregate model. A clustering algorithm for recognizing approximate synchrony is presented, and improvements over the standard slow-coherency recognition algorithm are noted. Using for illustration a 23-generator power system model with 325 state variables, the paper demonstrates the effectiveness of a synchrony-based approach to decomposing the eigenanalysis of the electromechanical modes, separating the computation of inter-area and intra-area modes in the style of multi-area Selective Modal Analysis. Key Words - Synchrony, Coherency, Slow-Coherency, Aggregation, Dynamic Equivalencing, Eigenanalysis, Selective Modal Analysis.

This paper presents an efficient methodology for the analysis of small signal stability in large electric power systems. It is based on the Selective Modal Analysis approach and assumes a multi-area structure of the small signal behavior in large power systems. In the proposed procedure, the system modes are separated into two categories (the inter-area and the intra-area modes) and are independently determined. Results of the application to a realistic large power system with 266 generators and 1472 buses are discussed.

With the steady expansion of renewable energy sources (RES), the provision of ancillary services is becoming an increasingly challenging task within system operation. In order to add regulation capacity, battery energy storage systems (BESS) have been recognized as an efficient tool in recent literature. In this context, this article proposes a novel BESS control strategy to improve dynamic performance of automatic generation control (AGC). Such control strategy is applicable to power systems which evaluate AGC performance utilizing regulation response accuracy margins, which typically define the slowest and fastest permitted power response to a regulation signal. The primary goal consists in enhancing the regulation service by minimizing the rate of non-compliance with respect to the corresponding dynamic performance criteria and its defined accuracy margins. Additionally a comprehensive State-of-Charge-control (SoC-control) is included in the strategy. By utilizing the BESS only during designated regulation assistance periods, preventing extreme levels of charge and furthermore identifying and avoiding futile regulation efforts, BESS degradation is kept to a minimum. To verify effectiveness a case study is conducted with 400 hours of real AGC operation of various BESS sizes. The obtained results show that a BESS is able to minimize the rate of non-compliance considerably, whilst preserving low BESS usage and degradation. [ABSTRACT FROM AUTHOR]

This study presents a thermo‐electric model to implement the energy management system (EMS) of an off‐grid system. The EMS is formulated as a unit commitment problem where thermal and electrical components are dispatched simultaneously to minimise system operation costs. Particular attention is paid to the operation of heat pumps. The model contemplates a variable coefficient of performance (COP) of heat pumps. COP is different for the cold water and hot water circuit and depends on the output temperatures of the heat pump and the heat pump operation mode. Furthermore, the heat pumps' ability to switch operation modes is also considered as well as the limitation of heat pumps to operate when hot water thermal energy storage is nearly full. The developed model has been successfully applied to an existing off‐grid system at Norvento Enerxia's headquarters in Spain by simulating its operation during a winter week. The impact of modelling multi‐mode operation of heat pumps with variable COP has been compared with single‐mode operations with either fixed or variable COP. The proposed integrated approach of solving the electrical and thermal dispatch simultaneously has also been compared with a sequential resolution of first the thermal and then the electrical dispatch. [ABSTRACT FROM AUTHOR]

Excitation boosters (EBs) are aimed to improve fault ride through capability of synchronous generators equipped with bus fed static excitation systems by means of adding a supplementary voltage or current source to the rectifier. However, this paper shows that EBs governed by local on–off controls may result in adverse effects in case of remote faults from generators that may occur in multi-machine systems. To sort this problem out, a novel EB Wide Area Control System (WACS) that modulates the EB voltage using the generator speed deviation with respect to the speed of the center of inertia (COI) is proposed. The speed of the COI is obtained using a Wide Area Measurement System (WAMS), whereas the control laws are deduced from the Lyapunov stability theory. Results show that transient stability of multi-machine systems can be enhanced using EB WACS. [ABSTRACT FROM AUTHOR]

This paper presents a fundamental study of voltage collapses that occur on a post-fault trajectory of a stressed power system in seconds after large disturbances. The focus of the study are voltage collapses that are induced by certain load models. Using an n-machine-N-bus power system model, the paper explicitly shows that the voltage collapse is caused by the non-existence of a real, positive solution for a load voltage magnitude in different areas of a relative rotor angle space when the load is of non-linear type. These “areas without voltage solution” are denoted as Voltage Impasse Regions (VIR) and are mathematically characterized as trigonometric functions of (n -1) relative rotor angles. Once the post-fault trajectory enters a VIR, voltage magnitude solutions become complex or negative, the algebraic Jacobian becomes singular, and the behaviour of a system becomes undefined. The case study has been carried out using a simple 3-machine-1-load system with static load models. In the study, VIR appeared and enlarged as the non-linear (constant power and constant current) load increased. Furthermore, the non-convergence of time-domain solution occurred exactly at VIR, thereby confirming that the problem is of structural nature. [ABSTRACT FROM PUBLISHER]

This paper presents a simple induction motor (IM) model for system frequency response (SFR) simulations. SFR simulations are widely used to analyze frequency stability of small isolated power systems. Isolated power systems are especially sensitive to generation-load imbalances due to their small size. The IM shows a proportional-derivative like behavior to active-power disturbances. The response of the proposed IM model is compared with the response of a detailed non-linear IM model. Responses of the proposed and detailed IM models show a good agreement. Further, the impact of load modeling on the performance of the UFLS scheme of a Spanish small isolated power system is shown. Finally, the impact of load shedding on load responsiveness, ie, load characteristics, is shown. [ABSTRACT FROM AUTHOR]

This paper proposes a new control strategy for the reactive-power injections of voltage-source converters (VSCs) in high-voltage direct-current multi-terminal systems to improve power system transient stability. A reactive-power supplementary signal is provided for each converter. Its value is proportional to the frequency deviation of its corresponding AC bus with respect to the weighed-average frequency of the multi-terminal system stations. The purpose is to increase (decrease) the electromagnetic torque of generators close to those terminals, in which the frequency is above (below) the weighed-average frequency used. The AC frequency for all VSC stations is always available locally for synchronization purposes and could be used by a central controller. Simulations have been carried out using PSS/E, and the results have shown that transient stability can be improved using this strategy. Since this approach uses global measurements of all VSC stations, the impact of the communication delays has been analyzed, concluding that the negative effect is small, for realistic latency values. [ABSTRACT FROM PUBLISHER]

This paper proposes a complete approach to modeling, sizing, and controlling ultracapacitor-based excitation boosters (EB) aimed at improving the fault ride-through capability of synchronous generators equipped with bus-fed static excitation systems. The EB model is a simplified one suitable for transient stability simulations. Sizing is performed using transient stability simulations. It is aimed at maximizing the machine critical clearing time subject to machine rotor insulation and ultracapacitor features. The developed control system includes fast voltage sag and loss of synchronism detectors. The proposed approach can be used to design ultracapacitor-based EB in real-world applications. A dedicated test environment based on 10-kVA synchronous machine has been used to validate the proposed approach. [ABSTRACT FROM PUBLISHER]

Multi-terminal High Voltage Direct Current (HVDC) using Voltage Source Converters (VSC-HVDC) is a promising technology which provides flexible control of active and reactive power and facilitates remote renewable energy integration, above all using long cables. This paper analyses an active power control strategy for multi-terminal VSC-HVDC systems tailored to enhance transient stability of hybrid AC/DC grids. The proposed strategy controls each VSC using frequency measurements of all terminals. Its performance is compared to a strategy in which each VSC is controlled using only local frequency measurements of the AC side, proving that the proposed strategy shows better performance, even taking into account reasonable communication delays. The paper also shows that the proposed strategy generally gives similar results to those obtained when each VSC is controlled using the speed of the centre of inertia (COI). The speed of the COI is a more comprehensive and richer figure than the one proposed in this paper but it is also much more complex to obtain. Simulation results with PSS/E of a test system have been used to illustrate the comparisons and the main contributions of the proposal. [ABSTRACT FROM PUBLISHER]

This paper studies the scalability and replicability of smart grid projects. Currently, most smart grid projects are still in the R&D or demonstration phases. The full roll-out of the tested solutions requires a suitable degree of scalability and replicability to prevent project demonstrators from remaining local experimental exercises. Scalability and replicability are the preliminary requisites to perform scaling-up and replication successfully; therefore, scalability and replicability allow for or at least reduce barriers for the growth and reuse of the results of project demonstrators. The paper proposes factors that influence and condition a project's scalability and replicability. These factors involve technical, economic, regulatory and stakeholder acceptance related aspects, and they describe requirements for scalability and replicability. In order to assess and evaluate the identified scalability and replicability factors, data has been collected from European and national smart grid projects by means of a survey, reflecting the projects' view and results. The evaluation of the factors allows quantifying the status quo of on-going projects with respect to the scalability and replicability, i.e., they provide a feedback on to what extent projects take into account these factors and on whether the projects' results and solutions are actually scalable and replicable. [ABSTRACT FROM AUTHOR]

This paper studies the impact of the size and controller settings of an ultracapacitor (UC) on frequency stability. Underfrequency load-shedding (UFLS) schemes play an important role in protecting the system against frequency instability and their performance is used as a measure for the enhancement of frequency stability thanks to the UC. To this end, a simplified but still accurate model of the UC is developed. In addition, a meaningful procedure is proposed to size the UC's power and energy storage capacities in function of the system parameters and operation constraints in order to improve frequency stability. Finally, the impact of the UC on the UFLS scheme is studied for many generation dispatch scenarios and for all possible single generating unit outages of a Spanish isolated power system. An analysis with respect to the UC's control parameters and its size in terms of power and energy storage capacities is carried out, too. Appropriate control parameter settings improve the UC's impact on frequency stability. [ABSTRACT FROM PUBLISHER]

Units within a control area, participating in the secondary frequency control, are usually spinning generating units already connected to the network and operating outside their range of optimal performance. This paper deals with an alternative method of providing secondary frequency control called rapid-start (RS). It consists in assigning a regulation band to several offline units (RS units) which are capable of being started and connected rapidly, therefore allowing the online units to function more closely to their nominal power. RS units have commonly been used for peaking generation and for tertiary control reserve, and have been rarely used for secondary control reserve. As RS operation may have economic benefits, since it allows for better dispatch of the other units in the control area, an appropriate algorithm to start up an RS unit needs to be developed. This paper proposes a machine learning based system (MLBS) to be employed in the decision to start up an RS unit while being used to provide secondary frequency control. The decision-making procedure is carried out by a decision tree. The building and implementation of the RS machine learning based system is illustrated for a secondary frequency control zone within the Spanish power system. [ABSTRACT FROM PUBLISHER]

This paper presents the principles of a centralized underfrequency load-shedding (UFLS) scheme based on a frequency stability boundary curve suitable for small isolated power systems. The centralized UFLS scheme is illustrated in case of two small isolated Spanish power systems. [ABSTRACT FROM PUBLISHER]

This paper fills the gap between the well-known in control theory model reduction techniques based on the balanced realization and the structure preserving dynamic model equivalencing approaches used in power systems. The relations between the synchrony and the loss of controllability and observability are investigated and, from that, new aggregation methodologies are proposed for two distinct situations. The first one corresponds to the case, already treated in the literature, where a full model is available for the power system which must be reduced. For the second one, which is new, it is considered that part of the data of the power system is not available when the reduction is performed. Both small theoretic and large-scale realistic examples are considered. [ABSTRACT FROM PUBLISHER]

This paper studies an approach to identify representative operating and contingency (OC) scenarios for the design of underfrequency load-shedding (UFLS) schemes. In small isolated power systems, contingency scenarios are outages of generating units. Usually, only N - 1 outages are considered. In this paper, simultaneous outages of several units are also taken into account. Data mining techniques such as K-Means and Fuzzy C-Means algorithms are used to group scenarios in terms of system frequency and to identify representative OC scenarios. The approach has been applied to the design of UFLS schemes of two of the Spanish isolated power systems. The results have also been compared to the common practice of scenario selection. Clustering techniques yielded to satisfactory results, i.e., representative OC scenarios can be identified. Furthermore, these representative OC scenarios cover a wider range of possible system responses than the scenarios selected following the common practice. [ABSTRACT FROM AUTHOR]

Large frequency deviations due to a number of disturbances are frequent in small isolated power systems. The maximum frequency deviation in the system is limited to prevent other generator tripping. It is important to have an accurate model to calculate it, both for system planning and operation. A new simplified model to calculate the maximum frequency deviation when either a generator or load-related disturbance occurs in these systems is presented. This model takes into account the response of governor-prime mover even when different technologies are present in the power system. Model parameters can be easily obtained from either more complex models or from test records. Simulation results for an actual power system aimed at checking the model accuracy are presented. High accuracy is obtained while computation time is reduced due to the simplicity of the model. [ABSTRACT FROM AUTHOR]

A simple discrete time model of a thermal unit has been formally developed for designing automatic generation control (AGC) controllers. This model has been developed using data obtained from specific tests and historical records. This model consists of a nonlinear block followed by a linear one. The nonlinear block consists of a dead band and a load change rate limiter, while the linear block consists of a second-order linear model and an offset. Although most of these elements have already been included in unit models for AGC presented in the literature, a certain mix up exists about which of them are necessary. This is clarified in this paper. It has been found that the unit response is mainly determined by the rate limiter, while the other model components are used for a better fitting to the real response. An identification procedure is proposed to estimate the values of the model's parameters. [ABSTRACT FROM AUTHOR]

Power transmission is the main purpose of high voltage direct current systems based on voltage source converters (VSC-HVDC). Nevertheless, this type of system can also help to improve transient stability by implementing suitable supplementary controllers. Previous work proposed active- (P) and reactive-power (Q) control strategies in VSC-HVDC multi-terminal systems (VSC-MTDC, for short) to improve transient stability, producing significant improvements. In those strategies, each VSC station of the MTDC system compares its frequency measurement with the average of the frequencies measured by all converter stations of the MTDC system (weighted-average frequency, WAF) in order to modulate its own P and Q injections. Hence, a communication system is required. This paper presents a detailed analysis of the impact of communication latency on the performance of those control strategies. The communication delays have been modelled using a Padé's approximation and their impact on the performance of the control strategies have been assessed by means of time-domain simulation in PSS/E. The effect of the control strategies on transient stability has been quantified with the critical clearing time (CCT) of a set of faults. Results show that the control strategies analysed present good results for realistic values of communication delays. [ABSTRACT FROM AUTHOR]