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Renewable sources are expected to be a key element of future power systems. With many challenges ahead, thinking out of the box is necessary in order to equip the power system with additional advanced functionalities and capabilities to combat the anticipated challenges. The associated equipment of distributed generation can be seen as a grid asset upgrade because of the increased number of elements connected to the grid, such as grid side converters (GSC) of photovoltaics and battery storage systems. It therefore makes sense to think of ways to diversify the role of distributed renewables in a way that benefits the grid, consumers, and prosumers in terms of power quality. The work proposes an advanced, multi-function GSC control technique for improving the distribution grid power quality by enabling the injection of asymmetric, DC and harmonic currents in case it is needed. Injecting for example such currents locally through the GSC of a residential photovoltaic system can compensate undesired characteristics of the prosumer loads in order to allow the flow of symmetric and high quality currents between the prosumer and the distribution grid. Experiments and simulation results are presented to validate the performance of proposed control technique.

The integration of modern renewable energy sources is enabled by the grid side converter (GSC) based on power electronic technology. The operation of the GSC is properly regulated by the GSC controller according to sensor measurements (i.e., of the grid voltage, of the line currents, and of the voltage at the dc-link). However, in case of current sensor faults, the operation of the GSC and of the entire renewable system can be critically affected and catastrophic failures may occur if the sensor fault is not accommodated on time. This paper proposes a model-based Fault Detection, Isolation, and Accommodation (FDIA) scheme, which enables the GSC to overcome faults that may occur on the associated current sensors. The sensor fault detection and isolation scheme has been designed based on analytical redundancy relations, while the accommodation of the faults is based on an adaptive estimation scheme. The proposed FDIA scheme has been applied on a modern GSC and the effectiveness of the scheme has been tested under several multiple current sensor faults and under several grid conditions. Furthermore, the FDIA scheme enables the GSC to operate properly (without causing failures or damages to its components) under current sensor faults and thus, the reliability of the GSC is enhanced.

Synchronization is a crucial aspect in grid-tied systems, including single-phase photovoltaic inverters, and it can affect the overall performance of the system. Among prior-art synchronization schemes, the Multi Harmonic Decoupling Cell Phase-Locked Loop (MHDC-PLL) presents a fast response under grid disturbances and high accuracy under harmonic distortions. However, major drawbacks of the MHDC-PLL include increased complexity and inaccurate response under non-nominal frequencies, which may occur in practical applications. Thus, this paper proposes strategies to address these issues. At first, a novel re-formulation of an equivalent decoupling cell is proposed for reducing the implementation complexity, and then a frequency adaptive quadrature signal generator for the MHDC-PLL is proposed to enable an accurate response even under non-nominal frequencies. Simulation and experimental results are provided, which show that the proposed synchronization (i.e., the frequency adaptive MHDC-PLL) can achieve a fast and accurate response under any grid disturbance and/or severe harmonic conditions., This work was supported by the Research Promotion Foundation of Cyprus (RPF, Cyprus, Project KOINA/SOLAR-ERA.NET/0114/02), by Energinet.dk (ForskEL, Denmark) and the SOLAR-ERA.NET (European Union's Seventh Framework Programme). © 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. L. Hadjidemetriou, Y. Yang, E. Kyriakides and F. Blaabjerg, "A synchronization scheme for single-phase grid-tied inverters under harmonic distortion and grid disturbances," IEEE Trans. Power Electronics, vol. 32, no. 4, pp. 2784-2793, April 2017. DOI: 10.1109/TPEL.2016.2581019. https://www.ieee.org/publications_standards/publications/rights/rights_policies.html

Microgrids composed of solemnly AC-coupled distributed energy resources can be found in many real-life applications while their control has not been researched nearly enough to address some fundamental challenges, the most important of which is overall system reliability and fault tolerance. This paper proposes a droop-based coordinated control scheme for microgrids with AC-coupled units, a method that enables distributed energy resources units to hot swap between current source and voltage source grid-supporting control modes for satisfying load demand and ensuring energy storage systems will constantly be able to form the grid during islanded operation. The proposed control scheme has been realised in MATLAB/Simulink simulation model of a small-scale microgrid of AC-coupled units that corresponds to a real testbed in Northern Greece. Preliminary simulation results, in islanded mode, demonstrate the effectiveness of the proposed control scheme regarding power-sharing accuracy among the resources and state-of-charge balancing among storage units. 1 6

This paper presents an optimal scheduling for the utilization of the battery energy in order to minimize the cost of the electricity bill in a non-residential building equipped with a photovoltaic-storage system. The battery storage device is connected at the DC-link of the photovoltaic system for enabling a more efficient grid integration through a common central inverter. For the optimal scheduling method, the day-ahead electricity pricing and the next-day forecasted building load and PV generation for each quarter hour are taken into consideration. The problem is formulated as a mixed-integer linear programming model and is solved using commercial solvers. Furthermore, the efficiency curve of the inverter is approximated by a discretized step function and along with the battery efficiency are included in the formulation. Two case studies are carried out using real data and useful results according to the energy flow in the system and the profit are obtained.

Recent decentralization of electricity systems together with the decarbonization and several changing societal demands are giving rise to different application scenarios such as microgrids. A microgrid is a small-scale electrical system which consists of several loads and sources (conventional and renewables) that can either operate autonomously in a stand-alone mode or interconnected with the main grid. The design and development of such a smart microgrid in a university campus is proposed within the 3DMicroGrid project (funded through the ERANETMED European Union’s initiative). This paper reviews the main components and characteristics of similar microgrids developed around the world. Furthermore, this study provides the design guidelines, the main functionalities, the key components and the control architecture for developing the microgrid proposed by the 3DMicroGrid project. A simulation model has been developed and initial results are demonstrated for the operation of this microgrid. The recommendations and insights are replicable to any solar priority country for future microgrids pilots.

During the last decade, microgrids are attracting a significant attention due to their numerous advantages. Amongst them, the most important one is their ability to operate either in grid-connected or in islanded (autonomous) mode. However, the transition between the two modes can be challenging in terms of maintaining the stability and integrity of the microgrid. In this paper, an optimization-based islanding methodology is developed to ensure a timely and smooth transition from the grid-connected to the islanded mode. This is achieved through shedding loads, by defining the generation level ofthe photovoltaics (energy spill) and by regulating the charging/discharging rate of batteries. A resynchronization method is also presented along with the requirements that need to be satisfied for the smooth reconnection of the microgrid back to the main grid. The effectiveness of the proposed approach is demonstrated through simulation results for the events of surplus energy production, and excess energy demand of the microgrid. 1 6

Intentional Controlled Islanding (ICI) has been proposed as a corrective measure of last resort to split the power system into several sustainable islands and prevent cascading outages. This paper proposes a novel ICI algorithm based on a Linear Programming (LP) formulation that directly determines an islanding solution with minimal power-flow disruption for any given number of islands, while ensuring that each island contains only coherent generators. In addition, the proposed algorithm enables operators to constrain any transmission line to be excluded from the solution, allows the control of the size of islands and ensures their connectivity. The basis of the proposed LP formulation is an exact Mixed Integer Linear Programming (MILP) Formulation. A Search Space Reduction (SSR) procedure that generates additional constraints for reducing the search space of the MILP is also proposed. In most cases, these additional constraints are enough for the relaxed MILP formulation (LP formulation) to generate optimal solutions. Nonetheless, the proposed LP formulation is executed as a part of a recursive linearization procedure which ensures that optimal solutions are always obtained. Multiple simulation results demonstrate the ability of the proposed LP ICI algorithm to meet the requirement of real-time controlled islanding in large-scale power systems., A. Kyriacou, P. Demetriou, C. Panayiotou and E. Kyriakides, "Controlled Islanding Solution for Large-Scale Power Systems," in IEEE Transactions on Power Systems, vol. 33, no. 2, pp. 1591 - 1602. doi: 10.1109/TPWRS.2017.2738326 © 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

Low-voltage distribution grids face several challenges due to the high penetration of renewable energy sources, the increasing demand (electrification of thermal and transportation sector), and the limited observability (absence of smart metering infrastructure). These challenges impose problems regarding the integrity, stability, power quality and efficiency of distribution grids. Therefore, intelligent solutions are required in order to overcome these problems. This paper proposes a centralized control scheme for phase balancing of low-voltage distribution grids at the substation level. The proposed solution utilizes the advanced capabilities of the grid tied inverters of photovoltaics and storage systems to increase the utilization of distribution lines and to compensate the asymmetric loading conditions of the substation. The solution is based on only one smart meter installed at the distribution substation and utilizes available information and communication technology infrastructure to integrate the solution. The solution achieves significant benefits for the distribution grids, especially regarding their power quality and the effective utilization of their capacity.

This work proposes a sensor-less controller for grid tied photovoltaic (PV) inverters to enable phase balancing functionalities for compensating asymmetric loading conditions imposed by building loads in low voltage distribution grids. For enabling such an advanced functional operation by PV inverter, the first step is to enable the inverter to estimate the equivalent grid impedance. Then, the grid impedance is utilized to approximate the nearby load asymmetries without using any additional current sensors. Finally, advanced control schemes have been developed for PV inverters in order to enable the new phase balancing operation mode, where the inverter can compensate the asymmetric loading conditions of a distribution feeder. The effectiveness of the proposed method has been experimentally validated in a prototype where the grid tied PV inverter is able to compensate nearby load asymmetries and maintain a purely symmetrical interaction with the grid. Further, a simulation-based investigation in a realistic distribution feeder has been performed in order to highlight the benefits of the proposed approach regarding the power quality, the energy losses and the effective utilization of distribution grid capacity.

A grid-connected power electronics based distributed generator can act as an active element by providing support to the grid under various disturbances. In this paper, a voltage support scheme dedicated for distributed generators installed in the low voltage distribution network is proposed. The voltage support considers the resistive characteristics of the distribution line grid impedance in order to maximize its effectiveness. Therefore, the optimal active and reactive current references are estimated based on the resistance to reactance ratio (R/X). Moreover, a droop curve is suggested such that the intensity of voltage support will be defined according to the voltage drop conditions. The proposed voltage support concept is applied on a photovoltaic system to validate its operation. The effectiveness of the proposal is confirmed by means of simulations in a realistic low voltage distribution network.

Fault recorders can track the transients in the power system during a fault. However, they cannot provide a wide area picture of the power system operating condition. The provision to track the transients is of great interest for the power system operators. In this paper, a two-stage state estimator (SE), based on both conventional and phasor measurement unit (PMU) measurements, is presented. The estimator is able to track the dynamics of the power system states during a fault. The advantage of the proposed SE is that it does not require full power system observability by the PMU measurements. The two-stage SE was successfully tested on the IEEE 14 and 118 bus systems.

The accurate representation of the transmission line parameters is essential for several monitoring and control applications accommodated in the control center of the power system. In this sense, it is always important to update the database of the transmission line parameters which might be obsolete and/or contain erroneous parameters. The validation and the correction of any inconsistencies related to the power system model is one of the natural applications of the Synchronized Measurement Technology. One way for line parameter refinement is the use of Phasor Measurement Units at the two ends of the transmission line. In this paper, the error that is associated with the calculation of the line parameters is examined. The effect of uncertainties that are introduced to the PMU measurements by both instrument transformers and PMUs on the calculation of the transmission line parameters is investigated by performing several case studies in the IEEE-14 bus system., "© 2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works." M. Asprou, E. Kyriakides, and M. Albu, "The effect of PMU measurement chain quality on line parameter calculation," 2017 IEEE International Instrumentation and Measurement Technology Conference (I2MTC), Turin, May 2017, pp. 1-6.

The accurate knowledge of the transmission line parameters can be beneficial for several monitoring and control applications accommodated in the power system control center. Actually transmission line parameters are stored in the control center databases (assuming that they are time invariant); however, the databases might be obsolete and contain erroneous parameters. This paper proposes a methodology for identifying and estimating the erroneous transmission line parameters using measurements provided by phasor measurement units (PMUs) and estimated states provided by a state estimator. The main advantage of the proposed methodology is that for the identification of the erroneous transmission lines and the estimation of the line parameters only one PMU is required for the monitoring of the transmission line (i.e., capture measurements related to the transmission line). The proposed methodology is tested in the IEEE 14- and 118-bus systems.

Load shedding constitutes the very last resort for preventing total blackouts and cascading events. Conventional load shedding schemes, which are massively applied in industrial practice, adopt a step-wise approach that usually causes overshedding or fails to prevent frequency decay above the allowable limits. Recently proposed schemes based on real time intelligent control and neural networks achieve the control objective, but fail to minimize the amount of load to be shed due to the delay incurred in consecutive control decisions. This letter proposes a new load shedding scheme, decoupled from the conventional scheme. This scheme, based on the equivalent swing equation of the system, determines the minimal amount of load that should be shed immediately (in a single step) after the load event/disturbance occurs, in order to guarantee system stability.

The capacitive network representation of voids in electric power cables has long been used for the study of the transients generated by a breakdown in an enclosed void. As an alternative to the capacitive representation of the void, the induced charge concept has been introduced by Pedersen and his colleagues. This work examines the induced charge concept from an electrostatic point of view and from measured transients in relation to the change in the capacitance of the system by opposing Pedersen's views.

The state estimator constitutes the cornerstone of the supervisory control and data acquisition system since it provides the power system operating situation in consecutive time intervals. Furthermore, the output of the state estimator is used by other tools responsible for the monitoring and control of the power system. Therefore, there is a need for the power system state estimator to be as accurate and reliable as possible. The main source of uncertainty that may deteriorate the accuracy of a weighted least squares (WLS) state estimator, provided that the network parameters are perfectly known, is the uncertainty that is encompassed in the measurements. It is well known that the measurement chain is not ideal and this information is passed to the state estimator through the measurement weights. In this paper, the effect of the measurement weights, which are calculated by considering both the standard uncertainties associated with the measurement devices and the instrument transformers, on the WLS state estimator is examined.

Diversifying the role of grid side converters (GSC) associated with distributed generation (DG) may have a technical and economic impact. The increasing number of residential photovoltaic systems can be exploited by distribution network (DN) operators in order to support and improve the power quality of the grid without considerable further capital investment. The GSC can support the grid during abnormal conditions but may also be used to improve the DN power quality by balancing out asymmetries and alleviating the grid from undesired harmonics. The proposed advanced control technique demonstrates through simulation and experimental results that the role of GSC can be upgraded to improve the power quality of the grid. Furthermore, the proposed control scheme is validated on a realistic low-voltage DN using data from the Cyprus Electricity Authority. Consequently, conventional practices by distribution network operators for improving the power quality can be re-assessed as the necessary power electronics hardware for mitigating grid problems are already present within the GSCs of DG systems.&nbsp

Improving the efficiency of thermoelectric devices is critical to their widespread adoption. Here a design methodology, formulated on computational and analytical modeling, derives the optimum efficiency and geometry of segmented Bi2Te3–PbTe Thermoelectric Generators (TEGs) between ≈298 K and ≈623 K (ΔT ≈ 325 K). Comparisons between the different TEG designs, in terms of the electrical load to TEG electrical resistance ratio (m = RL/RTEG), are simplified thanks to the devised maximum efficiency to temperature gradient (βmax = η/ΔT) metric. Quasi-computational results of βmax show that the collective Seebeck coefficient Bi2Te3–PbTe ( α ˜ ) design sustains a higher electrical load in relation to the homogeneous Bi2Te3 and PbTe materials. The average ( α ¯ ) and collective ( α ˜ ) Seebeck coefficient Bi2Te3–PbTe configurations, in comparison to Bi2Te3 and PbTe, exhibit a considerably higher (60–68%) source and sink thermal resistance matching (ΘTEG = ΘHx). The proposed segmented Bi2Te3–PbTe ( α ˜ ) TEG yields a peak efficiency of 5.29% for a ΔT of 324.6 K.

The delay of Phasor Measurement Unit (PMU) measurements due to the communication network may affect the performance of real time applications accommodated in a Wide Area Monitoring and Control system. In order to mitigate the effect of the PMU measurement delays, a data delay compensation technique based on a linear predictor is proposed in this paper. The proposed method incorporates autocorrelation Linear Predictive Coding (LPC) to predict the future values of the measured signals and a data delay compensator to compensate the data delays actively. The proposed data delay compensation technique is applied to the IEEE 9-bus test system where it is indicated that the proposed method can compensate the data delays in wide area signals effectively.

The ability of monitoring a laboratory scale power system is crucial for research and educational purposes. This paper proposes the development of a low-cost real-time platform for monitoring a laboratory scale power system. The design and development of this novel monitoring system is based on a TMS320F28335 microcontroller of Texas Instruments. The microcontroller reads voltage and current measurements from several locations of the laboratory scale power system, processes these measurements and then sends the results to a personal computer. The user can monitor in real-time the operation of the power system through the implemented graphical user interface. The developed system can achieve an accurate and real-time monitoring of a laboratory scale power system which is particularly useful in the educational process for understanding the main power engineering practices. Moreover, novel algorithms regarding the stability, the state estimation, the operation and the monitoring of power systems can be experimentally validated by utilizing the proposed monitoring system. The demonstration of the proper operation of the prototype monitoring system is available online as a video.

A Renewable Energy Source (RES), enhanced with Fault Ride Through (FRT) capability, can provide proper voltage and frequency support to the power grid under several disturbances. This paper investigates how a realistic dynamic power system with high penetration of RES reacts under different operating conditions. More specifically, the power system operation when RES operates according to the current grid regulations of the Cyprus TSO is demonstrated. For the purposes of this investigation, a RES is interconnected with the IEEE 9-bus test system, reinforced with an appropriate directional over-current protection scheme, and dynamic electromagnetic transient simulation results are provided.

The controllers of single-phase grid-tied inverters require improvements to enable distribution generation systems to meet the grid codes/standards with respect to power quality and the fault ride through capability. In that case, the response of the selected synchronization technique is crucial for the performance of the entire grid-tied inverter. In this paper, a new synchronization method with good dynamics and high accuracy under a highly distorted voltage is proposed. This method uses a Multi-Harmonic Decoupling Cell (MHDC), which thus can cancel out the oscillations on the synchronization signals due to the harmonic voltage distortion while maintaining the dynamic response of the synchronization. Therefore, the accurate and dynamic response of the proposed MHDC-PLL can be beneficial for the performance of the whole single-phase grid-tied inverter.

The common feature of many monitoring and control applications that will be accommodated in the future Wide Area Monitoring and Control system will be the quasi real time execution, exploiting the fast reporting rate of the Phasor Measurement Unit (PMU). A critical parameter for the timely execution and the good performance of such real time applications is the delay that the PMU measurements experience during their transfer between successive processing layers — for example, from the substation level to the control center. The associated delays, mainly due to the communication infrastructure, should be quantified and considered in the process of a real time application. Therefore, in this paper the delay associated to real PMU measurements that are transferred through a heterogeneous communication network (3G) is estimated for a specific measurement set-up used in the Romanian distribution system. Further, the respective delay will be considered in a numerical simulated measurement environment which is used by a real time state estimator to investigate its impact on the real time performance.

The main task of the state estimator in power systems is the provision of the power system state accurately. In order to achieve this, the measurement set has to be validated by identifying erroneous measurements. This is usually achieved by using either the Chi-square or the Largest Normalized Residual test. In both tests, the estimated states that are provided by the state estimator and the measurement variances included in the measurement error covariance matrix are used for detecting the erroneous measurements. The measurement error covariance matrix is usually formulated using the accuracy of the measurement device only, ignoring the quality of other elements in the measurement chain such as the instrument transformers. In this paper, the effect of instrument transformer measurement quality on the bad data detection algorithm is investigated. Further, simulation results based on the IEEE 118-bus system are provided.

Monitoring utility-scale solar arrays was shown to minimize cost of maintenance and help optimize the performance of the array under various conditions. In this paper, we describe the design of an 18 kW experimental facility that consists of 104 panels fitted with smart monitoring devices. Each of these devices embeds sensors, wireless transceivers, and relays that enable continuous monitoring, fault detection, and real-time connection topology changes. The facility enables networked data exchanges via the use of wireless data sharing with servers, fusion and control centers, and mobile devices. Research planned at this stage includes developing machine learning methods for fault detection. Preliminary simulation results on fault detection using machine learning are given in this paper.

This paper presents a nonlinear parameter estimator for synchronous machines based on the unscented Kalman filter. The proposed methodology uses voltages and current signals recorded from the stator and the field winding to update the parameters of the classical model of the synchronous machine for stability studies. The methodology can be applied without interrupting the normal operation of the generator. Park's Transformation is included in the estimation process to relate the stator measurements (in abc components) to the nonlinear voltage equations in the qd0 reference frame. The proposed robust methodology has been validated using real and simulated data to estimate the model parameters of a 483-MVA round rotor machine.

This paper presents a new hybrid constrained power system state estimator. The conventional and synchrophasor measurements are simultaneously incorporated in the estimation problem without using any transformation of measurements. This constrained formulation makes it possible to take advantage of information from phasor measurement unit (PMU) branch current and voltage measurements, improving the accuracy of the existing estimators. The methodology is applied on the IEEE 14-, 57-, and 118-bus test systems, and its performance is compared with other hybrid estimators.

This article deals with the synchronized measurement technology that has the potential of becoming the backbone for real-time monitoring system. The secure and reliable operation of modern power systems is an increasingly challenging task due to the ever-increasing demand for electricity, the growing number of interconnections, penetration of variable renewable energy sources, and deregulated energy market conditions. Power companies in different parts of the world are therefore feeling the need for a real-time wide area monitoring, protection, and control (WAMPAC) system. Synchronized measurement technology (SMT) has the potential of becoming the backbone of this system. The major advantages of using SMT are that 1) the measurements from widely dispersed locations can be synchronized with respect to a Global Positioning System (GPS) clock, 2) voltage phase angles can be measured directly, which was so far technically infeasible, and 3) the accuracy and speed of energy management system (EMS) applications increases manifold.

The increasing penetration of renewable energy sources (RESs) in the power grid requires high-quality power injection under various grid conditions. The synchronization method, usually a phase-locked loop (PLL) algorithm, is directly affecting the response of the grid-side converter of the RES. This paper proposes a new PLL algorithm that uses an advanced decoupling network implemented in the stationary reference frame with limited requirements for processing time to enable a fast and accurate synchronization even under harmonic distorted voltage and low-voltage grid faults. The robust response of the proposed PLL is validated, and the effect of the proposed synchronization on the performance of the grid-connected renewable energy system is investigated. This investigation proves that the robust, accurate, and dynamic response of the new PLL can enhance the quality of the injected power from the RES and can also enable an appropriate fault-ride-through operation under harmonic distorted voltage and grid faults.

Grid-connected renewables are increasingly developed in recent years, e.g. wind turbine systems and photovoltaic systems. Synchronization of the injected current with the grid is mandatory. However, grid disturbances like voltage sags, harmonics, and frequency deviations may occur during operation, becoming inevitable challenges to the synchronization of the grid-connected renewable energy systems. In order to ensure the quality of the power generation from the renewables, robust and reliable synchronization methods are in demand. Among the prior-art solutions, Phase Locked Loop (PLL) based synchronization methods have gained much popularity in grid-connected applications. However, an appropriate selection and thus a proper design of the selected PLL synchronization remain of interest in practice, especially for single-phase systems. Therefore, in this paper, a benchmarking of the main PLL synchronization methods for single-phase grid-connected inverter systems in terms of accuracy, dynamic response, harmonic immunity, etc., has been conducted. Experiments on a 1-kW single-phase grid-connected system, suffering from different grid disturbances, are performed for the benchmarking. The experimental results have verified the discussions.

The paper presents a methodology to estimate the rotor angle of a synchronous generator by using measurements from phasor measurement units (PMUs). Based on the available measurements, an observer for the damper currents is designed. Using the observed damper currents, known machine parameters, and PMU measurements, stator reactances of the machine are estimated during transients following a disturbance. These updated reactances are then used to find the rotor angle of the generator. Application of the proposed method on a test power system shows promising results.

The integration of renewable energy sources in power systems has steadily increased in the recent years in the vision of significantly decreasing the carbon emissions produced by the conventional power plants. Although the incorporation of the renewable energy sources in the power system network is environmentally beneficial, at the same time it poses some challenges regarding their smooth operation and control. In this sense, the renewable energy sources should be considered not only as passive elements in the power system network, but, if possible, as ancillary services for the mitigation of evolving contingencies. In this paper, a methodology to preserve the voltage stability of the power system after the occurrence of a contingency utilizing the renewable energy sources is presented. The proposed methodology was applied to the IEEE 14-bus system and simulation results are provided.

The complete integration of the Synchronized Measurement Technology (SMT) in the power systems will bring about fundamental changes to the applications of the control center. One of the natural applications of the SMT is the state estimation. Although the state estimator is used for monitoring the steady state operating condition, it is envisioned that in the near future, a PMU-based state estimator will be able to monitor the dynamic behavior of the power system states. In such an application, the transfer delay of the PMU measurements should not be ignored. In this work, the effect of the PMU measurements transfer delay on the accuracy of a PMU-based state estimator is examined. Further, the optimal waiting time of the Phasor Data Concentrators (for sending information to the control center), and the time execution of a PMU-based state estimator for capturing accurately the state transients during fault conditions are also examined.

The Fault Ride Through (FRT) capability of renewable energy sources (RES) for providing support to the power grid under several disturbances allows us to consider them not only as passive elements in the power system network, but also as ancillary services for the mitigation of evolving contingencies. In this paper, an investigation of the FRT operation of RES according to current grid regulations highlights some impacts on the system response that were not considered previously and require to be addressed. Slight modifications of the current grid codes are therefore necessary to solve these problems. A critical dead band zone modification for smother fault recovery is suggested. Moreover, this paper proposes the use of an adjustable parameter into the FRT strategy for a fair compromise of voltage and frequency support. The proposed FRT strategies are applied to a RES that is interconnected with the IEEE 14-bus test system and dynamic electromagnetic transient simulation results are provided.

Branch parameter errors may affect the success of voltage stability enhancement schemes. This work considers errors of varying sign and magnitude in the parameters of transmission lines. The considered voltage stability enhancement scheme is based on a central state estimator and on a distributed monitoring of individual lines. The state estimator incorporates synchronized phasor measurements. The applied voltage stability enhancement scheme is based on monitoring individual branches and on load shedding, for various thresholds of the applied stability indices. A discussion on voltage stability enhancement schemes and branch parameter errors is followed by simulations on two case studies involving two IEEE studies and both static and transient contingencies. In conclusion, dealing with branch parameter errors in the context of voltage stability enhancement schemes is discussed.

Transmission line parameters are affected by several factors, such as ambient temperature, mutual coupling of parallel lines, and erroneous transmission line length. Further, the several joints, especially at the line ends, cause additional changes in the calculated line parameters. These factors are rarely taken into consideration resulting to obsolete line parameter databases. The error that is encompassed in the values of the transmission line parameters can be up to 30% of their nominal value. In this paper, a methodology for estimating the actual parameters of transmission lines represented as a nominal pi model is proposed. In this methodology the PMU measurements at the one end of the line and the estimated states from a hybrid state estimator for the other end of the line are required. The proposed methodology is applied to the IEEE 14 bus system and the results are demonstrated in this paper.

The modeling of damping for power system stability analysis is revisited, and types of power system damping are described. Current practices in modeling damping are considered, including the representation of damper windings in generators. The effect of not modeling damping accurately in stability studies is considered, and some misconceptions are pinpointed. Finally, some suggestions are presented to improve the damping representation in power systems and to improve the validity of results obtained from power system stability analyses.

This paper presents a method to identify synchronous generator parameters from on-line data. An observer for estimation of synchronous machine damper currents is designed. The observer-estimator is used in a graphical user interface (GUI) application. Possible internal machine fault conditions can be detected and remedial action can be undertaken. It is desired that an algorithm be developed such that it will enable bad measurement detection and rejection so as to increase the reliability of the results. Secondary objectives include calculation of the error characteristics of the estimation; development of an index of confidence; study of which machine parameters can be estimated, and which cannot; and evaluation of alternative GUI features.

The occurrence of contingencies in power systems and their impacts are non-deterministic, justifying probabilistic approaches to power system security assessment. In this work, a probabilistic methodology to assess the security of electrical power systems is applied. The IEEE 39 and 118 bus systems are used as case studies. The distributions of security indices such as Expected Demand not Served are derived by performing nonsequential Monte Carlo simulations and by considering remedial actions such as load shedding and line tripping. In order to account for the uncertainty in the knowledge of their values, various failure probabilities for the individual elements of the power system are considered. Results are discussed in terms of evaluating both the method applied as well as the security of the examined systems.

Monitoring of the power system states in real time, especially after the occurrence of a fault, can enhance the situational awareness of the power system operators. Although the state estimator is designed for monitoring steady state conditions of the power system, the advent of Synchronized Measurement Technology (SMT) can make the state estimator a helpful tool for monitoring the power system operating condition in quasi real time. In this paper, the use of a linear state estimator using only synchronized phasor measurements will be presented. The innovation lies in the use of the linear state estimator for the estimation of the states of the system in the presence of a fault. With the proper PMU placement, all system buses can be monitored accurately and the voltage/angle behavior can be tracked with precision.

This paper presents an adaptive frequency remote control of an islanded autonomous grid equipped with a 5.5 kW wind generator, 7.2 kW solar panels, and 58 kWh energy storage. The islanded grid is located at VTT premises in Finland. The grid was controlled remotely from Cyprus using a communication protocol. The ultimate aim of the work described in this paper is to control and balance the generation and demand in the islanded grid through direct load management based on a single point measurement of the electric frequency.

Disasters, whether natural or man-made, compromise the quality of life for all involved. In such situations, expeditious recovery activities are deemed imperative and irreplaceable for the restoration of normalcy. However, recovery activities rely heavily on the critical infrastructures that supply basic needs like electricity, water, information, and transportation. When disasters strike, it is likely that the critical infrastructures themselves are affected significantly, hampering efficient recovery processes, thus presenting a Catch-22 conundrum. In this article, we present examples from different parts of the world where distributed energy resources, organized in a microgrid, were used to provide reliable electricity supply in the wake of disasters, allowing recovery and rebuilding efforts to occur with relatively greater efficiency.