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This paper develops, evaluates, and verifies a Supervisory Controller (SC) to transfer from the grid-connected mode and operate a battery-enhanced Electric Vehicles (EVs) DC Fast Charging (DCFC) station in the autonomous mode, when the supply grid is not available. The SC is designed based on the Supervisory Control Theory (SCT) of discrete event systems and is (i) based on a rigorous mathematical process, (ii) non-blocking, i.e., avoids entering an operational deadlock scenario that leads to the system collapse, (iii) minimally restrictive with respect to the station's discrete behavior, (iv) modular, and (v) scalable. The SC also operates the station in the grid-connected mode and provides seamless transition between the two modes and thus the DCFC station is also synonymous with a DC-microgrid. The SC is implemented on an industrial Programmable Logic Controller (PLC) and its performance is verified in a real-time Hardware-In-the-Loop (HIL) environment using an OPAL-RT testbed.

This paper presents and evaluates a discrete-event control-based Supervisory Controller (SC) for a grid-connected, battery-enabled, DC Fast-Charging (DCFC) station for Electric Vehicles’ (EVs). The SC enhances operational resiliency of the DCFC station and its host grid by decoupling the dynamics and limiting the steady-state power between the station and its host grid. The charging station includes multiple DCFC units, a Battery Energy Storage System (BESS), a solar-PV unit, and a grid-interface Voltage-Sourced Converter (VSC). The SC (i) limits the station-imported power from the grid based on the grid-imposed requirements, (ii) dynamically dispatches power flow among the grid, PV unit, DCFC units, and BESS to meet charging needs of EVs and when applicable charges BESS or exports power to the grid, (iii) protects the BESS by enforcing its state of charge upper and lower limits and the maximum charge/discharge rates, and (iv) enables the VSC to provide grid ancillary services, if required. Simulation results in PLECS software platform are given to assess the dynamic performance of the proposed SC in response to various discrete events/transitions. The SC algorithm is implemented on a Programmable Logic Controller (PLC) and verified based on real-time Hardware-In-the-Loop approach using an OPAL-RT platform.

This paper presents, develops, and evaluates a system architecture and its control structure that mitigates impact of the Electric Vehicles’ (EVs) DC fast-charging station on its host weak AC-grid. The charging station incorporates a battery energy storage and (i) limits the imported power based on grid-imposed requirements and (ii) its control system decouples dynamics of the station from those of the grid. The charging station can operate in either grid-connected mode or islanded mode. The dynamic models of the charging station, in both modes, are presented and eigenvalue analyses performed to design the Local Controllers (LCs). Time domain simulation results in PLECS software are presented to verify that the station's internal dynamics, based on the proposed architecture and designed controllers, are sufficiently decoupled from the host AC-grid dynamics.

This paper presents an FPGA-based Digital Real-Time Simulation (DRTS) environment for the analysis of Electromagnetic Transients (EMTs) of power systems that include multiple Power Electronic Converters (PECs). The main features of the proposed DRTS platform are that it i) employs two-value resistor switch model for PECs and thus avoids inaccuracies of Associate Discrete Circuit (ADC) model and ii) does not require separation of PECs by transmission lines (stub lines) and thus accurately represents PECs in close electrical proximity of each other. These features respectively result in a time-variant and large system admittance matrix which significantly increases the computation burden in each simulation time-step and can render it infeasible for real-time applications. To overcome this problem this paper introduces i) Switching-Network Partitioning (SNP) method which is tailored for PECs representation in FPGA and ii) Reformulated Modified Nodal Analysis (RMNA) method to enable computational parallelism between the solution of network and component models in FPGA. SNP enables solution-level partitioning of the PECs and the rest of the system. The developed DRTS platform enables sub-microsecond simulation time-step and provides a fixed hardware irrespective of the system configuration. Performance of the platform is evaluated for a study system and verified versus off-line simulation studies.

This paper investigates impact of high-depth penetration of wind power, i.e., up to 30%, on low-frequency (0–2 Hz) oscillatory modes of interconnected power systems. The wind power is integrated as large Wind Power Plants (WPPs) based on Type-3 and Type-4 wind units represented by enhanced generic nonlinear and linearized models. The studies are based on eigen analysis of the 16-machine/68-bus NPCC equivalent system with multiple WPPs added to it. The case studies consider different ratios of Type-3/Type-4 WPPs to investigate impact of interaction between WPPs adopting different technologies on power system low-frequency dynamics. The studies reveal that WPPs can (i) noticeably impact the damping ratios of existing inter-area oscillatory modes at low-depth penetrations more than at high-depth ones, (ii) introduce new low-frequency oscillatory modes and (iii) cause instability by interacting with existing controllers in the system.

With protective relays exclusive for microgrid applications yet to be developed, microgrids are currently limited mostly to directional overcurrent relays (DOCRs) for protection. This paper unveils that the long-known small fault current of electronically coupled distributed generation (DG) units is merely one of several phenomena that DOCRs suffer from; the current magnitude in microgrids with DG units may even provide misleading indications of fault conditions. More important, commercial-grade torque- and impedance-based directional elements are shown to be adversely affected by the fault behavior of DG units. The misoperation may include incorrect fault direction identification or deactivation of directional elements by false detection of loss-of-voltage condition, ruling out existing impedance relays as workable solutions for microgrid protection as well. This paper also devises a new directional element that addresses these problems. The proposed element detects the direction of asymmetrical faults using the magnitude and angle of the superimposed negative-sequence impedance. For symmetrical faults, the proposed directional element uses the magnitude of superimposed positive-sequence impedance along with the positive-sequence current and torque angle. PSCAD/EMTDC simulation studies verify the performance of this new method.

This work presents a new class of cyber-physical switching attacks that targets power transmission systems. The proposed approach relies on exciting inter-area oscillation modes in a coordinated manner to drive groups of system generators out of step. Our paradigm targets inter-area oscillations by switching a relatively small part (in the order of 2%) of the system load at a (low) frequency that resonates with one of the inter-area oscillation modes observed in the power system. The switching frequency of the targeted mode is chosen through measurement-based analysis of the frequency deviation at a select bus that is observable by the adversary. The inter-area switching attack is implemented as single-load switching and coordinated multi-load switching and is studied with a variety of switching signals. Numerical results show the potential and characteristics of the proposed switching exploitation when applied to the four-machine two-area power system and the Northeast Power Coordinating Council 68-bus system.

This paper extends the dynamic coherency determination (DCD) method by including Type-3 wind power plants (WPPs) models in the coherency evaluation process and quantifies impacts of the high depth of wind power penetration on the coherency phenomena of interconnected power systems. The method is based on frequency-deviation signals, measured at nongenerator buses and terminal buses of WPPs and synchronous generators. Salient features of the presented method are its ability to account for the hybrid nature of enhanced generic models of Type-3 WPPs and expanding notion of the power system coherency beyond the classical definition by including models of Type-3 WPPs. This paper also investigates impacts of the high depth of wind power penetration; i.e., up to 30%, on the coherency structure of the 16-machine/68-bus NPCC equivalent system. The studies are based on time-domain simulations in PSS/E software. The investigations reveal that Type-3 WPPs can introduce new coherent areas, significantly change areas’ boundaries and number of areas, and alter frequencies/dampings of interarea modes.

This paper 1) introduces a new approach for dependable and secure detection of loss-of-field (LOF) events based on estimating rotor signals, 2) implements digital algorithm of the corresponding LOF relay in an industrial hardware platform, and 3) evaluates performance of the LOF relay in a real-time-digital-simulator (RTDS) based control-hardware-in-the-loop (CHIL) environment. The main features of the proposed relay as compared with the conventional approaches include 1) capability to discriminate between system disturbances and LOF events, and 2) dependability and higher speed of the LOF detection mechanism. To evaluate the proposed approach performance for various types of LOF events, a detailed excitation system is modeled and implemented in the RTDS-based CHIL environment. This paper also verifies superior performance of the proposed LOF relay as compared with that of a conventional impedance-based LOF relay, using the RTDS-based CHIL platform.

This paper presents i) an equivalent model of the half-bridge modular multilevel converter (HB-MMC) which is suitable for real-time applications, ii) a hybrid central-processing unit/field-programmable gate array (CPU/FPGA)-based architecture for real-time simulation of electromagnetic transients of systems which include HB-MMC, and iii) a novel arrangement for sorting results referred to as the “sub-module (SM) rank list”, which tackles the bottleneck for parallel implementation of the MMC arm model solver on the FPGA. The Adam–Bashforth (AB) method is used for numerical integration of the HB-SM capacitor model. The second-order AB method provides a constant admittance matrix of the HB-MMC and, thus, reduces computational burden while offering the same accuracy as that of the widely used Trapezoidal method. The CPU/FPGA-based architecture is optimized to obtain maximum parallelism of the HB-MMC model implementation, adopting a standard, single-precision, floating-point computational engine. The proposed sorting arrangement is independent of the utilized sorting algorithm and its application to the odd–even bubble sorting scheme is presented in this paper. The proposed architecture offers a simulation time-step of 825 ns while including the sorting module as the SM capacitor voltage-balancing control unit. This enables accurate analysis of MMC controls based on either software-in-the-loop or hardware-in-the-loop approaches. Performance and accuracy of the MMC model and the hybrid CPU/FPGA-based architecture are evaluated based on a set of case studies on a 401-level HB-MMC-based HVDC station and verified based on offline simulation results in the PSCAD/EMTDC environment.

This study presents an innovative control scheme to improve the power sharing among doubly-fed induction generator (DFIG) wind units in a medium-voltage (MV) microgrid. The control objectives of DFIGs in an islanded mode of microgrid operation are to achieve: (i) stabilisation of the microgrid voltage amplitude and frequency, (ii) proper active/reactive power sharing among wind units. To satisfy these requirements, the DFIG control loop based on the traditional droop control is designed. This method, however, cannot satisfactorily operate in a MV microgrid with dominantly resistive line impedances from power sharing point of view. To overcome this problem, a modified control strategy is proposed in this study. The mathematical modelling is developed, and time-domain simulations are presented to verify the novel control scheme in a typical microgrid case study.

This paper develops enhanced hybrid generic (nonlinear) models of Type-3 and Type-4 wind power plants (WPPs) and extracts the corresponding linear (small-signal) dynamic models for power system transient stability analysis. The models are hybrid in nature since they consider both continuous states and discrete logic-controlled variables. The introduced enhancements include (i) a freezing function to reactivate reactive power emulator of Type-3 and Type-4 WPPs, (ii) active-current command recalculation step for Type-3 WPP and (iii) elimination of an activating logic of PI-controller limits in real current control path. The main feature of the enhanced models is that they can replicate the field-verified responses of the built-in PSS/E software models in any adopted software platform. It should be noted that the generic models described in the technical literature do not necessarily provide such replication. The paper also deduces small-signal dynamic models of Type-3 and Type-4 WPPs and addresses the multiple eigen structures of the linearized enhanced generic model of Type-3 WPP, which has not been comprehensively discussed in the technical literature. The enhanced nonlinear hybrid models and the corresponding linearized models are evaluated and verified based on time-domain simulation studies in PSS/E and MATLAB platforms, using NPCC system as the test bed.

In this paper, a new extendable multi-input step-up DC–DC converter (MISUC) topology is proposed to efficiently interface multiple (renewable/non-conventional) energy sources of different output characteristics with a common load. This topology provides higher voltage gains by increasing the number of inputs thereby being suitable for a wide range of applications from low to high voltage/power in hybrid energy systems (HESs). Input energy sources are controlled simultaneously to supply the load while adjusting the output voltage at the desired level. The operation and design of the MISUC are presented in different modes and design criteria are fulfilled to achieve a low output voltage ripple (OVR). The performance of the proposed converter is validated by simulation and experimental results.

This paper proposes a frequency-based synchronization control (FSC) approach for the voltage-sourced converter in rectification and inversion modes of an HVDC station that is connected to a weak ac grid. Similar to the conventional vector current control (VCC) approach, FSC includes a conventional phase-locked loop (PLL), and its outer loop controller generates the current reference for the inner loop. Different from VCC, the inner loop of FSC utilizes a supplementary frequency-based control loop to provide damping for the oscillations due to the PLL and generates the angle/magnitude for the converter ac-side voltage. The inner loop of FSC switches to VCC to limit over-current during fault scenario. This paper develops a linearized dynamic model of VSC-HVDC station, analytically evaluates behavior of the FSC dynamics, and validates the results based on time-domain simulation studies in the PSCAD platform. The eigenanalysis and simulation results show that FSC can effectively mitigate the instability issues of the PLL even under very weak ac grid condition.

This paper presents a systematic method to determine the feasible range and optimal value of the virtual impedance of the droop-based control to enhance a microgrid system performance with respect to power decoupling, reactive power sharing, system damping, and node voltage profile. A modified power flow analysis and an augmented small-signal dynamic model of the droop-based controlled microgrid, considering the impact of the virtual impedance, are developed. Subsequently, based on the developed methods, the feasible range of the virtual impedance, which can satisfy all the system performances requirements, is determined and presented. Based on a particle swarm optimization technique, an optimization process is introduced to select a virtual impedance value within the feasible range to achieve the overall optimal microgrid performance. Finally, simulation results in the PSCAD/EMTDC platform are provided to validate the feasibility and effectiveness of the proposed methods.

This paper introduces an enhanced droop-based dc-voltage control method, including dead-band, for applications to the high-voltage direct-current (HVDC) grid that utilizes the voltage-sourced converter (VSC) technology. The proposed droop-control structure also autonomously imposes energy balance between the HVDC grid and its host ac system. The droop-control method (1) divides the VSC stations into four groups, (2) activates the droop-control of each group based on a prespecified voltage margin, and (3) introduces an improved power-voltage characteristic for desirable VSC station dynamic performance. Feasibility and performance of the proposed control method are evaluated based on time-domain simulation studies in the PSCAD platform, using the IEEE-39-Bus system that imbeds a five-terminal VSC–HVDC grid. Each VSC station is a monopolar modular multilevel converter (MMC). The study results show that the proposed droop-control method enables the HVDC-AC system to reach a new steady state after transient events.

This paper proposes a non-isolated medium-voltage DC-DC converter with high step-up ratio. It aims to interconnect the low-voltage (LV) generation unites and medium-voltage (MV) power-collecting grid in DC renewable system. The proposed DC-DC converter is composed by a switch network and a few cells. The switch network alternatively configures the cells in parallel connection for charging from low-voltage input side (V L ) and in series connection for discharging to high-voltage output side (V H ). This helps step up the low generation-unite voltage to power-collecting-grid level. Meanwhile, the embedded cells in converter possess voltage adjusting ability, which enables the immediate change of output voltage. As compared to other megawatt-class non-isolated DC-DC converter, the proposal not only achieve high step-up ratio, but also integrates the capabilities of output voltage modulation and DC fault blocking. The feasibility of the proposed DC-DC converter is verified by simulations performed in MATLAB/SIMULINK.

Performance requirements and/or operational integrity of a microgrid subject to major changes, e.g., transition from a grid-connected to an islanded condition, transfer switch operation within the microgrid, or control communication failure, can necessitate the controllers (or a subset of them) to change their modes of operation, e.g., P/Q mode to V/f mode. Transfer in the control mode can further exacerbate the transients due to the original disturbance. This paper proposes mechanisms for smooth transition between control modes. Emphasis of the reported work is on control mode change subsequent to (1) grid-connected to islanded condition and (2) control communication failure. The proposed mechanisms are based on (1) an observer (or state estimator) and (2) an auxiliary tracking controller. The effectiveness of the proposed mechanisms are investigated based on digital time-domain studies in the PSCAD/EMTDC environment and verified in an RTDS-based hardware-in-the-loop platform.

This paper presents the development and validation of an accurate and computationally efficient wideband reduced-order dynamic equivalent model for the Type-3-based wind power plant (WPP). The proposed Type-3 WPP equivalent model reproduces the dynamic behavior of the WPP in response to an electromagnetic transient in the host power system and is composed of two parts: 1) a static frequency-dependent network equivalent model which represents the response of all passive components of the WPP in a wideband frequency range, and 2) a dynamic low-frequency equivalent model that represents the aggregated dynamic model of the doubly-fed asynchronous generator (which is also referred to as doubly-fed induction generator) wind turbine generators, their local controls, and the WPP supervisory control. The proposed model significantly reduces the hardware/software computational burden and the computation time, as compared to the WPP detailed models, without compromising the accuracy of the simulation results. The proposed model is incorporated as a software module in the PSCAD/EMTDC environment and its computational efficiency and accuracy are verified based on comparing the results with those of a detailed model.

Accommodation of DGs (distributed generations) close to loads has led to the concept of MG (micro-grid) for better reliability and quality of energy supply. MG as a clump of consumers and DGs can operate in stand-alone and grid-connected modes, and often needs ESS (energy storage system) to handle generation surplus/shortage. Variations of renewable sources and consumptions along with economical and environmental issues necessitate an efficient OM (operation management) of MG for short-term scheduling of energy outputs of DGs, ESS and exchange route to upstream macro-grid. This paper presents MOUWCA (multi-objective uniform water cycle algorithm) for optimal OM of MG considering operation cost and emission as objectives. The problem is casted to find 24 POFs (pareto-optimal fronts) corresponding to 24 h of the day (unlike previous studies giving one POF per day) to provide more flexibility for selecting hourly compromise solutions. Through an interactive process, charging/discharging of ESS is balanced over a day based on the desired order of hours for discharging ESS. MOUWCA is examined on some benchmark problems and compared with NSGA-II (non-dominated GA-II), MOPSO (multi-objective particle swarm optimization) and NCA (normal constraint algorithm) to verify its effectiveness. MOUWCA is then applied to a typical MG where its superiority is confirmed in comparison to other previously used algorithms.

This paper investigates the dynamic interactions of a modular multilevel converter (MMC) -based HVDC grid and its host ac system during ac-side disturbances. The HVDC grid is composed of five MMC stations that are interconnected through a network of cables and overhead lines. The ac grid includes ten turbine generators. The studies are conducted in the PSCAD platform, including the detailed models of the ac and dc subsystems. The reported case studies indicate that without the HVDC supplementary control functions and based on the conventional $dq$ -frame control structure: 1) each MMC behaves as almost a fixed sink/source of power during the ac-side electromechanical transient oscillations; 2) the MMC-HVDC grid effectively improves dynamic interactions between ac subsystems that are connected only by the HVDC grid; 3) without appropriate provisions, the embedded MMC-HVDC grid can aggravate the transients of the host ac system, for example, interarea oscillations. The observations also contradict the widely accepted notion that even without supplementary controllers, the MMC-HVDC grid necessarily improves the dynamics of the ac system. The study results provide a comprehensive insight into the transient behavior of the MMC-based HVDC-AC grid. Such an understanding is essential to design the protection and control systems of the future HVDC-AC grids.

The dc cables of the voltage-sourced converter-based HVDC link, including the modular multilevel converter (MMC) configuration, must be protected by surge arresters against transient overvoltages. The selection of the appropriate surge arresters requires a comprehensive understanding of the phenomena that cause the overvoltages and accurate calculation of energy discharged in the arresters. This paper investigates the mechanism that causes transient overvoltages, due to dc-side line-to-ground faults, in a cable-connected MMC–HVDC link. This paper also evaluates the impacts of various system parameters, for example, the prefault power transfer level, the MMC arm inductance, the transformer leakage inductance, the fault location, the ac system short-circuit capacity, the cable inductance, and the MMC blocking delay, on the peak transient overvoltages and the amount of energy discharge in the cable surge arresters. The transient overvoltage studies are also conducted when the dc cable is replaced by an overhead line. The studies are performed in the PSCAD platform. The studies conclude that the prefault power transfer level and the MMC blocking delay have the most significant impacts on the surge arrester energy discharge under dc-side line-to-ground faults in the MMC–HVDC link. The study results also provide essential information for the design of the MMC–HVDC link components.

Effective estimation of voltage flicker components plays an important role in distribution systems for either flicker meters or flicker compensators. A novel approach has been presented in this paper to accurately estimate voltage flicker components by using the improved Teager energy operator (ITEO) and the improved chirp-Z transform (ICZT). The error correction factor $K$ of the Teager energy operator is presented and ITEO is established to reduce the extraction errors of voltage flicker waveform. ICZT is used to extract the frequency and magnitude of the voltage envelope which is corrected by the $K$ factor of ITEO. The effects of signal sampling rate, sampling number, spectrum subdivision points of ICZT, voltage harmonics and interharmonics, frequency fluctuation, and white noise are investigated. The implementation of the proposed approach in the digital-signal-processor platform is also introduced. Multiple simulation and experimental test application results validate the accuracy and efficiency of the proposed approach.

A DC microgrid (DC-MG) provides an effective mean to integrate various sources, energy storage units and loads at a common dc-side. The droop-based, in the context of a decentralised control, has been widely used for the control of the DC-MG. However, the conventional droop control cannot achieve both accurate current sharing and desired voltage regulation. This study proposes a new adaptive control method for DC-MG applications which satisfies both accurate current sharing and acceptable voltage regulation depending on the loading condition. At light load conditions where the output currents of the DG units are well below the maximum limits, the accuracy of the current sharing process is not an issue. As the load increases, the output currents of the DG units increase and under heavy load conditions accurate current sharing is necessary. The proposed control method increases the equivalent droop gains as the load level increases and achieves accurate current sharing. This study evaluates the performance and stability of the proposed method based on a linearised model and verifies the results by digital time-domain simulation and hardware-based experiments.

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

This study presents a field programmable gate array (FPGA)-based real-time simulation platform for realistic-size modular multilevel converters (MMCs). This development is primarily intended for controller hardware-in-the-loop (HIL) testing for MMC-based high-voltage direct current (HVDC) transmission systems. Although the real-time simulation of MMC-based systems is particularly challenging due to the presence of thousands of power electronic switches, yet, it is still possible based on adopting a computationally efficient MMC model and the use of a high-performance parallel computational engine. This study focuses on the design and implementation of the parallel computational engine that solves the mathematical model of an enhanced computationally efficient MMC model on the FPGA platform. Moreover, provided are the implementation results and their verifications corresponding to the real-time simulation of a 401-level MMC and the HIL testing of the controllers of the 401-level MMC-HVDC link between France and Spain.

The dynamic voltage restorer (DVR) is a series compensator for distribution system applications, which protects sensitive loads against voltage sags by fast voltage injection. The DVR must estimate the magnitude and phase of the measured voltages to achieve the desired performance. This paper proposes a phasor parameter estimation algorithm based on a recursive variable and fixed data window least error squares (LES) method for the DVR control system. The proposed algorithm, in addition to decreasing the computational burden, improves the frequency response of the control scheme based on the fixed data window LES method. The DVR control system based on the proposed algorithm provides a better compromise between the estimation speed and accuracy of the voltage and current signals and can be implemented using a simple and low-cost processor. The results of the studies indicate that the proposed algorithm is insensitive to noise, harmonics, interharmonics, and dc offset unlike the LES method, while both methods estimate the phasor parameters within 5 ms. The performance of the control scheme based on the proposed method is evaluated by multiple case studies in the PSCAD/EMTDC environment and experimentally validated based on a laboratory setup.

This paper proposes and investigates a new line-commutated current-sourced converter (LCC)-high voltage dc current (HVDC) global supplementary control (GSC) strategy for stabilizing and enhancing the dynamic performance of a large ac system that is segmented by LCC-HVDC links. The GSC is designed based on a linear quadratic Gaussian (LQG) method and enables coordinated supplementary control action of multiple HVDC links that participate in segmentation. The GSC can stabilize the ac system while either minimizing the propagation of oscillatory dynamics from one segment to other segments (GSC1) or enabling their controlled transfer from a disturbed segment to other segments (GSC2). The study results show that in a fully-dc-segmented system: 1) GSC1 and GSC2 are able to stabilize the system; 2) under GSC1, each segment can experience major disturbances without causing adjacent segments to experience the disturbances with the same degree of severity; and 3) GSC2 enables controlled transfer of the oscillations among the segments and, depending on the system configuration, can further reduce the magnitude and duration of oscillatory dynamics. The studies are conducted on a three-segment ac system including three interconnecting LCC-HVDC links.

Hopf bifurcation phenomenon of a power system results in oscillatory dynamics which can lead to instabilities in the system. Therefore, it is desirable to operate the system such that a sufficient margin to Hopf bifurcation is ensured. This paper presents a methodology based on the adjustment of the setpoint values of the HVDC link controllers, to prevent instability or increase the stability margin of the system subject to Hopf bifurcation. In this paper, the first-order sensitivities of the Hopf stability margin to the setpoint values of the HVDC links are presented. These sensitivities identify the optimum direction to change the HVDC setpoints to steer the system away from instability, increase the stability margin, and improve the damping of oscillatory modes. The proposed method is evaluated on various system configurations subject to Hopf bifurcation phenomena caused by a variety of events, such as load and line impedance variations. Simulation results show that at the optimum operating point, for a variety of Hopf bifurcations, the stability margin and damping of the oscillatory modes improve.

The current status of international microgrid research, development, and demonstration (RD&D) was on display and open for review and debate in Tianjin, China, 13?14 ?November 2014 at the Tianjin 2014 Symposium on Microgrids. Sponsored by the U.S. Department of Energy Office of Electricity Delivery and Energy Reliability (U.S. DOE-OE) and the Energy Foundation, organized by Tianjin University, Hefei University of Technology, and supported by other generous sponsors, the tenth in the influential series of microgrid symposiums was held at the Geneva Grand Hotel. Broad international participation was achieved, with 106 attendees from 18 countries (Figure 1). There were 26 technical presentations by speakers from around the globe as well as a panel session on standards development and technical tours to the Tianjin ?Eco-City sustainable city development, Tianjin University?s Microgrid Laboratory, and the Yanqing dc microgrid near Beijing. The formal welcome was offered by Zhipeng Liang, deputy director of the ?Renewable Energy Department, Chinese National Energy Administration.

In the developing countries, women generally have fewer shares in worker organizations. Even when women join the syndicates, they rarely get the high positions as in management. Employers have extremely limited women's access to development. They (employers] usually complain about women absenting from work due to their pregnancy and delivery. They claim that they have suffered costs such as paid leave during their pregnancy or after delivery, and replacement of other workers for them is so difficult and does not have that productivity. Usually, they believe, women are less productive because of their home responsibilities. Another factor that affects the public employment is the age structure, migration, pregnancy rate, population growth, and worker-supply level. However, economic, social, cultural, and political factors affect women's employment for more than population growth factor. War-time and early revolution economic depression brought about changes in the economic system of the country and resulted in transitions in the work market. These transitions and economic depression seriously limited industrial production and productive employment where women's employment was quantitatively and qualitatively affected.

This paper demonstrates the need for and provides an improved fitting of the line parameters when a wide range of frequencies with emphasis on the dc response is the objective. The improved fitting is based on a low-order fitting of a function error for each of the line parameters, that is, the characteristic admittance and propagation function, in the low-frequency range. The function error represents the difference between the analytical evaluation of the line parameters, that is, obtained from a frequency-dependent distributed parameters line model, and their initial rational approximation, for example, given by an electromagnetic transients software tool. The approximated function error, added to the initial rational approximation of the line parameters, provides the enhanced fitted line model. The enhanced fitting improves the dc response of the line and ensures numerical stability in time-domain simulations. This paper applies the proposed fitting approach to the universal line model for time-domain simulations. The simulation results by the enhanced fitting are validated by comparing the results with those obtained from a frequency-domain line model (FDLM). Two case studies are presented to illustrate the proposed fitting approach.

This paper presents the fundamental concepts of a generalized central power management system and a decentralized, robust control strategy for autonomous mode of operation of a microgrid that includes multiple distributed energy resource (DER) units. DER units are divided into voltage-controlled and power-controlled DER units. The frequency of each DER unit is determined by its independent internal oscillator and all oscillators are synchronized by a common time-reference signal based on a global positioning system (GPS). The power management system (PMS) specifies the power and voltage set points for the local controllers of each DER unit. A linear, time-invariant, multivariable, robust, decentralized control system is designed to track the setpoints. Each control agent guarantees fast tracking, zero steady state error, and robust performance despite uncertainties of the microgrid parameters, topology, and the operating point. Existence conditions, control design procedures, eigenanalysis and robust stability analysis of the closed-loop system, and performance of the control strategy based on digital time-domain simulation studies in PSCAD/EMTDC platform, are reported. Performance of the control system is also verified based on hardware-in-the-loop (HIL) studies in the RTDS environment.

This letter presents the extension of a previously proposed frequency-domain (FD) approach for the analysis of electromagnetic transients (EMTs), primarily due to a set of discrete switching events, in an electric power network. The method is based on dividing the analysis time window into a set of subwindows with equal or unequal time lengths and solving each in the FD. The sampling time step is also independently selected for each subwindow. The extended method, formerly proposed for a single transmission line, can readily use the concept of network equivalents, achieving efficient EMT solutions of large systems, and can simultaneously accommodate slow and fast dynamics.

This paper presents two add-on features for the voltage control scheme of directly voltage-controlled distributed energy resource units (VC-DERs) of an islanded microgrid to provide overcurrent and overload protection. The overcurrent protection scheme detects the fault, limits the output current magnitude of the DER unit, and restores the microgrid to its normal operating conditions subsequent to fault clearance. The overload protection scheme limits the output power of the VC-DER unit. Off-line digital time-domain simulation studies, in the EMTDC/PSCAD software environment, demonstrate the feasibility and desirable performance of the proposed features. Real-time case studies based on an RTDS system verifies performance of the hardware-implemented overload and overcurrent protection schemes in a hardware-in-the-loop environment.

Article history: Received May 2, 2013 Received in revised format 25 June 2013 Accepted 7 July 2013 Available online July 11 2013 This paper presents an empirical investigation on the effect of public attitudes toward the role of arts and cultural institutions for the cultural development of the country. The study performs the study among urban and rural families in city of Esfahan, Iran and all questions are designed in Likert scale of 1-5. Using a sample of 700 people, the study finds that people generally have negative impression toward the role of arts and cultural institutions for the cultural development of the country. In their opinions, the nature and quality of the effects play the most important role, which must be improved. The study also finds the people’s style of living and place of residency are also important issues on the perception toward cultural development. © 2013 Growing Science Ltd. All rights reserved.

This paper introduces, formulates and evaluates an approach for damping inter-area oscillations of power systems based on supplementary current control of a line-commutated HVDC link. The proposed control is based on the model predictive control (MPC) strategy. The salient feature of the MPC as compared with other optimal control strategies, e.g., the linear quadratic Gaussian (LQG) control, is that it adjusts the control signal to achieve the objectives while explicitly respecting the plant constraints. This paper also compares the performance of the MPC with that of the LQG control. The two approaches are tested on the Western System Coordinating Council (WSCC) 9-bus system and the IEEE 14-bus system. Small-signal disturbance and large-signal disturbance stability studies are performed to demonstrate and compare the performances of the LQG and MPC methodologies. The study results show the effective and superior performance of the MPC for damping poorly damped oscillatory modes of the test systems.

Summary form only given. This paper introduces an accurate and computationally efficient reduced-order dynamic equivalent of a Type-4 based wind power plant (WPP) for the analysis of electromagnetic transients (EMTs) in the power system external to the WPP. The proposed model significantly reduces the computational resources and the simulation run time while preserving the WPP response-fidelity in the desired frequency range, e.g., 0 to 50 kHz. The proposed WPP equivalent model is composed of two parts; (i) a frequency-dependent equivalent model which represents the WPP passive component in the entire frequency range, and (ii) a dynamic equivalent model that represents the WPP supervisory control and the aggregated low-frequency dynamics of wind-turbine generator (WTG) units. The latter can be constructed from the (i) dynamic generic models, (ii) vendor-specific models, or (iii) user-defined models. The proposed model is incorporated as a software module in the PSCAD/EMTDC environment and its efficiency and accuracy are verified based on comparing the results with those of a detailed model.

The current transformer (CT) is prone to the magnetic core saturation during the power system faults. The CT saturation can prolong and/or miscoordinate operation of relays and thus may result in the system stability problems and equipment damage. This paper proposes a new hardware-based method to suppress or prevent the CT saturation. The proposed approach is based on adding a controllable resistor at the CT secondary circuit which dynamically generates a flux to oppose the time varying flux associated with the decaying dc component of the fault current. Part I of this two-part paper presents the concept, the controllable-switch operational strategy, the design steps, and the parameters of the controllable resistor. Part II provides a comprehensive evaluation of the controllable resistor based on offline digital time-domain simulation and control-hardware-in-the-loop studies.

Part I of this two-part paper introduces a hardware-based mechanism to prevent/suppress the saturation phenomenon of the current transformer (CT). The mechanism includes an electronically switched resistor which is in series with the CT secondary winding. Part II performs a set of 1) offline digital time-domain simulation studies in the PSCAD/EMTDC environment and 2) control-hardware-in-the-loop (CHIL) test cases in a real-time digital simulation platform, to demonstrate technical feasibility of the proposed approach. The investigations also report the effect of CT saturation and the proposed desaturation mechanism on a digital distance relay. The digital algorithms of the relay and the control of the electronic switch are implemented in two NI-CRIO platforms for the CHIL studies. The investigation results demonstrate and verify the effectiveness of the proposed CT desaturation mechanism.

This paper describes the challenges and lessons learned when developing industrial-grade data translators aimed for the interfacing of power-flow programs with Electromagnetic Transients Program-type programs. It has been found that the greatest challenges to overcome include: the lack, in the databases used in power-flow programs, of vital pieces of information necessary to perform transient studies; inconsistency in the format of data files; the presence of data entry mistakes in very large databases; the validation of the translated data; and the analysis of the large amount of data that transient simulations provide. Several examples are presented to show the implemented solutions. Finally, recommendations based on experience are made to help future developers of interfacing tools.

This paper introduces and evaluates an auxiliary control strategy for downstream fault current interruption in a radial distribution line by means of a dynamic voltage restorer (DVR). The proposed controller supplements the voltage-sag compensation control of the DVR. It does not require phase-locked loop and independently controls the magnitude and phase angle of the injected voltage for each phase. Fast least error squares digital filters are used to estimate the magnitude and phase of the measured voltages and effectively reduce the impacts of noise, harmonics, and disturbances on the estimated phasor parameters, and this enables effective fault current interrupting even under arcing fault conditions. The results of the simulation studies performed in the PSCAD/EMTDC software environment indicate that the proposed control scheme: 1) can limit the fault current to less than the nominal load current and restore the point of common coupling voltage within 10 ms; 2) can interrupt the fault current in less than two cycles; 3) limits the dc-link voltage rise and, thus, has no restrictions on the duration of fault current interruption; 4) performs satisfactorily even under arcing fault conditions; and 5) can interrupt the fault current under low dc-link voltage conditions.

The reconfigurable-hardware real-time power system simulator (RH-RTS) is a field-programmable gate-array (FPGA)-based real-time simulator that is developed based on the concept of simulators hardware reconfigurability (i.e., to change the underlying hardware architecture of the simulator to accommodate various power system topologies). The uniqueness of the RH-RTS is the underlying hardware architecture. The RH-RTS has a massively parallel customized hardware architecture that is tailored to the solution of the mathematical model of the power system under consideration. The RH-RTS enables the simulation of large power systems with a computation-time per simulation time-step in the range of tens of nanoseconds. Not only does the RH-RTS provide a means for real-time operation (e.g., for closed-loop testing of physical control/protection platforms in hardware-in-the-loop (HIL) configuration), it also provides a means for faster-than-real-time operation (e.g., for statistical switching studies). This paper provides validation and evaluation of the performance of the RH-RTS. This paper presents a case study involving the simulation of a power system in both real time and faster than real time. The computation time per simulation time-step is as low as 24 ns, for realistic size systems, which is, by far, the lowest computation time reported in the technical literature.

Part I of this paper has proposed a duty-cycle-space-vector (DCSV)-based modulation strategy for three- and four-leg matrix converters (MCs). This paper develops a digital DCSV-based modulation strategy for the three- and four-leg MCs based on the results of Part I of this paper. The digital DCSV-based modulation strategy is implemented in a DSP-field programmable gate array (FPGA) platform. The objective is to reduce the calculation time of the modulation algorithm by performing the following: 1) reducing the total number of instructions needed to implement the modulation algorithm in the DSP and 2) implementing parts of the modulation algorithm in an FPGA to generate the switching pattern. Therefore, the calculation process of the modulation algorithm is split between the DSP and the FPGA.

The generalized modulation strategy of the conventional matrix converter (MC), i.e., three-leg MC, was previously developed using the duty-cycle space vector (DCSV) representation of switch duty cycles. This paper uses the DCSV representation of switch duty cycles to develop a new formulation of the generalized modulation strategy for one-, three-, and four-leg MCs. The four-leg MC, unlike the three-leg MC, is capable of providing a neutral connection at the output side of the MC. The neutral connection enables the four-leg MC to operate as a power supply providing a balanced, regulated, and fixed-frequency voltage for balanced and unbalanced loads, including the single-phase load. The generalized modulation strategy of the four-leg MC in this paper is developed in the context of power supply applications. Using the new formulation of the generalized modulation strategy, the direct duty-cycle modulation strategies based on DCSV calculation are developed for one-, three-, and four-leg MCs, which are referred to as the DCSV-based modulation strategy of the MC. Part I of the paper formulates the DCSV-based modulation strategy of the three- and four-leg MCs.

Article history: Received October 3, 2012 Received in revised format 30 November 2012 Accepted 30 November 2012 Available online December 2 2012 For almost a century, radio stations have been some primary sources for presenting arts, entertainment, news, etc. and the primary concern on many studies is to understand audience attitude on this media. This paper attempts to investigate audience attitude on radio programs broadcasted in city of Esfahan, Iran. The proposed study selects random sample of 600 out of 1,745,428 residence of this city where 345 people actively were listening to these programs, actively. All questions were designed in Likert scale and Cronbach alpha was 0.941, which was well above the desirable level and validated the survey. According our survey, the most popular part of radio programs was associated with news where 78.3% of the audiences were listening to these programs. According to our survey, audiences were mostly satisfied with family oriented programs and the mean score was 4.05. The other observation indicates that people have good attitude towards educational guidance programs (mean=3.37) and programs related to people and officials (mean=3.41). The results of Freedman test with Chi-Square value of 52.507 determines that there is meaningful difference among different components (mean difference = -0.23 and P-value =0.022). We have also performed an investigation to find out whether there is any difference between different components of this survey in terms of participants’ personal characteristics such as age, gender, educational background and job. Our survey only confirms the mean difference in terms of job specifications and other personal characteristics did not have any impact on people’s attitude. The other observation in our survey indicates that there is a difference between students and people with no job or housekeeper on their attitudes towards educational programs. © 2013 Growing Science Ltd. All rights reserved.

The restricted earth fault (REF) relay is one of the protection units of the transformer digital protection system which typically has a fast response and thus is prone to maloperation. Based on measurement results and time-domain simulation results, this paper provides a comprehensive performance evaluation of the REF relay which is an augmented-based adaptive restraint current approach for performance stabilization. The investigation results provide justifications to introduce modification to the relay to improve its performance during transients and, thus, improve the relay security. This paper also introduces and evaluates a scheme to modify the REF relay performance. The proposed scheme has been implemented in the field, and the results indicate performance improvement.

This study introduces a fast locating algorithm for a switched shunt capacitor based on the slopes of the voltage/current space vectors at monitoring points of a power system. Compared with the other suggested slope-based methods, such as signal-processing and filter-based methods, the proposed algorithm herein offers a robuster and more reliable locating method because of using the information of all three phases, whereas other methods separately process individual phase signals. The method is training free and established based on algebraic calculations with a simple realisation algorithm, which is independent of the power system dynamics and parameters. The proposed method is analytically investigated and then verified based on the time-domain simulation of two study systems. The study systems cover various capacitor-switching scenarios, harmonic effects and measurement noise and power system dynamics including dynamics of rotating machines and power electronic converters.

This paper presents the implementation of the time-frequency analysis tools, based on the S-transform and the modified S-transform, into a Raspberry Pi for a real-time computation. The aim of this approach is to provide an embedded system acquisition and measurement at low costs that would permit higher sampling resolution, recording and extraction of the data measurements in order to analyze electrical harmonics and transients. We discuss the modified S-transform proposed in literature and the different optimized Gaussian windows forms in order to obtain better energy concentration measurement. The implemented algorithm is described and applied to synthetic and real electrical signals. The modified S-transform used brings to light the enhancement of the time-frequency resolution. We conclude with future enhancements and perspectives of this study.