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Renewable Energy Source (RES)-based power plants need to control the active and reactive power at the Point of Common Connection (PCC) with the grid, in order to comply with the requirements of the Transmission System Operators (TSOs). This point is normally far away from the power converter station, and the cables and step-up transformers have a non-neglectable influence on the delivered power. In order to overcome this drawback, this paper presents a control algorithm that permits one to control remotely the power injected at the PCC, by adjusting the local controller of the Voltage Source Converters (VSCs). In this work, the synchronization with the grid is done based on the Virtual Flux (VF) concept. The results reveals that the VF estimation is able to produce a reliable estimation of the grid voltage in any point of the network, and makes it possible to calculate the necessary current reference for injecting a desired active and reactive power at a point that can be some kilometres away. In this paper the main principle for this remote power control is presented. Likewise, the simulation and experimental results will be shown in order to analyse the effectiveness of the proposed system. [ABSTRACT FROM AUTHOR]

The control system of a doubly-fed adjustable-speed pumped-storage hydropower plant needs phase-locked loops (PLLs) to obtain the phase angle of grid voltage. The main drawback of a comb-filter-based phase-locked loop (CF-PLL) is the slow dynamic response. This paper presents a modified comb-filter-based phase-locked loop (MCF-PLL) by improving the pole-zero pattern of the comb filter, and gives the parameters' setting method of the controller, based on the discrete model of MCF-PLL. In order to improve the disturbance resistibility of MCF-PLL when the power grid's frequency changes, this paper proposes a frequency-adaptive modified, comb-filter-based, phase-locked loop (FAMCF-PLL) and its digital implementation scheme. Experimental results show that FAMCF-PLL has good steady-state and dynamic performance under distorted grid conditions. Furthermore, FAMCF-PLL can determine the phase angle of the grid voltage, which is locked when it is applied to a doubly-fed adjustable-speed pumped-storage hydropower experimental platform. [ABSTRACT FROM AUTHOR]

Powerline communications (PLC)-based smart meter deployments are now a reality in many regions of the world. Although PLC elements are generally incorporated in smart meters and data concentrators, the underlying PLC network allows the integration of other smart grid services directly over it. The remote control capabilities that automation programs need and are today deployed over their medium voltage (MV) grid, can be extended to the low voltage (LV) grid through these existing PLC networks. This paper demonstrates the capabilities of narrowband high data rate (NB HDR) PLC technologies deployed over LV grids for smart metering purposes to support internet protocol internet protocol (IP) communications in the LV grid. The paper demonstrates these possibilities with the presentation of the simulation and laboratory results of IP communications over international telecommunication union: ITU-T G.9904 PLC technology, and the definition of a PLC Network Management System based on a simple network management protocol (SNMP) management information base (MIB) definition and applicable use cases. [ABSTRACT FROM AUTHOR]

This paper presents a microgrid stability controller (MSC) in order to provide existing distributed generation units (DGs) the additional functionality of working in islanding mode without changing their control strategies in grid-connected mode and to enhance the stability of the microgrid. Microgrid operating characteristics and mathematical models of the MSC indicate that the system is inherently nonlinear and time-variable. Therefore, this paper proposes an adaptive robust total sliding-mode control (ARTSMC) system for the MSC. It is proved that the ARTSMC system is insensitive to parametric uncertainties and external disturbances. The MSC provides fast dynamic response and robustness to the microgrid. When the system is operating in grid-connected mode, it is able to improve the controllability of the exchanged power between the microgrid and the utility grid, while smoothing the DGs' output power. When the microgrid is operating in islanded mode, it provides voltage and frequency support, while guaranteeing seamless transition between the two operation modes. Simulation and experimental results show the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

A distorted grid voltage or nonlinear behavior in the current control loop can cause low frequency current harmonics in a grid-connected voltage source inverter (VSI). Many efforts have been made to mitigate such phenomena, including hardware and/or control structure improvements. A well-known suitable strategy to reduce current harmonics in a selective manner is to apply a Proportional Multi-Resonant (PMR) current controller. Inverter-grid stability is another common issue when dealing with grid-connected VSI. Stability is influenced by the inverter impedance, which depends on the controller parameters. This paper presents a simplified tuning strategy for the PMR controller, taking into consideration the inverter-grid stability issue. The obtained controller was implemented and tested in a 10 kW three-phase inverter with a passively damped LCL filter. A significant reduction of current harmonics emission from the inverter up to 650 Hz was achieved without any hardware modification. The limits of PMR controllers to mitigate current harmonics were studied, and the influence of the grid impedance was verified. [ABSTRACT FROM AUTHOR]

This paper aims to develop the generalized dynamical model of multiple photovoltaic (PV) units connected to the grid along with the dynamic interaction analysis among different PV units. The dynamical models of multiple PV units are developed by considering three different configurations through which these PV units are connected to the grid. These configurations include: (a) the direct connection of multiple PV units to the grid; (b) the connection of multiple PV units to the grid through a point of common coupling (PCC); and (c) the connection of PV units without a PCC. The proposed modeling framework provides meaningful insights for analyzing dynamic interaction analysis where these interactions from other PV units are expressed in terms of voltages and line impedances rather than the dynamics of currents. The dynamic interactions among different PV units for all these configurations are analyzed using both analytical and simulation studies. Simulations are carried out on an IEEE 15-bus test system and dynamic interactions are analyzed from the total harmonic distortions (THDs) in the current responses of different PV units. Both analytical and simulation studies clearly indicate that the effects of dynamic interactions are prominent with the increase in PV units. [ABSTRACT FROM AUTHOR]

This paper presents the design, fabrication and experimental power analysis of a novel double-stator magnetic geared permanent magnet (DSMGPM) machine which comprises of a doublestator permanent-magnet (PM) machine integrated a with triple rotor magnetic gear. The proposed machine can upscale the low-speed rotating magnetic field of the prime PMs on the prime rotor by using the modulation effect produced from the pole-pieces to high-speed rotating magnetic field from the field PMs. The low-speed prime rotor can also increase the speed of the field PM rotor and excite the coil windings to induce an electromotive force (EMF) resulting in electrical power. Thus, the machine is proposed for power generation in low-speed renewable energy applications such as wind turbines and tidal power generators. The proposed machine topology is presented and discussed while the performance power characteristics are evaluated experimentally. A prototype is fabricated and the measured results are in good agreement with the calculated results, therefore validating the proposed magnetic geared machine design. [ABSTRACT FROM AUTHOR]

After a brief historical introduction to the hosting-capacity approach, the hosting capacity is presented in this paper as a tool for distribution-system planning under uncertainty. This tool is illustrated by evaluating the readiness of two low-voltage networks for increasing amounts of customers with PV panels or with EV chargers. Both undervoltage and overvoltage are considered in the studies presented here. Probability distribution functions are calculated for the worst-case overvoltage and undervoltage as a function of the number of customers with PV or EV chargers. These distributions are used to obtain 90th percentile values that act as a performance index. This index is compared with an overvoltage or undervoltage limit to get the hosting capacity. General aspects of the hosting-capacity calculations (performance indices, limits, and calculation methods) are discussed for a number of other phenomena: overcurrent; fast voltage magnitude variations; voltage unbalance; harmonics and supraharmonics. The need for gathering data and further development of models for existing demand is emphasised in the discussion and conclusions. [ABSTRACT FROM AUTHOR]

According to the theory of passivity-based control (PBC), this paper establishes a port-controlled Hamiltonian system with dissipation (PCHD) model for a doubly fed induction generator (DFIG) system under unbalanced grid voltage conditions and proposes a method of interconnection and damping assignment passivity-based control (IDA-PBC) of the system under such conditions. By using this method, the rotor-side converter and grid-side converter can be controlled simultaneously in order to improve fault ride-through capability of the DFIG system. Simulation results indicate that this IDA-PBC strategy effectively suppresses fluctuations of output current and power in the DFIG system during unbalanced grid voltage sag/swell, enhances dynamic performance, and improves the robustness of the system. [ABSTRACT FROM AUTHOR]

The ever-growing penetration of local generation in distribution networks and the large diffusion of energy storage systems (ESSs) foreseen in the near future are bound to affect the effectiveness of interface protection systems (IPSs), with negative impact on the safety of medium voltage (MV) and low voltage (LV) systems. With the scope of preserving the main network stability, international and national grid connection codes have been updated recently. Consequently, distributed generators (DGs) and storage units are increasingly called to provide stabilizing functions according to local voltage and frequency. This can be achieved by suitably controlling the electronic power converters interfacing small-scale generators and storage units to the network. The paper focuses on the regulating functions required to storage units by grid codes currently in force in the European area. Indeed, even if such regulating actions would enable local units in participating to network stability under normal steady-state operating conditions, it is shown through dynamic simulations that they may increase the risk of unintentional islanding occurrence. This means that dangerous operating conditions may arise in LV networks in case dispersed generators and storage systems are present, even if all the end-users are compliant with currently applied connection standards. [ABSTRACT FROM AUTHOR]

This paper describes the detailed analysis of a reactor application for a power plant to mitigate the voltage rise of a distribution line (DL) caused by the connection of distributed resources (DRs). The maximum capacity of renewable energy generators (REGs) that meets the acceptable voltage rise of a DL and the necessary capacity of the reactor to mitigate that voltage rise according to the different types of REGs are analyzed. The re-coordination of a protection relay and the loss of generation revenue as well as the installation location of a reactor are described. Finally, the ON/OFF conditions of the reactor, such as the magnitudes of the grid voltage and generator voltage, and the duration time of the voltage rise are analyzed. As the voltage rise is mitigated and self-limited in small power plants, it is confirmed that the capacity of the DRs connected to the DL can be increased through a field demonstration. [ABSTRACT FROM AUTHOR]

A grid connected inverter usually requires voltage and current measurements to control the active and reactive powers as well as the inverter output currents. While voltage sensors are essential to obtain reliable information on the phase angle, these additional components certainly increase the production costs and complexity. In this paper, a voltage-sensorless control scheme for a grid connected inverter using a disturbance observer (DOB) is presented. The grid voltages are estimated by DOB in the stationary reference frame using the current measurements and reference signals. Even though the DOB estimates the grid voltages with reasonable accuracy in the presence of the uncertainty such as the unbalanced condition and harmonic distortion, the resultant waveform shows a phase lag depending on the estimation bandwidth. To overcome this limitation, a phase lead compensation is introduced. By using these techniques, the phase angle of grid voltages can be completely restored even if the phase angle of grid is initially unknown. The proposed scheme is simple and straightforward. In addition, it does not require any additional hardware. The feasibility of the proposed voltage-sensorless control scheme is demonstrated through simulations and experiments using 2 kVA prototype inverter. [ABSTRACT FROM AUTHOR]

This paper presents a seamless transition method for a droop-controlled inverter. The droop control is suitable to make the inverter work as a voltage source in both islanded and grid-connected modes, however, the transfer between theses modes can result in a big inrush current that may damage the system. The proposed method allows the droop-controlled inverter to improve the transient response when transferring between modes, by detecting the inrush current, activating a current control loop during transients, and then transferring back to droop-controlled mode smoothly by using a virtual inductance loop. In addition, a local phase-locked-loop (PLL) is proposed to align the inverter voltage with the grid in order to reduce the transient current during the transition. Therefore, the droop-controlled inverter is able to operate in both grid-connected and islanded modes, providing as well a smooth transition between them, requiring neither synchronization signals nor grid-side information. The control algorithm and design procedure are presented. Experimental results from a laboratory prototype validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

High level penetration of renewable energy sources has reshaped modern electrical grids. For the future grid, distributed renewable power generation plants can be integrated in a larger scale. Control of grid-connected converters is required to achieve fast power reference tracking and further to present grid-supporting and fault ride-through performance. Among all of the aspects for converter control, the inner current loop for grid-connected converters characterizes the system performance considerably. This paper proposes a unified current loop tuning approach for grid-connected converters that is generally applicable in different cases. A direct discrete-time domain tuning procedure is used, and particularly, the selection of the phase margin and crossover frequency is analyzed, which acts as the main difference compared with the existing studies. As a general method, the approximation in the modeling of the controller and grid filter is avoided. The effectiveness of the tuning approach is validated in both simulation and experimental results with respect to power reference tracking, frequency and voltage supporting. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new Zero Direct Power Control (ZDPC) technique for active compensation of harmonics and reactive power, using shunt active power filter (SAPF), based on cancellation of instantaneous active and reactive power disturbances by comparison with their zero references. To separate harmonic and fundamental components of the line voltage and current a highly selective filter (HSF) has been used. Depending on the power errors and line voltage vector position, a switching table produces the appropriate control vectors leading to the active and reactive power variation required to reach the zero power references, even under grid voltage unbalanced and distorted conditions. The experimental validation of the proposed ZDPC has been performed. The results are compared to other recent techniques to demonstrate the superiority and feasibility of the proposed strategy. [ABSTRACT FROM AUTHOR]

When large wind farms are centrally integrated in a power grid, cascading tripping faults induced by voltage issues are becoming a great challenge. This paper therefore proposes a concept of static voltage security region to guarantee that the voltage will remain within operation limits under both base conditions and N-1 contingencies. For large wind farms, significant computational effort is required to calculate the exact boundary of the proposed security region. To reduce this computational burden and facilitate the overall analysis, the characteristics of the security region are first analyzed, and its boundary components are shown to be strictly convex. Approximate security regions are then proposed, which are formed by a set of linear cutting planes based on special operating points known as near points and inner points. The security region encompassed by cutting planes is a good approximation to the actual security region. The proposed procedures are demonstrated on a modified nine-bus system with two wind farms. The simulation confirmed that the cutting plane technique can provide a very good approximation to the actual security region. [ABSTRACT FROM AUTHOR]

This paper proposes a maximum power point tracking (MPPT) and voltage regulation method based on model predictive control (MPC) for the two-stage grid-tied photovoltaic (PV) system, which can achieve MPPT and output voltage regulation of a PV system simultaneously. The MPPT algorithm based on MPC is implemented in a DC-DC boost converter. The reference voltage at maximum power point is obtained by dual step Incremental Conductance (I&C) algorithm under the rapidly varying illumination intensity, and the MPPT controller only needs to minimize one cost function of PV current, without pulse width modulation (PWM) module. To inject the generated PV power into the grid with high quality, this paper designs voltage regulation controller based on MPC to maintain the output voltage of the PV system at the desired value. The MPC controller outputs the optimal duty signal with the input and state constraints in the inner loop, and the PI controller in the outer loop is designed to improve the dynamic performance. The proposed method based on MPC was demonstrated using the SimPower systems tool in MATLAB/Simulink. Analysis and simulation results for the PV system show possible improvements on the closed-loop performance such as fast response and low overshoot. [ABSTRACT FROM AUTHOR]

An experimental evaluation of a digital control system based on a dual Digital Signal Processor (DSP) architecture is proposed for a three phase Unified Power Quality Conditioner (UPQC). A classical UQPC is constituted by two power conditioners, connected in series and shunted with the power grid, and sharing a common DC-link. In a smart grid scenario of operation, a UPQC will be fundamental for compensating power quality problems, also contributing to improving the efficiency of the electrical grids from a global perspective. The UPQC operation requires a bidirectional energy transfer between the two power conditioners, however, respecting some constraints, they can be controlled independently. In order to take advantage of this characteristic, the control algorithms can be executed on two independent DSPs, without any communication between them and maintaining the operational characteristics of the UPQC. Comparing with the classical control architecture based on a single DSP, with the proposed dual DSP architecture, the computational effort of each DSP is decreased of about 35%, allowing to increase the sampling rate. Therefore, the main advantages of the proposed approach are the minimization of delays caused by the processing time, which are very common in digital control systems, as well as the increment of the UPQC performance. Along the paper, detailed analysis of the processing speed and memory requirements to implement the UPQC control algorithms in both DSPs is presented. The paper also presents a set of detailed experimental results, obtained with a developed 5 kVA laboratory prototype of UPQC, which was used to evaluate the performance of the proposed dual DSP architecture. [ABSTRACT FROM AUTHOR]

As a proportion of generation using renewable energy increases in a power system, the need for facility investment is increasing in not only the distribution system but also the transmission system. However, it is inefficient to invest in new equipment to deal with the short peak output of renewable energy generation. For this reason, this paper proposes a method for operating the grid more flexibly, through an optimal operating strategy of the direct current (DC) line at medium-voltage (MV) level, thereby delaying the facility investment in the transmission line. In order to determine the optimal operating point of the converter connected with a DC line, the swarm intelligence-based optimization technique, which can minimize multi-objective function, is used. In addition, in order to overcome the communication dependency of the centralized control, which needs to receive information about the optimal operating point computed by the system operator, this paper proposes a decentralized emergency control (DEC) method in case of a communication fault. [ABSTRACT FROM AUTHOR]

Grid-connected voltage source inverters (VSIs) with LCL filters have been widely used for distributed generation systems (DGs). Various control methods have been studied to achieve good performance. Among them, sliding mode control has been applied to LCL-filtered grid-connected VSIs for its fast, dynamic response and strong robustness. However, LCL networks can easily cause instability problems under weak grid conditions such as grid impedance variation. At the same time, the stability design of sliding mode control applied for LCL-filtered grid-connected inverters are important, but they lack detailed parameters design in recent papers. In this paper, a design of observer-based sliding mode control to improve the stability of three-phase LCL-filtered grid-connected VSIs was proposed. The theoretical stability analysis was developed to consider the effect of the system discretization and grid impedance variations. Finally, a 3-kW, 110-V, 50-Hz experimental setup has been built to demonstrate the validation of the proposed method. [ABSTRACT FROM AUTHOR]

This paper deals with the topic of power quality in a local distribution grid. It is aimed to analyze the individual influences, which aggravate the power quality of the distribution grid. Based on the analysis, the most adverse effects were determined, and they were the voltage drops and supply voltage interruptions, supply voltage unbalance, load power factor, and also higher harmonics. These influences cause the technical losses in a distribution grid, which subsequently have a financial impact not only on the distribution, but also on the transmission of electricity. Only the load voltage unbalance, the load power factor, and the higher harmonics, which mainly cause the technical losses, were analyzed in this paper. The measurement of the influences of the adverse effects was performed on the model of a 22-kV distribution grid. The measurement was performed on the basis of three types of power line conductors and their different lengths, three types of active power consumption, and the different values of these adverse effects. According to this measurement, a simulation in program Matlab-Simulink was created. This simulation represented part of a 22-kV distribution grid, which was influenced by the abovementioned adverse effects. The results of the measurements were compared with the results of the simulation. Based on the evaluation of the technical losses from the measurement and the simulation, the financial losses during a certain period were calculated for the distribution system operators. [ABSTRACT FROM AUTHOR]

This paper addresses the output current and circulating current control of the modular multi-level converter (MMC). The challenging task of MMCs is the control of output current and circulating current. Existing control structures for output and circulating current achieve control objectives with comparatively complex controllers and the designed parameters for the controller is also difficult. In this paper, an adaptive proportional integral (API) controller is designed to control the output current and the circulating current. The output current is regulated in α β axes while the circulating current is regulated in the a b c stationary frame to enhance MMC performance. The output and circulating current control results using an API controller are compared with the conventional proportional resonant (PR) controller in terms of transient response, stability, optimal performance, and reference tracking for results verification. The API control architecture significantly improve transient response, stability, and have excellent reference tracking capability. Moreover, it controls output current and converges the circulating current to a desired value. The control structure is designed for a three-phase MMC system, simulated and analyzed in MATLAB-Simulink. [ABSTRACT FROM AUTHOR]

In a neutral ungrounded system, the high voltage fuses used to protect voltage transformers (VTs) often abnormally blow out, causing unbalanced VT operation. Fuses also fail to blow out in time, resulting in further damage to the VT. This paper reported the results of steady-state current testing, breaking characteristics, X-ray measurements, fuse corona testing, and electromagnetic transient impact testing for VT fuses. This paper comprehensively examines and analyzes the quality and electrical performance of VT fuses and provides new guidance for the use of high voltage fuses in voltage transformers. This paper recommends that 35 and 10 kV systems use fuses rated for a current of 1 A based on a single fuse, which is not easily oxidized and has a wound skeleton composed of an Ag or Ni melt. [ABSTRACT FROM AUTHOR]

The constructive asymmetry of the untransposed overhead lines of a high voltage alternating current is the cause of a great number of difficulties in their operation and modeling. In order to model the operating regimes of such lines, the symmetrical component method, based on constructive symmetry and thus the symmetry of the equivalent phase parameters, is inappropriate, which is why many research papers have been dedicated to either setting up improved modeling methods or to returning to phase coordinate modeling. This paper intends to justify a paradox found on some untransposed overhead lines of a high voltage alternating current during the no-load operating conditions by performing phase coordinate modeling. In such a situation, the transmission or distribution operators measured significant negative values for the active powers on one or two phases at the beginning of the lines. Considering the case of a real untransposed overhead line operating under no-load conditions, the paper starts from presenting the recorded electrical values. Then, the paper moves on to outlining the Carson's simplified computing relations for calculating the series and shunt primitive equivalent parameters and Kron's transformation relationships for calculating the phase equivalent parameters. After applying them to the real line, the calculation of the power flow for the no-load operating conditions, which is applied to an equivalent scheme of the line consisting of nine identical octopoles, is performed. Both the untransposed line and its transposed variant are studied here. The values of the electrical amounts obtained by the calculation for the untransposed line are basically similar to those obtained by measuring on the real line, which gives a mathematical confirmation of the so-called paradox. Its occurrence represents the effect of the asymmetry of the equivalent phase capacities, which causes a redistribution of the active powers between the phases of the network to which the overhead line operating in no-load conditions is connected. [ABSTRACT FROM AUTHOR]

In this paper, the basic principle and characteristics of a phase-locked loop (PLL) in a single phase grid-connected system are analyzed, and this paper introduces one type virtual orthogonal voltage vector method based on a third order generalized integrator (TOGI) to construct an alpha and beta static coordinate system. The TOGI structure can eliminate the DC offset in a voltage signal or zero offset in the sampling process, and ensure the amplitude of the virtual orthogonal signal is consistent. At the same time, the adaptive frequency estimation unit is introduced, which can effectively deal with the power grid voltage frequency changes and ensure the accuracy of PLL. MATLAB (R2012a, MathWorks, Natick, MA, USA) is used to simulate the variation of power grid voltage frequency, DC component injection, harmonics injection and other parameters, and the performance of PLL is adequately verified. In addition, a 5kW single-phase energy router experimental platform is built to verify the proposed PLL. The experimental results show that the PLL can well track the frequency change of the grid voltage and eliminate the DC offset, so as to achieve accurate phase tracking. [ABSTRACT FROM AUTHOR]

This paper presents a procedure to estimate the impacts on voltage harmonic distortion at a point of interest due to multiple nonlinear loads in the electrical network. Despite artificial neural networks (ANN) being a widely used technique for the solution of a large amount and variety of issues in electric power systems, including harmonics modeling, its utilization to establish relationships among the harmonic voltage at a point of interest in the electric grid and the corresponding harmonic currents generated by nonlinear loads was not found in the literature, thus this innovative procedure is considered in this article. A simultaneous measurement campaign must be carried out in all nonlinear loads and at the point of interest for data acquisition to train and test the ANN model. A sensitivity analysis is proposed to establish the percent contribution of load currents on the observed voltage distortion, which constitutes an original definition presented in this paper. Initially, alternative transient program (ATP) simulations are used to calculate harmonic voltages at points of interest in an industrial test system due to nonlinear loads whose harmonic currents are known. The resulting impacts on voltage harmonic distortions obtained by the ATP simulations are taken as reference values to compare with those obtained by using the proposed procedure based on ANN. By comparing ATP results with those obtained by the ANN model, it is observed that the proposed methodology is able to classify correctly the impact degree of nonlinear load currents on voltage harmonic distortions at points of interest, as proposed in this paper. [ABSTRACT FROM AUTHOR]

In order to solve the neutral-point voltage unbalancing problem for neutral-point clamped (NPC) three-level inverters under non-ideal grid conditions, a novel zero-sequence component injection method for modulation signals was proposed in this paper. Based on the mathematical expressions of neutral-point voltage unbalancing, the fluctuation of the neutral-point voltage under different grid conditions was studied. The proposed method dynamically calculates the zero-sequence component for modulation signals by region selections based on sinusoidal pulse width modulation (DCR-SPWM). Under non-ideal grid conditions, this DCR-SPWM method can control the neutral-point voltage balance for NPC three-level inverters with different grid and output power conditions, and improves the grid current quality. The simulation results verified the correctness of the theoretical analysis in this paper, and the feasibility and effectiveness of this method were verified by the experiments in the experimental platform of the NPC three-level grid-connected inverter based on DSP-CPLD controllers. [ABSTRACT FROM AUTHOR]

As is well known, the high installation and operational costs of the unified power flow controller (UPFC) restrict its widespread applications in power grid. An economical “Sen” transformer (ST) that is composed of a three-phase four-winding transformer and three sets of tap changers provides similar power flow control functionality as the UPFC. Unfortunately, the ST is not suitable for high-voltage systems because of its high-voltage insulation requirement on all secondary windings and tap changers. To overcome this problem, a high-voltage “Sen” transformer that consists of a three-phase two-winding transformer and two sets of tap changers is proposed in this paper. In high-voltage power system applications, the high-voltage “Sen” transformer (HVST) offers several advantages over the ST, such as fewer windings, fewer tap changers, bigger phase-shift range, and larger power flow control region. This paper focuses on the configuration, principles, and applications of the HVST. The contrastive simulation results in a two-area test system and an IEEE 39 bus system prove the validity and advantages of the proposed HVST. Moreover, a 10 kV HVST device and a corresponding experiment platform are developed to verify the feasibility and effectiveness of the HVST. The HVST can provide a flexible voltage and power flow control. [ABSTRACT FROM AUTHOR]

Voltage profile is one of many aspects that affect power quality in low-voltage distribution feeders. Weak grids have a typically high line impedance which results in remarkable voltage drops. Distribution grids generally have a high R/X ratio, which makes voltage regulation with reactive power compensation less effective than in high-voltage grids. Moreover, these networks are more susceptible to unbalance and harmonic voltage disturbances. This paper proposes an enhanced coordinated control of active and reactive power injected in a distribution grid for voltage regulation. Voltage drop mitigation was evaluated with power injection based on local features, such loads and disturbances of each connection. In order to ensure disturbances rejection like harmonic components in the grid voltages, a repetitive vector-control scheme was used. The injection of coordinated active and reactive power with the proposed control algorithm was verified through simulations and experiments, demonstrating that it is a promising alternative for voltage regulation in weak and low-voltage networks subject to inherent harmonic distortion. [ABSTRACT FROM AUTHOR]

With the large-scale centralized PV clusters connected to grid, the grid power flow has certain randomness. Considering the fluctuation of PV output, an improved Krawczyk-Moore algorithm in a mixed coordinate system is proposed to solve the uncertain power flow problem. Firstly, aiming at the special structure of a centralized PV cluster with only load node and no generator node, this paper proposes a power flow calculation in the mixed power flow coordinate, and then the Krawczyk-Moore operator is used to combine interval and affine arithmetic to overcome the shortcoming of over-conservative interval algorithm. Finally, the voltage operating condition under different volatility and different partial shading conditions is studied through the simulation of a practical example, and the out-of-limit voltage problem inside the centralized PV cluster is analyzed. Meanwhile, the effectiveness of the proposed algorithm is verified. [ABSTRACT FROM AUTHOR]

The increased penetration of renewable energy sources in the electrical grid, due to the rapid increase of power demand and the need of diverse energy sources, has made distributed generation (DG) units an essential part of the modern electrical grid. The integration of many DG units in smart grids requires control and coordination between them, and the grid to maximize the benefits of the DG units. Smart grids and modern electronic devices require high standards of power quality, especially voltage quality. In this paper, a new methodology is presented to improve the voltage quality and power factor in smart grids. This method depends on using voltage variation and admittance values as inputs of a controller that controls the reactive power generation in all DG units. The results show that the controller is efficient in improving the voltage quality and power factor. Real data from an electrical network have been used in the simulation model in MATLAB Simulink to test the new approach. [ABSTRACT FROM AUTHOR]

The cascaded H-bridge static var generator (SVG) has been employed to provide reactive power and regulate grid voltages for many years because of its good modularity, easy scalability, and improved harmonic performance. A novel cluster-balancing power control method combining negative-sequence currents and zero-sequence voltage is proposed to redistribute the unbalanced active powers and eliminate the power oscillation under asymmetrical conditions. Simultaneously, the dynamic performance of the SVG power balance control can be improved under asymmetrical conditions with the zero-sequence voltage expression derived in this paper. On the basis of the proposed method, the fault ride through capability of star-connected SVG under asymmetrical conditions is compared among active power oscillation elimination (APOE), reactive power oscillation elimination (RPOE), and balanced positive sequence current (BPSC) injection references calculation strategies from the perspective of the zero-sequence voltage, maximum phase voltage, and maximum phase current. The method provides the theoretical reference for power control under asymmetric conditions and the analysis results show that under asymmetrical conditions, the current of BPSC is minimal and symmetrical, while the RPOE has the least voltage and no zero- sequence voltage needs to be injected. Finally, the results of simulation and experiment have been given to verify the theoretical studies. [ABSTRACT FROM AUTHOR]

This paper proposes an alterable structure power router (PR) topology which shares the ability of general Alternating Current (AC) and Direct Current (DC) port. This alterable structure PR aims to better implement the interconnections among micro-grids, renewable energy, and traditional grids. The PR's converter structure is alterable according to the system power. Based on the operation analysis of the PR, the mathematic model of the power router is established and the relationship between switch states and port voltage is analyzed. According to the PR mathematical model, the control method is designed to realize current control in both AC and DC mode. With the proposed power router topology, the PR's port can be used in both AC and DC situations, which saves the cost of traditional PRs and increases economic efficiency. The simulation and experimental results prove the good steady-state and dynamic performance of the proposed power router topology. [ABSTRACT FROM AUTHOR]

The ground current of a high-voltage direct current (HVDC) transmission system can cause DC bias in transformers near the grounding electrode during monopole operations, which affects the alternating current (AC) power system operation. Owing to multiple bias current flow paths, a capacitance blocking device installed at the neutral point of a transformer may increase the DC bias in adjacent transformers, while suppressing the DC current in that transformer. This paper introduces the use of an effective bias current indicator to describe the effect of the grounding current on transformers in a network, considering the wiring characteristics of the autotransformers and the power system topology. Additionally, a combination optimization method for the capacitance and resistance is applied in order to determine the minimum number of installed devices that restrict the maximum effective bias current throughout the network to a permissible range. A genetic algorithm based on an improved roulette selection method is adopted to solve the optimal configuration problem. The method is validated by using a test case based on the Xizhe HVDC transmission receiving-end grid near the Jinsi grounding electrode. The configuration of the capacitance and resistance was optimized by the improved genetic algorithm. This method can achieve the desired level of DC bias management with fewer devices than the conventional method, which verifies the feasibility and superiority of the proposed optimization method. [ABSTRACT FROM AUTHOR]

In most smart grids, load management techniques are required to handle multiple loads of several types. This paper studies decentralized demand-side management (DSM) in a grid with different types of appliances in two service areas: one with many residential households, and one bus with commercial customers. Each building runs an individual optimal DSM to reschedule the usage time of its flexible appliances to reduce its electric energy cost at a manageable sacrifice of inconvenience according to the forecasted time-varying electricity price. Using the developed model, we examined the effectiveness of decentralized DSM by comparing its performance on the operation status of the grid in terms of electricity cost saving, rooftop photovoltaic (PV) utilization efficiency, voltage fluctuation, power loss, voltage rises, and reverse power flows, which can easily be seen at the commercial load bus. [ABSTRACT FROM AUTHOR]

This paper proposes an optimal operational scheduling of a reconfigurable multi-microgrid (MG) distribution system complemented by demand response programs and Energy Storage Systems (ESSs) in an uncertain environment. Since there is a set of competing players with inherently conflicting objectives in the system under study such as the Distribution System Operator (DSO) and MG owners, a one-leader multi-follower-type bi-level optimization model is proposed. In this framework, the upper-level player as a leader minimizes the total cost from DSO's point of view, while the lower-level players as multi-followers maximize the profit of MG owners. Since the resulting model is a non-linear bi-level optimization problem, it is transformed into a single-level mixed-integer second-order cone programming problem through Karush–Kuhn–Tucker conditions. The satisfactory performance of the proposed model is investigated on a real-test system under different scenarios and working conditions. [ABSTRACT FROM AUTHOR]

With the increasing integration of huge-size generators and tighter interconnection among power grids, in internal fault, the fault current of Series Compensated (SC) lines may be reversed. At this time, the differential protection of SC lines may fail to operate due to insufficient sensitivity. In this research analyzes the reasons for the current reverse in SC lines and its effect on differential protection. The operation characteristics of differential protection are studied by focusing on multiple factors, including different series compensation degree, fault types, system operation mode, power angle difference, fault points, and transition resistance. An improved criterion based on the current amplitudes and phases on both sides of the series compensated lines is proposed. Further, to strengthen the operation characteristics of differential protection, the triple-fold line is chosen to improve the sensitivity for internal faults in this paper. The criterion is effective to improve the protection sensitivity of SC lines during internal fault. The PSCAD/EMTDC simulation and the power system dynamic physics simulation demonstrates that protection sensitivity is obviously increased in the improved scheme. [ABSTRACT FROM AUTHOR]

The energy internet (EI) is a wide area power network that efficiently combines new energy technology and information technology, resulting in bidirectional on-demand power transmission and rational utilization of distributed energy resources (DERs). Since the stability of local network is a prerequisite for the normal operation of the entire EI, the direct current (DC) bus voltage stabilization for each individual DC microgrid (MG) is a core issue. In this paper, the dynamics of the EI system is modeled with a continuous stochastic system, which simultaneously considers related time-varying delays and norm-bounded modeling uncertainty. Meanwhile, the voltage stabilization issue is converted into a robust H ∞ control problem solved via a linear matrix inequality approach. To avoid the situation of over-control, constraints are set in controllers. The problem of finding a balance between voltage regulation performance and constraints for the controllers was also extensively investigated. Finally, the efficacy of the proposed methods is evaluated with numerical simulations. [ABSTRACT FROM AUTHOR]

In order to systematically shift existing control and management paradigms in distribution systems to new interoperable communication supported schemes in smart grids, we need to map newly developed use cases to standard reference models like Smart Grid Architecture Model (SGAM). From the other side, any new use cases should be tested and validated ex-ante before being deployed in the real-world system. Considering various types of actors in smart grids, use cases are usually tested using co-simulation platforms. Currently, there is no efficient co-simulation platform which supports interoperability analysis based on SGAM. In this paper, we present our developed test platform which offers a support to design new use cases based on SGAM. We used this platform to develop a new scheme for wide area monitoring of existing distribution systems under growing penetration of Photovoltaic production. Off-the-shelf solutions of state estimation for wide area monitoring are either used for passive distribution grids or applied to the active networks with wide measurement of distributed generators. Our proposed distribution state estimation algorithm does not require wide area measurements and relies on the data provided by a PV simulator we developed. This practical scheme is tested experimentally on a realistic urban distribution grid. The monitoring results shows a very low error rate of about 1 % by using our PV simulator under high penetration of PV with about 30 % error of load forecast. Using our SGAM-based platform, we could propose and examine an Internet-of-Things-based infrastructure to deploy the use case. [ABSTRACT FROM AUTHOR]

A virtual synchronous machine (VSM) is a converter which, compared to other types of converters, has more friendly interactions with the power grid because it is able to simulate the external characteristics of a synchronous machine, which can provide virtual inertia and damping. When the grid voltage is unbalanced, there will be negative sequence current and power oscillations. There will also be double-frequency ripples on the DC bus, which affect the normal operation of the DC power source or load. In order to solve these problems, a comprehensive control strategy is proposed in this paper. The principle of a VSM operated as a current source converter, also called VISMA, is used in the design. A complex coefficient filter is applied to separate the positive and negative sequence components of the grid voltage. By analyzing the reasons of power oscillations under unbalanced voltage, the electrical simulation part of the VSM is improved to achieve several objectives: to suppress negative sequence current and DC voltage ripples. Additionally, the rated voltage in the reactive control part is adaptively adjusted to stabilize the system. The validity of the proposed control strategy is verified by simulation and experiment. [ABSTRACT FROM AUTHOR]

Voltage sag is one of the most serious problems in power quality. The occurrence of voltage sag will lead to a huge loss in the social economy and have a serious effect on people's daily life. The identification of sag types is the basis for solving the problem and ensuring the safe grid operation. Therefore, with the measured data uploaded by the sag monitoring system, this paper proposes a sag type identification algorithm based on K-means-Singular Value Decomposition (K-SVD) and Least Squares Support Vector Machine (LS-SVM). Firstly; each phase of the sag sample RMS data is sparsely coded by the K-SVD algorithm and the sparse coding information of each phase data is used as the feature matrix of the sag sample. Then the LS-SVM classifier is used to identify the sag type. This method not only works without any dependence on the sag data feature extraction by artificial ways, but can also judge the short-circuit fault phase, providing more effective information for the repair of grid faults. Finally, based on a comparison with existing methods, the accuracy advantages of the proposed algorithm with be presented. [ABSTRACT FROM AUTHOR]

A waveform contains recognizable feature patterns. To extract the features of various equipment disturbance conditions from a waveform, this paper presents a practical model to estimate distribution line (DL) conditions by means of a multi-label extreme learning machine. The motivation for the waveform learning is to develop device-embedded models which are capable of detecting and classifying abnormal operations on the DLs. In waveform analysis, power quality waveform modeling criteria are adopted for pattern classification. Typical disturbance waveforms are applied as class models, and the formula-generated waveform features are compared with field-acquired waveforms for disturbance classification. In particular, filtered symmetrical components of the modified varying window scale are applied for feature extraction. The proposed model interacts suitably with the parameter update method in classifying the waveforms in real network situations. The classification result showed disturbance features on model with the real DL waveform data holds a potential for determining additional DL conditions and improving its classification performance through the update mechanism of the learning machine. [ABSTRACT FROM AUTHOR]

This paper proposes a two-dimensional finite element mathematical model taking the mf-15 simulation generator in the dynamic model laboratory as the research object. The electromagnetic performance of the motor under no-load and rated load was analyzed to verify the correctness of the model. By using the method of field-circuit coupling combined with simulation analysis, the limit value of the generator's phase-in depth is determined, and the variation law of the electric and magnetic field distribution with the depth of the generator's established Leading phase operation is analyzed, as well as the influence of the phase-in operation on the system's operation stability. The results obtained provide a reference for the phase-in operation analysis of large generators. [ABSTRACT FROM AUTHOR]

A novel nonlinear current-limiting controller for three-phase grid-tied droop-controlled inverters that is capable of offering voltage support during balanced and unbalanced grid voltage drops is proposed in this paper. The proposed controller introduces a unified structure under both normal and abnormal grid conditions operating as a droop controller or following the recent fault-ride-through requirement to provide voltage support. In the case of unbalanced faults, the inverter can further inject or absorb the required negative sequence real and reactive power to eliminate the negative sequence voltage at the point of common coupling (PCC) whilst ensuring at all times boundedness for the grid current. To accomplish this task, a novel and easily implementable method for dividing the available current into the two sequences (positive and negative) is proposed, suitably adapting the proposed controller parameters. Furthermore, nonlinear input-to-state stability theory is used to guarantee that the total grid current remains limited below its given maximum value under both normal and abnormal grid conditions. Asymptotic stability for any equilibrium point of the closed-loop system in the bounded operating range is also analytically proven for first time using interconnected-systems stability analysis irrespective of the system parameters. The proposed control concept is verified using an OPAL-RT real-time digital simulation system for a three-phase inverter connected to the grid. [ABSTRACT FROM AUTHOR]

To eliminate current harmonics and to obtain enhanced dynamic behaviors, the voltage feedforward scheme is normally adopted in the control process of transformerless grid-tied converters. However, voltage measurement errors caused by parameter variation of sampling resistance and zero-voltage drift in analog devices will result in DC current injection and distorted grid currents in grid-tied systems. Conventional compensation strategies based on plug-in repetitive controllers and multiple resonant controllers are considered to be effective solutions to solve this problem. Even though undesired components in grid currents can be partially mitigated with these conventional compensation schemes adopted, degraded reference-tracking process with oscillations and slow dynamics are inevitably caused by the controller coupling. To obtain decoupling between reference tracking and disturbance rejection, this paper proposes a dual-loop controller to achieve current regulation and to suppress the disturbances caused by voltage measurement errors. With the proposed dual-loop controller adopted, DC current injection and distortion of grid currents can be effectively attenuated while excellent transient performance with negligible overshoot and fast step response can be simultaneously guaranteed. Frequency-domain analysis and experimental validations are both conducted to verify the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Effective state estimation is critical to the security operation of power systems. With the rapid expansion of interconnected power grids, there are limitations of conventional centralized state estimation methods in terms of heavy and unbalanced communication and computation burdens for the control center. To address these limitations, this paper presents a multi-area state estimation model and afterwards proposes a consensus theory based distributed state estimation solution method. Firstly, considering the nonlinearity of state estimation, the original power system is divided into several non-overlapped subsystems. Correspondingly, the Lagrange multiplier method is adopted to decouple the state estimation equations into a multi-area state estimation model. Secondly, a fully distributed state estimation method based on the consensus algorithm is designed to solve the proposed model. The solution method does not need a centralized coordination system operator, but only requires a simple communication network for exchanging the limited data of boundary state variables and consensus variables among adjacent regions, thus it is quite flexible in terms of communication and computation for state estimation. In the end, the proposed method is tested by the IEEE 14-bus system and the IEEE 118-bus system, and the simulation results verify that the proposed multi-area state estimation model and the distributed solution method are effective for the state estimation of multi-area interconnected power systems. [ABSTRACT FROM AUTHOR]

For fast and accurate faulty line selection (FLS) in single phase-to-ground (SPG) faults for resonant grounded distribution networks (RGDNs), this paper proposes a FLS method based on zero-sequence power characteristics. Through analysis of the zero-sequence current of SPG faults, it is found that the transient zero-sequence current in the faulty line contains abundant attenuated DC components, while that in the sound line is weak, which could be utilized as a technically practical way of judging faulty lines. Thus a FLS scheme using a transient zero-sequence DC component energy integral is put forward, which highlights the difference of the DC components in the faulty line and the sound line. A series of simulation test results indicate that the proposed scheme shows favorable performance under various SPG fault conditions, and should be helpful to improve the success rate of FLS significantly for RGDN in the future. [ABSTRACT FROM AUTHOR]

Accurate identification of critical nodes and regions in a power grid is a precondition and guarantee for safety assessment and situational awareness. Existing methods have achieved effective static identification based on the inherent topological and electrical characteristics of the grid. However, they ignore the variations of these critical nodes and regions over time and are not appropriate for online monitoring. To solve this problem, a novel data-driven dynamic identification scheme is proposed in this paper. Three temporal and three spatial attributes are extracted from their corresponding voltage phasor sequences and integrated via Gini-coefficient and Spearman correlation coefficient to form node importance and relevance assessment indices. Critical nodes and regions can be identified dynamically through importance ranking and clustering on the basis of these two indices. The validity and applicability of the proposed method pass the test on various situations of the IEEE-39 benchmark system, showing that this method can identify the critical nodes and regions, locate the potential disturbance source accurately, and depict the variation of node/region criticality dynamically. [ABSTRACT FROM AUTHOR]

With the increasing demand of transmitting bulk-power over long-distance, the ultra high-voltage direct-current (UHVDC) transmission systems become an attractive option. Nowadays, not only the line commutated converter (LCC) based systems, but also the modular multilevel converter (MMC) based systems have reached UHVDC levels. The converter stations of UHVDC systems normally utilize two series-connected valve-groups to reduce the difficulties of device manufacturing and transportation. This high-voltage and low-voltage valve-group configuration allows the UHVDC systems to achieve a full-voltage to half-voltage operation which increases the flexibility of the systems. However, the existing research only focuses on the full-voltage to half-voltage control of LCC-UHVDC systems. The control strategies for hybrid LCC/MMC UHVDC systems are underresearched. Moreover, the approaches to reduce the load-shedding caused by the full-voltage to half-voltage control for both LCC and hybrid LCC/MMC based UHVDC systems have not been investigated. In this paper, full-voltage to half-voltage control strategies for both LCC and hybrid LCC/MMC based UHVDC systems have been proposed. Moreover, to avoid load-shedding caused by the half-voltage operation, a power rescheduling method that re-sets the power references of the half-voltage operating and full-voltage operating poles has been proposed. The proposed full-voltage to half-voltage control strategies and power rescheduling method can achieve a stable and fast control process with a minimum power loss. The proposed methods have been verified through the time-domain simulations conducted in PSCAD/EMTDC. [ABSTRACT FROM AUTHOR]

In this paper, we propose a voltage control technique for the seamless transfer algorithm of a microgrid inverter. When the microgrid inverter is switched from grid-connected mode to standalone operation mode, an output voltage transient may occur, and adversely affect the load. To solve this, seamless transfer algorithms that complete mode switching within 4 ms have been studied. However, previous control techniques have very complex structures. In this study, we find the cause of the voltage transient occurring in the mode conversion, through state average modeling of the inverter. Subsequently, we propose a feed-forward compensator for seamless transfer. The cause of the voltage transient is removed by the compensation, and smooth mode switching is achieved. The proposed controller is verified by Powersim (PSIM) simulations. [ABSTRACT FROM AUTHOR]