Abstract: We propose a consensus-based distributed voltage control (DVC) that solves the problem of reactive power sharing in autonomous inverter-based microgrids with dominantly inductive power lines and arbitrary electrical topology. Opposed to other control strategies available thus far, the control presented here does guarantee a desired reactive power distribution in steady state while only requiring distributed communication among inverters, i.e., no central computing nor communication unit is needed. For inductive impedance loads and under the assumption of small phase angle differences between the output voltages of the inverters, we prove that the choice of the control parameters uniquely determines the corresponding equilibrium point of the closed-loop voltage and reactive power dynamics. In addition, for the case of uniform time constants of the power measurement filters, a necessary and sufficient condition for local exponential stability of that equilibrium point is given. The compatibility of the DVC with the usual frequency droop control for inverters is shown and the performance of the proposed DVC is compared with the usual voltage droop control via simulation of a microgrid based on the Conseil International des Grands Reseaux Electriques (CIGRE) benchmark medium voltage distribution network.

Abstract: Traditional reactive power optimization aims to minimize the total transmission losses by control reactive power compensators and transformer tap ratios, while guaranteeing the physical and operating constraints, such as voltage magnitudes and branch currents to be within their reasonable range. However, large amounts of renewable resources coming into power systems bring about great challenges to traditional planning and operation due to the stochastic nature. In most of the practical cases from China, the wind farms are centrally integrated into active distribution networks. By the use of conic relaxation based branch flow formulation, the reactive optimization problem in active distribution networks can be formulated as a mixed integer convex programming model that can be tractably dealt with. Furthermore, to address the uncertainties of wind power output, a two-stage robust optimization model is proposed to coordinate the discrete and continuous reactive power compensators and find a robust optimal solution that can hedge against any possible realization within the uncertain wind power output. Moreover, the second order cone programming-based column-and-constraint generation algorithm is employed to solve the proposed two-stage robust reactive power optimization model. Numerical results on 33-, 69- and 123-bus systems and comparison with the deterministic approach demonstrate the effectiveness of the proposed method.

Abstract: In this paper, the overvoltage problems that might arise from the integration of photovoltaic (PV) panels into low-voltage (LV) distribution networks is addressed. A distributed scheme is proposed that adjusts the reactive and active power output of inverters to prevent or alleviate such problems. The proposed scheme is model-free and makes use of limited communication between the controllers in the form of a distress signal only during emergency conditions. It prioritizes the use of reactive power, while active power curtailment is performed only as a last resort. The behavior of the scheme is studied using dynamic simulations on a single LV feeder and on a larger network composed of 14 LV feeders. Its performance is compared with a centralized scheme based on the solution of an optimal power flow (OPF) problem, whose objective function is to minimize the active power curtailment. The proposed scheme successfully mitigates overvoltage situations due to high PV penetration and performs almost as well as the OPF-based solution with significantly less information and communication requirements.

Abstract: Multilevel cascaded H-bridge converters are promising candidates for large-scale photovoltaic power plants. They allow direct connection to medium-voltage distribution networks without the presence of bulky line frequency power transformers. Owing to the stochastically variable nature of irradiance level, ambient temperature, and other factors, power levels in the three phases are expected to be unequal. The power imbalance condition creates unexpected problems with this topology, which was initially designed to operate under balanced power conditions. To deal with this issue, the paper proposes three novel zero-sequence injection methods as an expansion to the conventional zero-sequence injection method. Results obtained from simulations and a 430-V 8-kW three-phase seven-level cascaded H-bridge prototype are presented to verify the effectiveness and feasibility of the proposed methods.

Abstract: In this paper, a new mode of operation has been introduced for packed U-cell (PUC) inverter. A sensor-less voltage control based on redundant switching states is designed for the five-level packed U-cell (PUC5) inverter, which is integrated into switching process. The sensor-less voltage control is in charge of fixing the dc capacitor voltage at half of the dc source value results in generating symmetric five-level voltage waveform at the output with low harmonic distortion. The sensor-less voltage regulator reduces the complexity of the control system, which makes the proposed converter appealing for industrial applications. An external current controller has been applied for grid-connected application of the introduced sensor-less PUC5 to inject active and reactive power from inverter to the grid with arbitrary power factor, while the PUC auxiliary dc bus is regulated only by sensor-less controller combined with new switching pattern. Experimental results obtained in stand-alone and grid-connected operating modes of proposed PUC5 inverter prove the fast response and good dynamic performance of the designed sensor-less voltage control in balancing the dc capacitor voltage at desired level.

Abstract: This paper proposes the use of a least mean fourth (LMF)-based algorithm for single-stage three-phase grid-integrated solar photovoltaic (SPV) system. It consists of an SPV array, voltage source converter (VSC), three-phase grid, and linear/nonlinear loads. This system has an SPV array coupled with a VSC to provide three-phase active power and also acts as a static compensator for the reactive power compensation. It also conforms to an IEEE-519 standard on harmonics by improving the quality of power in the three-phase distribution network. Therefore, this system serves to provide harmonics alleviation, load balancing, power factor correction and regulating the terminal voltage at the point of common coupling. In order to increase the efficiency and maximum power to be extracted from the SPV array at varying environmental conditions, a single-stage system is used along with perturb and observe method of maximum power point tracking (MPPT) integrated with the LMF-based control technique. The proposed system is modeled and simulated using MATLAB/Simulink with available simpower system toolbox and the behaviour of the system under different loads and environmental conditions are verified experimentally on a developed system in the laboratory.

Abstract: A cooperative distributed secondary/primary control paradigm for AC microgrids is proposed. This solution replaces the centralized secondary control and the primary-level droop mechanism of each inverter with three separate regulators: voltage, reactive power, and active power regulators. A sparse communication network is spanned across the microgrid to facilitate limited data exchange among inverter controllers. Each controller processes its local and neighbors' information to update its voltage magnitude and frequency (or, equivalently, phase angle) set points. A voltage estimator finds the average voltage across the microgrid, which is then compared to the rated voltage to produce the first-voltage correction term. The reactive power regulator at each inverter compares its normalized reactive power with those of its neighbors, and the difference is fed to a subsequent PI controller that generates the second-voltage correction term. The controller adds the voltage correction terms to the microgrid rated voltage (provided by the tertiary control) to generate the local voltage magnitude set point. The voltage regulators collectively adjust the average voltage of the microgrid at the rated voltage. The voltage regulators allow different set points for different bus voltages and, thus, account for the line impedance effects. Moreover, the reactive power regulators adjust the voltage to achieve proportional reactive load sharing. The third module, the active power regulator, compares the local normalized active power of each inverter with its neighbors' and uses the difference to update the frequency and, accordingly, the phase angle of that inverter. The global dynamic model of the microgrid, including distribution grid, regulator modules, and the communication network, is derived, and controller design guidelines are provided. Steady-state performance analysis shows that the proposed controller can accurately handle the global voltage regulation and proportional load sharing. An AC microgrid prototype is set up, where the controller performance, plug-and-play capability, and resiliency to the failure in the communication links are successfully verified.

Abstract: Voltage problems are challenging in modern power systems with a high penetration of renewables integrated via power electronics. This paper extends the transient time-scale classification to identify voltage dynamics in modern power systems. Voltage dynamics in the current control time-scale is firstly proposed and then the mechanism of terminal voltage change is elaborated. After that the significant influences of the voltage source converter (VSC), current control (CC) loop and voltage feed forward (VFF) on the voltage dynamics in the current control time-scale are discussed. The VSC current control loop provides positive damping on the terminal voltage, while the VFF scheme results in an additional loop that deteriorates terminal voltage dynamic performance and stability. In addition, a sensitivity analysis was carried out to investigate the influence of critical parameters on stability. Finally, simulation results of a current-controlled VSC attached to different strength of AC grids (including a weak grid) are presented to validate the phenomenon and the influencing factors of voltage dynamics in the current control time-scale.

Abstract: A large-scale power grid's ability to transfer energy from producers to consumers is constrained by both the network structure and the nonlinear physics of power flow. Violations of these constraints have been observed to result in voltage collapse blackouts, where nodal voltages slowly decline before precipitously falling. However, methods to test for voltage collapse are dominantly simulation-based, offering little theoretical insight into how grid structure influences stability margins. For a simplified power flow model, here we derive a closed-form condition under which a power network is safe from voltage collapse. The condition combines the complex structure of the network with the reactive power demands of loads to produce a node-by-node measure of grid stress, a prediction of the largest nodal voltage deviation, and an estimate of the distance to collapse. We extensively test our predictions on large-scale systems, highlighting how our condition can be leveraged to increase grid stability margins.

Abstract: In this paper, we propose a frequency and voltage control strategy for a standalone microgrid with high penetration of intermittent renewable generation systems, which might cause large frequency and voltage deviation in the system due to unpredictable output power fluctuations. To this end, a battery energy storage system (BESS) is suggested for generating the nominal system frequency instead of a synchronous generator, from a frequency control perspective. This makes the system frequency independent of the mechanical inertia of the synchronous generator. However, a BESS has a capacity limitation; a synchronous generator is used to maintain the state of charge (SOC) of the BESS at a certain value. For voltage control, we proposed that a reactive power/active power (Q/P) droop control be added to the conventional reactive power controller. By adding a Q/P droop control, renewable generation acquires a voltage-damping effect, which dramatically alleviates the voltage fluctuation induced by its own output power fluctuation. Simulation results prove the effectiveness of the proposed control strategy from both frequency and voltage control perspectives.

Abstract: This paper proposes an improved voltage regulation method in a multisource-based dc electrical power system in the more electric aircraft. The proposed approach, which can be used in terrestrial dc microgrids as well, effectively improves the load sharing accuracy under high droop gain circumstance with consideration of cable impedance. Since no extra communication line and controllers are required, it is easily implemented and also increases the system modularity and reliability. By using the proposed approach, the dc transmission losses can be reduced and the system stability is not deteriorated for normal and fault scenarios. In this paper, optimal droop gain settings are investigated and the selection of individual droop gains as well as the proportional power sharing ratio has been described. Simulation and experimental results validate the effectiveness of the proposed method.

Abstract: With the wide application of voltage source converters (VSCs) in power system, dc-bus voltage control instabilities increasingly occurred in practical conditions, especially in weak ac grid, which poses challenges on stability and security of power converters applications. This paper aims to give physical insights into the stability of dc-bus voltage control affected by ac-bus voltage control in VSC connected to weak grid. The concepts of damping and restoring components are developed for dc-bus voltage to describe the stability of dc-bus voltage control. The impact of ac-bus voltage control on dc-bus voltage control stability can be revealed by investigating the impact of ac-bus voltage control on damping and restoring components essentially. Furthermore, the detailed analysis for the impact of ac-bus voltage control on damping and restoring components is presented considering varied ac system strengths, operating points, and ac-bus voltage control parameters. The simulation results from 1.5-MW full-capacity wind power generation system are demonstrated which conform well to the analysis. Finally, the experimental results validate the analysis.

Abstract: Energy storage systems (ESSs) are essential in future power systems because they can improve power usage efficiency. In this paper, a novel coordinated control algorithm is proposed for distributed battery ESSs (BESSs). The neighboring BESSs of a simulation system are grouped and controlled by a main control center. The main control center sends charging or discharging operation signals to each BESS. The primary objective of the proposed coordinated control scheme is to mitigate voltage and frequency deviations. In order to verify the proposed algorithm, the BESSs are connected to a distribution system of the Korea Electric Power Corporation. The results are compared with those obtained using uncoordinated control scheme with on-load tap changer considering aspects of power quality (voltage and frequency variation). The simulation results show that the voltage and frequency deviations are reduced with the proposed coordinated control algorithm.

Abstract: This paper proposes an adaptive dc-link voltage control method for the two-stage photovoltaic inverter during the low voltage ride-through (LVRT) operation period. The dc-link voltage will be controlled to follow the change of grid voltage during the LVRT operation to maintain the high modulation ratio so that the high frequency harmonics injected into the grid can be attenuated significantly. Besides, when suffering the asymmetrical grid faults, the proposed control method could to some extent attenuate the double-line-frequency dc-link voltage ripple to keep the dc-link voltage in the safe operational range by shifting the double-line-frequency power ripple to the front-end dc input source, which can be achieved by intentionally fluctuating the dc input power or employing a bidirectional dc–dc converter depending on the voltage drop ratio and the input power level. The theoretical findings were verified by MATLAB simulations and the constructed experimental prototype.

Abstract: The high penetration level of power electronics interfaced distributed generation (DG) systems creates great ancillary services potential through the DG interfacing converters, such as the grid unbalanced voltage compensation. However, the unbalanced voltage compensation may cause adverse effects on the DGs' operation, such as output active power oscillation and dc-link voltage variations. Moreover, since the compensation is realized through the available rating of DGs' interfacing converters, it is equally important to consider the effectiveness of control strategy for unbalanced voltage compensation. Considering these challenging issues, two grid unbalanced voltage compensation strategies for three-phase power electronics interfaced DG systems are proposed in this paper. Especially, the first control strategy aims at minimizing the DG's active power oscillation and reducing the adverse effects of unbalanced voltage compensation on DG's operation. The second control strategy focuses on the effectiveness of unbalanced voltage compensation by controlling DG's negative sequence current to be inphase with the grid negative sequence current. Performances of the two proposed control strategies under different grid conditions and DG operating conditions are studied, and recommendations for appropriate control strategy utilization under various conditions are provided. Finally, validity of the proposed strategies is verified by both simulations and experimental results.

Abstract: Managing power delivery from distributed generation systems is challenging when the grid voltages are unbalanced, since the negative sequence voltage causes power oscillations at twice the fundamental grid frequency. Current regulation using sequence components can be used to manage these real and reactive power oscillations, and thus help mitigate the unbalanced network voltages. This paper presents a consolidated control scheme using double sequence frame current regulators that can readily adjust between eliminating real or reactive power oscillations created by unbalanced grid voltages, or alternatively can simply balance the three-phase currents. The mitigation influence of these alternative control strategies on unbalanced grid voltages is then experimentally examined for a distribution feeder with a resistive and/or reactive series impedance.

Abstract: In this paper, we consider the impact of cyber attacks on voltage regulation in distribution systems when a number of photovoltaic (PV) systems are connected. We employ a centralized control scheme that utilizes voltage measurements from sectionizing switches equipped with sensors. It is demonstrated that if measurements are falsified by an attacker, voltage violation can occur in the system. However, by equipping the control with a detection algorithm, we verify that the damage can be limited especially when the number of attacked sensors is small through theoretical analysis and simulation case studies. In addition, studies are made on attacks which attempt to reduce the output power at PV systems equipped with overvoltage protection functions. Further discussion is provided on how to enhance the security level of the proposed algorithm.

Abstract: This paper proposes a coordinated direct power control (DPC) scheme for the rotor-side converter (RSC) and the grid-side converter (GSC) of the doubly fed induction generator (DFIG) under unbalanced grid voltage conditions. In order to eliminate the coupling interactions between the phase-locked loop (PLL) and the local unbalanced network, a virtual phase angle is used to replace the actual one acquired by the PLL. Thus, the PLL is removed out of RSC and GSC in the proposed DPC scheme. During network unbalance, the RSC is controlled to reduce torque ripples, while three selectable control targets are identified for the GSC, i.e., constant total active power, constant total reactive power, and balanced currents. A single-side resonant controller with the frequency discrimination between the positive- and negative-sequence signals of the same frequency is employed in the coordinated DPC scheme to avoid the complex calculations of the power compensating components. Meanwhile, the sequential separations of the voltages and currents are also eliminated. Then, the control performance, including the limits of the dc-link voltage, the dc-capacitor power oscillations, the impacts of the frequency deviations, and the grid synchronization of the proposed DPC strategy, is discussed. Finally, the experimental results of DFIG system verify the effectiveness of the proposed DPC strategy under unbalanced grid voltage conditions.

Abstract: This letter proposes an improved nearest-level modulation (NLM) method to enhance the quality of output voltage of a modular multilevel converter (MMC) with the low amount of submodules, as well as restrain the voltage fluctuation of the submodules. By adding a small offset, which is alternating at the double fundamental frequency to the reference signals, a small phase shift of the step-changing moment between upper and lower arms’ voltage emerges. As a result, an odd-level difference between the output voltage of lower arm and that of upper arm occurs, which can increase the level number of output voltage from $N + 1$ to $2N + 1$ , where N is the number of submodules per arm. With the proposed method, the total harmonic distortion (THD) of the output voltage is mitigated without increasing the switching frequency of IGBTs or changing the average voltage of submodules’ capacitor. In addition, within a special range of power factor angle, the circulating current can be reduced by choosing the proper phase between the fundamental and the double fundamental frequency. Simulation and experimental results verify the effectiveness and validity of the proposed NLM scheme.

Abstract: This paper provides a theoretical and experimental discussion about the performance, availability, and flexibility of a battery energy storage system (BESS) using a modular multilevel cascaded converter based on single-star bridge cells (MMCC-SSBC). The SSBC-based BESS produces three-phase multilevel voltage waveforms, and eliminates both harmonic filters and a complicated zig-zag transformer from the ac side. The circuit modularity of the SSBC enables the usage of multiple individual low-voltage battery modules. Along with control strategy based on zero-sequence-voltage injection, this modularity enhances the availability and flexibility of the BESS. A three-phase laboratory downscaled system rated at 140 V, 10 kW, and 21 kW·h is designed, constructed, and tested to verify the operating principles and performance. The tested BESS produces a current total harmonic distortion (THD) of 3.0% even when it operates with different active-power commands for individual bridge cells. In addition, zero-voltage ride-through capability is verified for the severest single-phase, two-phase, and three-phase voltage sags. Mathematical analysis and experimental verification validate the downscaled system and performance, making the BESS prospective.

Abstract: The high penetration of photovoltaic (PV) generators leads to a voltage rise in the distribution network. To comply with grid standards, distribution system operators need to limit this voltage rise. Reactive power control is one of the most proposed remedies. A popular form of reactive power control is an active power dependent characteristic to define the reactive power control of a PV generator. This standard Q ( P ) characteristic is a simple curve, which is not adapted to the specific situation in the grid. Therefore, this work proposes a method to define the optimal Q ( P ) curve. The optimal Q ( P ) curve is represented as a piecewise constant or a piecewise linear function. The parameters are optimized based on historical smart meter information, to obtain a Q ( P ) curve that keeps the voltage within limits throughout the whole year, with a minimal amount of reactive power. An easy to solve convex optimization problem defines the parameters. The method is applied to unbalanced three-phase four-wire systems. Several simulations with realistic data are performed on an existing distribution network to compare the optimal Q ( P ) curve with standard Q ( P ) and Q ( V ) curves.

Abstract: In this paper a brief review on different multilevel inverter topologies are discussed. Inverter is a power electronic device that converts DC power into AC power at desired output voltage and frequency. Multilevel Converters nowadays have become an interesting area in the field of industrial applications. Conventional power electronic converters are able to produce an output voltage that switches between two voltage levels only. Multilevel Inverter generates a desired output voltage from several DC voltage levels at its input. The input side voltage levels are usually obtained from renewable energy sources, capacitor voltage sources, fuel cells etc. The different multilevel inverter topologies are: Cascaded H-bridges converter, Diode clamped inverter, and Flying capacitor multilevel inverter. Multilevel inverters nowadays are used for medium voltage and high power applications. The different field of applications include its use as UPS, High voltage DC transmission, Variable Frequency Drives, in pumps, conveyors etc. The disadvantages of MLI are the need for isolated power supplies, design complexity and switching control circuits.

Abstract: This article proposes an improved pseudo-gradient search-particle swarm optimization (IPG-PSO) approach for solving the optimal reactive power dispatch (ORPD) problem. This ORPD problem is to determine optimal control variables, such as generator bus voltages, settings of shunt VAR compensators, and tap settings of on-load tap change (OLTC) transformers, for minimizing the real power loss, voltage deviation, and voltage stability index satisfying power balance equations and generator and network operating limit constraints. The proposed method is an improved PSO using a linearly decreasing chaotic inertia weight factor and guided by a pseudo-gradient search, which determines an appropriate direction of particles toward a global optimal solution. The proposed IPG-PSO method is used to minimize three different single-objective functions, including real power loss, voltage deviation, and voltage stability index. Test results on the IEEE 30-bus and 118-bus systems indicate that the proposed IPG-PSO me...

Abstract: The proper operation of single-phase and three-phase grid-connected power converters depends on the synchronization with utility networks. The major challenge of the synchronization is how to quickly and precisely extract the ac signal and fundamental positive sequence in single- and three-phase power systems, respectively. This paper proposes a new detection technique based on a modified Kalman filter and the generalized averaging method. The method has an open-loop structure, and uses the orthogonal signals which are obtained directly from the Kalman filter. The resulted detection system is very simple and robust even in the presence of power quality disturbances, such as voltage imbalance, harmonics, and voltage fluctuations. The proposed technique can detect the fundamental and harmonics frequencies within or less than half a cycle in all situations, such as small and considerable frequency variations. Meanwhile, the method guarantees the zero steady-state error in complicated harmonic scenarios, including all typical single-phase and three-phase harmonics. Various case studies are assessed and the performance of the proposed detection method is verified by experiments.

Abstract: The cascaded H-bridge (CHB) topology is ideal for implementing large-scale converters for photovoltaic (PV) applications. The improved quality of output voltage waveforms, high efficiency due to transformer-less connection, and ability to employ multiple instances of a maximum power point tracking (MPPT) algorithm are just some advantages. An important disadvantage is the required over-rating to ensure balanced three-phase currents at times of unequal PV generation. Unequal generation occurs due to shading, temperature inhomogeneity, faulty H-bridges, etc. Capacitor voltage balancing under such conditions requires zero-sequence voltage injection which increases the required number of series connected H-bridges. However, leakage current and safety requirements often dictate a need for isolation between PV arrays and the cascaded converter. Therefore, this paper proposes a converter topology that avoids the cost of extra series connected H-bridges by extending the function of dc–dc converters that provide isolation. Second harmonic power oscillations seen in typical cascaded topologies can also be eliminated or reduced through use of the proposed topology. Simulation and experimental results are presented that confirm correct operation of the proposed approach.

Abstract: Efforts towards energy-harvesting solutions are targeted for wireless sensor node applications and focus on performing maximum power extraction and storing power, yet efforts to deliver a regulated supply to voltage-sensitive blocks in power-limited applications has yet to be fully achieved. This paper presents a low-power, autonomous power management unit (PMU) able to perform maximum power point tracking for dc-type renewable sources. It includes a startup circuit fed directly from the renewable source. The PMU delivers a regulated output voltage through a digital LDO. The main step-up operation is performed through a dynamically controlled, power-aware, capacitive dc–dc converter that performs the required voltage gain procedure. Then, the digital LDO receives the EH source power density information from the dc–dc converter and provides regulation. Information about the source-power density availability is passed on to the digital LDO in order to select the best pass device size from a bank of three arrays. The PMU allows power consumption decrease by reducing the gate driving losses associated with large pass transistor devices, and it enhances efficiency. The system was fabricated in 180 nm CMOS process, and maximum end-to-end efficiency was measured at 57% with 1.75 mW of input power.

Abstract: Compared with conventional power transformer, the power electronic transformer (PET) or solid-state transformer has many attractive additional features. This paper focuses on an H-bridge-based three-phase three-stage modular PET, which consists of an input stage with series-connected H-bridge converters, an isolation stage with several independent dual-active-bridge converters, and an output stage with parallel-connected H-bridge converters. This PET suffers dc-link capacitor voltage unbalancing issue, and the parallel-connected module current unbalancing sharing issue. In this paper, a system control structure is proposed for the PET to deal with these issues. Different input-stage individual module dc-link voltage balancing control methods are analyzed and compared. It is found that the one implemented by directly trimming module output voltage amplitude is most suitable for PET. Moreover, a downscaled laboratory prototype is designed, built, and tested to verify the control strategy.

Abstract: In this paper, a transformerless hybrid series active filter using a sliding-mode control algorithm and a notch harmonic detection technique are implemented on a single-phase distribution feeder. This method provides compensation for source current harmonics coming from a voltage fed type of nonlinear load (VSC) and reactive power regulation of a residential consumer. The realized active power filter enhances the power quality while cleaning the point of common coupling (PCC) from possible voltage distortions, sags, and swells initiated through the grid. Furthermore, to overcome drawbacks of real-time control delay, a computational delay compensation method, which accurately generates reference voltages, is proposed. Based on an improved compensation strategy, while the grid current remains clean even with a small compensation gain, voltage disturbances initiated by the power system are obstructed by the compensator, and the PCC became free of voltage harmonics and protected from sag and swell. Simulation and experimental results carried on a 1.6-kVA prototype are presented and discussed.