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This paper proposes a coordinated optimal control strategy of PV generators and air-conditioning loads, in order to handle the possible voltage beyond limit issues resulting from high penetration of PV generators in the distribution networks. This strategy is achieved via coordinately considering the node voltage sensitivity and the adjustment-compensation bid model, to improve the economy of the whole system. As a result, the shortage of the PVs’ reactive power control capability is compensated by the adjustable air-conditioning loads, so that the waste of the photovoltaic power can be reduced or even avoided. The case study using an IEEE standard 33-node system, which is further updated with the installation of 5 PV generators and 5 air-conditioning loads, validates the correctness and effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Summary: The increasing penetration of renewable energy resources in power systems has improved microgrid's implementation. A microgrid is a localized grouping of electricity sources and loads that normally operates connected to and synchronous with the traditional centralized grid but can disconnect and function autonomously as physical and economic conditions dictate. Main factors to control in a microgrid are mainly frequency and voltage regulation, energy management, operation, and control scheme. This high penetration of renewable generation systems with their intermittent nature and unpredictable output power fluctuations might cause many control problems and large frequency/voltage deviation in microgrid stand‐alone operation. Thus, to maintain the microgrid stability, an efficient and highly reliable control scheme is required. In this paper, a newly proposed fuzzy logic‐based robust control mechanism is used to stabilize the frequency and direct current bus voltages in large fluctuations caused by sudden changes in power generation or load side. Also, supercapacitor and battery energy storage system are used to stabilize direct current bus voltages. This proposed method ensures the system efficiency and stability by reducing the system complexity and transient time, minimizing the frequency deviations, and preventing synchronous generator units from surpassing their power ratings in response to these disruptions. Simulation results also validate the effectiveness of the proposed controller in comparison with previous techniques. [ABSTRACT FROM AUTHOR]

This paper proposes an offset-free proportional-type output voltage-tracking algorithm embedding the disturbance observers (DOBs) for the N-phase interleaved DC/DC boost converter through a systematical multivariable approach. The contributions of this article fall into two parts. The first one is to design the first-order nonlinear DOBs for exponentially estimating the disturbances caused by the model-plant mismatches. The second one is to prove that the proposed proportional-type controller equipped with the DOBs guarantees the performance recovery property as well as the offset-free property. The performance of the proposed method is evaluated through simulations and experiments using a 3-kW four-phase interleaved DC/DC boost converter, comparing the proposed and feedback linearizing (FL) methods. [ABSTRACT FROM AUTHOR]

The control of bus voltage is a crucial task for the stable operation of islanded DC microgrids. To improve the DC bus voltage control dynamics and stability, this paper proposes an adaptive sliding-mode control method based on large-signal model. The sliding-mode control, adaptive observation, and fix-frequency pulse width modulation technology are adopted and combined efficiently, which guarantee stable bus voltage and the constant switching frequency of closed-loop system, regardless of how the parameters vary with the variable constant-power loads and uncertainties. In addition, the reference values can be quickly tracked by the state variables using the proposed method without any additional sensors/hardware circuits. Therefore, this method is beneficial for the scalability and plug-play of the distributed generators and loads within the DC microgrids. The performance of the proposed control method has been successfully verified in simulation. [ABSTRACT FROM AUTHOR]

In this paper, a developed voltage control method for distributed single-phase photovoltaic (PV) generators in unbalanced distribution networks is proposed. The reactive power capability of the PV inverters has been used to regulate bus voltages during irradiance or load variations. The amount of the reactive power required to optimize phase voltages is calculated using fuzzy logic control function, and hence, the power factor is precisely determined. The proposed control for the PV inverters is based on the dq-current control to regulate the DC bus voltage and the amount of the reactive power. A hysteresis current control is used as a pulse width modulation gate generator. The proposed control precisely determines the amount of the required reactive power to regulate the system bus voltages to lie within standard limits. The efficiency of the proposed control is evaluated with a 10-node, 5 kV/phase unbalanced three phase distribution system. The analysis includes different test cases under transient and steady state operations to ensure accurate representation of the actual system. The obtained results assess the quality of the network voltages of the studied system regarding voltage levels and the degree of the unbalance. Moreover, the proposed control is capable of operation under different power factors of the PV inverter. [ABSTRACT FROM AUTHOR]

Due to the global concerns regarding the climate change, integration of renewable energy sources is considered as a mitigation approach in electric power generation. This requires advanced frequency and voltage control methodologies to overcome the challenges especially in microgrids. This paper presents a 2-step frequency and voltage control methodology for microgrids with high penetration of variable renewable energy sources. An optimized Proportional-Integral controller is designed for a Superconductor Magnetic Energy Storage System to minimize the transient frequency deviations. In cases that the Superconductor Magnetic Energy Storage System cannot stabilize the microgrid frequency in the isolated mode, the microgrid controller activates the next level of the frequency control. In the second level, an intelligent fuzzy-logic frequency controller is designed to adjust controllable loads, controllable generation units as well as perform load shedding. In the interconnected mode, the microgrid controller is able to activate the second level to contribute to the system frequency control. Finally, an intelligent fuzzy-logic voltage controller, realized through distribution static synchronous compensator, is devised to control the voltage magnitude of the main feeders of the microgrid. In this work, a real-time operation algorithm for frequency as well as voltage control is proposed and has been tested by set of simulations on a low voltage benchmark network. [ABSTRACT FROM AUTHOR]

In this paper, a synchronous generator excitation controller based on a novel feedforward control action is proposed. The main purpose of the new feedforward action is to enable a more robust excitation of the synchronous generator operating in varying operating conditions when connected to a power grid, for any superimposed stator voltage controller. Namely, the dynamic performance of the proposed controller is compared with a conventional proportional-integral (PI) controller, and with the PI controller combined with a feedforward action based on the direct component of the stator current. It is analytically shown that the novel controller improves the robustness when compared to 2 aforementioned solutions. Also, by simulation and experimental tests, it is shown that for the loaded synchronous generator, the novel controller, which is designed to have a similar response speed in open circuit as conventional controllers, exhibits up to 2 times faster dynamic performance. [ABSTRACT FROM AUTHOR]

This paper identifies a new mode called 'critical voltage mode" in emerging distribution systems. The small-signal stability analysis indicates that load voltage dynamics significantly influence the damping of the newly identified mode. This mode has frequency of oscillation between the electromechanical and subsynchronous oscillation of power systems. A novel voltage controller is designed to damp this voltage mode of the system. The controller is designed using the linear quadratic Gaussian (LQG) method with norm-bounded uncertainty. The approach considered in this paper is to find the smallest upper bound on the H ∞ norm of the uncertain system and to design an optimal controller based on this bound. The design method requires the solution of a linear matrix inequality. The performance of the designed controller is demonstrated on a distribution test system for different types of induction motors. Simulation results show that the proposed controller with a careful uncertainty modeling has significant performance to improve the voltage profile of the distributed generation system compared to the conventional excitation controller and standard LQG controller. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY This paper is concerned with determining the compensation capability of an interline dynamic voltage restorer (IDVR) under various balanced and unbalanced voltage sag scenarios. An IDVR consists of two or more DVRs with a common dc link which enables active power exchange among DVRs. This feature increases the compensation range of an IDVR as compared with separate but otherwise similar DVRs with no energy storage. The steady-state modeling and analysis of an IDVR is briefly explained in this paper. The expressions governing power requirement for compensating various voltage sags are derived. Consequently, the constraints associated with an IDVR operation are obtained and an algorithm is developed to calculate the compensation range of an IDVR and two similar storage-less DVRs. Illustrative graphs demonstrate the operating characteristics of an IDVR and compare these characteristics with those of two similar storage-less DVRs. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The five-phase fault-tolerant flux-switching permanent magnet (FT-FSPM) motor can offer high efficiency and high fault-tolerant capability. In this paper, its operation principle is presented briefly and its mathematical model is derived. Further, a new adaptive control for an FT-FSPM motor, based on the backstepping method and the sliding mode control strategy, is proposed. According to the backstepping method, the current controllers and voltage control laws are designed to track the speed and minimize the current static error, which enhance the dynamic response and the ability to suppress external disturbances. In order to overcome the influence of parameter variations, according to sliding mode control theory, the virtual control variables and the adaptive algorithm are utilized to approach uncertainty terms. Three Lyapunov functions are designed, and the stability of the closed-loop system is analyzed in detail. Finally, both simulation and experimental results are presented to verify the proposed control method. [ABSTRACT FROM AUTHOR]

Integrated volt/Var control (IVVC) is an important tool to optimize voltage and VAR resources and improve energy efficiency and, therefore, plays an important role to support the smart grid vision. This paper investigates the application of support vector machines (SVMs) to control switched capacitor banks and voltage regulators of distribution systems in order to minimize system losses and maintain acceptable voltage profile. The objective is to develop an approach that can use voltage/current measurements provided by active and reactive monitors to IVVC control without the need to perform retraining or repeat optimization simulations. The performance of the SVM scheme for different input selection (i.e., active and reactive power and node voltage) is discussed. Moreover, the generality of the proposed SVM scheme is investigated under noisy environment as well as under network reconfiguration. Studies on a typical distribution system show that the proposed approach has been able to achieve satisfactory results. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

RF MEMS devices are known to be superior to their solid state counterparts in terms of power consumption and electromagnetic response. Major limitations of MEMS devices are their low switching speed, high actuation voltage, larger size, and reliability. In the present paper, a see-saw single pole double throw (SPDT) RF MEMS switch based on anchor-free mechanism is proposed which eliminates the above-mentioned disadvantages. The proposed switch has a switching time of 394 nsec with actuation voltage of 5 V. Size of the SPDT switch is reduced by utilizing a single series capacitive switch compared to conventional switches with capacitive and series combinations. Reliability of the switch is improved by adding floating metal and reducing stiction between the actuating bridge and transmission line. Insertion loss and isolation are better than −0.6 dB and −20 dB, respectively, for 1 GHz to 20 GHz applications. [ABSTRACT FROM AUTHOR]

This paper analyzes the operation characteristics of the modular multilevel converter when the arms are asymmetrical with different sub-module number caused by sub-module faults. The research reveals the problems caused by asymmetrical arms, including the asymmetrical capacitor voltages, the asymmetrical direct current (dc) components of the arm currents, the asymmetrical circulating currents, the dc current fluctuation, and the voltage offset at the dc or alternating current (ac) side. Then, it is demonstrated by the theoretical analysis that the series of problems can be solved by rebalancing the fundamental frequency components of the upper arm voltage and the lower arm voltage. Therefore, a fault-tolerant control strategy is proposed by introducing a fundamental frequency resonant controller to the traditional second-order frequency proportional resonant circulating current controller. Besides, the research shows that the coordination of the proposed control strategy and the negative sequence current suppressing control can keep the output voltages of the three phases balanced. A 201-level modular multilevel converter-high-voltage direct current time-domain simulation model is established based on PSCAD/EMTDC, and the simulation studies confirm the efficiency of the proposed controller under balanced and unbalanced ac grid conditions. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In extra/high-voltage power systems, electromagnetic transient phenomena (e.g. from switching events) are analysed by time-domain simulation based on Electromagnetic Transient Program-type simulation tools. For the particular combined power cable-overhead line studied in this paper, it can be time-consuming especially if the effect of design parameters are studied systematically. A tailored solution based on solving the system's impedance and admittance matrix in frequency domain can serve as alternative approach. It also allows for including design details, which can be hard to implement in Electromagnetic Transient Program-type simulation tools designed for general application. The method is applied to analyse transient behaviour (upon no-load energising and no-load switching surge response) of a mixed double circuit connection including a section with essentially four parallel cables per phase, regularly cross-bonded over a length of 10.8 km. It is validated by comparison with pscad/emtdc simulation. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents a method to determine whether an operative voltage control action will have the steady-state expected effect or not. Sensitivity indexes are designed to correlate the controlled voltage with the controlling physical variable in generators, synchronous and static compensators, LTCs and shunt capacitors/reactors. An illustrative numerical example is provided as well as a real life one. The heavily loaded Brazilian system was operating without any problem. However, a small increase in generation caused voltage control devices to work unexpectedly. A harmful interaction among nearby control devices was also noticed. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper presents a simplified voltage control of paralleling doubly fed induction generators (DFIGs) connected to the grid by using static Var compensator (SVC). The DFIG converters are based on a voltage source converter in the grid side converter (GSC) and current source converter in the rotor side converter (RSC). The GSC is employed to work like static synchronous compensator (STATCOM) to provide precise control to maintain the DC link voltage under control. The RSC is oriented to control real and reactive powers of the DFIG. Instead of adding two SVCs in every DFIG bus it connects only one SVC in the point of common coupling of the generators to enhance the voltage stability at a steady state and dynamic operation by decreasing the reactive power hence to increase the voltage amplitude. The comprehensive simulations are carried out using PSCAD/EMTDC program. The results show that the SVC has improved voltage stability and promote the system transient response by compensating reactive power. Furthermore, this study has presented the possibility of using one SVC with low capacity based on the SVC characteristics adjusted only by choosing a suitable transformer rating. Moreover, the system model simulation was tested at steady state and dynamic mode during constant and random wind speeds. Using one SVC with low capacity leads to decrease power dissipation, and this directly reduced the power cost. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

For the obvious advantages in integrating wind power, multiterminal HVDC transmission system (VSC-MTDC) is widely used. The priority control strategy is proposed in this paper considering the penetration rate of wind power for the AC grid. The strategy aims to solve the problems of power allocation and DC voltage control of the DC system. The main advantage of this strategy is that the demands for wind power of different areas can be satisfied and a power reference for the wind power trade can also be provided when wind farms transmit active power to several AC grids through the DC network. The objective is that power is well distributed according to the output power of wind farm with the demand of AC system and satisfactory control performance of DC voltage is obtained. [ABSTRACT FROM AUTHOR]

Low-voltage direct current (DC) microgrids will have a significant position in smart grids. Of course, currently, DC microgrids are mainly used in the areas with sensitive electrical loads. The main issue in these microgrids is the voltage control. In this paper, a practical, reliable, modular, autonomous, and decentralized voltage control method, namely the voltage-based power-voltage (VbPV) control method, is proposed for the most common type of DC microgrids, that is, the DC microgrids with sensitive loads. The proposed voltage control method leads to the flattest voltage profile, which is essential for the DC microgrids. Besides, the presented method minimizes the power loss and maximizes the line capacity release. It is noteworthy that the VbPV control method requires no telecommunication infrastructure. Furthermore, contrary to some other DC voltage control methods, it is applicable to different real DC microgrids. Here, some numerical examples are mentioned to discuss and clarify the remarkable achievements of the VbPV control method. The analytic and simulation results confirm the successful performance of the VbPV control method. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The most common protection scheme for synchronous generators against stator windings fault is the differential protection scheme. In this paper, a new voltage-controlled overcurrent protection scheme is proposed as a backup protection method for the differential protection method. This scheme is designed to operate in conditions in which differential protection fails to do so and is implemented by fuzzy controllers. The fuzzy controllers set the plug setting of the overcurrent relay. In the proposed method, the fault conditions can be determined regardless of variations of voltage and the current of terminal in different operating states of the synchronous generator. This is realized by measuring not only the terminal voltages and currents of the generator, which are usually used in conventional voltage-controlled overcurrent protection schemes but also other variables that promote the accuracy of the proposed scheme. For presenting this protection method, a synchronous generator with internal fault model is used. The fault model is based on the direct-phase representation that uses the conventional and readily available machine data. Simulations for various types of stator faults of synchronous generator validate the proposed method. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY Voltage stability analysis and control are key applications for maintaining and enhancing the voltage security of bulk power systems. In today's control center, as power systems become more stressed and the penetration of renewable energies increases, system operators need to analyze voltage security of the systems based on actual operating conditions, contingency and power transactions of the system. In this paper, the model used to represent a power system losing stability is described. It is shown that the voltage collapse point is reached at saddle-node bifurcation or a structure-induced bifurcation. Computation of P-V curves showing system load margin to voltage collapse, system load margin to voltage-limit violation and system load margin to thermal-limit violation is presented. The architecture and example results of the Voltage Stability Analysis and Enhancement system at the California Independent System Operator implementing this research are shown. The continuation power flow engine at the core of this work is presented in detail, and study results are shown. Other key functions, contingency selection (insecure or critical) and ranking, preventive control to increase the load margin of insecure contingencies such that it is secure and enhancement control to increase the load margin of critical contingencies, are presented. Finally, a novel method to enhance the continuation power flow in handling uncertainty of renewable energy is shown. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY This paper presents a platform for integrating a voltage stability assessment and enhancement (VSA&E) application into wide-area situational awareness (WASA) based on international standards. VSA&E is a key tool for maintaining and enhancing voltage security of bulk power systems. As power systems become more complicated and the penetration of renewable energies increases, system operators require powerful tools to analyze voltage security of the system based on actual online system operating conditions. We have proposed a concept of the international standard-based WASA that can provide such a next generation VSA&E application platform with no vendor lock-in. The effectiveness and practicability of the international standard-based WASA are demonstrated with VSA&E application integrated into a WASA evaluation system. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY In this paper, several numerical examples to illustrate limitations of Quasi Steady-State (QSS) model in long-term voltage stability analysis are presented. In those cases, the QSS model provided incorrect stability assessment. Causes of failure of the QSS model are explained and analyzed in nonlinear system framework. Sufficient conditions of the QSS model for correct approximation are suggested. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY This paper deals with an improved decoupling control of the current components applied to three-phase modular multilevel converter (MMC) via rotating d-q frame theory based on the mathematical model of the system. In order to apply independent controllers for the two axes in d-q coordinates, the impact of the q-axis on the d-axis component and vice versa has to be removed. This is done by creating a decoupling block consisting of modified coupling terms. The feed-forward ac-side voltages compensation can effectively reduce any harmonic disturbance of the load side. Furthermore, we employed the strategy to reduce the capacitor voltages variation in each arm. A comparison of simulation results for both existing and improved decoupling current control algorithms using a (6.6 kV, 6 MW) induction motor drive verifies the superior performance of the proposed controller especially for good current reference tracking and disturbance rejection properties despite the variations in the system parameters. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY This paper focuses on voltage fluctuation mitigation and fault-tolerant control methods for modular multilevel converters (MMC). The reason for MMC output fluctuation caused by the floating capacitor voltage fluctuation at low frequency is carefully analyzed. To reduce voltage fluctuation, two control freedom degrees of high-frequency zero-sequence voltage and phase-to-phase circulating current are proposed. In fully implementing MMC functionalities, we comprehensively investigated its fault-tolerant operation without redundant submodules for fail-safe energy transfer. A fundamental-frequency zero-sequence-based neutral-shift method is adopted to enable continuous operation and avoid complete shutdown of the MMC system. These methods enhance both the reliability and availability of MMC while avoiding the undesired unbalanced stresses among the healthy submodules. Rigorous simulation studies are performed to demonstrate the validity and effectiveness of the proposed control methods. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY Plug-in electric vehicles (PEVs) will spread as environmental awareness continues to grow. As a result, many PEV batteries are likely to be charged in residential areas overnight, but this will cause a significant voltage decrease in the distribution network. As a countermeasure that does not require capital investment to strengthen the network, this paper proposes an autonomous decentralized voltage control scheme using a PEV battery charger. The scheme can continuously estimate household loads consumption and calculate the reactive power required for compensating the voltage decreases caused by household load consumption and PEV charging. Because only the measured voltage and current at the connecting point of the PEV battery and the impedance between this point and the nearest distributing substation are required to calculate the necessary amount of reactive power compensation, the control scheme can be implemented merely by modifying the control logic circuit built into the PEV battery charger at almost no extra cost. A realistic model of the distribution network and the standard daily demand curve in residential areas in Japan are used to validate the performance of the new scheme. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Maintaining a constant voltage in polymer electrolyte membrane fuel cells (PEMFCs) has always attracted the attention of many researchers, and many articles have been published on this issue. Furthermore, water management in PEMFC has become an important challenge because it can improve cell efficiency and lifetime. This paper will develop a one-dimensional dynamic model for a single PEMFC, which correlates changes in the cell voltage to changes in the cell current density and humidification rate. Subsequently, a recurrent neural network controller based on the approximation of nonlinear autoregressive moving average model is proposed. The controller manipulates the anode and the cathode water mole fractions in order to fix cell voltage and preserve cell water content within a satisfactory interval regardless of the varying cell current. The model and the controller are simulated in MATLAB/Simulink (Mathworks Inc., Natick, MA) software, and the results are compared with a PID controller from different viewpoints such as current disturbance and plant parameter variation. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

SUMMARY In this paper, we have studied different strategies for managing voltage fluctuations in distribution networks originating from decentralized electricity generation systems (DEGS) or increased loads, which are highly important issues in the smart grid context. A starting point for system design when increasing load or local power production could be to limit the voltage fluctuations to ±5% from nominal voltage. Strategies to regulate voltage include cable improvement, transformer management, Demand side management, storage, and line interconnection. We present a mathematical model applicable for both a static and dynamic analysis to quantify effects from these measures, though the best solution will depend on local conditions and needs to be determined case by case. Combining several voltage control options simultaneously may lead to further positive effects. Strategies when doubling the load and increasing DEGS production to twice the electricity demand were analyzed here in detail. It also needs to be pointed out that other factors related to power quality besides voltage may need consideration when large amounts of DEGS are integrated to distribution networks. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The power source is a critical obstacle for wireless sensor network nodes. In order to prolong the lifetime of wireless sensor networks, this paper presents an energy supply system that uses a specially designed broadband piezoelectric energy harvesting technology for sustaining the operation of wireless sensor network nodes. The proposed energy supply circuit can apply an optimal control voltage to the piezoelectric element to ensure impedance matching between the vibration source and the energy supply system in nonresonance frequencies. As compared to the conventional piezoelectric energy harvesting circuit, it is shown that the efficiency has been increased 4 times. In this work, the overall system structure, the function modules design, and the performance testing analysis are illuminated in detail. Experimental results reveal that this energy supply system can significantly improve power within the wide bands by the active piezoelectric energy harvesting technology and enable wireless sensor network nodes to operate normally. [ABSTRACT FROM AUTHOR]

The power source is a critical obstacle for wireless sensor network nodes. In order to prolong the lifetime of wireless sensor networks, this paper presents an energy supply system that uses a specially designed broadband piezoelectric energy harvesting technology for sustaining the operation of wireless sensor network nodes. The proposed energy supply circuit can apply an optimal control voltage to the piezoelectric element to ensure impedance matching between the vibration source and the energy supply system in nonresonance frequencies. As compared to the conventional piezoelectric energy harvesting circuit, it is shown that the efficiency has been increased 4 times. In this work, the overall system structure, the function modules design, and the performance testing analysis are illuminated in detail. Experimental results reveal that this energy supply system can significantly improve power within the wide bands by the active piezoelectric energy harvesting technology and enable wireless sensor network nodes to operate normally. [ABSTRACT FROM AUTHOR]

Summary: Modular multilevel converter (MMC) is easy to realize more voltage levels, and its structure is easy to connect back to back. It is suitable for high‐voltage direct current (HVDC) transmission system. However, the unbalance of grid voltage will seriously affect performance of HVDC transmission. The Lyapunov‐function‐based control strategy proposed in this paper can well solve the existing problems of MMC under unbalanced power grid. First of all, according to the topological structure of MMC, this paper establishes the mathematic model under the condition of grid unbalance and balance. Secondly, aiming at three different control objectives: (1) eliminate negative sequence current components and keep AC currents balance, (2) eliminate the active power double base frequency ripples, and (3) eliminate the reactive power double base frequency ripples. Then the Lyapunov‐function‐based control strategy is designed, and the circulating current control and capacitors voltage balance control are added to make the overall control strategy. Finally, the simulation of Lyapunov‐function‐based control strategy and traditional PI control strategy based on Matlab/Simulink platform is given. The validity and superiority of the Lyapunov‐function‐based control strategy are proved. And experiments on the hardware platform further verify the feasibility and correctness of the control strategy. [ABSTRACT FROM AUTHOR]

Summary: This paper presents optimal coordination and management of on‐load tap changer (OLTC), distribution static compensator (D‐STATCOM), distribution generation (DG), and reconfiguration to maximize annual cost savings and improve voltage profile using hybrid optimization. An analytical approach is developed to find optimal setting of OLTC. Optimal reconfiguration of the network is derived using genetic algorithm. Optimal location of DG and D‐STATCOM is determined using gravitational search algorithm, and optimal ratings are obtained using interface with MATLAB and GAMS environment. Fourteen realistic case studies are investigated in this paper work. In the first scenario, regulating devices are disabled to monitor the voltage profile and energy loss of the test system. In the remaining cases, influence of OLTC, DG, D‐STATCOM, and reconfiguration is independently and simultaneously studied. In the last scenario, OLTC, DG, D‐STATCOM, and reconfiguration are enabled to maintain voltage security and minimize power losses. Simulation results are obtained for IEEE 69‐bus test system and compared with other existing methods available in the literature. The proposed technique is reliable and efficient to obtain global optimal solution. Simulation results clearly indicate that optimal coordination and management among OLTC, DG, D‐STATCOM, and reconfiguration is required to maximize annual cost savings and maintain voltage at each node as per statutory requirements. Moreover, the analysis has been carried out for time‐varying distribution system with realistic load model. [ABSTRACT FROM AUTHOR]

Summary: This paper presents an improved version of the direct power control (DPC) for the distributed generation systems. The improvement is exemplified in using an adaptive proportional‐integral (PI) controller, whose parameters are recursively tuned at any operating condition to reach the minimum error in the shortest possible transient time. This improved DPC is applied to a distributed generation unit that is based on the 5‐level diode‐clamped inverter so that its output power can be easily controlled in a grid‐connected mode. This paper also introduces an innovative technique called direct voltage control to stabilize the loads' voltage in a stand‐alone mode at balanced and unbalanced loads. Both control schemes for DPC and direct voltage control depend on this new combination of the self‐adaptive PI controllers and estimation of the feedback parameters. The simulation results are provided to show the superior performance of the proposed control schemes compared with some other common techniques such as voltage oriented control, conventional DPC, and conventional DPC operated by regular PI controllers. Experimental results are also presented to prove the practicality of the presented adaptive PI controller in a grid‐connected mode. [ABSTRACT FROM AUTHOR]

Summary: This paper presents a low switching frequency–based pulse width modulation technique for a 3 × 3 indirect matrix converter. The nontriplen lower‐order harmonics are considered for minimization using differential evolution–based metaheuristic technique. The mathematical equations are formulated while considering minimization of nontriplet lower‐order harmonics along with control of fundamental voltage. An objective function is constructed by considering all the constraints on the feasible region and then it is minimized by using differential evolution technique. It is observed that the solution in the entire modulation range exists along with multiple solutions. Various cases of switching angles are considered, and solution trajectories are reported in practically feasible range. This type of modulation and control scheme is extremely useful in high power applications such as unified power flow controller as it minimizes the switching losses in the converter. The theoretical findings are verified by simulation and hardware results. [ABSTRACT FROM AUTHOR]

Summary: DC‐link voltage control is an eminent attribute of grid‐tied photovoltaic (PV) systems, and it is maintained at 10% to 15% higher than the peak of supply voltage to control the flow of active power. However, large variations in amplitude of supply voltage, compensating current, and PV current affect the recital of DC‐link voltage controller, which further deteriorates the performance of control algorithm. This paper presents a small signal analysis using instantaneous power balance on both sides of inverter for DC‐link voltage stabilization. A novel feed forward compensation (FFC) technique is proposed for mitigating the effects of external disturbances. Active current detection technique is also proposed for load current decomposition and compensating current detection which is needed for FFC technique design. An integral time absolute error criterion is used for collecting the error generated by the floating nature of DC‐link voltage reference. This generated error is minimized by optimal tuning of the proportional controller using gravitational search algorithm. The effectiveness of the proposed controller is validated using sensitivity analysis. The dynamic performance of the proposed control technique is tested and verified using MATLAB/Simulink. Hardware prototype using a cost‐effective Arm Cortex M4 microcontroller (STM32F407VGT6) is also presented to validate the simulation results. [ABSTRACT FROM AUTHOR]

Summary: The current quality of the utility in distribution system is improved by using distribution static compensator (DSTATCOM). The voltage source inverter (VSI) of DSTATCOM is connected to utility through an interfacing inductor for smoothing the current. But the required inductance value is high in conventional DSTATCOM. Due to this, the requirement of DC‐link voltage of VSI is increased to improve the current quality. In this paper, a hybrid DSTATCOM topology is proposed to reduce the inductance value, which will reduce the requirement of the DC‐link voltage. In conventional hybrid DSTATCOM, DC‐link voltage is maintained to a fixed value; due to this, the switching losses is more under the reduced load conditions. In order to minimize the switching losses, a variable DC‐link voltage control is proposed, which is based on the ac series capacitor voltage and the compensation performance. The effectiveness of the proposed method is validated by simulation and experimental studies. [ABSTRACT FROM AUTHOR]

Summary: This paper proposes a novel methodology for controlling voltage and frequency of islanded microgrids and meanwhile improving the system power quality. The proposed methodology is based on applying a multi‐stage H‐Infinity (H∞) controller with different control loops to restore the nominal values of voltage and frequency after any load variation. These control loops include power control, current control, and voltage control loops. The H∞ is also applied with an LCL filter to enhance the system power quality. The proposed controller is applied to a test system under three different loading conditions and the obtained results are compared with that obtained by applying the model predictive control (MPC) method. The results are compared with that obtained by applying a proposed traditional control method in the same test system. A comparison between the droop controller and the proposed controller, MPC is performed based on fitness value, controller response time, and maximum frequency deviation. In addition, the system stability is analysed using root locus, system step response, singular value decomposition, and bode diagram. The results prove the effectiveness and the efficiency of the proposed method in restoring voltage and frequency and improving the system power quality. [ABSTRACT FROM AUTHOR]

Summary: On‐load tap changer (OLTC) is utilized in regulating the voltage in electric systems. Recently, electronic on‐load tap changer (EOLTC) has been presented to eliminate the OLTC limitations and drawbacks. This paper presents an intelligent controller for EOLTC. The proposed controller aims to regulate the load voltage via fast and accurate adjusting the EOLTC when load voltage or power system voltage is changed. A multilayer feed forward neural network (MFFNN) controller is used in this proposal. The designing, training, and testing of the proposed intelligent controller are done in a MATLAB/Simulink environment. The performance of the EOLTC with the proposed MFFNN controller is achieved using two test distribution systems. The first one consists of a 220‐V source installed with a 50‐kW electrical load during a 220/210 V transformer and a transmission of short line. The second test system consists of an 11‐kV source connected to a (500 kW + 300 kVAR) industrial load through a 11/0.4‐kV, 800‐kVA transformer. This test system is a part of a real distribution feeder picked from North Delta Electric Distribution Company in Egypt. Several cases for the load voltage decreasing and increasing are used to verify the effectiveness of the proposed intelligent controller. The proposed controller shows fast and accurate response. [ABSTRACT FROM AUTHOR]

Summary: Microgrid inverters are characterized by nonlinearities, strong coupling, and switching flexibility, by which traditional proportional‐integral control is inadequate to realize a satisfactory control performance due to its local linearization. This paper attempts to design a modified linear active disturbance rejection control (MLADRC) for voltage control of microgrid inverters. Under the framework of extended state observer, voltage error derivative is obtained indirectly by the measurement of filter capacitor current, such that the observation performance of extended state observer could be significantly improved. Furthermore, a first‐order inertial link is employed in the perturbation loop to effectively suppress observation noises. A thorough frequency response analysis of MLADRC, linear active disturbance rejection control, and proportional‐integral control is undertaken, which shows that MLADRC owns superior disturbance rejection performance than that of others. Lastly, hardware experiment validates the effectiveness and implementation feasibility of MLADRC. [ABSTRACT FROM AUTHOR]

Summary: This paper analyzes the equivalent impedance characteristics of the ac microgrid with distributed renewable energy generations (DRGs) based on the droop control and proposes a decoupled power flow calculation method. The droop control strategy with adaptive virtual impedance has become an effective solution to utilize the DRG. To analyze the characteristics of the DRG, a microgrid consisting of multi‐DRGs based on distributed consensus‐based secondary voltage droop control with adaptive virtual impedance is researched. Considering the characteristics of the DRGs, a decoupled power flow calculation can be proposed to satisfy the quick power flow calculation in order to achieve economic dispatch and stability recovery for the microgrid. Furthermore, the accuracy of the power flow calculation with considering virtual impedance is improved. Moreover, a small microgrid, including three DRGs, has been modeled and simulated using the MATLAB/Simulink. Eventually, the decoupled power flow calculation is tested on three test case systems. [ABSTRACT FROM AUTHOR]

Summary: Traditional volt‐VAR control (VVC) devices such as on‐load tap changers (OLTC), voltage regulators (VRs), and shunt capacitor banks (SCBs) may not be capable to handle the sudden voltage violations because of slow response and large delay time. The voltage fluctuations may result from various disturbances such as intermittence in power output from distributed energy sources (DERs) such as photovoltaic (PV) and wind generation, change in network configuration, and load demand (especially in the case of flexible loads). Hence, there is a need of fast‐acting voltage regulation device such as smart inverter along with traditional VVC devices to encounter these issues. However, without proper coordination, these devices may cause a detrimental impact on distribution operations and network assets. In order to resolve this issue, a hierarchical coordinated volt‐VAR optimization (VVO) methodology has been introduced in this paper. In the proposed methodology, centralized as well as local control algorithm has been considered. The VVO objective of present study is to minimize the total operating cost considering conservation voltage reduction (CVR) and voltage deviation at all nodes simultaneously. Besides, the impact of battery energy storage (BES) on total operating cost, voltage deviation, and CVR has been examined. The ϵ‐constraint method and fuzzy decision‐making method have been employed for the solution of the abovementioned multi‐objective optimization problem. The performance of the proposed scheme has also been verified considering the forecast errors in PV generation and load demand. The proposed VVO methodology has been validated on a well‐known 33‐bus distribution systems. The test results demonstrate the significance of the proposed scheme on minimization of energy consumption, losses, OLTC switching operations, and voltage deviation. [ABSTRACT FROM AUTHOR]

Summary: Reactive power management and voltage control are essential for improved operational performance of distribution system (DS). With increasing distributed generation (DG) penetration in DS, reactive power compensation (RPC) becomes more significant, as most of the renewable‐based DGs are not able to supply reactive power. This paper presents new graph‐theoretic method for placement of reactive power compensator in radial DS using "average electrical centrality index." The proposed method is efficient with less compute intensive because appropriate location for the compensator is decided by centrality measures, unlike the existing methods that use optimization techniques to determine location. The proposed method is tested in a modified 69‐bus radial system to demonstrate its applicability in practice. The robustness of the proposed method is demonstrated in case of renewable generation uncertainty for modified 69‐bus radial DS. The performance of the proposed method is compared with existing multipoint RPC methods also. [ABSTRACT FROM AUTHOR]

Summary: In this paper, a 13‐level cascaded H‐bridge medium voltage static synchronous compensator (STATCOM) has been designed for dynamic reactive power compensation. The dynamic performance of STATCOM has been enhanced by finding the optimal PI controller parameters using particle swarm and gravity search algorithms. In order to avoid capacitor voltage drifting, individual voltage balance control loop is introduced. The results of optimization methods on finding optimal PI controller parameters of STATCOM are verified and compared by simulation studies. The results show that each optimization technique renders effective tracking of both, DC‐link voltage reference values, direct current, reactive current, and reactive power reference values during the closed‐loop operation of STATCOM. [ABSTRACT FROM AUTHOR]

Summary: This study presents a method for optimally selecting the location and size of switched shunt capacitors equipped with stand‐alone voltage control systems and fixed shunt capacitors in radial distribution networks. The main contribution of this paper is the introduction of a novel algorithm specifically designed for placement of capacitors equipped with stand‐alone voltage control systems. The considered objective function is the net saving calculated for every potential solution by using a backward‐forward load flow technique. A genetic algorithm is used to maximize the objective function while taking into account the technical constraints of the distribution network and the maximum capacitors' sizes depending on the set points of the stand‐alone voltage control system and on the locations of capacitors. The effectiveness of the proposed optimization method is verified and compared with other prevalent methods by simulation results carried on 69‐bus and 28‐bus distribution networks with three load levels. [ABSTRACT FROM AUTHOR]

Summary: Performance improvement is an important challenge for distribution network (DN) operator in many aspects. On the other hand, progress of DN performance is very valuable by allocating the optimal cost. This paper considers simultaneously optimal allocation of D‐STATCOM and reconfiguration in order to decrease active power loss, improve voltage profile, and increase the loadability in radial DNs by economic considerations in a multi‐objective optimization framework. In order to make a decision from Pareto front solutions based on their attributives importance, a multi‐attribute decision making procedure, is used. This procedure is based on the technique for order performance by similarity to ideal solution. The proposed methodology has been evaluated on IEEE 33 node and IEEE 69 node radial DNs. [ABSTRACT FROM AUTHOR]

Summary: The deployment of smart grid concept at the power distribution level has great potential for improvements to the consumers, e.g., better power quality and higher reliability. The available data allow designing a more efficient control framework aiming to improve operational variables such as power losses and nodal voltage levels. This paper presents a multiobjective control framework for a multifeeder distribution grid, which uses the data gathered from available voltage control devices, power, and current measures, and offline databases. The goal is to determine the best positions of taps in voltage control devices aiming to decrease power losses, switching operations, and fines due to low‐quality steady‐state voltages in a multiobjective problem. Different academic and real distribution networks with multiple feeders are used to test the proposed framework. [ABSTRACT FROM AUTHOR]