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Microgrid inverters in the presence of faults, unavoidable modelling uncertainties, disturbance and harmonic current resulting from nonlinear loads should have small steady state tracking error, small THD and high robustness hence this subject has turned one of the motivations of this investigation. To achieve the stated goals, fractional adaptive sliding mode controller (FASMC) is proposed in this paper. The proposed controller increases the robustness, flexibility and degree of freedom. As far as in practice it is not easy to define the bounds of disturbances and guarantee the system stability, hence in next step, to overcome this challenge the adaptation laws are suggested. Then for the problem of determining the controller parameters, Particle Swarm Optimization (PSO) algorithm is used. The performance of the proposed technique is investigated for an islanded microgrid under different disturbances, also to verify the advantages of the proposed controller, the results are compared with other controllers. Finally, the proposed controller and PID controller are implemented experimentally on Arduino mega 2560 microcontroller. [ABSTRACT FROM AUTHOR]

This paper proposes a WECS based on a back to back diode‐clamped multilevel inverter systems (DCMLI) fired comparatively by sinusoidal pulse width modulation (SPWM) and third harmonic injection pulse width modulation (THIPWM) techniques. DFIG performance is compared using these technologies for different wind speed under normal operation condition. The proposed approach shows that the DCMLI systems generate a nearly sinusoidal voltage with reduced total harmonic distortion (THD) thus upgrading the power quality of that produced by DFIG. As SPWM and THIPWM MLI are most common usage techniques in research, we applied them in the system to discuss a comparison of using those techniques to determine the number of levels that achieve the IEEE 519 criteria of THD which is less than 5% for both techniques (5th level for SPWM and 4th level for THIPWM) in all operation wind speed. Compared with simple PWM, and THIPWM have many advantages as lower THD, minimal number of switching to decrease switching losses and the output fundamental voltage is increased. Lastly, the paper investigates the variation of the frequency of induced rotor voltage and the active power flow due to the wind speed change when the rotor speed changes from super‐synchronous to sub‐synchronous speeds. [ABSTRACT FROM AUTHOR]

Grid-connected converters are expected to provide ancillary service utilising their spare capacities during operation. This study proposes a novel control strategy for clustered micro-grids to compensate voltage-unbalance of the power grid at the point of common coupling. Every micro-grid are interfaced to the power grid through a micro-grid inverter (MGI). All MGIs are designed independently with positive-sequence (PS) and negative-sequence (NS) control loops. Every micro-grid autonomously performs grid synchronisation and mode switching. After grid-connecting, a dynamic capacity allocation method is proposed for the MGIs to optimise its PS and NS capacity according to the grid voltage unbalance factor. Furthermore, a V–I droop control of the NS control loops is developed, with which each MGI can regulate its NS current according to the NS voltage in a droop manner. With the proposed method, all micro-grids participate in grid voltage unbalance compensation according to their own potential capacities without communications. Finally, a micro-grid cluster model with three micro-grids is built on Matlab/Simulink to validate the proposed control strategy. [ABSTRACT FROM AUTHOR]

The variable step size modified least mean square (VSSMLMS) learning algorithm based multifunctional double‐stage grid interactive photovoltaic (PV) system is implemented in this paper which feeds power to the grid and the load, ensures unity power factor operation and compensates reactive power. Along with this, it mitigates high grid neutral current during unbalanced loading condition. This algorithm uses variable step sizes to have a smooth dynamic and steady state performances. The boost converter (booster) is used in the first stage to extract peak power from a PV array using perturb and observe peak power point tracking technique. Next to this, a four leg voltage source converter (VSC) is placed to interlink the solar PV system to the grid and the loads. VSSMLMS‐based control technique is used to filter the fundamental component of load current and hence gate pulses are generated for the four‐leg VSC. This system is simulated in SIMULINK/MATLAB software under several working conditions and the obtained simulation results are validated on a developed laboratory prototype. The performance of the VSSMLMS‐based control technique is compared with that of the traditional LMS‐based technique. All results validate the IEEE‐519 standard. [ABSTRACT FROM AUTHOR]

In this paper, a decentralized control strategy for inverter‐based distributed generation (DG) is proposed to enhance the accuracy of power sharing under unbalanced load conditions in low‐voltage microgrids. The proposed method includes two important saliences: (1) accurate unbalanced current injection from DGs, and (2) reduction of circulating real and reactive powers in an islanded microgrid. The features are complied with the proposed droop strategy utilized in the control system of each phase of DG converters. Based on the proposed triple‐droop control strategy, the real power sharing in an AC low‐voltage microgrid is significantly improved via P–V control strategy and the reactive power sharing accuracy is enhanced via Q–f control strategy. By this method, the amplitude of the output voltage of each phase of the converters is determined based on the amount of load connected to that phase. In addition, the efficiency of the microgrid is increased due to the circulating current reduction between DGs. A virtual impedance concept is also utilized to improve the accuracy of the proposed power sharing method. The feasibility and effectiveness of the proposed strategy are validated using MATLAB/SIMULINK software for International Council for Large Electric Systems (CIGRE) low‐voltage distribution network benchmark. [ABSTRACT FROM AUTHOR]

This study presents an analysis method of harmonic interaction characteristics between direct‐drive wind turbines (DDWTs) based on αβ coordinates and the relative gain array (RGA). First, considering the digital delay of inverters, the capacitive current feedback active damping is applied to suppress the resonance of LCL filter, and the output current of DDWT is regulated by quasi‐proportional resonant controller in the αβ coordinates and then combined with the topology of the direct‐drive wind farm (DDWF), the transfer function matrix of the DDWF is established. Second, based on the RGA principle, the relationship between parameters and harmonic interaction characteristics is quantitatively analysed. The results show that with the proportional coefficient increasing, the degree of negative interaction between DDWTs is strengthened, which increases the harmonic content in the negative interaction frequency band. However, the increase of resonance coefficient and number of DDWTs will weaken the degree of negative interaction. In addition, with the increase of capacitive current feedback coefficient and grid inductance, the interaction frequency gradually decreases. Finally, the DDWF model is built both on Matlab/Simulink and StarSim + PXI hardware‐in‐the‐loop platform to verify the analytical results. The harmonic interaction characteristics can be used to guide the parameter design of DDWTs, so as to optimise power quality of the wind farm. [ABSTRACT FROM AUTHOR]

Virtual oscillator control (VOC) is an emerging non‐linear technique to control the terminal voltage of inverters in ac micro‐grids. This study compares two small‐signal models of VOC, obtained from average VOC dynamics over one ac cycle. The first model from previous literature is based on the approximation of real and reactive power in the average dynamics by the corresponding instantaneous values of power. The second model proposed in this study, in contrast to the first, approximates the average power by filtered power from a second‐order low‐pass filter. The two models are evaluated by eigenvalue and participation analyses. Simulations and experimental results are used to verify the models, by observing transient responses of a three‐phase inverter subject to a step change in the load. The proposed linearised model can tracks transients of the output voltage and current more accurately than the existing technique both in the simulations and experimental tests. The proposed linearised VOC model enables a more precise small‐signal stability and transient analysis of the average VOC dynamics. In conclusion, this improvement benefits stability studies when system parameters are changed or load disturbances are added to virtual oscillator‐controlled inverter‐based micro‐grids. [ABSTRACT FROM AUTHOR]

This study proposes a decentralised robust fuzzy control strategy for islanded operation of an AC microgrid with voltage source inverters. The objective is to design a robust controller for regulating the load voltage and sharing power among distributed generators (DGs) in the presence of uncertainties in the system and non‐linear loads. The AC microgrid consists of parallel DGs connected to a main AC bus. A Takagi‐Sugeno fuzzy approach is developed in this article to achieve stability and desired performance in dealing with non‐linearities in the islanded microgrid and H∞ criterion is used to obtain a robust control strategy in presence of uncertainties raised by unmodelled and high frequency dynamics. Therefore, a non‐convex condition in H∞ optimisation problem is converted to a convex linear matrix inequality (LMI) condition and is solved by MATLAB LMI toolbox. In order to develop a decentralized system, P–f and Q–V droop controllers are used to specify set‐points for local inverter controllers in each DG. The effectiveness of the proposed control strategy in presence of constant power load and DG accidental outage is validated by the simulation of a microgrid test system in MATLAB SimPowerSystems toolbox. Comparison with cascaded proportional integral controller shows advantages of robust T‐S controller. [ABSTRACT FROM AUTHOR]

This study proposes a distributed hierarchical coordination control paradigm for parallel inverters, which not only enhances the flexibility of plug and play architecture but also strengthens the accuracy of current sharing with low sensitivity to the different line impedances and with fast response to the transient current change in islanded microgrid. Two kinds of distributed coordinated controller are designed to remove the voltage and frequency deviation from primary control. One is proportional–integral–derivative‐based control, which detects the error between the segmented reference current and the actual measured current of the local inverter to calculate the compensating control input of the primary controller. The other is consensus‐based control, which uses distributed control actions to spread local information among neighbouring distributed units. To improve the robustness of consensus algorithm, the authors take a relative value instead of absolute value from the primary control as the consensus variable to achieve accurate current sharing among different nominal power rating inverters. Then, a detailed small‐signal state‐space model is developed with the proposed secondary cooperated controller to achieve the stability analysis and parameters design purpose. Finally, an islanded MG test system is built in MATLAB/Simulink and control performances of two strategies are verified and compared. [ABSTRACT FROM AUTHOR]

Photovoltaic (PV) grid‐connected inverter is the core component of PV generation system; quickly and accurately obtaining the parameters of inverter controller has great significance in analysis of transient characteristics of PV generation system. Parameter identification relies on disturbance signals and measurements selection, and the trajectory sensitivities of inverter controller parameters are calculated with different system outputs as the measurements, the disturbance signals include AC three‐phase short‐circuit fault and DC voltage reference jump, accordingly, a two‐step identification method is proposed, the first step use a three‐phase fault to identify all voltage loop parameters and the proportional coefficient of current loop, and the second step consider a DC voltage reference jump disturbance to identify the integral coefficient of current loop and the setting inductance value. The adaptive differential evolution is taken as the identification algorithm, which adopts adaptive strategies for algorithm parameters in order to speed up the convergence and enhances the balance between the global search and local mining. The simulation analysis validates the effectiveness of this method. In addition, the impact of the inductance error on the identification is discussed. [ABSTRACT FROM AUTHOR]

Droop control is a common method in the universal microgrid applications. Conventional droop control is unpractical for low‐voltage microgrid, where the line impedance among distributed generation units (DGs) is mainly resistive to generate the active and reactive power of DG is coupled. Besides, accurate reactive power sharing is not achieved due to the voltage deviation of DGs caused by disparate line impedance in microgrid. An improved droop control based on the virtual power source (VPS) and composite virtual impedance, which is constituted by a negative resistance and a negative inductance, is proposed for low‐voltage microgrid. The virtual negative resistance counteracts the line resistance to decouple power. The reactive power sharing accuracy based on the virtual negative inductance and VPSs control increases through analysing the line and virtual voltage drop. Finally, the fractional‐order PID controller is adopted to control the inverter voltage. The optimal controller parameters are obtained by the differential evolution algorithm. To verify the viability and availability of improved control strategy, simulations are carried out on MATLAB/Simulink. [ABSTRACT FROM AUTHOR]

This study presents a trajectory tracking control strategy that modulates the active power injected by geographically distributed inverter-based resources to support transient stability. Each resource is independently controlled, and its response drives the local bus voltage angle towards a trajectory that tracks the angle of the centre-of-inertia. The centre-of-inertia angle is estimated in real time from wide-area measurements. The main objectives are to stabilise transient disturbances and increase the amount of power that can be safely transferred over key transmission paths without loss of synchronism. Here the authors envision the actuators as utility-scale energy storage systems; however, equivalent examples could be developed for partially-curtailed photovoltaic generation and/or Type 4 wind turbine generators. The strategy stems from a time-varying linearisation of the equations of motion for a synchronous machine. The control action produces synchronising torque in a special reference frame that accounts for the motion of the centre-of-inertia. This drives the system states toward the desired trajectory and promotes rotor angle stability. For testing, a reduced-order dynamic model of the North American Western Interconnection is employed. The results show that this approach improves system reliability and can increase capacity utilisation on stability-limited transmission corridors. [ABSTRACT FROM AUTHOR]

This study proposes controls and power sharing design and architecture for a 100% inverter-based transmission system. Such an operation scenario has already occurred for short periods in portions of several systems in the United States, Europe, and Australia, and is likely to be more frequent in the future. The proposed algorithm enables the inverter-based resources (IBR) to participate in power sharing based on an angle droop method that explicitly takes into account the IBR ratings and preferred set points. This strategy results in an essentially constant-frequency operation of the power system without relying on secondary controllers or communication for frequency restoration. The performance of the proposed architecture under different operating conditions is evaluated via extensive simulation case studies in PSCAD/EMTDC software. [ABSTRACT FROM AUTHOR]

This study presents the design and analysis of an adaptive second-order generalised integrator (SOGI) technique for controlling voltage source inverter in a three-phase three-wire distribution system. The function of the proposed shunt active power filter involves compensation of reactive power for power factor correction, harmonic elimination, load balancing, voltage regulation under non-linear load condition. In this study, an effective approach for load compensation is proposed by designing an adaptive SOGI trained using Levenberg–Marquardt technique and compared with conventional techniques which ensure smooth operation under normal and distorted grid condition. An experimental setup is developed and its performance is analysed during different loading conditions. The experimental results are in line with the expected simulation results. Experimental and simulation results are verified and analysed using Bode plot. [ABSTRACT FROM AUTHOR]

To cater unceasing supply to the modern AC and DC loads, renewable energy resources integrated hybrid DC/AC microgrid (HMG) are considered as a viable technological solution. In this study, a multiple parallel-connected bidirectional converter (MPBC) topology has been proposed to enhance the solid-state transformer (SST) power handling capability with flexible control of the HMG. To improve the high-frequency AC-link power quality and to control the power transfer through the SST in each dual active bridge a split-step phase shift (SSPS) control technique has been used. A power management control strategy has also been proposed for efficient power transfer between the sources and loads of the HMG. The proposed MPBC-based SST is suitable for interconnecting multiple utility feeders. Multiple redundant utility feeders ensure stiff bus voltages and omit the power supply reliability issues. The operation of the proposed MPBC-SST converter system with corresponding control algorithm is verified through the simulation results in both grid feeding and grid fetching operating modes. Furthermore, experimental results are presented to validate the performance of the proposed MPBC-SST converter along with its proposed controller strategy. [ABSTRACT FROM AUTHOR]

This study proposes a new modelling approach for studying low‐frequency oscillations (LFOs) in a droop controlled islanded microgrid. Due to the absence of inertia, these sources are more vulnerable to power and frequency oscillations. Their quick response can introduce faster electrical dynamics in the system which should be monitored from time to time. To ease the analysis process, this study proposes a simplified method for finding LFO in the microgrid. Inspired from the small‐signal automatic generation control model of the conventional grid, a transfer function based closed‐loop small‐signal model of inverter‐based islanded microgrid is presented in this study to study power‐sharing among the various inverters. The proposed model uses the concept of dynamic power flow through the network to find the power output of each source following load perturbations in a system. Time domain simulation results and eigenvalue analysis is provided to verify the effectiveness of the proposed small‐signal model. By comparing the results obtained with actual system simulation in MATLAB/Simulink, it is found that the proposed simplified model is able to predict the stability margin and the LFO of the system without using actual detailed state‐space modelling procedure. [ABSTRACT FROM AUTHOR]

Using inverter-based topologies and lack of rotational masses can lead to a noticeable reduction in the inertia of modern systems and have detrimental effects on the resiliency, stability and strengths of microgrids. Effective frequency control ancillary services and modern adaptive control mechanisms can be proposed to resolve the mentioned challenges practically. From this perspective, several flexible and intelligent control approaches have been recently introduced to create a balance between generation and load demand during various operational conditions in low-inertia power systems. This study suggests a supportive collaboration between two distributed generations including virtual inertia of wind turbine generator and fast speed micro-turbine based on an adaptive optimal model predictive control (AOMPC). To demonstrate the effectiveness of the proposed framework, the results are compared with the previous controllers like optimal proportional–integral, optimal fractional order proportional–integral–derivative (PID), optimal fuzzy PID, the optimised membership function of fuzzy and adaptive MPC controller during multiple load variations, changes in the weather patterns, unwanted time-varying uncertainties and collapse of power generation units. [ABSTRACT FROM AUTHOR]

This study presents a model predictive control (MPC)-based centralised control approach for maintaining the voltages of the nodes within permissible limits in the presence of high photovoltaic (PV) penetration. The proposed control scheme coordinates the actions of on-load tap changer (OLTC) and PV inverters optimally to fulfil the desired objectives. The objectives are minimisation of change in control variables, slack variables, energy loss, and voltage error. These objectives are weighted to form the overall objective function. Three rules are formulated based on the severity of voltage magnitudes. The weights of the objectives are adjusted according to these pre-defined rules. Simulations are performed in an active distribution network (ADN) integrated with and without microgrids, where both demands and generations vary hourly over the day. As power is injected by the microgrids during most of the time of the day, the excursions of node voltages are slightly higher in the microgrids integrated ADN. Moreover, the incorporation of the proposed rule-based MPC drastically reduces the energy loss due to active power loss in distribution networks, as is evident from the simulation results obtained by comparing the proposed approach with an existing MPC-based approach. [ABSTRACT FROM AUTHOR]

In distribution networks, the voltage unbalance occurrence is inevitable. The voltage unbalance can have negative impacts on the customer equipment and the network performance if it exceeds the permissible limit. This study proposes a novel control strategy to compensate for the voltage unbalance in stand-alone microgrids. The proposed control method applies to the inverter-interfaced distributed generation units. The suggested control strategy is based on the decouple double synchronous reference frame (DDSRF) that is modified to improve its effectiveness. The modified DDSRF can effectively decouple the positive and negative sequence components. The simulation results depict that the proposed control strategy provides faster dynamic response and more effective voltage unbalance compensation comparing to the DSRF-based control method. [ABSTRACT FROM AUTHOR]

Integrated photovoltaic (PV) distribution systems voltage stability is of great significance in supporting all connected equipment smooth functioning in the distribution network. Voltage profile maintenance is one of the challenging tasks in PV integration. To maintain a constant voltage profile to a sensitive load of 22 kVA is the main idea of this study. A single‐phase PV‐integrated distribution system is selected for the study. The novelty is that differential inverters are used for dynamic voltage restorer and distribution static synchronous compensator of the unified power quality conditioner (UPQC). Active power decoupling facility is the main advantage using the differential inverter. The research work objective is to synchronise a 10 kW solar PV system to the distribution system using this new UPQC. The research work discusses and derives the most suitable control strategy for the UPQC with battery energy storage system. A 20 kVA UPQC is designed for the PV integration and to increase voltage stability of the distribution system. The frequency, voltage and reactance/resistance ratio of the distribution system is assumed to be constant. A prototype model of differential UPQC is developed. Experimental and simulation results validate the main objective. [ABSTRACT FROM AUTHOR]

In this study, power quality (PQ) improvement has been addressed, in the form of total harmonic distortion (THD) minimisation, as well as, voltage regulation using two cascaded schemes. The first scheme is the optimised triple action controller (TAC)‐based pulse width modulated voltage source inverter. TAC consists of a proportional resonant controller, selective harmonic compensator, and a new added current‐assisted feed forward controller. The second scheme is the optimised cascaded dual‐level control of a standalone microgrid. Cascaded level control consists of droop, secondary, and synchronisation control loops. The two approaches have been optimised for best parameter selection out of the possible solution space using a particle swarm optimisation algorithm to satisfy the study objectives. The optimisation objectives/constraints were to minimise THD and minimise overshoot/undershoot, rise time, and steady‐state error for voltage compensation under two disturbance scenarios, sudden load change, and voltage flicker injection as a power frequency disturbance. These research results have been compared to other existing simulation and experimental work. The results proved to be better in output voltage, frequency, response time, and THD. Furthermore, the proposed schemes ensure power factor improvement, high efficiency, overall system PQ, and reliability at various load conditions. [ABSTRACT FROM AUTHOR]

In this study, energy management and damping improvement of a DC microgrid are proposed by using an interconnection and damping assignment–passivity‐based control (IDA–PBC) technique. The control technique is applied for (i) the grid connected inverter and (ii) the DC–DC converters connected with fuel cell and battery of the DC microgrid to control the DC bus voltage. The IDA–PBC generates the control laws with the desired energy function for the current controller of the converters based on the generation from the renewable power generators and load demand. In addition, an integral action is added with the IDA–PBC control laws to reduce the steady‐state error in the DC bus voltage. The parameters of the IDA–PBC technique are tuned based on the state of charge of the battery and grid availability for a smooth transition between the operating modes and better energy management. It is proved that the derived energy function satisfies the Lyapunov stability criterion. Simulations are carried out in MATLAB/Simulink to test the performance of the proposed control technique. The results show that the proposed technique provides the desired damping and energy management. [ABSTRACT FROM AUTHOR]

To ensure the safe operation of the grid, there are some requirements to be taken into consideration to connect power converters. During abnormal conditions – e.g. during voltage sags –, the control of the converters is a very important key to guarantee power quality and good behaviour of the distributed generation system. The aim of this study is to employ two possible control strategies for a grid‐connected inverter according to the Spanish grid code, and to analyse the behaviour of the output voltages during both symmetrical and unsymmetrical voltage sags. The analytical development shows the sag influence on currents, voltages, active and reactive powers. These influences are explained through Ku transformation in the synchronous reference frame, thus giving a representation for electrical variables easiest to analyse. The results show how control strategies affect the converter behaviour and how they can support the main grid during faults through the control of active and reactive power injection. Sags with different durations and depths have been taken into account, which can provoke critical values for electrical magnitudes and can lead to the violation of the grid code. The proposed control strategies study has been validated by means of both simulations in MatlabTM –Simulink and experimental results. [ABSTRACT FROM AUTHOR]

This study introduces a distributed secondary control method‐based adaptive pinning control for inverter‐based distributed generations (DGs) in an isolated microgrid (MG) structure with frequency and voltage compensation and power sharing. Without MG centre controller or energy management system, local DGs' controllers have no global information of the whole communication network, which is the second smallest eigenvalue of the Laplacian matrix. In practice, the large coupling weight gains are mostly selected. However, a sufficient large coupling weight may cause instability. It is difficult to choose the coupling weight in the fully distributed complex network. To solve the above problems, the authors investigate a pinning control for synchronisation using the local adaptive pinning algorithm on coupling weight gains. The proposed method has the advantage that the consensus controller design is independent of the Laplacian matrix associated with the communication network. And the collective dynamics of the communication network can be adjusted to steady states without global information. However, the adaptive pinning control scheme is not suitable for the multi‐disturbance situation, which will accumulate the coupling weight and even cause integral saturation. Therefore, an improved strategy is proposed to adapt to the load changing conditions for islanded MGs. [ABSTRACT FROM AUTHOR]

Due to reactive power irrational sharing issue resulting from the mismatched line impedances and unbalanced local loads of the inverters, the hierarchical coordination control strategy based on fuzzy adaptive compensation is designed to improve the accuracy of the reactive power sharing in the islanded AC microgrid. Firstly, in the local controller, the total active power and the average reactive power are obtained by the initial power sharing stage and the secondary power sharing stage, respectively. Then, in the power balancing stage, the average reactive power as the input of the fuzzy control can be used to adaptively adjust the voltage reference value in real time, so the rational sharing of reactive power may be achieved by using the designed control strategy. Furthermore, based on the classification of load fluctuation, the coordinated operation strategy is designed to deal with reactive power sharing for different load fluctuation conditions. Noteworthy, only the low‐bandwidth synchronisation signal from the central controller is used to the hierarchical control strategy, which can maintain characteristic of plug‐and‐play in the microgrid. Simulation and experiment results are presented to validate the effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

Synchronous reference frame (SRF) control strategy for solar photovoltaic (SPV) sources is widely used to deliver maximum power to the grid. However, poor inertia support just after a disturbance and improper phase angle tracking in presence of the harmonics and system unbalance are noticed when the conventional phase‐locked loop (PLL) based SRF control structure is used. In this study, an inertia enhancement method for inverter interfaced SPV sources is proposed, which adjusts only the PLL parameters with a notch filter (NF) in the SRF controller. NF in PLL is used due to its disturbance rejection potential and accurate phase angle tracking, even during system unbalance. The dynamic equation of pseudo induced voltage (PIV) vector with respect to the point of common coupling is derived. The kinematic equation of the PIV angle vector correlates inertia contribution and inverter terminal voltage. The impact of the change in PLL parameters on inertia enhancement is analyzed by validating the proposed technique on a test system in the real‐time digital simulator. The frequency response by the proposed method has been compared with two state‐of‐art methods to prove the superiority of the proposed approach in enhancing the inertia of the SPV source in a microgrid. [ABSTRACT FROM AUTHOR]

Utilisation of variable distributed energy sources in a microgrid necessitates control of active power and is enhanced by the control of reactive power. A control technique to regulate the active and reactive power of a virtual oscillator controlled inverter is presented. Current feedback gain of the virtual oscillator controlled inverter is used to regulate the active or reactive power. In order to achieve the desired power set‐point, a PI controller is used to tune the current feedback gain. An approximate local stability analysis is presented using the system linearisation and eigenvalues analysis. Constraints on the virtual oscillator controller and system parameters are identified to ensure the stability of the system. Global stability of the system is investigated by determining a Lyapunov function using the sum‐of‐squares technique. Simulation and experimental results are presented for both the cases of active and reactive power control of a virtual oscillator controlled inverter to verify the analytical analysis and to demonstrate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

The electrical energy required by the remote communities can be supplied efficiently and effectively using a decentralised renewable energy source (RES). However, the stochastic variations in power output of RESs, i.e. solar photovoltaic and wind turbine, and deviations in demand make it difficult to preserve the balance between generation and demand which may jeopardise the stable operation of the system. Moreover, in the islanded systems the lack of inertia due to the replacement of conventional power plants with inverter‐based sources cause undesirable influence on the frequency of the supply. Generally, various energy storage systems (ESSs) are proposed in such a grid to overcome this problem. This study investigates the implications of the hybrid ESS (HESS) on the frequency regulation (FR) of an islanded system. Battery ESS and a supercapacitor has been used to form a HESS for the islanded power system. Analysis has been conducted in MATLAB/Simulink simulation platform. Simulation results demonstrate the efficacy of the proposed HESS in FR of the islanded system. [ABSTRACT FROM AUTHOR]

Growing trends in the deployment of inverter‐based renewable energy will decrease the inertia and frequency control capability of electric power systems by replacing conventional power plants; thus, the frequency of future power systems might be dynamic. This study proposes a capability‐coordinated frequency control (CCFC) scheme of a virtual power plant (VPP) including adjustable‐speed pumped storage hydropower (AS‐PSH), a wind power plant (WPP), and an energy storage system to support the frequency nadir and reduce the steady‐state error of system frequency. The CCFC scheme is based on a hierarchical‐control structure in which a CCFC organises the output of local frequency control units. To support the frequency nadir, the CCFC dispatches weighted frequency errors that are proportional to the available headroom of the units; thus, the errors are forwarded separately with a system frequency error to the primary control loop of each unit and thereby arrest the frequency nadir at a higher value than a system without the CCFC. To reduce the steady‐state error of the system frequency, the CCFC determines a partial active power command by additionally feeding an integrator of the CCFC with a modified frequency error that depends on the unit with the largest control. [ABSTRACT FROM AUTHOR]

In this study, a ring‐type multi‐microgrid (MG) system comprising of three MGs with solar energy‐based generation, wind energy‐based generation, and static synchronous compensator as primary sources along with batteries as emergency back‐up is proposed. The voltage‐source converters at the individual MG level serve as the interface between the neighbouring MGs and the grid. An augmented proportional sliding‐mode‐control‐based approach is proposed in the primary control level to improve the power quality of the system during balanced and unbalanced loading conditions. A negative‐sequence controller is also included to mitigate the negative‐sequence voltage from the inverter output voltage. The multi‐MG (MMG) system is capable of operation in grid‐connected, islanded, or interconnected mode with balanced and unbalanced loads. The bidirectional DC ‐DC converters interfacing the batteries at the DC link is controlled by a hysteresis‐function‐based sliding‐mode control. A comparison of the proposed inverter primary control with proportional ‐integral controller, quasi‐proportional ‐resonant controller, and hysteresis‐band current controller is carried out. The comparison reveals that the total harmonic distortion and voltage unbalance factor improves by employing the proposed control when compared with conventional control. The proposed MMG system with its control strategies is modelled in MATLAB/Simulink environment and is experimentally verified using OP4510 from OPAL‐RT. [ABSTRACT FROM AUTHOR]

Stability of inverter‐dominated microgrids (MGs) faces many challenges such as inertia shortage, mutual interactions among converters and sudden external disturbances. To enhance the stability of MGs, a distributed non‐linear stabiliser is designed in this study based on synergetic control theory. First, inverter interfaces are uniformly modelled as virtual synchronous generators, whose small‐signal stability is then analysed with the state‐space average method. Also, the objective of the following control strategy, i.e. the system unstable domain, is obtained. Second, macro‐variables applicable to droop control are constructed based on the typical dynamic model and then the analytic expression of the controller output could be derived. When external disturbances occur, or droop coefficients with high gains are chosen, power system stability could be guaranteed by adding a non‐linear stabiliser. Finally, taking a 9‐node MG as an example, simulation verifies the feasibility of the proposed strategy in cases of small disturbances, large disturbances and mode transition. [ABSTRACT FROM AUTHOR]

Building‐integrated photovoltaics (PVs) is one of the most promising and elegant ways of producing on‐site electricity. The compact and cost‐effective PV integration system needs to be developed to convert dc voltage into ac compatible with the grid. A single‐stage PV inverter is proposed, and its design and control system implementation are focused on. The proposed system is able to transfer active power to the grid with maximum power point tracking. At the same time, the PV inverter can control reactive power and suppress harmonics due to non‐linear loads. By coupling to grid via a series LC branch, the proposed system operates on a dc bus voltage much lower than that of the conventional PV inverter, so that less PV panels need to be connected in series. Therefore, the proposed system is a low‐cost and multi‐functional alternative to PV inverters in building‐integrated applications. Simulation and experimental results are provided to validate its effectiveness. [ABSTRACT FROM AUTHOR]

The microgrid with cascaded H‐bridge micro‐converters (cascaded‐type microgrid) is an effective way to integrate the distributed generators (DGs) into medium/high‐voltage distribution energy system. Just like the islanded microgrid composed of paralleled inverters, achieving accuracy in power sharing and high voltage quality is a serious challenge in cascaded‐type microgrid without communication. In this article, a decentralised control scheme is proposed to share the active and reactive power accurately under the resistance‐inductance and resistance‐capacitance loads. The power factor angle of each DG is assigned to be consistent in the steady state via regulating both the frequency and voltage. The proposed scheme can be easily implemented only based on the local measured signals. Meanwhile, excellent load voltage quality is achieved. Small‐signal analysis method is performed to verify the effectiveness of the proposed scheme, and a guide for designing the power sharing coefficient is given. The cascaded‐type microgrid model is developed through simulations and experiments to verify the performance of the proposed scheme. [ABSTRACT FROM AUTHOR]

This study presents an improved power management control strategy of a hybrid direct current (DC) micro‐grid (MG) system consisting of photovoltaic cell, wind turbine generator, battery energy storage (BES), fuel cell (FC), and electrolyser. Based on the voltage and state of charge of BES, FC, and electrolyser, the proposed control scheme improved the dynamics of the DC‐link voltage and contributes a better power management between each generation/source and load. A gain control technique is implemented in the grid‐side inverter controller to regulate the modulation index and improving the voltage stability of the DC‐link. Furthermore, the PI‐controller gains of BES are tuned dynamically based on the deviation in voltage and its derivative using Takagi–Sugeno‐fuzzy control to enhance the transient response of the voltage. For a reliable operation of the DC MG under standalone or prolonged islanding mode of operation, a priority‐based load shedding algorithm is proposed for maintaining proper power coordination between different energy sources and storage devices. Owing to smoother and faster voltage response, the proposed control schemes can comply with the grid code requirements of the changing configuration of the modern renewable energy integrated DC MG. The effectiveness of the proposed control strategy is tested by comparing the existing scheme through MATLAB/Simulink®. [ABSTRACT FROM AUTHOR]

The mismatched feeder impedance of the voltage‐sourced converter (VSC) units may result in reactive power sharing error in the islanded microgrids. This study presents a reactive power sharing strategy for single‐phase inverter‐based microgrids that eliminates the circulating currents and reduces the coupling between real and reactive powers. The proposed controller is based on an adaptive complex virtual impedance that aims to equalise the E−Q characteristic output of VSCs. The resistive term regulates the virtual voltage drop using the proposed ΔRv−P droop characteristic, while the inductive one controls the phase angle of VSC current and increases the X/R ratio of the microgrid. The proposed strategy does not require a central controller and knowledge of the microgrid lines and loads data and is applicable for the various microgrid topologies. Furthermore, it employs the unidirectional low bandwidth communication link and is robust against the delay and failure of communication channels. The proposed method is implemented using the conventional droop control scheme and presents the medium plug and play capability. Both simulation and experimental case studies using various microgrid topologies are conducted to demonstrate the merits of the proposed scheme. [ABSTRACT FROM AUTHOR]

This study proposes a new application and a novel control scheme for utilising Smart PV and Smart Park inverters to mitigate temporary overvoltage (TOV) phenomenon in distribution systems. As more number of the inverter‐based distributed generators such as PV are connected to a medium‐ and low‐voltage distribution systems, TOV phenomenon becomes more prevalent during single line to ground fault. Currently, the IEEE 142 'Effective grounding' technique has been the reference for various grounding schemes to prevent TOV. This study explores the potential of low‐cost and a novel solution for the first time that utilises a specialised controller associated with PV/SmartPark inverters (Smart inverters) as a TOV suppressor. The efficacy of the aforementioned novel application has been demonstrated on a simplified benchmark system of the IEEE Standard 399‐1997 with few modifications. A 4 MW conventional PV plant is considered along with 10 MVAR Smart inverter connected to the point of common coupling. During normal operation of the distribution systems, the Smart inverter controller acts in a voltage regulation mode to curtail the voltage rise due to reverse power flow from the 4 MW PV plant. During a fault condition, the TOV sensor enables the TOV control to suppress the violated limits of voltages in the healthy phases to an acceptable value. [ABSTRACT FROM AUTHOR]

This study addresses the robust stability analysis for an islanded microgrid with droop‐controlled inverter‐based distributed generators (DGs). Owing to large load changes, microgrid structure reconfiguration, and higher‐power demands, the low‐frequency (LF) dominant modes of a microgrid stir toward unstable zone and make the system more oscillatory or even unstable. In this study, a robust two‐degree‐of‐freedom (2DOF) decentralised droop controller, which is the combination of the conventional droop with a robust transient droop function, is utilised for each inverter‐based DG unit. Unlike conventional tuning of 2DOF droop controllers, a new design procedure is proposed to robustly determine the transient droop gains to effectively damp the LF oscillatory modes of the islanded microgrid irrespective of disturbances, equilibrium point variations, and uncertain parameters of a microgrid. To mitigate the LF power oscillations at different microgrid conditions, inspired by Kharitonov's stability theorem, a robust D‐stability analysis is performed to determine the specific ranges of the transient droop gains to provide a robustness margin for the disturbances, equilibrium point variations, and uncertain parameters of the islanded microgrid. Finally, digital time‐domain simulation studies are performed in MATLAB/SimPowerSystems software environment to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study presents the fuzzy logic integrator gain‐tuned improved second‐order generalized integrator (GI) for a double‐stage grid‐interfaced photovoltaic (PV) system. The proposed system includes the functionalities of feeding active power to the grid, power factor correction, grid currents balancing and system isolation under grid side faults. Moreover, the smooth system operation is ensured under weak distribution grids where grid voltage is subject to huge diversions. Furthermore, automatic protection scheme for the system under grid‐side faults is also established with the proposed algorithm for increased reliability. The fuzzy‐tuned GI provides advantages of efficient and effective extraction of load current fundamental component under steady‐state and dynamic grid conditions. The non‐linear frequency error variation is compensated here using fuzzy logic‐based self‐tuning integrator gain of the controller. The controller is improved to mitigate the possible DC component in the load current. The neutral current in the loads is nullified by using a four wire system. The adaptive DC bus voltage helps to minimize the switching losses and prevents unexpected tripping of the PV inverter. The system is experimentally verified using a prototype built in the laboratory. [ABSTRACT FROM AUTHOR]

A novel system frequency support strategy is proposed for the two‐stage three phase photovoltaic generation system, which involves simultaneously utilising a direct current (DC)‐link capacitor and deloading generation output. When frequency variation is detected, the energy stored in an inverter DC‐link capacitor is utilised to enable virtual inertia characteristic for system frequency support. At the same time, a deloading control strategy is also activated to provide system frequency support based on reserved generation beforehand. To address impacts due to the changing working conditions with respect to irradiance and ambient temperature, a variable droop scheme is designed for dynamically adjusting the frequency regulation ability based on deloading. Case studies using a microgrid system embedded with a solar and diesel generator are carried out to verify the effectiveness of the proposed control strategy. [ABSTRACT FROM AUTHOR]

In the outside environment, the partial shading on the photovoltaic (PV) arrays occurs frequently and the generated power of PV arrays is far lower than the rated power of inverters, thereby it will cause the total output power of PV system decline sharply. In this study, a flexible topology structure of PV power generation network, connecting PV arrays and inverters by the switching matrix is presented. Then, it introduces the working principle of topology structure and puts forward the corresponding multi‐population genetic algorithm to realise the group management of the multi‐objective control, so as to achieve the power matching optimisation between PV arrays and inverters during partial shading. Finally, the simulation and experiment show that the conversion efficiency of PV system used by the control strategy is much better than that of conventional distributed structure of PV power for the shade of PV arrays, and provide a new way for the effective use of solar energy. [ABSTRACT FROM AUTHOR]

Electric system operators are increasingly concerned with the potential system‐wide impacts of the large‐scale integration of distributed energy resources including voltage control, protection coordination, and equipment wear. This prompts a need for new simulation techniques that can simultaneously capture all the components of these large integrated smart grid systems. This study describes a novel platform that combines three emerging research areas: power systems co‐simulation, power hardware in the loop (PHIL) simulation, and lab–lab links. The platform is distributed, real‐time capable, allows for easy internet‐based connection from geographically‐dispersed participants, and is software platform agnostic. The authors demonstrate its utility by studying real‐time PHIL co‐simulation of coordinated solar photovoltaic (PV) firming control of two inverters connected in multiple electric distribution network models, prototypical of US and Australian systems. The novel trans‐pacific closed‐loop system simulation was conducted in real time using a power network simulator and physical PV/battery inverter at power at the National Renewable Energy Laboratory in Golden, CO, USA and a physical PV inverter at power at the Commonwealth Scientific and Industrial Research Organisation's Energy Centre in Newcastle, NSW, Australia. This capability enables smart grid researchers throughout the world to leverage their unique simulation capabilities for multi‐site collaborations that can effectively simulate and validate emerging smart grid technology solutions. [ABSTRACT FROM AUTHOR]

Microgrids are used widely in electric power systems for enhancing the power system operation in both grid‐connected and island modes. One of the main problems with microgrid operations in power systems is maintaining the microgrid voltage and frequency within permissible ranges and sharing microgrid loads among participating distribution generations (DGs) in an island mode. The droop control method will pose a degraded performance when feeder impedances of DGs are different. In this study, a new control method based on the virtual impedance and the compensating voltage is proposed and the simulation results show that this method combined with the droop control will offer a balanced power sharing among DGs with negligible voltage and frequency drops. Here both single‐bus and multi‐bus microgrids with distributed loads have been considered. The simulation results are based on the MATLAB Simulink which shows that the proposed method has a good load sharing performance among DGs with varying feeder impedances and droop gains. [ABSTRACT FROM AUTHOR]

One of the main concerns of utilities is to minimise voltage variations along their feeders when they enhance their networks by various distributed generation (DG) systems that have uncertainty in their output. Furthermore, such DG systems can provide reactive power to the grid. Thus, the objective of this study is to optimally allocate locations and capacities of DG systems, particularly photovoltaic systems in this study, on distribution networks that host DG inverters with the capability of reactive power control, in other words, Volt/Var management and control. For this purpose, this study proposes a hybrid method based on optimal reactive power control and genetic algorithms. The proposed method is verified in well‐known test feeders. [ABSTRACT FROM AUTHOR]

Decentralised control methods of voltage‐source inverters‐based multi‐distributed energy resource (DER) microgrids and microgrids hierarchical control scheme studied in earlier researches limited to primary and secondary control levels while the proposed control scheme is completely analysed so that the three hierarchical control levels modelled for both grid‐connected mode and islanded modes. In this study, the proposed hierarchical control scheme adds new control loop to control the reactive power reference by a fuzzy logic controller to have the benefit of increasing the system stability margins. In addition to the small‐signal events, it maintains the stability of microgrids faced with large‐signal disturbances such as short circuits, line outage, heavy motor starting etc., and improves fault ride through capability. However, power sharing to loads and network is sufficiently done. To demonstrate the effectiveness of the proposed hierarchical controller, simulation studies have been performed on a microgrid consisting of four units of distributed generation with local loads and in presence of main grid using MATLAB/SIMULINK software (offline simulations) and OPAL‐RT real time digital simulator for verification. [ABSTRACT FROM AUTHOR]

An improved control methodology for off‐grid, three‐phase induction machine working as a single‐phase generator is presented in this study. A three‐phase machine made to work as a single‐phase generator has advantages of lower size and cost than their single‐phase equivalent in almost all ratings except in the lower fractional kilowatt (kW) ranges. The three‐phase windings are configured as isolated two equivalent phases in the proposed system for this purpose. The machine uses photovoltaic (PV)‐assisted single‐phase inverter to one of its two phases for variable excitation requirement to cater dynamic loads at different rotor speeds besides having a fixed capacitor to provide bulk excitation. A storage battery is used along with the PV panels to improve the reliability of the scheme during low or no insolation periods. A stator‐flux‐oriented control is implemented for regulating the terminal voltage within specified limit at a fixed frequency. The control strategy ensures fixed load voltage and fixed frequency even with perturbations of load or rotor speed. Different simulation results, verified by suitable experiments using a 1 kW, 415 V laboratory prototype, confirm the suitability of the proposed concept. [ABSTRACT FROM AUTHOR]

This study presents a novel adaptive control scheme for variable‐speed wind turbine (VSWT) driven permanent magnet synchronous generator (PMSG) to ensure its operation under different operating conditions. The adaptive control scheme is based on the affine projection algorithm (APA) which provides a faster convergence and less computational complexity than the least‐mean‐square algorithm. The proposed adaptive controller is used to control both the generator‐side converter and the grid‐side inverter without giving additional tuning efforts. Each vector control scheme for the converter/inverter has four APA‐based adaptive proportional‐integral (PI) controllers. Detailed modelling and the control strategies of the system under study are demonstrated. Real wind speed data extracted from Hokkaido island, Japan is used in this study. The dynamic characteristics of a grid‐connected VSWT‐PMSG are investigated in details to ensure the proposed controller operation under different operating conditions. The effectiveness of the proposed adaptive controller is compared with that obtained using optimised PI controllers by Taguchi method. The validity of the adaptive vector control scheme is verified by the simulation results which are performed using PSCAD/EMTDC environment. [ABSTRACT FROM AUTHOR]

A simple technique is proposed in this study, aimed at the islanding detection of an inverter‐based distributed generation (DG). The main advantage of the proposed technique is its reliance on a simple approach for islanding detection with negligible non‐detection zone and keeping the unchanged output power quality. This work attempts to detect islanding conditions through computing the rate of change of voltage phase angle (ROCOVPA) of the point of common coupling by phase‐locked loop (PLL) in inverter controllers. Because almost all inverter systems are equipped with the PLL, its implementation for inverter‐interfaced DGs is simple and economical. Simulation results show that the durability energy of ROCOVPA is different for islanding and non‐islanding conditions. In fact, in non‐islanding condition the energy of ROCOVPA reduces to a negligible value after a certain time, but in islanding conditions it has considerable variation. The proposed method is evaluated using the conditions dictated by the IEEE 1547 and UL 1741 standards. The simulation results are presented to highlight the effectiveness of the proposed technique. In addition, the proposed method is compared with several passive and intelligent methods and it is confirmed that the proposed method has better performance and more accuracy. [ABSTRACT FROM AUTHOR]

This study presents a high‐performance photovoltaic (PV)‐battery hybrid power conversion system (HPCS) which is integrated to a microgrid using a quasi‐Z‐source inverter (qZSI). The battery system is directly connected to the intermediate dc link of the qZSI without any extra converter which reduces the size and the cost of the power electronic interface. A controller equipped with a harmonic compensator is designed to control the battery current in the grid‐connected mode and to regulate the microgrid voltage in the islanded mode in the presence of unbalanced and non‐linear loads. Furthermore, the proposed controller adjusts the shoot‐through duty cycle of the qZSI to regulate the PV voltage to its reference value provided through the maximum power point tracking algorithm. Moreover, the proposed controller provides the microgrid with the capability to transfer between the two operation modes. A new strategy is adopted to keep the HPCS operational in case of severe reduction in the available power of the PV modules and its consequent voltage drop. The performance of the proposed control system is evaluated for the control of a typical microgrid simulated in MATLAB/SIMULINK environment. [ABSTRACT FROM AUTHOR]

The continuous expansion of consumer‐driven installations of residential rooftop photovoltaic (PV) systems causes serious power quality, notable voltage variations and unbalance, which limit the number and capacity of the further connections. The latent reactive power capability of PV inverters can increase the network PV hosting capability. This study proposes both reactive power control and real power curtailment as a comprehensive inverter control strategy to improve the operating performance of unbalanced three‐phase four‐wire low voltage distribution networks with high penetrations of residential PV systems. A multi‐objective optimal power flow (OPF) problem that can simultaneously improve voltage magnitude and balance profiles while minimising network loss and generation costs is defined. The solution is found using the global solver based on Sequential Quadratic Programming algorithm with multiple starting points in Matlab. Detailed simulations are performed and analysed for typical operating scenarios over 24‐h period on a real three‐phase four‐wire unbalanced distribution network in Perth Solar City trial, Australia. Smart meter readings are used to justify the accuracy and validation of the network model and the proposed multi‐objective OPF model. [ABSTRACT FROM AUTHOR]