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A cluster with multiple AC or DC microgrids (MGs) based on flexible DC interlinking and distributed energy storage systems is studied in this paper, which has been identified to have the following issues in terms of its primary control scheme: coordinated power control among slack terminals in the MG cluster, seamless operation mode transition for the interlinking converters and the common DC bus voltage control. A novel flexible control scheme that unifies the control of all the MGs only using local measured signals without further communications is proposed in this paper, which aims to improve control stability, and operation reliability and flexibility. Theoretical introduction and analysis are presented to explain the proposed control. The detailed average value model and small‐signal model of an MG cluster with two AC MGs and two DC MGs are simulated, respectively, with the proposed control in power systems computer‐aided design/electro‐magnetic transient design and control to verify the effectiveness in control stability and flexibility enhancement, and the impact of control parameters on the small‐signal stability of the considered cluster is analysed through root locus. [ABSTRACT FROM AUTHOR]

Uncertainities and variations of power generation through photovoltaic (PV) sources are major challenges for their integration with the distribution grid. Voltage rise and voltage drop issues limit the increase in PV penetration and loading levels, respectively. Moreover, it is important to maintain voltage levels as per grid code while ensuring that the PV power generation is not curtailed. In this paper, a voltage control method using smart transformer (ST) via dynamic optimal setpoints and limit tolerance is proposed in a residential distribution network. Performance indicators are developed to understand the impact of the proposed method on the system voltage profile. A method to determine the optimal inputs for voltage control methods depending on the day-ahead predictions of load demand and PV power profiles is developed. Further, a voltage control method of switching among three setpoints based on the voltage that determines the load side ac reference voltages for ST is proposed. The proposed method is compared with the method of switching between two setpoints based on current. The proposed method provides an improved voltage profile in the distribution network, which is tested on a CIGRE low voltage residential distribution network using PSCAD [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]

This study proposes an approach to evaluate a practical margin for photovoltaic (PV) generation hosting capacity (HC) of low voltage distribution networks. This HC is determined considering the randomness of the connection points and is supposed to be the maximum value of PV penetration up to which the utility can authorise any interconnection without performing additional case studies. Smart inverter control strategies and battery storage systems are used to avoid costly network expansion solutions. The simulations are performed using actual solar radiation data and residential demand profiles. The results show an increase in the network HC, bringing benefits by deferring network investments such as conductors and asset upgrades. [ABSTRACT FROM AUTHOR]

With the proliferation of intermittent renewable energy sources, power systems need to withstand an increasing number of disturbances that may affect system frequency. In this paper, we focus on the effects of high penetration level of wind turbine‐generators (WTG) on the power system operational planning from the frequency point of view. Specifically, an exact formulation for the minimum frequency calculation is presented to increase accuracy and speed of analyses. Then, the effect of WTG's frequency response on the power system frequency stability is investigated to compute the maximum acceptable generation outage as a function of penetration level. Finally, as an application to power system operational planning, the obtained frequency dynamics of the power system and the wind turbine frequency response are integrated into the unit commitment problem. Simulations are performed on the IEEE 39‐bus test system to study the effects of the WTGs on the frequency stability of the system. Results illustrate a more secure and reliable operation of the power system when the wind turbines participate in the frequency regulation task. Moreover, including the frequency response of the wind turbines in the unit commitment problem can reduce the ancillary cost, meanwhile increase the frequency stability of the system. [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]

The controlled islanding problem is typically considered only for pure ac power systems, with the ultimate objective of either minimising power imbalance or power-flow among islands. However, as ac/dc hybrid power systems are becoming popular worldwide, current controlled islanding schemes should be redesigned to take into account the presence of high-voltage dc (HVDC) links, possibly connecting different islands. Accordingly, in this study, the authors solve the classic islanding problem combining HVDC power modulation with the conventional ac cutset search. The flexibility of the HVDC allows the authors' strategy to jointly capture the previously mentioned objectives, and provides even lower power imbalances than the minimum imbalance strategy, at a relatively small cost of increasing power-flow impact. Then, the optimisation problem is formulated as a mixed-integer linear programming problem, which can be conveniently solved with existing commercial solvers. Finally, they validate their proposed strategy in two case studies, corresponding to a modified IEEE-118 system and to the China Southern Power Grid system. In both cases, dynamic simulations show that their proposed approach outperforms classic algorithms where the presence of HVDC power modulation is not explicitly taken into account. [ABSTRACT FROM AUTHOR]

The increasing penetration of distributed generation (DG) inverters supplying power to the networks bring challenges to the power system under the grid faulty and harmonic grid since negative sequence components lead to double oscillations. Reference current generation (RCG)‐based flexible control strategies ensure solutions to mitigate oscillations on the control signals and to maintain a more stable, secure and reliable power system operation. In this direction, this study proposes an RCG‐based optimal control strategy that reduces oscillations of active and reactive power at optimal control parameter values and ensures minimum harmonic contents at the grid side under an unbalanced and harmonic distortion grid. The main cause of the power oscillations is comprehensively demonstrated, optimised and examined with numerical studies and theoretically. Control parameters are accurately selected by the courtesies of optimisation function to minimise power oscillations. A fast and robust third‐order sinusoidal signal integrator (TOSSI) extracts positive–negative sequence components for the calculation of the RCG. To provide a fast transient response and achieve almost current tracking errors at a zero level, proportional complex integral (PCI) current regulation controller is selected. Various case studies have been performed to show the validity and availability of the proposed entire system, including flexible control, optimisation, TOSSI and PCI. [ABSTRACT FROM AUTHOR]

The electronically interfaced photovoltaic (PV) system and its control approach form an intrinsic part of the distributed generation (DG) system. This study proposes pseudo affine projection (PAP)‐assisted multitasking control scheme for grid‐integrated DG inverter under input grid voltage disturbances. The PAP approach is based on the convergence of active weighted values and is independent of the input signal property, which manifests a discrete characteristic. Additionally, it exhibits a unique capability of updating weight based on multiple, delayed input signal vectors. The fundamental distinguishing feature of the proposed PAP is its intrinsic learning behaviour and therefore it provides an optimal solution to the ingrained paradox between steady‐state misadjustment and convergence speed compared to its conventional counterpart affine projection. The PAP‐assisted algorithm practically enables DG inverter to perform multiple tasks including suppression of source current harmonics, unbalancing, and neutral current. It also provides load reactive current support with unity power factor operation while transferring the maximum active power into the utility grid. The practicality of the proposed PAP algorithm is investigated through MATLAB/Simulink software. Finally, the experimental results recorded with the dSPACE‐1104 based real‐time control platform show good to excellent agreement with simulations in the steady‐state, as well as transient phenomena. [ABSTRACT FROM AUTHOR]

The increasing penetration of photovoltaic (PV) energy in power grids will impose system instability issues, especially in the occurrence of faults. However, very limited research has been conducted on the low‐voltage ride‐through (LVRT) control of PV systems in the low‐voltage distribution networks (LVDNs) with predominantly resistive line impedances. To fulfil this remaining gap, the effects of active current injection (ACI) on the grid voltage support in LVDN is mathematically analysed in this study. Subsequently, a novel LVRT control scheme for the PV grid‐forming inverter is proposed, where the control distinguishes itself from other existing methods due to its optimisation of ACI and PV energy harvesting with the premise of system safety and specified reactive current injection as per grid codes. Multi‐mode control modes are involved in the proposed method for dealing with different environmental conditions and voltage dips. Meanwhile, the DC‐link voltage is adaptively operated in a self‐adjustable control structure for improving grid resilience. The effectiveness of the proposed control method is verified by simulations in MATALB Simulink and hardware experiments on a PV microinverter. Compared with the traditional LVRT control, the post‐fault power recovery and voltage support capability can be significantly improved. [ABSTRACT FROM AUTHOR]

In this study, a proportionate power sharing among the parallel inverters operating in an islanded microgrid is achieved using droop and virtual inductance control. The same droop coefficients are used to achieve the frequency regulation as well. The frequency changes which are inevitable in droop control are measured and used to emulate the behaviour of damping and inertia to the DC link voltage using hybrid energy storage system consisting of battery and supercapacitor units. The proposed DC link voltage regulator restores the DC link voltage quickly by providing power corresponding to the rate of change of frequency and frequency deviation. This reduces the impact of voltage variations on the DC‐load and keeps modulation index within the linear range for voltage source inverter. The design aspects of DC link voltage regulator, damping and inertia constants, selection of battery and supercapacitor units based on rating of the DC link voltage are discussed. The proposed decentralised droop control and DC link voltage restoration methods are validated through detailed simulation and experimental studies. [ABSTRACT FROM AUTHOR]

For low‐voltage autonomous microgrids, the essential goal is maintaining stable voltage and frequency throughout its network for the smooth operation of distributed loads. The other goal is to maintain proportionate power‐sharing between multiple sources connected through voltage source converters (VSCs). For the particular case of a heavily loaded microgrid, any further increase in demand poses new problems. The VSCs proximate to the loads may overshoot beyond its maximum ratings. Owing to which the stability of the power supply, as well as the longevity of VSCs, may get compromised. Also, these sources provide power under varying composite loading conditions comprising both linear and non‐linear loads resulting in unbalancing in its network. To overcome these problems, a decentralised fast terminal sliding mode control strategy for active power‐sharing between parallel VSCs has been proposed. A detailed mathematical model based on Lyapunov's stability theory‐based study is designed to establish the operating stability of the proposed controller. To validate the proposed control strategy, MATLAB‐based simulation results are provided for possible loading scenarios. A comparative study for performance evaluation between the proposed controller and a conventional proportional ‐integral‐based controller is further carried out. Finally, the performance of the proposed controller is also validated in real‐time scenario. [ABSTRACT FROM AUTHOR]

Apart from the basic renewable generation function, the multi-functional grid-connected inverter (MFGCI) derived from traditional grid-connected inverter could provide ancillary power quality enhancement functions for the interfaced grid. However, with multiple functions integrated into one control system, the control scheme of MFGCI would become inevitably more complicated and vulnerable to the disturbance in distribution networks. Here, a control scheme of MFGCI for rejecting both the steady-state and dynamic-state disturbance is designed. The proposed control scheme includes an inner loop controller and an improved reference current generation algorithm, which constitutes an overall MFGCI control scheme. To reject the steady-state disturbance which would distort the output current of MFGCI, the inner loop controller by combining the linear adaptive disturbance rejection controller (LADRC) and repetitive controller (RC) is designed. While for the dynamic-state disturbance which would amplify the output current of MFGCI or even destroy the stability of the system, an improved reference current generation algorithm with enhanced dynamic disturbance rejection capability is proposed. The proposed MFGCI could provide overall improvements in both dynamic- and steady-state disturbance rejection capability. Finally, some experiments are conducted to verify the superiority of the proposed MFGCI. [ABSTRACT FROM AUTHOR]

In this study, nested backward/forward sweep (NBFS) algorithm, is proposed to solve the load flow problem of islanded microgrids. The proposed algorithm is designed in such a way that the voltage magnitude of the angle reference bus and the system frequency become additional variables in the power flow analysis. In the absence of the slack bus, new loops and equations are introduced in the proposed backward/forward sweep (BFS) method to fulfil the requirement of droop characteristics in the power flow analysis. In each loop of the proposed algorithm, modified equations of BFS are employed to update the voltages of every bus except angle reference bus (slack bus). The voltage of angle reference bus and the system frequency are updated at the end of each loop operation. The proposed NBFS method is applied on several test systems including IEEE 22‐bus, 38‐bus, 69‐bus, and 160‐bus and the obtained results are compared with the result obtained from other BFS‐based algorithms including direct BFS and modified BFS, Jacobian‐based algorithms including Newton trust region (NTR) algorithm and modified Newton ‐Raphson (MNR) algorithm, and time‐domain simulation in PSCAD/EMTDC. The proposed algorithm for droop‐regulated AC microgrid exhibits faster convergence with better accuracy as compare to other algorithms. [ABSTRACT FROM AUTHOR]

A microgrid is a well known solution to increase the technical and economical capability of distributed generations. Recently, DC microgrids, as small‐scale DC networks, have attracted more attention due to lower losses and simple control structure. Stability of DC microgrids can be an important issue under high penetration of constant power loads (CPLs). In this study, stability analysis of the DC microgrid system including hybrid wind/battery and CPLs is studied, and then three different types of stabilising compensators are presented in two groups to increase the stability margin and to stabilise the system under high penetration of CPLs. The proposed stabilising compensators modify the voltage reference, current reference and duty cycle by adding active damping signals in the external, middle and internal parts of the interfaced converter control loops. These active damping signals are obtained by the feedback of the output voltage and current and modify the DC microgrid control structure. To evaluate the system stability under different stabilising compensators, small signal and frequency response analyses are done with and without active damping signals. Simulation results based on a detailed DC microgrid model are given to verify the effectiveness of the proposed compensators. [ABSTRACT FROM AUTHOR]

Micro-grid can be used as a supplement to power supply of large power grids. Combined with smart park, users can easily manage the micro-grid. In the tourist scenic area, the local distributed new energy is mainly used to provide energy for the system. The micro-grid control system as the core of the system controls the optimal operation of the entire smart park. In order to ensure the efficient operation of the entire system, the energy management system is needed for smart control and automatic scheduling. In the grid-connected mode, the main-grid complements the system energy and charges the park energy storage device. In the island mode, the distributed energy is fully used to provide energy for the smart park system. The energy power fluctuates greatly, and the energy management system needs to schedule the micro-grid energy, energy storage system, electric vehicle load and so on, and power quality needs to be monitored. Use of micro-grid control system in the smart parking deploying photovoltaic power generation, wind power generation, charging and exchanging devices and other devices, real-time monitoring of the park of cars, charging cars automatically complete charging, automatic processing of visitor cars into and out of smart park, and improve work efficiency. [ABSTRACT FROM AUTHOR]

The distribution network connected with photovoltaic (PV) power generation may show high voltage under strong light and low voltage under weak light. The influence of distributed PV generation on the grid voltage profile is analysed first, and then, the sensitivity of the grid voltage to the PV inverter output power is deduced. Aiming at overhead line distribution network, the local voltage regulation strategy based on the power control of the grid-connected PV inverter is proposed. The PV inverter works under the maximum power point tracking (MPPT) mode with unity power factor when the grid voltage is at the normal level. However, when the voltage is higher than its upper limit, the grid-connected PV inverter will be co-ordinately controlled to output limited active power and necessary reactive power to restrain the line voltage. Also, when the voltage is below than the lower limit, only the reactive power of the grid-connected PV inverter was controlled to sustain the line voltage, and the reactive power is no more than the residual capacity of the grid-connected PV inverter operated in the MPPT mode. Simulation and results show that the voltage quality of the distribution network with PV generation is improved by using the voltage regulation strategy presented in this study. [ABSTRACT FROM AUTHOR]

The presence of distributed generation (DG) in a power distribution network can cause difficulties in managing the voltage, power flow and fault level as the network was designed for supplying passive loads. This study focuses on the control of a back-to-back (B2B) voltage source converter operated as a synchronverter and a soft open point (SOP) between adjacent feeders to mitigate the voltage fluctuation caused by DG and to increase the reliability of the supply without increasing the fault level. Better reliability is achieved as the B2B converter can continue to supply the loads when a loss of mains occurs on one side. The proposed control of the two DC–AC converters, which also needs to regulate the DC link voltage and adapts the synchronverter algorithm to the new application requirements, so that events will cause minimum disruption. Simulation shows that the B2B-synchronverter is capable of regulating the load voltage under normal mains connected condition, and also limiting the fault current and supplying the loads within its rating during islanding, providing seamless transition to the islanded mode. [ABSTRACT FROM AUTHOR]

Increasing penetration of photovoltaic (PV) generation may significantly impact existing voltage regulation schemes. In particular, the lack of coordination between control devices and fast change in PV generation can cause voltage magnitude to go beyond limits. This study addresses the issue by providing a novel real‐time centralised voltage control algorithm to tackle voltage fluctuations caused by variability of PV power generation. The new method considers autonomous response of upstream voltage regulation devices and provides optimised set‐points for the chosen downstream controllers. Furthermore, the investigated method was tested in the real‐time digital simulator on a model of 11 kV grid located in a semi‐rural area of Queensland, Australia. The control algorithm was designed in MATLAB with a multidimensional, non‐linear, mixed‐integer optimisation. The parallel computing implementation of the KNITRO optimiser helped to provide quicker and more accurate response than the existing uncoordinated control schemes. As a result, optimised control coordination leads to a substantial reduction in voltage fluctuations and decrease in regulation time. Moreover, the proposed method detects and eliminates unnecessary control operations. Thus, it can also prevent voltage hunting in distribution networks. [ABSTRACT FROM AUTHOR]

The reactive power coordinated optimisation problem for distribution network (DN) with the integration of renewable distributed generation (DG) is investigated in this study. By actively participating in reactive power control together with an under‐load tap changer and shunt capacitors, DG can operate more effectively in the DN. Non‐dispatchable DG is considered in the coordinated optimisation method. The objective of the proposed reactive power coordinated optimisation method is to minimise active power loss and to decrease the number of switching device operations while maintaining the grid voltage within the allowable range. The optimisation problem is to solve a variable combination including integers and continuous variables, which will formulate a mixed integer non‐linear programming (MINLP) problem. The control variable includes operations of traditional reactive power control devices and the reactive power adjustments of DGs. To solve the MINLP problem, a heuristic searching method, an improved harmony search algorithm is proposed with adaptive parameters process. Numerical tests on a standard 10‐kV distribution system and a real distribution system demonstrate the applicability of the proposed reactive power coordinated optimisation method. [ABSTRACT FROM AUTHOR]

The high‐voltage multi‐terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage‐source converter‐HVDC grid‐side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two‐layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low‐level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid. [ABSTRACT FROM AUTHOR]