With the continuous increase of renewable energy generation and high voltage direct current (HVDC) power transmission, the system inertia is declining, thus resulting in greater risks of frequency instability. To solve this issue, a dc-link inertia control is firstly proposed for a point-to-point modular multilevel converter based HVDC (MMC-HVDC) link, which enables the MMC-HVDC system to provide emulated inertia support by employing the electrostatic energy stored in the dc capacitors. Moreover, in order to maintain reasonable submodule capacitor size as well as provide more emulated inertia support, a virtual capacitor control for MMC-HVDC connected an offshore wind farm (OWF) is proposed. With the virtual capacitor control, the OWF is able to provide a large capacity virtual capacitor in dc side of MMC-HVDC link and supply desired emulated inertia support for onshore power system by utilising the offshore wind turbines’ rotor rotational kinetic energy. Furthermore, a procedure for selecting the appropriate virtual capacitor control parameters is proposed. Finally, the performance of the proposed coordinated control is researched in PSCAD/EMTDC, and the simulation results show that the onshore power system inertia is effectively improved with the coordinated control.

Based on inherent dynamics similarity between synchronous generator (SG) and DC capacitor power port, this study proposes an improved synchronisation control method of grid-connected PV inverter based on DC capacitor voltage control where a DC-link capacitor voltage is regulated to provide the synchronisation angle. The proposed control method uses the estimated grid angle feedforward to improve the damping performance of the system under disturbances. To assess the robustness of the proposed control method, a two-step procedure is followed. Firstly, a reduced-order model of the system was derived. Secondly, the performance and stability of the proposed control method were fully investigated through the eigenvalue analysis. The offline simulations and real-time simulations are carried in PLECS software and PLECS RT Box, respectively in order to validate and demonstrate the effectiveness of the proposed control method. It has been found that the proposed control method yields a large band controller parameter setting while maintaining system stability. The results showed that the proposed method improved the operating capability and enhanced the performance of the system under disturbances and weak grid operation.

Considering that the low-voltage and small capacity ratings of the traditional distributed energy storage system (ESS) cannot meet the increasing capacity of distributed generations, this study describes a cascaded multilevel converter-based ESS. In addition, the virtual synchronous generator (VSG) as a promising solution to enhance the system's inertia is studied. However, some studies indicate that VSG suffers from the inherent resonance near synchronous frequency under weak grid conditions, which may result in power oscillation and even deteriorate the system stability. In this work, the resonance mechanism is investigated with an established wide-band small-signal dynamic model. It is revealed that the resonance poles caused by the small impedance ratio on the AC side introduce a phase lag jump of 180°, which decreases the phase margin of VSG, leading to synchronous power oscillation (SPO). The influence of control parameters on the power oscillation is investigated with the eigenvalue analysis method. Accordingly, a differential feedforward compensation-based SPO suppression strategy is proposed. The stability margins are significantly enhanced with this method. The validity of the SPO analysis and suppression method is verified with simulation and a 10 kW experimental prototype.

This work presents an improved phase-locked loop (IPLL)-based control for grid-integrated photovoltaic (PV) system (GIPVS). It is used to extract amplitude, frequency, and phase angle of distorted load currents to control PV-grid-interfaced voltage source converter (VSC). The VSC controls active and reactive powers at unbalances and distortions in the grid voltages. The control algorithm of VSC includes power feed-forward term, DC-link voltage, and grid voltages. The feed-forward term improves the dynamic performance of a GIPVS and enhances the utilisation of VSC. Three-phase IPLLs extract the in-phase components of three-phase load currents at utility frequency. Further, these in-phase components of load currents are used for estimation of reference-grid currents. The proposed IPLL-based control algorithm offers good dynamic response even under unbalanced load condition. The efficient working of IPLL-based control algorithm is realised by extracting maximum power from PV-grid system and through its power-quality improvement. The perturb and observe (P & O) control algorithm is used for optimum utilisation of a PV array. A developed prototype of PV-grid system is used to implement an IPLL-based control algorithm under different operating conditions.

This work proposes a design method for robust state feedback grid current controller applied to single-phase grid-connected photovoltaic (PV) systems. The proposed method achieved high system reliability even dealing with grid distorted voltages and other power quality issues. The robustness of the controller was verified in the face of parametric uncertainties at the first-order output filter, and also a satisfactory disturbance rejection performance was achieved. The proposed design is based on linear matrix inequalities associated with constraints involving optimisation and a pole placement analysis on the continuous-time domain. A discrete-time analysis is also presented in order to indicate the simplicity of its implementation by a microcontroller or similar devices. Based on the presented results and in order to validate the proposed controller, its robust performance was compared with two linear and well-known control methods: a classical PI controller, in terms of robustness; and a robust resonant controller, regarding the power quality at the connection point with the grid-utility. Although the conclusions were listed regarding a low voltage PV grid-interactive multilevel power converter, the proposed control design (and its concepts) can be applied to a great variety of renewable energy resources systems.

This study describes a current controller, based on a Lyapunov control law and a limit cycle oscillator (LCO) to provide stability into the grid-connected photovoltaic (PV) system during grid faults. The current controller also ensures an efficient current injection and active power regulation. The proposed controller offers a high degree of immunity and robustness against perturbation on the grid, due to the three-phase LCO – frequency locked loop (FLL) synchronisation technique. The LCO-FLL is used to compute the positive and negative sequence components of the grid, which are used by a Lyapunov control law in order to control the injected currents into the grid. Moreover, the inverter input voltage is considered in the controller design. The control configuration is proved by two different strategies, balanced injected currents and constant active power in the presence of an unbalanced grid voltage. Simulation and experimental test results are presented to demonstrate the proficiency and performance of the proposed technique in grid-connected PV systems.

Modular multilevel converters (MMCs) have the advantages of high-power density and small-harmonic distortion because of their modularity and flexibility, thus providing a new avenue for research into scalable superconducting magnetic energy storage (SMES) in renewable energy generation. This study presents coordinated control for a three-phase four-wire (3P4W) MMC-based SMES system under unbalanced voltage applications. First, the positive- and negative-sequence mathematical models and the port-controlled Hamiltonian with dissipation model of the 3P4W MMC-SMES system are established by introducing an additional path for zero-sequence current. Second, a multi-objective passivity-based control strategy that can effectively improve the power quality and system robustness and eliminate both double-frequency active and reactive power fluctuations or double-frequency active power fluctuation and negative-sequence current is proposed. The simulation results based on MATLAB/Simulink demonstrate the effectiveness of the proposed topology of the SMES and its control strategy.

Doubly fed induction generator (DFIG) driven wind turbines experience poorly damped stator flux oscillations at frequencies around the fundamental, limited low voltage ride through capability and output power fluctuation. In this study, a proposed controller to mitigate stator flux oscillations of DFIG is proposed during autonomous operation mode. In addition, super-capacitors based energy storage is integrated to manipulate wind speed vagaries and contribute to load demand. A 53rd-order small-signal analytical model and eigenvalues study is carried out to optimally determine control system parameters and pinpoint latent system dynamics and stability margins under various operation conditions. Also, a detailed non-linear system is modelled and simulated in Matlab/Simulink environment to assure small-signal model results. The results obtained confirm the accuracy of the analytical model and explore the significance of the proposed damping controller to mitigate stator flux oscillations even during contingencies.

Short-circuit faults and asymmetrical power production and consumption result in the appearance of unbalanced voltages. The change in voltage profiles could result in grid-connected converter's overcurrent protection triggering if power references are kept at the pre-disturbance level. This study provides a framework for current references calculation that can result in virtually any active and reactive power profiles that are within the domain of the converter's physical capabilities. This is accomplished by introducing three references calculation algorithms, which are all based on the same underlying concept. The first algorithm gives the grid operator the option to choose to prioritise the production of either active or reactive power and to curtail the power production only to the extent that converter's current ratings are not surpassed. Simultaneously, the oscillations in one of the powers are managed. The second approach also provides power production prioritisation feature, but here power production is realised through adjustable ratio between positive and negative current components. The third scheme is an enhancement of the previous two schemes and allows for the constant power factor to be produced before and during the disturbances. Distribution network model emulated in the hardware-in-the-loop environment was used for the validation of proposed solutions.

In this study, a control algorithm based on non-linear adaptive second-order Volterra filter (NLVF) is utilised to operate the voltage-source converter (VSC). The main role of VSC is to supply compensating current to improve the power quality in wind-based off-grid distributed power generation system. This off-grid system consists of three-phase induction generator, VSC and non-linear loads. The proposed control algorithm is adaptive as well as non-linear in nature. The characteristic of Volterra series is exploited to generate the reference source current for VSC operation. The NLVF estimates the active and reactive weights of fundamental load current for reference current generation. It mitigates the power quality problems created by disturbances in non-linear load or wind oscillations on off-grid system. The solution of power quality problems such as the presence of harmonics in supply current, load unbalance and reactive power compensation is obtained. Nevertheless, voltage and frequency control are also achieved. The VSC is used as a shunt compensator in the system. The entire off-grid system is designed and validated through the laboratory prototype development. The objective of the system is achieved and the steady state and dynamic performance is found satisfactory.

This study aims to propose optimised and robust current controller for the transformerless grid-integrated photovoltaic inverter. Challenge of ensuring improved power quality while minimising the DC-component under non-ideal grid condition is at the forefront in the photovoltaic system. In this context, an advanced current controller is designed and developed in the stationary reference frame with proportional-integral (PI) plus four-vector PI (4VPI) controllers. The PI-controller modulates the shape of fundamental current to follow its reference induced from outer DC-link loop. Meanwhile, 4VPI controllers are tuned at the multiple of grid frequencies including third-(), fifth-(), seventh-(), and ninth-() to introduce infinite gain and thereby alleviating stationary errors in the current loop of the photovoltaic inverter. The most distinguishing feature of the proposed controller is its inherent control robustness, and therefore, it operates satisfactorily against control delay time and frequency excursion. Traditionally, cumbersome trial-and-error techniques have been largely employed to adjust the PI-VPI controller gains. Nonetheless, tuning of these parameters has made many practitioners switch-off from employing the proposed controller. Given this, the particle swarm optimisation algorithm is proposed. Finally, the practicality of the proposed controller is verified through comprehensive set of simulation and experimental tests conducted on a laboratory-developed prototype using dSPACE-1104.

A parallel static var generator (SVG) system based on cascaded H-bridge topology in wind afflux station is analysed in this study to reveal the mechanism of harmonic current and DC voltage instability due to the phase difference between the PWM carriers. First, the inherent relation of harmonic current and low-frequency disturbance is revealed by carrier-phase-shift pulse-width-modulation (CPS-PWM) theory. Second, to identify the negative impact of low-frequency disturbance, a small-signal circuit model is built to research the electric coupling relation between SVGs and the grid, then pointing out the main factors which affect the harmonic circulation among the two-paralleled SVGs. Third, the output impedance and the transfer function of SVG give an insight into the interaction between SVG and low-frequency resonance characteristics, based on which a virtual resistance method is proposed to shape the resonant peak. Finally, theoretical analysis and system experiment prove that the proposed control strategy can rapidly attenuate low-frequency resonant peaks of the system, avoiding the DC voltage instability in the SVG system when harmonic circulation occurs.

Reactive power control loop (RPCL) in virtual synchronous generator (VSG) plays the role of excitation circuit in conventional SG. The control strategy improvement or parameters configuration method of RPCL is always the emphasis of analysis. However, the impacts of RPCL on VSG system performance are usually overlooked. On the basis of that, this study aims to investigate the effects of RPCL on power control performance. First, a simplified SG model is established and implemented to VSG. Then, VSG system modelling with state equations in power control time scale is elaborated. The excitation current in the modelling is regarded as one crucial state variable to identify the effects of the excitation circuit on VSG system. Eigenvalue locus analysis and eigenproperties analysis reveal how the excitation circuit affects power oscillation damping performance and even results in power coupling issue of power control. The effects of operating points on system power control are also taken into account. Finally, the experimental results are given to verify the theoretical analysis.

This research proposed the methods based on the neural network (NN) to build the digital twins (DT) of the inverter model. The proposed methods can be divided into two groups: firstly, an online tuner for the proportional–integral (PI) controller is formulated through backpropagation (BP) algorithm; and secondly an NN-based identifier is used to approximate the non-linear functional dynamics of the targeted control loop of the inverter. The design of PI tuner is based on the deviation between the output of the model and the reference output from measurement data. Then, according to the difference, the tuner can calculate the appropriate parameter of the current and voltage controller to track the dynamic behaviour of the reference model. The NN identifier is, however, to replicate the dynamic character of the reference model by NN which is initially trained offline with extensive test data and afterwards is applied to online tuning. In order to compare the advantages of the methods by NN with the traditional ones, the system identification and parameter estimation are also utilised to build the DT of inverter model. The performance of these methods will contribute to illustrating that NN identifier is effective in the building of DT.

Renewable energy-based energy conversion technologies have become more relevant due to environmental considerations even though they are intermittent in nature. As a result, the concept of microgrid and microgrid control techniques have been evolved as major areas of power system research. Among different inverter control methods, the droop-based control method is more popular in microgrid systems due to its simplicity and non-requirement of expensive communication systems. The transient performance, power-sharing accuracy and decoupling between real and reactive power are improved by modifying the natural droop control method. In this study, the selected microgrid system consists of two inverters operating in parallel, two interconnecting lines and three loads. A state-space model of the microgrid is created based on the small-signal stability and the transient response is improved by introducing virtual impedance and dynamic droop gains. The different controller parameters are optimised using particle swarm optimisation ensuring stability. Eigenvalue analysis is done to analyse stability. The analysis of the response of the system for various disturbances validates the effectiveness of the proposed controller. The strategy developed ensures improved power-sharing capability with high values of natural droop gains without compromising stability by using optimised dynamic droop gains.

The continued rise of renewable power generation requires the invention of smart grid technologies, among which communication and remote control are key. Multilevel control strategies improve grid stability and provide the necessary flexibility to address changing demand, contingency, and renewable generation variability. To test these strategies while avoiding damage to existing infrastructure or lab equipment, simulation is employed to facilitate large-scale control and protection applications. In this work, a hierarchical control strategy is tested in a real-time simulation environment implementing a moderately large microgrid with 100% renewable generation penetration, using both physical and software-based controllers, in both local- and remote-control configurations. Additionally, an adaptation is made to the existing hierarchical control strategy to accommodate energy storage balancing amongst distributed resources. The scalability of the testbed and adaptability of the control strategy are tested, and stable operation is observed.

This article presents dual-mode control of a single-stage utility interactive microgrid based on a photovoltaic array and battery energy storage with improved power quality. For protection reasons, grid tied solar inverters are mandatory to shut down at loss of the grid. However, proposed multitasking microgrid is developed to provide energy to critical loads, even at disappearance of grid. The microgrid is integrated the BES and a PV array at DC link of voltage source converter of it to provide the harvested energy to local loads. In islanding, the amplitude and waveform of load voltages are maintained sinusoidal by the voltage controller. The transfer of modes (islanded to grid connected and vice-versa) is implemented using the synchronisation controller as per the accessibility of the grid. The instantaneous characteristic-based adaptive filter is utilised to control the microgrid in the grid-integrated mode. Moreover, a PV feed-forward term is incorporated in current control for injection of active power to the utility as well as to improve the dynamic response of microgrid. The phase detection controller estimates phase, frequency of utility and load voltages accurately and capability of disturbance rejection is improved. The effectiveness of PV-BES-based microgrid is validated experimentally.

This study presents two proposed adaptive and intelligent control schemes for accurately adjusting the MG voltage and frequency in islanded mode and ensuring the seamless transition between islanded and grid-connected modes. The two proposed controllers are based on improving the performance of the droop control method depending on H-infinity and model predictive control methods. However, when the MG connected to the utility grid, the MG is related to the grid voltage and frequency and the active and reactive power transfer is adjusted using the P/Q control method. Firstly, this study presents a comprehensive review of the commonly used control schemes in MGs. Then, the structure of the two proposed controllers is described and the Simulink model is presented. The controller is applied to an MG test system to validate its capability to meet requirements. In addition, a comparison between the two proposed controllers is made based on the integral of square error (ISE), integral of absolute error (IAE), and integral time-weighted absolute error (ITAE). The results prove that the two proposed control schemes have the ability to adjust the MG in its two operating modes and also provide a seamless transfer between the islanded and grid-connected modes of MGs.

This study proposes a dual-stage cascaded operational control to attain a seamless transition of a standalone microgrid to a grid-connected microgrid system. In order to safeguard the microgrid's stability, the strategy incorporates a bi-directional converter. A dynamic reference frequency loop-based resynchronisation controller is proposed to control the operation of the bi-directional converter. The stage-I operates to maintain the converter end system parameters equal to the utility. This creates a pseudo-equalisation between the converter connected microgrid parameters and the utility. As a result, the simultaneously operating synchroniser controller switches the resynchronisation switch. The stage-II of the proposed strategy addresses the transition controller, which switches the microgrid's mode of operation from standalone mode to grid-connected mode. The grid connected bi-directional converter-based microgrid builds up its parameter equal to the utility. When microgrid builds up to a level that it satisfies the set values of the bypassing controller, it signals the bypassing switch (BS) to operate. In this study, the BS plays an essential role in bypassing the bi-directional converter. The test system designed to study the proposed strategy is simulated on the podium of Matlab 2015b/Simulink.

This study considers the hierarchical control of microgrid and harmonic compensation of non-linear load specially in transition from grid-connected to unplanned islanding. The proposed controllers are based on the stationary reference frame with hierarchical control levels (inner, primary, and secondary). In the inner control loop, proportional and multi-resonant voltage and current controllers for three-phase voltage source inverters are proposed. Moreover, a droop control is used to share active and reactive power in the primary control loop. To achieve decoupled active and reactive power sharing, virtual impedance loop is implemented, too. The mentioned procedure increases the total impedance and consequently reduces the initial current and subsequently facilitates the continuous connection of DGs to the MG. To attain smoother transition, selective harmonic virtual impedance is applied by multiple second-order generalised integrator module to extract the fundamental positive sequence and harmonic components of the output current of the DGs. In secondary control level, synchronisation procedure is considered to make connection and restoration by the advanced three-phase synchronous reference frame PLL with in-loop moving average filter which significantly enhances its disturbance rejection capability. The results are shown smoother transition, robustness, and high performance of the proposed hierarchical control VSIs during variation of system configuration.

Along with the increasing demand of large power transfer and integration of renewable energy sources, voltage source converter based multi-terminal high-voltage DC (VSC-MTDC) technology has represented both challenges and opportunities. However, the DC voltage of the droop-controlled VSC station may exceed the DC voltage limits when facing large disturbances, thereby losing the DC voltage control and power sharing capabilities. To tackle this problem, a DC voltage deviation-dependent voltage droop control method (VDM) is proposed to change the droop coefficients adaptively when large disturbances occur, not only retaining the advantages of the fixed VDM in the steady state, but also preventing frequent adjustments of the droop coefficients in the transient state. The basic analytical model of droop-controlled VSC is analysed first, and then the generalised linear model of VSC-MTDC is derived based on the restrict requirements of DC voltage deviation. The DC voltage deviation-dependent VDM is derived from the generalised linear model of the VDC-MTDC. Case studies and simulations of a ±320 kV VSC-MTDC system are conducted in PSCAD/EMTDC software to validate the effectiveness of the proposed DC voltage deviation-dependent VDM.

Power oscillation damping (POD) is one of the ancillary services expected from high-voltage direct current (HVDC) converters. When providing POD to the ac side, converters draw power from the dc side, which can cause distortion to the dc voltage especially in the case of limited dc capacitance. In meshed/multi-terminal HVDC grids, where dc voltage regulation is distributed using a dc voltage droop control strategy, the distortion due to the POD controller action is propagated to other connected ac grids because of the droop action. This propagation can be reduced by using the inherent energy storage capability of the modular multilevel converter (MMC), which is a common converter topology for HVDC. Different methods to utilise the energy stored in the MMC for the purpose of POD have been proposed in the literature. This study presents a detailed analysis and experimental validation of one of these methods, referred to as virtual capacitance support, which increases the effective dc grid capacitance by using the stored energy of multiple MMCs connected in the same grid. The experiments, which were carried out using power hardware in the loop setup, demonstrated the effectiveness of the method.

This study presents a flexible model and testing arrangement that can be used to mimic the fault infeed characteristics of power electronics converters and evaluate the performance of transmission protection schemes to faults when the system is ‘converter-dominated’. That is sources (e.g. renewables) and infeeds (e.g. HVDC interconnectors) that are interfaced to the main power system via converters. Actual protection relays are injected, in real-time, using the outputs of the various fault simulations. A range of potential protection issues, including slow tripping, erroneous discrimination and non-operation, are illustrated for particular scenarios. The main contributions of the study include knowledge of how different protection schemes may be affected by converter-interfaced sources and guidance on possible solutions to the observed problems. Two solution options are highlighted: changes to the fault detection methods used within relays, and/or defining the fault-response elements of future grid codes (and therefore future converter fault responses) to ensure that existing protection schemes will not be adversely affected in future low-carbon, converter-dominated systems.

From the electrical point of view, the concept of smart community (SC) was defined as a distributed system consisting of a set of smart homes, distributed energy resources (DERs) and energy storage systems (ESSs) using SC controllers to enable smart power management. In this context, the SC energy management system (SCEMS) acts as aggregator of these resources, aiming to assure benefits for every SC stakeholder by setting the SC operation. The references given by the SCEMS must be accurately tracked by the energy routers (ERs), intended as one of the key components of the SC, acting as smart interface between the utility grid and the prosumers’ DER and ESS. This study proposes a flexible, robust and simple control strategy for a single-phase ER. The ER regulates the active and reactive powers in grid-connected (GC) mode, and the voltage and frequency when operating in stand-alone (SA) mode. A seamless transition between SA and GC is demonstrated, avoiding undesired transients. The design and implementation of the proposed control strategy is progressively explained. Finally, this is tested via simulation (in PSCAD/EMTDC software) and verified with the experimental prototype.

This study presents a Laguerre-based stochastic economic predictive functional control (L-SEPFC) method for real-time power dispatch of balance responsible parties (BRPs). In this regard, at the control layer, an L-SEPFC method operates on the energy time scale and optimises the expected economic performance of the BRP. Calculated set points in this layer are average power injection of controllable generators, as well as known disturbance of prediction wind generation. The novel L-SEPFC scheme is able to cope with the wind uncertainty occurring in the real-time operation of BRPs. Furthermore, the proposed algorithm is able to take into account the technical constraints of generators and transmission line constraints of the power network. Due to the parameterisation of the future control signal based on the Laguerre function, the proposed method is faster than the nominal predictive algorithms. Simulations on an IEEE 118-bus test system with proposed method shows improvements inability to track reference production, economic performance, imbalance energy and computational burden with respect to nominal predictive controllers.

When several microgrids are interconnected into microgrid cluster (MGC), the network topology of MGC system would be more complex. Traditional hierarchical control based on droop control tends to make full use of all networks so that the costs of energy exchange could be high. Considering the voltage stability and power sharing of MGC system, a distributed cooperative control method based on network topology optimisation for MGC is proposed to control the power flow of multi-microgrid system. Specially, preliminary Q-leaning algorithm is utilised with establishment of new adjacency matrices for the distributed cooperative control to decrease the costs. Moreover, multi-level controls based on consensus protocol are proposed in the distributed cooperative MGC system to eliminate the necessity of a central controller. The adaptive regulation based on the state of charge among the battery groups will be taken as the first-level control. The second-level control including the current controller, voltage controller and power controller is implemented among the DGs and a novel third-level control based on consensus is applied in eliminating the voltage deviation caused by the first-level control and the second-level control. The effectiveness of the proposed method is verified through the simulation by simulink.