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Abstract This paper proposes a coordinated control strategy to enhance the low/high‐voltage ride‐through (L/HVRT) capability of grid‐tied micro‐grids (MGs). The novel control scheme, which is based on grid‐following concepts, contains two layers for each power source of MG. The former, known as the primary layer, has reverse‐droop and non‐linear robust sliding mode control. Furthermore, the secondary layer detects voltage perturbations based on L/HVRT curves. Then, the reactive power required for supporting the main grid is determined through the dynamic‐voltage‐support requirement of the grid codes. Finally, coordinated reactive power‐sharing among the sources is performed based on the presented method and free capacity of the power sources. Several case studies are carried out to assess the effectiveness of the proposed control scheme, which points out that the aims of this study, such as compliance with the grid codes, sharing reactive power among the sources, tracking the reactive power precisely and feeding local loads, are achieved. The superiority of the presented strategy against its grid‐forming counterparts is demonstrated in comparison tests for the studied grid‐tied MG. Simulations are performed in an offline MATLAB/Simulink environment and then validated by experimental real‐time results.

Abstract A fault current compensation technique is proposed in this paper for resonant grounded power distribution systems in bushfire prone areas. Arc suppression devices with residual current compensation inverters are used to compensate fault currents due to single line‐to‐ground faults in order to mitigate powerline bushfires. The main contribution of this paper is the design of a compensation technique for the T‐type residual current compensation inverter using a non‐singular terminal sliding mode control scheme. The main objective of the proposed scheme is to reduce the fault current and bring its value to a level so that it cannot ignite fires. The proposed controller is designed based on the selection of a sliding surface in a way the singularity problem can be avoided and chattering effects in existing sliding mode controllers can be eliminated. The desired current injection through the residual current compensation inverter is ensured by enforcing the control law into the terminal sliding surface where the control law is determined by satisfying the Lyapunov stability criteria. The performance of the non‐singular terminal sliding mode controller is compared with an integral sliding mode controller by considering different values of fault currents where these values are varied by changing fault resistances. Results for simulation in the software and processor‐in‐loop simulations are verified against operational standards which are essential for mitigating powerline bushfires. This work focuses to design a non‐singular terminal sliding mode controller for the residual current compensation inverter which is used in an arc suppression device to compensate both active and reactive components of the fault current and keeps its value below 0.5 A within 2 s after activating the residual current compensation inverter which is a requirement as per the operational standard. This controller is designed based on the selection of a terminal sliding surface while satisfying the condition for avoiding the singularity problem.

Abstract The constant growth of distributed energy resource (DER) in the electric power system (EPS) is a reality on account of some aspects, for example, technical, financial, environmental. The insertion of DERs can be positive, providing ancillary services, for example, harmonic mitigation, or negative, causing overvoltage, depending on the proper control and the characteristics of the grid. This paper proposes a pq‐based control strategy for distributed generation, wherein the studied system is composed by a photovoltaic DER and a three‐phase converter for processing the generated power and providing ancillary services (ASs) to a three‐wire EPS. The power control is based on the power balance between the grid and the converter. Besides supplying active and reactive power, the proposed strategy can provide ASs by controlling the grid currents from a pq transformation. Some advantages over other solution can be mentioned, such as reduced sensor number and instantaneous control response. In addition, neither additional harmonic‐extraction method nor a phase‐locked loop is required. To validate this study and theoretical considerations, simulation results are presented and evaluated. From tests in a downscaled laboratory setup, experimental results are also presented to verify the effectiveness of the proposed control strategy.

Abstract Frequency droop control is a simple and effective frequency control method. However, it is not appropriate as a primary frequency control for microgrids with energy storage systems and voltage source converters (VSCs) owing to their slow response speed. In addition, it is difficult to determine the grid balance only with frequency deviation. In this paper, a new converter‐fed microgrid primary frequency control method based on the estimated active power imbalance for fast response and less variability in the transmission system is proposed. The active power imbalance is estimated via frequency deviation, frequency drop speed and the modified swing equation. The estimated imbalance indicates the status of the power imbalance more quickly and accurately than the frequency deviation. The proposed method does not require communication devices, responds quickly to frequency drops, and prevents unnecessary operation of the grid‐connected VSC because it uses the estimated imbalance instead of the frequency or angle for droop control. Simulation results show that the proposed method is significantly faster than frequency droop control methods, improves the rate of change of frequency and frequency nadir and reduces variability in the transmission system.

Abstract This paper studies the electromechanical transient modelling techniques of the modified modular multilevel converter (MMC), named active MMC, which is equipped with embedded energy storage in submodules. Firstly, the mathematical model of the active MMC and its equivalent circuits at the AC and DC sides are established. Then, the control scheme of active MMC that focus on the cooperation of the MMC converter and the energy storage submodules is illustrated. The proposed electromechanical transient model are implemented on PSS/E and compared with the electromagnetic transient model on PSCAD in a two terminal active MMC sytem; the results of the active MMC system under AC and DC fault prove the validity of the proposed model. Lastly, stability studies of the practical system are carried out, and the simulation results prove the improvement by the application of the active MMC on the rotor angle stability and frequency stability.

Abstract An islanded microgrid is cyber‐physical system, and the control relies on the communication system significantly. Improper parameters of the cyber system can result in instability of a microgrid system. To evaluate the impact of a networked control system on control performance, a cyber model is developed to represent data acquisition periods and communication delays. Simplification of the networked control system model is proposed to enhance the computational performance, making the analytical method applicable for large‐scale systems. Based on the analysis, a two‐dimensional stability region of a microgrid in the space of cyber parameters can be obtained. To validate the proposed method, a microgrid control scheme is proposed for power dispatch and regulation based on the droop and proportional‐integral (PI) feedback control. The analytical method is compared to the time‐domain simulation, and it is shown that the stability regions are nearly identical. The critical values of cyber parameters are determined based on the analytical results. The proposed control strategy with the given cyber parameters is validated for transient stability following dynamic events. Simulation results indicate that the design of a microgrid as a cyber‐physical system needs to be guided by critical values for cyber parameters to prevent system instability.

Abstract One of the crucial challenges of the distribution network is the unintentionally isolated section of electricity from the power network, called unintentional islanding. Unintentional islanding detection is severed when the local generation is equal to or closely matches the load requirement. In this paper, both ensemble learning and canonical methods are implemented for the islanding detection technique of synchronous machine‐based distributed generation. The ensemble learning models for this study are random forest (RF) and Ada boost, while the canonical methods are multi‐layer perceptron (MLP), decision tree (DT), and support vector machine (SVM). The training and testing parameters for this technique are the total harmonic distortion (THD) of both current and voltage signals. THD is the most important parameter of power quality monitoring under islanding scenarios. The parameter and data extraction from the test system is executed in a MATLAB/Simulink environment, whereas the training and testing of the presented techniques are implemented in Python. Performance indices such as accuracy, precision, recall, and F 1 score are used for evaluation, and both ensemble learning models and canonical models demonstrate good performance. Ada‐boost shows the highest accuracy among all the five models with original data, while RF is robust and gives the best results with noisy data (20 and 30 dB) because of its ensemble nature.

Abstract Fault analysis is an essential prerequisite for fault detection, location and isolation. A detailed fault analysis method on the steady state of the pole‐to‐pole fault for voltage source converter (VSC) based DC systems is presented in this paper. According to fault characteristics under different transition resistances (TRs), the steady‐state response is divided into three scenarios: low transition resistance with blocked converter (LTRBC), medium transition resistance with blocked converter (MTRBC) and high transition resistance with unblocked converter (HTRUC). When TR is relatively low, fault currents through insulated gate bipolar transistors (IGBTs) are large and the converter is blocked. After an in‐depth study of freewheel diodes’ conduction states, the iterative method is utilized to solve the steady state under blocked converter. When TR is relatively high, fault currents through IGBTs are not large enough to block the converter. With the unblocked converter, control strategy and pulse width modulation (PWM) are investigated to obtain steady‐state fault currents. Finally, a reasonable simulation model was built in PSCAD/EMTDC software. The simulation results verified the fault analysis accuracy, with much less calculation error than that of the conventional method. Besides, the proposed analysis method was proved applicable to fault scenarios under different TRs.

Abstract Online experimental systems that include remote and virtual experiments based on the Internet has become a new form of experiment in engineering education. This paper proposes a virtual experiment for the complex process control system. It is based on the networked control system laboratory (NCSLab), which provides a reliable framework for virtual thermal experiment. Taking the combustion system of 1000 MW ultra‐supercritical coal‐fired once‐through boiler‐turbine units as an example, the framework and the implementation process of the online system are given in this paper. A new presentation form for complex processes control experiment is provided, which could be a solution for simulating physical experimental equipment in virtual environments. The potential danger in practical experiments could be avoided. Additionally, the real‐time operation for complex control systems could be presented in a 3D scene. Meanwhile, the flexibility and efficiency could be increased in engineering experimental education. Moreover, using the experiment evaluation method, the references and feedback for teaching and assessment are provided in this paper.

Abstract This paper proposes a vehicle‐to‐grid (V2G) operation methodology of electric vehicles (EVs) for frequency regulation, EV charging, and state‐of‐charge (SOC) management. This V2G operation methodology exhibits multifunctionality in regard to instantaneous power adjustment of EVs for primary frequency control during charging. The proposed method can stabilize grid frequency under the high penetration of renewable energy by simultaneously managing the desired state‐of‐charge (SOC) of the batteries. The results indicate that both charging and charged EVs can assist in frequency regulation so that V2G services can help embracing renewable energy sources and stabilize a power system. The feasibility of this method is verified by a practical power grid model to demonstrate the efficacy of EV charging control for SOC management using DIgSILENT/PowerFactory.

Abstract Development of advanced grid‐synchronization technique for unbalanced and distorted grid is considered in this paper. In this context, self‐tuning filter (STF) is considered to extract the fundamental component from the measured unbalanced and distorted voltages. Standard STF considers balanced grid voltages which is not always possible to ensure in the actual power grid. To mitigate this issue, an extended STF is proposed by analyzing the standard STF in the state‐space framework. To make the proposed ESTF grid‐following, a robust open‐loop frequency estimator is also applied. The closed‐loop system enjoys excellent filtering benefit from the extended STF side while taking advantage of the fast convergence speed property of the open‐loop frequency estimation technique. Numerical simulation and experimental results with double second‐order generalized integrator phase‐locked loop are provided to validate the theoretical developments.

Abstract In this paper, a multi‐objective optimization technique is proposed for the planning of a networked microgrid based on peer‐to‐grid (P2G) and peer‐to‐peer (P2P) energy trading schemes. Two different criteria's including annual profit and energy index of reliability are taken into consideration to form a multi‐objective function. The networked microgrid consists of three individual microgrids containing their own combinations of generation resources, batteries and residential loads. All microgrids are connected together and also to the main grid to meet the energy exchange requirements of P2P energy trading. A cooperative game theory technique based on a particle swarm optimization algorithm is used to model the networked microgrid, and to find the suitable sizes of the players that simultaneously maximize the payoff values of both objective functions. Besides, a comparative analysis is carried out for both P2G and P2P energy trading schemes. The results show that the outcomes are maximum when both criteria are considered in the optimization and P2P energy trading is carried out. The sensitivity analysis is performed on the selected parameters and verified the right change 0.003% and 4.5% in discount rate and electricity prices, respectively.

Abstract Enlarging the speed range of wind turbines is an effective way to increase power production in low‐frequency band wind which contains more energy and is very challenging for DFIG‐based wind energy conversion system (WECS). The analysis shows that current controllability of the converter is the main factor limiting bidirectional speed range extension (BSRE). This problem can be solved by increasing dc‐bus voltage, but a constant and high dc‐bus voltage will increase the electrical stress and switching losses of the converter. This paper proposed a dynamic minimum dc‐bus voltage (DMDCV) technique in which the dc‐bus voltage can automatically adjust to enable the back‐to‐back voltage source converter to work in linear modulation mode at its minimum value, thus supporting BSRE while also achieving multiple goals including low electrical stress and low switching losses. Two implementation methods of DMDCV with and without modulation depth loop are proposed and discussed. The co‐simulation results verified the feasibility of DMDCV and the advantages of BSRE in improving power generation.

Abstract A novel agent‐based online prediction method is presented in this paper, to predict the status of both transient and short‐term voltage (STV) stability, against fault occurrence. In the proposed method, the trajectories of the relative frequency deviation (ΔF) and the power imbalance (ΔP) are estimated by employing the third‐degree polynomial curve fitting method. By tracking the estimated trajectories on ΔF–ΔP plane and checking some simple defined rules, an early prediction of both transient and STV instability is achieved in an organized multi‐agent system (MAS). In order to evaluate the performance of the proposed algorithm, the method has been tested on IEEE 39‐bus system, IEEE 118‐bus system and IEEE Nordic test system. Based on the obtained results, the proposed algorithm has an overall accuracy of 99.5% under symmetrical and asymmetrical faults, PMU measurement error, different operating points and topological changes.

Abstract With the large‐scale increasing demand for passenger transportation, the alternating current (AC) 25 kV traction power supply system (TPSS) of electrified railway is introduced into the construction of urban rail transit in China instead of the direct current (DC) TPSS. In order to suppress the negative sequence current and cancel the neutral section at the exit of traction substation of the AC TPSS, a novel AC integrated power supply system (IPSS) of the urban rail transit based on single‐phase static var generator (SVG) is proposed, which the capacity of SVGs can be further reduced. The topology of the novel AC IPSS is introduced. By defining the constraint factors of negative sequence current and reactive power compensated, the comprehensive compensation model of negative sequence current and reactive power of the novel system is established based on the corresponding standards, which the full compensation and satisfactory compensation can be achieved. The method of determining the compensation scheme and the control strategy are proposed. Taking the actual data as an example, the effectiveness of the compensation scheme is proved. In addition, the correctness of the control strategy proposed is proved by simulation and experiment.

Abstract This paper proposes a decentralized robust dispatch method for multi‐area AC/DC system considering wind power uncertainty. The accommodated wind power interval model is adopted to handle the inherent uncertainty and volatility of wind generation, which is modelled based on the affinely adjustable robust optimization method. The accommodated wind power interval and the automatic generation control participation factors are optimized simultaneously to accommodate more wind energy and ensure operational reliability of the entire multi‐area system. The flexible adjustment model of HVDC converter station is also presented to accommodate more wind power. Then, the resulting economic dispatch model for multi‐area system is solved in a fully decentralized and parallel manner using accelerated alternating direction method of multipliers algorithm to preserve the information privacy and dispatch independence of each region. Case studies illustrate the effectiveness of the proposed model.

Abstract Multi‐tap and multi‐capacitors type arc suppression coils are commonly used in distribution systems. Due to their low compensation accuracy and limited arc suppression capability, it is hardly to avoid personal safety accidents and wildfires absolutely. In this paper, a novel single‐phase grounding fault voltage full compensation topology based on antiphase transformer is proposed, which is composed by an antiphase transformer, a phase‐selection switch and a multi‐tap single‐phase voltage regulator. The proposed topology can control the neutral point voltage, thus cause the fault voltage being zero. By this way, the lack of arc suppression coil is overcome. The novel compensation topology features low costs, high precision of fault voltage compensation. The principle of the novel compensation topology is explained. The mathematical expressions of residual voltage and current are derived in detail. Finally, the simulation model and experimental platform were built. The results show that the proposed novel compensation topology can effectively make up for the shortcomings of traditional arc suppression coils when it is used with them.

Abstract This paper discusses how frequency‐stable operational decision‐making of asynchronous grids with intensive high‐capacity HVDC tie‐lines, can be achieved leveraging a novel total transfer capability (TTC) evaluation approach. This problem is formulated as a hierarchical optimization model with multiple decision‐makers, in which the master problem is solved by the coordinator to determine the base case maximum acceptable transmission power of each HVDC tie‐line, while the sending‐ and receiving‐end grid's local TSO each autonomously solves a frequency constrained optimal power flow sub‐problem. Coordinated fast and slow frequency arrest strategies are fully integrated into the TTC assessment framework, which not only ensure that last‐resort protections are not triggered following the HVDC bi‐pole block contingency, but also take advantage of the potential control capability of the sending‐ and receiving‐end grids and would therefore increase the TTC of HVDC tie‐lines. A decentralized algorithm based on accelerated analytical target cascading is developed to solve the formulated model. Case studies on a modified two‐area RTS‐96 system and a practical large‐scale system demonstrate the effectiveness of the proposed approach.

Abstract Recently, a new type of subsynchronous oscillation (SSO) has occurred in doubly‐fed induction generator (DFIG)‐based wind farms. This oscillation is also called the subsynchronous control interaction (SSCI), which is mainly caused by the interaction between the transmission series compensation and the DFIG controller. Therefore, the oscillation frequency of the SSCI is susceptibly impacted by DFIG operating conditions, which presents major challenges to conventional mitigation strategies. To solve this problem, a grid‐connected series‐compensated DFIG‐based wind farm is established, and the formation process of the SSCI is analysed. The strategy of active disturbance rejection control (ADRC), which can automatically estimate and compensate for the total disturbance of the system in real‐time, is introduced into the rotor‐side converter to mitigate the SSCI in the DFIG. ADRC is designed based on dynamic parameter tuning and chaos optimization. Finally, a detailed model‐based time‐domain simulation is conducted to evaluate the performance of the proposed optimized ADRC (O‐ADRC) compared to conventional virtual impedance and ADRC. The simulation results show that O‐ADRC can effectively mitigate the SSCI under different operating conditions and provide sufficient damping for the DFIG‐based wind farms.

Abstract As an essential apparatus in resonance grounding systems, the arc suppression coil is widely used in rejecting the single‐line‐to‐ground (SLG) fault of distribution networks. However, the resistive ground‐fault current cannot be compensated by an arc suppression coil (ASC). As a novel active arc suppression device, the hybrid flexible arc suppression device (HFASD) composed of ASC and flexible arc suppression device (FASD) can eliminate the capacitive component of ground‐fault current and the resistive component of ground‐fault current. After the SLG fault occurs, the FASD from stand‐alone mode to grid‐connected mode may lead to switching transients, affecting the stability of distribution networks. The soft grid connection (SGC) strategy can achieve low or without transient response during the grid synchronization of FASD to the grid. Thus, the HFASD based on SGC strategy and segmented proportional‐integral‐differential (SPID) and second‐order generalized integrator phase‐locked loop (SOGI‐PLL) control method is proposed for rejecting the current and voltage overshoot responses during the process of grid synchronization of FASD in this paper. Simulation and experimental results verify the effectiveness of the proposed method, and the benefit of the proposed method is indicated in comparison with the non‐SGC (NSGC) strategy.

Abstract The poor damping of bidirectional dc (bi‐dc) converter caused by constant power load makes power system prone to oscillation, and non‐minimum phase characteristic also jeopardises voltage stability. To solve these challenges, the interconnection and damping assignment passivity‐based control (IDA‐PBC) is utilised to improve transient response. The influences of the right‐half‐plane (RHP) zero on the stability margin and controller design are illustrated by zero dynamics analysis. Then the port‐controlled Hamiltonian modelling is used to obtain the IDA‐PBC control law, which is suitable to the bi‐dc converter and independent of the operation mode. The system dissipation property is modified, and thus the desired damping is injected to smooth the transient voltage. To remove the voltage error caused by RHP zero and adjust the damping ratio, an energy controller with an adjustment factor is introduced. Besides, a virtual circuit is established to explain the physical meaning of the control parameter, and the parameter design method is given. Passivity analysis assesses the controller performance. Simulation results are analysed and compared with other control strategies to test the proposed IDA‐PBC strategy.

Abstract An inertia identification approach has been proposed for diesel generators by using the modified load rejection procedures. Large frequency deviation problems often occur in the Kinmen island power system where many diesel generators are used. In order to avoid erroneous evaluation of frequency deviation caused by generator tripping, one of the critical issues in system planning is to accurately estimate the maximum frequency deviation during system outage events. The main factors that influence the frequency deviation include the generator inertia (H), damping constant (D), and governor controller parameters. In order to improve the accuracy of the transient stability analysis to obtain more actual frequency deviation, it is critical to have more accurate inertia values of these diesel generators. This paper proposes a useful approach to evaluate reasonable inertia values. With the identified inertia values of diesel generators, the frequency responses of the Kinmen power system obtained from PSS/E simulation are more consistent with the field measurement results.

Abstract The present paper devises a second‐order system frequency response (SFR) model for power systems integrating virtual synchronous variable speed wind turbines (VSWTs). The inertial response model of full converter VSWTs, in which grid‐side converters are controlled using a virtual synchronous generator concept, is developed. The derived model is incorporated into the traditional SFR model deployed to estimate the frequency behaviour of the synchronous generators following infeed loss contingencies. The proposed SFR model formulates interaction between the VSWTs rotor speed reduction and the system frequency drop. The system loads inertia is also included in the SFR model. Additionally, an analytical approach to derive the suggested SFR model for the large power system including VSWTs with different capacities and operating points is presented. Solving the proposed model gives the analytical functions for estimating deviations in the system frequency as well as VSWTs rotor speed variations. Finally, the accuracy of the devised SFR model is verified through the time‐domain simulations of a modified IEEE 39‐bus system. Particularly, impact of the VSWTs virtual inertia on the efficiency of the proposed SFR model is studied.

Abstract Low voltage ride through (LVRT) requires wind generation systems (WGS) to maintain continuous operation and provide reactive power support under grid voltage dips. This paper proposes a novel disturbance attenuation control (DAC) approach based on state‐dependent Riccati equation (SDRE) technique to enhance the LVRT capability of doubly fed induction generator‐based (DFIG‐based) WGS. The DAC problems are formulated with the control objectives for rotor side converter and grid side converter, and the weighing matrices are designed with fully studied principles to balance the control effect and cost. The SDRE technique is adopted to solve the DAC problems, and an alternative feasible state dependent coefficient construction algorithm is applied to improve computational efficiency. An active Crowbar circuit with overcurrent limiting mechanism is applied to ensure the rotor current and DC link voltage within the secure zone. Comparisons with conventional PI controller, exact linearization controller and coordinated control strategy are performed, the results demonstrate the proposed DAC approach has a better transient performance and enhances the LVRT capability of DFIG‐based WGS.

Abstract In the islanded microgrid, it is necessary to distribute the harmonic powers correctly among generators and mitigate the voltage harmonics at the point of common coupling (PCC) for effective microgrid operation. These control objectives are hard to satisfy at the same time with only one virtual impedance component. To simultaneously improve the microgrid voltage quality and share the distortion powers, a consensus‐based distributed power‐sharing scheme is proposed along with PCC harmonics mitigation. In the proposed control scheme, the controllable parts of output impedance at the fundamental and harmonic frequencies are separately tuned by simple integral controllers without depending on feeder impedance parameters. Compared to conventional control schemes, the proposed method is less complicated while ensuring a high voltage quality despite sudden changes in load power. A small‐signal state‐space model of the microgrid is analysed to validate the stability of the proposed control scheme, and its effectiveness is verified by experiments with a scaled‐down microgrid system.

Abstract In the transactive energy (TE) paradigm, the devices of participative consumers, or prosumers, may be aggregated and employed to drive operational objectives at the network level. Home heating, ventilation and air‐conditioning (HVAC) systems in particular are well‐suited to modulate their behaviours based on both home thermal flexibility and requests from the utility grid. This paper develops a robust, hierarchical power dispatch scheme in the context of a residential distribution network. The formulation couples a unique, multiphase linear distribution optimal power flow (OPF) at the upper level with model predictive control (MPC)‐based HVAC fleet controllers at the lower level. The proposed approach is tested on nearly 2000 homes with a three‐phase distribution network in an intraday market setting, where two major applications are explored and analysed.

Abstract Multi‐vendor interoperable HVDC grid protection is key to build large‐scale HVDC grids in a step‐by‐step manner. On the one hand, various protection strategies and technologies have been developed in the past decade to address the challenges associated with HVDC grid protection. On the other hand, state‐of‐the‐art HVDC technologies are often vendor‐specific and there is a general lack of standardisation on HVDC systems as the majority of existing HVDC systems have been built by single vendors as turn‐key projects. In recent years, driven by the need to develop multi‐vendor HVDC grids, both national and international standardisation bodies have been working on guidelines and pre‐standardisation for HVDC systems. This paper provides a comprehensive review of the recent progress on HVDC grid protection focusing on multi‐vendor interoperability and identifies the main challenges to achieve interoperable HVDC grid protection. Compared to AC system protection, the fundamental differences of multi‐vendor interoperability in HVDC grid protection are the possible co‐existence of multiple protection philosophies and the extended scope of the fault clearing process in terms of protection and control. The challenges and research needs related to multi‐vendor HVDC grid protection due to these fundamental differences are identified.

Abstract Classic droop control ensures the synchronization of distributed generation (DG) units inside a microgrid without requiring any deployment of communication links, however it causes unwanted frequency fluctuations and degrades system dynamics. Angle droop control and V−I control have been developed as two major global positioning system (GPS)‐based control methods, both of which realize a fixed‐frequency operation of the microgrid through synchronizing DG units with GPS timing technology and brings improvement to the dynamic response of the overall system. This paper reveals the similarities and the correlations between these two methods that they can both be regarded as different forms of virtual impedance control. A novel adaptive virtual impedance control method is proposed accordingly to generalize GPS‐based control strategies into a unified control frame. An impedance and inner controller design approach considering both stability constraints and power quality requirements based on the small‐signal model of GPS‐based microgrids is presented for practical implementation. An adaptive transient resistance concept is adopted to enhance the system stability during large disturbances and grid faults. Case studies are presented to validate the system performance and fault ride‐through abilities of the proposed control scheme.

Abstract This paper presents a sequential network‐flow graph‐based method for a steady‐state power flow solution in hybrid multi‐terminal power systems. The proposed method is a unique and novel one, which differs from other established methods that involve the use of modified versions of classical power flow methods. The proposed method formulates a power flow problem as a maximum network‐flow problem and solves it using a push‐relabel max‐flow algorithm. The solution procedure solves AC and DC parts sequentially, while accounting for voltage source converter losses using a generalised converter model. The proposed flow‐Augmenting method solves the power flow problem using matrix vector multiplication in its most abstract form, and it is independent of system parameters and network configuration. The proposed formulation is computationally efficient, as it is based on matrix vector multiplication, and is scalable, because the formulation works as a graph‐based method, which, inherently, allows for parallel computation for added computational speed. Further, unlike previously reported methods, the proposed method does not rely on Jacobian matrix formulation or any matrix inversion. This proves to be a strong advantage for the proposed method, as a significant reduction in computational time is observed, as a result. The proposed method is validated on 5‐bus hybrid system and CIGRE B4 DC system.

Abstract Hybrid HVDC consisting of line commutated converter (LCC) on rectifier side and modular multilevel converter (MMC) on inverter side is the favourable future trend of HVDC projects utilising the advantages of both converter technologies. However, fault clearing and adaptive restart capabilities of such hybrid HVDC are not much investigated in the literature. In this paper fault clearing of hybrid HVDC transmission line is investigated using mechanical DC breaker at MMC terminal and forced firing angle retardation at LCC terminal. Then, the characteristics of the line voltages are analysed during the fault blocking states of converters to clear the transient fault. Characteristics of the line voltages of LLC rectifier have shown prominent difference under permanent and temporary fault states, and also dependent on the severity of fault. Therefore, based on line voltage characteristics, a restart strategy is proposed for hybrid HVDC which is adaptive to the fault characteristics. Several PSCAD/EMTDC based simulation studies are conducted to validate the proposed fault recovery strategy. Simulation results validate the effectiveness of the proposed strategy for hybrid HVDC under DC line faults.

Abstract This paper investigates coordinated design of multiple wide‐area damping controllers (WADCs) for damping multiple inter‐area oscillations considering communication delays in multiple control loop. Two approaches have been investigated: the sequential approach which uses the stability criteria for single delay and design the WADC independently, and the coordinated approach which employs a less conservative and fast Lyapunov stability criterion for linear system with multiple delays to calculate delay margins simultaneously and coordinately design the multiple WADCs. The sequential approach cannot reveal the interaction among several WADCs loops with multiple delays and may lead to inaccurate delay margins, while the coordinated approach takes into account interactions of delays between different WADC control loops and can result in accurate delay stable region and relationship between delay margins and the WADC gains. Gains of WADCs are determined based on a trade‐off between damping performance and delay margin. Case studies are based on the modified IEEE 10‐machine 39‐bus power system equipped with two FACTS devices and two WADCs. The feasibility and effectiveness of the proposed approach is verified by simulation studies.

Abstract To enable reliable power delivery through transmission tie‐lines, total transfer capability (TTC) must be calculated and regulated to accommodate the transferred amount. However, the traditional optimal power flow (OPF)‐based total transfer capability calculation is computationally expensive for efficient total transfer capability control due to the inclusion of a large set of differential‐algebraic equations (DAEs) to verify transient stability constraints. In order to enable practicable total transfer capability regulation, a novel risk‐aware deep learning‐assisted paradigm is proposed here. First, a deep belief network (DBN) is employed to establish the total transfer capability predictor and surrogate the computation‐intensive differential‐algebraic equations in original optimal power flow formulas, simplifying the high‐dimensional and intractable constraints deep belief networks without loss of nonlinearity. Particularly, in order to be aware of control risk from the predictive error of the deep belief networks, prediction intervals (PIs) are produced improved by using ensemble learning and used to disclose the probability of insufficient actions, further guaranteeing the sufficient and cost‐effective control by compromising the tradeoff between cost and risk. Symbiotic organisms search (SOS) is then applied to solve the proposed risk‐aware deep belief network‐assisted total transfer capability control problem globally. The numerical studies testify that the proposed method enables economical, reliable, and full nonlinearity‐retained dynamic total transfer capability regulation control within a risk‐free surrogate‐assisted and tractable physical model‐driven hybrid framework.

Abstract The motivation of this paper is to introduce a new Hebbian least mean square control algorithm for two level self‐supported voltage source converter based distributed static compensator. The Hebbian least mean square controller is employed on the various aspects such as, weight updating, direct and quadrature unit voltage template, active and reactive part of reference supply currents and switching signal generation. Firstly, a set up equations using Hebbian least mean square are premeditated to compute both active and reactive weight updating. Secondly, DC and AC proportional integral controller outputs are involved with the updating weight to generate the reference supply current. The error signal obtained from both reference and actual source current is processed through the hysteresis current controller for the gate signal firing. The MATLAB/Simulink results as well as experimental results prove that the proposed algorithm is competent to afford the better voltage regulation, voltage balancing, source current harmonic reduction and power factor correction. Finally, performance obtained from the proposed controller reveals the applicability for various loading condition in the distribution grid.

Abstract Frequency stability is important for microgrid with renewable energy. However, the deterioration of frequency and the increased energy storage equipment are caused by source‐load uncertainty. Therefore, a frequency coordinated control strategy based on sliding mode method for a microgrid with hybrid energy storage system (HESS) is proposed. First of all, the detailed frequency regulation is designed, which divides deviation of frequency and area control error into different components as power reference value of different power supply. Secondly, the power threshold of HESS, which is consisted of ultra‐capacitor and battery, is set by designed fuzzy controller to reduce reserve power of HESS and avoid unreasonable power output. Thirdly, a load frequency control model with HESS is built and a sliding mode control is designed by using the detailed frequency regulation. Finally, the effectiveness of the proposed frequency coordinated control strategy is verified through comparison of different cases.

Abstract A novel online system‐identification‐based control design for damping inter‐area oscillations in power systems using supplementary wide‐area excitation control is presented. The identification is based on a partly reduced‐order and a partly full‐order model of the grid. Assuming the grid to be divided into coherent areas with phasor measurement units located across the grid, the coherent states of the generators are averaged while the non‐coherent states, such as the internal states of each power system stabilizer, are not. A hybrid state vector consisting of these averaged and non‐averaged states is computed, and used for identifying a hybrid linear time‐invariant state‐space model. A linear quadratic regulator based on this hybrid model is, thereafter, designed. The reduced‐order part of the model is found to save significant amount of online time for both learning and control, while the full‐order part serves to enhance closed‐loop stability. N4SID with randomized singular value decomposition (rSVD) is used for making the identification loop fast. The reduced‐dimensional controller is finally implemented using a broadcast control strategy. The design is extended to non‐linear models of power systems using Carleman bilinearization. Results for linear and bilinear control designs are validated using the IEEE 68‐bus power system model.

Abstract The optimal controlled islanding of power systems is a practical solution to prevent system blackouts if the boundary of islands and corrective control actions in each island are carefully specified. This paper establishes a model for the controlled islanding problem by a proposed mixed integer linear program (MILP). The frequency‐arresting (FA) and frequency‐stabilising (FS) constraints are linearised and incorporated in our FA‐FS‐constrained MILP model to prevent triggering of load shedding (LS) relays. This achievement makes our model capable of handling low‐inertia networks. Intentional LS and stepwise generation curtailment are corrective actions accommodated for frequency control and power mismatch considerations. These corrective actions increase the degree of freedom and broaden the feasible space of the MILP model under the envisaged tough operating conditions. Our model's computational efficiency is improved using a proposed graph theory‐based network reduction technique. The basic groups of coherent generators, determined in the offline mode, are aggregated as equivalent buses using an extended Steiner tree method. A graph‐path determination technique is also proposed to generate disconnection constraints (between equivalent buses of incoherent areas) and bus‐allocation constraints. Simulation results on the IEEE 39‐bus test system and a 76‐bus case study verify the proposed network reduction technique's effectiveness and the MILP model.

Abstract As the grid codes of wind turbines note, doubly fed induction generator (DFIG)‐based wind energy conversion system (WECS) should maintain uninterrupted operation during certain voltage faults. To meet the codes, a novel configuration of DFIG‐based WECS is proposed in which a superconducting magnetic energy storage (SMES) and a series transformer are incorporated. The control strategies of the WECS are redesigned to smooth the output power fluctuations and ride through different levels of voltage faults. The grid‐side converter (GSC) is controlled to smooth power fluctuations caused by random wind speeds under normal conditions. The GSC and protection circuit are controlled to switch into the series compensation mode to fully compensate the stator voltage during voltage swells and slight voltage sags. While under severe voltage sags, the protection circuit operates in the current limiting mode, which significantly limits the stator and rotor fault overcurrent, improves the stator voltage and supresses fluctuations of the electromagnetic torque. The different faults are applied, the simulation results demonstrate that the presented scheme can effectively smooth output power fluctuations and strengthen the voltage fault ride‐through capability (FRT) of WECS.

Abstract Various disturbances in voltage source converter based high‐voltage, multi‐terminal DC grids will significantly cause DC voltage deviations when the classical voltage droop control is adopted. The unsatisfactory voltage deviations weaken the voltage regulation ability and increase the risk of instability. Two flexible controls at the top of the classical voltage droop control to reduce voltage deviations are proposed, and the power–voltage (P–V) curves of the classical voltage droop control are translated co‐ordinately. Firstly, a global DC voltage oscillation damping control designed based on the Lyapunov theory is proposed for transient voltage deviations. Secondly, a constant weighted voltage control is designed for reducing steady weighted voltage deviations. The steady active power injections keep almost unchanged under proposed controls, consistent with those under only the classical voltage droop control. The stability influence and the power flow impact are carried out to evaluate the proposed controls theoretically. Tests are conducted on a multi‐terminal DC grid with five‐terminal voltage source converters. Simulation results show the proposed controls' effectiveness and superiority under various fault conditions, including both the outage of a constant power voltage source converter and a droop voltage source converter.

Abstract An accurate inter‐circuit fault location algorithm for double‐circuit transmission lines compensated by a generalized unified power flow controller based on phasor analysis is presented. The proposed algorithm utilizes synchronous currents and voltages of the buses on both sides of the line to accurately calculate the fault location. Using the equations of sequence component circuits, the algorithm calculates voltage and current of the fault point on both circuits. Then, using boundary conditions equations for 98 types of inter‐circuit fault on the double‐circuit transmission lines, the accurate location of the fault is obtained. The number of boundary conditions equations varies depending on the type of faulty phases of the two circuits and the number of unknowns in the current and voltage equations of the fault point. The calculation of these boundary equations are also described. The results presented in the simulation section verify the validity and accurate performance of the proposed algorithm under different fault conditions.

Abstract In view of the growing load demand in developing countries, overloading in existing transmission lines is a major concern. In addition to the voltage drop due to overloading, the proliferation of non‐linear loads in recent years distort the shape of sinusoidal voltage. Stable transmission of power through a transmission line under such condition is a challenge. In the presence of harmonics, control of transmittable power using static synchronous series compensator needs additional efforts, where increased power demand can be fulfilled by taking into consideration various other parameters. If an attempt is made to increase transmittable power through a transmission line using SSSC, power loss, the magnitude of voltage and its shape deteriorate in the presence of non‐linear loads. Thus, the trade‐off between transmittable power, transmission loss and voltage quality is tackled by optimal control of the compensator. In this study, a novel optimal control of SSSC is proposed with the objective of minimising transmission loss by maintaining the voltage within the limits while enhancing the transmittable power under non‐sinusoidal conditions. A two‐machine model of power system with 400 kV transmission line with parameters as per standards issued by the Central Electricity Authority is considered for validation of the algorithm using MATLAB platform and results are presented.

Abstract This paper presents a non‐unit protection method for long transmission lines of multi‐terminal HVDC (MTDC) grids based on the magnitude and direction of modal voltage and current travelling waves. The method overcomes the difficulty resulting from high attenuation and distortion as reported in previous methods using fault‐generated transients for protection of long HVDC lines. The proposed method determines the faulted pole using multi‐resolution morphological gradient of the pole voltage. Then, the multiplication of multi‐resolution morphological gradient of the modal travelling waves is utilised as a criterion to discriminate between internal and external faults and non‐fault events. The criterion is a negative high value for internal faults, a negative low value for forward external faults and events, and a positive value for backward external faults and events. A four‐terminal HVDC grid with modular multilevel converters including long HVDC lines is modelled using the PSCAD/EMTDC software. The simulation study validates effectiveness of the method for detection and discrimination of high resistance faults on long lines, as well as stability against external DC and AC faults, and DC circuit breaker opening and reclosing transients. The proposed method uses a relatively low sampling frequency and local data, i.e. no requirement for a communication link.

Abstract The main remedial actions to prevent voltage instability problems are load shedding and rescheduling of generating units. In AC/DC networks, the presence of high voltage direct current (HVDC) links equipped with voltage source converter (VSC) provides an efficient active and reactive power support. As the voltage stability is a local phenomenon, the rescheduling of an HVDC link could change the arrangement of critical buses and the stability margin of the entire power system as a remedial action. Considering this problem, a new optimization algorithm is proposed to provide the desired voltage stability margin in interconnected AC/DC networks taking into account the post contingency conditions and minimizing the cost of remedial actions. The regulation of the converter operating point is presented as a novel remedial action. The well‐known loading margin (LM) index is used as the voltage stability criterion and different VSC‐HVDC structures, i.e. single‐infeed, multi‐infeed and multi‐terminal HVDC systems are considered. The effectiveness of the proposed algorithm is evaluated using an AC/DC system comprising the IEEE 118‐bus and New England 39‐bus test systems. The results show that the proposed algorithm could provide the desired value of LM with the minimum value of load shedding and the control cost.

Abstract This paper proposes a power sharing control scheme for power electronics interfaced distributed generation to attain accurate harmonic power dispatch as well as unbalanced power sharing. The proposed controller is composed of two main blocks: 1) a harmonic power sharing unit, which leads to an accurate harmonic power sharing, and 2) individual triple‐droop control units separately applied to the control unit of each phase, which properly provide unbalanced power sharing. For this purpose, a virtual harmonic conductance concept is developed to control the inherent impedance of the inverter under non‐linear load conditions. As a result, the fundamental power sharing performance as well as the harmonic power dispatch accuracy is ensured. Simulation results from an islanded microgrid prototype using the CIGRE low‐voltage distribution network are provided to validate the effectiveness of the proposed method.

Abstract PV plant power excursions can have adverse implications on grid frequency. This phenomenon is observable due to inherently uncertain cloud transients across a local PV plant. Hence, provision of decision‐based controllers for centralized power inverters becomes imperative for supporting local grid operations. Such controllers can be improved to better counteract minutes‐based PV power deviations from its stable equilibrium. Thus, grid frequency deviations require further investigation at PV plant point of interconnection to the grid. In this research, single and spatially distributed utility‐scale PV plants operation is studied on a real‐time power system simulator, under fast‐changing meteorological conditions at different PV site loading levels (PPV−ref). Software‐in‐the‐loop Monte Carlo simulation is conducted and an empirical approach is proposed for characterizing minutes‐based variations in grid frequency originating from PV plant operation, that is, power fluctuations at different PPV−ref. The power−frequency curve obtained at the PV site can be incorporated in form of an empirical frequency droop function in characteristics curve of adjacent auxiliary power source(s). A prominent feature of this adaptive frequency droop is that it considers PV site loading levels during different hours, giving it leverage over common practice constant droop(s). A hardware‐in‐the‐loop platform is presented allowing field derivation of adaptive frequency droop curves using hardware PMU time‐series data analytics.

Abstract In the isolated wind‐diesel hybrid power system, the system voltage and frequency may be affected simultaneously by the fluctuated wind turbine output power and variable load, which can make the system crash in extreme cases without control strategy. Therefore, in order to reduce the voltage and frequency deviation caused by system power fluctuation, both the battery and super twisting algorithm sliding mode state observer (STATCOM) are equipped to adjust the active power and reactive power for load power demand in the isolated small‐scale power system. Then, the system mathematical model is established which considers the cross‐coupling effect exist in the small‐scale power system. In order to improve the control effect of battery and STATCOM, the unified control strategy is designed for battery and STATCOM by using the super twisting sliding mode state observer and the adaptive sliding mode controller according to the constructed cross‐coupling effect model. Furthermore, the parameters of controller are further optimized by using the fuzzy method. Finally, the simulation experiments are performed by the real‐time digital simulator to validate the effectiveness of the proposed control strategy under different operation points.

Abstract Traditional permanent magnet synchronous generator (PMSG)‐based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG‐based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid‐side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC‐link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC‐link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small‐signal analysis of the proposed scheme indicates the DC‐link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non‐linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid‐forming control.

Abstract A new adaptive scheme is proposed in this paper to design excitation controllers for feedback linearized models of synchronous generators in multimachine power systems in order to ensure the stability during large disturbances. The proposed scheme uses speed deviations of synchronous generators, readily available measured physical properties of multimachine power systems, to make all generators within a power network as partially linearized as well as to provide more damping. An adaptive scheme is then used to estimate all unknown parameters which appear in the partial feedback linearizing excitation controllers in order to avoid parameter sensitivities of existing feedback linearization techniques. The overall stability of multimachine power systems is ensured through the excitation control and parameter adaptation laws. The Lyapunov stability theory is used to theoretically analyse the stability of multimachine power systems with the proposed scheme. Simulation studies are presented to evaluate the performance of the proposed excitation control scheme for two different test systems by different operating conditions including short‐circuit faults on key locations along with variations in parameters for a large duration. Furthermore, comparative results are presented to highlight the superiority of the proposed adaptive partial feedback linearizing excitation control scheme over an existing partial feedback linearizing excitation controllers.

Abstract It is distinguished that there should be a balance between power generation and load demand, thereby maintaining the frequency and tie‐line power of the multi‐source multi‐area interconnected power system (MS‐MA‐IPS) in a determined limit to safeguard the entire system from failure. Hence, load frequency control (LFC) by adjusting the megawatt output of generators is applied, which has been considered as the most recent research work in this field. Under these circumstances, this paper intends to construct a dual‐mode‐switch‐controller‐based LFC in a multi‐area power system, which obviously considers the impact of incremental control action along with the system dynamic constraints such as capacitive energy storage, generation rate constraint, and governor with dead band. In order to achieve this effect, in this research work, the operations of the proposed controller of the MS‐MA‐IPS are based on the dual‐mode switch, and here, switching is carried out with respect to a threshold value ξ. Based on switching, the control varies between proportional–integral (PI) control and model‐predictive control. Moreover, in order to make the performance elegant, the proportional gain of the PI controller Kpg and the threshold of the switch ξ are optimally tuned by introducing a novel optimisation algorithm referred to as particle updated dragonfly algorithm, which is the conceptual hybridisation of the traditional dragonfly algorithm and particle swarm optimisation. Finally, the performance of the proposed model is evaluated by varying the control parameters such as Cg, TCD, Tr, and Trb to minimize the undesired deviations in power flows between control areas.

Abstract Cascaded H‐bridge inverter (CHBI) is widely used in the industry application for its outstanding features. However, CHBI needs a large number of isolated DC sources. If some of the DC sources are out of work, the DC voltage cannot maintain the desired value and even decreases to zero, thus degrading the quality of the output voltage of the CHBI. Thus, a soft‐switching modulation strategy based on the carrier phase shift SPWM is proposed in this paper to cope with the DC source failure. The strategy is proposed to achieve two goals: 1) one is to adjust the DC voltage of faulty cells back to the normal; 2) the other one is to make the pulse width modulation (PWM) pulses of each cell change without cell voltage level‐skip. The correction and efficiency are verified by a prototype of three‐cell CHBI built in the laboratory.

Abstract The current paper presents a multi‐objective manta ray foraging algorithm (MO‐MRFA) for efficient operation of hybrid AC and multi‐terminal direct current (MTDC) power grids. The multi‐objective framework aims at achieving economical, technical and environmental benefits by minimising the total production fuel costs, minimising the transmission power losses and minimising the environmental emissions in the AC/MTDC transmission systems. The MRFA imitates three separate independent foraging organisations of the manta rays. It is updated incorporating an additional Pareto archive to preserve the non‐dominated solutions. A dynamic adaptation of the fitness feature is employed by iteratively varying the form of the employed fitness function. Furthermore, a fuzzy decision‐making technique is activated to finally pick the appropriate operating point of the AC/MTDC power grids. The proposed technique is compared with other reported algorithms in the literatures. The applications are conducted on three test systems. These systems are IEEE 30‐bus, IEEE 57‐bus test power systems in addition to real part of the Egyptian grid at West Delta region. Numerical results demonstrate that the proposed MO‐MRFA has great effectiveness and robustness indices over the others. Nevertheless, the proposed MO‐MRFA is successfully extracting several Pareto solutions that meet the techno‐economic requirements with accepted environmental concerns.