This study presents a comprehensive survey on the reliability evaluation of the electrical network system. The impacts of integration of new and renewable energy sources (electric vehicle, energy storage system, solar, and wind) on the reliability of electrical power system (EPS) are discussed. The impacts of these renewable sources have merits/demerits when these sources are integrated with the conventional electric power system. However, the merits are predominant as it includes unlimited, free, and cost-effective resources. The recent researches depict that the uncertainties of renewable energy resources leads to the probabilistic and reliability analyses of EPS. EPS includes offshore and onshore wind farms, micro-grid, energy storage system, and other high voltage grids. It also contains the failure-prone components related to the power systems. For the accomplishment of these aspects, the handling methods of uncertainty parameters in generation, transmission, and distribution systems are discussed. The incorporation of electric vehicles, wind energy system, and energy storage system for reliability assessment is also discussed briefly. This study also presents the scope of a new research area for the researchers on the reliability assessment of renewable energy integrated power system.

In this study, a novel voltage stability assessment algorithm (VSAA) is proposed, which can accurately detect the stability stiffness of power systems. To this purpose, VSAA uses only three measured samples of a bus voltage and the power flowing out of the bus to detect the maximum loadability limit of that bus. Although VSAA has a low computational burden and uses only locally measured data, it can consider the impact of small and/or large disturbances that change the Thevenin equivalent network (i.e. PV curve) seen from the target bus and is able to accurately assess the stability status. To examine the performance of VSAA, it was applied and tested on a 4-bus, IEEE 39-bus, Nordic32 and IEEE 33-bus distribution test systems using DIgSILENT PowerFactory software. The static and dynamic simulation results show the advantages of VSAA over other stability indices proposed in the literature and verify its efficiency and accuracy for online applications.

Modular multilevel converter (MMC) station is expected to be able to operate under unbalanced grid conditions (UGCs) and to satisfy all kinds of limits under UGC. To investigate the allowable region of MMC under UGC, this study presents a point-scanning method, which directly solves the steady-state values of MMC from non-linear dynamic equations for each possible operating point, and therefore, can access the constraints of internal variables including the sub-module capacitor voltage, arm current and modulation index. First, the complete 28th-order dynamic model of MMC, which contains the zero-sequence second-order modulation signal is derived. Next, as the premise to solve the steady state of MMC, point of common coupling voltages are solved from a proposed unbalanced grid model, which incorporates the effect of ac fault. Following that, a peak-value seeking algorithm is proposed to seek the peak values of internal dynamics of irregular waveforms to implement the constraint of internal dynamics. Next, the allowable region of MMC under UGC is depicted and analysed. Also, the power limiting control is proposed to avoid MMC under UGC, violating the limits of internal dynamics. Finally, the simulation validates the effectiveness and correctness of the proposed point-scanning method and power limiting control.

This study presents an algorithm for detection, classification, and location of transmission line faults. A fault index based on features extracted from current signals using the alienation coefficient and Wigner distribution function has been proposed for the detection and classification of faults. Double line and double line to ground faults have been classified from each other using ground fault index based on negative sequence current. Statistical relations are proposed for the estimation of fault location using peak values of the proposed fault index. The results of different case studies established the effectiveness of the algorithm. The algorithm is found to be effective for providing protection to transmission line against various faults. This is achieved using current signals recorded on one terminal of the line. This makes the protection scheme less complex, fast and more economic due to the elimination of the requirement of communication channel and global positioning system synchronisation. The proposed protection scheme is also validated on a real-time network of transmission utility. The effectiveness of the algorithm is established by comparing performance with reported algorithms.

Active power oscillations and frequency fluctuations appear simultaneously in virtual synchronous generator (VSG) systems with fixed rotor inertia and damping coefficients. Thus, an optimisation control strategy to self-adjust the parameters is proposed in this study to suppress the power oscillations and frequency fluctuations. A small-signal model of parallel VSG systems is established, and the impact of rotor inertia and damping coefficients on the output active power is analysed. Then, the parameter optimisation control for the self-adjusting inertia and damping coefficients is introduced according to the rate of change and the deviation of virtual rotor electrical angular velocity. Notably, the changes and deviations are limited in a range to ensure the dynamic and steady-state performance of the system. Based on the principle of the equivalent synchronous generator, a virtual inertia matching method for parallel VSG systems is accordingly obtained. The feasibility of the proposed control method is verified by extensive simulations. The results confirm that this method can not only suppress the power oscillation in the transient connection of the VSGs to the AC bus, but also improve the frequency support. In turn, it ensures that the load power is proportionally distributed among the VSGs.

Power system automation is an effective tool from both economical and technical aspects to improve the optimal operation of power generators. In this regard, Security Constrained Unit Commitment (SCUC) incorporating Dynamic Thermal Line Rating (DTLR) of overhead transmission lines can boost the system security effectively. Using Transmission Switching (TS) tool in SCUC problems leads to cost reduction. Still, one of the main challenges arisen in TS problems is the excessive number of switching in lines, which decreases the lifespan of power switches. In this paper, an Improved Linear AC Optimal Power Flow (ILACOPF) is proposed by using TS and considering Heat Balance Equation (HBE) as a security constraint. Merging dynamic thermal line rating (considering the weather conditions) in SCUC with TS, besides decreasing the number of switching and increasing the lifespan of power switches, causes a remarkable reduction in operating costs. In this power flow, system losses are modeled by linear formulations. Moreover, a linear approximation of the heat losses due to power flow through lines is proposed. To solve the proposed model, Benders' decomposition approach is applied. The performance of the proposed framework has been evaluated on 6-bus and 118-bus IEEE test systems.

Superconducting magnetic energy storage (SMES) system is one of the efficient pulsed power compensators. Its response to the load fluctuations is faster and competent. This study proposes a non-linear dynamic evolution control (NDEC) approach for SMES system. The control approach is implemented to regulate the error of three-phase compensating current and DC-link voltage. The control functions required for voltage source inverter (VSI) and bi-directional DC–DC converter of the SMES system are derived using the NDEC control approach. A proportional–integral (PI) controller is replaced by the NDEC in the inner loop of a traditional pulse-width-modulation control strategy of VSI for the quick response and effective energy exchange. The elementary idea of implementing NDEC is to govern the system dynamics by maintaining a zero-error state irrespective of load uncertainties quickly. Detrimental high rating stress on the system due to pulsed power load is substantially reduced with the proposed NDEC strategy. Source current is maintained almost balanced and harmonic-free in order to ensure a quality and reliable supply to surrounding loads. The variation in source current is drastically reduced from 260 to 6% using the proposed system. Moreover, a comparative study between PI and NDEC-based control scheme is presented using MATLAB simulation and experimental results.

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

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

The hybridisation of two or more algorithms is recently emerging to detect superior solutions to the optimization troubles. In this study, a new hybrid cuckoo search algorithm and grey wolf optimiser (CSA–GWO) optimisation technique is exercised and exhibited to optimally design and tune the controller parameters installed in the voltage source converter (VSC) of an offshore wind farm (OWF). One of the widely used control strategies for VSC is the proportional–integral (PI) closed-loop control system. The new hybrid optimisation algorithm is used to design and tune the PI controllers' parameters to improve the performance of OWF. It shall be mentioned that these parameters are usually hard to obtain owing to the high level of embedded non-linearity in such energy systems. The performance of such optimally designed PI controllers is presented in both dynamic and transient conditions. To examine the realistic stability of the proposed algorithm, real wind speed pattern has been captured from Egypt wind farm at Zafarrana and simulated. The obtained results from this new hybrid optimisation CSA -GWO control system reflect its superiority over other traditional algorithms, such as genetic algorithm, especially during symmetrical and unsymmetrical faults. CSA–GWO algorithm was examined using MATLAB/Simulink.

To improve the utilisation of renewable energy and the power transmission capacity in the power system, the concept of power flow router (PFR) is proposed by scholars worldwide. However, there lacks an optimal planning method for PFR based on the economic principle, which impedes the development of PFR. To solve the above problem, the optimal location and capacity planning method of PFR is researched in this study, where a bi-level algorithm based on the convex relaxation optimisation and sensitivity analysis method is proposed. First, the definition and the general power flow model of PFR are introduced. According to the principle of engineering economy, the allocation model of PFR is established by introducing the binary variables. To solve the non-linear and non-convex mixed-integer programming problem, this study proposes a bi-level algorithm with the convex relaxation optimisation and sensitivity analysis method. Finally, the case studies are conducted in IEEE 30, IEEE 57 and IEEE 118 systems. The calculation results of the optimal power flow, planning results and economical analysis of PFRs are verified in IEEE 30, 57 and 118 systems, which shows the accuracy of the proposed algorithm.

Due to the low converters rating and cost of doubly fed induction generator (DFIG) along with its ability to function under variable wind speed, DFIG has been widely employed in wind energy conversion systems (WECSs). Unfortunately, the performance of DFIG is sensitive to the variation in the operating conditions and disturbance events at the grid side. This includes wind gust, voltage fluctuation and faults at the point of common coupling of the DFIG and the grid. In this study, a model-free adaptive control (MFAC) is developed for a unified power flow controller (UPFC) in order to improve the overall dynamic performance of a DFIG-based WECS during wind gusts and enhance the fault ride through capability of the DFIG during various disturbance events. The effective performance of the proposed controller is assessed through a comparison with a conventional proportional–integral (PI) controller optimised by a modified flower pollination algorithm. Results reveal the superiority of the proposed UPFC-MFAC technique over the conventional PI controller currently used in most of the UPFC-WECS applications.

This study introduces a robust orthogonal implementation for a new class of information theory-based state estimation algorithms that rely on the maximum correntropy criterion (MCC). They are attractive due to their capability to suppress bad data. In practice, applying the MCC concept amounts to solving a matrix equation similar to the weighted least-squares normal equation, with difference that measurement weights change as a function of iteratively adjusted observation window widths. Since widely distinct measurement weights are a source of numerical ill-conditioning, the proposed orthogonal implementation is beneficial to impart numerical robustness to the MCC solution. Furthermore, the row-processing nature of the proposed solver greatly facilitates bad data removal as soon as outliers are identified by the MCC algorithm. Another desirable feature of the orthogonal MCC estimator is that it avoids the need of post-processing stages for bad data treatment. The performance of the proposed scheme is assessed through tests conducted on the IEEE 14-bus, 30-bus, 57-bus and 118-bus test systems. Simulation results indicate that the MCC orthogonal implementation exhibits superior bad data suppression capability as compared with conventional methods. It is also advantageous in terms of computational effort, particularly as the number of simultaneous bad data grows.

In modular multilevel converter (MMC)-based high-voltage direct current transmission system, hundreds of submodules (SMs) with large SM capacitance in each arm result in expensive cost and bulky volume. In full-bridge submodules (FBSMs)-based MMC (FB-MMC), a novel capacitor voltage ripple suppression method based on three available variables manipulation is proposed to reduce SM capacitance requirement. In the interaction of these available variables, the dominant fundamental-frequency and second-order harmonic fluctuations of SM capacitor voltage can be eliminated under different power factors. Under the proposed method, three-to-five times frequency fluctuation components will be reduced as much as possible. By establishing capacitor voltage fluctuation model, the concrete mathematical expressions of the three desired available variables under different power factors can be derived, which are easily implemented to suppress capacitor voltage ripple. Compared with normal operation, SM capacitor voltage ripple can be reduced by 80% with the proposed method when the power factor of 0.9–1 is considered. In other words, SM capacitance requirement can be reduced by 80% under the same constraint of capacitor voltage ripple. Therefore, the cost and volume of SM capacitors can be significantly reduced. Simulation results confirm the feasibility and validity of the proposed method.

This study provides a new perspective to study the formation and wholeness of the stability region of power systems. The main idea of the approach proposed in this study is to project the system into a closed area and then investigate the behaviours of the field in a neighbourhood of infinity on a sphere that is topologically equivalent to the original system. The invariant manifolds and singularities at infinity are described in detail to indicate the boundary of the entire stability region, based on the concept of the basin of attraction. The proposed method is applied to the single-machine model, two-machine model and multi-machine system to illustrate the complex geometry structure of the transient stability region. A singularity variation index is presented to estimate the size of the entire region and indicate the transient stability degree of the power system. Simulation results verify the correctness and effectiveness of the theoretical analysis.

Currently, an issue of relay protection (RP) settings determination that ensures its correct operation in electric power systems (EPSs) has not been resolved completely. The main reason for this is the lack of methods and tools for adequate consideration of transients during a fault existence for RP settings calculation. In this study, a novel approach for RP settings determination is presented. Its main feature is the application of detailed mathematical models reproducing simultaneously processes in the entire set of elements in the RP scheme, including instrumental transformers, and in large-scale EPS mathematical model as a whole. This key feature can be achieved via the hybrid power system simulator – hybrid real-time power system simulator – implementing a methodically accurate analogue–digital solution of the EPS mathematical model. The numerical transformer differential protection (NTDP) based on this approach was set up via the standard dual-slope tripping characteristic. Also, a novel approach to the formation of the tripping characteristic is proposed. The key feature of this approach is applying a flexible curve that enveloping all fault characteristics I _diff = f(I _rest) of those modes in which the NTDP should not trip.

Dynamic line rating (DLR) technologies are fast becoming a key instrument in power systems with high penetration of wind farms, because power companies can reduce the value of wind power spillage and also can postpone the investments of transmission lines by using DLR equipment. However, what the authors know about DLR is largely based on empirical studies that investigate how to change the capacity of transmission lines with changing the weather data. Therefore, lack of a mathematical model for estimating the capacity of lines by considering the weather conditions across the line and wind farm generations can be accounted as a research gap of DLR problems. This study proposes a solution for transmission expansion planning considering DLR technologies. Also, this study seeks to develop a model, which will help to address the research gap of DLR problems. The geostatistical analysis is used to develop a mathematical model, which can provide the relationship between wind farms as well as the capacity of lines calculated based on the weather conditions across the line. The efficiency of the proposed model is examined on a modified IEEE 118-bus test system.

Hydrogen as an energy carrier holds promising potential for future power systems. An excess of electrical power from renewables can be stored as hydrogen, which can be used at a later moment by industries, households or the transportation system. The stability of the power system could also benefit from electrolysers as these have the potential to participate in frequency and voltage support. Although some electrical models of small electrolysers exist, practical models of large electrolysers have not been described in literature yet. In this publication, a generic electrolyser model is developed in RSCAD, to be used in real-time simulations on the real-time digital simulator. This model has been validated against field measurements of a 1 MW pilot electrolyser installed in the northern part of The Netherlands. To study the impact of electrolysers on power system stability, various simulations have been performed. These simulations show that electrolysers have a positive effect on frequency stability, as electrolysers are able to respond faster to frequency deviations than conventional generators.

Generally, the mathematical formulation of the dynamics governing multi-area power systems with Unified Power Flow Controller (UPFC) is a challenging task owing to the presence of both differential and algebraic sub-systems. The proposed research work attempts to integrate the two subsystems by replacing the algebraic subsystem with a differential approximant that leads to a non-linear system of differential equations. Solution of the proposed model with a properly chosen Lyapunov function produce a nonlinear control signal which damps inter-area oscillations effectively. The non-linear control signal is realised using the backstepping method. Moreover, the new formulation enables utilisation of the law for uncertain parameters using the standard parametric feedback form, such that the advantage of such a controller is unaffected by these parameters. In addition to this major contribution, full utilisation of UPFC, by using a lone multi-variable PI controller which eliminates negative interaction between the controllers, is also achieved. Empirical verification of the proposed approach is done by simulating various scenarios with varying degrees of complexity – from dual area power networks to 39 buses New England system. The results of the experiments indicate the efficacy of the method.

The present work focuses on the design of optimal proportional–integral–derivative with filter controller for the frequency regulation in single and multi-area thermal power systems considering effective utilisation of the network resources. The proposed strategy uses global neighbourhood algorithm (GNA), a meta-heuristic algorithm, to obtain optimal controller parameters. Since the controller, in load frequency control (LFC) systems, communicates via open channels there is a need for effective utilisation of channel bandwidth. Therefore, for effective communication, the decentralised adaptive periodic event-triggered scheme is adopted in this study. In order to find the appropriate values of sampling time and to study the effects of time-delay introduced by the communication channel, delay-dependent stability is investigated using Lyapunov theory. The theoretical concepts, developed in this study, are verified using the time-domain simulations carried out using MATLAB which validates the superiority of the GNA-based approach for LFC systems.