This study covers research and development efforts to develop, design and test a novel very low frequency cosine-rectangular voltage generator (CRVG) specialised not only for a withstand test, but also a partial discharge (PD) test of standard medium voltage (MV) cross-linked polyethylene power cables. The system schematic and operating principle of the proposed system are presented and described. The cosinusoidal polarity reversing process is realised through L–C series resonance. A charging and discharging switch is developed by means of an insulated gate bipolar transistor series-connected semiconductor switch. A laboratory prototype is assembled and tested. Maximum output voltage of the proposed CRVG is measured as 20 kV, which is in accordance with the expected acceptance test voltage level given in IEEE standard 400.2. Finally, the proposed system is applied to PD detection of a 300 m cable circuit with a built-in artificial defect, and the PD locating results are presented, proving the system's feasibility.

Vast majority of already-installed grid-connected wind turbines is fixed-speed wind generators (FSWGs). Their initialisations for power-system dynamic simulations are mostly performed using conventional power-flow approach, causing an unavoidable reactive-power discrepancy between power-flow result and generator dynamic model. To eliminate this problem, unified Newton–Raphson (NR) power-flow approach together with wind-turbine aerodynamic power coefficient is applied for accurate initialisation of the FSWG under passive-stall regulation. However, FSWG initialisations relating to two other active-stall and active-pitch regulations under the same approach have not yet been addressed. Therefore, in this study, the unified power-flow approach is straightforwardly extended to yield efficient and precise dynamic initialisations of active-stall- and active-pitch-regulated FSWGs. In the proposed model, variables of generators and wind turbines are fully solved all together with the power coefficients. The obtained power-flow result is compared with the published result for model verification. Efficiency of the extended algorithm is demonstrated using IEEE-300bus and 118bus networks with large groups of wind power plants. The simulation results reveal that the extended algorithm not only gives precise steady-state initialisation of the FSWGs but also retains NR quadratic convergence characteristics.

This study proposes the application of combinatorial multi-objective optimisation (MOO) in an electrical power distribution system. Conventional electrical power systems do not consider reverse power flow, in which the power flows toward the feeder in the distribution system. However, reverse power flow toward the substation transformer is caused by voltage deviation with high penetration of distributed generators into a distribution system. Consequently, this causes faults in electric devices and may even lead to a massive blackout. To resolve voltage deviation problems, it is necessary to consider some trade-offs. With this background, this study reveals three points. The first and second contributions regard general engineering research issues such as the definition of a new optimisation problem framework. To solve the problems discussed in this study, a new method of MOO was required. This method of MOO is applied to the power system to minimise voltage deviation while simultaneously minimising the number of required voltage control devices and operation. In addition, a new MOO method to determine the optimal placement of control devices while retaining operation diversity is proposed. Finally, each optimisation method is compared with numerical simulation and the advantages are summarised from the simulation results.

This paper describes an intelligent sensor fault detection and compensation (FDC) scheme for a hybrid grid involving renewable energy (RE) sources with power electronic interfaces. To evaluate the sensor FDC scheme, a wound rotor induction generator (WRIG)-based wind energy system connected to the hybrid grid is examined. The system employs a WRIG with excitation from DC RE grid via a single, rotor side converter. To analyse the dynamic performance, a decoupled voltage vector control is employed. The sensors involved in the control algorithm may not be ideal and if fault occurs might lead to system collapse. To overcome electrical sensor failure, a robust, intelligent sensor fault control scheme is proposed with ANFIS. A hardware-free solution with PLL block is proposed for mechanical sensor fault. The ANFIS-based FDC scheme involves a bank of observers that computes residuals, detects and isolates faults. When fault occurs, the observer output is utilised for the execution of control algorithm thereby avoiding system collapse. The focused system is simulated in MATLAB and experimentally tested with a dSPACE controller for various fault scenarios. The results show the effectiveness of the FDC scheme.

The recovery of power system after a large area blackout is a critical task. To speed up the recovery process in a power grid with multiple black-start units, it would be beneficial to partition the system into several islands and initiate the parallel self-healing process independently. This study presents an effective network partitioning algorithm based on the mixed-integer programming technique and considering the restoration process within each island. The proposed approach incorporates several criteria such as self-healing time, network observability, load pickup capability, and voltage stability limits. It can quickly provide multiple partitioning schemes for system operators to choose based on different requirements. Experimental results are provided to demonstrate the effectiveness of the proposed approach for IEEE 39-bus and IEEE 118-bus standard test systems. Also, the sensitivity of the partitioning solution with respect to the various parameters is presented and discussed. Ultimately, the advantage of the proposed method is demonstrated through the comparison with other references.

Evaluation of available transfer capability (ATC) is a complicated process involving the determination of the total transfer capability (TTC) and the existing transfer commitments (ETC). Considering the uncertain renewable generation such as wind power will bring more challenge to this process. Previously the uncertainty of wind power is considered either in the ED model or the following TTC calculation, but not included in two process simultaneously. Therefore, the ATC output may not be accurate since the uncertainty impact two problem simultaneously. To consider the uncertainty and correlation of wind power in both ISO’s ED and ATC evaluation, this paper proposes a bi-level optimization framework in which the ATC evaluation is formulated as the upper level problem and the ED is the lower level. The bi-level model is first converted to a mathematic program with equilibrium constraints (MPEC) by recasting the lower level problem as its Karush-Kuhn-Tucker (KKT) optimality conditions, and then transformed to a mixed-integer linear programming (MILP) problem which can be solved by existing optimization tools. Case studies on the PJM 5-bus and IEEE 118-bus systems are presented to verify the proposed methodology and the impacts of correlation of wind power on ATC are analysed.

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

Smart grid technologies require advances in various conventional solutions where efficient control of the inductance of reactors is of a significant interest for utilities. Novel solution is presented for inductance control of power reactors which is carried out by developing a virtual air gap (VAG) within the core. The impact of VAG on the magnetisation behaviour of the transformer core has been analysed by using simulation environment of finite-element method (FEM). Experimental investigation is carried out to observe the performance of VAG for intended inductance variation. Using proposed technique reactor inductance can be tuned in the range of 1 : 20 in less than a millisecond with a significant reduction in harmonics. Implementation of such fast, linear and magnetically controlled inductance along with suitable intelligence can be valuable to enhance smart grid technology for improved integration of renewables and grid optimisation.

This study presents a mixed integer linear programming-based approach for determining the optimal strategy for reliability improvement in radial distribution networks where island operation of distributed generators (DGs) is allowed. The proposed approach simultaneously defines the optimal number, type, and location of new automation devices (ADs) to be installed in a network, the optimal relocation of the existing ADs and the optimal creation of DG islands. The load curtailment possibility in creating islands is considered along with the multi-level load model. The presented numerical results indicate the effectiveness of the proposed approach and the importance of simultaneous consideration of the aforementioned individual strategies in determining the best reliability improvement strategy in radial distribution networks with DGs.

Feeder reconfiguration is commonly used in the distribution system for real loss minimisation. However, in the stressed system condition, loadability limit enhancement is an important issue. This aspect has not been paid much attention in the literature. Metaheuristic methods can solve this problem at a high computational cost. In this view, an efficient two-stage algorithm is proposed for the same. In the first stage, an efficient reactive power loss minimisation-based reconfiguration plan is obtained. Using heuristics from this solution, the second-stage employees improved harmony search algorithm for reconfiguration with loadability enhancement. It is important to note that this algorithm is used only when system load is near the critical load; otherwise, the conventional real power loss minimisation reconfiguration plans can be used. Results for a 33-bus balanced test system and a 25-bus unbalanced distribution system confirm the potential of the proposed method.

This study presents an approach based on short-circuit theory and on modern monitoring infrastructure for locating permanent and temporary faults in power distribution systems. The main innovative aspects of the proposed approach are: (i) no fault classification is required if phasor measurements are adopted, (ii) both current and voltage measurements may be included in the problem, (iii) in the fault location criterion the measurements are weighted according to their proximity to the fault location, (iv) a refinement scheme allows finding the fault on branches and not only at buses. The proposed approach can cope with magnitude measurements, synchronised phasors obtained via micro phasor measurement unit and unsynchronised phasors, which are being used for the first time for fault location purposes. Case studies are carried out considering a real 134-bus feeder. Initially, the performance of the proposed approach is assessed with respect to fault location, fault type and fault resistance. Next, studies regarding the influence of the number, position and type of measurements are presented. Statistical simulations are performed for assessing the influence of the random errors inherent to measurements, load forecasting and distribution system parameters. The results indicate that the proposed approach is accurate and very promising.

Due to the importance of global warming and environmental impacts that accumulated from emission of gaseous pollutants of fossil-fuelled power plants, the modern combined emission economic dispatch (CEED) is applied. This study proposes a new evolutionary lightning flash algorithm to solve dual-objective CEED problem considering different scenarios with wind power penetration, multiple fuel options and operation constraints on the generators. The lightning flash algorithm is formulated based on the movements of the cloud to ground lightning strikes in a thunderstorm. This method is tested on 11 benchmark functions and then it is applied on six different practical case study systems for solving non-convex CEED. The results of LFA on benchmark functions and the case study systems are compared with other methods and confirm the effectiveness and applicability of the proposed method with higher quality solution, less emission, less costs and better convergence against other methods for solving non-convex practical economic dispatch, CEED and dynamic dispatch problems.

Converting the surplus power into rotor kinetic energy by changing the control functions of machine side converter (MSC) and grid side converter (GSC) is considered to be an efficient way for permanent magnet synchronous generator (PMSG)-based wind turbine generator system (WTGS) to accomplish low-voltage ride through (LVRT) without additional hardware components. However, because of the time delay of grid fault detection and the control system inertia, the dc-link capacitor may still be subjected to overvoltage in the early tens of milliseconds after a grid fault happens. This study proposes a compositive control method of LVRT for PMSG-based WTGS, in which the surplus power during LVRT is converted into rotor kinetic energy by controlling the electrical power of the PMSG according to the grid-connected power of the GSC, and a crowbar circuit is reserved to consume the surplus power before MSC reacts to the grid fault. The proposed method does not change the control functions of MSC and GSC, and the related control parameters resetting can be avoid. Besides, it can also provide reactive power support to assist the grid voltage recovery. Simulations and experiments proved the proposed method to be feasible and effective.

This study shows that with fast and flexible power electronic converter interfaces, micro-grids can quickly re-energise blacked-out transmission lines without overvoltage. After the transmission lines are re-energised, micro-grids can speed up (i) the reconnections of generators and loads; (ii) the re-synchronisation of severed transmission line sections back to the original grid. Micro-grids have active power to energise auxiliary motors of large thermal plants in black start. Faster restoration is limited by steam-turbine plants because they cannot be speeded up beyond thermodynamic constraints.

The stability of electric power system (EPS) insures energy security of consumers. Therefore, saving the stability of EPS is the main task for professionals around the world. For its solution, a variety of activities is used. This publication deals with the description of the authors' experience for solving the task in the Tyumen power system. The hybrid real-time power system simulator (HRTSim) developed in Tomsk Polytechnic University was used as a platform for research carrying out. The study presents the research result fragments of the momentary and sustained fast turbine valving control (FTVC) which is one of the most effective ways of an automatic emergency control of power units in the thermal power station. Using the simulator HRTSim, one of the greatest challenges of electric power industry in Russia has been solved, i.e. FTVC adequate setting of the power units in the thermal power station Surgut GRES-2 in the Tyumen power system. An FTVC operation largely determines the reliability and stability of the EPS. Therefore, an incorrect setting may result in substantial disturbance or disruption of the stability that already occurred in the past. The adequacy of turbine and FTVC simulations is verified by comparing it with the real data.

Thermal behaviour is a prime factor in the accurate performance assessment of power transformers as well as in the prediction of their life expectancy. This study presents a computer modelling tool based on an electro-thermal equivalent circuit of transformers that is able to predict the hot-spot temperature and average surface temperatures for all internal layers of distribution-class toroidal transformers. Temperature is the limiting factor that prevents running transformers for hours or days in overload conditions. The modelling tool presented in this study is capable to identify the safe maximum overload current and duration that a transformer can handle without introducing damage or loss of life. The model is helpful to predict the short-term (few hours) and long-term (few days) overload capabilities of transformers. The electro-thermal model can also be used as a tool to optimise the design and evaluate the performance of transformers. This study is specifically focused on the implementation of the proposed method on dry-type distribution-grade toroidal transformers. The model is built using circuit components (lumped R and C) obtained from the thermal–electrical analogy. The model is validated with numerous finite-element method simulations and laboratory tests with transformers of various power ratings.

This study describes a solution to discriminate the original sources of bad data which are concerned by the substation maintainers. Bad data caused by defects of monitoring system, such as mal-setting of monitoring devices or time skew, are concerned here. These bad data can be identified mostly in electric power control centre by bad data identification (BDI) methods including robust state estimation; however, these methods can hardly explain the relevant original sources of bad data in substations. Here, the authors propose a data-driven method named abductive identification (AbI) to discriminate the original sources of bad data already identified by BDI methods. In the AbI method, feature patterns of bad data, based on their residuals, are investigated. A feature pattern selection method for bad data is then proposed, in which both the absolute residual and relative residual are considered. Based on this feature selection approach, a Bayesian classifier is developed to identify the original sources of bad data. A two-level architecture for the AbI approach is introduced to deal with diverse bad data patterns of wide-area networks. Simulation and field test results demonstrate the applicability of the proposed method.

Battery energy storage system (BESS) is one of the best solutions to compensate the wind power fluctuations and forecasting errors for participation in the power markets by contribution in the energy production several hours ahead. Based on coordination of BESS and wind power, this study aims to present a new strategy to optimise the BESS size and increase the battery lifetime. In the proposed strategy, the average wind power is considered as the dispatch power to minimise the battery capacity and two back up battery sets are utilised to avoid shallow charge–discharge cycles for saving the battery efficiency and lifetime. In addition, the short-term operation criterion is chosen to deal with the prediction error effects. The proposed method is applied to a 2 MW wind turbine as a case study with 2 years wind data. Simulation results show the effectiveness of the proposed strategy.

A method to investigate dc-network instabilities in multi-terminal high-voltage DC (HVDC) systems is presented. The method consists of defining subsystems, called in this Letter Z and Y , which can be interpreted as a dc-network impedance matrix and as set of converter admittances, respectively. Through the analysis of the Z and Y terms, it is shown that resonances originated from the system's dc network can be amplified if the converters show a negative dc-side conductance. Moreover, it is also shown that converters located at different terminals can contribute to instability if the resonances to which they are exposed are of the same frequency. The analysis method is applied to a four-terminal HVDC system as an example and, using the theory developed in this Letter, mitigation measures are applied. The results are verified through time-domain simulations.

In order to address the wind power uncertainty in the security constrained economic dispatch (SCED), this study proposes a two-stage data-driven distributionally robust reserve and energy scheduling (DDRRES) model that considers the loss of wind spillage and load shedding. This model aims to minimise the total cost while ensuring that the operating constraints are satisfied on the adjustable uncertainty set, of which the boundaries are decision variables. Unlike the previous approaches, which assumed that the underlying true probability distribution (PD) of uncertainty is known, the proposed model does not rely on the specified distribution but extracts the information from historical data directly. The ambiguity sets, i.e., Wasserstein balls are constructed to contain the possible PDs. Fixing the boundaries of adjustable uncertainty set, the operational risk is the expected loss under the worst-case PDs over Wasserstein balls. Thus the operational cost and operational risk can be balanced by adjusting the adjustable uncertainty set. After the tractable formulation of DDRRES is obtained, such a model is solved with the column-and-constraint generation method. The performance of the proposed approach is verified on a 6-bus test system and a 118-bus system.

Clearing power grid faults swiftly and selectively offers higher security, reliability and sustainability. Accomplishing this aim by deploying directional overcurrent relays (DOCRs) is one of the major challenges in meshed and multisource distribution networks. To overcome this challenge, the current study elaborates a new coordination strategy which concentrates on minimising overall operation time of relays. In this strategy, an auxiliary variable is added to classical operation time model of each DOCR. All the auxiliary variables are considered as coordination constraints which help to yield new and well-defined time current characteristics (TCCs). In other words, more flexibility is attained in adjusting relay's characteristic which helps to alleviate clearing time of faults. The obtained TCCs can be easily performed by numerical relays. In addition, a new objective function is defined to steer the relay settings towards optimal solutions conveniently. It was shown that the proposed approach not only reduces the operation time of relays, but also prevents miscoordinations. This approach demonstrates a non-linear programming model tackled by particle swarm optimisation algorithm. The effectiveness of the proposed approach is verified via 8-bus and IEEE 14-bus test systems and results are discussed in depth.

A hybrid scheme for enhancing fault ride through (FRT) capability of doubly fed induction generator under symmetrical and asymmetrical grid faults is presented. The hybrid strategy is composed of switch type fault current limiter (STFCL), braking chopper and energy storage system (ESS). The hybrid scheme guarantees the safety and controllability of rotor side converter and DC-link capacitor. The proposed FRT scheme has ability to overcome the transients and oscillation produced during the voltage dip. The hybrid strategy is capable of keeping the fault current, DC-link voltage, rotor current and voltage and electromagnetic torque within the limits and damps the oscillations during the voltage dip. The results of hybrid scheme are compared with STFCL only and are found better for the former case. MATLAB/Simulink is used for simulation of 9 MW wind turbine interfaced with grid.

Here, a new travelling wave-based fault location method is proposed for branched networks. A mathematical morphology-based filter is used to analyse fault-induced transient signals and detects the arrival time of travelling waves. In the proposed algorithm, using the first surge arrival time of travelling waves at each of the travelling wave detectors installed in the network, a set of linear equations is constructed. By solving the equations, the probable fault occurrence points on the main line of the network are determined. Finally, proper evaluation criterions are introduced to determine accurate faulted lateral branch or fault location on the main lines. Extensive simulation studies using EMTP and MATLAB are carried out to examine the effectiveness of the proposed method. The obtained results verify the high accuracy, noise immunity, and fault impedance robustness of the proposed method.

Incorporation of the short-circuit level constraints in the transmission expansion planning (TEP) has a direct impact on the final solution. Conventionally, in TEP models, a substation is tackled as a single node where are all transmission lines, power transformers, and generating units are connected to each other. However, bus splitting is a common option in most substations that divides the connected elements into two groups. Doing so, bus splitting can alter the flow path impedances and accordingly manage the short-circuit levels. Here, the short-circuit level constraints are modelled in terms of conventional TEP and bus splitting decision variables. The modelling process is very complex and some non-linear terms appear in the models. To have a tractable model in real-world problems, the non-linearities are converted to linear equivalents making the final TEP model conforming the mixed-integer linear programming format. The performance of the proposed model is examined on the 24-bus reliability test system. The results show that incorporation of the bus splitting option in the TEP problem decreases the total cost of the optimal expansion plan.

This study presents a digital differential busbar protection scheme based on generalised alpha plane approach which combines the benefits of percentage differential approach and two-restrain alpha plane approach. The proposed scheme utilises one cycle current transformer (CT) secondary current signals of all the bays connected to the busbar to map the operating points on a complex alpha plane. The proposed scheme has been evaluated by modelling an existing 400 kV Indian power generating station in PSCAD/EMTDC software package. The performance of the proposed scheme has been evaluated on large numbers of cases with wide variation in system and fault parameters. In order to verify authenticity of the proposed scheme, a laboratory prototype of the proposed busbar protection scheme has been developed. From the developed prototype, CT secondary current signals are captured during internal faults and external fault with CT saturation condition. Comparison between the simulation and prototype results clearly shows the effectiveness of the proposed scheme in terms of higher sensitivity during internal faults and better stability in case of external faults. The proposed protection scheme has a high response speed (around 5 ms) and hence can be considered on par with modern busbar protection schemes.

This study presents the analysis of transient states generated along the traction catenary during switching operation in the main supply substation unit. For this purpose, a Swedish autotransformer railway system operating at 15 kV and 16.7 Hz has been chosen as a reference. The simulations of abovementioned system were conducted using EMTP-ATP software. This study presents results of simulations for various conditions and at different loads. The main outcome of the analysis is determination of transient currents and voltages in the system during catenary energisation and de-energisation process. Due to the presence of saturable autotransformers along the catenary the flow of excessive inrush current may lead not only to the overloading and as a consequence main breaker tripping but may also cause slow-front overvoltages. In order to represent the worst case conditions parametric study was performed. It includes variable tripping time of the main circuit breaker and pre-existing residual core flux. The results of the analysis show that the system response is strongly dependent on the breaker closing and opening times. As a consequence, it is possible to determine safe switching conditions in order to avoid large overvoltages or accidental tripping.

The optimal power flow (OPF) problem for active distribution networks with distributed generation (DG) and a variety of discretely adjustable devices (e.g. on-load tap-changers, OLTCs) is essentially a non-convex, non-linear, mixed-integer optimisation problem. In this study, the quadratic model of three-phase OLTCs is proposed by adding branch currents as unknown variables, which guarantee a constant Hessian matrix throughout iterations. This study proposes a three-phase OPF model for active distribution networks, considering a three-phase DG model. The OPF model is solved by an interior point method incorporating a quadratic penalty function as opposed to a Gaussian penalty function. Furthermore, a voltage regulator is also incorporated into the OPF model to form an integrated regulation strategy. The methodology is tested and validated on the IEEE 13-bus three-phase unbalanced test system.

The detailed models of renewable energy resources based distributed generation (DG) unit, namely, wind energy conversion system, photovoltaic and diesel engine are presented in this study. Combination of different DG units in three microgrid (MG) structures is considered to investigate small signal stability and possible interaction between sensitive modes, particularly in autonomous mode of MG operation. Evaluation of oscillatory condition suggested that gain controller variation significantly influenced MG stability and system dynamic response. Moreover, since modal interaction potentially occurred due to gain change, it is necessary to identify the interaction accurately to ensure stable MG operation. The conventional identification method of eigen-interaction is conducted by observing the movements of engaged eigenvalues. However, the eigen-trajectories method is less sensitive to identify the occurrence of weak interaction. To provide more sensitive identification method, cross-participation factor (CPF) and modal interaction index (MII) analysis are proposed. Deviation of eigen-trajectories after approaching a particular interaction point, higher values of CPF and MII confirmed the occurrence of interactions. The presented works contribute for MGs gain setting consideration and proposing novel methodologies in identifying modal interaction.

For grid-connected doubly fed induction generators (DFIGs), fault ride-through (FRT) capability and transient stability are vital problems that urgently need to be addressed. To overcome these problems, a novel superconducting magnetic energy storage-fault current limiter (SMES-FCL)-based protection scheme by modified control of superconducting coil (SC) and rotor-side converter (RSC) is investigated here. A rotor-side mathematical model is used to estimate and optimise the SC inductance parameter. Two modified control strategies of SMES-FCL cooperative operation and positive-sequence d–q current modification (PCM) are proposed to control the SC and RSC for enhancing the transient stability of the overall DFIG system. In addition to maintaining good FRT performance from the SMES-FCL on stabilising the electromagnetic torque, DC-link voltage, active and reactive power, and stator and rotor current during grid fault, the PCM control is also demonstrated in the simulations to suppress the transient oscillations and thus to shorten the recovery time after grid fault. Meanwhile, compared with conventional SMES-FCL scheme, the requirement of SC current capacity is much lowered for smoothing the DFIG output power under a varying wind speed condition and for enhancing the FRT performance under a grid fault condition.