In power system electromechanical transients, wind power (WP) with power electronic interfaces is more capable of controlling active power compared to synchronous generators (SGs), but less capable of controlling voltage. WP is beneficial to transient rotor angle stability because it improves the deceleration of SG, but it is detrimental to transient voltage stability. A mechanism of WP inducing voltage instability is studied. The fast dynamics related to power electronic control transients can be neglected, and the WP is simplified to static power injection model in electromechanical transients. The WP is modelled with power balance algebraic equations, and the system dynamic equations are differential algebraic equations (DAEs). The trajectory encountering a singular point of the DAE corresponds to transient voltage instability in the system. The power balance equation of the WP becomes unsolvable at the point. Simulation results validate the mechanism, and reveal the variation of instability mode of a wind-thermal-bundled sending system. When the proportion of WP in the system increases, the dominant stability problem changes from rotor angle stability to voltage stability, and the stability of the system first gets better then gets worse. An emergency control strategy against the transient voltage instability induced by WP is proposed. WP is shed to shift the singular surface and avoid encountering singular points. An approach to select the most effective locations for WP shedding is proposed.

A grid-connected double fed induction generator(DFIG) for wind power generation can affect power system small-signal angular stability in two ways: by changing the system load flow condition and dynamically interacting with synchronous generators (SGs). This study presents the application of conventional method of damping torque analysis (DTA) to examine the effect of DFIG's dynamic interactions with SGs on the small-signal angular stability. It shows that the effect is due to the dynamic variation of power exchange between the DFIG and power system and can be estimated approximately by the DTA. Consequently, if the DFIG is modelled as a constant power source when the effect of zero dynamic interactions is assumed, the impact of change of load flow brought about by the DFIG can be determined. Thus the total effect of DFIG can be estimated from the result of DTA added on that of constant power source model. Applications of the DTA method proposed in this study are discussed. An example of multi-machine power systems with grid-connected DFIGs is presented to demonstrate and validate the DTA method proposed and conclusions obtained in this study.

Installed capacity of solar photovoltaics (PV) continues to grow rapidly in the US. High penetrations and can create operational and reliability challenges. PV integration shares some attributes and mitigation strategies with wind, and differs in some key ways that will be discussed. This study investigates PV integration challenges and examines mitigation issues for utilities planning for high penetrations of PV, with a focus on the Western Interconnection of the US. System balancing becomes more difficult due to a duck-shaped net load curve. Flexibility in the remaining generators, load, and the PV itself, is critical in balancing the system. Reliability impacts from PV have been studied less and addressed less than system balancing issues, but become more important at high penetrations. Understanding essential reliability services from PV, weak grid issues with PV, and distributed PV impacts on bulk power system reliability is critical for planning of high penetrations of PV.

A fast power restoration operational scheme and relevant stabilising control is proposed for active distribution power systems with multi-terminal DC network in replacement of the conventional normal open switches. A nine-feeder benchmark distribution power system is established with a four-terminal medium power DC system injected. The proposed power restoration scheme is based on the coordination among distributed control among relays, load switches, voltage source converters and autonomous operation of multi-terminal DC system. A DC stabiliser is proposed with virtual impedance method to damp out potential oscillation caused by constant power load terminals. The proposed system and controls are validated by frequency domain state-space model and time-domain case study with Matlab/Simulink.

This study presents a comprehensive analysis and a suppression method of sub-modules’ (SMs’) voltage stress in the hybrid modular multilevel converters when riding through zero DC voltage faults in high-voltage direct-current (HVDC) systems. First, the general DC fault ride through (FRT) strategy considering the redundancy of the arm voltage generation scheme under a reduced dc-link voltage is derived. Then full-bridge sub-modules (FBSMs’) voltage stress for the conventional DC-FRT schemes with and without common-mode voltage injection are analysed. Finally, on the basis of the available full-bridge SM capacitor energy control strategy, an improved method implemented by energy interaction between half-bridge sub-modules and FBSMs is presented. The proposed DC-FRT scheme can make all SM capacitor voltages balanced at their rated values during the zero DC voltage conditions. Simulated results are provided to demonstrate the validity of the analytical results and the feasibility of the proposed DC-FRT scheme.

This study presents a new concept for full-capacity wind turbines (WTs) to emulate inertial response behaviour by optimising phase-locked loop (OPLL). The proposed OPLL-based concept can make full-capacity WT spontaneously respond to frequency events in grid with no more need for frequency measurement and calculations of its variation or differential, which are usually required to calculate the desired extra support power or chosen to trigger a predetermined active power shape etc., in common methods. Comprehensive analysis and comparisons are presented for full-capacity WT with two different generic control structures by establishing corresponding motion equations with regard to the active current dynamics. The established motion equations theoretically indicate that PLL dynamics and the related active power controls are in the equally important positions and greatly influence the frequency response behaviours of full-capacity WT, which constitutes the theoretical basis of the proposed OPLL-based methodologies. Simulated results demonstrate the feasibility and correctness of the OPLL-based methodologies. The influences of key parameters concerned corresponding to each proposed OPLL-based methodology are studied. Finally, the proposed methodologies for two generic control structures are also briefly compared and evaluated.

Hundreds of cascading trip faults have occurred in recent years in Chinese wind farms, leading to challenges in the secure operation of the power system. To prevent cascading trip faults, the concept of a voltage security region (VSR) has been discussed in previous studies. However, the VSR proposed in previous work cannot provide decoupled voltage control ranges for each wind farm, which would be a more promising and applicable solution for wind farms in China. In addition, with increasing wind farm active power penetration, the VSR of a wind power integration area may not exist. In this situation, the control centre must curtail some wind power to guarantee security. Thus, this study proposes an iterative VSR model to guarantee secure operation and obtain a larger VSR in wind farms considering wind power curtailment. In each wind farm, the VSR is designed to guarantee normal operation of each wind turbine generator, while in the control centre, the region is designed to guarantee normal operation both under normal conditions and N–1 contingencies considering wind power curtailment. The proposed VSR demonstrated good performance in a simple test case and in a real system in Northern China using a Monte Carlo method.

Coordination of hydropower with multiple renewable sources can effectively reduce the adverse effects of large-scale renewable energy integration in power systems. A yearly scheduling model was proposed for a hydro-photovoltaic (PV)-wind hybrid system, maximising its generation interval and considering firm generation limits. Firm generation ensures reliable monthly generation and reduces generation fluctuations. Because the probability distributions of wind power, water inflow and solar radiation are difficult to predict precisely, interval optimisation is employed to solve this problem by considering only the possible intervals of random variables. It reduces errors introduced by distribution assumptions, acquires possible generation intervals and reduces computation burden. To handle the scheduling model using intervals, a pessimistic preference ordering of interval numbers is employed. The proposed interval optimal scheduling model is illustrated and analysed for a realistic case study in Yunnan.

This study presents a wind energy conversion system based on a dual stator-winding induction generator. The generator can work at variable wind speed and can also be connected to the grid. The proposed energy conversion system presents three operation zones that depend on the wind speed and are delimited by the power ratio assigned to each of the two stator windings. The generator design starts from a three-phase squirrel-cage induction machine and allows a simple assembling of the new windings, what results in an economical, robust and reliable machine. These features make the generator appropriate for wind energy conversion systems located in remote places or in developing areas with limited resources, as well as for distributed generation systems and microgrids. This report also includes experimental and simulation results of the generator operation in the three working areas.

Non-buoyant type of wave energy converter is an innovative method to harness ocean waves. In this paper the assessment of heave displacement for non-buoyant type wave energy converter is investigated by means of artificial neural networks (ANNs). The significant water wave amplitude and time period are chosen as a basis for the heave displacement, and thus these two parameters are considered as effective parameters for the development of ANN model. For this purpose a wide range of dataset of about 4500 data (water wave amplitude and time period (seconds) is considered as input parameter and corresponding heave amplitude of non-buoyant body is considered as output parameter) is obtained from an extensive laboratory campaign and is used to develop an ANN. The developed model has the capability of predicting the position of non-buoyant five seconds ahead of actual heave displacement. The AI technique chosen for the model is a nonlinear autoregressive network with exogenous inputs (NARX) trained with the Levenberg-Marquardt algorithm. A comparative study of 30 different architectures is carried out and finally the best performing ANN architecture is selected and successfully validated based on the measured data.

This study presents a new chattering free full-order terminal sliding-mode controller (FOTSMC) for maximum power point tracking of photovoltaic cells. The proposed system consists of two loops, namely seeking loop and tracking loop. The seeking loop utilises an adaptive perturb and observe method for maximum power point (MPP) searching. Since the accuracy of the seeking loop depends on the tracking loop, a new FOTSMC is used in the tracking loop for tracking the MPP. The proposed control law is continuous, and therefore a chattering free system is achieved. Furthermore, since the derivatives of terms with fractional powers do not exist in the control law, the singularity problem, which exists in traditional terminal sliding-mode controllers (TSMCs), is avoided. Stability and robustness of the proposed system is presented. In order to evaluate the results of the system, a traditional TSMC is provided. Afterwards, the performance of the control system is verified through simulation and experiment.

Most maximum power point tracking (MPPT) techniques based on sliding-mode control (SMC) use another method such as perturb and observe or incremental conductance (IncCond) to provide current or voltage reference which makes the system more complex. To reduce the complexity and to increase the photovoltaic (PV) array efficiency, a direct control high-performance MPPT based on improved SMC has been investigated in this study. Using two different step sizes can follow the PV peak power at different operating conditions with rapid convergence and greater accuracy. The new SMC-based MPPT designed for boost-type DC/DC converters is compared with a conventional and modified IncCond method, and to a classical SMC method which is very similar to that applied by Chu et al. The proposed PV-MPPT system is tested during a stringent profile of sunshine variation as recommended by the European Norm 50530, by simulation within MATLAB/Simulink^TM tools and verified by implementation using a test bench based on DS1104 R&D controller board. The obtained results are satisfactory and demonstrate that the new SMC can track the MPP quickly within 0.003 s and with good accuracy close to 99%.

The dynamic processes of a power system during grid fault are being changed by the large-scale applications of wind generation. The transient characteristics of a single doubly fed induction generator (DFIG) have been widely researched. However, the overall characteristics of a wind farm have not been obtained because the existing models of a wind farm cannot cover the differences caused by normal instruction values, controller responses, and protections of the DFIGs. The equivalent model of a doubly fed wind farm used for the electromagnetic transient analysis is studied. The feature information of the stator short-circuit current (SCC) of a DFIG was excavated based on transient behaviour analysis. The structure similarity indices are established to evaluate the envelope curves of the SCCs. A clustering method based on transient physical process is then proposed to distinguish coherent DFIGs. The equivalent model represented by a small number of DFIGs is obtained by aggregating coherent groups. The simulations indicate that the method effectively recognises transient state differences of the DFIGs. Thus, the model meets the requirements of the transient analysis.

Traditionally, active and reactive power sharing between parallel inverters based on droop control method is considered as a high priority for operating an islanded microgrid. An issue in droop control of parallel inverters in the islanding mode is the voltage drop associated with reactive power sharing within a microgrid. However, maintaining microgrid voltage stability while sharing active power between parallel inverters should be the primary objective for a microgrid. This study proposes to integrate droop and direct-current vector control (DCVC) techniques for active power sharing and bus voltage control within a microgrid. The proposed approach consists of a droop-only controlled unit whose function is similar to a traditional slack bus generator and the rest inverters controlled by using a combined droop-DCVC technique. The droop-DCVC controlled units provide active power sharing among parallel inverters and at the same time maintain microgrid voltage level by injecting the needed reactive power that is determined automatically from the droop-DCVC controllers. Parallel inverter units with loads for a microgrid in the islanding mode are simulated using Matlab/Simulink and Opal-RT real-time simulation system. The hardware experiment is also conducted for two parallel inverters. Results show the effectiveness and excellent performance of parallel inverters in the microgrid by combining DCVC and droop methods.

Pressure retarded osmosis (PRO) is a renewable energy conversion process with potential for sustainable power production. This study introduces this resource to the electrical engineering community and presents the first electric equivalent circuit of the process. This provides a useful tool for integrated osmotic power plant analysis and design. The model illustrates the influence of several important non-ideal effects on power output including concentration polarisation, spatial variation, and pressure drop. The dynamic influence of salt storage by water is also considered. The influence of feed and draw input currents and hydraulic load on power output are investigated and suggest that there is some operating point that will yield maximum power. This illustrates the need for maximum power point tracking in PRO energy conversion.

Hot spot of photovoltaic (PV) panels leads to early degradation and even permanent damage of them. Partial shading is the main cause of hot spotting. Conventional bypass diodes are not able to rectify hot spotting perfectly and more efficient techniques are necessary. In this study, a simple technique is proposed for detection of partial shading and protection of the panels against hot spotting. Moreover, a strategy is used for discrimination between permanent and temporary partial shadings. Equivalent DC impedance of the panel is utilised to detect partial shading. Then, the shaded panel is open circuited using a simple relay to prevent hot spotting. The detection is confirmed or declined by measuring Thevenin impedance of the panel in a reclosing strategy. In the reclosing strategy, the panel is reconnected to the PV system for temporary partial shadings or disconnected from the system for permanent partial shading. The Thevenin impedance is calculated using open circuit voltage and short circuit current of the panel measured by a current sensor and a simple resistive voltage divider. The performance of the proposed method is evaluated using some experiments. The results confirm the performance of the technique for prevention of hot spotting.

This study presents a modified vector auto-regressive moving average (VARMA) modelling procedure to model spatially and temporally correlated wind speed time series over wide geographical areas. The standard VARMA is normally used to model stationary time series with Gaussian distribution. However, wind speed is non-stationary (mean and variance varies over time) and non-Gaussian. Hence, a method that can be used to transform wind speed data into a stationary and Gaussian time series is introduced in the modified procedure. To show the applicability of the procedure for different scenarios, six cases are investigated in the North and the Baltic Sea. The results show that the procedure can be used to model spatially and temporally correlated wind speed over a large geographical area. In addition, the resulting model can capture probability distribution and periodic characteristics of the wind speed data. Furthermore, based on the investigated case, it is shown that a vector auto-regressive model of order three is a reasonable model structure which can be used to model spatially and temporally correlated wind speed in the North and the Baltic Sea area provided that the power transformed wind speed data is normalised by its monthly mean value and its variance.

The main contribution of this study is to propose a novel optimisation technique for optimal sizing of hybrid power generation systems (HPGSs). In power system, size optimisation of an HPGS is a complex optimisation problem which needs a superior optimisation technique to be effectively solved. In this study, a photovoltaic–diesel (PV/diesel) HPGS is designed to supply the electrical load of an off-grid remote area. In such system, size optimisation is defined by the determination of the size of the diesel generator and the PV surface area to have a cost-effective and reliable HPGS. In this study, a Levy flight-based particle swarm optimisation (PSO_levy) is proposed for solving the sizing problem. In PSO_levy, the velocity term of the original PSO is replaced by a Levy flight term to promote the population diversity. Simulation results indicate that PSO_levy produces comparative results in comparison with the other PSO variants.

The main contributions of this study are to (i) incorporate tidal power into a hybrid PV/wind/battery renewable energy system and (ii) introduce a new metaheuristic technique named crow search algorithm (CSA) for optimisation of the PV/wind/tidal/battery system. For this aim, power equations of the different components are introduced and an objective function is defined based on the economic analysis of the system. The proposed CSA is then used to optimally size the PV/wind/tidal/battery system. On the case study, simulation results show that using tidal energy decreases the total cost of the system. Moreover, the proposed CSA produces better results in comparison with two well-known metaheuristic methods, namely, particle swarm optimisation and genetic algorithm in terms of accuracy and run time.

As photovoltaic (PV) integration increases in distribution systems, to investigate the maximum allowable PV integration capacity for a district distribution system becomes necessary in the planning phase, an optimisation model is thus proposed to evaluate the maximum PV integration capacity while guaranteeing the entire system operating constraints (e.g. network voltage magnitude) within reasonable ranges in this study. Meanwhile, optimal inverter dispatch is employed to further improve the PV integration by ensuring the optimal set-points of both active power and reactive power for the PV inverters. However, the intermittency of solar PV energy (e.g. due to passing clouds) may affect the PV generation in the district distribution network. To address this issue, the voltage magnitude constraints under the cloud shading conditions should be taken into account in the optimisation model, which can be formulated as a mixed integer non-linear non-convex programming. Furthermore, a sequential interior-point method is utilised to solve this problem. Case studies on the IEEE 33-bus, 69-bus distribution networks and two practical distribution networks in China demonstrate the effectiveness of the proposed method.

In this study, wind energy potential in the South of Turkey was investigated statistically by using the Turkish State Meteorological Service's hourly wind speed data between 2009 and 2013. The wind data used in this study were gathered from the Meteorology Station in Hatay and Osmaniye. In this study, different numerical methods were analysed and their performances were compared for effectiveness in determining the shape ‘k’ and scale ‘c’ parameters of the Weibull distribution function for two different regions. Six different methods: namely, graphical method, empirical method, maximum likelihood method, energy trend method, energy pattern method and moment method were used to estimate the Weibull parameters. The following statistical indicators were used for comparing the efficiency of all the methods used: the root mean square error, analysis of variance (R ²) and mean percentage error. Wind power densities were also calculated for all numerical methods used in this study. The power density is the key issue for the suitable use of wind energy. The calculated power densities for all methods used were compared with wind power density derived from measured wind data for two regions.

This study deals with a double-stage single-phase grid-connected photovoltaic (PV) system operating with an additional feed-forward control loop (FFCL). Owing to the PV array being constantly subjected to abrupt solar irradiance change, the DC-bus voltage varies and can interfere in adequate PV system operation. Therefore, an FFCL is proposed to improve the DC-bus voltage dynamic response, and reduce the settling time and overshoot. The FFCL acts on the generation of the inverter current reference, such that the dynamic behaviour of the current injected into the grid is also improved. Furthermore, the PV system performance is affected by problems associated with mismatching phenomena such as partial shading. This problem can be overcome using the maximum power point tracking (MPPT) technique based on particle swarm optimisation (PSO). The PSO-based MPPT is compared with the conventional perturb-and-observe MPPT technique, in order to highlight its effectiveness. In this study, the PV system also performs active power-line conditioning. Thereby, whereas the step-up DC–DC converter carries out the MPPT, the proper inverter current reference is computed to inject active power into the grid, as well as perform power-line conditioning. The performance and effectiveness of the PV system are evaluated through extensive experimental tests.

With the development of the energy internet, the standalone wind–solar–battery hybrid power system has gradually become an effective means for the complete utilisation of clean energy for power generation. The allocation of the various types of power sources for ensuring a reliable power supply and for improving the economic efficiency of a system is a crucial issue in the system planning and design stages. The models of the power load and the power sources, containing random components, are first established in this study, based on which the power supply reliability and constraint conditions for the equipment quantities are considered for establishing various evaluation indexes for improving the system performance. Then, according to the combinations of the natural selection particle swarm optimisation algorithm and the weight coefficient transform method, a multi-objective optimisation algorithm is proposed for optimising the configuration of a standalone wind–solar–battery hybrid power system. Finally, it is demonstrated by an example that the optimisation method not only meets the requirements of the multi-objective optimisation system but is also significant in reducing energy wastes and fluctuations, cost saving, and in ensuring the reliability of the power supply.