The Kriegers Flak combined grid solution (KF CGS) is the first hybrid-asset project utilising an offshore high voltage alternating current/high voltage direct current (HVAC/HVDC) interconnector for large offshore wind power plants between Eastern Denmark and Germany in the Baltic Sea. On the Danish side, above 200 km 220 kV cables are arranged into a meshed offshore transmission grid (MOG). In order to maintain good voltages in the MOG and wind power plants, on the Danish side, the control is realised by an automatic voltage regulation and reactive power control (AVR/RPC) of several switchable and variable reactors and transformers with tap-position controllers, and the wind power plant controllers (WPPC); on the German side, the voltage control is achieved by the HVDC back-to-back converter, WPPC, and part of the optimal power flow by the master controller for interconnector operation. This paper presents the proposed coordinated voltage and RPC of the HVAC MOG on the Danish side of the KF CGS interconnector. The efficiency and constraints of the proposed coordinated control scheme of the KF CGS at fluctuating voltage and reactive power on both ends of the MOG are presented by simulations using the entire set up of the 220 kV AC MOG.

Based on the two-stage-type configuration of direct current (DC) photovoltaic (PV) power collection network, the key issues of coordination start-up and operation mode switching for the converters in different stages are investigated thoroughly in this study. With the assistance of start-up resistors on low-voltage DC (LVDC) side, the asynchronous start-up, master–slave synchronous start-up and droop synchronous start-up for the first-stage maximum power point tracking (MPPT) converters are proposed along with the corresponding inter-stage coordination scheme. Besides, two start-up strategies for the second-stage DC step-up converter are proposed with the considerations of different scenarios. According to the proposed scenario-driven coordination strategies, the start-up of DC collection network could be realised without relying on real-time communication. The principles, logical flow and parameter design methods of the proposed scenario-driven coordination start-up strategies are explained in details while a comparative study is performed. The correctness and effectiveness of the proposed strategies have been verified by the system simulation. The work of this study could be beneficial for the future engineering applications of DC PV generation and integration.

As the share of renewable energy sources increases, the grid frequency becomes more unstable. Therefore, grid balancing services will become more important in the future. Dedicated devices can be installed close to the point where off-shore wind farms are connected to the transmission grid on land. There, they can be used to attenuate power variations, reduce congestion and offer grid balancing. The provision of these ancillary services can create considerable additional economic revenue. In this study, the provision of the primary reserve by means of a large hydrogen electrolyser of 25 MW is investigated for the specific case of the Belgian transmission system. The revenue of the provision of the frequency containment reserve (FCR) is analysed on a techno-economic model, including capital costs, operational costs, the revenue of the generated hydrogen and oxygen products and the ancillary service income. The revenue depends strongly on the contracted power band. Therefore, it is optimised to yield maximum revenue. The results show that providing FCR creates considerable additional revenue. Therefore, a large electrolyser can be a good candidate to buffer excess renewable energy into green gas while simultaneously providing the grid support.

The life loss of batteries caused by the daily operation implies a reduction in capital value, which is essential for the economic performance of storage-containing systems. Most of current studies rarely considered it or simplified it to be proportional of throughput electricity, due to its multi-factor dependence and complexity to be incorporated into dispatch models. This study presents a dynamic loss evaluation model for batteries that considers the cumulative effect of state of charge (SOC) changes. First, based on the results of battery aging test, the loss coefficient subject to SOC is derived. The general formulation of analytical battery life loss is further presented by integrating the damage effect during the change in SOC. Finally, by means of self-optimal piecewise linearisation, the resultant life loss term is embedded in the online wind-storage integrated scheduling. Case studies demonstrate the desirable computational complexity of the proposed evaluation method and the adaptability in general economic dispatch.

The phase-locked loop (PLL) is an available synchronisation technique that can extract the fundamental frequency and phase of the grid. Benefited from the filter properties, pre-filter PLL is widely applied in voltage source inverters (VSIs) to enhance the accuracy of phase extraction. With the consideration of inter-harmonic perturbation, the principles between pre-filter PLL and VSI performance as well as the effects of pre-filter PLL on VSI are investigated in this study. Based on the conventional PLL, the small signal of pre-filter PLL is proposed. To fully represent the pre-filter PLL effects, the simplified VSI model is established and the current performance of VSI could be presented by the pre-filter PLL. From the angles of current total harmonic distortion, the effects of pre-filter PLL under inter-harmonic perturbation are clarified. The effects of pre-filter PLL on VSI are validated under the inter-harmonic perturbation based on the experiment platform. The results indicate that the current performance of VSI could be improved with the decreased bandwidth of both pre-filter and PLL, which shows a valuable application reference for the optimised design of VSI.

As a new highly penetrated power source of modern power systems, doubly fed induction generator (DFIG)-based wind turbine (WT) has a different fault current contribution when compared with a synchronous generator. DFIG-based WT's fault current has significant impacts on distance relay's performances and may result in maloperation. In this study, the impacts of DFIG-based WT's fault current on the dynamic performances of two typical distance protection elements, mho and polarisation, are studied in terms of EMTP simulations and fault current expressions in symmetrical fault scenarios. It is discovered that the zone-1 distance relays of the lines connecting to DFIG-based wind power plants tend to overreach. Also, the following analysis indicates that the transient components of DFIG-based WT's fault current and the phasor estimation unit are the underlying reasons. With those understanding, this study is expected to help design new setting methods and for distance protection in power systems integrating wind power plants to prevent this kind of possible overreach.

This study investigates the model-free assessment of small-disturbance stability determined by the DC-link voltage dynamics in a weakly grid-connected wind farm with M permanent magnet synchronous generators (PMSGs). A reduced-order model of the PMSG wind farm is established, which is the approximate representation of the wind farm dominated by the DC-link voltage dynamics. Based on the reduced-order model, limit of small-disturbance stability of the PMSG wind farm determined by the DC-link voltage dynamics is derived. Calculation of derived stability limit does not need to have the parametric model and can be used to approximately assess the small-disturbance stability of the PMSG wind farm. Hence, the assessment made by using the derived stability limit is model-free and useful in practice when it is usually difficult to establish the parametric model of the wind farm. In addition, the derived stability limit clearly indicates and explains that when the grid connection weakens or/and the number of PMSGs increases, the DC-link voltage dynamics are more likely to cause the instability of the PMSG wind farm. In the study, an example AC grid with a grid-connected PMSG wind farm is presented to demonstrate and evaluate the model-free assessment of small-disturbance stability of the wind farm.

With the increase of grid-connected renewable power generation units, the three-level converter (TLC) is widely utilized. However, when unbalanced grid voltage occurs, the converter should provide a certain of reactive power to satisfy the grid code, which decreases the power factor and degrades neutral-point (NP) voltage balance. The existing model predictive control methods for NP voltage balance in TLC mainly focus on high power factor operation, which are not applicable to low power factor operation. To solve this problem, this study proposes a generalized model predictive control (GMPC) scheme for the TLC. By analyzing the influence of P-type and N-type small vectors on NP voltage, it is indicated that the charging and discharging operation of the dc-link capacitors are determined by the grid currents directions with the defined small vectors. Since the directions of grid currents are changed with different power factors, the effect of one small vector on NP voltage is change. In the proposed GMPC scheme, the direction of grid currents and the capacitor voltage difference are simultaneously used to select the appropriate small vector to mitigate the NP voltage unbalance effectively in the full power factor range. The validity is verified by simulation and experimental results.

This study presents a grid-integrated solar photovoltaic system (SPVS). The objective of the work is to explore the scope of cascaded H-bridge multilevel inverter (CHBMLI) in the high infiltration of photovoltaic (PV) in the distribution system. A five-level CHBMLI is implemented with a balanced power-sharing and equal switch stress mode, which increases the system power rating with longer life. The power conversion efficiency is elevated by reduced switching frequency, compared to two-level inverters. The damped second-order generalised integral (DSOGI) control-based grid synchronisation is used to critically damp system oscillations and overshoots at dynamic operating conditions. Hence, safer service of lower rating power electronic switches at high power operation is ensured. It also has better bandpass filtering at the nominal grid frequency. A single-input multiple-output single-ended primary inductance converter realises the isolated DC sources for the CHBMLI. It extracts maximum power from the PV array by using centralised incremental conductance algorithm. The SPVS also considers the nonlinear unbalanced loads at the bus of common coupling and provide shunt active filter functionality. A minimum number of voltage and current sensors are used in the configuration. An extensive analysis is carried out to validate the theoretical claims.

In order to ensure the uninterrupted power supply, it is necessary for the microgrid to operate in both grid-connected mode and islanded mode. To address this concern, this study proposes a unified decentralised control strategy for both grid-connected and islanded operation of the cascaded-type microgrid (CMG). It can realise the smooth mode switching without the need of changing controllers. In terms of the proposed scheme, the system always holds a unique equilibrium point regardless of the grid-connected or islanded operation. Since the CMG is performed in a decentralised manner without any communication, it is a reliable and cost-effective solution. Moreover, the self-synchronisation of each DG is obtained under both the resistance–inductance (RL) and resistance–capacitance (RC) loads. The small-signal stability is proved and the design of control parameters is given. Finally, the feasibility of the proposed method is verified by simulation and OPAL-RT-based real-time simulation results.

Microgrids are often accomplished with power-dense and energy-dense hybrid storage systems for the assured improvement in the stability and the awake time. The conventional droop based steady-state power bifurcation strategies for the energy-dense storage elements can efficiently improve the awake time. However, the improvement in the stability and the robustness demands accurate transient demand allocation. The constant cut-off frequency-based physical power decomposition filter cannot ensure an accurate transient demand allocation, particularly, while the demand is fluctuating and uncertain. Therefore, the study proposes the concept of pseudo-power decomposition filter. The concept of the pseudo-decomposition filter has been executed for the microgrid through the novel selected power component droop. The real-time performance of the proposed concept has been evaluated in the BTS-7960 packaged-Metal Oxide Semiconductor Field Effect Transistor (MOSFET)-based hardware platform, regularised through the ARM Cortex A-72 based digital signal processor. In respect to the conventional linear droop, the improvement through the concept in the power decomposition and the allocation of transient demands is significant.

Though perovskite solar cells have outstanding commercialisation potential, perovskite materials are very vulnerable to moisture and oxygen. So the main issue for perovskite solar cell is very fast degradation. In order to elaborate on this concern, the stability of perovskite solar cells were investigated in this study. The effect of oxygen and moisture on perovskite solar cells decreased with encapsulation of the cells. Encapsulation has been carried out with ultraviolet cured epoxy resin. Stability tests were taken place in laboratory conditions under solar simulator and also shelf-life tests. Perovskite solar cells showed the highest power conversion efficiency (PCE) of 9.77% with a short circuit current density of 22.5 mA/cm², the open-circuit voltage of 0.94 V and fill factor of 0.46. After 2800 h, perovskite solar cells were still 85% stable in atmospheric conditions. Perovskite solar cells exhibited distinctive durability for shelf-life test in laboratory conditions. The PCE of perovskite solar cells were decreased only 15% of its initial value after 2800 h. In order to elaborate the degradation of perovskite solar cells, the cells are exposed to solar irradiation AM1.5G. Perovskite solar cells degraded gradually with time under solar irradiation; however, this stability test indicated that the solar cells were durable 20% after 500 h exposure to solar irradiation.

Partial shading in photovoltaic (PV) power generating systems is practically unavoidable and is a menace to the PV system. Non-uniform shading can be caused due to clouds, tree branches, buildings or other nearby items. The non-linear output characteristics of a PV source varies with temperature and solar insolation. During partial shading condition, the conventional hill-climbing methods of maximum power point tracking (MPPT) prove to be inefficient. In partial shading, the power–voltage curve exhibits multiple peaks. Conventional MPPT methods get stuck on one of the local maxima and fail to attain the global maximum power point (GMPP). Particle swarm optimisation (PSO) has proved to be a very accurate and powerful technique to find GMPP. In this study, the authors improved the conventional PSO by reducing swarm size, as well as, the number of iterations to achieve MPP at a fast pace with remarkable accuracy, thus making the system more efficient with less computational effort. Simulation and hardware implementation for performance evaluation of the proposed technique is carried out to verify various qualities under different partial shading conditions, such as flexibility, reliability, system-independence and high accuracy in tracking the GMPP under non-uniform insolations.

The generated power of the Sea-wave slot-coned generator (SSG), as one of the high capacity wave energy converters, is dependent on the period and height of the waves. The generated power of this energy-converter changes due to the variation in these parameters. Thus, different aspects of the power systems may be affected that must be investigated by increasing the share of these converters for electricity production in the power-systems. In this paper reliability assessment of composite power-systems containing large-scale SSG is performed. The Xie and Beni (XB) index is calculated for obtaining the optimum number of states in the reliability model of the SSG and fuzzy c-means clustering technique is proposed for reducing the number of states of the model. The contingency analysis method is proposed for calculating the reliability indices of the composite power-system incorporating large-scale SSG. DC load-flow method, which considers line capacity, is used to perform load-flow in the composite power-system including generation and transmission networks. In the proposed technique, a load-shedding program with the goal of minimizing the value of lost load is solved using of the linear programming method based on the interior-point-approach for contingency states leading to the load curtailment.

The uncertainty of wind power brings great challenges to large-scale wind power integration. The conventional point forecasting of wind power is difficult to meet the demand of power grid planning and operation. A novel two-stage short-term hybrid wind power interval forecasting model is proposed in this study. In the first stage, the original wind power data is automatically decomposed and divided into three different classes based on the data preprocessing method combining variational mode decomposition with sample entropy. In the second stage, the prediction model is established using the probabilistic regularised extreme learning machine (PRELM) and particle swarm optimisation (PSO). In view of the different characteristics of the subseries in the above three classes, prediction intervals (PIs) are constructed for each subseries. A novel interval evaluation index is used as the objective function of PSO to optimise the PRELM output weight matrix to find the optimal PIs. Also the prediction results of each subseries are reconstructed to obtain the final wind power prediction results. The numerical results based on actual wind power data show that the proposed model shows better performance compared with other methods and can effectively improve the prediction accuracy.

A wind-storage system (WSS) can use the synergy between wind farms and battery storages to mitigate the uncertainty of wind. With the flexible ramping capability of battery storages, a WSS can diversify its portfolio by selling energy, committing regulation services and offering flexible ramping products in a power market. The portfolio management problem of a WSS is studied here. The uncertainties of market prices and wind power are considered. In order to maximise the expected profit in the worst case, a distributionally robust optimisation model is proposed. The model is then reformulated as a solvable semidefinite optimisation problem. Real market data are used in the simulation, which validates the proposed method.

A photovoltaic energy harvesting system has been developed for application in smart clothing for mountain rescuers. The generator has been assembled from flexible organic photovoltaic modules and integrated with a specially designed clothing. A power conversion and storage system has been prototyped, providing a USB standard output for supplying active clothing components or standalone devices. The clothing with the integrated energy harvester has been comprehensively tested in respect of both electrical performance and ergonomics. An electrical test methodology has been developed, based on the characteristics recorded in the field and their simulation in a laboratory. This enabled repetitive tests under identical real-life conditions. Ergonomic tests involved diverse physical activities performed by mountain rescuers, simulating their true operations, but conducted in a training room for the sake of standardisation. The prototype system has substantially extended the operating time of a realistic load (equivalent to a smartphone) by providing an extra 1.91 Wh of energy in the best case of irradiance in autumn and 0.77 Wh for a typical case with varying irradiance and temperature. Ergonomics ratings by end users have been good to very good and they have been generally in favour of using power harvesters integrated with their clothing.

The excess hydraulic pressure in water supply network pipes is one of the major problems in design and implementation stages of these projects. To reduce the excess pressure and maintain network safety, a variety of pressure reducing valves are used, which waste all the excess pressure. However, having used the appropriate water microturbines, while reducing the pressure loss to an optimum value, the system could recover the wasted energy. In this study, numerical simulation and experimental investigation of microturbine in a water supply system have been performed to evaluate the electrical power generation capacity of an over-pressured network. The numerical simulation of the microturbine was conducted using Ansys-Fluent software. The microturbine has been experimentally investigated with different inlet flow rates, the effect of different post-microturbine heads and also the effect of different opening angles of guide plate on its performance, three different scenarios were defined. Based on numerical simulation and laboratory results, the best performance of the microturbine was obtained when the inlet flow rate was 0.01184 m³/s and also opening angle 20° for the guide plate, in which the microturbine output was equal to 59.01 W. The generated power by the microturbine can meet the electrical needs of sensors and other network monitoring equipment.

An essential non-linear flow-energy converter based on fully-passive oscillating-foil is proposed to improve the adaptation under varied flow conditions. The converter consists of a C-shape foil elastically supported by two springs, an arm that transfers the plunging motion into swing motion and two dampers to absorb mechanical energy. For the purpose of improving performance within a broader range of exciting frequency under varied flow speeds the authors employ two essential non-linear springs in the converter. The adaptation and performance of an essential non-linear converter were assessed numerically. The effects of non-linear stiffness coefficients, varied damping factors, and speed ratios are investigated in detail. The results indicated that the effective range of speed ratio (range of synchronisation) is broadened from (0.4, 2.0) to (0.2, 4.0) with appropriately tuned non-linear stiffness, so the adaptation of the flow energy converter based on oscillating-foil is improved, rendering it more suitable for varied flow conditions and real applications. Besides, performance gains at the design point can be realised as compared with the linear converter. It is found that two fundamental mechanisms are advantageous to enhance the adaptation and performance of the converter: the increased effective stiffness with oscillating amplitude and the non-sinusoidal swing motion.

This study establishes a set of frequency-domain phase margin-oriented quantitative design guidelines for deriving coefficients of optimised multi-resonant AC current regulators, consisting of series-connected terms. Such an arrangement allows straightforward extension of existing design guidelines for developing coefficients of optimised single-resonant AC current regulators. Practical restrictions imposed by, relatively, low switching frequency are also considered, yielding corresponding design guidelines generalisation.

This study introduces a new biodiesel blend as an alternative for diesel using waste cooking oil methyl ester by adding tyre pyrolysis oil and cerium oxide. Despite the conventional biodiesel blending models, this study made an effort to efficiently measure the prediction rate of these blended fuels by modelling through the deep belief network (DBN). To attain the accurate prediction, this study moves on with the new logic of optimal tuning of the count of hidden neurons in DBN. The optimal selection is carried out by introducing a new algorithm named lioness updated crow search algorithm (LCSA), which hybrids the concept of the lion algorithm (LA) and crow search algorithm (CSA). Finally, the proposed work is analysed and compared over other conventional models with respect to emission analysis and error analysis. From the analysis, the proposed model in terms of mean deviation (MD) measure has gained betterment and is 75.57, 17.71, 85.55, and 74.19% better than grey wolf optimiser (GWO), whale optimisation algorithm (WOA), LA, and CSA, respectively. For the mean absolute error measure, the implemented model is 42.38, 24.42, 43.53 and 36.72% improved than GWO, WOA, LA, and CSA, respectively.

Voltage profiles may exceed the upper limit in a distribution network with a high penetration of distributed generation (DG). Voltage can be effectively regulated by curtailing the active power of DG and adjusting the reactive power. Soft open points (SOPs) have flexible active power transfer and reactive power regulation capabilities that can help reduce the active power curtailment of DG. Asymmetric network parameters, single-phase loads, and DGs cause voltage unbalance in distribution networks. This study proposes an optimal power control model of SOPs and DG converters in unbalanced distribution networks (UDNs). First, the individual phase power control characteristics of SOPs and DG converters in UDNs are modelled. Second, an optimisation model is constructed with the minimum active power curtailment of DG and the total power loss and voltage unbalance of UDNs as the objective function and with consideration of the networks and SOP operation constraints. The original optimisation model with non-convex and non-linear characteristics is transformed using power flow linearisation and linear relaxation into a linear programming model that can be efficiently solved. Finally, the feasibility and efficiency of the proposed model are verified using an IEEE 33-node system.

This work deals with the implementation, control and operation of a reconfigurable small hydro-solar photovoltaic (PV)-battery energy storage (BES)-based distributed generation system (DGS) working in isolated mode (IM) and grid connected mode (GCM). The DGS consists of a hydro generation-based on a PMSG integrated with a PV array supported by the BES. It operates in an IM during the grid outage and when the grid is available, it synchronizes and operates in GCM. A frequency adaptive comb filter-frequency locked loop (FLL)-based control is adopted to estimate errorless phase angles of the hydro generator and grid voltages for quick and accurate grid synchronization. A third-order band pass filter-based control technique is used to extract fundamental component of load currents and determine switching pulses to VSC. The overall multi-objective control is used to maintain voltage and frequency of the system at the point of common coupling during IM and GCM. The power quality of grid and generator currents are maintained at non-linear loads. The modified perturb and observe-based control technique is used to extract maximum power from the PV array and it provides drift-free operation during the change in insolation with de-rating capability to protect the BES from overcharging in IM.

The limited reserve of fossil fuels and public awareness of environmental issues prompt the rapid development of renewable energy generation. However, the centralised utilisation of renewable energy in bulk power systems is impeded mainly by its volatile nature and transmission congestion, leading to the spillage of renewable power. The energy storage unit is expected to be a promising measure to smooth the output of renewable plants and reduce the curtailment rate. This study addresses the energy storage sizing problem in bulk power systems. To capture the operating status of the power system more accurately, the authors use a dedicated power flow model which involves voltage and reactive power. The uncertainty of renewable generation is described via inexact probability distributions encapsulated in a data-driven Wasserstein-metric based ambiguity set, based on which the renewable energy curtailment rate is formulated as a distributionally robust chance constraint. The objective is to minimise the total investment cost, and the optimal sizing problem gives rise to a distributionally robust chance-constrained program, and is reformulated as a tractable linear program via conservative approximation. Case studies conducted on the modified IEEE 30-bus and 118-bus systems demonstrate the effectiveness and performance of the proposed approach.