Today's residential battery energy storage systems (BESSs) are off the shelf products used to increase the self-consumption of residential photovoltaic (PV) plants and to reduce the losses related to energy transfer in distribution grids. This work investigates the economic viability of adding a BESS to a residential grid-connected PV plant by using a methodology for optimising the size of the BESS. The identification of the optimal size which minimises the total cost of the system is not trivial; indeed, it is a trade-off between OPEX and CAPEX, which are mainly affected by the battery technology, usage profile, expected lifetime, and efficiency. Here, an analysis of the opportunity to install a storage system together with a grid-connected residential PV plant is performed. Three typical low-voltage prosumers (Italy, Switzerland, and the UK) are investigated in order to take into account the different legislative and tariff framework over Europe. Numerical results reported here show that present costs of storages are still too high to allow an economic convenience of the storage installation. Moreover, an indication of the necessary incentives to allow the spreading of these systems is given.

Partial shading, commonly observed in domestic rooftop solar photovoltaic (PV) deployments, can be highly detrimental to the performance ratio (PR) of a PV system. Typically, for domestic installations, string-inverter or module micro-inverter configurations are deployed. While module level micro-inverters generally present a better response to non-uniform distributions of sunlight, they are still less common and therefore, costly in many emerging markets. String-level implementations, on the other hand, are widely deployed as they are less complex and cost efficient. In this work, the authors present an analytical and simulation framework for improving PR under partial shading conditions through alteration of string connections in a string-level inverter system. Results show up to 4.6% higher PR in winter months for a 42.24 kWp system installed at Lahore University of Management Sciences, Lahore, Pakistan.

A high-efficiency, three-phase, solar photovoltaic (PV) inverter is presented that has low ground current and is suitable for direct connection to the low voltage (LV) grid. The proposed topology includes a three-phase, two-level (2L) voltage source inverter (VSI) and an active common-mode (CM) filter. The VSI utilises silicon (Si) insulated-gate bipolar transistors with silicon carbide (SiC) diodes to reduce switching losses and provide very high efficiency. The active CM filter reduces the high level of CM voltage associated with the three-phase 2L VSI. The active CM filter is controlled so that the PV ground current is reduced to acceptable levels, even when the PV inverter is connected directly to a LV grid with low-impedance grounding. 50 and 100 kW prototypes are presented that demonstrate high efficiency.

Perfect power system voltage stability is not possible in practice. Generally, the power grid is continually exposed to changes in its load and operating conditions. Therefore, dynamic stability analysis is one the most important and effective elements for greater security, stability and reliability of planning, design, operation and economic aspects of electric power networks. This study investigates and reports on the dynamic stability of the power system with a large-scale photovoltaic system (L-S PV). Two different scenarios with centralised PV power plants are considered in the medium voltage level without voltage regulation capabilities. Simulation results with these scenarios will show how the voltage instability decreases with the L-S PV based on the bus status, disturbance location, and disturbance duration. In addition, the study discusses network terminal voltage, generator's rotor angle, generator's terminal current, generator's reactive power and electrical power output. This study is an extension of the earlier published conference paper.

The increased net-demand uncertainty and volatility observed in power systems with large-scale penetration of intermittent renewables has translated into the deployment of larger volumes of reserve and into the need for procuring new sources of flexibility. In order to cope with increased uncertainty and risk of experiencing low profits, wind farm owners must adopt flexible bidding strategies such as coordinating its operation with energy storage systems (ESS). Besides managing wind imbalances, ESS are also capable of providing ancillary services such as spinning reserve (SPR) and frequency response to improve profitability. In this context, this paper proposes a novel two-stage stochastic mathematical programming model that allows considering different degrees of risk aversion when optimising the day-ahead energy and SPR bidding strategy of a wind farm with on-site ESS. Uncertainty is modelled through prices and wind generation forecasts, while the conditional-value-at-risk metric is used to handle day-ahead profit risk. The developed case studies provide evidence of the value of combined wind farm and ESS bidding not only through increased daily profits but also through reduced offer uncertainty which improves the position of a wind farm in the day-ahead markets.

The marine energy industry is in its early stages but has a large potential for growth. One of the most significant challenges is the reduction of operation and maintenance costs. Magnetic gears (MGs) offer the potential for long periods between maintenance intervals due to their frictionless torque transmission which could reduce these costs. This study presents a summary of the state of the art in MG technology and then investigates its potential for marine energy applications. A brief overview is given of the state of the marine energy industry and the environment in which marine energy converters (MECs) operate. A short history of MG development over the past century is then presented followed by a discussion of the leading MG technologies and their relative advantages. In order to demonstrate the potential of MGs in marine applications, the current technologies, i.e. mechanically geared and direct drive machines, are examined in terms of sizing, reliability and economic value using previous studies on a similar technology, namely wind. MGs are applied to four types of MECs to demonstrate how the technology can be incorporated. The potential to deploy at scale and potential obstacles to this are then discussed.

The majority of electrical failures in wind turbines occur in the generator-side semiconductor devices. This is due to temperature swings affecting the layers of insulated-gate bipolar transistors in different ways; these effects are aggravated by the variation of wind speed. The implementation of accurate on-line condition monitoring mainly relies on on-line tracking of the temperature variation of components. The maximum temperature stress is observed at the junction terminal of the devices, which cannot be easily measured. Additionally, it is difficult to track the exact dynamic of temperature due to the slow response of sensors. Thus, several methods have been presented in the technical literature to estimate the junction temperature of the semiconductor devices. Each method has merits and disadvantages in terms of accuracy and complexity, as failure mechanisms have their own effects on the variation of junction temperature and some or all of the electrical parameters. Therefore, detection algorithms have to determine the root cause of the temperature variation. This study comparatively reviews the condition monitoring methods presented so far and gives directions on the future steps that should be addressed by research in this area.

In this study, an improved voltage stability index that can evaluate the unstable behaviour of the power system with doubly-fed induction generator (DFIG) wind parks integration is presented. Accordingly, voltage stability constrained optimal power flow (VSC-OPF) is studied including an improved impedance-based (IB) index. In particular, this study has two main contributions. First, it proposes an IB index with consideration of DFIG capability curve limits and on-load tap changer (OLTC) behaviour. The proposed voltage stability index can detect precisely the voltage collapse, especially after the occurrence of a given contingency due to the dynamic elements of the system. In this study, a model is proposed for DFIG capability curve limits that can be integrated to the internal circuit of the generator. In particular, the proposed model can be appended to impedance matching theory. Furthermore, the OLTC model is added to this index. The index uses the concept of the coupled single-port circuit. The second contribution proposes a VSCOPF with an improved IB index that is also compared with three existing VSC-OPF methods in stressed and single line outage conditions. These approaches are tested and validated on modified WSCC test system, IEEE 39-bus, IEEE 57-bus and Polish 2746-bus systems.

This study proposes a coordinated voltage control scheme based on model predictive control for voltage source converter (VSC)-based high voltage direct current connected wind power plants. In the proposed scheme, voltage regulation capabilities of VSC and wind turbine generators (WTGs) are fully utilised and optimally coordinated. Two control modes, namely the operation optimisation mode and corrective mode, are designed to coordinate voltage control and economic operation of the system. In the first mode, the control objective includes the bus voltages, power losses and dynamic Var reserves of WTGs. Only the terminal voltages of WTGs are considered in the second mode. The predictive model of the system including VSC and WTGs is developed firstly. The calculation of sensitivity coefficients is done by an analytical method to improve the computational efficiency. Simulation results are presented to demonstrate the effectiveness of the proposed controller and the control performance is compared with conventional optimal control and loss minimisation control. Besides, the robustness of the proposed controller to communication time delay and measurement errors is investigated in the last.

This study deals with a new version of hill-climbing (HC) algorithm for maximum power peak estimation of the solar photovoltaic (PV) panel, which has self-estimation (SEn) and decision-taking ability. Moreover, it is based on a single sensor, which is applicable for the PV array-fed battery charging. The working principle of Self-Estimated HC (SEHC) algorithm is based on three consecutive operating points on the power-current characteristic. By using perpendicular line analogy (PLA), these points decide direction, and an optimised operating position for next iteration, which is responsible for quick maximum power point tracking as well as improved dynamic performance. Moreover, in every new iteration, the step size is reduced by 90% from the previous step size, which provides an oscillation-free steady-state performance. Owing to a single sensor, the computational burden, as well as calculation complexity of the SEHC algorithm is less, so it can be simply implemented on a cheaper microcontroller. The effectiveness of the proposed technique is validated by MATLAB simulation as well as tested on the experimental prototype. Moreover, the performance of SEHC algorithm is compared with recent state-of-the-art techniques. The highly suitable and satisfactory results of SEHC algorithm show the superiority over the state-of-the-art methods.

Intentional islanding operation (IIO) is a feasible solution to improve the reliability of active distribution network (ADN) by supplying critical loads through the local DG when a fault occurs. Aiming at this goal, a new two-stage methodology is proposed to supply critical loads based on cost-effective improvement. In the first stage, the interruption cost is proposed as the load priority and the ON/OFF status of switches are considered as the binary decision variables. Therefore, IIO is considered as a mixed integer linear programming (MILP) problem to minimise the interruption cost. At the second stage, the power flow calculation is performed on the initial islands for the real-time operation. The proposed method can be utilised for both long- and short-term studies. In the long-term study, the inherent uncertainty of ADN is considered in MILP by using a Monte-Carlo simulation. This concept is used for clustering ADN into self-sufficient microgrids. Moreover, by taking a snapshot of the ADN status and performing operational feasibility, the proposed method can be considered as a real-time power regulation method. The proposed methodology is implemented on the IEEE 33-bus distribution network, and the results are discussed in detail.

A hybrid microgrid has numerous decentralised control loops. Therefore, coordination among hybrid microgrid subsystems with desired performance is essential. This study presents a practical control approach for efficient tuning of proportional–integral (PI) controllers and leads compensators in islanded hybrid microgrids. This method is based on the frequency response characteristic and root-locus trajectory. It is used to minimise the frequency deviations of an AC hybrid microgrid. The presented well-tuned controllers are tuned based on droop mechanism, and coordination among hybrid microgrid subsystems with desired damping coefficient and stability margin. Then, the system performance is analysed under several disturbances. The results are compared with PI controllers tuned by Ziegler–Nichols method. As well, the robustness of the proposed approach in a wide range of parameter changes is investigated. Eigenvalue analysis and simulation results show that the minimum frequency deviations and desired relative stability of the hybrid microgrid subsystems are achieved by the proposed controllers. To show generality and efficiency of the proposed approach, the presented method is applied to a different hybrid microgrid model used in the literature. For this purpose, in order to control the frequency deviations in the stand-alone mode, presented well-tuned controller is compared with intelligent fuzzy and particle swarm optimisation-fuzzy controllers.

Technologies such as the integration of distributed generations (DGs) and shunt capacitors (SCs) are key for realising smart distribution systems. These technologies may be coordinated together to get better solutions so that distribution systems can achieve optimum performance. This study proposes a long-term distribution system planning methodology to determine the optimal sizing and siting of SCs and mix of dispatchable and intermittent DGs. More practical formulations are suggested while considering realities of modern distribution systems with reference to the stochastic nature of load demand and power generation among distribution buses on account of diversity, variability and uncertainty. The long-term costs in the proposed planning model include several techno-economic and social objectives pertaining to investment, operation and maintenance of these distributed resources and the revenue generated by sale of electricity to customers and the utility grid. The proposed planning model is more practical as it fully considers the system uncertainties and variability in load and power generation which are efficiently handled by introducing deterministic self-adaptive uncertainty model. The case study on a standard test distribution system being modified by wind turbines, photovoltaic generators and microturbines demonstrates the effectiveness of the proposed methodology. The results of study are investigated and presented.

This study presents the performance analysis of a new asymmetrical multi-level inverter using reduced number of switches for a single-phase grid-tied photovoltaic (PV) system. The solar PV panels of unequal power rating are connected in an appropriate manner to obtain the DC link voltages of suitable ratio for an asymmetrical cascaded multi-level inverter. The PV power, voltages as well as the current injected into the grid have been controlled using the separate maximum power point tracking, voltage controllers and a current controlled technique to achieve the maximum power with sinusoidal current with a unity power factor. The variations of DC link voltages, inverter voltage and injected grid current are simulated and are experimentally verified under the variable irradiation as well as grid voltage fluctuation. The simulation and the corresponding hardware results of the proposed reduced switch asymmetrical seven-level inverter for a low-power residential grid-tied PV system is also presented.