The connection of high penetrations of renewable generation such as wind to distribution networks requires new active management techniques. Curtailing distributed generation during periods of network congestion allows for a higher penetration of distributed wind to connect, however, it reduces the potential revenue from these wind turbines. Energy storage can be used to alleviate this and the store can also be used to carry out other tasks such as trading on an electricity spot market, a mode of operation known as arbitrage. The combination of available revenue streams is crucial in the financial viability of energy storage. This study presents a heuristic algorithm for the optimisation of revenue generated by an energy storage unit working with two revenue streams: generation-curtailment reduction and arbitrage. The algorithm is used to demonstrate the ability of storage to generate revenue and to reduce generation curtailment for two case study networks. Studies carried out include a single wind farm and multiple wind farms connected under a ‘last-in-first-out’ principle of access. The results clearly show that storage using both operating modes increases revenue over either mode individually. Moreover, energy storage is shown to be effective at reducing curtailment while increasing the utilisation of circuits linking the distribution and transmission networks. Finally, renewable subsidies are considered as a potential third revenue stream. It is interesting to note that under current market agreements such subsidies have the potential to perversely encourage the installation of inefficient storage technologies, because of increased losses facilitating greater “utilisation” of renewable generation.

Wind power plants (WPP) are for power system stability studies often represented with aggregated models where several wind turbines (WT) are aggregated into a single up-scaled model. The advantage is a reduction in the model complexity and the computational time, and for a number of study types the accuracy of the results has been found acceptable. A large WPP is, however, both modular and distributed over a large geographical area, and feasibility of aggregating the WTs, thus, have to be reassessed when new applications are introduced for WPPs. Here, the power oscillation damping capabilities are investigated for a WPP, which includes the full layout of the collector grid and where the WTs are represented individually. With this approach, the influence of the WT control in terms of impact on oscillatory modes is assessed for the WTs individually. The initial results encourage that park level control is possible. Time domain simulations support that each WT contribute to a common WPP response. Park level active and reactive power-based power oscillation damping controllers (POD) are designed and the positive damping contribution is demonstrated. Keeping the POD designs unchanged, the impact of WPP aggregation is investigated and it is shown that the level of WPP aggregation only has limited impact on the resulting modal damping. The study is based on a non-linear, dynamic model of the 3.6 MW Siemens Wind Power WT.

A standalone wind/solar/battery hybrid power system, making full use of the nature complementarity between wind and solar energy, has an extensive application prospect among various newly developed energy technologies. The capacity of the hybrid power system needs to be optimised in order to make a tradeoff between power reliability and cost. In this study, each part of the wind/solar/battery hybrid power system is analysed in detail and an objective function combining total owning cost and loss of power supply probability is built. To solve the problems with non-linearity, complexity and huge computation, an improved particle swarm optimisation (PSO) algorithm is developed, which integrates the taboo list to broaden the search range and introduces ‘restart’ and ‘disturbance’ operation to enhance the global searching capability. The simulation results indicate that the proposed algorithm is more stable and provides better results in solving the optimal allocation of the capacity of the standalone wind/solar/battery hybrid power system compared with the standard PSO algorithm.

Wind turbines are increasingly being expected to provide oscillation damping to the power system to which they are connected. In this study, power oscillation damping control of variable speed wind turbines is studied. An energy storage device with a bidirectional DC/DC converter connected to the DC link of a fully rated converter-based wind turbine is proposed. As system oscillation is often induced by an AC fault, it is desirable for wind turbines to ride through the fault first and then provide a damping effect. During the fault period, the energy storage system (ESS) is controlled to assist the fault ride through process, and the line side converter (LSC) is controlled to provide AC voltage support in accordance with the grid code. Methods based on regulating the active power output of the ESS and modulation of reactive power output of the LSC are proposed so as to damp the oscillations of the power system. Matlab/Simulink simulations based on a simplified Irish power system demonstrate the performance of the ESS and LSC during fault periods and validate the damping effect of the proposed system.

The droop control method is usually selected when several distributed generators (DGs) are connected in parallel forming an islanded microgrid. This is because of the advantages it offers such as flexibility, absence of critical communications etc. Besides, several studies add a fictitious impedance to improve the performance of the original droop method. However, only a few studies deal with the design of this fictitious impedance, which is necessary to ensure an improvement in the dynamics and stability of the microgrid. In addition, these studies do not consider load variations for the design of the fictitious impedance, which is a habitual event in these systems. On the other hand, some studies propose a restoration control to bring the frequency and voltage amplitude of the microgrid to their nominal values. However, these do not deal with the design of the dynamics of this control to maintain a good transient and to ensure the stable performance of the microgrid. This study proposes the design of a fictitious impedance that ensures the stable operation of an experimental microgrid without power oscillations during load jumps and throughout its entire load range. This study also proposes a new restoration control that allows to take into account the possible inertias, delays etc. of the DGs and reduces the bandwidth of the required communications. Moreover, the proposed restoration control is properly designed to guarantee a good transient and the satisfactory performance of the microgrid. Experimental results confirm the validity of the proposed controls.

The study presents a wind generator system based on a new converter configuration with a rectifier with near sinusoidal input currents (RNSICs-1 converter with DC capacitors connected in parallel with diodes). A detailed analysis of the system for different values of the load current is presented and the advantages of the solution are emphasised. The new converter configuration is characterised by smaller power losses, reduced electromagnetic interference (EMI) problems, low harmonic input currents, high reliability, as well as reduced costs. This original configuration could also be used for small hydro interconnection with squirrel cage induction generator (SCIG) and partial variable-speed wind turbine (typically 60–100% synchronous speed).

As more renewable energy sources, especially more wind turbines (WTs) are installed in the power system; grid codes for wind power integration are being generated to sustain stable power system operation with non-synchronous generation. Common to most of the grid codes, wind power plants (WPPs) are requested to stay connected and inject positive-sequence reactive current in order to boost positive-sequence grid voltage during short-circuit grid faults, irrespective of the fault type; symmetrical or asymmetrical. However, as shown in this study, when WPPs inject pure positive-sequence reactive current in case of asymmetrical faults, as a conventional method (CM) in accordance with the grid code requirement, positive-sequence grid voltage is boosted, but also higher negative sequence voltage in the grid and higher overvoltages at the non-faulty phases occur. In this study, an alternative injection method, where WTs are injecting both positive and negative sequence currents during asymmetrical faults, providing improved grid support, is given and compared with the CM. In addition, effect of coupling between positive, negative and zero sequences when WPPs are injecting currents during asymmetrical faults, is investigated, which was not considered in the wind power impact studies before.

Small wind turbine generator (WTG) below 300 W capacity is suitable to generate power in residential or city area, where no large space for installing kilowatt scale WTG. The modularised micro-inverter which can feed WTG power to the grid directly has the merit of increasing the installed flexibility and omitting the battery in this application. This study proposes a grid-tied micro-inverter combining the boost and flyback-based inverter topology and specially designed for the small WTG with permanent magnetic (PM) generator. The boost converter is used to step up the input voltage and track the maximum power point (MPP) of the WTG. The flyback-based inverter is used to feed power to the grid with unity power factor. A fast MPP tracking (MPPT) method is presented to track the wind power to fit for the wind speed with fast variation. The variable frequency peak current mode control method is adopted for the control of the flyback-based inverter. An interleaving technique is also applied to the inverter for increasing its power capacity. Quantitative controller design of the boost converter with the WTG input and the grid-tied flyback-based inverter is presented. A 240 W experimental system is built. A WTG emulator implemented with the M–G set is developed for testing the proposed system. Some simulation and measured results are provided for demonstrating the effectiveness of the proposed system.

How much capacity credit should be given to wind power in generation system adequacy analysis is a question of great interest around the world. Both a theoretical analysis and an accurate evaluation on the wind power capacity credit are essential for understanding its contribution to power system reliability. Current evaluation techniques usually rely on numerical calculation procedures that do not provide an analytical analysis, or are based on assumptions that are valid only for small wind penetration. This study presents a rigorous model based on the definition of the reliability function. The derivation of the model is presented and a fast and accurate method for calculating the capacity credit is developed based on this model. The proposed method does not require strong hypotheses and is thus widely applicable, especially when current evaluation techniques might cause large errors, for example, when the wind power penetration is large and the wind power and load profile are not statistically independent. The model is used to explain how the statistical characteristics of the load and wind power affect the capacity credit, from both a statistical and chronological perspective. Numerical tests demonstrate the correctness of the proposed model and its potential applicability under different circumstances.

Measurements of breaking wave forces on vertical circular columns have been re-analysed, where substantial magnification over forces because of non-breaking non-linear waves may occur, by a factor of up to 2.8. The analysis shows that this factor increases as the depth parameter kd decreases, being close to unity for kd > 1.5 (k is the wave number and d is depth). Non-breaking wave forces are predicted reasonably by non-linear stream function wave theory using Morison's equation with empirical drag and inertia coefficients. A study was then made of the magnitude of wave overturning moment in relation to hub moment due to wind on standard 2 and 5 MW turbines with a 6 m diameter column. This showed that the wave moment in extreme conditions is greater than wind ‘hub’ moment for depths greater than about 7 and 13 m for the 2 and 5 MW turbines, respectively. Monopiles are normally used in depths below about 30 m and extreme moments due to waves occur when waves are depth-limited and defined by the Miche criterion, well below the limiting value of kd for the onset of depth-induced breaking. The maximum moment for these depths is expected to be predicted reasonably.

The use of renewable energy sources has increased year-on-year. Thus, there is an increasing rate of small generating units connected directly to distribution networks and micro-grids close to consumers. At the same time, these micro-sources must provide stability and reliability of electrical energy to the power network to which they are connected. In the technical literature, several studies have been done to ensure power systems with traditional generating sources to operate in a stable and reliable way, but there is an issue regarding generation uncertainty when a distribution system has many micro-sources. This is because of the uncertainty of primary sources, for example, wind and radiation intensity, and could result in intermittent generation. In this study, stability and reliability of voltage in a power system with distributed generation is analysed using simulation techniques. In the proposed method in this study voltage security analysis is jointly considered with probability laws. Moreover reliability theory is also considered in the proposed voltage collapse analysis methodology. The responsibility of generator in the voltage collapse process, the probabilistic risk of voltage collapse of each operating point and the probability of enlarging the system load as a function of different operating points are the outcome of the methodology, and it is validated by using the IEEE34 test feeder.

This study analyses the integration impact of battery energy storage systems (BESSs) on the short-term frequency control in autonomous microgrids (MGs). Short-term frequency stability relates with the primary or speed control level, as defined in the regulations of the classical grids. The focus is on autonomous MGs that dynamically behave similarly to the classical power systems. This is the systems case with classical distributed generators (DGs), but which can also contain renewable energy sources (RESs) in a certain penetration level. During MG islanded operation, the local generators take over most of the frequency control process, by means of their automatic generation control, which include inertia response and primary control. However, RES-based DGs are rarely able to provide grid frequency support, as they lack controllability and usually the power conversion chain does not have the possibility of storing and releasing energy when required by the system. Therefore the need of boosting the MG power reserves by adding energy storage systems is often a requirement. The study highlights the improvement in the MG short-term frequency stability brought by an original BESS control structure enhanced with both inertial response and an adaptive droop characteristic during battery state-of-charge limitations. The conducted analysis is accomplished by adopting aggregated models for the involved control mechanisms. The developed model is analysed in frequency domain, whereas an experimental test bench including a real-time digital simulator with BESS controller in a hardware-in-the-loop structure is used for assessing the system performances.

This study presents a second-order sliding-mode control (2-SMC) scheme for a wind turbine-driven doubly fed induction generator (DFIG). The tasks of grid synchronisation and power control are undertaken by two different algorithms, designed to command the rotor-side converter at a fixed switching frequency. Effective tuning equations for the parameters of both controllers are derived. A procedure is also provided that guarantees bumpless transfer between the two controllers at the instant of connecting the DFIG to the grid. The resulting 2-SMC scheme is experimentally validated on a laboratory-scale 7 kW DFIG test bench. Experimental results evidence both the high dynamic performance and the superior robustness achieved with the proposed control scheme.

An intelligent controlled three-phase squirrel-cage induction generator (SCIG) system for grid-connected wind power application using wavelet fuzzy neural network (WFNN) is proposed in this study. First, the indirect field-oriented mechanism is implemented for the control of the SCIG system. Then, an AC/DC power converter and a DC/AC power inverter are developed to convert the electric power generated by a three-phase SCIG from variable-voltage and variable-frequency to constant-voltage and constant-frequency. Moreover, the intelligent WFNN controller is proposed for both the AC/DC power converter and DC/AC power inverter to improve the transient and steady-state responses of the SCIG system at different operating conditions. Three online trained WFNNs using backpropagation learning algorithm are implemented as the tracking controllers for the DC-link voltage of the AC/DC power converter and the active power and reactive power outputs of the DC/AC power inverter. Furthermore, the network structure and the online learning algorithm of the WFNN are introduced in detail. Finally, some experimental results are provided to demonstrate the effectiveness of the proposed SCIG system for wind power.

A coupled electromechanical and hydrodynamic time-domain simulation of a direct-drive generator connected to a heaving buoy for wave energy conversion is presented. The system is based on a novel direct-drive power take-off unit referred to as snapper. The simulation is based primarily in MATLAB using its built-in ordinary differential equation solvers. These solvers act on the data derived from electromagnetic finite element analysis and from the WAMIT wave interaction simulation software. Test results of a generator prototype for comparison with the electromechanical simulation are presented. Results from wave tank tests of a full system incorporating the power take-off are also provided for comparison with the hydrodynamic model.

This study presents the dynamic modelling of a linear permanent magnet generator for extracting energy from ocean waves. Translator position, calculated from measured generator voltage, is used as input for the simulation model. Instantaneous power from the simulation model has been compared with the measurements from the Lysekil research site. The power output from the model considering the air gap flux variation is precisely matching with the measured values before core saturation. The generator dynamic model is modified by including the saturation effect. Although a simple mathematical expression is considered for representing the saturation, the model gives accurate power spectrum close to the experimental results. The presented model is a first step towards the system model that can simulate the entire electric system including electric grid. As such, the generator model can be used for further analysis of the wave energy conversion system.