This study reports the methodology and results of a renewables (REs) integration grid study for the 2030 Japanese power system. In light of the Japanese energy policy outlook for 2030, two scenarios were compared: the government's target scenario with 22–24% RE penetration (64 GW solar and 10 GW wind), and a scenario with higher RE penetration (100 GW solar and 36 GW wind). The impact of these two RE integration scenarios on the grid in terms of frequency stability and power flows were simulated and compared. The evaluation outcomes showed that a RE penetration more ambitious than the government's plan can be achieved without compromising the evaluated elements of grid security and without additional technical measures in situations with at least up to 62% variable REs, i.e. wind and solar, instantaneous penetration in western Japan and at least up to 36% in eastern Japan. Furthermore, technical solutions like soliciting fast frequency response from solar and wind power plants, and setting a system non-synchronous penetration limit for grid management can be applied to improve grid security for higher RE penetrations.

The North Sea region offers large amounts of investable offshore wind generation and several interconnection lines are located in the area, with more planned. Optimal transmission and generation investments are modelled in the region towards 2050. A project-based scenario, where each offshore wind power plant is connected individually, is first analysed. Then, an integrated offshore grid is modelled to investigate the viability of connecting future transmission and offshore wind generation investments. These two scenarios are then compared. It is shown that going towards an integrated solution can increase offshore wind investments by several GWs, with Germany seeing the biggest difference between the two scenarios. It is shown that the integrated solution leads to overall cost minimisation and shows somewhat higher variable renewable energy generation by 2050 compared to the project-based scenario.

The penetration of power electronic interfaced generation (PEIG) is expected to reach up to 65% in some parts of the European power system by 2030 (at least during some hours of the year). Under such grid conditions, system security challenges are observed with frequency stability, voltage stability and undamped converter control interactions being among the most important issues. This study presents a short-term voltage stability assessment of the Great Britain synchronous area under EMT modelling assumptions. The study provides a mapping of system stability and identifies the critical penetration level of PEIG that instabilities are observed. In addition, an application of a grid forming control scheme (namely the enhanced direct power control) is proposed as a mitigation option which is applied here on full-converter interfaced wind power plants (type-4). The simulation results reveal that the application of the grid forming control to a part of the total wind power generation fleet can mitigate the instabilities observed, while enabling the system operation with 100% PEIG.

Gotland is a Swedish island that is connected to the grid only by two high voltage direct current (HVDC) cables. The growth of installed wind power on Gotland has been temporarily stopped due to concerns regarding the reliability of the operations. This study investigates the capability of a centralised energy storage system along with or without wind curtailment to support the growth of installed wind power capacity. The energy storage system is tested for maintaining frequency stability during unintentional islanding through dynamic studies using power system simulator for engineering (PSS/E). The results assess the ability of energy storage to prevent frequency instabilities and provide primary frequency response albeit of the absence of any rotating inertia. The analysis determines the requirements for the power and energy capacity of the energy storage system in relation to the exported power from the HVDC cables at the instant of fault, which eventually relates to the wind power capacity. Moreover, the study examines wind power curtailment as primary and/or secondary frequency response and the impact on the energy capacity. Finally, the study did not explore the effect of fast power ramps on the energy storage to leave the door open for all types of inverter-based energy storage technologies.

The increasing penetration of converter-based generation in many power systems around the world has sparked a discussion about how to operate these power systems with the usual levels of efficiency, reliability and cost-effectiveness. Current grid-following converter-based generators have proven to run stably in parallel to one another, even if there are thousands of them connected in a power system, and even in very small isolated power systems with extremely low system inertia. Discussions around the necessity of additional converter performance, usually under the ‘grid-forming’ and ‘Virtual Synchronous Machines’ concepts, have recently been transferred from the academic sphere to national and international industry fora. Formal discussions have started in Great Britain, in Germany and at ENTSO-E level. However, there is still a lot of uncertainty about the real and not simulated performance of grid-forming converters, whilst the needs case for requiring this radically different control method has not been adequately justified. With the present paper we raise key questions that will serve towards an objective discussion about power system needs, grid infeed technologies and their interaction.

The connection of renewable energy sources to local low-voltage networks is becoming more accepted as electrical power networks progress to higher renewable penetration. Renewable energy resources, for example, wind and solar are highly dynamic and intermittent compared with more traditional generation sources, which imposes increasing challenges to the electrical network operator in terms of effectively managing the resource to maximise energy transfer and maintaining system stability. Therefore, transient energy storage systems (TESSs), for example, electrochemical batteries with fast charging/discharging capabilities are suitable candidates to improve the availability and reliability of connected renewable systems. In this study, two potential TESS technologies are presented, the lithium-ion (Li-ion) and sodium–nickel chloride (NaNiCl₂) battery, and their feasibility to improve power systems in terms of power delivery and frequency fluctuations are compared. Experimentally validated battery models are presented and used to investigate the TESS performance in terms of state-of-charge, terminal voltage variation, peak current, power, energy and efficiency. The models and general design procedure may be applied to systems of different ratings and duty variations.

Small-signal sequence impedance models have been developed for different types of wind turbines and used in industry to study wind farm and high-voltage direct current (HVDC) system harmonics and resonances. Compared to other small-signal methods, a major advantage of system modelling and analysis based on sequence impedance is its scalability: an equivalent impedance model can be easily built for any complex system by aggregating the impedance of individual turbines and the network. A recent development in the small-signal sequence impedance theory is the modelling of frequency coupling to improve the accuracy of system analysis and explain a common characteristic of harmonics created by system resonance. In light of this new development, this study presents and compares different aggregation methods to build farm-level impedance models in the presence of coupling in individual turbine responses. The objective is to identify practical methods that can meet the needs of different system analyses and are easily to use. Experiences of China State Grid and TenneT with the application of sequence impedance models in renewable energy and HVDC system resonance and harmonic analysis are also discussed, along with an overview of their ongoing efforts to develop new grid codes and system analysis tools based on sequence impedances.

Small-scale systems are vulnerable to changes. The development of solution strategies for changing conditions should, therefore, be considered from the outset in planning. For planners and installers of mini-grids for rural villages without grid connection, first of all, the question of the expected electricity demand arises. So far, this is determined primarily by interviewing the population. However, studies have shown that there are still major differences between the power demand estimated and the real load after electrification. Reinforcing here is the inadequate assessment of load development through immigration and increased prosperity. On the basis of a currently developing mini-grid in the Eastern Cape of South Africa, we could show that a load increase of ∼60% within 10 years is to be expected. The following possibilities were outlined as possible ways of counteracting the increase in load: (i) communicating with the population, i.e. on energy efficiency to raise awareness and understanding, (ii) early planning of capacity building and identification of key performance parameters to trigger the expansion based on local socio-cultural conditions and grid supporting qualities. An adequate database for the initial electrification and power development should also be established and available on open-source basis for researchers, developers, communities, and installers.

In this study, a resilient distributed control algorithm is proposed for the secondary voltage and frequency restoration of an autonomous inverter-based microgrid considering simultaneous relative state-dependent noises and communication time-delays (CTDs). The proposed algorithm is robust against potential time-varying stochastic noises and CTDs, which corrupt the data exchanges in the secondary control layer. Additionally, this study considers the contribution of both distributed generation and distributed energy storage (DES) units in islanded AC microgrids. The presence of DES creates the need for an extra control algorithm to provide state-of-charge (SoC) balancing for these units and having precise active power-sharing. The theoretical concepts of the proposed control algorithm, including the mathematical modelling of microgrid, basic lemma, and controller design procedure, are outlined. The performance assessment of the presented control algorithm is evaluated through simulations on a test microgrid in MATLAB/Simulink software. Then, some previously-reported algorithms are selected to compare the proposed control algorithm with them. The obtained results show the effectiveness and robustness of the presented algorithm in regulating the voltage and frequency, matching SoCs, and as a result, having precise active power-sharing.

This study proposes an approach to evaluate a practical margin for photovoltaic (PV) generation hosting capacity (HC) of low voltage distribution networks. This HC is determined considering the randomness of the connection points and is supposed to be the maximum value of PV penetration up to which the utility can authorise any interconnection without performing additional case studies. Smart inverter control strategies and battery storage systems are used to avoid costly network expansion solutions. The simulations are performed using actual solar radiation data and residential demand profiles. The results show an increase in the network HC, bringing benefits by deferring network investments such as conductors and asset upgrades.

Hysteresis-based energy management strategy for microgrid containing photovoltaic, ESS and heating loads is proposed in this study. In this real-time optimisation method, economic cost, operation cost, comfort level, renewable energy penetration and other performance indices are optimised in real time. To solve the complex energy management problem, when power forecasting is unavailable, different operational cases are clarified and calculated accordingly. In this way, the management process is turned into a finite-solution problem. Furthermore, a hysteresis loop is also established based on the cost function to optimise the system continuously, of which the loop width is offline designed. As a result, the computation burden is reduced. Finally, simulation results are presented to verify the effectiveness of the proposed energy management method. The application of the proposed method can reduce the economic cost and improve the operational performances of a microgrid significantly, including comfort, power quality and health of ESS.

With the growth of distributed generators in the power grid, replacing the fossil-fuel-based conventional generation sources with renewable power resources, namely photovoltaics (PVs) or wind turbines, is inevitable. However, when rotating-kind synchronous machines that usually possess high inertia are replaced by PVs that have an inherent static nature, grid stability issues such as reduced system inertia, lack of reactive power, and decreased system damping may arise. This study investigates the stability impacts with high-PV penetration in the Texas 2000-bus network. From the impact analysis conducted under transient conditions, it is observed that high-PV penetration could negatively adversely affect the voltage and frequency stability of the system which is mainly due to the replacement of conventional generators that results in reduced network inertia and lack of reactive power support. Hence, to alleviate the adverse stability implications of PVs on power systems with reduced inertia, a long-term generator scheduling strategy is proposed considering the criticality of synchronous generators for optimally decommitting and scheduling the generators. Extensive case studies are conducted to determine the ideal dispatch strategy for the test system based on transient stability criterions.

Asymmetrical faults in the grid cause oscillations in dc-link capacitor voltage or overcurrent in the converters. In this study, an optimal controller with a fixed structure is introduced based on the command generator tracker (CGT) to restrict the fault currents and increase stability performance under asymmetrical grid faults. For this purpose, first, the integrated dynamical model including grid-connected converters and dc-link voltage are introduced. Then, using the optimal control techniques, a CGT is designed to enhance the fault ride through performances. Next, the proposed controller is applied to the system including converter and dc-link at the source side and the dynamic performances of the closed-loop system under different fault conditions are determined. Finally, the simulation results are compared with the other conventional controllers and the dynamic performances of the proposed controller are evaluated. The results indicate that the proposed controller provides stability for the system and satisfies allowable constraints of converter current for different grid fault types such as voltage sags, asymmetrical and symmetrical faults.

The energy storage system is the backbone of an isolated microgrid (MG), which helps to provide a secure and reliable power supply. The operation of the MG systems operating without storage is affected highly due to intermittent generation and sudden load demand fluctuations. This study investigates the operational behaviour of an isolated MG system in terms of frequency and power balance by incorporating the Micro Pump Hydro Energy Storage (MPHES) system. The investigated MG system consists of biodiesel, solar and wind-based generating units with MPHES and battery as energy storage systems. The realistic data for solar and wind resources have been used to consider the practical scenario of any area. The recently developed coyote-optimisation algorithm is employed for optimal tuning of the proportional–integral controller using the integral of time multiplied absolute error criterion. The simulation studies have been conducted to investigate the performance of the MG system by incorporating the MPHES system for a typical day (36 h) under various operating conditions. The analysis reveals that the MG system exhibits smoother and consistent dynamic performance by including the MPHES unit and efficiently utilises renewable power.

To reduce the voltage fluctuations inside the wind farm cluster (WFC), a combined active and reactive power control schemes based on model predictive control is proposed. The aims of the proposed control scheme are to maintain the collector bus and wind turbine (WT) terminal voltages within the feasible range. With the size of WFC increasing, conventional centralised optimal control may no longer be suitable for a large-scale WFC due to the high computation burden of the WFC central controller. To improve the calculation efficiency and protect information privacy, a two-tier control structure with alternating direction method of multipliers (ADMMs) algorithm is used to solve the large-scale optimisation problem in distributed/hierarchical manner. In the upper-tier distributed control, the active and reactive power outputs of the WFs are coordinated to enhance the voltage control performance. In the lower-tier control, an ADMM-based hierarchical control is developed to minimise the voltage deviation of the WT terminals. Case studies demonstrate the efficacy of the proposed two-tier combined active and reactive power controls.

This study proposes a seven-level power conversion system for a solar power generation system. This seven-level power conversion system consists of a DC–DC power converter and a cascade DC–AC inverter. The cascade DC–AC inverter comprises a full-bridge inverter and a T-type inverter. The T-type inverter generates a three-level quasi-square-wave voltage and the full-bridge inverter provides a three-level high-frequency pulse voltage. The amplitude of the three-level quasi-square-wave voltage is double that of the three-level high-frequency pulse voltage, so seven levels of AC voltage are synthesized. Only the full-bridge inverter with lower DC bus voltage uses high-frequency switching so switching losses are effectively reduced. The novelty of the proposed cascade DC–AC inverter is that the output power of T-type inverter is controlled by regulating the DC bus voltage, so no real power is supplied from the full-bridge inverter and an independent power source is not required to provide power to the full-bridge inverter. The negative terminal voltage for solar cell array keeps almost constant to reduce the leakage current of proposed seven-level power conversion system. A hardware prototype with a digital signal processor controller is developed to verify the performance of seven-level power conversion system for a solar power generation system.

This study presents a security-constrained optimisation problem for co-expansion planning of integrated electric power and natural gas (SCEP-EPNG), taking into account multiple uncertainties of wind power generation and electric system load demand. The proposed model is a bi-level and bi-directional approach, which considers the flow of electric power to the natural gas network and vice versa through the power to gas and gas to power technologies, respectively. The security of the system is investigated from the static voltage stability point of view using the L-index approach. Also, the information-gap decision theory technique is adopted for modelling the uncertainties. The model aims at minimising the investment and operation costs as well as voltage stability index in a multi-objective optimisation problem, solved by the ɛ-constrained method, and the best compromise solution is calculated using the fuzzy-based min–max method. The proposed SCEP-EPNG model is implemented on the integrated Garver six-node electric power and seven-node natural gas networks, as well as integrated IEEE 24-node electric power and 12-node natural gas networks. Simulation results indicate the importance of voltage stability constraints in long-term energy planning. Furthermore, the model shows the effects of multiple uncertainties on co-expansion planning decisions.

Precise daily forecasts of photo-voltaic (PV) power production are necessary for its planning, utilisation and integration into the electrical grid. PV power is conditioned by the current amount of specific solar radiation components. Numerical weather prediction systems are usually run every 6 h and provide only rough local prognoses of cloudiness with a delay. Statistical methods can predict PV power, considering a specific plant situation. Their intra-day models are usually more precise if rely only on the latest data observations and power measurements. Differential polynomial neural network (D-PNN) is a novel neuro-computing technique based on analogies with brain pulse signal processing. D-PNN decomposes the general partial differential equation (PDE), being able to describe the local atmospheric dynamics, into specific sub-PDEs in its nodes. These are converted using adapted procedures of operational calculus to obtain the Laplace images of unknown node functions, which are inverse L-transformed to obtain the originals. D-PNN can select from dozens of input variables to produce applicable sum PDE components which can extend, one by one, its composite models towards the optima. The PDE models are developed with historical spatial data from the estimated optimal numbers of the last days for each 1–9-h inputs-output time-shift to predict clear sky index in the trained time-horizon.