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Energy storage systems (ESSs) are needed in the smart grids both at the generation, transmission, and distribution levels, and different types of ESSs have widely different characteristics and are suitable for different tasks and situations. In recent years, the smart grid concept has been dramatically developed in different applications, such as islands, shipboards, aircraft, microgrids and grid integration of renewables. With the wide application of ESS in smart grids, significant technical challenges remain along with the enhancement of smart grid operations and services. These challenges can be categorised as control of ESS, optimal operation, and energy management systems, optimal design of hybrid ESS, protection of ESS, and power electronics for ESS connections. This necessitates suitable design and control of the interfaces between ESSs in smart grids, as well as consideration of different applications of smart grid systems. This special issue of IET smart grid is focused on research ideas, articles, and experimental studies related to ‘Energy Storage for Green Transition of Electrical Grids’ from contributors in universities, industries, and research laboratories to develop and propose novel solutions on applications of ESS in smart grids.

It is well-known that the accurate measurement of input voltage and current, as the feedforward and feedback terms, plays a crucial role in the nonlinear controller design for power factor compensation of an AC-DC boost converter. This paper addresses the problem of the simultaneous estimation of the input voltage and current from the output voltage in a full-bridge AC-DC boost converter. In the lossless model of the system, those variables are unobservable from the output voltage when the control input is zero. To overcome this, the system dynamics are immersed in a proper form by a new filtered transformation. The phase and amplitude of the input voltage, along with the input current, are globally estimated from the output voltage by a fifth-dimensional estimator. Unlike some existing results, the stability of the proposed estimator does not rely on a priori knowledge about the parasitic resistances and is guaranteed exponentially under the persistence of excitation conditions on the control signal. An application of the proposed estimator is presented in conjunction with a dynamic controller to form a sensorless control algorithm that does not require any sensor on the input side and controls the system only by the feedback from the output voltage. Processor-in-the-loop (PIL) studies conducted by OPAL-RT OP 5700 are used to assess the performances of the proposed estimator and controller.

A secondary control method is proposed for accurate reactive power sharing as well as frequency and voltage restoration in islanded AC microgrids (MGs). The proposed method consists of an MG secondary controller, local secondary controllers for distributed energy resources (DERs), and a low-bandwidth communication link for broadcasting common data from the MG secondary controller to DERs. The broadcasted data includes the MG point of common coupling voltage magnitude and a common vertical shift for frequency and voltage restoration. Local secondary controllers calculate specific shifts for the Q-V droop characteristic of each dispatchable DER and the V-Q reverse droop characteristic of each photovoltaic (PV) system, aligning their operating points with the Q-VPCC and VPCC-Q droop characteristics, respectively. By employing VPCC as a common global variable, coordination of reactive power generation of all dispatchable DERs and PV systems is achieved, enabling accurate reactive power sharing. Importantly, in the proposed scheme, the required communication bandwidth and the communication burden are minor and are not increased with the number of DERs. Additionally, the DERs are relieved of the need for data transmission capability. The small signal stability of the proposed method is examined and its effectiveness is validated through Hardware-in-the-Loop (HIL) experimental results.

Some microgrid challenges arise when operating isolated from the main grid precisely because the local distribution network becomes an ungrounded delta circuit and the microgrid power sources have low short-circuit capacity. The main problems are the non-detection of ground overcurrent in an earth fault event, the unbalance between voltages measured by voltage transformers connected between phases-to-earth, and black start. These issues were resolved through adjustments in the protection relays of the sources and medium voltage circuit breakers, in addition to the implementation of effective control strategies in the Energy Management System.

Rumor has it that all technologies needed to build energy islands are ready. Wind turbines are spinning in many large offshore parks, while combinations of sand and concrete have given birth to several entirely new islands. However, not all rumors are true. Not only has the Danish parliament mandated the largest ever infrastructure project in the history of our country. The first Danish artificial island built for energy production will also become the world’s largest renewable energy project. On top of the technical and logistical challenges associated with building something of an unprecedented scale and nature come new concerns. The energy islands are an extreme version of the power system we know today, and therefore represent a Mars mission for the energy system. More than once have large infrastructure projects been plagued by delays and significant additional costs. Often such problems have been rooted in overly optimistic planning, limited knowledge regarding the complexity and interdependencies involved, and not giving enough attention to the development phase relative to the construction phase. For many reasons, it is highly desirable for the energy island projects to perform well. Therefore, we have teamed up to map the key challenges and suggest R&D initiatives to address them. Importantly, these initiatives are not intended as an inserted step before construction. Given the urgency in green transition and ending the reliance on fossil fuels, research and construction must be conducted in parallel. A solid foundation for energy islands On the following pages we invite you to delve into the complexity of constructing and operating offshore hubs for renewable energy. As you will hopefully agree, we are by no means saying that it cannot be done. It can. But only if decisions are based on a solid foundation of knowledge.

In this study, distributed event-and self-triggered (ST) control methods are proposed for accurate load sharing and voltage control in islanded AC microgrids with resistive line impedances. The proposed control framework is comprised of a two-level control structure. At the primary level, the V-I droop control method is adopted. In this method, the distributed energy resources (DERs) are synchronised to a common synchronous reference frame and load sharing is accomplished through voltage-current droop characteristics in this frame. The secondary level, which has a distributed structure, mitigates the voltage deviations induced by the primary level and eliminates the current sharing error caused by the line impedances. The data exchange among the DERs is controlled using event- and ST algorithms. In order to reduce the communication burden, the feedback control laws are realised using estimated variables, which are updated at certain event times. The event times are extracted based on Lyapunov stability analysis and by consideration of the V-I droop characteristics. Comparison of event and ST algorithms shows that while the event-triggered strategy offers superior dynamic response, the ST approach requires lower network traffic. The efficacy of the proposed scheme is verified through real time simulation results.

In this paper, a control scheme is proposed to compensate nonlinear and unbalanced loads currents and mitigate neutral current in a weak distribution network utilizing a four-leg converter. Two separate Complex Coefficient-Filters (CCF) have been used to extract the fundamental frequency positive sequence component of the load current and the converter to compensate for and control the dc- link voltage. Also, due to the presence of unbalance and harmonics the voltage of the weak grid, a CCF-based Phase Locked Loop (PLL) has been used for proper synchronization with grid voltage. Owing to absence of a low-pass filter in the CCF structure does not reduce the system dynamics and increases the speed and accuracy of estimation of the frequency and phase angle of the grid voltage. The Triple-CCF (TCCF) control strategy proposed in this article is compared with the same case, Triple-Reduced Order Generalized Integrator (TROGI). Simulation results show the performance and efficiency of the four-leg active filter control strategy based on TCCF.

By increasing the number of inverter-based resources, for instance wind turbines (WTs), the overall inertia of the network and the dynamic frequency and voltage stability are decreased. Proper battery energy storage system (BESS) control can improve stability in this case. In this paper, by considering a low-inertia power grid facing loss of generation, the effect of BESS on stability enhancement is evaluated. The study network is a modified Kundur's four-machine system modeled in PowerFactory. The results show that BESS can reduce fluctuations and increase the stability of the system.

High penetration of rooftop PV systems in distribution feeders can give rise to overvoltage during peak generation hours. A well-known solution for this problem is Volt-VAR control method, in which PV inverters mitigate the overvoltage by reactive power absorption. The conventional Volt-VAR scheme calculates the reactive power reference based on a limited bandwidth V-Q droop controller. Although the conventional method is capable of steady-state voltage regulation, it is not effective against voltage overshoots caused by fast solar irradiance variations. In this paper, a new V-Iq based Volt-VAR control scheme is presented to suppress the transient voltage overshoots during cloud passing. In this method, the bandwidth of the Volt-VAR controller is increased to enhance its responsiveness to voltage variations. To prevent instability caused by unwanted interaction between the high bandwidth Volt-VAR controller and the voltage feedforward section of the inner control loop, a low pass filter is added to the feedforward path. Small signal analysis of the proposed method provides a guideline for designing the bandwidths of Volt-VAR controller and feedforward filter. The stability of the proposed method is assessed based on Lyapunov method. Hardware in the Loop (HIL) experimental results are presented to validate the proposed scheme.

This paper presents a new distributed event- triggered control mechanism for accurate load sharing and voltage regulation in islanded AC microgrids (MGs). The control structure is composed of two layers. The primary control level coordinates the output current of the Distributed Energy Resources (DERs) by utilizing the V-I droop control strategy. In this method, all DERs are synchronized to a common rotating reference frame. To realize proportional load sharing among the DERs, for each DER, the d and q components of the output voltage are determined in accordance to droop characteristics of the d and q axis currents. To eliminate the load sharing caused by the line voltage drops, the slope of the droop characteristic of each DER is altered by means of a distributed secondary control scheme. The secondary controller utilizes an event-triggered communication strategy, which remarkably decreases the exchanged data and saves bandwidth. Simulation results show that the proposed method favors smooth dynamic performance and efficient network utilization.

In this paper, transient current and voltage responses, caused by low and high impedance faults in a power system with distributed generation (DG) systems, are investigated. The proposed approach utilizes a novel transient index (TI) based on the voltage during fault at the point of common coupling. The proposed technique uses voltage signals at DG connection point to provide the value of TI. The performance of the proposed scheme is evaluated on a power system consisting of photovoltaic systems and wind turbine generators as the case study by using a real-time simulator. The simulation results, which will be included in the full paper, show the fast and reliable operation of the proposed fault detection technique as compared to the existing fault detection methods, not only for low impedance transient faults but also for high impedance faults.