The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.

https://research.aalto.fi/files/6557604/lund_et_al_review.pdf

Review of energy system flexibility measures to enable high levels of variable renewable electricity

Energy storage systems (ESSs) are enabling technologies for well-established and new applications such as power peak shaving, electric vehicles, integration of renewable energies, etc. This paper presents a review of ESSs for transport and grid applications, covering several aspects as the storage technology, the main applications, and the power converters used to operate some of the energy storage technologies. Special attention is given to the different applications, providing a deep description of the system and addressing the most suitable storage technology. The main objective of this paper is to introduce the subject and to give an updated reference to nonspecialist, academic, and engineers in the field of power electronics.

Energy Storage Systems for Transport and Grid Applications

The wind and solar energy are omnipresent, freely available, and environmental friendly. The wind energy systems may not be technically viable at all sites because of low wind speeds and being more unpredictable than solar energy. The combined utilization of these renewable energy sources are therefore becoming increasingly attractive and are being widely used as alternative of oil-produced energy. Economic aspects of these renewable energy technologies are sufficiently promising to include them for rising power generation capability in developing countries. A renewable hybrid energy system consists of two or more energy sources, a power conditioning equipment, a controller and an optional energy storage system. These hybrid energy systems are becoming popular in remote area power generation applications due to advancements in renewable energy technologies and substantial rise in prices of petroleum products. Research and development efforts in solar, wind, and other renewable energy technologies are required to continue for, improving their performance, establishing techniques for accurately predicting their output and reliably integrating them with other conventional generating sources. The aim of this paper is to review the current state of the design, operation and control requirement of the stand-alone PV solar–wind hybrid energy systems with conventional backup source i.e. diesel or grid. This Paper also highlights the future developments, which have the potential to increase the economic attractiveness of such systems and their acceptance by the user.

A current and future state of art development of hybrid energy system using wind and PV-solar: A review

In this paper, we present new distributed controllers for secondary frequency and voltage control in islanded microgrids. Inspired by techniques from cooperative control, the proposed controllers use localized information and nearest-neighbor communication to collectively perform secondary control actions. The frequency controller rapidly regulates the microgrid frequency to its nominal value while maintaining active power sharing among the distributed generators. Tuning of the voltage controller provides a simple and intuitive tradeoff between the conflicting goals of voltage regulation and reactive power sharing. Our designs require no knowledge of the microgrid topology, impedances, or loads. The distributed architecture allows for flexibility and redundancy, eliminating the need for a central microgrid controller. We provide a voltage stability analysis and present extensive experimental results validating our designs, verifying robust performance under communication failure and during plug-and-play operation.

/pdf/secondary-frequency-and-voltage-control-of-islanded-34ymy3dp1h.pdf

Secondary Frequency and Voltage Control of Islanded Microgrids via Distributed Averaging

Advances and trends of energy storage technology in Microgrid

Distributed energy storage systems in combination with advanced power electronics have a great technical role to play and will have a huge impact on future electrical supply systems and lead to many financial benefits. So far, when Energy storage systems (ESSs) are integrated into conventional electric grids, special designed topologies and/or control for almost each particular case is required. This means costly design and debugging time of each individual component/control system every time the utility decides to add an energy storage system. However, our present and future power network situation requires extra flexibility in the integration more than ever. Mainly for small and medium storage systems in both (customers and suppliers) side as the storage moves from central generation to distributed one (including intelligent control and advanced power electronics conversion systems). Nevertheless, storage devices, standardized architectures and techniques for distributed intelligence and smart power systems as well as planning tools and models to aid the integration of energy storage systems are still lagging behind.

Challenges in integrating distributed Energy storage systems into future smart grid

Review of control techniques for inverters parallel operation

This paper demonstrates that the introduction of a Flywheel Energy Storage System (FESS) into a isolated hybrid grid increases the renewable energy penetration. The isolated power system studied includes diesel and hydro generators and wind turbines. The simulated and measured results clearly show the positive influence of the energy storage on the system performance. The system has been operating with the FESS for 1 year and has demonstrated improvements in renewable energy penetration, reduced diesel fuel usage, and improved power system stability and power quality.

Increasing Renewable Energy Penetration in Isolated Grids Using a Flywheel Energy Storage System

One of the desirable characteristics of inverters in three-phase systems is the ability to feed unbalanced/non-linear loads with voltage and frequency nominal values. Therefore three-leg four-wire inverters are expected to play an essential role in future power systems because of their ability to handle the neutral current caused by unbalanced or non-linear loads. This study introduces an original control method in combination with three-dimensional space-vector modulation (3D-SVM) strategy. The steps for the 3D-SVM implementation are identified. The switching vectors, 3D-SVM diagrams and the boundary planes equations, as well as the matrices for the duty cycles and symmetric switching sequences are discussed in detail. Experimental results including different loads are presented to validate the proposed SVM control strategy for three-leg four-wire voltage source inverters. The experimental results of this study show that the developed control scheme in combination with three-leg four-wire inverters can carry out the grid feeding requirements and supply good power quality to loads under extreme unbalanced conditions efficiently.

Control strategy and space vector modulation for three-leg four-wire voltage source inverters under unbalanced load conditions

Future power distribution requires advanced expandability and flexibility in the integration of distributed energy resources which normally require interfacing units to provide the necessary crossing point to the grid. The core of these interfacing units is power-electronics grid front end, namely, inverters. The inverter is the primary interface that provides not only their principal interfacing control function but also various utility functions. This paper presents the flexible control methodology of inverters as grid front end using an isochronous control function which is used by synchronous generators in conventional power systems to provide load sharing and control.

Egon Ortjohann

Papers

Challenges in integrating distributed Energy storage systems into future smart grid

Review of control techniques for inverters parallel operation

Increasing Renewable Energy Penetration in Isolated Grids Using a Flywheel Energy Storage System

Control strategy and space vector modulation for three-leg four-wire voltage source inverters under unbalanced load conditions

Control Strategy for Three-/Four-Wire-Inverter-Based Distributed Generation