Short-term energy recovery control for virtual inertia provision by renewable energy sources

The proliferation of Converter-Interfaced Renewable Energy Sources (CIRES), which are inertia-less, and the gradual decommissioning of synchronous generation have posed several challenges to the electric power system. This has motivated a complete a shift in the CIRES design and its corresponding control philosophy. Integrating Energy Storage Systems (ESS) within CIRES enables the implementation of different operating modes allowing them to provide ancillary services (AS) in a similar way to the synchronous generation. In order to tackle with those short-term response AS, such as virtual inertia, fast ESS (FESS) solutions with high power-to-energy ratio, particularly flywheels and supercapacitors, are preferred. In spite of several control algorithms have been proposed to provide such fast AS, very little research effort has been paid on the proper FESS energy recovery after the AS provision. This task is particularly challenging, since supercapacitors must be operated at a certain state of charge to guarantee that the required AS can be provided within its operational limits. This paper aims to fill this gap by proposing a new energy recovery control scheme for supercapacitors after the provision of short-term AS, such as virtual inertia. The proposed control is validated via simulations which clearly highlights its adequate performance.Horizonte 2020 (Unión Europea) 764090Ministerio de Economía y Competitividad (MINECO). España ENE2017-84813-R

Methodology for the Techno-Economic Assessment of Medium-Voltage Photovoltaic Prosumers Under Net-Metering Policy

Net-metering has been introduced as an alternative to the feed-in tariff scheme to encourage consumers to act as prosumers by installing photovoltaics. Originally, net-metering referred mainly to household photovoltaic systems, however, larger prosumers connected to medium-voltage networks are also considered. Scope of this paper is to present a techno-economic assessment methodology to evaluate the viability of net-metering policy in medium-voltage prosumers. Unlike most relevant methods, the proposed methodology combines techno-economic analysis with a quasi-static simulation model incorporating the real-world operational properties of the distribution network, where the medium-voltage prosumers are connected. The analysis focuses on nine university campuses operating under net-metering policy. The impact of several important parameters on the viability of the investment as well as on the optimal size of the system is investigated; among them special emphasis is given on the applicability of the decentralized voltage regulation techniques applied to the prosumer's photovoltaic system

Optimizing the Installation of Hybrid Power Plants in Non-Interconnected Islands

This paper proposes a comprehensive optimization approach based on linear programming (LP) for the installation of multiple hybrid power plants (HPPs) in non-interconnected islands. Contrary to the current state-of-the-art solutions, the proposed approach optimizes simultaneously the size, location, and technology of each HPP in order to minimize the long-term electricity cost of the island. The optimization problem is formulated as a LP problem to ensure convergence and global optimum solution. Moreover, a series of system constraints are included in the optimization problem, e.g., power reserves, transmission constraints, etc., to ensure the secure and reliable operation of the grid; this is compatible with the actual preventive measures imposed by the network operator in real non-interconnected islands. Simulations are executed in a real Greek Island (Rhodes), confirming the applicability of the proposed method as an optimization tool for network planning studies in non-interconnected islands.</p

A Control Method for Balancing the SoC of Distributed Batteries in Islanded Converter-Interfaced Microgrids

In a low-voltage islanded microgrid powered by renewable energy sources, the energy storage systems (ESSs) are considered necessary, in order to maintain the power balance. Since a microgrid can be composed of several distributed ESSs (DESSs), a coordinated control of their state-of-charge (SoC) should be implemented, ensuring the prolonged lifespan. This paper proposes a new decentralized control method for balancing the SoC of DESSs in islanded microgrids, without physical communication. Each DESS injects a current distortion at 175 Hz, when its SoC changes by 10%. This distortion is recognized by every DESS, through a phase-locked loop (PLL). In order to distinguish the origin of the distortion, each DESS injects a distortion of different time duration. This intermediate frequency has been selected in order to avoid the concurrence with the usual harmonics. The DESSs take advantage of this information and inject a current proportional to the SoC. Implementing this strategy, a comparable number of charging/discharging cycles for each DESS are achieved. Furthermore, an active filter operation, implemented in the dq rotating frame for each individual harmonic, is integrated in the control of the distributed generation units, supplying nonlinear loads with high-quality voltage. The effectiveness of this method is verified by detailed simulation results

Evaluation of Decentralized Voltage Harmonic Mitigation through DRES converter active filtering capability

The increased penetration of Converter-Interfaced Distributed Renewable Energy Sources (CI-DRES) has posed power quality challenges into the distribution system. In the future CI-DRES may have the responsibility to cope simultaneously with several issues. One function that has already been presented in the literature, but has not yet been included in Standards, is the operation of the DRES converters as active harmonic filters. This paper firstly develops an active filtering control for the CI-DRES, taking into account its thermal limit. This parameter is important, since DRES are already prescribed by Standards to participate in the voltage regulation process by providing reactive power. If the CI-DRES operates also as active filter, it may exceed its thermal capacity. Secondly, the decentralized operation of CI-DRES is evaluated in the CIGRE Benchmark LV distribution system by considering several scenarios of DRES/Non-linear loads penetration and mixture. Time-domain (TD) simulations are carried out in PowerSim Software to demonstrate the contribution of each CI-DRES in the active filtering process. The derived conclusions will serve as a basis for the development of a new coordinated algorithm, so as the CI-DRES can mitigate properly voltage harmonic distortion (HD) in the most efficient techno-economic way. In this way, the active filtering can be treated as a new ancillary service to be introduced in respective markets.Horizonte 2020 76409

Ramp-Rate Control of DRES employing Supercapacitors in Distribution Systems

The increased penetration of Distributed Renewable Energy Sources (DRES) has posed several challenges into the electric power systems, the most important of which is the instability caused by the intermittent and stochastic nature of the primary sources (wind, sun). Since the System Operators have started to specify limits for the restriction of the Ramp-Rate (RR) at the Point of Common Coupling (PCC) of the DRES with the grid, several algorithms have been proposed to mitigate the power fluctuations using energy storage systems (ESS). Some drawbacks are: high computational effort in the calculation of RR, increased ESS size/decreased ESS operational life, etc. In this paper a new RR Limitation (RRL) algorithm is proposed to address gaps in the current state-of-the-art. This algorithm is based on the two-point calculation of RR and is performed considering the connection of a Supercapacitor (SC) at the DC-link of a DRES converter. The relationship between the SC voltage and the degree to which the RRL is achieved is established, which is something missing from the current state of the art and is essential if the RRL is to be treated as a new tradeable AS. The RR control is validated in a real experimental testbed. Finally, this control is modelled in Simulink in order to perform investigations on the influence that several parameters have on the achieved RRL at the DRES PCC. This is important, since the evaluation of the RRL impact at DRES level will allow for further investigations to evaluate the RRL impact at distribution system level and defer costs related to the installation of large-scale ESS.European Union (UE). H2020 GA 764090Centro para el Desarrollo Tecnológico Industrial CER-2019101

Ancillary Services Offered by Distributed Renewable Energy Sources at the Distribution Grid Level: An Attempt at Proper Definition and Quantification

Article number 7106The gradual displacement of synchronous generators driven by conventional power plants, due to the increasing penetration of distributed renewable energy sources (DRES) in distribution grids, is creating a shortage of crucial ancillary services (AS) which are vital for the frequency and voltage stability of the grid. These AS, and some new ones, could now be offered by the DRES, particularly those that are converter interfaced, in a coordinated way in order to preserve the grid stability and resilience. Although recent standards and grid codes specify that the DRES exhibit some system support functions, there are no specifications on how to measure and quantify (M & Q) them both at DRES level and in aggregated form. The M & Q of AS is crucial, since it would allow the AS to be treated as tradable AS in the current and future AS markets. This paper attempts to define a number of AS that can be offered by converter-interfaced DRES and suggests methods for their M & Q. The new AS addressed are: (1) inertial response; (2) primary frequency response; (3) active power smoothing (ramp-rate limitation); (4) exchange of reactive power for voltage regulation; (5) fault-ride-through (FRT) and contribution to fault clearing; (6) voltage harmonic mitigation. Additionally, a rough estimation of the additional investment and operational cost, as well as the financial benefits associated with each AS is provided in order to form the basis for the development of business models around each AS in the near future.Unión Europea Proyecto EASY-RES (Horizonte 2020) 76409