The variability of the output power of distributed renewable energy sources (DRESs) that originate from the fast-changing climatic conditions can negatively affect the grid stability. Therefore, grid operators have incorporated ramp-rate limitations (RRLs) for the injected DRES power in the grid codes. As the DRES penetration levels increase, the mitigation of high-power ramps is no longer considered as a system support function but rather an ancillary service (AS). Energy storage systems (ESSs) coordinated by RR control algorithms are often applied to mitigate these power fluctuations. However, no unified definition of active power ramps, which is essential to treat the RRL as AS, currently exists. This paper assesses the various definitions for ramp-rate RR and proposes RRL method control for a central battery ESS (BESS) in distribution systems (DSs). The ultimate objective is to restrain high-power ramps at the distribution transformer level so that RRL can be traded as AS to the upstream transmission system (TS). The proposed control is based on the direct control of the ΔP/Δt, which means that the control parameters are directly correlated with the RR requirements included in the grid codes. In addition, a novel method for restoring the state of charge (SoC) within a specific range following a high ramp-up/down event is proposed. Finally, a parametric method for estimating the sizing of central BESSs (BESS sizing for short) is developed. The BESS sizing is determined by considering the RR requirements, the DRES units, and the load mix of the examined DS. The BESS sizing is directly related to the constant RR achieved using the proposed control. Finally, the proposed methodologies are validated through simulations in MATLAB/Simulink and laboratory tests in a commercially available BESS.

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The increasing penetration of renewable energy sources is strongly linked to the development of voltage source converters used in their connection to the grid. As a result, in the near future of an inverter-dominated power system it will be a requirement for renewable generation to provide ancillary services in order to compensate for the absence of synchronous generation. In this scenario, the use of energy storage systems along with advanced control algorithms mimicking the dynamic behaviour of the traditional generators will be of utmost importance. This paper deals with a renewable energy source interfaced with a voltage source converter and comprising an energy storage system in the DC bus. Particularly, a new energy management algorithm for the DC-bus based on a three levels hierarchical control is proposed, which is able to simultaneously provide ancillary services, maintain the state of charge of the storage system within the permissible limits and use it to control the DC bus voltage. The control strategy is validated experimentally using a prototype with results evidencing a reliable and stable operation.@en

Renewable Energy Sources (RES), and especially wind turbines (WT) as the most mature technology, will provide a large share of energy generation in a future sustainable power system. This development introduces several challenges that need to be addressed in order to ensure a safe transition away from the fossil-fuel based generation. One of the segments where RES will have a major impact is ancillary services, such as frequency support. Until now, RES (Wind & Solar) did not participate at frequency regulation of the system, which will change as their numbers in the grid increase. The contribution of this paper is the investigation of the system impact of wind generation, when wind turbines are equipped with a control system that provides frequency related ancillary services. More specifically, we analyze primary frequency response and inertial response in the case when Doubly Fed Induction Generator (DFIG) wind turbines are equipped with Energy Storage Systems (ESS). The results are presented for different levels of RES penetration and different levels of ESS deployment@en

Generation from Wind Turbines has a minimal impact on the environment compared to traditional energy sources. Renewables can effectively tackle the problems which arise with fossil fuel usage. The most considerable disadvantage of the Renewable Energy Sources (RES), and also Wind Turbines, is that, since they are inverter-based they cannot inherently support system’s frequency. Thus, when their penetration is increased, the systems stability and reliability are endangered. Moreover, nowadays RES are mostly rewarded only for providing Active Power to the grid; thus, they did not participate in voltage regulation. This situation can risk the voltage stability of the system with high renewable penetration. The contribution of this paper, is an integrated control system which is capable of providing frequency and voltage support to the grid, while operating the Wind Turbine at its optimal point, by utilizing an Energy Storage System. The design and the simulation of this system was performed in real-time with RTDS.@en