Provision of Primary Frequency Response as Ancillary Service From Active Distribution Networks to the Transmission System

This paper deals with the provision of primary frequency response (PFR) as ancillary service (AS) from active distribution networks (ADNs) to the transmission system (TS). In particular, two methodologies are developed. The first one aims to quantify the PFR capability range of the ADN. This range is defined by determining the range of the aggregated, i.e., equivalent, active power - frequency P(f ) droop curves that can be provided at the point of interconnection (POI) with the TS. The second one targets to optimally control P(f ) droop curves of individual distributed energy resources (DERs), installed in the premises of the ADN, to guarantee specific frequency regulation characteristic at the POI. This frequency regulation characteristic is expressed by means of a P(f ) droop curve. Both methods are tested on two discrete distribution systems. Several test cases are examined to demonstrate their implementation. Additionally, comparisons against conventional approaches and time series simulations are conducted to evaluate the performance of the proposed methods.Unión Europea Subvención 76409

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

An enhanced role for an energy storage system in a microgrid with converter-interfaced sources

An enhanced role for the energy storage system (ESS), strategically placed at the point of common coupling (PCC) of the microgrid with the utility grid, is proposed. During island operation, the ESS ensures that the frequency and magnitude of the voltage will remain within the limits specified by the Standard EN 50160. By implementing an adjustable droop control method, the distributed energy resources (DERs) adjust their active and reactive powers in order to fulfil the load demand. When the grid is recovered, the ESS detects its presence and achieves a seamless synchronisation of the microgrid with the main grid, without any kind of communication. In grid-connected mode, the DERs deliver their available active power, whereas their reactive power is determined by a zero-sequence voltage. This voltage is injected by the ESS and aims to the zeroing of the amount of reactive power at the PCC. In this way, a reduction of power losses in the distribution lines of the microgrid is achieved. The effectiveness of the proposed control method in all operation modes, without any physical communication means, is demonstrated through detailed simulation in a representative microgrid with DERs fed by photovoltaics

A short communication to define the overcurrent protection system of the CIGRE European benchmark distribution networks for RES penetration studies

The use of clean energies in distribution networks is an unstoppable trend which has a significant positive impact on the progressive power system decarbonization. However, it has to be considered that conventional distribution systems have been designed to operate as passive networks. Therefore, a massive penetration of distributed generation may create several operational problems, such as malfunction of the protection systems, which may limit the deployment of this technology. The analysis of these limitations by means of representative benchmark networks is of utmost importance. Most of the proposed benchmark networks proposed so far, however, lack of information about their protection system. To overcome this shortcoming, this letter specifies the protection system of the benchmark European distribution networks proposed by the CIGRE Task Force C06.04.02 for this purpose. In this way, this letter facilitates the analysis of the possible impact that renewable energy sources may have in the distribution system protections.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea 764090Centro para el Desarrollo Tecnológico Industrial CER-2019101

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

Provision of Ramp-rate Limitation as Ancillary Service from Distribution to Transmission System: Definitions and Methodologies for Control and Sizing of Central Battery Energy Storage System

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