Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409

Development of cost-functions for the remuneration of new ancillary services in distribution networks

INSPEC Accession Number: 21129600The higher penetration of intermittent and volatile Distributed Renewable Energy Sources (DRES) in distribution grids is gradually replacing the bulk synchronous generators (SGs) of the transmission system, resulting in poor reaction after a grid event. Conventionally, in order to ensure the stable and safe operation of the electrical grid, the Transmission System Operators (TSOs) request the participation of SGs in Ancillary Services (AS) markets. On the other side, the DRES are mainly regarded as negative loads complying with certain grid codes requirements and no participation to such markets. However, new control algorithms are emerged, including the DRES operation with P-f droop curves, their reactive power contribution according to voltage variations, ramp-rate limitation and fault-ride-through (FRT) capability. Moreover, other support functions have also been proposed in the literature, e.g. provision of virtual inertia, power smoothing and harmonic mitigation. Such support functions, provided that they are properly quantified, can be traded in the AS markets already existing at transmission system level. This paper develops cost-functions for the procurement of the aforementioned AS in a parametric form after conducting a cost-benefit analysis per AS, considering several factors, such as the location, the size and the capability for providing the AS.Horizonte 2020 76409

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

Ancillary Services Market Design in Distribution Networks: Review and Identification of Barriers

The high proliferation of converter-dominated Distributed Renewable Energy Sources (DRESs) at the distribution grid level has gradually replaced the conventional synchronous generators (SGs) of the transmission system, resulting in emerging stability and security challenges. The inherent characteristics of the SGs are currently used for providing ancillary services (ASs), following the instructions of the Transmission System Operator, while the DRESs are obliged to o er specific system support functions, without being remunerated for these functions, but only for the energy they inject. This changing environment has prompted the integration of energy storage systems as a solution for transfusing new characteristics and elaborating their business in the electricity markets, while the smart grid infrastructure and the upcoming microgrid architectures contribute to the transformation of the distribution grid. This review investigates the existing ASs in transmission system with the respective markets (emphasizing the DRESs’ participation in these markets) and proposes new ASs at distribution grid level, with emphasis to inertial response, active power ramp rate control, frequency response, voltage regulation, fault contribution and harmonic mitigation. The market tools and mechanisms for the procurement of these ASs are presented evolving the existing role of the Operators. Finally, potential barriers in the technical, regulatory, and financial framework have been identified and analyzed.Unión Europea (Programa Horizonte 2020) 76409

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

Reactive power consumption in photovoltaic inverters: a novel configuration for voltage regulation in low-voltage radial feeders with no need for central control

High photovoltaic (PV) system generation in low-voltage feeders can cause voltage rise especially in low demand conditions. The conventional way of coping with voltage violation is disconnection of the PV systems or curtailment of the generated power. To address this issue, a novel configuration for voltage management in a radial feeder via regulated reactive power capability in PV inverters is presented. The novelty of the proposed configuration is based on the fact that all the PV inverters with the ability to consume reactive power are involved in voltage regulation without being centrally controlled. In order to apply the configuration, a reference voltage is initially estimated for each PV system and the PV inverters are calibrated accordingly. These settings depend on the feeder topology and can be calculated by the distribution network operator with a simple power flow modelling tool. Finally, the article presents a sensitivity analysis in order to examine how reactive power consumption in a single inverter influences PV penetration and inverter sizing at various PV topologies along the feeder.JRC.F.6-Energy Technology Policy Outloo