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

Frequency-adaptive control of a three-phase single-stage grid-connected photovoltaic system under grid voltage sags

The low-voltage ride-through service is carried out in this paper according to the voltage profile described by the IEC 61400-21 European normative when short-duration voltage sags happen, and some instantaneous reactive power is delivered to the grid in accordance with the Spanish grid code; the mandatory limitation of the amplitude of the three-phase inverter currents to its nominal value is carried out with a novel control strategy, in which a certain amount of instantaneous constant active power can also be delivered to the grid when small or moderate voltage sags happen. A Multiple second order generalized integrator frequency-locked loop synchronization algorithm is employed in order to estimate the system frequency without harmonic distortions, as well as to output the positive- and the negative- sequence of the {\alpha}\b{eta} quantities of the three-phase grid voltages when balanced and unbalanced voltage sags happen in a frequency-adaptive scheme. The current control is carried out in the stationary reference frame, which guarantees the cancellation of the harmonic distortions in the utility grid currents using a Harmonic compensation structure, and the implementation of a constant active power control in order to protect the DC link capacitor from thermal stresses avoiding the appearance of large harmonic distortions at twice the fundamental frequency in the DC link voltage. A case study of a three-phase single-stage grid-connected PV system with a maximum apparent power about 500 kVA is tested with several simulations using MATLAB/SIMULINK firstly, and secondly, with some experiments using the Controller hardware-in-the-loop (CHIL) simulation technique for several types of voltage sags in order to do the final validation of the control algorithms

Provision of voltage ancillary services through enhanced TSO-DSO interaction and aggregated distributed energy resources

The electrical energy generated from renewable energy resources connected to transmission and distribution systems and the displacement of synchronous generators continues to grow. This presages a paradigm-shift away from the traditional provision of ancillary services, essential to ensure a robust system, from transmission-connected synchronous generators towards provision from synchronous and non-synchronous generation (including distribution-connected resources). Given that the available resources at the disposal of system operators are continuously increasing, the flexibility for operating the network can be enlarged. In this context, this paper introduces a dedicated voltage ancillary services strategy for provision of reactive power. A main feature of the proposed strategy is that it is technology-neutral, unlike existing ones that are focused on synchronous generators. The system need for voltage stability is placed at the core of this strategy, which is translated into a requirement for reactive power provision. The proposed strategy achieves, through the combined utilization of distributed generation and traditional resources, to defer the investments in reactive compensating equipment. Dynamic and transient studies are conducted to demonstrate the technical benefits of the strategy, while its practical feasibility is also validated through hardware-in-the-loop testing

Feeder flow control and operation in large scale photovoltaic power plants and microgrids : Part I Feeder ow control in large scale photovoltaic power plants : Part II Multi-microgrids and optimal feeder ow operation of microgrids

This thesis deals with the integration of photovoltaic energy into the electrical grid. For this purpose, two main approaches can be identified: the interconnection of large scale photovoltaic power plants with the transmission network, and the interconnection of small and medium-scale photovoltaic installations with the distribution network. The first part of the thesis is focussed on the interconnection of large scale photovoltaic power plants. Large scale photovoltaic power plants are required to provide different ancillary services to the electrical networks. For this purpose, it is necessary to control the active and reactive power injected by photovoltaic power plants at the point of interconnection, i.e. to control the power flow through the main feeder. In this direction, it is developed a central controller capable of coordinating the different devices of the photovoltaic power plants as photovoltaic inverters, FACTS, capacitor banks and storage. The second part is focused on the distributed generation, consisting on small and medium-scale generation facilities connected to the distribution system. In this context, distribution grids, traditionally operated as passive systems, become active operated systems. In this part, the microgrid concept is analysed, which is one of the most promising solutions to manage, in a coordinated manner, the different distributed energy resources. Taking into account the possible transformation of the current distribution system to a multi-microgrid based system, the different architectures enabling microgrids interconnections are analysed. For the multi-microgrid operation, it could result interesting that a portion of their networks operate so that the power exchange is maintained constant, i.e. controlling the power flow at the main feeder. In this thesis, an optimal power flow problem formulation for managing the distributed generation of these feeder flow controlled microgrids is proposed