Electric power systems are moving toward more decentralized models, where energy generation is performed by small and distributed power plants, often from renewables. With the gradual phase out from fossil fuels, however, Distribution Energy Resources (DERs) are expected to take over in the provision of all regulation services required to operate the grid. To this purpose, the opening of national Ancillary Service Markets (ASMs) to DERs is considered an essential passage. In order to allow this transition to happen, current opportunities and barriers to market participation of DERs must be clearly identified. In this work, a comprehensive review is provided of the state-of-the-art of research on DER integration into ASMs. The topic at hand is analyzed from different perspectives. First, the current situation and main trends regarding the reformation processes of national ASMs are analyzed to get a clear picture of the evolutions expected and adjustment required in the future, according to the scientific community. Then, the focus is moved to the strategies to be adopted by aggregators for the effective control and coordination of DERs, exploring the challenges posed by the uncertainties affecting the problem. Coordination schemes between transmission and distribution system operators, and the implications on the grid infrastructure operation and planning, are also investigated. Finally, the review deepens the control capabilities required for DER technologies to perform the needed control actions.
Nowadays, traditional synchronous generators are replaced by distributed renewable energy sources (DRESs), which are connected to the grid via power converters. This shift towards non-synchronous generation leads to low inertia power systems and affects considerably the frequency control procedure. To provide an inertial response and to enhance grid stability, DRESs can be equipped with fast discharging energy storage systems, such as ultracapacitors. This feature allows distribution system operators (DSOs) to provide an inertial response as ancillary service to transmission system operators (TSOs) by coordinating the operation of many DRESs. For this purpose, DSOs should develop tools to quantify and control the provision of inertial response. Towards this objective, in this study, two new methodologies are proposed. The first one aims to evaluate the maximum aggregated inertial capability of active distribution networks (ADNs). The second one aims to dispatch DRESs to provide specific, TSO-defined, inertial response with the minimum cost. Both methods are tested on a medium voltage grid. Several cases are examined highlighting the impact of different parameters, such as converter limitations, line congestions, on the inertial capability of ADNs. Results indicate that the proposed methods can fully exploit operational limits of ADNs and maximise their inertial capability.
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