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Development of innovative ancillary services for distribution system operators (Doctoral thesis)

Πιππή, Καλλιόπη/ Pippi, Kalliopi

Scope of this doctoral dissertation is the development of innovative control strategies for the provision of advanced ancillary services (ASs) for active distribution networks (ADNs). The proposed ASs are based on the use of the available active and reactive power of the converters of distributed renewable energy sources (DRESs) and distributed battery energy storage systems (DBESSs) to solve technical issues related to voltage regulation (VR) and congestion of distribution lines. A new framework is also proposed for the coordinated operation of the primary/secondary distribution network (DN) aiming to solve voltage violation events. Furthermore, emphasis is given on the development of new methodologies for the assessment of the long-term performance of the provided ASs and grid operation. Initially, a new generalized framework for the impact assessment of DRESs-DBESSs on the operation of ADNs is presented. The proposed assessment methodology combines quasi-static simulations, with statistical analysis tools and technical indices, in order to evaluate the load matching capabilities and DRES-DBESS utilization of end-users (end-user oriented analysis) as well as to quantify the impact of DRES-DBESS on the network performance, in terms of voltage profiles, power losses and reverse power-flow incidents (network oriented analysis). The applicability of the proposed methodology is investigated considering two different ADN topologies. In addition, a new voltage control scheme is developed aiming to cope with voltage violation and voltage unbalance issues. The proposed strategy consists of two innovative algorithms, one for the provision of ASs related to VR and one for voltage unbalance mitigation (VUM). Both algorithms are based on the use of the active and reactive power of DRES/DBESS. For their operation, the use of reactive power is prioritized against the active power to ensure decreased DBESS utilization. The key feature of the proposed algorithms is their implementation in the symmetrical components domain removing any coupling interaction between them. The applicability of the proposed voltage control scheme is validated by time-domain simulations, as well as by conducting real-time simulations on a laboratory-scale DN. The long-term performance of the proposed VR and VUM algorithms is examined by performing quasi-static simulations in a software package; the obtained results are also compared with several control schemes of the relevant literature. Α new method is proposed to evaluate the provision of ASs by DRES/DBESS within a multi-services perspective considering the DBESS capacity degradation. The core of the proposed methodology is the combination between quasi-static simulations and a DBESS ageing model. Considering that the DRES/DBESS provide ASs related to VR and power smoothing, the performance of the proposed methodology is assessed using a low-voltage test feeder. Furthermore, a unified control strategy (UCS) for the provision of multiple ASs is introduced. Aim of the proposed UCS is to solve technical issues related to voltage violations, voltage unbalances, fluctuations of the active power at the output of DRESs and overloading of network components, using specialized algorithms. The effectiveness and the long-term performance of the proposed UCS are investigated by conducting time-domain and quasi-static simulations, respectively. Finally, as the provision of ASs by DRES/DBESS converters at a local level can not fully ensure the reliable operation of the entire DN, a new algorithm is introduced for the coordinated operation of the primary/secondary DN. Specifically, the proposed coordination algorithm uses the available active and reactive power of DRES/DBESS converters, located in the secondary DN, to deal with over- and undervoltage incidents detected at the primary/secondary DN connection node. The performance of the proposed algorithm is evaluated by performing quasi-static simulations considering an analytical primary/secondary DN model.