Analysis and realization of a new device for power quality and custom power improvement: open UPQC

Power Quality (PQ) in Low Voltage (LV) distribution networks is already a concern in many European countries especially where there is a strong presence of renewable energy generation. Therefore there is a growing interest in new solutions able to improve the power quality level of such a system providing regulated power to the end user within standard definition. In other hand, for different reasons, customer may need or require customized power which may not fit into standard definition. PQ and Custom Power may follow the same patch or possibly those may contradict each other going in opposite directions. Therefore, it is not an easy task for Distribution System Operator (DSO) to respect both standard PQ definitions and custom power requirements. Several solutions have been introduced by electrical power engineers in order to perform PQ compensation tasks and provide custom power requirements. Among those, Uninterruptible Power Supply (UPS) is the most interesting and prosperous solution which is able to provide both PQ and custom power requirements of a customer. However a UPS system is quite an expensive solution and several alternatives are proposed in order to decrease solution expenses in the cost of reducing its functionality and flexibility. Series and Shunt conditioners with different topologies have been studied and those can furnish the grid with their voltage and current compensation capabilities however, a series or shunt conditioner alone, is an incomplete solution due to their compensation and service restrictions. To deal with series and shunt conditioners shortcomings, a combination of series and shunt conditioner is introduced by Akagi, which is able to tackle both voltage and current impureness and it is called Unified Power Quality Conditioner (UPQC). Although UPQC looks a well designed solution to manage PQ and custom power requirements, however ever increasing renewable energy types, Distributed Generation (DG) and storage integration into power system and especially at LV level, have changed the PQ conditioners paradigm into new era. By introducing bidirectional power flow in newly introduced modern and Smart Grid systems, DSOs started to look for system level and modular solutions which are able to operate within this modern and Smart Grid systems communicating with other active participants. Open Unified Power Quality Conditioner (Open UPQC) is a novel proposal and special design of original UPQC and it is a distributed solution which is able to provide high power quality to the installed area and provide custom power services to the end users and several auxiliary services to the DSO. The proposal is to split the UPQC series and shunt units, move the series unit to the Medium Voltage (MV)/LV substation, in order to support all the installed area, and to split the shunt unit into several units according to end user needs and install each shunt unit at front end of customer property, providing PQ and custom power improvements to the end user and different auxiliary services to the grid and DSO. The series and shunt units are able to communicate with each other within a generic Information and Communications Technology (ICT) system. The proposed Open UPQC is compatible to be easily integrated within Smart Grid systems. This thesis discusses the working principle, hardware and controller design of Open UPQC.Working philosophy of Open UPQC is explained in detail and series and shunt units responsibilities are introduced. Series unit is meant to work with pure non-active power compensation strategy to inject compensation voltage in quadrature to the line current. This working principle is meant to reduce system losses and its realization cost. This non-active power working concept, will impose operation limits on series unit. Series unit operation limits is analyzed in deep, considering possible contribution that shunt units can have to improve series unit performance and this led to the co-operation idea between series and shunt units as happens in the UPQC. Series and shunt units should be able to work independently so, both units controller should be fast enough in order to deal with transient events. Series unit is designed to work as self-supported Dynamic Voltage Restorer (DVR) system in order to compensate both fast and slow voltage variation of the grid. In other hand, shunt unit’s function is like a line interactive UPS system which is able to give some ancillary services to the grid. Beside series and shunt unit local controller, ICT based controller is also described in order to enable co-operation between series and shunt units. The performance of the designed Open UPQC is verified by MATLAB based simulation and Laboratory experimental tests prior to real field tests. The whole Open UPQC has been realized as a part of Smart Domo Grid (SDG) project and it has been installed and tested in a real LV distribution network in the city of Brescia, north of Italy. This thesis presents several original and innovative concepts to the research area. A common solution to answer both PQ and Custom Power issues are investigated. Open UPQC as a unique and innovative distributed solution is proposed to be implemented in distribution LV Smart Grid. Working principle is analyzed in detail and especially series unit operation limits is addressed and continuous operation is verified by simulation and experimental tests. Based on best knowledge of author, for first time Open UPQC is realized and the only worldwide available prototype, has been installed and tested in real LV network. PQ and custom power conditioning has been perform in LV distribution network and several scenarios have been defined and tested within Smart Grid system.

Questa tesi discute di principio di funzionamento, l'hardware e il design del controller del dispositivo elettronico chiamato Open UPQC. La filosofia di controllo dell’Open UPQC è spiegata in dettaglio e in particolare le funzioni dell’unità serie e delle unità shunt. Al fine di ridurre le perdite di sistema e il costo di realizzazione l’unità Series lavora con una strategia di compensazione che sfrutta la potenza non attiva. Questo impone però dei limiti operativi all’unità serie. Tali limiti di funzionamento vengono approfonditamente analizzati nella tesi, considerando anche il possibile contributo delle unità shunt che possono essere impiegate per migliorare le prestazioni della stessa unità serie. Questo concetto ha portato all’idea di cooperazione tra le unità della serie e shunt come avviene nel sistema UPQC. L’unità serie e le unità shunt devono essere in grado di lavorare in modo indipendente. Quindi il sistema di controllo deve essere abbastanza veloce per poter far fronte ad eventi transitori. L’unità Series è progettata per funzionare come un Dynamic Voltage Restorer (DVR) per compensare variazioni sia veloci che lente della tensione di rete. L’unità shunt è progetta per funzionare come un sistema UPS interattivo on-line che è in grado di dare alcuni servizi accessori alla rete. In parallelo al sistema locale di ogni unità, è stato sviluppato un sistema di comunicazione ICT al fine di consentire la cooperazione tra le unità serie e shunt. Le prestazioni dell’Open UPQC progettato sono state dapprima verificate da simulazioni basate su piattaforma MATLAB poi confermate con prove sperimentali di laboratorio e infine da prove reali sul campo. L'intero Open UPQC è stato realizzato come parte del progetto Smart Domo Grid (SDG) ed è stato installato e collaudato in una vera e propria rete di distribuzione in bassa tensione (BT) nella città di Brescia, a nord di Italia.