Assessment of Voltage Regulation Methods in Low Voltage Distribution Networks

Originally, the power systems were designed to deliver the produced power from the large-scale centralized power stations to the end-users of the distribution grids. Since the centralized power plants usually use fossil fuels as primary energy sources, the growing energy demand has necessitated the use of low-carbon energy resources. Hence, most European utilities have already implemented programs to support the exploitation of renewable energy sources (RES). Due to the growing power demand and the increasing RES usage, the power systems, specifically the distribution networks, are rapidly changing. The traditional policy of installing distributed energy resources (DERs) is based on the passive “fit-and-forget” philosophy. Under this regime, the high penetration of DERs in distribution networks is expected to cause several issues related to power quality, reliability, security and stability. The high investment cost required to upgrade the current network assets has led to the transition from the passive network operation to the active distribution network management by the distribution network operators (DNOs). In terms of DNO responsibilities, voltage management is of primary concern, since voltage quality directly affects the quality of power provided to the grid customers. The concept of active network management (ANM) has mainly been implemented in the medium voltage (MV) distribution networks, while, at the LV level, voltage management was achieved by passive control devices, such as off-load tap changers and capacitor banks. This thesis evaluates the performance of the existing active voltage management technologies, when being applied in LV distribution networks. In particular, the technologies being assessed in this study are: (i) MV/LV transformers fitted with an OLTC device, (ii) distributed static synchronous compensators (D-STATCOMs), (iii) active in-line voltage regulators (IVRs) fitted with an OLTC device, (iv) DERs, and (v) battery energy storage systems (BESSs). In general, the LV networks consist of single-phase elements, either loads or generators, therefore, the growing trends of demand and generation can lead to several considerable problems, such as over-voltages, under-voltages, high voltage unbalances and grid losses rise. The main goal of this dissertation is to examine to which degree the evaluated voltage control technologies can mitigate the aforementioned issues. First, different centralized voltage control strategies are proposed, when applying each technology individually. Besides that, the focus is to evaluate to which extent the applied techniques can perform remote voltage management under high levels of DER integration and load demand. Particularly, the integration of OLTCs into MV/LV transformers is assessed considering the voltage constraints, as well as the thermal limits of the transformer and the cables. In case of radial feeders with both large number of branches and long distribution lines, the voltage control via MV/LV transformers might be infeasible. In such cases, the IVR contribution might be crucial for the voltage regulation. In this study, its performance is evaluated with respect to various power quality issues, while its optimal placement and sizing are investigated with respect to the operating voltage margin and the total grid losses. Next, different power management technologies (D-STATCOM, DERs and BESSs) are used to regulate the voltage remotely. As for the D-STATCOM, two control schemes are evaluated, the former for the regulation of the positive-sequence voltage component, and the latter for the regulation of the voltage unbalances. Moreover, the coordination of D-STATCOM’s with the OLTC is also examined. Concerning the DERs, three localized voltage control schemes for single-phase units are presented, based on reactive and active power management. Crucial parameters for the implementation of those methods are the sensitivity factors of voltage magnitude with regard to the power injections. Since the reactive power is insufficient to mitigate fully the over-voltage and under-voltage issues, two active power curtailment methods are also evaluated, following the concepts of “hierarchical” and “fair” contribution, respectively. Finally, the dissertation assesses the performance of hybrid voltage management schemes. Different coordinated schemes are examined, utilizing the centralized control through the MV/LV transformer with an OLTC mechanism and the localized power management methods through single-phase DERs. The main focus on using the coordinated schemes is to eliminate the curtailed active power, the OLTC wear and the reactive power provision. Furthermore, a novel optimization scheme to control both the voltage magnitudes and voltage unbalances is investigated, by using the MV/LV transformer with an OLTC device, single-phase DERs and single-phase BESSs. The main objective of this approach is to develop the optimal interaction between the aforementioned devices, while minimizing a set of objectives related to grid losses, OLTC wear, battery ageing and reactive power provision.status: publishe