Environmental targets set by governments around the world are leading to high penetrations of small to medium-scale renewable distributed generation (DG). High penetration of DG in distribution networks, however, can result in voltage and thermal issues among other technical problems. The traditional 'Fit & Forget' approach that refers to the passive use of assets with limited or no control, in the context of distribution network planning, is used to meet maximum demand or generation requirements. However, to ensure that more renewable generation is cost-effectively connected to distribution networks, it is imperative to adopt a more active control of network elements and participants. The active control of future distribution networks requires understanding the corresponding dependencies between voltage magnitudes and DG active/reactive power outputs to mitigate voltage issues. One classical method to calculate these dependencies is to use sensitivity approaches such as those based on the Jacobian matrix. However, during operation, updating the Jacobian matrix requires the network to be fully observable making it unfeasible for decentralized control approaches. Therefore, it is critical to develop a sensitivity approach only requiring local real-time information. This thesis proposes a novel approach to produce voltage sensitivity coefficients using the surface fitting technique based solely on knowledge of network characteristics and, therefore, no remote monitoring is required. To assess the performance of the proposed voltage sensitivity approach, a decentralized (local) voltage control algorithm that simultaneously caters for both the active and reactive power outputs of a single DG plant is adopted. Comparisons with classical sensitivity approaches are carried out using the 16-bus UK GDS test network, 1-min resolution demand and wind generation data. Persistence forecasting (i.e., assuming no changes in demand and wind in a short time period) is considered in this case. The lower Mean Squared Error (MSE) shows that the coefficients of the proposed sensitivity approach are close to those of the Jacobian matrix and better than the perturb-and-observe approach. In the context of voltage management, results highlight that the proposed sensitivity approach is more effective than the Jacobian matrix inverse and perturb-and-observe, resulting in better voltage compliance and energy harvesting (better capacity factor). It should be highlighted that this performance is achieved without the need of full network observability. Furthermore, to cater for the more realistic and complex case of multiple DG plants, this thesis proposes a time-delay based decentralized control algorithm. A comparison with an ideal AC Optimal Power Flow (OPF) is carried out using the same 16-bus UK GDS network but with seven DG plants. The results demonstrate that the proposed sensitivity approach and time delays are very effective when compared to the AC OPF. This, in turn, proves that the combined use of the proposed voltage sensitivity approach and the decentralized controller is an implementable, cost-effective solution to manage DG plants in distribution networks without the need of further communication infrastructure. Finally, a decentralized DG control logic with the capability of using wind forecasting techniques is proposed to tackle the unpredictable nature of wind power. In this work, a time-series based forecasting technique is incorporated to the proposed decentralized controller. The results confirm that the use of more advanced forecasting technique can further improve the management of renewable DG plants.

Recent advancements in Voltage Source Converters (VSCs) of high-voltage and high-power rating had a significant impact on the development of Multi-Terminal HVDC (MTDC) power transmission systems. The U.S. Navy has proposed Multi-Zonal Medium Voltage DC (MVDC) Shipboard Power System (SPS) architecture for the next generation of their surface combatant. A Multi-Zonal MVDC SPS consists of several VSCs exchanging power through a DC network. Following a system fault or damage, the current flow pattern in the DC distribution grid will change and the DC voltages across the VSCs will assume new values. DC voltage reference or power reference settings of VSCs have to be determined, in advance, which can maintain the DC voltage within desired margins (usually 5% around the nominal value) in steady state, under the prefault as well as the postault conditions. In this work, the reference settings have been pre-determined by: (1) Development of a sensitivity based algorithm for voltage control of VSCs of the DC system and (2) Development of an optimal algorithm for voltage and power control of the VSCs. The algorithms have been tested on a simplified representation of the MVDC SPS architecture.

Diversification of the energy mix is one of the main challenges in the energy agenda of governments worldwide. Technology advances together with environmental concerns have paved the way for the increasing integration of Distributed Generation (DG) seen over recent years. Combined heat and power and renewable technologies are being encouraged and their penetration in distribution networks is increasing. This scenario presents Distribution Network Operators (DNOs) with several technical challenges in order to properly accommodate DG developments. However, depending on various factors, such as location, size, technology and robustness of the network, DG might also be beneficial to DNOs. In this thesis, the impact of DG on network planning is analysed and the implications for DNOs in incorporating DG within the network planning process are identified. In the first part, various impacts of DG to the network, such as network thermal capacity release, security of supply and on voltage, are quantified through network planning by using a modified successive elimination method and voltage sensitivity analysis. The results would potentially assist DNOs in assessing the possibilities and effort required to utilise privately-owned DG to improve network efficiency and save investment. The quantified values would also act as a fundamental element in deriving effective distribution network charging schemes. In the second part, a novel balanced genetic algorithm is introduced as an efficient means of tackling the problem of optimum network planning considering future uncertainties. The approach is used to analyse the possibilities, potential benefits and challenges to strategic network planning by considering the presence of DG in the future when the characteristics of DG are uncertain.

Long term supporting schemes for photovoltaic (PV) system installation have led to accommodating large numbers of PV systems within load pockets in distribution grids. High penetrations of PV systems can cause new technical challenges, such as voltage rise due to reverse power flow during light load and high PV generation conditions. Therefore, new strategies are required to address the associated challenges. Moreover, due to these changes in distribution grids, a different response behavior of the distribution grid on the transmission side can be expected. Hence, a new equivalent model of distribution grids with high penetration of PV systems is needed to be addressed for future power system studies. The thesis contributions lie in three parts. The first part of the thesis copes with the PV modelling. A non-proprietary PV model of a three-phase, single stage PV system is developed in PSCAD/EMTDC and PowerFactory. Three different reactive power regulation strategies are incorporated into the models and their behavior are investigated in both simulation platforms using a distribution system with PV systems. In the second part of the thesis, the voltage rise problem is remedied by use of reactive power. On the other hand, considering large numbers of PV systems in grids, unnecessary reactive power consumption by PV systems first increases total line losses, and second it may also jeopardize the stability of the network in the case of contingencies in conventional power plants, which supply reactive power. Thus, this thesis investigates and develops the novel schemes to reduce reactive power flows while still keeping voltage within designated limits via three different approaches: decentralized voltage control to the pre-defined set-points developing a coordinated active power dependent (APD) voltage regulation Q(P)using local signals developing a multi-objective coordinated droop-based voltage (DBV) regulation Q(V) using local signals   In the third part of the thesis, furthermore, a gray-box load modeling is used to develop a new static equivalent model of a complex distribution grid with large numbers of PV systems embedded with voltage support schemes. In the proposed model, variations of voltage at the connection point simulate variations of the model’s active and reactive power. This model can simply be integrated intoload-flow programs and replace the complex distribution grid, while still keepingthe overall accuracy high. The thesis results, in conclusion, demonstrate: i) using rms-based simulations in PowerFactory can provide us with quite similar results using the time domain instantaneous values in PSCAD platform; ii) decentralized voltage control to specific set-points through the PV systems in the distribution grid is fundamentally impossible dueto the high level voltage control interaction and directionality among the PV systems; iii) the proposed APD method can regulate the voltage under the steady-state voltagelimit and consume less total reactive power in contrast to the standard characteristicCosφ(P)proposed by German Grid Codes; iv) the proposed optimized DBV method can directly address voltage and successfully regulate it to the upper steady-state voltage limit by causing minimum reactive power consumption as well as line losses; v) it is beneficial to address PV systems as a separate entity in the equivalencing of distribution grids with high density of PV systems. / <p>The Doctoral Degrees issued upon completion of the programme are issued by Comillas Pontifical University, Delft University of Technology and KTH Royal Institute of Technology. The invested degrees are official in Spain, the Netherlands and Sweden, respectively. QC 20141028</p>

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