Low-frequency HVac transmission and distribution systems : planning and operation

The rapid load growth coupled with large-scale renewable generation sources remote from population centers demands future transmission grids to carry larger amounts of bulk power. For long distance transmission, high voltage direct current (HVdc) technology is superior to the conventional 50/60-Hz high-voltage alternating current (HVac) approach. Unfortunately, the lack of dc circuit breakers limits the application of HVdc technology to point-to point transmission links only. Thanks to advances in semiconductor materials and control methods, modern power converters make low-frequency HVac (LF-HVac) transmission systems possible. This new method of bulk power transmission overcomes the challenges in forming practical HVdc grids. Similarly, future distribution systems are expected to accommodate growing load demand in addition to increasing number of local inverter-based photovoltaic (PV) generations. Based on the aforementioned motivation, the objective of this dissertation is to develop power flow (PF) and optimal power flow (OPF) analysis methods for planning and operation of multi-frequency transmission systems that inherently employ a large number of converters. Similar steady-state analysis for a future distribution grid with high PV penetration, either as a standalone system or in coupling to a transmission grid, defines the second subject of the presented work. LF-HVac transmission scheme is recently proposed for long-distance bulk-power transmission by reducing the operating ac frequency to a low and variable value as determined by operational objectives and constraints. A multi-frequency HVac - HVdc system is formed by interconnecting conventional 50/60-Hz HVac grids to LF-HVac grids and HVdc lines. With respect to the first and major focus of this dissertation, a novel concept of an LFHVac grid employing converters with a centralized control is proposed. In addition, PF and OPF in a multi-frequency HVac - HVdc system are formulated by completely representing the steady-state models of HVac, HVdc, and LF-HVac grids as well as power converters, subject to all planning and operational constraints. The PF and OPF problems are solved by efficient algorithms based on the Newton-Raphson and predictor-corrector primal-dual interior-point methods (PCPDIPM), respectively. The proposed approach is applicable for a multi-frequency power system having arbitrary numbers of buses and topologies. Based on the PF results, the dependence of system MW losses on converter dispatch as well as the operating voltage and frequency in an LF-HVac is discussed and compared to that in HVdc transmission. On the other hand, the OPF analysis is applied to determine a suitable rated voltage in planning phase as well as optimal real-time operating frequency and dispatch of generators, shunt capacitors, and converters in operation phase. At the distribution side, high PV penetration might introduce voltage violations and reverse power flow. Besides the primary function of providing local generation, inverter-based PVs operating in grid-supporting mode can mitigate these consequences and minimize total losses with suitable dispatch. Therefore, the second focus of this work is to propose an exact OPF formulation and PCPDIPM-based solution algorithm to determine real-time dispatch of all inverters, switched capacitors, and voltage regulators with tap changers. The objective is to minimize total system losses, PV curtailment, and operations of capacitors and voltage regulators, in addition to elimination of voltage violations and reverse power flow. Effective computational strategies are proposed to allow real-time applications of the solution approach with a large number of constraints and variables. The accuracy and quality of the numerical solution in improving system performance are validated using practical distribution circuits with 15-minute load and PV data. High PV penetration also makes distribution systems more active and increases their impacts on the upstream transmission grids. As an extension of the work in distribution systems, a PF formulation as well as unified and sequential solution algorithms for joint transmission and distribution (T&D) systems are proposed, considering their physical coupling at substations. The potential effects of distributed PVs on transmission performance are also investigated. In addition, an OPF formulation and unified solution approach are proposed to determine the optimal operation for a joint T&D system. The objective is to minimize the total system losses while satisfying all operational constraints from both transmission and distribution sides.