Inverter-based voltage regulation is gaining importance to alleviate emerging reliability and power-quality concerns related to distribution systems with high penetration of photovoltaic (PV) systems. This paper seeks contribution in the domain of reactive power compensation by establishing stability of local Volt/VAr controllers. In lieu of the approximate linear surrogate used in the existing work, the paper establishes existence and uniqueness of an equilibrium point using nonlinear AC power flow model. Key to this end is to consider a nonlinear dynamical system with non-incremental local Volt/VAr control, cast the Volt/VAr dynamics as a game, and leverage the fixed-point theorem as well as pertinent contraction mapping argument. Numerical examples are provided to complement the analytical results.
This paper considers the problem of voltage regulation in distribution network. The primary motivation is to keep voltages within pre-assigned operating limits by commanding the reactive power output of distributed energy resources (DERs) deployed in the grid. We develop a framework for developing local Volt/Var control that comprises of two main steps. In the first, exploiting historical data and for each DER, we learn a function representing desirable equilibrium points for the power network. These points approximate solutions of an Optimal Power Flow problem. In the second, we propose a control scheme for steering the network towards these favorable configurations. Theoretical conditions are derived to formally guarantee the stability of the developed control scheme and numerical simulations illustrate the effectiveness of the proposed approach.
As the electric power supply systems are undergoing major changes with the integration of renewables, the issues related to voltage regulation and system protection are arising. In this scenario, advanced voltage regulation technologies that provide voltage control at the grid-edge, that is at the low-voltage secondary side of the distribution circuit, have emerged as a potential solution to address the shortcomings of traditional voltage control practices in distribution systems. In this work, these technologies are modeled and algorithms are developed to strategically deploy them, tune their control parameters, and evaluate their voltage regulation performance. A two-stage optimization framework is proposed for optimal placement and real-time control of the low-voltage static var compensators to minimize the energy losses while maintaining the voltage regulation. Integration of high levels of distributed generation such as photovoltaic (PV) systems impacts the voltage regulation by causing steady-state voltage variations and transient voltage fluctuations. This work further develops a procedure to tune the control parameters of PV smart inverters to mitigate these voltage issues. Furthermore, the PV penetration levels in a distribution network can be increased without creating voltage problems by dynamic controlled reactive power absorption at several strategic buses. This concept is modeled and demonstrated in this work. Furthermore, the high levels of PV generation can interfere with the overcurrent protection schemes prevalent in distribution networks. An analytical approach is proposed in this work to estimate the distribution feeder PV accommodation limits with respect to overcurrent protection issues as the impact criteria, without needing to simulate numerous PV screening scenarios to assess the impact
Voltages in distribution networks are subject to variations primarily due to varying demand and the intermittent nature of growing renewable generation. Conventionally voltages are regulated only at certain locations such as distribution substations with some fixed settings for control equipment such as voltage targets or tap changers of under-load tap changing (ULTC) transformers and switching status of shunt capacitors. These settings are normally determined by operators in control rooms and are changed on the condition that power supply cannot be adequately delivered. A fixed constant setting for the controllable devices may not guarantee secured voltage profiles for all load centres at all times during the course of a day and may lead to frequent operations due to fluctuations in generation and demand profiles, which will eventually lead to wear on the control equipment. This thesis investigates the operational control of voltages on distribution networks and proposes a control strategy that manages voltage control devices from an operational planning perspective in order to maintain a desired level of voltage security by applying the most cost-effective control actions. The proposed methodology integrates power system sensitivity analysis and an artificial intelligence (AI) planning approach to schedule voltage control actions for a given electrical system across a specific planning time period based on known generation and demand profiles. The concept of a failsafe mode is incorporated into the proposed control strategy to deal with the potential loss of data communication or ultimate failure of the planned solutions. A typical radial distribution network model was studied under a range of scenarios and the simulation results demonstrated that the proposed methodology was capable of automatically planning control settings for ULTC transformers and MSCs to maintain requisite voltage limits and outperformed the conventional methods by eliminating the number of voltage violations and also reducing the number of control operations. The flexibility of the proposed methodology allows it to be integrated to the existing software platforms used by some of the UK distribution network operators (DNOs).
Power distribution networks are rapidly evolving as active distribution systems, as a result of growing concerns for the environment and the shift towards renewable energy sources (RESs). The introduction of distributed generations can benefit the distribution network in terms of voltage support, loss reduction, equipment capacity release, and greenhouse gas (GHG) emission reduction. However, the integration of RESs into electric grids comes with significant challenges. The produced energy from renewable sources such as wind and solar is intermittent, non-dispatchable and uncertain. The uncertainty in the forecasted renewable energy will consequently impact the operation and control of the power distribution system. The impact on Volt/Var control (VVC) in active distribution systems is of particular concern, mainly because of reverse power flow caused predominantly by RESs. RESs can influence the operation of voltage control devices such as on-load tap changers (OLTCs), line voltage regulators (VRs) and shunt capacitor banks (ShCs). It is mainly because of reverse power flow, caused predominantly by RESs. Reverse power flow or injecting power between the regulator and the regulation point can confuse the local regulator controller, which leads to inappropriate or excessive operations. Some of the potential adverse effects include control interactions, operational conflicts, voltage drop and rise cases at different buses in a network. This research project aims to carry out an in-depth study on coordinated Volt/Var control strategies in active distribution networks. The thesis focuses on the problem of Volt/Var optimization in active distribution networks, operated under different operating conditions, by taking into consideration the current distribution system requirements and challenges in the presence of high RESs penetration. In the initial phase of the research project, a generic solution to the VVC problem of active distribution systems was first developed. The primary goal of this generic solution involved the determination of an optimal control strategy based on system status, which was identified from bus voltages. As such, there are three different operating states; normal, intermediate and emergency state. Each operating state has its own control strategy that includes state-related objective functions, such as minimization of power losses, operational control costs, and voltage deviation. For both normal and intermediate state operations, a heuristic-search based optimization algorithm is implemented. In order to be able to take control actions rapidly, a novel rule-based control strategy is developed for the emergency state. In the second phase of the research project, the proposed zone-oriented convex distributed VVC algorithm was developed to address the limitations of heuristic optimization algorithms, including long solution times and the non-global optimal solution. The proposed algorithm is based on chordal-relaxation semi-definite programming (SDP), and divides distribution systems into areas based on customer types, wherein, each zone has its own priorities, characteristics, and requirements. The primary goal is to achieve optimal voltage control for each zone, according to its operational requirements and characteristics. Furthermore, in contrast to many decentralized approaches that require iterative solutions to update global multiplier and a penalty parameter to convergence, this method proposes a novel multi-period hierarchical convex distributed control algorithm, requiring no iterative process and no penalty parameter. Eliminating the iterative solution makes convergence fast, while having no penalty parameter allows for the algorithm to be less human and system dependent. In the final phase of the research project, a 2-stage control algorithm aiming to minimize VR tap movements in convex VVC formulation was developed. In the first stage, the VVC problem is solved for hourly intervals, and VR tap positions are obtained. In the second stage, control horizon is divided into 15 minutes intervals, and the voltage is controlled only by the RESs' active and reactive power adjustment. The tap movement minimization and 2-stage control algorithm eliminates the excessive use of VRs, prolongs the operational life of VRs and reduces the system operational cost. The optimal operation of Volt/Var control devices was investigated in the presented Volt/Var optimization methodology. The proposed research will pave the way for managing the increasing penetration of RESs with different types, technologies and operational modes, from a distribution system voltage control perspective. The proposed methodologies in this thesis have been tested on sample distribution systems and their effectiveness is validated.
