Utilizing smart inverter capabilities for management of high penetration renewable distributed generation integration in active distribution networks
Abstract
With a strong indication of high renewable distributed generation (DG) penetration increase in the near future, distribution system operators (DSOs) are burdened with challenges to manage their operation while also trying to solve the power quality issues emerging due to reverse power flow and their stochastic behavior. This work focuses on both distributed control as well as centralized control techniques while utilizing the capability of the smart inverter that controls the power flow as well as provides power quality services. In order to signify the importance of smart inverter capability while integrating renewable DGs on active distribution networks, applications are proposed for distribution networks that are either connected to the main grid or operates as a standalone microgrid that requires both power quality as well as power flow control to perform economic operation. This dissertation work begins with presenting the negative impacts of high scale distributed generation on the grid specifically during high ramp rate events. Initial results show the inability of either the conventional voltage regulators as well as the dynamic reactive power support from a smart inverter to maintain node voltages within a desired band. A passive ramp rate control is presented that depends solely on the battery state of charge (SoC) and does not take into consideration the system modeling. Next, a distributed voltage control (DVC) scheme is proposed on the same active distribution network (ADN) in the presence of high penetration photovoltaic (PV) generation to mitigate high ramp-rate issues actively based on a sensitivity tool strategy. Multiple voltage regulating devices, are coordinated in various ways based on a distributed coordination management scheme. This work presents the relationship of the active and reactive power outputs of a PV plant in the presence of the conventional voltage regulating devices, and proposes a distributed control strategy to coordinate the SVRs with the PV inverter’s capability to improve power quality. In order to plan the PV penetration ahead of time, a zone based multistage time graded operation of the cascaded load-tap changing (LTC) transformers and capacitor banks (CBs) is developed. The objective of the proposed centralized optimization algorithm is to regulate the voltage in a medium voltage (MV) unbalanced distribution system while trying to relax the tap operations of voltage regulators that are cascaded in series, and thereby, minimize the curtailment of PV output when necessary. A technique for changing Mixed-integer nonlinear programming (MINLP) to nonlinear programming (NLP) and then to binary-MINLP optimization is suggested to meet the different objectives at successive stages. The first stage is associated with the on-load tap changing (OLTC) transformer taps and capacitor bank operations. The second, third and fourth stages deal with the step voltage regulator (SVR) operations in their respective zones. A week long simulation was performed for various scenarios of PV and load profile variations. Finally a microgrid system is tested with a hybrid DG/battery system to prove its ability to dispatch based on set-points from a secondary level. A multi-stage power regulation scheme for different Distributed Energy Resources (DERs) in a standalone microgrid is proposed. During the on-peak period for islanded mode, a two stage operation based on cost optimization and frequency regulation is proposed. In the standalone mode, a microgrid energy management system (M-EMS) aims to sustain the loads while performing an economic generation scheduling based on an optimization control strategy with the help of several droop enabled sources that includes two hybrid distributed generator (DG)/battery systems. In addition when the load demand is low, a multi-objective optimization strategy is proposed to charge the batteries back to their nominal values while the other dispatchable sources operate in droop mode with economical operation. Numerical results from the 33 bus distribution system, when operated as a microgrid, show the effectiveness of the strategy.