Sensitivity Based Cooperative Operation for Grid Connected Converter and Distributed Generator Considering Losses in the Radial DC Distribution System

Abstract

This dissertation proposes a cooperative control scheme for grid connected converter (GCC) and distributed generator (DG) considering line and conversion losses in the radial DC distribution system and novel formulations of approximate expression for voltage sensitivity and line loss sensitivity. Interesting and possibilities of the DC distribution system is increasing due to its advantages. In the DC distribution system, transmittable power can be increased and overall system loss is smaller comparing with the AC distribution system. The DC distribution system is also suitable for smart grid (SG) environment from the perspective of energy efficiency, sustainability, and flexibility and this is one of important technical issues in SG. Possibility and feasibility of the DC distribution system is studied in many researches. However, there are technical issues on the DC distribution system and these should be considered from the perspective of stable and economic operation. Loads in the DC distribution system are fed through DC/AC or DC/DC converter, which have constant-power load (CPL) behavior, and CPL behavior cause voltage instability due to negative incremental resistance characteristics of CPL. Voltage control methods have studied to prevent voltage instability. However, these studies have drawbacks of limit of voltage regulation capability and efficient voltage control. Especially, line and conversion loss is an important factor in the DC distribution system from view point of efficient operation because large power delivered by power converter. However, most of conventional researches do not focused on both line loss over the conductor and conversion loss. The Purpose of research in this dissertation is to control voltage within suitable range considering total loss reduction in the radial DC distribution system. Before establishing control scheme, novel sensitivity expressions on voltage and line loss are formulated based on linear approximation and reasonable assumptions. Verification on proposed formulation is presented and it is indicated that an accuracy of proposed expression presents a good result. Analytic analysis of voltage sensitivity is presented and results are utilized to construct a voltage control strategy. In addition, these formulations are useful for fast speed controller because the computation effort can be reduced. Based on approximate sensitivity expression and analysis result, cooperative voltage control scheme considering total losses in the radial DC distribution system is proposed. The structure of the overall control scheme is illustrated and roles and interaction of voltage control equipment are explained. A model for an optimal scheduling is presented and objective function and constraints are formulated. The Control strategies for loss reduction and voltage regulation are established and control modules are designed to realize proposed strategy. In addition, the loss sensitivity on total with regard to voltage control equipment is formulated and it represents the actual loss sensitivity due to changes in electrical quantities of the voltage control equipment. The proposed method applied to the test system to verify effectiveness and tested under various conditions using MATLAB and PSCAD/EMTDC. The results have shown that the entire bus voltages are regulated and loss can be reduced under varying load and generation condition. The cooperative voltage control scheme can be implemented within the radial DC distribution system and provide a useful control scheme from the perspective of stability and economic operation.