Research On DC Distribution Network With Meshed Structure And Its Voltage Control Stratagy

A large numble of distributed energy resources (DER) are connected to DC distribution network which required high power supply reliability, and the extensive electronics equipment with the nonlinear characteristics of control system pose a challenge to the operating of DC distribution grid. For the construction and development of DC distribution system, this thesis presents the meshed power grid structure for medium voltage DC distribution network and a method to determine the voltage level sieries. Additionally, a new model calculating the power flow and the layered control strategy for multi-voltages grid are also proposed. The main works and innovations are as follows:The medium voltage DC distribution network with meshed topology was put forward to satisfy both the requirements of massive DER intergrating and high power supply reliability, besides the key technologies and feasibility of which have been analysed. The unbanlance degree index of voltage and line comprehensive utilization are used to evaluate the ability of meshed DC distribution networks for DER integrating, and validated by simulations.With the development of social and technology and the increasing requirement of DER, A method is proposed to evaluate the adaptability of DC voltage class series, then DC380 V is identified as the optimal voltage level for the low voltage DC distribution network. The presented voltage class sequence that 2n times of 380 V can be a guide for choosing the voltage grades of DC distribution network in future. Considering the adaptability of power supply sources and loads intergrating to the DC distribution system, as well as the insulation voltage of cables used in 10 kV,20 kV AC distribution network, DC12 kV or DC24 kV chosen from the proposed voltage class series could be satisfied for the medium voltage DC distribution network.The steady-state models of VSC and DC-DC converters and the power flow analysis and calculation models are established in the DC distribution network. Equivalent circuits are simplied to represent the DC-DC and VSC converters under different control modes and the buses nodes are reclassified. Taking the power losses of converters account, the improved Newton-Raphson algorithm is presented and verified to calculate the power flow of the DC distribution network.For the needs of massive intergrating DER and loads with multi-voltages, the layered control strategy is studied for the DC distribution network under different voltage levels and operating conditions. This paper put forward to the layered control strategy that the voltage droop control is adopted for the medium voltage DC distribution grid and taking the minimum power loss of whole network as the control objective, while the master-slave control is for the micro grid and taking the maximized voltage quality of whole network for its control objective.This results revealed the meshed structure of DC distribution network and the method for determing the voltage level sieries can satisfy the requirments for high power supply reliability and massive intergrating DER. In addition, the power flow algorithm and layered control strategy presented for DC distribution network with multi voltage grades could provide a reference for the planning of DC distribution network and the developing of control system equipment.