A Decentralized Optimal Operation of AC/DC Hybrid Distribution Grids

Abstract

In an AC/DC hybrid distribution grid (AC/DC HDG), different AC distribution grids are connected through a voltage source converter (VSC) based DC grid to operate the entire system in a secure and economic manner. This paper presents a decentralized optimal operation approach for running AC/DC HDG which can either work individually or collaborate with each other. Using the analytical target cascading (ATC) approach, a three-level hierarchy consisting of DC grids, VSC stations and AC grids, is formulated to decompose the operation problem of AC/DC HDG. The obtained subproblems for these grid levels, as well as the interactions between them, are modeled and solved with a nested solution process.

Existing System

Evolutionary Algorithm, Optimal power flow algorithm.

Proposed System

AC/DC HDG is a closed-loop network which is composed of interconnected AC and DC distribution grids. As an interface between AC and DC distribution grids, VSC station allows fast and reversible control of active and reactive power and provides flexibilities on the coordination of AC/DC HDG. In this paper, a decentralized optimal operation approach based on ATC frame was proposed to optimize the operation of AC/DC HDG. A three-level hierarchy was presented and the subproblems for the AC distribution grid, the DC distribution grid and the VSC station were formulated respectively. A hierarchical algorithm including the outer loop and inner loop was applied to solve the decentralized optimization problem and coordinate the operating points of the interconnected systems. The contributions of this paper include: Taking advantage of the structure features of AC/DC HDG, this paper develops a three-level hierarchy including the AC and DC distribution grids, and the VSC stations to decompose the original AIO optimization problem into several subproblems. Using this hierarchy, the optimization subproblems of AC and DC distribution grids, which are relatively complicated and time consuming, can be solved in a parallel manner, whereas the subproblem of the VSC station, which is simple, can be formulated as a coordinator. Different from most existing methods that take voltage magnitudes and angles as coupling (shared) variables, this paper adopts the active and reactive power as the shared variables, which renders the advantages, including It is more suitable for the operation of AC/DC HDG as the VSC station takes active and reactive power as its control variables. Therefore, the optimized results can be utilized directly as the references for the VSC station level control; and It is easier to address the connections between AC and DC grids, because the coupling interface can be simply regarded as a pseudo generator or load for each distribution grid, and different AC distribution grids do not need to coordinate their swing bus angles.

Architecture

BLOCK DIAGRAM