A Second-Order Cone Programming (SOCP) Based Co-Optimization Approach for Integrated Transmission-Distribution Optimal Power Flow

The advancement of grid technology with high integration of distributed energy resources (DERs) has led to dynamic changes in the distribution network with a substantial impact on the transmission network. With this impact of DER integration, separate optimal power flow (OPF) analysis in transmission and distribution (TD) networks is not always adequate for the optimal operation of the power system. Hence, this paper proposes a coordinated TD-OPF co-optimization method with second-order cone programming (SOCP) to address this challenge. This co-optimization method provides a feasible solution for an integrated TD system, bridging the gap between transmission and distribution network challenges with renewable-based DERs through data exchange between the transmission and distribution networks. Moreover, each distribution network employs a distributed OPF (D-OPF) algorithm, which supports decentralized operation, thereby enhancing the reliability and scalability of the proposed OPF model for large TD networks. The goal is to reduce the overall system planning expenses, optimizing the microgrids in distribution networks and the major generation units within the transmission network. The efficacy of the proposed TD-OPF co-optimization approach has been verified through numerical analysis.

I. Introduction

Active distribution networks incorporate a large set of distributed generators (DGs) and renewable distributed energy resources (DERs) into the power distribution grid [1]. These active power sources make the distribution network complex, and the network needs more flexibility for stable network operation. The rising integration of DERs and DGs within their capacities necessitates coordinated operations between transmission and distribution networks. Traditionally, transmission and distribution networks have been managed and controlled independently. Although physically coupled, the transmission and distribution systems are managed by two distinct entities, the transmission system operator (TSO) and the distribution system operator (DSO), with minimal collaboration. As a result, there is minimal exchange of information regarding system conditions and control strategies between the TSO and DSO. Furthermore, the dynamic nature of the loads and the significant integration of DERs can lead to changes or even reversals in energy flow in specific feeders, posing challenges to traditional power system operations [2]. This integration of DERs could also complicate the utilization of flexible resources across the dual-layered grids of both networks. Due to these challenges, optimal coordination of transmission and distribution grids with DERs can provide a better and optimal point of operation for both networks [3]. However, there are significant challenges concerning the transmission-distribution network co-optimization for optimal power flow analysis. •

First, the existing AC-OPF model does not allow the decoupling of active and reactive power flows in active distribution grids. Disregarding the reactive power flow may result in errors in the OPF analysis.

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DERs integration is restricted by reverse power flow issues in the active distribution network. Large power injections into the distribution grids result in over-voltage [4]. So, OPF analysis is challenging within the network operation limits.

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Considering both the transmission and distribution grid, the scale of the problem is very extensive for the existing models for OPF analysis.