Unit Commitment with AC Power Flow Constraints for a Hybrid Transmission Grid

Abstract

A unit commitment formulation satisfying AC optimal power flow constraints models the resource optimization problem accurately, but is challenging to solve for traditional transmission systems. Integration of generation based on renewable energy sources is often limited by transmission congestion issues in existing grids. For such grids, AC to HVDC conversion schemes are viable and attractive options for capacity expansion. In this respect, we utilize the structural properties of a hybrid AC/HVDC grid architecture with a specific topology to enable an exact mixed integer conic relaxation of the aforementioned problem. The simulation results for the PJM 5-bus system show that the relaxation is exact for the hybrid architecture. Further, it alleviates the network congestion leading to a substantial reduction in generation cost at normal and increased load conditions compared to the reference AC grid. Moreover, the hybrid architecture improves the utilization of the grid to accommodate more demand and generation.

Minimum Frequency and Voltage Stability Constrained Unit Commitment for AC/DC Transmission Systems

“…2) Constant voltage (CV). The whole HVDC is modeled by Equations (1)- (7). The converter Equations (1) and (2) express V dR (V dI ) in terms of V m (V n ), k R (k I ), α (γ), and other converter variables while the coupling transformers are assumed to be lossless.…”

“…Equation (3) represents the DC voltage-current relationship which depends on the DC transmission system configuration (i.e., V dR , V dI , and I d ). After getting the active power of both rectifier and inverter by Equations (4) and (5), the exchange reactive power between AC and DC system could be calculated through Equations (6) and (7) by considering reactive compensation capacity Q CI and Q CR .…”

“…Another method is establishing the detail model of the HVDC during operation optimization. For example, authors in [7] introduced comprehensive representation of HVDC transmission in operation optimization of the AC/DC hybrid system while increasing the complexity of the model. To solve such problem efficiently, linear power flow equations were used to model the AC and DC transmission grids so as to decrease the complexity of the whole model in [8].…”

Robust optimal operation of future power systems under uncertainties

“…Typically, the former involves mixed-integer variables, and the latter involves nonlinear constraints. As the practical power systems are generally larger in scale, the combined problem has computational challenges due to the problem structure and the larger dimensionality [5][6][7].…”

“…On this basis, an HTG architecture comprising an AC subgrid with multiple multi-terminal DC subgrids was recently proposed in [1], Chapter 1. Introduction which serves as an upgrade strategy for traditional AC transmission grids to enhance its economic utilization and loadability [1,6,7,12]. However, integrating AC and DC systems further demands new modeling methodologies and optimization approaches which are capable of accurately representing their electrical behavior while maintaining the computational tractability.…”