Double-time-scale distributed voltage control for unbalanced distribution networks based on MPC and ADMM

Abstract

With the increasing penetration of distributed generators (DGs) in the distribution system, the distributed coordinated optimal control of DG and voltage control devices has become imperative. This paper proposes a double-time-scale distributed voltage control scheme for unbalanced distribution networks with distributed generators based on model predictive control (MPC) and alternating direction method of multiplier (ADMM) to regulate the voltage profile across a network. To better coordinate the economical operation and voltage regulation, the voltage control devices with different temporal characteristics are separately adjusted. In the fast-time-scale control, the decision variables are the active and reactive power outputs of DGs. In the slow-time-scale control, the on-load tap changer, step voltage regulators and capacitor banks are optimized. A tailored solution method based on McCormick envelopes and branch-and-bound algorithm is developed to efficiently solve the optimization problems by transforming the mixed-integer nonlinear program (MINLP) into a second order cone program (SOCP). The voltage control scheme is a combinatorial optimization problem whose computational complexity grows exponentially with the number of binary variables. To resolve this issue, a distributed optimal voltage control strategy is developed based on the alternating direction method of multipliers (ADMM). Through the ADMM, the optimization problem is distributed among the zones of the distribution networks, without requiring a central controller. The modified IEEE 123-node system is used to verify the effectiveness of the proposed control scheme as a test system.