Comparative Performance Analysis of the DC-AC Converter Control System Based on Linear Robust or Nonlinear PCH Controllers and Reinforcement Learning Agent

Abstract

Starting from the general topology and the main elements that connect a microgrid represented by a DC power source to the main grid, this article presents the performance of the control system of a DC-AC converter. The main elements of this topology are the voltage source inverter represented by a DC-AC converter and the network filters. The active Insulated Gate Bipolar Transistor (IGBT) or Metal–Oxide–Semiconductor Field-Effect Transistor (MOSFET) elements of the DC-AC converter are controlled by robust linear or nonlinear Port Controlled Hamiltonian (PCH) controllers. The outputs of these controllers are modulation indices which are inputs to a Pulse-Width Modulation (PWM) system that provides the switching signals for the active elements of the DC-AC converter. The purpose of the DC-AC converter control system is to maintain ud and uq voltages to the prescribed reference values where there is a variation of the three-phase load, which may be of balanced/unbalanced or nonlinear type. The controllers are classic PI, robust or nonlinear PCH, and their performance is improved by the use of a properly trained Reinforcement Learning-Twin Delayed Deep Deterministic Policy Gradient (RL-TD3) agent. The performance of the DC-AC converter control systems is compared using performance indices such as steady-state error, error ripple and Total Harmonic Distortion (THD) current value. Numerical simulations are performed in Matlab/Simulink and conclude the superior performance of the nonlinear PCH controller and the improvement of the performance of each controller presented by using an RL-TD3 agent, which provides correction signals to improve the performance of the DC-AC converter control systems when it is properly trained.

Distributed Control Algorithm for DC Microgrid Using Higher-Order Multi-Agent System

“…Furthermore, when loads in the system are supplied with DC power, conversion losses from sources to loads are lower than in AC microgrids. Inverters are required for DC output type sources, gas engine cogeneration, and wind turbines to convert DC to AC power to match output voltages and frequencies to those of utility grids [10]. In addition to reducing losses in AC/DC conversions, DC microgrids provide continuous high-quality electricity when the voltage of the utility grid collapses or blackouts strike.…”

“…By distributing the control functions across the microgrid's components, the system can respond quickly to changes in energy supply and demand, improve energy efficiency, and reduce energy losses. In the literature, different control strategies are proposed for microgrids [7,9,10,13,14,16,18]. These control strategies include control of power system elements and quantities, such as different sorts of power converters control and current, voltage, and frequency control.…”