The Control of Parallel-Connected Inverters for U.S. Navy Shipboard Applications

Abstract

Concepts such as the Integrated Power System (IPS) and the DC Zonal Electric Distribution System (DC ZEDS) have motivated the study of power inverter control and paralleling strategies by the U.S. Navy. For example, in DC ZEDS a Ship Service Inverter Module (SSIM) converts DC power into AC power for use within a zone. For reasons of redundancy and logistics, multiple paralleled inverter submodules are used to implement the SSIM. The current effort supports ongoing SSIM hardware development by the SatCon Corporation. This report documents the modeling and control analysis of a set of parallel-connected PWM inverters. Although initially focusing on a static three-phase R-L load, the analysis is extended to three-phase symmetrical induction machine loads and three-phase round- rotor permanent-magnet synchronous machine loads. In each case, an algorithm for designing the local inner current control is developed by employing reference frame theory, replacing the paralleled inverters by an equivalent unit, and executing a nonlinear pole-placement strategy. Upon reviewing the equations necessary for AC machine vector control, the report concludes by documenting the analysis and design of a simple speed control loop for this aformentioned machine loads.