Optimization-based Control of Variable Speed Diesel Generators

Constant-speed fuel-fired prime movers driving synchronous generators are ubiquitous in electric power generation systems. When coupled to a synchronous generator, the electrical system frequency is a function of generator RPM, and the prime mover must operate at a fixed speed to maintain rated electrical frequency. However, hybrid AC-DC architectures that rectify AC generator output to DC can take advantage of variable speed operation by using power conversion to maintain a stable DC bus with a variable frequency AC bus. With the engine speed constraint removed, digital engine control systems can manage the prime mover RPM as a function of power demand. Diesel engines are commonly used as prime movers due to high reliability, availability, and maintainability. Diesel engines have a fuel consumption curve that is characterized by lower fuel efficiency at low loads, typically less than 50% of rated power. Fuel efficiency is also a function of engine speed. By allowing variable speed operation, an optimization problem of generator speed as a function of power demand with the objective of minimal fuel consumption can be solved real-time to generate a speed schedule for minimum fuel. A shipboard electric plant control system with an optimization-based control can operate the AC generator's prime mover across its entire RPM operating range, achieving improved fuel efficiency while maintaining electrical stability at the DC bus. Such a system could be retrofitted to existing ships already using AC-DC architectures to realize cost savings from reduced fuel consumption.