Design and Development of a Two-megawatt, High Speed Permanent Magnet Alternator for Shipboard Application

Conventional gas turbine generator sets consist of a high speed turbine coupled to a low speed alternator through a speed reduction gearbox. This is required to maintain the alternator output frequency at 50 Hz or 60Hz, as output frequency is directly proportional to speed. Since power is also directly proportional to speed, the conventional system is bulky and possesses a very large footprint. The advent of solid-state inverters with their unique ability to efficiently and cost effectively change the alternator output frequency has made it possible to eliminate the need to link the alternator speed to the required 50/60 Hz output frequency. This output can be produced with a high-speed alternator, eliminating the need for a gearbox and greatly reducing the size, complexity, and weight of the machine by trading speed for torque. A direct drive system in which an alternator is coupled directly to a gas turbine is much more compact and highly efficient and requires much less maintenance. In this paper we will review the design and development of a high-speed permanent magnet alternator in an advanced cycle gas turbine system for shipboard applications. In addition to the alternator’s design features, we will discuss design considerations including electromagnetic design, thermal design and structural design of high speed electrical machines, and review the alternator development including risk mitigation. INTRODUCTION The British Royal Navy has embarked on a development program to evaluate advanced cycle low power gas turbine alternator (ACL-GTA) technology as an alternative to the diesel generation (DG) system for shipboard applications. The ACL-GTA system is being considered to provide propulsion, ship’s service power at sea and in harbor, and independent emergency power generation. Target platforms include future surface combatants and future carriers. ACL-GTA technology promises numerous advantages compared to DG technology including smaller foot print, environmentally compliant emissions, low maintenance, and therefore reduced manning levels. The goal of this development program is to demonstrate the ACL-GTA system as a “diesel beating” solution, such that the ACL-GTA system should be able to compete in performance and cost with DG systems while providing the added benefits mentioned above. The ACL-GTA system shown in Figure 1 is an integrated unit with all major components mounted on a common skid. The skid is spring mounted to reduce the shock input to the system. The major components include the gas turbine, the heat recuperator, the high speed alternator (HSA), and the electronic unit (EU) for power conditioning. Figure 1 – ACL-GTA Configuration The HSA couples directly to the gas turbine and operates at the same speed as the gas turbine. The use of a high-speed alternator instead of a conventional low speed synchronous machine operated through a speed reduction gearbox is one of the unique features of this ACL-GTA system. By trading speed for torque, the direct-drive turbo alternator technology provides a much more compact, highly efficient and low maintenance solution for the ACL-GTA system. This direct drive approach is further enhanced by the advent of low cost, high power semiconductor switching devices. This paper will review the design features of the two-megawatt, permanent magnet HSA, discuss design considerations including electromagnetic design, thermal design and structural design and review the HSA development including risk mitigation. DESCRIPTION AND UNIQUE DESIGN FEATURES The HSA, shown in Figure 2, is a permanent magnet synchronous machine with surface mount magnets on the rotor. It couples directly to the gas turbine through a flexible mechanical coupling. The HSA functions as a motor to provide starting torque to the gas turbine and as a generator producing electrical power as the gas turbine operates at higher running speeds. In generation mode, the HSA is designed to provide electrical output power compatible with mechanical power supplied from the gas turbine at various speeds and ambient conditions up to rated power of 2MW. Table 1 summarizes the requirements for the HSA. Figure 2 – HSA Configuration The HSA is a four-pole, three-phase machine. An integral cooling fan mounted on the main rotor shaft provides cooling air for the rotor, and a liquid cooling loop in the housing cools the stator. The following describes the machine’s unique design features. Table 1 – HSA Requirements Max Output Power 2030 kW Speed Range 19krpm to 22.5 krpm