What Are Vertical Generators?

Vertical generators are synchronous electrical machines in which the rotating shaft is oriented along a vertical axis, designed primarily to be coupled to hydraulic turbines in hydroelectric power stations. Unlike horizontal-axis turbo-generators driven by steam or gas turbines, vertical generators are configured to match the natural orientation of water turbines, where water flows downward through a penstock to turn a runner mounted on a vertical shaft. The IEEE standard for these machines, IEEE Std 1095, covers installation procedures for synchronous generators and generator/motors rated 5 MVA and above coupled to hydraulic turbines or hydraulic pump/turbines with vertical shafts.

Vertical generators operate at relatively low rotational speeds compared to thermal-plant generators, which requires a salient-pole rotor construction with many pole pairs to achieve grid frequency. A machine delivering 60 Hz power at 120 revolutions per minute, for example, requires 25 pairs of magnetic poles. This low-speed, high-pole-count design is a defining characteristic that separates vertical hydro generators from the cylindrical-rotor machines found in steam turbine plants.

Mechanical Structure and Bearing Systems

The principal mechanical components of a vertical generator are the stator assembly, the rotor assembly, the thrust bearing, and one or two guide bearings supported by structural brackets. The stator consists of a frame, a laminated magnetic core, and three-phase windings; the rotor consists of a shaft, a spider frame, a rim of solid or laminated steel, and salient field poles wound with direct-current excitation coils. Because the shaft is vertical, the thrust bearing carries the full axial weight of the rotating assembly as well as any hydraulic downthrust transmitted up from the turbine runner. Three thrust bearing designs are commonly used: the adjustable-shoe type, the spring-supported type, and the self-equalizing type. Each provides a thin film of oil between stationary and rotating surfaces to limit friction under sustained axial loads that can reach thousands of tonnes in large units.

Electrical Design and Excitation

The stator winding is configured as a three-phase circuit assembled from single-turn or multiturn coils, or from half-turn Roebel bars in larger machines. Insulation systems must withstand the combined thermal, electrical, and mechanical stresses of continuous operation. Field excitation was historically supplied by shaft-mounted DC exciters, but modern installations use static excitation systems or brushless exciters fed by a small auxiliary alternator on the main shaft. The excitation system controls terminal voltage and reactive power output, and its dynamic response is critical to grid stability during load transients. Generator/motor configurations, in which the same machine can operate as either a generator consuming upstream water or as a pump-motor driving water back into a reservoir, are used in pumped-storage hydroelectric facilities to provide grid-scale energy storage.

Installation and Commissioning

Installation of a vertical generator proceeds in stages governed by the precise alignment requirements of a vertical rotating shaft. The foundation and sole plates are set first, followed by the lower bracket and guide bearing housing, the stator frame and core, the shaft and rotor spider, field poles, and finally the thrust bearing assembly. Dimensional tolerances are tight: shaft runout and air-gap uniformity between rotor poles and the stator bore are verified at each stage. Commissioning tests include insulation resistance measurements, high-potential tests on the stator winding, open-circuit and short-circuit characteristic tests, and vibration surveys as specified in USBR power plant maintenance standards.

Applications

Vertical generators have applications across a range of energy and grid infrastructure contexts, including:

• Conventional run-of-river and reservoir hydroelectric stations
• Pumped-storage facilities providing grid frequency regulation and peak shaving
• Tidal and marine current turbine installations using vertical-axis runners
• Small and micro-hydro systems in remote or island grids
• Reversible generator/motor units in combined generation and pumping schemes