Hydrogenerators - Features

Special design characteristics

Winding design
The stator winding is lap or wave type depending on the optimum connection configuration. Each bar is fabricated from a series of drawn and annealed strands of electrolytic copper, Roebel transposed in the slot portion to eliminate losses due to circulation currents. The ground insulation is formed of glass fabric and continuous mica sheet pre-impregnated with epoxy resin filler (resin-rich system). Corona discharges in the slot are eliminated by tape treated with conducting varnish glued over the ground insulation.

Overhang corona protection is provided by glass fabric tape, epoxy resin and silicon carbide, joined to the corona slot protection system by means of a glass fibre tape, silicon resin and graphitic powder. The Ansaldo Energia “System 4675™” insulation system was introduced in 1969 used up to 20 kV, with no failure during operation.

Mechanical design
Design solutions for high speed machines and high capacity medium speed generators are highly dependent on stress and areas of maximum stress concentration, making high quality materials an essential. The material used to manufacture the rotor rim of the 310 MVA San Giacomo generator, for example, has a yield point
of 650 N/mm2, with 600 N/mm2 yield point material used for the rotor pole. The rotor is made up of two stub shafts, a welded spider and a laminated rotor rim. As it is a large capacity medium speed unit, construction features typical of both low and high speed machines are incorporated.

A typical feature of high speed machines is the trapezoidal pole body, which has inclined longitudinal sides to allow the corresponding coil bar centre-line to be positioned at a right angle to the rotor radius. This configuration is rarely adopted for reasons of cost but may be the only option open for designers to solve problems caused by centrifugal forces acting on the rotor winding.


While Ansaldo adopted the solution for the first time on a 16 pole machine rotating at 375 rpm, it is usually applied to machines with rated speeds higher than 400 rpm. Clearly the relative costs and benefits need to be weighed carefully. While the San Giacomo generator rotates at only 300 rpm, trapezoidal poles were adopted to achieve optimum behaviour for all machine components.
The basic reason for using this approach is the very long rotor core and problems encountered with self-supporting brackets due to overheating in the central part of the rotor. This is because the brackets act as an obstacle to the axial ventilation system air flow.
Overheating of the rotor copper caused by limited thermal exchange through the insulated part of the bracket is also a key factor. However, as the function of the brackets is to support the tangential component of the centrifugal force acting on the rotor winding, they can be eliminated by adopting trapezoidal poles.
Using the trapezoidal solution, the centrifugal forces only stress the rotor winding radially, which is beneficial from both thermal and mechanical standpoints. This means the dimensions of the pole and the pole tip in particular can be reduced. The rotor rim weight can also be reduced significantly, which is extremely important in terms of critical speed.

Rotormorphology
In low speed hydrogenerators the rotor comprises the poles, the laminated rotor rim, the welded spider and the shaft. Both through shaft and stub shaft solutions are available. The most frequent configuration for high speed generators and generator-motors involves forged rings with cast hub and stub shafts. Recently, when stresses allow, the trend has been towards laminated rotor rims in high speed machines too.

Ventilation system
All hydrogenerators and generator-motors manufactured by Ansaldo Energia have indirect air cooling. Both axial and radial self-ventilating configurations are adopted. While radial ventilation is an option for generator-motors,
cooling by electric blowers is advisable.
Rim ventilation systems offer a more typical design solution for low speed machines but are not exclusive to them.
The solution adopted for the San Giacomo plant on the Vomano river (the most powerful hydrogenerator in service
in Italy) is an unusual mixed configuration for a machine with such a long axial core.
Two axial fans mounted on the rotor initiate an axial airflow which combines with an additional flow from the rim and the spider.
Optimum cooling is achieved using a duct in the central part of the rim and holes in the spider disks. This reduces aerodynamic resistance in the circuit by opening a route parallel to the space between the poles. The idea is to keep the temperature as uniform as possible along the rotor and stator and avoid any possible hot spots in the central part of the rotor winding.