Experiences of air cooled generator stator flexible

Reduction of forced outage is one of the major steps towards reliability enhancement of power utilities. Generator failures attracts special attention in reduction of forced outage as the time required for repair is considerably high. So proper prevention of failures is the remedy. This paper illustrates our experiences with stator flexible failures and remedial measures for failure prevention in air cooled generators.

The utility under mention is having a number of combined cycle power plants, having air-cooled generators of various makes. Copper flexibles are provided in Generators for inter-connecting bus bar and bushings. The copper flexibles are number of thin copper laminations electrical resistance welded at ends. These flexible connections take care of the thermal expansions in the machine, dampen the electromechanical vibrations from stator and carry the required electrical current from stator to the bushing. This way flexibles act as a link between two stationary parts of generator - Generator stator overhang winding and Bushing. Failure of flexibles due to high vibration was experienced by the utility. The flexible vibration is due to the mechanical force created by rotor rotation and the electromagnetic forces. Cracking of laminations can lead to entrapment of broken copper pieces in the air gap and winding space which has the potential of causing serious damage to the generator. Experience of machine manufactures had established that resonating conditions due to matching of natural frequency of flexible assembly with operating frequency causes high flexible vibration and flexible failures.

This paper highlights the need for properly designed flexible connections, forcing unit shutdowns at high economic cost. This feedback also highlights need for checking natural frequency of the assembly at site after machine erection so as to prevent excessive vibrations, which can cause serious equipment damage. The possibility also needs to be examined for type testing along with flexible assembly at works so as to minimize potential hazards on account of failures at site after commissioning of the unit. Taking proper preventive measures like natural frequency measurements and flexible vibration measurements shall eliminate these failures. This will serve as a useful tool for utilities in reduction of forced outage in air-cooled generators.