Large capacity turbo-generators in nuclear Power Plants technologies and operational experience

Rotating electrical machines, such as synchronous generators and induction motors, are critical assets to any generating station. In Nuclear Power Plants (NPP) apart from large capacity turbo generators to generate electrical power, induction motors are used as prime mover to drive various auxiliary systems. These prime movers are required to perform their intended function both during normal operating conditions and during or even after, a postulated accident condition. Though redundancy and fail safe design is followed in electrical power supply distribution system within in the power plant, tripping of the generating station on electrical equipment failure is undesirable and damage to the rotating machine is far worse. The economic loss for the station due to such an incident is not only the cost of repair or replacement of the damaged machine and huge revenue loss of lost generation, but it is also a challenge to the safety systems of the nuclear power plant. In this context, failure of a non-redundant, large capacity turbo-generator is critical even to the connected power grid.

The stator windings of the synchronous generator are connected in star with neutral grounded through external impedance, to limit the stresses and damage due to ground fault within the machine. The stator core of the machine is made up of a large number of thin steel laminations, insulated on both sides to limit the losses. During operation of machine, in addition to the normal radial exciting flux from the rotor field winding, there are end region axial flux at the core ends due to end winding of stator and rotor. Additionally, there is an interaction between rotor and stator magnetic fields. During lagging power factor these fluxes tend to oppose and during leading zone of operation, generally encountered during reduced demand accompanied by increased grid voltage, tend to sum up creating higher flux, losses and core temperature. To compensate the increased heating, additional slotting, slitting and thinner core packets are generally employed at the core-ends of the generator. Depending on the machine rating and design, losses in the stator core and windings are removed by either by air or hydrogen or by a combination hydrogen and water. For any machine, inadequate removal of losses results in rapid deterioration of insulation system, leading to fault in the affected winding and or core. A fault in stator core is more serious because apart from its own damage, it has potential to damage the winding before its detection & isolation and it takes long time to repair to bring back the station into operation. Also due to the stored energy in mechanical and magnetic field, machine continue to feed on stator or rotor fault, and fault current does not stop even after the machine is isolated from the grid and field breaker is opened. The paper also covers brief description of the types of turbo generator installed along with its state of art monitoring and operating limits. It also discusses operating experience of the machines and prevailing grid condition. A case study of one of the turbo generators in nuclear power plant suffering major core damage and its repair is also highlighted.