A Presentation On Summer Training At Bharat Heavy Electricals Limited, Haridwar

CONSTRUCTION OF TURBO GENERATOR

INTRODUCTION  A turbo generator is a turbine directly connected to electric generator for the generation of electricity. They are mostly used as large capacity generator driven by steam/gas turbine.

PRINCIPLE OF OPERATION  In case of turbo generator, Rotor winding is supplied with DC current (through slip rings or brushless exciter) which produces constant magnetic field.  3 phase stator winding is laid in stator core.  When generator rotor is rotated (by a turbine) magnetic flux produced by rotor winding also rotates.  Voltage is induced in stator winding according to Faraday’s law.  3 phase stator winding also produces magnetic flux revolving at synchronous speed ((=120*f/p ). Rotor also rotates at synchronous speed. Both the magnetic fields are locked and rotate together.

GENERATOR MODULES

COMPONENTS USED IN TURBO GENERATOR 1. STATOR  Stator frame  Stator core  Stator winding  End cover  Bushings  Generator terminal box

Stator frame  Rigid fabricated cylindrical frame and is the heaviest section in the generator  Withstands weight of core & winding, forces & torques during operation  Provisions for H2/CO2 filling  Provision for temperature measurements  Foot plates for supporting on foundation  Provision for H2 coolers

STATOR FRAME

STATOR CORE PURPOSE:-  Support the stator winding  To carry the magnetic flux generated by rotor winding. The stator core is made from the insulated electrical sheet lamination to minimize eddy current losses. Each lamination layer is made of individual sections.

Stator Winding  The stator winding consist of two layers made of individual bars. To minimize losses, bars are composed of separately insulated strands which are transposed by 360°.  To minimize stray losses in end winding, strands of top and bottom bars are separately brazed.

Bushings  A Generator bushing can be the weak link in the electricity supply chain. It is generally anticipated to be maintenance free but several clues prior to failure must be watched.  The bushings are bolted to the stator frame at the exciter end.

ROTOR  Rotating part of turbo generator  A high strength alloy steel single forging prepared by vacuum cast steel.  Longitudinal slots for housing field winding  Damper winding is provided which safeguards the asymmetrical and asynchronous operative conditions.  Rotor of cylindrical type used in turbo generator.  Supported on two journal bearings.  Provision of axial fan for forced ventilation.

MAIN PARTS OF ROTOR  Rotor Body  Damper Bar  Silver Plated Rotor End Wedge  Rotor Winding  Retaining Ring  Silver Plated Shrink Seat  Transverse Slots  Rotor Wedge

ROTOR WINDING  Rotor coils are made of silver- bearing strap copper for greater creep resistance.  The coil is accurately formed to precisely fit in the rotor slot with minimum distortion.  Ventilating slots are punched in the straight portion of the coils to allow for radial discharge of cooling air

Rotor Slots  Slots for field winding are step milled over the entire length of the rotor body.  Step milling minimizes stress level at the root of the teeth.  Grooves for slot wedges are accurately machined into the slot walls.

Retaining Rings  Retaining rings support the rotor coil end turns against the centrifugal forces encountered during high-speed operation.  They are made from forgings that meet rigid specifications and are resistant to stress corrosion cracking.  Circular steel plates are shrunk into the outer end of the retaining ring to add section and to reduce distortion.

Rotor Fans  Steel axial flow fans are fabricated, tested and balanced. The fans are keyed and shrunk on the rotor body outboard of the retainer rings.

Conductor Construction Steps 1. Insulation check 2. Conductor cutting 3. Transposition 4. Cross over insulation 5. Stack Pressing 6. Forming 7. End Conductor cutting 8. Pickling Process 9. Contact sleeve mounting 10. Coil lug brazing 11. Testing 12. Reforming 13. Insulation 14. Finishing

Brushless Excitation The main parts of brushless excitation system are as follows:  Pilot exciter  Main exciter  Rectifier wheel  Automatic voltage regulator

AIR COOLED TURBO GENERATOR  In Air Cooled Turbo generator stator winding is indirectly air cooled whereas the rotor winding and stator core is directly air cooled.  This type of cooling is applicable for rating of 30 MW- 60 MW generators.  In this type of turbo generator there are vertically sides mounted cooler in a separate housing.

TESTING OF TURBO GENERATOR  SHORT CIRCUIT TEST  OPEN CIRCUIT TEST  INTER STRAND TEST  HIGH VOLTAGE TEST ON ROTOR AND STATOR WINDING  HELIUM TEST