GENERATOR PKG ERECTION PART-I, GENERAL FEATURES OF GENERATOR WITH PHOTOES - ABC OF THERMAL POWER PLANT.

PART -I

Generator with exciter is the ultimate  output of all the system put together. This is very important item to be installed with turbine system. Generator is the heaviest item to be installed in the power house in single lift. In earlier days generators ( up to 300 MW) class were installed by two EOT crane by using common hook  and tandem operation. But 500 MW onwards this practice was discarded because the structural design and cost required to be done is much higher than the conventional one. Therefore a method was involved to lift the generator which has no bearing with the structural load bearing capacity. Use of Portal crane and more popular is strand jack method gained popularity for lifting stator . We will discuss the same with photographs on later dates. A brief description of 500 MW generator  is given below for new comers.

The two-pole generator uses direct water cooling for the stator winding, phase connectors and bushings and direct hydrogen cooling for the rotor winding. The generator frame is pressure-resistant and gas tight and equipped with one stator end shield on each side. The hydrogen coolers are arranged vertically inside the turbine end stator end shield.

The generator consists of the following components:

• Stator

Stator frame ,End shields ,Stator core ,Stator winding and Hydrogen coolers.

• Rotor

Rotor shaft , Rotor winding, Rotor retaining rings and Field connections

• Bearings

Shaft seals

The following additional auxiliaries are required for Generator operation :

• Oil system , Gas system , Primary water system and  Excitation system.

Cooling System.

-The heat losses arising in the generator interior are dissipated to the secondary coolant (raw water,

condensate etc.) through hydrogen and primary water.

-The hydrogen is circulated in the generator interior in a closed circuit by one multi-stage axial-flow fan arranged  on the rotor at the turbine end. Hot gas is drawn by the fan from the air gap and delivered to the coolers, where it is re-cooled and then divided into three flow paths after each cooler.

-For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at the turbine and exciter ends. The rotor winding is symmetrical relative to the generator centre line and pole axis. Each coil quarter is divided into two cooling zones.

For cooling of the stator core, cold gas is admitted to the individual frame compartments via separate cooling gas ducts. From these frame compartments the gas then flows into the air gap through slots in the core where it absorbs the heat from the core.

The treated water used for cooling of the stator winding phase connectors and bushings is designated as primary water in order to distinguish it from the secondary coolant (raw water, condensate, etc.). The primary water is circulated in a closed circuit and dissipates water is circulated in a closed circuit and dissipates the absorbed heat to the secondary cooling water in the primary water cooler. The pump is supplied with hot primary water from the primary water tank and delivers the water to the generator via the coolers. The cooled water flow is divided into two flow paths .

Stator Frame

 The stator frame consists of a cylindrical centre section and two end shields which are gas-tight and pressure-resistant. The stator end shields are joined and sealed to the stator frame with an O-ring and bolted flange connection. The stator frame accommodates the electricity active parts of the stator, i.e., the stator core and the stator windings.

The stator core is stacked from insulated electrical sheet-steel laminations and mounted in supporting rings over insulated dovetailed guide bars. Axial compression of the stator core is obtained by clamping fingers, pressure plates, and non-magnetic through-type clamping bolts, which are insulated from the core.

The rotor shaft is a single-piece solid forging manufactured from a vacuum casting. Slots for insertion of the field winding are milled into the rotor body. The longitudinal slots poles are obtained. The rotor poles are designed with transverse slots to reduce twice system frequency rotor vibrations caused by deflections in the direction of the pole and neutral axis.

The hydrogen cooler is a shell and tube type heat exchanger which cools the hydrogen gas in the generator. The heat removed from the hydrogen is dissipated through the cooling water. The cooling water flows through the tubes, while the hydrogen is passed around the finned tubes.

The sleeve bearings are provided with hydraulic shaft lift oil during start-up and turning gear operation. To eliminate shaft currents, all bearings are insulated from the stator and base plate, respectively. The temperature of the bearings is monitored with thermocouples embedded in the lower bearing sleeve so that the measuring points are located directly below the BABBITT. Measurement and any required recording of the temperatures are performed in conjunction with the turbine supervision. The bearings have provisions for fitting vibration pickups to monitor bearing vibrations.

Typical 500MW Generator Specification

KW 500,000 ; KVA 588,000 Hz 50 ; R.P.M 3000 ; P.F. 0.85 LAG

Stator  : Volts 21000 ;  Amps 16200 ;  Phase 3; Conn. Y Y ;  Spec. IEC-34

Rotor  :  Amps 4040 ; Volts 340  : Coolant Hydrogen & Water; Gas Pressure 3.5 Kg/cm2 (g)

         Typical 500MW Generator cross section

 Typical 500MW Generator outline drawing and 660 MW  generator. Difference of construction

will be outlined later.

courtesy  BHEL

Continued to PART -II

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