BY

K.PRABHU

12BEEE1039

STUDY OF TURBO GENEATORS

AT

BHEL, HYDERABAD

INTRODUCTION ABOUT BHEL BHEL, Hyderabad unit manufactures Turbo

Generators of rating up to 200 MW for industrial applications and for power generation in Steam Power Plants.

Turbo Generators manufactured here range from 4 MW to 200 MW

RATINGS OF TURBO GENERATOR Rated output:200MW

Rated kva capacity:247KVA

Rated voltage:15.75kv

Rated current:9050A

Rated speed:3000rpm

Rated frequency:50hz

efficiency:98.5%

INTRODUCTION TO TURBO GENERATORS A turbo generator is a turbine directly connected to an

electric generator for the generation of electric power.

OPERATING PRINCIPLE:” FARADY’S LAW OF ELECTRO MAGNETIC INDUCTION

whenever conductor cuts magnetic flux, dynamically induced EMF is produced. This EMF causes a current flow if the circuit is closed

e = -N d/dt

The general components of a turbo generator are Stator- Stator Frame

- Stator Core- Stator Windings

Rotor-

Rotor Shaft

- Rotor Windings

Cooling System

STATOR FRAME

STATOR FRAMEThe stator frame is of welded steel single piece construction.

It supports the laminated core and winding.

Guide bars are welded or bolted inside the stator frame over which the core is assembled

STATOR CORE Stator core is made up of silicon steel sheets which are

suspended in the frame from guide bars Purpose of stator core: 1.to support the stator winding To carry the magnetic flux generated by rotor winding Losses in core: 1.hysteresis 2.eddy current In order to reduce hysteresis losses steel alloy is used for

the building up of core In order to reduce eddy currents thin laminations(0.5mm)

are used.

STATOR CORE

PREPARATION OF LAMINATIONS COLD ROLLED NON GRAINED ORIENTD (CRNGO)

Steel sheets are used

PRESS TOOLS are used in manfacturing of laminations

1.compounding tools

2.Blanking and slot notching tools

Compounding tools: in this method stamping with all the core bolt holes and winding slots in single operation

Used for the machines rated above 40 MW

BLANKNG AND NOTCHING OPERATION:

Core bolt holes are made using blanking tools

Winding slots are made using notching tools

Used for smaller rated machines

500 tons of crank press is used for this purpose

VARNISHING: After this process laminations should be varnished for the insulation

Varnishing thickness should be 12-18mm

Sheets are dries at a temperature of 260-350 deg centigrade.

STATOR COILS AND WINDINGstator coil is made up of Copper bars or conductorsHigh voltage insulation(resin poor mica tape) is provided

over the barsWinding:The bars are located in slots of rectangular cross section which are uniformly distributed on the circumference of the stator core

To minimize the stator losses in the winding, the strands of the top and bottom bars are separately brazed and insulated from each other

It is three phase layer lap winding

STATOR WINDING

RESISTANCE TEMPERATURE DETECTORS The temperature measurements on the generator are

made with RTDs They are placed at various sections of the core and winding

The RTD works on the principle of the change in electrical resistance of a conductor due to temperature

R=R0(1+Kt)

R0=resistance refer to room temperature

K=coefficient of resistance

T=temperature difference

INSULATION OF STATOR High voltage insulation(resin poor mica tape) is

provided over the bars

VACCUM PRESSURE IMPREGNATION SYSTEM(VPI)

1.stator coils are taped with poor resin mica type before inserting in the slots of cage stator subsequently wounded stator is subjected to vpi process in which first the stator is vacuum dries and then impregnated in resin bath under the pressure of nitrogen gas

Process:

1.stator is initially heated minimum for one hour to remove moisture

Heated stator shall be inserted in heated impregnating vessel. stator shall be dried under vaccum (should not exceed 50hrs.

Stator is to be flooded with impregnation resin mixture so that highest point of winding lies at least 100mm below resin level pressure is applied so that resin penetrates in the tape to fill all the gaps

Vacuum Pressure Impregnation Plant

ROTOR Solid rotors are manufactured from forged alloy steel

with suitable alloying elements to achieve very high mechanical and superior magnetic properties

Rotor consists of electrically active portion and two shaft ends

60% of rotor body circumference has longitudinal slots which hold the field winding

Rotor Winding: The field winding consists of several series connected coils inserted into the longitudinal slots of the rotor body. The coils are wound so those two poles(one north pole and south pole) are obtained

COILS MATERIAL: made up of copper with silver content of approximately 0.1% .

Silver alloyed copper have a high strength properties at high temperature so that coil deformation due to thermal stresses are eliminated.

INSULATION: The coils are insulated from the rotor body with L-shaped strips of glass fibre

ROTOR SLOT WEDGES:To protect the winding against the effects of centrifugal forces, the winding is secured in the slots with wedges. The slot wedges are made of copper alloy featuring high strength and good electrical conductivity.

ROTOR

VENTILATION AND COOLING Losses in the generator raise the temperature of the

generator .due to temperature the insulation will be effected .

Various methods of cooling:

1.air cooling

2.hydrogen cooling

3.hydrogen and water cooling

Closed circuit hydrogen cooling mounted axially on stator frame

For the rotor two axial fans are located at each shaft end.

Draws the cool air from the atmosphere

Flowpath1:to the rotor winding

Flow path 2:to the stator winding

TESTING MECHANICAL RUN AND MEASUREMENT OF

VIBRATIONS AT RATED SPEED: machine should be run at rated speed and the vibrations are measured in all directions.

Vibrations should be less than 5 microns and noise level should be in b/w 75-90db

SHORT CIRCUIT TEST: in this machine should run at rated speed. output terminals of stator are short circuited and excitation gradually increases up to the rated current in the winding. winding temperature should be less than 60deg centigrade. we will get copper loss

Open circuit test: machine should be run at rated speed and excitation increases gradually up to the rated voltage

From this test:

1.core loss can be calculated

Shaft voltage :when the rotor shaft rotates inside the stator there will be induced emf in rotor in addition to this voltage will go to the bearings at the shafts of rotor . Shaft voltage measures with high impedance voltmetres.

This increase the heating in rotor .to avoid this bearings placed on insulation HGL

FLUX TEST ON STATOR CORE: before winding in the slots flux is generated in the core by passing current .if there any break of inter laminations or shorts between laminations eddy current flows which raises the temperature called hotspots . This test is used to detect the hot spot.

INTER TURN INSULATION TEST: insulation between the windings of rotor is tested by applying current at a frequency of 500HZ . Insulation should be withstand to this test

HIGH VOLTAGE TEST ON STATOR AND ROTOR WINDINGS: high voltage is applied to windings by increasing gradually to required value and mainted for 1 minute and reduces gradually

High voltage test levels:

Stator winding-(2*ratedvoltage+1)kv

Rotor winding-(10*excitation voltage)v

1.INSULATION RESISTANCE MEASUREMENT TEST: Resistance is measured by placing two copper plates above and below the laminations .maximum acceptable value should be 1Mohm per kv of rated voltage. This test ensures the quality of varnish on laminations

TESTING OF STATOR BARS: after laying of top and bottom bars their balancing is done and then they are tested for voltage of 2(rated voltage)+3kv

In front of customer test it is carried out 2(rated voltage)+1KV

ADVANTAGES AND DISADVANTAGES The life span of machine is long

Cost is low compared to the other generators

In US 90% electrical generation done by turbo generators.

Disadvantages:

Repairing and maintaining cost is high