ntpc project report

GENERATOR AND GENERATOR AUXILIARIES

1. INTRODUCTION

It is not unusual for well maintained and operated generators to be in use long

after the OEM’s designed life expectancy. In fact, most large utilities in the United

States have generators in their fleets that have been in operation for well over 30

years. With a growing dependence on electric power and the prohibitive cost of outage

extensions, there have been increased efforts to obtain higher operating voltages

while increasing reliability. To accomplish this, it may require the replacement of major

components such as armature windings, field windings or a complete rotor. Upgrading

generator components can be a complicated process that requires a considerable

amount of pre-planning, extended outage time and a significant financial investment.

Not all generator service issues are solved with a rewind. Often ignored during

generator upgrades are the auxiliary systems; seal & lube oil, hydrogen cooling, gas

supply & controls, stator cooling water and monitoring systems. The auxiliary system

equipment is critical to ensure efficient, reliable and safe operation of the generator.

Time and wear of auxiliary system components have a direct impact on the generator

availability and could potentially lead to a generator problem that could lead to a

catastrophic failure. A proactive approach to upgrading these critical auxiliary systems

will help secure your return on investment involved in rewinding a generator. This

paper identifies the benefits for evaluating and upgrading generator auxiliary systems.

2. GENERATOR AUXILIARIES

All large generators require auxiliary systems to handle such things as

lubricating oil for the rotor bearings, hydrogen cooling apparatus, sealing oil,

demineralized water for stator winding cooling and excitation systems for field-current

application. Not all generators require all these systems and the requirement depends

on the size and nature of the machine.

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2.1GENERAL DESIGN FEATURES

Make : BHEL

Type : THDF 115/59

Code : IEC 34-1, VDE 0530

Cooling ,stator winding : Directly water cooled

Stator core ,rotor : Directly hydrogen cooled.

Rating:

Apparent power : 588 MVA

Active power : 500 MW

Power factor : 0.85(LAG)

Terminal voltage : 21 KV

Permissible variation in voltage : +5%

Speed/Frequency/Hz : 3000/50

Stator current : 16200

Hydrogen pressure : 4 Kg/Cm2

Short circuit Ratio : 0.48

Field Current(calculated value) : 4040 A

Class and Type of Insulation : MICALASTIC (similar to class F)

No. of terminals brought out : 6

Resistance in Ohms at 20°C: U-X 0.0014132

Stator Winding between terminals : V-Y 0.0014145

: W-Z 0.0014132Rotor Winding : F1-F2 0.0672

Main Exciter:

Active Power : 3780 KW

Current : 6300 A

Voltage : 600V

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Pilot Exciter:

Apparent power : 65 KVA

Current :195 A

Voltage : 220 V(1+10%)

Frequency :400 Hz

Torque, Critical Speeds:

Maximum short circuit torque of stator at line to :488 kpm

line single phase short circuit

Moment of inertia of generator shaft :10,000kgm2

Critical speed (calculated):

nk1 : 14.4 rps(V-GEN)

nk2 : 30.1rps(V-EXC)

nk3 : 39.8rps(S-GEN)

2.2 GENERAL DESCRIPTION

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 losses

in the remaining generator components, such as iron losses windage losses and stray

losses, are also dissipated through hydrogen. 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

Hydrogen coolers

Rotor

Rotor shaft

Rotor Winding

Rotor retaining rings

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Field connections

Bearings

Shaft seals

The following additional auxiliary systems are required for generator operation:-

Oil systemGas systemPrimary water system

Excitation system

3. 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. Direct

cooling essentially eliminates hot spots and differential temperatures between adjacent

components which could result in mechanical stress, particularly to the copper

conductors, insulation rotor body and stator core.

3.1 HYDROGEN COOLING CIRCUIT

The hydrogen is circulated in the generator interior in a closed circuit by one

multistage 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 path after each cooler.

3.2 FLOW PATH I

Flow path I is directed into the rotor at the turbine end below the fan hub for

cooling of the turbine end half of the rotor.

3.3 FLOW PATH II

Flow path II is directed form the coolers to the individual frame compartments

for

cooling of the stator core.

3.4 FLOW PATH III

Flow path III is directed to the stator end winding space at the exciter end

through

guide ducts in the frame for cooling of the exciter end half of the rotor and of the core

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GENERATOR AND GENERATOR AUXILIARIESend portions. The three flows mix in the air gap. The gas is then returned to the

coolers via the axial-flow fan.

The cooling water flow through the hydrogen coolers should be automatically

controlled to maintain a uniform generator temperature level for various loads, and

cold water temperatures.

Fig-1 Hydrogen Cooling system

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4. COOLING OF ROTOR

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 oil quarter is divided into two cooling

zones. The first cooling zone consists of the rotor end winding and the second one of

winding portion between the rotor body end and the mid-point of the rotor. Cold gas is

directed to each cooling zone through separate openings directly before the rotor body

end. The hydrogen flows through each individual conductor in closed cooling ducts.

The heat removal capacity is selected such that approximately identical temperatures

are obtained for all conductors. The gas of the first cooling zone is discharged from the

coils at the pole centre into a collecting compartment within the pole area below the

end winding. From there the hot gas passes into the air gap through pole face slots at

the end of the rotor body. The hot gas of the second cooling zone is discharged into

the air gap at mid-length of the rotor body through radial openings in the hollow

conductors and wedges.

4.1 COOLING OF STATOR CORE

For cooling of the stator core, cold gas is admitted to the individual frame

compartments via separated 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. To dissipate the higher losses in the core ends, the cooling gas slots are

spaced in the core end sections to ensure effective cooling. These ventilating ducts

are supplied with cooling gas directly from the end winding space. Another flow path is

directed from the stator end winding space past the clamping fingers between the

pressure plated and core end section into the air gap. A further flow path passes into

the air gap along either side of the flux shield. All the flows mix in the air gap and cool

the rotor body and stator core surfaces. The gas is then returned to the coolers via the

axial-flow fan. To ensure that the cold gas directed to the exciter end cannot be

directly discharged into the air gap, an air gap choke is arranged with in the range of

the stator end winding cover and the rotor retaining ring at the exciter end.

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5. PRIMARY COOLING WATER CIRCUIT IN THE

GENERATOR

The treated water used for cooling of the stator winding phase connectors and

bushing 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 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 as

described in the following paragraphs.

5.1 FLOW PATH I

Flow path I cools the stator windings. This flow path first passes to a water

manifold on the exciter end of the generator and from there to the stator bars via

insulated hoses. Each individual bar is connected to the manifold by a separate hose.

Inside bars the cooling water flows through hollow strands. At the turbine end, the

water is passed through similar hoses to another water manifold and then returned to

the primary water tank. Since a single pass water flow through the stator is used, only

a minimum temperature rise is obtained for both the coolant and the bars. Relative

movements due to different thermal expansions between the top and bottom bars are

thus minimised.

5.2 FLOW PATH II

Flow path II cools the phase connectors and the bushings. The bushings and

phase connectors consists of thick walled copper tubes through which the cooling

water is circulated. The six bushings and the phase connectors arranged in a circle

around the stator end winding are hydraulically interconnected. The secondary water

flow through the primary water cooler should be controlled automatically to maintain a

uniform average generator temperature level for various loads and cold water

temperatures.

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6. STATOR FRAME

The stator frame consists of a cylindrical centre section and two end shield

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 connections. The stator

frame accommodates the electrically active parts of the stator, i.e. the stator core and

the stator windings. Both the gas ducts and a large number of welded circular ribs

provide for the

rigidity of the stator frame. Ring shaped supports for resilient core suspension are

arranged between the circular ribs. The generator cooler is subdivided into cooler

sections arranged vertically in the turbine side stator end shield. In addition, the stator

end shields contain the shaft seal and bearing components. Feet are welded to the

stator frame and end shields to support the stator on the foundation. The stator is

firmly connected to the foundation with anchor bolts through the feet.

6.1STATOR CORE

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 supporting rings

form part of an inner frame cage. This cage is suspended in the outer frame by a large

number of separate flat springs which are tangentially arranged on the circumference

in sets of three springs each, i.e. two vertical supporting springs on both sides of the

core and one horizontal stabilising spring below the core. The springs are so arranged

tuned that forced vibrations of the core resulting from the magnetic field will not be

transmitted to the frame and foundation.

The pressure plates and end portions of the stator core are effectively shielded against

stray magnetic fields. The flux shields are cooled by a flow of hydrogen gas directly

over the assembly.

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Fig-2 STATOR WATER SYSTEM

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7. MICALASTIC HIGH - VOLTAGE INSULATION

High-voltage insulation is provided according to the proven Micalastic system.

With this insulating system, several half-overlapped continuous layers of mica tape are

applied to the bars. The mica tape is built up from larger mica splitting which are

between two polyester backed fabric layers with epoxy as an adhesive. The number of

layers, i.e., the thickness of the insulation depends on the machine voltage. The bars

are dried under vacuum and impregnated with epoxy resin which has very good

penetration properties due to its low viscosity. After under vacuum, the bars are

subjected to pressure, with nitrogen being used as medium. The impregnated bars are

formed to the required shape in moulds and cured in an oven at high temperature. The

high-voltage insulation obtained is nearly void - free and is characterised by its

excellent electrical,

mechanical and thermal properties in addition to being fully water proof and oil -

resistant. To minimize corona discharges between the insulation and the slot wall, a

coat of semiconducting varnish is applied to the surfaces of all bars within the range.

In addition, all bars are provided with an end corona protection to control the electric

field at the transition from the slot to the end winding and to prevent the formation of

creepage spark concentrations.

8. ROTOR

8.1 Rotor Shaft

The high mechanical stresses resulting from the centrifugal forces and short-

circuit torque call for a high quality heat-treated steel. Therefore, the rotor shaft is

forged from a vacuum cast steel ingot. Comprehensive tests ensure adherence to the

specified mechanical and magnetic properties as well as homogeneous forging. The

rotor shaft consists of an electrically active portion, the so-called rotor body, and the

two shaft journals. Integrally forged flange couplings to connect the rotor to the turbine

and exciter are located outboard of the bearings. Approximately two-thirds of rotor

body circumference is provided with longitudinal slots which hold the field winding. Slot

pitch is selected so that the two solid poles are displaced by 180 deg.

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GENERATOR AND GENERATOR AUXILIARIESDue to the non-uniform slot distribution on the circumference, different moments of

inertia are obtained in the main axis of the rotor. This is turn causes oscillating shaft at

twice the system frequency. To reduce these vibrations, the deflections in the direction

of the pole axis and the neutral axis are compensated by transverse slotting of the

pole.

After completion, the rotor is balanced in various planes at different speeds and then

subjected to an overspeed test at 120 % of rated speed for two minutes. The solid

poles are also provided with additional longitudinal slots to hold the copper bars of the

damper winding. The rotor wedges act as a damper winding in the area of winding

slots.

8.2 COOLING OF ROTOR WINDING

Each turn is subdivided into eight parallel cooling zones. One cooling zone

includes the slots from the centre to the end of the rotor body, while another cover, half

the end winding.The cooling gas for the slot portion is admitted into the hollow

conductors through milled openings directly before the end of the rotor body and flows

through the hollow to the centre of the rotor body. The hot gas in then discharged into

the air between the rotor body and the stator core through radial openings in the

conductors and the rotor slot wedges. The cooling gas passages are arranged at

different levels in the conductor assembly so that each hollow conductor has its own

cooling gas outlet.

The cooling gas for the end windings is admitted into the hollow conductors at the

ends of the rotor body. It flows through the conductors approximately up to the pole

centre for being directed into a collecting compartment and is then discharged into the

air gap via

slots. At the end winding, one hollow conductor passage of each bar is completely

closed by a brazed copper filler section. The enlargement of the conductor rigidity.

8.3 ROTOR WINDING

The rotor winding consists of several coils which are reinserted into the slots

and series connected such that two coil groups from one pole. Each coil consists of

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GENERATOR AND GENERATOR AUXILIARIESseveral series- connected turns, each of which consists of two half turns which are

connected by brazing in the end section. The rotor winding consists of silver bearing

de-oxidised copper hollow conductors with two lateral cooling ducts. L-shaped strips of

laminated epoxy glass fibre fabric with Nomex filler are used for slot insulation. The

slot wedges are made of highconductivity material and extend below the shrink seat of

the retaining ring. The seat of the retaining ring is silver plated to ensure a good

electrical contact

between the wedges and rotor retaining rings. This system has long proved to be a

good damper winding. The field winding are inserted into the longitudinal slots of the

rotor body. The coils are wound around the poles so that one north and one south

magnetic pole are obtained. The hollow conductors have a trapezoidal cross-section

and are provided with two cooling ducts of approximately semi-circular cross-section.

All conductors have identical copper and cooling duct cross-sections.

9. HYDROGEN COOLER

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 hydrogen cooler is subdivided into

identical sections which are vertically mounted in the turbine-end stator end shield.

The cooler sections are solidly bolted to the upper half stator end shield, while the

attachment at the lower water channel permits them to move freely to allow for

expansion.

The cooler sections are parallel connected on their water sides. Shutoff valves are

installed in the lines before and after the cooler sections. The required cooling water

depends on the generator output and is adjusted by control valves on the hot water

side.

Controlling the cooling water flow on the outlet side ensures a water flow through the

cooler sections so that proper cooler performance will not be impaired.

9.1 BEARINGS

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

stator and base plate, respectively. The temperature of the bearings is monitored with

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GENERATOR AND GENERATOR AUXILIARIESthermocouples 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 conjuction with the turbine supervision. The bearings

have provisions for fitting vibration pickups to monitor bearing vibrations.

10. OIL SUPPLY FOR BEARINGS AND SHAFT SEALS

10.1 BEARING OIL SYSTEM

The generator and exciter bearings are connected to the turbine lube oil supply.

10.2 SEAL OIL SYSTEM

Seal Oil Pump 1 & 2 Air Side

Kind of Pump : Screw Pump

Type : SNH210-R46 (Allweiler)

Capacity : 3.3 DM3/S

Discharge pressure : 15 bar

Pump motor –Type : 1LA3-133-4AA90 (Siemens)

Rating : 7.5 KW

Current : 141A

Type of enclosure : IP54

No. : 2 Nos.Full capacity

10.3 SEAL OIL PUMP AIR SIDE

Kind of pump : Screw pump

Type : SNH210-R46 (All weiler)

Capacity : 3.3 DM3/S

Discharge pressure : 15bar

Pump motor drive : 1 HA 4165-5JL20

(Siemens)

Rating : 8.5KW

Voltage : 220V, DC

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GENERATOR AND GENERATOR AUXILIARIESCurrent : 51A

Speed : 24.17RPS


No. : 1 No. full capacity

10.4 SEAL OIL PUMP H2 SIDE

Kind of pump : Screw pump

Type : SNH 210R46 (All Weiler)

Capacity : 2.17 DM 3/S

Discharge pressure : 15 bar

Pump motor : ILA3 133-6AA90 (Siemens)

Rating : 4 KW

Current : 7.2 A

Speed : 15.8 RPS


No. : 1 No. Full capacity

10.5 SEAL OIL FILTER, AIR SIDE AND H2 SIDE

Kind of filter : Strainer Type

Type : 2.62.9 MA (BOLL+MIIRCH)

Volume flow rate : 3.3 DM3/5

Degree of filter ation : 100 Microns

No. of air side : 2 Nos. full capacity

No. of H2 side : 2 Nos. full capacity

11. SEAL OIL SYSTEM CONSTRUCTION

The shaft seals are supplied with seal oil from two seal oil circuits which

consists of the following principal components.

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GENERATOR AND GENERATOR AUXILIARIES11.1 HYDROGEN SIDE SEAL OIL CIRCUIT

· Seal oil tank

· Seal oil pump

· Oil cooler 1

· Oil cooler 2

· Seal oil filter

· Differential pressure valve C

11.2 AIR SIDE SEAL OIL CIRCUIT

· Seal oil storage tank

· Seal oil pump 1

· Seal oil pump 2

· Standby seal oil pump

· Oil cooler 1

· Oil cooler 2

· Seal oil filter

· Differential pressure valve A1

· Differential pressure valve A2

11.3 HYDROGEN SIDE SEAL OIL CIRCUIT

The seal oil drained towards the hydrogen side is collected in the seal oil tank.

The associated seal oil pump returns the oil to the shaft seals via a cooler and filter.

The hydrogen side seal oil pressure required downstream, of the pump is controlled by

differential pressure valve C according to the preset reference value, i.e. the preset

difference between air side and hydrogen side seal oil pressures. The hydrogen side

seal oil pressure required at the seals is controlled separately for each shaft seal by

the

Exciter end or Turbine end pressure equalising valve, according to the preset pressure

difference between the hydrogen side and air side seal oil. Oil drained from the

hydrogen side is returned to the seal oil tank via the generator prechambers. Two float

operated valves keep the oil level at a predetermined level, thus preventing gas from

entering the suction pipe of the seal oil pump (hydrogen side). The low level float

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GENERATOR AND GENERATOR AUXILIARIESoperated valve compensates for an insufficient oil level in the tank by admitting oil from

the air side seal oil circuit. The high level float operated valve drains excess oil into the

seal oil storage tank. The hydrogen entrained in the seal oil comes out of the oil and is

extracted by the bearing vapour exhauster for being vented to the atmosphere above

the power house roof. During normal operation, the level float-operated drain valve is

usually open to return the excess air side seal oil, which flowed to the hydrogen side

via the annular gaps of the shaft seals, to the air side seal oil circuit.

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GENERATOR AND GENERATOR AUXILIARIESFig-3 GENERATOR SEAL OIL SYSTEM

12. GAS SYSTEM

The gas system contains all equipment necessary for filling the generator with

CO2, hydrogen or air and removal of these media, and for operation of the generator

filled with hydrogen. In addition, the gas system includes a nitrogen (N2) supply. The

gas consists of:

H2 supply

CO2 supply

N2 supply

Pressure reducers

Pressure gauges

Miscellaneous shut off valves

Purity metering equipment

Gas dryer

CO2 flash evaporator

Flow meters.

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Fig-4 HYDROGEN GAS SYSTEM

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12.1 HYDROGEN (H2) SUPPLY

a. GENERATOR CASING

The heat losses arising in the generator are dissipated through hydrogen. The heat dissipating capacity of hydrogen is eight times higher than that of air. For more effective cooling, the hydrogen in the generator is pressurized.

b. PRIMARY WATER TANK

Nitrogen environment is maintained above the primary water in the primary water tank:

To prevent the formation of a vacuum due to different thermal expansions of the primary water.

To ensure that the primary water in the pump suction line is at a pressure above atmospheric pressure so as to avoid pump cavitation.

To ensure that the primary water circuit is at a pressure above atmospheric pressure so as to avoid the ingress of air on occurrence of a leak.

c. CARBON DIOXIDE (CO2) SUPPLY

As a precaution against explosive hydrogen air mixtures, the generator must be filled with an inert gas (CO2) prior to H2 filling and H2 removal.

The generator must be filled with (CO2) until it is positively ensured that no explosive mixture will form during the subsequent filling or emptying procedures.

d. COMPRESSED AIR SUPPLY

To remove the CO2 from the generator, compressed air must be admitted into the

generator.

The compressed air must be clean and dry. For this reason, a compressed air filter is

installed in the filler line.

e. NITROGEN (N2) SUPPLY

Nitrogen is required for removing the hydrogen or air during primary water filling and

emptying procedures.

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13. PRIMARY WATER SYSTEM

The primary water required for cooling is circulated in a closed circuit by a separate

pump. To ensure uninterrupted generator operation, two full-capacity pumps are

provided. In the event of a failure of one pump, the standby pump is immediately ready

for service and cuts in automatically. Each pump is driven by a separate motor.

Fig-5 PRIMARY WATER SYSTEM

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All valves, pipes and instruments coming into contact with the primary water are made

from stainless material.

The primary water system consists of the following principal components:

Primary water tank

Primary water pumps

Primary water coolers

Fine filter

Ion Exchanger.

As illustrated in the diagram, the primary water admitted to the pump from the tank is

first passed via the cooler and fine filter to the water manifold in the generator interior

and then to the bushings. After having performed its cooling function, the water is

returned to the primary water tank. The gas pressure above the water level the primary

water tank is maintained constant by a pressure regulator.

13.1 PRIMARY WATER TANK

The primary water tank is located on top of the stator frame on an elastic support, thus

forming the highest point of the entire primary water circuit in terms of static head.

13.2 PRIMARY WATER TREATMENT SYSTEM

The direct contact between the primary water and the high-voltage windings call for a

low conductivity of the primary water. During operation, the electrical conductivity

should be maintained below a value of approximately 1 mmho/cm. In order to such a

low conductivity it is necessary to provide for continuous water treatment during

operation, a small quantity of the primary water should therefore be continuously

passed through the ion exchanger located in the bypass of the main cooling circuit.

The ion exchanger resin material requires replacement at infrequent intervals. The

resins can be replaced during operation of the generator, since with the water

treatment system out of service, the conductivity will rise very slowly.

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14. STATOR

To facilitate manufacture, erection and transport, the stator consists of the following

main components:

Stator frame

Bushing compartment

The stator frame with flexible core suspension components, core, and stator winding is

the heaviest component of the entire generator. A rigid frame is required due to the

forces and torque’s arising during operation. In addition, the use of hydrogen for the

cooling requires the frame to be pressure resistant up to an internal pressure of

approximately 10 bar (130 psig).welded stator frame consists of the cylindrical frame

housing, two flanged rings and axial and radial ribs. Housing and ribs within the range

of the phase connectors of the stator winding are made of non-magnetic steel to

prevent eddy current losses, while the remaining frame parts are fabricated from

structural steel. Arrangement and dimensioning of the rib are determined by the

cooling gas passages and the required mechanical strength and stiffness.

Dimensioning is also dictated by vibrational considerations, resulting partly in greater

wall thickness than required from the point of view of mechanical strength. The natural

frequency of the frame does not correspond to any exciting frequencies. Two lateral

supports for flexible core suspension in the frame are located directly adjacent to the

points where the frame is supported on the foundation. Due to the rigid design of the

supports and foot portion the forces due to weight and shot-circuit will not result in any

over-stressing of the frame. Manifolds are arranged inside the stator frame at the

bottom and top for filling the generator with CO2 and H2. The connections of the

manifolds are located side by side in the lower part of the frame housing. Additional

openings in the housing, which are sealed gas tight by pressure-resistant covers,

afford access to the core clamping flanges of the flexible core suspension system and

permit the lower portion of the core to be inspected. Access to the end winding

compartments is possible through manholes in the end shields. In the lower part of the

frame at the exciter end an opening is provided for bringing out winding ends. The

generator terminal box is flanged to this opening.

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14.1 GENERATOR TERMINAL BOX

The phase and neutral leads of the three-phase stator winding are brought out of the

generator through six bushings located in the generator terminal box at the exciter end

of the generator. The terminal box is a welded construction of non-magnetic steel

plate. This material reduces stray losses due to eddy currents. Welded ribs provide for

the

rigidity of the terminal box. Six manholes in the terminal box provide access to the

bushing during assembly and overhauling.

14.2 WINDING COOLING CIRCUIT

The end windings are enclosed by an annular water manifold to which all stators bars

are connected through hoses. The water manifold is mounted on the holding plates of

the end winding support ring and connected to the primary water supply pipe. This

permits the insulation resistance of the water-filled stator winding to be measured. The

water manifold is grounded during operation. For measurement of the insulation

resistance, e.g. during inspections, grounding is removed by opening the circuit the

stator frame. The hoses, one side of which is connected to ground, consists of a

metallic section to which the measuring potential is applied for measurement of the

insulation resistance of the water-filled stator winding. The cooling water is admitted to

three terminal bushings via a distribution water manifold, flows through the attached

phase connectors and is then passed to the water manifold for water outlet via the

terminal bushings on the opposite side. The parallel-connected cooling circuits are

checked for uniform water flows by a flow system covering all three phases. The

cooling primary water flows through the stator bars, which are hydraulically connected

in parallel, from the exciter end to the turbine end of the generator. This ensures a

minimum temperature rise of the stator bars, a minimum water velocity, and a

minimum head loss. Moreover, the thermal expansions of the stator bars are

completely uniform.

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15. EXCITATION SYSTEM 500 MW

In 500 MW Turbo-generator, brushless excitation system is provided. Brushless

exciter consists of a 3 phase permanent magnet pilot exciter the output of which is

rectified and controlled by the Thyristor Voltage Regulator to provide a variable d.c.

current for the main exciter.The 3 phases are induced in the rotor of the main exciter

and is rectified by the rotating diodes and fed to the field winding of generator rotor

through the D.C. leads in the rotor shaft. Since the rotating rectifier bridge is on the

rotor, the slip rings are not required and the output of the rectifier is connected directly

to the field winding through the generator rotor shaft. A common shaft carries rectifier

wheels, the rotor of the main exciter and permanent magnet of the pilot exciter.

The voltage regulation is effected by using thyrism 04.2, an automatic voltage

regulator. There are two independent control systems right up to the final Thyristor

element-an auto control and a manual control. The control is effected on the 3 phase

of the pilot exciter and provides a variable d.c. input to the main exciter. The feedback

of voltage and current output of the generator is fed to the AVR where it is compared

with the set-point generator volts set from the control room. The current feedback is

utilised for active and reactive power compensation and for the limiters. which act on

the AVR. A power

system stabiliser is also envisaged for damping oscillations in the power system. The

manual control system consists of an excitation controller which control the

as set on the manual set-point from the control room.

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25


Fig-6 EXCITATION SYSTEM

15.1 COOLING OF EXCITER

The exciter is air cooled. The cooling air is circulated in a closed circuit and re cooled

in two cooler sections arranged alongside the exciter. The complete exciter is housed

in an enclosure through which the cooling air circulates. The rectifier wheels, housed

in their own enclosure draw the cool air in at both ends and expel the warmed air to

the compartment beneath the base plate. The main exciter enclosure receives cool air

from the fan after it passes over the pilot exciter. The air enters the main exciter from

both ends and is passed into ducts below the rotor body and discharged through radial

slots in the rotor core to the lower compartment. The warm air is then returned to the

main enclosure via the cooler sections.

16. GROUND FAULT DETECTION SYSTEM

The field ground fault detection system detects high resistance and low-resistance

ground faults in the exciter field circuit. This is very important for safe operation of a

generator, because a double fault causes magnetic unbalances, with very high

currents flowing through the faulted part, resulting in its destruction within a very short

time. It is therefore an essential requirement that even simple ground faults should

activate an alarm and protective measures be initiated, if possible, before the fault can

fully develop. For this reason, the field ground fault detection system consists of two

stages and operates continuously. If the field ground fault detection system detects a

ground fault, an alarm is activated at . If the insulation resistance between the exciter

field circuit and ground either suddenly or slowly drops to the generator electrical

protection is tripped (2nd stage). The generator is thus automatically disconnected

from the system and de-excited.

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17. AUTOMATIC VOLTAGE REGULATORVOLTAGE REGULATING SYSTEM

Type : Thyrisiem 04-2

Maximum output voltage : 250V

Output current for field forcing : 152A

Output current for rated generator load : 88A

Auxiliary voltage from pilot exciter for thyristor : Three phase supply

Sets 220 V,400 Hz

D.C.voltage from station battery for conductor &

Drives : 220V

Power input continuously : < 0.1KW

Power input short time : < 1KW

DC current from station battery 2 X 24 V for : Max. 15A positive

control and regulation Max.6A Negative

Rated secondary voltage : 120V

Power input of voltage transformer per phase : 2 VA

Rated secondary current : 5A

Power input of current transformer per phase : 6.5 VA (plus losses in

connecting leads)

Accuracy of control : better than ± 0.5%

Setting range of voltage set point potentiometer :+5-10%of nominal

Gen.voltage

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GENERATOR AND GENERATOR AUXILIARIESSetting range of droop compensation or : ± 0-10% dependent on

the compounding setting of the

potentiometer and

proportional to reactive

current

17.1 BASIC MODE OF OPERATION

The THYRISIEM 04-2 voltage regulator is designed for excitation and control

brushless generators. The block diagram shows the circuit configuration. The machine

set of the generator and a direct coupled exciter unit with a three phase main exciter,

rotating rectifiers and a permanent magnet auxiliary exciter. The main of the voltage

regulator are two closed-loop control systems each followed by a separate gate control

unit and Thyristor set and a de-excitation equipment. In addition to this (but not

shown), a open-loop control system for the signal exchange the regulator and the

power station control room and other plant components is provided as well as power

supply equipment. Control system 1 for automatic generator voltage control (AUTO)

comprises the

following :

o Generator voltage control; the output quantity of this control is the set-

point for a following.

o Excitation current regulator, controlling the field current of the main

exciter (= output current of the co-ordinated Thyristor set)

o Circuit for automatic excitation build-up during start-up and field

suppression during shut-down; this equipment acts onto the output of the

generator voltage.control, limiting the set-point for the above excitation

current regulator. The stationary value of this limitation determines the

maximum possible excitation current set-point (field forcing limitation);

o Limiter for the under-excited range (under excitation limiter),

o Delayed limiter for the over excited range (over excitation limiter).

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29


Fig-7 VOLTAGE REGULATOR

18. CONCLUSION

Upgrading generator components can be a complicated process that requires a

considerable amount of pre-planning, extended outage times and a significant financial

investment. To help this significant investment, a proactive approach for evaluating

and up-grading critical auxiliary systems should also be considered. Utilizing a “flexible

philosophy” for prioritizing system up-grades should prove to be a valuable tool in this

process.

19. A FOOTNOTE ON SAFETY

Although the use of hydrogen gas as a cooling medium has several performance

benefits, it must be monitored carefully to prevent catastrophic oxidation. Colourless,

odourless, tasteless and nontoxic, hydrogen exists as a gas at atmospheric

temperatures and pressures. Hydrogen is flammable and burns in air with a pale blue,

almost invisible flame. In its gaseous form, hydrogen dissipates quickly. These unique

properties call for strict safety measures in hydrogen use and storage. Precautions

must be taken to safeguard against a hydrogen explosion. One such precaution is to

never permit an explosive mixture to exist. Hydrogen ignites over a wide range of

concentrations (from 4 to 74.2 percent by volume). Generators and auxiliary systems

are designed with many to avoid an explosive mixture. In order to ensure the design is

operating properly operators should monitor hydrogen gas purity on a continuous

basis. The following information is intended to define the basic components of a

hydrogen auxiliary system in addition to identifying some areas worth upgrading during

planned outages. In the power industry, millions of cubic feet of hydrogen gas are

used every year. The intent is that hydrogen is used under carefully controlled

conditions using specified procedures by trained. However, as equipment ages and

30

GENERATOR AND GENERATOR AUXILIARIESpersonnel changes occur with limited training, there becomes a potential for a serious

problem.

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