Turbo Generators

“OPERATION AND MAINTENANCEOF LARGE TURBOGENERATORS”

By: Nejarine Tio Torred

What is Turbo Generator?

steam and gas turbine-driven generators

Ratings: from few Megawatts (MW) up to 1900 MW

Two- and four pole design employing round rotors

What is Turbo Generator?

Rotational operating speed: 3600 and 1800 rpm ( North America, Japan, Asia)

3000 and 1500 rpm (Europe, Africa, Australia, Asia, and South America)

Synchronous machine with frequency of 60 or 50 Hz

Stator Core

Made up of thin sheets of electrical grade, 3% to 4% silicon or grain oriented, and 0.014 inch (0.355 mm) or 0.019 inch (0.483 mm) thick steel.

Stator CoreLamination is insulated on both sides with an organic (varnish) or inorganic (layer of oxidation) compound.

Hydrogen gas: coolant of the core

Stator Core

Note:Generators with a small outer core diameter (four pole) are usually lower fluxed machines while a large diameter (two-pole) are generally high flux machines.

Stator Frame

To provide support for the stator core and to act as a pressure vessel for the hydrogen cooling gas.

Stator Frame

Note:Ensure that the natural frequencies of the core and the frame are not near 60/50 Hz or 120/100 Hz.

Weight of the largest stator can reach up to 500 tons.

Stator WindingMade up of

insulated copper conductor bars that are distributed around the inside diameter of the stator core, called the stator bore, in equally spaced slots to ensure symmetrical flux linkage.

Stator Winding

OIndirectly cooled machines: strands are all solid

ODirectly gas-cooled machines: cooling ducts are insulated from the copper strands.

ODirect water-cooled: the strands are constructed as hollow, to carry liquid cooling, rectangular in shape and are insulated from each other.

Rotor Winding

OInstalled in the slots machined in the forging main body and is distributed symmetrically around the rotor between the poles.

Rotor WindingField voltage is

dc and can reach as high as 700V.

Rotor winding slot wedges are made of

aluminum or brass and do not usually sit tight in the slots to provide

thermal expansion during operation.

Retaining-Rings

Most highly

stressed componen

t

Hold the end-winding copper of the rotor winding

against centrifugal load during operation

Designed with a safety

factor of 150% up to 20%

overspeed.

Rotor Turning Gear

ORequired to slowly roll the rotor when the generator is taken off line.

Rotor Turning Gear

OFor two pole: 80 or 90 tons.

OFour pole: as much as 150 tons

Rotor Fans

For cooling gas circulation

Hydrogen CoolersHeat exchangers

mounted inside the generator in the

enclosed atmosphere

Generator Auxiliary Systems

Lubricating oil system

Hydrogen cooling system

Seal oil system Stator cooling

water system Excitation system

Lube Oil System

Generator Auxiliary Systems

provides oil for all the turbine and generator bearings the source of seal oil

Lube Oil System

Components:i. Main lube oil tankii. Pumpsiii. Heat exchangersiv. Filters and strainersv. Centrifuge or purifiervi. Vapor extractorvii. Check valves and

instrumentation

Hydrogen Cooling System

Reasons:O inherently better heat

transfer characteristics ( approx. 14 times)

O better the heat transfer with higher hydrogen pressure

O less windage and friction losses than air


Reasons:O suppression of partial discharges

with increased hydrogen pressureO significant increases of the

breakdown voltage of generator components


Control:O pressure regulators (below 500

cubic feet per day losses)O gas purity (maintained above 97%)O dew-point monitors (maintained

below -10 degree Celsius and not allowed to rise above 0 degree Celsius


Control:OC02

requirement

Since the explosive range of hydrogen/oxygen mixture is 4 to 75% hydrogen in air.

Seal Oil System

a portion of the lube oil, had its own pumps, motors, hydrogen detraining, and controls to regulate the pressure and flow

Maintained about 15 psi to stop hydrogen from leaking past the seals.

Stator Cooling Water System

To provide source of demineralized water to the generator stator winding for direct cooling

used in machines rated or above 300 MVA


System Components:

O Pumps (ac motor driven pumps used to deliver the cooling water, dc motor driven pump used for emergency shutdown)

O Heat exchangers ( for heat removal)


System Components:

O Filters and Strainers (removal of debris)

O De-Ionizing Subsystem ( maintained low conductivity)

O Makeup tank ( to hold sufficient makeup water as required)

OGas collection and venting required (venting off other gases like oxygen)

Exciter System

Types:O Rotating: motor or shaft driven,

self-excitedO Static: external source of powerO Brushless: consists of high-

frequency ac generator with a rotating fused diode assembly

Operation and Control

Machine Rating:O Gas turbine Gen.: 400 MVAO Steam turbine Gen.: 1600 – 2000

MVAPower factor: 0.85 to 0.90, lagging


Speed:O 60 Hz: 3600 rpm for 2-poles

gen: 1800 rpm for 4-

poles genO 50 Hz : 3000 rpm for 2-poles

gen: 1500 rpm for 4-

poles gen

Hydrogen Pressure: 75 psi (518 Kpa)


Hydrogen Temperature:O max allowable cold gas: 46 ºCO cold gas operating set point: 30-40

ºCO hot and cold hydrogen gas at full

load condition: 15 – 25 ºC


Overloading:O 10% overload condition will

increase the heat in the conductor by about 20%

O expected life of insulation: approx. halved for every 8 to 10 ºC increase in temperature


Overspeed: rotors of turbogenerators are

designed to withstand a 120% spin test

Shutdown Operation

The turbine should be tripped before the generator.

Make sure the generator does not motor the turbine.

Attain electrical separation by following clearly established safe routines.

Place the unit on turning gear as deemed necessary.

Generator Maintenance and Testing

Type of Maintenance Breakdown maintenance

- employed for short termeconomic gains with little regard to the future of the specific piece ofequipment.

- applied to smaller components,where repair is more costly than replacement


Type of Maintenance

Planned maintenance- entails the predetermined

maintenance schedules.- based on experience

acquired during many years of operation, onthe reliability of the equipment, as well as on load demands, weather, personnel availability, coordination with other plants of the same utility


Type of Maintenance Predictive maintenance

- schedule is based mainly on statistical calculations.

- age of the insulation components, type of insulation, load cycles, and abnormal operation events.


Type of Maintenance

Condition-Based maintenance

- determining whento inspect, maintain, and refurbish a generator and other plant equipment

-applied when equipment is monitored by a number of on-line, real-time sensors, as well as off-line periodic testing routines.


Stator Core Mechanical Tests

Core Tightness- is done by

inserting a thin, tapered, and hardened steel knife blade between laminations to determine the degree of looseness in the stator ring.

01


Stator Core Mechanical Tests

Core and Frame Vibration Testing

- is done by online testing, where vibration transducers are mounted on the core and frame internal to the machine.

01


Core and Frame Vibration Testing

Note:The maximum vibration of the stator core and core frame should be less than 50µm peak to peak, with no natural resonance within the frequency ranges of 50 – 75 Hz and 100 – 140 Hz for 60 Hz systems, and about 40 -65 Hz and 80 – 120 Hz for 50 Hz systems


Stator Core Electrical Tests

EL-CID Testing- (Electromagnetic-Core

Imperfection Detector)- The heat produced is detected

and quantified using established infrared techniques.

02



EL-CID TestingHow this will work?• A 120 or 230V with current capacity

of 15 to 20 A• The winding is energized

approximately 3 to 4% or 5V per meter.

• EL-CID detect fault in mA.

+-100mA( core has damage)

02



Rated Flux Test with Infrared Scan

- used to check the integrity of the insulation between the laminations in the stator core.

02



Core Loss Test- always

determined at the factory by the manufacturer, and is not a test that is generally done at site.

02


Stator Core Electrical Tests Through-Bolt

Insulation Resistance

- to ensure that the through-bolts do not create any short circuits across the stator core laminations.

- meggering at 500V dc. Good reading: hundreds of MΩ range.

02


Stator Core Electrical Tests Insulation

Resistance of Flux Screens

- to ensure that no additional circulating currents flow between the core and the flux screens.

- meggering at 500V dc. Good reading: hundreds of MO range.

02


Stator Winding Mechanical Tests

Wedge Tightness

- is done by tapping with a suitable hammer and recording the degree of looseness on a chart.

03


Wedge TightnessRe-wedging is required if:

• Less than 75% of the wedges are tight in the slot.

• Three or more adjacent wedges are fully loose.

• Fully loose end-wedges must be re-tightened.


Stator Winding Mechanical Tests Stator End-Winding Vibration

- “bump test” (temporary vibration transducers mounted on the end-winding and a calibrated hammer to measure the vibration signatures.

03


Water-Cooled Stator Winding Tests

Air Pressure Decay

- “pressurizing the stator winding to about 30 psi and watching that the drop in pressure does not exceed 0.1 psi over a 6 hour period.

04


Water-Cooled Stator Winding Tests Tracer Gases- “pressurizing

the stator winding with helium.

- used to “sniff” over the external surfaces of the exposed portions of the stator winding.

04


Water-Cooled Stator Winding Tests Vacuum Decay

- determine the leak sensitivity of the entire winding.

- by pulling vacuum to 29 in. of mercury, and watching gauge does not drop by more than 0.2 in Hg over 6 hours.

04



Pressure Drop

- to estimate the stator cooling water flow rate in each bar.

04



Flow Testing- to

determine if there are restrictions in the cooling water path and it is done by water.

04


Water-Cooled Stator Winding Tests Capacitance

Mapping- to identify the

presence of moisture in the ground-wall insulation from stator winding leaks.+-2 S.D. (good)+ 3 S.D. (suspect)+ 5 S.D. (wet)

04


Stator Winding Electrical Tests

Pre-testing Requirements• Completely dried• Three phases must be isolated• Generator CT windings should

be shorted• Consult vendor/pertinent

standards

05



Series Winding Resistance

- to measure the ohms resistance of the copper in each phase.

- to detect shorted turns, bad connections, and open circuits.Accepted test: 0.5% error from the average

05


Stator Winding Electrical Tests Insulation Resistance (IR)

- to measure the ohmic resistance between conductors in each of the three phases and the ground.

- from 500 to 5000V in 1 min.Accept: (VLL in KV + 1) MΩ.

05



Polarization Index (PI)- done on a per phase basis

at the same voltage as the IR test, 40°C.PI values:Class A insulation: 1.5Class B and F insulation: 2.0

05


Stator Winding Electrical Tests dc Hi-Pot- if the

winding is capable of sustaining the required rated voltage levels, capability to withstand overvoltages and transients, and maintain acceptable insulation life.

05

New windings: [(2*VLL + 1000) * 1.7] KV dc Field testing and maintenance: 125 to 150% of (VLL * 1.7) KV dc


Stator Winding Electrical Tests ac Hi-Pot- if the


05

New windings: (2*VLL + 1000) KV ac Field testing and maintenance: 125 to 150% of VLL KV ac



Dissipation/Power Factor Testing

- ac test used to measure the bulk quality of the ground-wall insulation, by measuring the dielectric loss per unit volume of the insulation.

05


Rotor Mechanical Testing

Rotor Vibration

06

MACHINE SPPED MAXIMUM AMPLITUDE

O-999 rpm 3 mils

1000-1499 rpm 2.5 mils

1500-2999 rpm 2 mils

3000 rpm & above 1 mil



Visual Inspection- applicable to surface

defects. It’s low cost, simple, and quick but the inspector must be knowledgeable and thorough.

06



Radiographic- done by

commercial X-ray or gamma units for welds, castings, and forgings. It can detects cracks, porosity, and poor welds.

06



Magnetic Particle

- for surface flaws in the range of 200 to 2000 amps. Magnetic powders are in dry or wet form and may be fluorescent for viewing under ultraviolet rays.

06



Liquid Penetrant

- primarily used for retaining-ring inspection, have greater sensitivity, and a cost-effective method.

06



Ultrasonic- able to detect surface and

subsurface flaws of about 2 to 3 mm

06



Eddy- able to

detect surface and near-surface defects, de-lamination of multi-layered components, and inclusions.

06


Rotor Electrical Testing

Insulation Resistance (IR)

- to measure the ohmic resistance between the total rotor winding insulation and ground.

- from 500 to 1000V in 1 min.Accept: (Vf in KV + 1) MΩ.

07



Polarization Index (PI)- to determine how clean

and dry the winding is.PI values:Class B insulation: 2.0Class F insulation: 2.0

07



dc Hi-Pot- if the


07

done between 1500 V up to 10 times the rated field voltage.



ac Hi-Pot- if the


07

Done at 10 times the rated field voltage at line frequency of 60 Hz.


Rotor Electrical Testing Shorted turns

Detection- to detect rotor

winding shorted turns that can occur from electrical breakdown of insulation and mechanical damage.

07


Hydrogen Seals

Liquid Penetrant- primarily used to determine

cracks, and other surface damage.

08

Turbo Generators

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