Thomas Kunz / Dr. A. Schwery - IEEE.CH · PDF fileThomas Kunz / Dr. A. Schwery. Grimsel, 11. th. ... −Manufacturing / Site −Project Management ... stator bar: 3-4 days

Generator RefurbishmentSwiss Chapter of IEEE 2008

Thomas Kunz / Dr. A. SchweryGrimsel, 11th of June 2008

IEEE PES Swiss Chapter - 11.06.2008- P 2

Content

• Challenges in Refurbishment

• Stator Winding Diagnosis

• Development of Insulation System

• Typical Scope / Case studies

• Introduction


• Investments in renewable energy sources become more and more interesting

Introduction

• Increaser in Energy consumption

• Increase in Oil and Gas price

• Opposition against Nuclear Power

• Climate change− CO2 emission reduction

Political and economical influence factors


• Old Hydro fleet− Few investments over the

last decades

• Market liberalization− Unbundling− More fluctuation in electricity

price− Change in operation mode of

power stations (intermittent operation)

• Hydro – Refurbishment, a topic in Europe

20061998 2002

Introduction

Political and economical influence factors

~150 Days


Large New Mini-hydroRefurbishment

Introduction

Market, Average volume 2007-11


Definition: Refurbishment, Upgrade, Repair

Time

Customer Value

Repair

RefurbishmentMaintenance

Upgrade / Uprate

Introduction


Content





• Introduction


Risks and Difficulties

• Missing or incomplete information− Incomplete set of OEM drawings

available− Not reliable or incomplete

measurements− Unknown material properties

• Old meets new− Changes in design

• Competitor design – own design− Different basic assumptions

• Physical laws can not be changed by a specification

Challenges in Refurbishment


• Data collection− OEM drawings− Measurement reports− Visual inspection

• Electrical re-computation (base line)− Adjustment of experience factors− Special computations

• New electrical design− Efficiency increase− Temperature levels

Addressing the Challenges - Engineering Process



• Check ventilation design − Avoid pressure drops in inactive

zones− Adjust volume flow to desired

temperature level• Mechanical validation

− Pole claws− Shaft flanges− Damper ring connections− …


Addressing the Challenges - Engineering Process


Content





• Introduction


New stator winding

• Build in a new winding in an existing stator core

• Same slot dimensions

• Improved insulation system− improved insulation material− increased filling factor

• Possibility to increase output power or reduction of copper losses

Typical scope / Case studies


New stator core and winding• Improved properties of steel

laminations− Reduced iron losses

• Improved core pressing system− Avoidance of buckling



New stator core and winding

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018-1.5

-1

-0.5

0

0.5

1

1.5x 104 line to line voltage (original and reconstructed signal) in [V] versus time in [s]

0 5 10 15 20 25 30 35 400

0.5

1

1.5fourier series coefficients in [%] thf factor in % : 1.8752

• Change of number of slots− Reduction of parasitic forces− Improved magnetic / thermal

utilisation balance− Checking of THF factor



Ventilation calculation of existing machine



• Ventilation network analyses:− Even Volume distribution on

stator core

• Coupled Temperature calculation− Uneven preheating of

cooling air axial direction




• Improved air distribution− Taking into consideration

the preheating of the cooling air

• Homogeneous temperature distribution on the active parts

• Systematic analyses of the machine is the base for the definition of an improved air flow distribution




• Located in Norway near the Glomfjord

• First operation in 1993

• Vertical shaft driven by a Francis turbine

• Stator and rotor winding directly water-cooled

• 410 MVA / 333 rpm

• 21 kV / 50 Hz



• Winter 2006− Several consecutive short-

circuits on the high voltage line

• Damage in August 2006− Smoke detection sensors

tripped the generator− Overheating of end

laminations

• Stator winding− Due to water-cooling only the

outer corona protection of some bars was damaged


Damages


• Repair of the old machine

• Computations on the stator core end laminations:− Magnetic field

distribution with 3D-FEM− Power losses

(analytical)− Temperatures (network)

PTooth

θAir

θCore

RAir

RCore

θYokeRYoke

θBar

RBar

PTooth

θAir

θCore

RAir

RCore

θYokeRYoke

θBar

RBar


Actions

• Minimise standstill time: Repair completed in 95 days


• Optimisation of the design:− Deeper core end stepping in radial direction− Deeper core end slitting with 3 slits per tooth

• Systematic analyses of the failure is the base for the definition of an improved new stator core end design

New design

24 mm 50 mm

Old design

h

50 mm

70 mm



• New Stator core and winding• New stator frame• New axial fans

− Cooling air volume reduced from 46 m3/s to 32 m3/s

− Reduced air friction and ventilation losses from 340 kW to 260 kW (measured)

• Considerable potential improvement in Ventilation losses for high speed machines

Witznau (Germany), 2 motor – generators 75 MW



• New Stator frame, core and winding

• Stator in two parts manufactured in workshop in Birr

• 60 MVA/ 13.8 kV/428.6 rpm

• 10 % apparent power increase

• Minimization of outage time

Soazza (Switzerland), 60 MVA



• Consistent and reliable set of data− Communication between

Customer and Supplier− Complete set of OEM drawings− Experienced engineers

• Short outage time− Assembly in workshop if possible− Optimized project schedule

• Quality− Calculations / Design− Manufacturing / Site− Project Management

Reliable data limits the customer risk

Refurbishment, key factors for success


Content




• Introduction



Stresses on the electrical insulation system

Thermal winding temperature, thermal cycling, etc.

Electrical grid over-voltage, discharges in voids, etc.

Ambient humidity, oil, carbon dust, etc.

Mechanical vibrations, electromagnetic forces, etc.

Change of insulating characteristic (ageing)Periodic condition assessments of the stator winding ensure a high reliability of the generating unit

During operation the stator winding is subjected to TEAM stresses

Stator Winding Diagnosis


Purpose and objectives

• Failures on the stator (particularly on the winding) cause more than 40% of the outage time

• Repair work is very time consuming especially in case of an unplanned outage


05

1015

20

2530

3540

45

Stator Rotor Bearings Excitation Components

Unavailability hours due to FaultsFrequency of faults

Failure on different components in 250 ENEL Hydro generators with an Output >20 MVA


Purpose and objectives

• Guideline values for downtime after insulation break-down− Dismantling of a pole: 2 days− Dismantling of the rotor: 1-2 week− Remove and installation of top

stator bar: 3-4 days− Remove of bottom bars: 2-3 weeks

(several bars necessary, all spare parts e.g. caps, corona protection, wedging, etc. available)



Standard test program


• Visual inspection• Charging and discharging current• Leakage current• Dielectric loss factor and

capacitance• Partial discharge• High Voltage


Test of stator slot wedging system


• The slot wedges loose their tightness during operation depending on

– Wedging system– Insulation– Operation condition

• Potential winding damages due to loose wedging system

– Vibration of stator bars leads to partial discharges between the stator winding and the stator core

– Deterioration of slot corona protection– Increased risk of stator earth fault


Interval for condition assessments


• 5 years (standard interval)– Stator winding – Wedging system

• Individual planning– Operation conditions – Current status of ageing– Design features– Environment– …


Strength properties on removed bars (example)


VET 26 kV Start 600 hours 1000 hours 1200 hours

(followed by VET 30 kV)

• Tanδ, real screen• Tapping diagram• Light-Out-Test• Currents on winding

overhang

• Tanδ, real screen• Tapping diagram• Light-Out-Test• Current on winding

overhang• PD measurement

• Tanδ, real screen• Tapping diagram• Light-Out-Test• Current on winding

overhang• PD measurement• Delamination

• Test samples– 2 aged stator bars (dismantled) – 2 spare bars without operation


Results of individual measurements


• Valuable information– The operation condition and reliability– Potential defects and weaknesses

• Further advantages for thepower producer

– Individual planning of overhaul work– Solid base for investment planning– Ensure safe operation and high reliability– Support Preventive Maintenance


Closed feedback loop in development of insulation system


Diagnostic methods;PD measurements

Endurance testing(Temperature T, el. Field E)

Endurance testing (T,E)

Material evaluation

System tests“Standard bar”

Prototype element 1:1(form wound coil, bar)

Complete machine(Manufacturing)

Endurance testingProperty testing:mech., electr., therm.

Customer feedbackSite feedback


Life time assessment


…the combination of both competences…


Life time assessment


• Customer benefits− Reliable base for planning of

investment− Risk analysis− Optimum use of existing equipment

Intervention is needed if the probability of failure is increasing super proportionally


Life time prediction


• Prerequisite for lifetime prediction− Design-, system and material know-how− Diagnosis data base with integrated

expert knowledge (historical data)− Experience about combination of

ageing factors− Test facilities, statistical methods and

optimized diagnosis strategies

Information from OEM and operation


Summary


• Powerful diagnostic methods in order to evaluate the condition of the insulation are available.

• Optimised diagnosis strategies allow to maximise the availability and reliability of the generating unit.

• So far the lifetime can not be predicted whereas the the probability for a failure can be calculated.

• Alstom continuously develop their products for winding diagnosis towards life time prediction


Content



• Introduction


• Development of Main Insulation System


Development of Main Insulation System

New requirements and challenges

• New market drivers− Government incentives, CO2 credits− Increase of primary energy price− Installed and planned capacity of

renewable energy plants (e.g. wind)

• New product requirements− Products offering grid stability

and higher flexibility

New operation conditions (e.g. cycling of machines)Higher loss evaluation


• New market requirements− Thermal cycling up to 155°C

according to IEEE 1310-1996− Voltage endurance test at 110°C

(US, Canada)

• Performance− Increased field strength− Corona protection system− Stator slot wedging system

Driver for product development (extract)



• Development in two main steps− Thermal cycling up to 40 - 130°C− Thermal cycling up to 40 - 155°C

• Basic investigations− Bonding between insulation and copper− Failure mechanism and cycling criteria− Impact of manufacturing process parameters− Life endurance test after cycling

Thermal cycling of stator winding



• Discussion and interpretation of cycling criteria− Higher probability to fail on tip-up criteria for an insulation with a

very flat tan delta curve (typically high quality systems)

− Bars that failed on TC have pass standard VET after tests

Thermal cycling of stator winding

0

5

10

15

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.

VOLTAGE (kV)

Tg D

ELTA

x 1

0(-3

)

Initial test

Customer Req. 60% of bars

Max values for 100% of bars

After 500

Tip-

up 0

cyc

les

All bars must be below this line

60% of bars must be below this line

Start point: max: 15%o, average: 10%o

Tip-

up 5

00 c

ycle

s



Summary

• New operation condition for the generation units drives new requirements for the product specifications

• Alstom continuously develop the core technologies towards the new market requirements

• In order to evaluate new design criteria further a close cooperation between utilities, suppliers and technical consultants is needed


www.alstom.com

Documents

Thomas Kunz / Dr. A. Schwery - IEEE.CH · PDF fileThomas Kunz / Dr. A. Schwery. Grimsel, 11. th. ... −Manufacturing / Site −Project Management ... stator bar: 3-4 days