Ageing Due to Start-stop cycles and Frequent Regulation in

Lifetime Estimation of Hydro Turbines and Generators

Société Hydrotechnique de France

Enhancing Hydropower Plant Facilities

Session B1

Grenoble, April 9‒11, 2014

Authors: Voitto Kokko and Jouni Ahtiainen

Content of presentation

• Motivation of the study

• Changes in the operation mode of hydro units

• Lifetime estimation principle

• Ageing effects of start-stop cycles and regulation

• Ageing models for turbines and generators

• Experimental study for Kaplan and low-head Francis runners

– Statistical approach to expected lifetime

– Correlation of failure modes against operation and design parameters

– Experimental results from lifetime estimations using ageing models

• Experimental study for hydro generator stator windings

– Statistics from lifetime effects of start-stops for asphaltic- and epoxy-mica insulation systems

– Experimental results from lifetime estimations using ageing models

– Case study: comparison of start-stop lifetime effects between asphaltic-mica and epoxy-mica stator winding insulation systems

• Conclusions

2

Motivation to lifetime estimation studies Fortum hydropower asset portfolio management

3

Ångermanälven

Indalsälven

Ljungan Ljusnan

Dalälven

Klarälven

Kemijoki

Oulujoki

Byälven

Gullspångsälven

Norsälven

Vuoksi

• Fortum has 139 hydro power plants in

Sweden and Finland (excl. Kemijoki)

– Total 257 units

– Normal annual production 18 600 GWh

– Installed power 4 275 MW

• Large age and condition distribution

between the production units

• This kind of large hydro fleet requires

systematic procedures both for Long Time

Investment planning, Asset Management

and Operation & Maintenance

Main operation tasks of hydropower plants

• Power production for electricity

consumption

• Ancillary services for electrical

networks

– Frequency response to maintain

system frequency by automatic and

fast responses

– Fast reserve

– Reactive power services

– Black start capability

• Operation mode has moved towards

high flexibility, in some cases to

almost continuous regulation

4

P

T

Operation mode and increase of start-stop cycles

• Operation mode of hydro units

include

– Start-stop cycles

– Steady state operation

– Power regulation

• Nordic hydro production

– At mid of 1990s big change

– In rivers with large reservoir and

flexible permits the increase of

start-stops was high

– In rivers following natural flow

change was smaller

5

0

50

100

150

200

250

300

Average

number

Yearly

starts

per unit

Year

Oulujoki (18 Kaplan)

Vuoksi (6 Francis, 5 Kaplan)

5 Moy. mobile sur pér. (Oulujoki (18 Kaplan))

5 Moy. mobile sur pér. (Vuoksi (6 Francis, 5 Kaplan))

Lifetime estimation principle for hydro turbines and generators Adaptive approach using ageing models

6

End-of-lifetime

statistics

Design &

Manufacture

Maintenance

improvements

Operation history

and

operation conditions

Existing&Future operation

mode (operation hours,

start-stops and

regulations)

Condition

inspections

and

measurements

Lifetime

analysis

with

ageing models

Lifetime

adjust

with

condition

information

Remaining

lifetime

estimate

Ageing effects of start-stop sequences with hydro power turbines Fatigue of runner blades

• High stress cycles on the turbulent area under partial load (ref. CEATI study)

– Can lead to increase of fatigue stress in runner blades/vanes

– Differences between the stresses with different start-up schemes (ref. Gagnon)

• In start-up the natural frequency of blades can be reached on partial load (ref.

Björndal)

• With a low-head Francis runner found most stresses at around 50% output (ref.

Virta)

• Frequent high fatigue stress can lead to crack development/propagation (ref.

Ahtiainen)

7

Ageing effects of regulation cycles with hydro turbines Wear of Kaplan and Bulb turbine blade bearings

• With earlier designs the lifetime of runner blade bearings and blades was nearly

similar

• Nowadays upgrades and frequent regulation have often increased wear stresses

in blade bearings

• This has in some cased led to accelerated blade bearing ageing

• Need to estimate separately lifetime of runner blades and runner blade bearings

8

Ageing effects of start-stop and regulation cycles with generators

• During start-stop and power regulation sequences stator windings are subject to

– Electrical stress and

– Mechanical shear stress in the interfaces of stator bars/coils caused by thermal cycling

• Temperature increase lead expansion of winding

• Cooling lead shrinking of winding

• Longer the winding in slot, greater the expansion and shrinking

– Conclusion: during transients generator windings operate under multi-stress ageing

• Thermal cycling of stator winding cause also thermal cycling of stator core

– This can lead to following consequencies

• Permanent change of the core form leading to asymmetric forces in generator gap

and increased vibration

• Buckling and degradation of core lamination insulation

9

Ageing models for hydro turbines

• Mechanical ageing model has been used in this study to estimate ageing by start-stop and regulation cycles (1).

• Frequent high fatigue stress can lead to development and propagation of cracks in blades. The Miner law gives estimate of total number of different load cycles which construction can stand (2).

• Crack length can be estimated as a function of applied load cycles. Foreman equation present one estimation for crack growth for crack under study (3).

• Wearing model can be used to estimate blade bearing wear. Archad’s law gives relation between slide wearing and distance of slide, loading on wearing surface and sliding materials.

10

tX

KR

K

KC

dN

daq

IC

m

)1(1

ts = tr k-n (1)

(2)

(3)

Electro-thermo-mechanical ageing model for generator stator windings

• Electro-thermo-mechanical ageing

model (4) has been used in this study

to estimate lifetime consumption due

to start-stop and power regulation

cycles

11

n

rm

e

m

re

w

rsl

l

E

Ett

(4)

Experimental study of turbines – database used in this study

Status Kaplan Vertical

Francis

Horizontal

Francis

Bulb

Kaplan

Propeller Francis &

Pump

Original 88 34 8 3 8 0

Replaced 37 26 0 4 0 1

Renewals 37 26 0 4 0 1

Total 162 66 8 11 8 2

12

Expected statistical lifetimes of Kaplan runners

• 34 Kaplan runners manufactured 1936 to 1955

– two manufacturers, similar design, 7 MW to 33 MW, head 6.5 to 32 m

• Statistical lifetime analysis by fitting with the maximum likelihood estimation the

operation history of turbines to Log-normal and Weibull distribution

13

Cracks in blade failure mode Blade bearing wear-out failure mode

Log-normal Weibull Log-normal Weibull

T50 sigma Characte

ristic life

shape

factor

T50 sigma Characte

ristic life

shape

factor

79 years 0.26 82 years 6 69 years 0.18 74 years 7

Average 79 years Average 78 years Average 69 years Average 70 years

Lifetime correlation of different operation and design parameters

Blade bearing wear out failure mode of Kaplan runners

• Correlation of blade bearing wear-out failure mode against operation and design parameters was studied

• 34 Kaplan runners, in 11 of them was detected “blade bearing wear out” failure mode

• Correlation of the parameters to lifetime in operation years and operation hours

• >0.65 = correlation, 0.4-0.65 = weak correlation, <0.4 = no correlation

• concurrent correlation (+): increasing value increases lifetime

• opposite correlation (-): increasing value decreases lifetime

Lifetime Rota-

tion

speed

Dis-

charge

Yearly

start-

stops

Regula-

tion

cycles

Output Opera-

tion

time

Total

start-

stops

Specific

speed

Head

Correlation

operation

years

0.13 -0.26 -0.18 -0.30 -0.37 0.40 -0.08 0.51 -0.37

Correlation

operation

hours

-0.40 -0.32 -0.37 -0.47 -0.61 0.58 -0.45 0.78 -0.73

14

Lifetime correlation of different operation and design parameters Runner Blade crack failure mode of Kaplan runners

• Correlation of crack failure mode against operation and design parameters was studied

• 34 Kaplan runners, in 8 of them was detected “blade crack” failure mode

• Correlation of the parameters to lifetime in operation years and operation hours

• >0.65 = correlation, 0.4-0.65 = weak correlation, <0.4 = no correlation

• concurrent correlation (+): increasing value increases lifetime

• opposite correlation (-): increasing value decreases lifetime

Lifetime Rota-

tion

speed

Dis-

charge

Yearly

start-

stops

Regula-

tion

cycles

Output Opera-

tion

time

Total

start-

stops

Specific

speed

Head

Correlation

operation

years

-0.09 -0.11 -0.58 -0.02 -0.44 0.95 -0.08 0.53 -0.47

Correlation

operation

hours

0.82 -0.94 -0.4 -0.36 -0.67 0.45 -0.42 -0.29 -0.24

15

Summary correlation of different operation and design parameters Cracks in blade and blade bearing wear of Kaplan runners

• Summary of the correlations for crack and blade bearing wear-out failure modes

• Correlation of the parameters to lifetime in operation hours

• ++ = correlation, + = weak correlation

Lifetime Rota-

tion

speed

Dis-

charge

Yearly

start-

stops

Regula-

tion

cycles

Output Opera-

tion

time

Total

start-

stops

Specific

speed

Head

Correlation

crack in

blade

++ ++ + ++ + +

Correlation

blade

bearing

wear-out

+ + + + ++ ++

16

Results from lifetime estimations using ageing models

Influence of operation parameters to lifetime of Kaplan runners

• Achieved lifetime of 24 Kaplan runners with same design

• Operation mode corrected lifetime calculation

• For start-stop and regulation has been used mechanical ageing model

• Red circles: cracks in blades (2nd World War time poor material)

• Blue circles: poor generator condition and high upgrade potential lead to whole unit renewal

17

0

100000

200000

300000

400000

500000

600000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Lifetime [Operation-

mode-corrected operation

hours]

Kaplan runners

Hours of operation Start and stop cycles Regulation

Expected statistical lifetimes of low-head Francis runners

• 34 Francis runners manufactured 1928 to 1967

– several manufacturers, 5 MW to 44 MW, head 20 to 80 m

• Statistical lifetime analysis by fitting with the maximum likelihood estimation the

operation history of turbines to Log-normal and Weibull distribution

18

Crack and wear-out failure modes – Low head Francis (head 20 to 80 m)

Log-normal Weibull

T50 sigma Characteristic life Shape factor

70 years 0.24 73 years 6.8

Average 70 years Average 70 years

Correlation of different operation and design parameters “Crack or extensive wear” failure mode of low head Francis runners

• Correlation of “crack or extensive wear” failure mode against operation and design parameters was studied

• 34 Francis runners, in 10 of them was detected “crack” or “extensive wear” failure mode

• Correlation of the parameters to lifetime in operation years and operation hours

• >0.65 = correlation, 0.4-0.65 = weak correlation, <0.4 = no correlation

• concurrent correlation (+): increasing value increases lifetime

• opposite correlation (-): increasing value decreases lifetime

Lifetime Rota-

tion

speed

Dis-

charge

Yearly

start-

stops

Regula-

tion

cycles

Output Opera-

tion

time

Total

start-

stops

Specific

speed

Head

Correlation

operation

years

-0.73 0.66 -0.95 0.14 0.05 0.57 -0.60 0.68 -0.63

Correlation

operation

hours

-0.73 0.44 -0.84 -0.25 -0.38 0.91 -0.58 0.83 -0.91

19

Summary correlation of different operation and design parameters in lifetime estimation of low-head Francis runners

• Summary of the correlations for “crack of extensive wear” failure mode

• Correlation of the parameters to lifetime in operation hours

• ++ = correlation, + = weak correlation

20

Lifetime

Opera-

tion

hours

Rota-

tion

speed

Dis-

charge

Yearly

start-

stops

Regula-

tion

cycles

Output Opera-

tion

time

Total

start-

stops

Specific

speed

Head

++ ++ ++ ++ + + ++ ++

Results from lifetime estimations using ageing models

Influence of operation parameters to lifetime of Francis runners

• Achieved lifetime of 25 low-head Francis runners with same design

• Operation mode corrected lifetime calculation – significant differences between lifetimes

• For start-stop and regulation has been used mechanical ageing model

• Red circles: sister units 9-10 high upgrade, 15-16 operation to end of lifetime

• For two units with high upgrade potential has been made early renewal

• For two remaining units (totally 7 units in power plant) lifetime has been significantly extended

21

0

100000

200000

300000

400000

500000

600000

700000

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Lifetime

[operation-

mode-corrected

operation hours]

Low head Francis runners

Hours of operation Start and stop cycles Regulation

Experimental study of generator stator windings Database used in this study

22

Status Asphaltic-

mica

Shellac-

mica

Epoxy-mica Early 1930s

design

Others

Original 57 16 55 0 4

Replaced 83 14 3 14 5

Re-windings 1 0 106 0 0

Total 141 30 164 4 9

Statistics from lifetime effects of start-stop cycles Group: asphaltic-mica stator windings

• Lifetime calculation for 73 stator windings

– with capacity 8 - 70 MVA, voltage 6 -16 kV, 70% reached end-of-lifetime

• Electro-thermo-mechanical ageing model used for estimations

• Average value was 9.4 h/start-stop

• Differences between units -> estimate windings separately

23

0

2

4

6

8

10

12

14

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Number of

asphaltic-mica

stator widnings

Lifetime consumption (h/start and stop cycle)

Statistics from lifetime effect of start-stop cycles Group: epoxy-mica stator windings

• Lifetime calculation for 90 stator windings

– with capacity 8 - 135 MVA and voltage 6 - 20 kV

• Electro-mechanical ageing model was used for estimations

• Average value was 4.2 h/start-stop

• Differences between units -> estimate windings separately

24

0

5

10

15

20

25

30

< 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Number of

epoxy-mica

stator windings

Lifetime consumption (h/start and stop cycle)

Results from lifetime estimations using ageing models Influence of operation parameters to asphaltic-mica stator windings

• Achieved lifetimes in ”Operation-mode-corrected operation hours”

• Multi-turn asphaltic-mica stator windings (1-36) and single-bar windings (37-49)

• In many cases lifetime effect of start-stop cycles is significant

• Multi-turn windings have shorter average lifetime

25

0

100000

200000

300000

400000

500000

600000

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49

Lifetime

[Operation-

mode-corrected

operation

hours

Stator windings

Hours of operation Starts and stops Regulation Electro-thermal ageing

Case study: Comparison of start-stop effects between asphaltic-mica and epoxy-mica windings - peak load generator

• With operation-mode-corrected method estimated for re-winded generator stator winding

– Reached lifetime of original asphaltic-mica winding

– Estimated total lifetime for a new epoxy-mica winding, been in operation 23 years

• Lifetime of new winding will be much longer than original winding reached

• With condition survey was verified the result for new winding

• Reached/estimated lifetimes in years

– Asphaltic-mica winding 34 years

– Epoxy-mica winding 72 years

26

0

50000

100000

150000

200000

250000

300000

350000

400000

450000

Reached

lifetime

original

asphaltic-mica

Estimated

total lifetime

re-winding

epoxy-mica

Lifetime

(h)

Stator winding

Operation hours Ageing start-stop cycles

Ageing regulation cycles Electro-thermal ageing

Conclusions

• Operation mode affects especially to lifetime of Kaplan runner blade bearings

• For various turbine types (Kaplan, Francis) the specific factors are needed for

estimation of ageing effects

• For various generator winding insulation types the specific factors are needed for

estimation of ageing effects

• It is recommended that the operation mode is taken into account in lifetime

estimation of hydro turbines and generators

• Significant differences between the lifetime effects of start-stops with various

generator stator winding insulation types

27

28

Thank you for your attention! Merci pour votre attention!