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Extended requirements on turbo-generators due to changed operational regimes
Matthias Baca, Siemens AG, Mülheim/Ruhr, Germany
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Page 2 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
Table of Content
• Evaluation of current operation regimes
• Extended requirements on turbo-generators Fast active & reactive load changes Load ramps Under-excitation Over-voltage
• Possible Solutions and Mitigations
• Conclusions
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Variable and specific operation stresses for generators of the same class
Increased demand on highly flexible load operation of conventional power plants
Evaluation of Operation Regimes Air Cooled Generators, 300 MVA Class
Worldwide disposition of the generators in the 50 Hz market Detailed evaluation from commissioning up to 2014 Strong dependency on renewable share and grid connection Increasingly frequent permanent load fluctuations
active power
reac
tive
pow
er
Summarized load capability diagram of the investigated generator fleet with relative frequency of operation points in % of all units
33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
9 8 7 6 5 4 3 2 1
Gen
erat
or N
r.
Distribution of reactive power operation of all 33 units over-excitation under-excitation
mean 20% mean 80%
Relative frequency of operation point (P, Q) [%]
>
• High number of start-stop cycles • Operation in whole released capability range • High share of reactive power for grid stabilization • Full use of under-excitation capability because of capacitive grid demands
One specific generator
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Extended requirements on turbo-generators Overview
Increased requirements Physical / technical challenges
Expected strain in respect to cooling method
Generator components Indirectly cooled
Directly cooled
Fast active & reactive load changes
High thermomechanical tension at windings
Main bushings of stator winding Mid Low
Carbon brushes and slip rings of static excitation Low Low
Stator core end zones (stepped teeth) Mid Low
Stator winding, especially overhangs High Low
Rotor winding, especially end-windings covered by retaining rings High Mid
Load ramps up to 24 % of rated MW / min Thermal cycling
Complete stator winding High Low
Complete rotor winding High Low
Under-excitation High magnetic flux in end region
End teeth, press finger, press plate High Mid
Stator winding in stepped core area High Low
Over-voltage High magnetic flux density
Stacking beams at stator core back High High
Rotor winding High Mid
Stator core insulation Low Low
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Fast active & reactive load changes, ramps Thermo-mechanical stress on the stator winding insulation system
T T
Copper conductor
Insulation
Stator winding bar
Generic cyclic thermo-mechanical loading
Positive load change + ΔP, ΔQ
Copper condctor
Insulation
Negative load change - ΔP, ΔQ
Steepness of current ramp
Rel
. occ
urre
nce
of c
urre
nt ra
mp
ΔI R
ST/Δ
t [%
]
Generator Nr.1: High amount of steep
current ramps
ΔP in MW or ΔQ in Mvar alteration of stator current ΔIRST alteration of stator
winding losses (ΔPV ~ ΔIRST2) change of stator winding temperature (ΔT ~ ΔPV)
Physical effect: Thermo mechanical stresses on the insulation system due to • Different thermal expansion coefficients of copper, insulation and steel • Different temperature levels
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Fast active & reactive load changes, ramps Detailed evaluation of thermo-mechanical stress on the stator winding insulation system
• Individual modeling of stator bar design including copper
conductor, insulation sleeve and interface • Challenging effort of „large“ end winding geometry compared
to thin/tiny insulation sleeve geometry • Detailed knowledge about temperature dependent mechanical
properties of insulation materials • Validation by strain and deflection measurements in operation
behaviour, continous calibration of design tools
stat
or c
ore
stat
or c
ore
High thermo-mechanical stress at first bend
Detailed assessment of highly stresses areas during load transients
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Fast active & reactive load changes, ramps Indirect cooled stator winding, inner/outer corona protection
Designed shear planes (ICP/OCP) reduce thermo mechanical stresses on groundwall insulation
Design characteristics of GVPI insulation system
Copper strands
Verification of designed shear plane (ICP) by
detailed material tests
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Page 8 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
Over-Voltage / Under-excitation / Start-stop cycles Stator Core, Generator Rotor
All requirements must be considered in the design work
Stator Core
• Risk of magnetic over-fluxing @ increased voltage and frequency fluctuation
• Capability to maintain leakage flux and circulating currents at the back of the core
• Under-excitation impact on end zone
Generator Rotor
• Mechanical integrity covered by extended analysis:
LCF (start-stop cycles)
Wider grid frequency range (natural frequencies)
Transient events
• Fast and frequent thermo cycling at the rotor winding:
Equal temperature distribution in the winding, no significant hot spots
Winding design allows fast thermal expansion and contraction of copper
Insulation materials are designed to sustain cyclic stresses for long term operation
Taken from: IEEE-PES-2012_WG8-Panel-paper_Grid Code Impact to Machine-design
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Possible Solutions and Mitigations Fast active & reactive load changes, ramps
Conventional static generator cooling system results in high variation gradient of winding temperature and thermo-mechanical stresses
Time t
Stator winding temperature, e.g. slot RTD
Generator Load
Pow
er S
, Tem
pera
ture
T
Simple Cooling Water System without active regulation
Generator Cooler
Variation of stator winding Temperature with conventional cooling system
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Fast active & reactive load changes, ramps Enhanced temperature control system
Time t
Dynamic control of cooling gas temperature with new water cooler system Reduced thermo-mechanical stress in winding materials
Controller
Generator Load
Pow
er S
, Tem
pera
ture
T
Process variable input e.g. slot RTD, warm gas
Stator winding temperature (slot RTD) with an active
operating control loop
Smoothing of temperature variation higher T level
Schematic diagram of active controlled generator cooling system
Less variation of stator winding temperature with load change
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Page 11 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
rotor
stator core r-axis
top coil
Possible Solutions and Mitigations Under-excitation / Radial flux effect
Flat stator core end region reduces “flux heating” in copper
strands in over-excited operation mode (lagging p.f.)
optimal design
optimal design
Steep stator core end region reduces heating in stepped iron in under-excited operation mode
(leading p.f.)
Indirectly cooled stator winding requires a compromise to stay within temperature limits of
• stator coil • stepped iron optimal design
meets future extended
requirements
Best design to meet extended requirements: Directly water cooled stator winding design Steep stator core end region
High magnetic flux in stepped core end
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Page 12 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
Possible Solutions and Mitigations Product life cycle philosophy, future targets
Robust Product Design Engineer toolbox Validation process Fleet experience
Power plant process Optimization
Improved process of plants Monitoring & Diagnostics Continous data assessment
Condition & Fleet experience based maintenance concept
Flexible inspection schedule Specific retrofit recommendation Probability to failure
XXX
XXX
XXX
XXX
XXX
Rotor
Stator Winding
Low
Life cycle assessment Aging of components
Risk evaluation
High Contingency risk
Dyn
amic
cou
nter
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Condition Based Maintenance Future Goal Example stator winding
Kind of loading Measurement Analysis Aging effect Thermo-mechanical loading
Stator current, Cold gas temp Static forces, strains
Debonding effects, loosening support structure
Dynamic vibration load Fiber optic vibration measurement at end windings
Dynamic forces Loosening end winding structure
Electrical field load Partial discharge Pattern comparison Degradation HV-insulation
Transients during electrical fault operation
All electrical data Short circuit forces, strains
Coil insulation at core end
High thermo-mechanical load at slot exit
1
1
Risk assessment
stator winding
Stator winding
Low High Contingency risk
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Condition Based Maintenance Future Goal Example stator winding
Kind of stressing Measurement Analysis Aging effect Thermo-mechanical stress Stator current, Cold gas temp Static forces,
strains Cracks in the HV-insulation material
Dynamic vibration load Fiber optic vibration measurement of end windings
Dynamic forces
Loosening end winding structure
Electrical field load Partial discharge Pattern comparison
Degradation HV-insulation
Transients during electrical fault operation
All electrical data Short circuit forces
Coil insulation at core end
Harmonic Stator End Winding Analysis
2
2
Risk assessment
stator winding
Stator winding
Low High Contingency risk
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Page 15 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
Condition Based Maintenance Future Goal Example stator winding
Kind of stressing Measurement Analysis Aging effect Thermo-mechanical stress Stator current, Cold gas temp Static forces,
strains Cracks in the HV-insulation material
Dynamic vibration load Fiber optic vibration measurement of end windings
Dynamic forces
Loosening end winding structure
Partial discharge tanδ0 values, Δtanδ0 rise Pattern comparison
Degradation HV-insulation and grading system
Transients during electrical fault operation
All electrical data Short circuit forces
Coil insulation at core end
Partial discharge measurement of HV winding insulation
3 Risk
assessment stator winding
Stator winding
Low High Contingency risk
3
new
aged
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Condition Based Maintenance Future Goal Example stator winding
Kind of stressing Measurement Analysis Aging effect Thermo-mechanical stress Stator current, Cold gas temp Static forces,
strains Cracks in the HV-insulation material
Dynamic vibration load Fiber optic vibration measurement of end windings
Dynamic forces
Loosening end winding structure
Electrical field load Partial discharge Pattern comparison
Degradation HV-insulation
Transients during electrical fault operation
All electrical data Short circuit forces
Coil insulation at core end 4 Transient Analysis of Fault conditions
4 Risk assessment
stator winding
Stator winding
Low High Contingency risk
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Page 17 Matthias Baca September 2015 VGB Congress AL:N; ECCN:N
New flexible grid demand has impact on whole system „generator“ with different amount of wear and aging at individual components
Changed requirements and remaining uncertainty for future increase of flexibility must be considered in the current generator development programs
Thermo-mechanical stresses on generator components require enhanced load dependent cooling technology, particularly at the stator winding
Based on new EOH calculation with load change factor (VGB R 167 – 2010) condition based maintenance is needed – new economic maintenance strategies for the generator
Extended requirements on turbo-generators Conclusions
Thank you for your Attention!
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Extended requirements on turbo-generators due to changed operational regimes Contact page
Matthias Baca Phone: +49 (208) 456 8222 Mobile: +49 (174) 1534169 E-mail: [email protected] Rheinstr. 100 45478 Mülheim an der Ruhr Germany
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Disclaimer
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