B07 Extended requirements on turbo-generators due to ... · PDF file9/19/2015 ·...

Unrestricted © Siemens AG 2015 All rights reserved. siemens.com

Extended requirements on turbo-generators due to changed operational regimes

Matthias Baca, Siemens AG, Mülheim/Ruhr, Germany

Unrestricted © Siemens AG 2015 All rights reserved.

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



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



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



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



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



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



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



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



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



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



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



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




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


All electrical data Short circuit forces


Harmonic Stator End Winding Analysis

2

2

Risk assessment

stator winding

Stator winding








Dynamic forces


Partial discharge tanδ0 values, Δtanδ0 rise Pattern comparison

Degradation HV-insulation and grading system




Partial discharge measurement of HV winding insulation

3 Risk

assessment stator winding

Stator winding


3

new

aged







Dynamic forces


Electrical field load Partial discharge Pattern comparison

Degradation HV-insulation



Coil insulation at core end 4 Transient Analysis of Fault conditions

4 Risk assessment

stator winding

Stator winding




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!



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

mailto:[email protected]



Disclaimer

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