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VGB PowerTech 6 l 2016 Changed operating regimes impact on turbo-generators

Authors

Kurzfassung

Erweiterte Anforderungen an Turbogeneratoren aufgrund eines veränderten Betriebsregimes

Weltweit ist ein ungewöhnlich schnell stattfin-dender Wechsel der Betriebsweise vom klassi-schen Grundlastkraftwerk zum höchst flexib-len Spitzenlastkraftwerk zu erkennen. Neben den geänderten Anforderungen an die Turbinen führen diese Randbedingungen dazu, dass sich Turbogeneratoren nicht mehr im thermisch ein-geschwungenen stationären Zustand befinden. Die aktuellen, hoch volatilen Randbedingungen des elektrischen Verbundnetzes – bedingt durch die Einspeisung erneuerbarer Energien – spie-geln sich insbesondere sowohl durch die hohe Anzahl an Start-Stopp-Zyklen als auch durch häufig auftretende und steile Lastwechsel von Gas- und Dampfturbosätzen wider.Aus dem täglichen Aufwärmen und Abkühlen (Start-Stopp-Zyklus) oder häufigen Ändern des Leistungspunktes (hohe Anzahl an steilen Last-gradienten, vermehrter Schwach- und Teillast-betrieb des Generators) resultieren auch neue Anforderungen an die Komponenten. Infolge der unterschiedlichen thermischen Ausdeh-nungskoeffizienten der verbauten Komponen-ten ergeben sich zyklische thermo-mechanisch getriebene Dehnungen, die zu einer beschleu-nigten Alterung der Komponenten im Genera-tor führen können. Bleiben diese Belastungen bei Auslegung und Konstruktion der jeweiligen Komponenten unberücksichtigt, kann es zu un-vorhergesehenen, langen und kostentensiven Stillstandszeiten kommen. Der vorliegende Beitrag stellt aktuelle Analysen der neuen Betriebsweisen und deren Auswir-kung auf die Turbogeneratoren, welche am häu-figsten in gasbefeuerten Turbosätzen eingesetzt werden, vor und beschreibt Lösungs ansätze für einen sicheren und kostenoptimierten Betrieb der Anlage mit Blick auf die aktuellen Entwick-lungen des Energiemarktes. l

Extended requirements on turbo-generators due to changed operational regimesMatthias Baca and Ana Joswig

Matthias BacaDr. Ana JoswigSiemens AG Power and Gas Mülheim a.d. Ruhr, Germany

Introduction

The integration of renewable energy sourc-es with preferential feed into the high-volt-age transmission system results in funda-mental changes in the load regime of the European grid. Conventional power plants will be signifi-cantly displaced from the market or used as reserve capacity for when the renew-able energy plants are unable to generate the total output required in the grid. In addition to grid operation requirements, the operating regime of the conventional power plants required to cover the supply gap is based especially on aspects of cost-effectiveness under consideration of the borderline implementation costs of the re-spective power plant. F i g u r e 1 shows an example how wind and solar energy contribute to the total generating capacity in the spring and win-ter over a period of one week in March and December 2014 [1]. During sunny weather in March, photovoltaics (solar – yellow) provide roughly ¼ of the required energy at midday, while hard coal power plants (HC – black) provide the regulating power reserve. With the change in weather over the weekend (Sat – Sun), wind power (gray) provides up to nearly 50  % of the power demand, and even nuclear power plants (Uranium – red) have to reduce their output. Most of the hard coal power plants are disconnected from the grid at this point. The situation for electric power generation is completely different in the se-lected week in December. No wind or solar energy are available from Tue to Sat due to the weather situation. The conventional coal- and gas-fired power plants now have to weigh in and close the supply gap.The contribution to electric power produc-tion from renewable energy sources (es-pecially wind power and photovoltaics) is increasing continuously in Germany. This gives rise to a large dependency on the weather and results in permanent fluc-tuations in the utilisation of conventional power plants, with the consequence of an extremely volatile operating mode at times. The number of start-up and shutdown cy-cles for peak load power plants increases significantly. Power plants remain in turn-ing gear operation more frequently and for

longer periods, complete shutdown times at 0 rpm are also increased. Load changes in operation are more frequent, and output gradients significantly steeper (e.g. due to unpredictable changes in weather and pri-ority of renewable energy sources). Over-all utilisation of the power plants is only partial or very low, oftentimes resulting in uneconomical operation. The evaluation of power plant operating data has revealed this special operating mode not only in Germany but also throughout Europe [11].

This energy market trend is highlighted by an evaluation of the operating mode of 32 power plant units in southwestern Europe. The evaluation shows the change from base load and lower medium load to peak load operation from 100 to 300 h/start to 10 to 70 h/start in many gas-fired power plants within only 4 years. Most new power plants are operated in peak load or upper medium load from the very first commissioning [2].

The conventional power plants contribute more and more to grid stabilization and power reserve. The new operational mode results in higher wear of the power plant components.

It is absolutely necessary to adjust the de-sign and maintenance of the component such as generators in power plants to this changed operating mode.

New grid demands and evaluation of operating regimes

The operating modes of conventional power plants and their generators can dif-fer greatly, even if the power plants are of the same type and output range.

The operating modes of 33 plants world-wide with generators of the same type (in-directly air cooled generators in the 300 MVA class) were analysed in a detailed investigation. The explored generators cover all operating modes from base-load to peak-load power plants ( F i g u r e 2 ) .

In addition to a daily start/stop cycle, it is clearly evident that the generator was not in a steady-state thermal condition in load operation, as the power plant frequently changed between full, part and minimum load.

The frequent and steep active power gra-dients form a further important charac-

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Changed operating regimes impact on turbo-generators VGB PowerTech 6 l 2016

teristic. These gradients are necessary to compensate the weather- and power de-mand related fluctuations. Reactive power input is similarly volatile. This provides the necessary voltage stabilisation in the grid, as renewable energy sources can gen-erally only provide limited grid support.

F i g u r e 3 shows a compact representation of the load points and gradients for one specific generator during entire operation lifetime. The left-hand image shows the relative frequency of the operating points in the capability curve. The broad scatter clearly reflects the operating modes de-scribed above:

– Operation in whole released capability range

– High share of reactive power for grid sta-bilisation

– Full use of under-excitation capability because of capacitive grid demand

Legend: Hydro Biomass Uranium Lignite Hard Coal Gas Pumped storage Wind Solar

Day

MW MW

70,00060,00050,00040,00030,00020,00010,000

70,00060,00050,00040,00030,00020,00010,000

Mo10.03.

Tu11.03.

We12.03.

Th13.03.

Fr14.03.

Sa15.03.

Su16.03.

Mo15.12.

Tu16.12.

We17.12.

Th18.12.

Fr19.12.

Sa20.12.

Su21.12.

Actual production December 2014Actual production March 2014

Day

Fig. 1. Weather-dependent fluctuations in the contribution to electric power generation from wind and solar plants (Fraunhofer Institute for Solar Energy Systems (ISE) Freiburg, 2015-01-07).

Time in h

Mea

sure

d va

lues

in %

Mea

sure

d va

lues

in %

Time in h

120110100

908070605040302010

0-10-20-30

120

110

100

90

80

70

60

50

40

30

20

10

0

-10

-20

-30

Active power PReactive power Q

Active power PReactive power Q

n [%]P [%]Q [%]TWG [oC]

n [%]P [%]Q [%]TWG [oC]

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 0 24 48 72 96 120 144 168

Fig. 2. Typical operating profile for a power plant in peak load operation (2 weeks period on the left, expanded view for 1st week on the right) [3]).

Relativefrequency of operation point (P, Q) in % Relative frequency of active and reactive power gradients in %

P in MW

Q in

Mva

r

DQ

/Dt i

n M

var/

min

DP/Dt in MW/min

200

150

100

50

0

-50

-100

>= 1.0 %

0.75 %

0.5 %

0.25 %

> 0.0 %

30

20

10

0

-10

-20

-30

>= 0.50 %

0.38 %

0.25 %

0.13 %

> 0.0 %0 50 100 150 200 250 300 -30 -20 -10 0 10 20 30

Fig. 3. Distribution of load points and load gradients for a generator with highly volatile utilisation [3].

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VGB PowerTech 6 l 2016 Changed operating regimes impact on turbo-generators

In addition to the load points, it is extreme-ly important to examine in detail what are the gradients of the load changes.

The image on the right in Figure 3 shows the relative frequency of the generator load gradient, distinguished between ac-tive and reactive load gradients, over the entire operating period. The broad scatter out to higher amplitudes reflects the high demand from the grid for generator flex-ibility.

New grid demands and effects on generator components

The additional demands on power plants resulting from the volatile grid demand are known and are accounted for in the current specifications of the European Network Code. The relevant specifications for be-haviour of the power generation units con-nected to the grid are especially defined in the Network Code “Requirements for Grid Connection applicable to all Generators” (NC RfG) [6]) and are further explained in a corresponding application guideline [7]. The determining requirements for the power plant/grid interface are significantly

more stringent in comparison with the pre-viously applicable regulations in NC RfG, and in part exceed the degree regarded as reasonable and necessary from the stand-point of the utilities.Major impact on generator components:

– High number of start-stop cycles – Operation in whole released capability

range with fast load changes – High share of reactive power for grid

stabilization incl. under-excitation capa-bility

– Wide grid frequency range – Wide voltage range

The degree of the impact is strongly related to the cooling system of the components as illustrated in Ta b l e 1 .Every change in active or reactive power leads to a change of stator current, which in turn influences the copper losses and also the temperature in the stator wind-ing. Due to different thermal expansion co efficients of the stator components (cop-per, insulation and steel), thermo-mechan-ical stresses will occur during every change of power. An evaluation of a generator fleet has shown that both relative load gradients and their occurrence will increase. It is ex-pected that this trend will continue in the future ( F i g u r e 4 ).

The generators have to withstand a “more exhausting” type of operation. These boundary conditions have to be considered in the future generator design.

In addition to the dynamic effects occur-ring during sudden load changes, the ther-mo-mechanical loads acting on the end winding structure also call for attention. Extensive finite element calculations are necessary to determine the detailed effects on the respective types of generators and end windings.

The extremely high degree of detail is nec-essary and requires superior modelling and calculation strategies (e.g. 3D FEM calcu-lation) to determine the transiently occur-ring maximum stresses in various compo-nents, particularly the insulation system on a stator bar.

The stator winding insulation system has to be extremely robust and ensure, that local thermo-mechanical delamination ef-fects have no negative influences on stator winding life time. Siemens GVPI Micalastic system fulfills this requirement by imple-mented Inner Corona Protection design (ICP) between copper strands and main insulation and by double layer Outer Coro-na Protection design (OCP) between main insulation and laminated core. Both inter-faces are free of electrical stress, which avoids the occurrence of partial discharges ( F i g u r e 5 ).

Favourable designs for highly volatile grid operation will need additional mitigation for further stress reduction and therefore require water-cooled stator bars.

By these measures thermo mechanical stresses are decreased significantly and the consequential impact on lifetime consump-tion is minimised.

Further solutions and mitigations

Several measures can be employed to ad-dress the new operating conditions for generators which have been in service for many years as well as for new apparatus generators:

– Online and offline monitoring with life-time consumption assessment

– Smarter inspection intervals, condition based maintenance

Tab. 1. Effects of new grid demands on generators [3].

Increased requirements

Physical/technical challenges

Expected stress based on cooling method

Generator components Indirectly cooled

Directly cooled

Fast active & reactive load changes

High thermo- mechanical stress

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- turns 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, clamping fingers, pressure plates High Mid

Overvoltage High magnetic flux density

Stator winding in stepped core area High Low

Dovetail bars at stator core back High High

Rotor winding High Mid

Stator core insulation Low Low

Steepness of current ramp ∆lRST/∆t

Rel.

occu

rren

ce o

f cur

rent

ra

mp

∆l RS

T/∆

t in

%

Generator Nr. 1: High amount of steep

current ramps

2.25

2

1.75

1.5

1.25

1

0.75

0.5

0.25

0 50 100 150 200

Fig. 4. Relative frequency and steepness of current ramps in stator for various units.

Siemens Competitor

Electrical stress

Thermo-mechanicalstress

Electrical discharges

Core

CoreResin

Double layer OCP

Main insulationICP

Copper

OCP

Main insulation

Copper

Fig. 5. Design of stress-resistant insulation system.

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Changed operating regimes impact on turbo-generators VGB PowerTech 6 l 2016

– Dynamic and active control of generator cooling system to reduce thermo-me-chanical stress

– Improved design of components and use of special features

– Advanced strategic spare parts planning – Specific generator design for flexible

grid demands and extreme peak load operation.

Dynamic and active control of cooling gas temperatureThe frequent load changes and start/stop cycles result in continuous thermo-mechanical stresses. Condition-oriented temperature control for the cold gas tem-perature in the generator is a means to counteract the actual cause of these ther-mo-mechanical stresses. The purpose of a cold gas temperature control of this type is to significantly reduce the temperature changes in the individual generator com-ponents due to load changes by controlling the cold gas temperature based on condi-tion (e.g. as a function of apparent power output). F i g u r e 6 shows a schematic RTD temperature of a non-regulated system (dashed line) which can be converted to a system with condition-oriented dynamic control through (solid line) the implemen-tation of new hardware.By its use during park load/turning gear operation, the cool-down times are ex-tended and the most severe thermal shift which occurs between ambient and hot condition is greatly reduced if the plant is cycling within reasonable periods of time.The development of an adaptive, dynamic control of the generator cooling system provides the following advantages:

– Improved thermal conditions of the gen-erator as a thermo-mechanically sensi-tive system

– Seamless integration in the power plant as an overall system

– Individual solution possibilities for exist-ing plants

Robust design of componentsDuring the analysis of the operation data of the generators it could be seen that the power plants will be operating more and more for the grid support and stabi-lisation. To withstand the higher thermal strain due the higher magnetic flux on the stepped core end during under excitation an improvement of its design is essential. The steeper design of the core end as for the indirect cooled stator winding normal-ly used can reduce the thermal stress in the core significantly during under excited operation. Yet at the same time it would

increase the losses (radial field losses) and the temperature in the stator winding when the generator operates in the over excited mode.

To remain within allowed temperature lim-its in the generator components it is recom-mended to switch from indirect to direct water-cooled stator winding. This design solution causes also the temperature relief in the whole winding not only in the end region. The direct water-cooled stator winding and steep stator core end region validated by long-term fleet experience is the best design solution to meet extended require-ments.

Product life cycle philosophy and future targetsA condition-based maintenance, refurbish-ment and replacement strategy is the key success factor to maximise the customer value and to reduce the forced outage risk for existing generators. The first step should be an economic decision-making process which evaluates operation and maintenance costs versus risks of loss of availability and efficiency of the generator. Siemens’ vision: based on multiple infor-mation and evaluation routines the utility could start a detailed residual life assess-ment together with the OEM to prepare the decision for possible lifetime extension of the generator. The future goal is to develop and built a monitoring system, that can be installed in the power plant to enable condition-based maintenance. This system should give a prediction on how the service life is reduced till now, which fatigue occurred, and at which point in time the generator should be serviced. The prediction is based on load characteristics in the past and on expected load scenarios in the future. Also

Stator winding temperature(slot RTD) with an activeoperating control loop

Generator load

Reduction oftemperature variation

Pow

er S

, Tem

pera

ture

T

Fig. 6. Example of an active/intelligent temperature control in the power plant.

CE - Bottom coils cenitroids (1/4 Basket)

Frequency in Hz

Ampl

itude

in m

m

450

400

350

300

250

200

150

100

50

020 40 60 80 100 120 140 160 180

Mid basket 2 lope

2 Lope

Torsion

Cantilever

Fig. 7. Natural frequency spectrum of a stator end winding structure.

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VGB PowerTech 6 l 2016 Changed operating regimes impact on turbo-generators

a risk prediction respectively risk rating for the generator components should be given by this system. The necessary evaluation data will be ob-tained during operation as well as dur-ing regular inspections. For example the possible changes in structural dynamics of stator end windings can be assessed by experimental modal analysis. The natural frequency spectra (F i g u r e 7) can be used for trending of on stator end winding’s be-haviour over time as well as comparison within a comparable fleet.Fiber optic vibration measurement of sta-tor end windings can be used to directly monitor online vibrations during opera-tion. Such data can be correlated for fur-ther analysis with operating parameters such as currents, temperatures, power factor etc. The combination of thermal, mechanical and electrical loading may induce degradation of the high-voltage insulation system. Trending of such degra-dation over the lifetime of a generator can be performed i.e. by means of recurring dissipation factor and partial discharge measurements. F i g u r e 8 shows the gen-eral difference between the dissipation fac-tor curves of a new and an aged insulation system. The increase in dissipation factor at low voltage indicates increased losses of thermally aged resin system. The tip up at higher voltages indicates delamination voids within the main insulation, which are not present in new generators.

Conclusion

To summarise the changes in grid require-ments and their impact on future generator design and service, it can be stated that:

– The new flexible grid demand has sig-nificant impact on the generator as an overall system with aging acceleration for individual components

– Changed requirements and remaining uncertainty for the future increase in flexibility must be considered in current generator development programmes

Accordingly mitigation measures have been evaluated and proposed:

– Thermo mechanical stresses on genera-tor components require enhanced load dependent cooling technology, particu-larly at the stator winding

– Special generator design features such as water cooled stator windings or modi-fied core end zone shapes help to adapt to flexible operation regimes

– Special online monitoring systems and offline maintenance tools are needed for economical inspection decision for gen-erator components and as early warning systems for weak points

– A refurbishment and replacement strat-egy is needed too to reduce the forced outage risk for older generators

References[ 1] Burger, B.: Electricity production from solar

and wind energy in Germany 2014. Fraun-hofer Institute for Solar Energy Systems ISE, Freiburg Dec 2014.

[ 2] Joswig, A., Krol, T., and Weidner, J.R.: Auswirkungen der neuen flexibleren Netz-anforderungen auf die zukünftige Be-triebsbeanspruchung der Turbogenerato-ren. (VGB KELI 2014 and VGB PowerTech 8/2015).

[ 3] Joswig, A., Steins, H., and Weidner, J.R.: Impact of new flexible load operation and grid codes on turbine generators focused on the end winding. (PowerGen 2015 Amster-dam).

[ 4] Joswig, A., Steins, H., and Weidner, J.R.: Auswirkung der Einspeisung von regenera-tiven Energien auf den Betrieb und die Le-bensdauer konventioneller Kraftwerke und mögliche Lösungen. (8. Essener Tagung “Turbogeneratoren in Kraftwerken” Feb. 2015).

[ 5] Mayor, K., Montgomery, L., Hattori, K., and Yagielski, J.: Grid Code Impact on Elec-trical Machine Design. (IEEE PES 2012).

[ 6] ENTSO-E Network Code for Requirements for Grid Connection Applicable to all Gener-ators, published by ENTSO-E AISBL (Brus-sels March 2013, Belgium).

[ 7] Implementation Guideline for Network Code “Requirements for Grid Connection Applicable to all Generators” (published by ENTSO-E AISBL, Brussels October 2013, Belgium).

[ 8] Geraerds, T.: The new European Network Codes on Electricity and their technical im-pact on our power plants. (VGB KELI 2014, Landshut, Germany).

[ 9] Eurelectric und VGB: Draft ENTSO-E Net-work Codes on Requirements for Genera-tors, Meeting with DNV KEMA (Arnhem 23 April 2013).

[11] Muntz, N., and Krol, T.: The new Gas Tur-bine Portfolio to meet the market require-ments for Distributed Generation. (Power-Gen June 2015, Amsterdam).

[12] Rosendahl, J., Joswig, A., and Steins, H.: Auswirkung des weltweiten Energiewandels auf den Betrieb und die Lebensdauer fossi-ler Kraftwerke – Lösungen für den sicheren Betrieb des Turbogenerators. (Kraftwerks-technisches Kolloquium, 13. – 14. Oktober 2015, Dresden). l

U/UN

tan

d *

10-3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

New

Aged

Fig. 8. Dissipation factor measurements of a new and an aged high voltage insulation system.

 

 

 

 

 

 

 

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