Water Power and Dam Construction - Issue December 2009

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WWW.WATERPOWERMAGAZINE.COM DECEMBER 2009 11

AUSTRALASIA

ITH the spectre of permanent climate change and

some 13 years of crippling drought impacting south-

ern Australian states, Queensland to the north is gear-

ing up its water security strategies. As Queensland’s

leading commercial water provider, one of SunWater Limited’s

most important ongoing activities is supporting the Queensland

Government’s Statewide Water Policy.

In the northwest region of Australia, SunWater is reviewing local gov-

ernment proposals for a 300,000 ML dam and 195,000 ML off-stream

storage to harvest seasonal ﬂoodwaters from the vast ﬂows of two mon-

soonal river systems. These will provide for two new irrigated agricul-

ture schemes to meet growing domestic and export food demands.

If proven to be economically and technically feasible, these two

projects have the potential for federal government Water for the

North funding under its A$12.8B (US$11.7B) programme to stimu-

late private sector investment opportunity.

The availability of secure and reliable water supplies is critical to

the future economic prosperity of Queensland. An integral part of

SunWater’s strategies to cater for growth and water security is main-

taining the safety and efﬁciency of its A$7B (US$) infrastructure assets,

which supply 40% of all water used commercially in Queensland.

This infrastructure comprises:

U 19 major dams.

U 63 weirs and barrages.

U 80 major pumping stations.

U 2500km of pipelines and open channels.

U 730km of drains.

SunWater manages a total water allocation of 2.82M ML, which

includes 1.92M ML of customer-owned allocations. It also under-

takes an important consulting role in contributing to cost-effective

water solutions in Queensland. Some recent major projects supported

with SunWater knowledge and expertise are:

UÊÊAs a sub-alliance partner providing design services to the Western

Corridor Recycled Water Project.

UÊÊPeer reviewer to the Hinze Dam Stage 3 raising.

UÊÊConsultant to Anglo Coal on a 20-year dam safety review for

Bundoora Dam.

SunWater’s hydraulic laboratory in Brisbane forms an integral part

of the company’s project management service, allowing internal and

external clients to test scale models before construction, enabling

considerable savings through design optimization. The models for

the A$348M (US$318M) Wyaralong Dam, which is scheduled for

completion by the end of 2010, are an example of the scale of the

projects for which the laboratory can cater. The dam needed two

models; the larger demonstrates the dam’s ability to safely pass ﬂows

up to the probable maximum ﬂood at a scale of 1:80. The smaller

model, at a scale of 1:30, provides crucial information related to the

hydraulic conditions at the dam’s outlet. SunWater is the owner’s

engineer for the project.

HYDRO BU SINESS

SunWater’s operational footprint across Queensland is broad. Its

business activities and interests spread from as far north as Bamaga,

west to Mount Isa, and south to St George. While the majority of its

business operations are in Queensland, SunWater also operates and

maintains water infrastructure in other states of Australia, including

urban supply storage for Australia’s national capital, Canberra.

Historically, Queensland irrigators were the majority of SunWater’s

clients; they still represent a signiﬁcant portion of its 5000-strong

customer base and will continue to be an important and signiﬁcant

market segment as agribusiness and water dependent food industry

moves north to escape drought and climate change.

Hydro power is another means of diversifying that SunWater

has been trialling over the last ﬁve years to add value to its stor-

ages without impacting on supply to customers. Tinaroo Falls

Dam in the far north was SunWater’s ﬁrst hydro power facil-

ity. It is a 1.6MW small hydro station, which began producing

power in May 2004. Subsequently, the design of Paradise dam

near Bundaberg included provision for a 2.8MW hydro station

completed in December 2006.

SunWater is continuing its work with Stanwell Corporation in the

preparation of Stanwell’s preliminary business case for a hydro gen-

eration scheme on either a raised Burdekin Falls Dam or at its current

height. In the wet season the Burdekin River sends immense ﬂood

volumes to the Coral Sea.

The Burdekin Falls Dam’s current capacity is 1.86M ML and at

its peak ﬂow in February this year catchment inﬂows were sufﬁcient

enough to enable the dam to reﬁll itself every one and a half days.

A proposed 2m raise of the dam would further enhance water

supply certainty for the Burdekin region, holding back a fraction

of the Burdekin’s massive ﬂood ﬂows both for posterity and green

power production. It would increase the storage capacity by 590,000

ML to a total of 2.45M ML.

An additional 150,000 ML of yield would provide for demand

from emerging and future economic development including mining

and agribusiness in northern and central Queensland.

Stage one of the investigation completed a pre-feasibility study and

preliminary business case. Stage two, including preliminary design

and environmental investigation, is on hold pending conﬁrmation of

commercial viability of the associated Water for Bowen project, or

for other signiﬁcant demand emerging.

Raising of Burdekin Falls Dam is just one of the major projects and

proposals SunWater currently has on its books, which in total are

worth in excess of A$3B (US$2.7B).

In July 2007, the Department of Infrastructure and Planning (DIP)

formally chose SunWater as proponent for the development of busi-

ness cases for seven major regional water supply projects to underpin

the Statewide Water Policy. SunWater’s investigations have included

prudent funding options which could include additional debt and

equity raising to maintain an appropriate capital structure, in the

event these projects proceed to construction. Two of the most strate-

gic of the proposed projects are highlighted below:

Connors River Dam and pipelines

The proposed dam is located on the Connors River some 235km

northwest of Rockhampton and 100km south of Mackay. The

storage is expected to have a maximum capacity of approximately

373,700 ML, which will conceptually provide a high priority yield of

49,500 ML/yr and 5,000 ML/yr lower priority product.

The majority of demand will be derived from the coal mining

industry in the northern and central Bowen basin and from the

Galilee basin (a massive undeveloped thermal coal deposit west of

the Bowen basin).

The project has been declared a signiﬁcant project under the State

Development and Public Works Organisation (SDPWO) Act 1971.

An environmental impact statement is scheduled for the Queensland

coordinator-general’s review in late 2009. Public release of the EIS will

follow. The project remains ‘on track’ for a ﬁnancial decision in late

2010. Construction completion remains scheduled for late 2013.

Nathan Dam and pipelines, Dawson River

The Nathan Dam site is located on the Dawson river. The storage is

expected to have a capacity of 1M ML, to provide high priority yield

of around 90,000 ML. The majority of demand is likely to be sourced

from the mining sectors in the southeast Bowen and Surat Basins.

A pipeline is currently under investigation to supply water to Surat

basin coal mining and power generation customers which could

extend as far as Dalby with a total length of 270km, forming the

spine for SunWater’s proposed Surat basin water grid. The grid

would initially transport treated coal seam gas water to customers

until that wanes and is replaced by Nathan dam construction and

supply. The dam and pipelines project has also been declared as being

of State signiﬁcance under the SDPWO Act.

Focused on

the future

Securing future water supplies is top of

the agenda for Australian water provider,

SunWater. Here the company gives IWP&DC

an insight into its hydro and dam activities

12 DECEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

AUSTRALASIA

Business cases and investigations for other signiﬁcant water infra-

structure projects and upgrades to existing assets include:

Glebe weir raising and pipeline, Dawson river

SunWater is working with Xstrata to deliver the supplementary EIS for

the Wandoan coal project, for which Glebe weir raising is one water

supply option under consideration. Feasibility and engineering investi-

gations are expected to progress at the end of 2009 or early 2010.

Nullinga dam, Walsh river Mareeba

The preliminary business case report submitted to the government

recommended no further work until clariﬁcation of the required envi-

ronmental ﬂows for the system.

Kinchant dam raising – Pioneer Valley MacKay

Following geotechnical investigation, SunWater’s preliminary busi-

ness case recommended no further work be undertaken because of

high costs.

Lower Fitzroy river infrastructure

The project focuses on the feasibility of constructing a weir near

Rookwood Crossing, and raising Eden Bann weir, an existing

SunWater asset, to augment short-medium term industrial, residential

and agribusiness supply for the Rockhampton and Gladstone Regions.

Completion of the business case is scheduled for June 2010.

Underpinning the long-term sustainability and cost-effectiveness of

water supplies in SunWater’s 23 water schemes relies on comprehen-

sive asset planning, incorporating new and emerging requirements for

safety, environment, and water efﬁciency.

SunWater’s most recent investment under its dam safety upgrade

programme has been a A$21M (US$19M) project to improve the

safety of Tinaroo Falls dam, commissioned in 1959 to supply water

the Mareeba-Dimbulah water supply scheme.

When the spillway capacity upgrade is completed in October

2010, the dam will be able to pass an extreme ﬂood equivalent

to 100% of the acceptable ﬂood capacity (AFC) by strengthening

the main dam structure and allowing ﬂoods to pass over the non-

spillway abutments.

Works will include the installation of stressed tendons through

the body of the dam which are anchored into the foundation rock,

and protection of the downstream foundation by construction of

an erosion protection slab along the downstream toe of the non-

spillway abutments.

Overall, the next decade promises some demanding challenges and

exciting business opportunities for SunWater.

For more information visit www.sunwater.com.au

IWP& DC

Caption

Above and left: During the peak wet season between January and February 2009,

Burdekin Falls Dam discharged more than 20M ML over its 500m spillway

14 DECEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

AUSTRALASIA

ROUGHT is an all too familiar concern for many

Australian communities. In fact, due to several years

of reduced rainfall and a continually increasing popula-

tion, the region of southeast Queensland, which includes

the Brisbane metro area, has experienced looming water shortages

that resulted in regulated water restrictions and a quest to ﬁnd

solutions to their potential water resource crisis. As a response,

the Queensland government invested resources into studying the

feasibility of precipitation enhancement via cloud seeding to sup-

plement water resources.

WHAT IS CLOUD SEEDING?

When a cloud forms, moisture in the form of water vapour is carried

into the cloud until it condenses on particles into liquid water, thus

forming cloud droplets (see Figure 1, part 1). The cloud droplets are

very small and cannot readily fall out of the cloud, therefore other

processes must occur to grow the liquid cloud droplets large enough

to fall as rain.

One such process is called collision and coalescence, in which cloud

droplets of a variety of sizes (which fall at different speeds to one anoth-

er) collide with each other and coalesce into a combined larger drop

(Figure 1, part 2). This process continues until drops are large enough

to overcome the updraft speed within the cloud and fall as rain.

In addition to processes that help the droplets grow, there are fac-

tors that can deplete the liquid water from eventually becoming rain

on the ground, such as evaporation of the droplets or the freezing of

small droplets (that were carried to the sub-freezing cloud top by the

updraft) into small ice crystals that are unable to fall as precipitation

(known as cloud glaciation). The amount of water vapour that enters

a cloud never all falls to the ground as rain.

The precipitation efﬁciency (or the percent of incoming cloud water

vapour that falls as rain) varies from cloud to cloud. Cloud seed-

ing technologies aim to convert more of the water vapour processed

in the cloud to more rainfall at the ground, thereby increasing the

cloud’s precipitation efﬁciency.

SCIENCE-BASED TECHNOLOGY

Cloud seeding is a science-based technology that aims to add particles

to a cloud that will help precipitation develop more efﬁciently, thus

hopefully yielding more rainfall. The type of particle added to the cloud

depends on the characteristics of the clouds that will be seeded.

Deep clouds that have portions growing in sub-freezing tempera-

tures may be seeded with silver iodide, a material whose properties

are very similar to an ice crystal and serve as good surfaces on which

ice can form.

Supercooled liquid water (liquid water that has been carried into

sub-freezing temperatures, but has yet to freeze) can freeze on these

silver iodide particles and initiate and enhance ice-based precipitation

formation processes at key levels in the cloud, and prevent loss of that

water to glaciation aloft.

Other clouds may be seeded with hygroscopic materials (particles

that take on water easily, such as salts) in the updraft region of the

cloud just below the cloud base (Figure 1, part 3).

These hygroscopic particles are then carried into the cloud by the

Investigating cloud-seeding

A world-ﬁrst cloud seeding research project is being used to help direct future water

resource management programmes in Australia

Above: Figure 1 – The effect of hygroscopic cloud seeding on rain formation.

Updraft

pdraft

Small cloud droplets

Collision-Coalescence

Small cloud droplet

(8-15 microns)

Large cloud droplet

(20-30 microns)

Large cloud droplet

(30-50 microns)

Raindrop

(>1mm)

Rain formation begins when

water molecules in a cloud

condense on naturally occurring

nuclei (i.e., dust particles) to

produce small cloud droplets,

around 8-15 microns in diameter.

R i

Cloud droplets may grow by a

collision-coalescence process;

that is, by colliding with other

droplets and coalescing into a

larger droplet.

However, this doesn’t occur until

the droplets are about 20-30

microns in diameter.

Hygroscopic seeding accelerates

the collision-coalescence

process to produce rain.

Hygroscopic flares are burned

at the base of a cloud into an

updraft. This releases hygro-

scopic nuclei into the cloud and

starts the collision-coalescence

process. The hygroscopic nuclei

produce larger cloud droplets

than would occur naturally

(30-50 microns in size).

Through collision-coalescence,

the droplets grow rapidly.

Once the droplets reach more

than approximately one

millimeter in diameter, their

terminal velocity is large

enough for them to fall from

the cloud as rain.

WWW.WATERPOWERMAGAZINE.COM DECEMBER 2009 15

AUSTRALASIA

updraft. Water vapour condenses on them to form additional liquid

cloud droplets, whose size depends on the size of the hygroscopic

particles introduced in the cloud. Adding additional particles of

larger sizes may help enhance collision and coalescence processes

that are responsible for rain formation and convert more of the

cloud water to rainfall. In essence, a more efﬁcient collision and coa-

lescence rain formation process yields more rainfall at the ground

(Figure 1, part 4).

The hypothesis for a propagation of the hygroscopic seeding effect

is summarised in the box below. Note that the seeding conceptual

model primarily focuses on aspects related to the sizes and concentra-

tions of water droplets and ice crystals (generally referred to as the

cloud microphysics) of the hypothesised response to cloud seeding.

However, based on previous hygroscopic seeding experimental

results from South Africa and Mexico, additional effects (seeded

clouds rained for longer periods over a larger area than natural

clouds) possibly related to cloud air motions (cloud dynamics such as

enhanced cloud growth due to possible modiﬁcation of the updraft/

downdraft structures and/or the development of progeny clouds) had

to be invoked to explain the statistical results (Foote and Bruintjes,

2000). These aspects are as yet not well understood.

ASSESSING FEASI BILITY

The potential for such manmade increases in precipitation is strongly

dependent on the natural microphysics and dynamics inside the clouds

that are being seeded. These factors can differ signiﬁcantly from one geo-

graphical region to another, as well as during and between seasons in the

same region. In some instances, clouds may not be suitable for seeding,

or the frequency of occurrence of ‘seedable’ clouds may be too low to

warrant the investment in an operational cloud seeding programme.

Measurements of air and cloud properties in a speciﬁc region are

also necessary to determine if the clouds are ‘seedable’. These factors

need to be evaluated from a climatological perspective as part of an

overall feasibility study. Such a study would also determine which

seeding method should be used in the given region.

Likewise, it is important to conduct hydrological studies relating

rainfall with river ﬂows and reservoir levels, to determine hydrologi-

cal regions where reservoir catchments are most efﬁcient, and to cal-

culate cost-beneﬁt ratios for the speciﬁc region. Seeding could then be

optimized by preferentially targeting the most efﬁcient watersheds.

The effects of the cloud seeding technique(s) then need to be evalu-

ated, which is often done using a randomization procedure to dem-

onstrate statistically if and how well the seeding method works, and

to quantify the increases in rainfall achieved. Typically this technique

has relied on statistical evaluation of the storm water mass, rain-

fall rate, duration, size, and radar reﬂectivity; parameters generally

derived from basic weather radar observations.

Nonetheless, because of the very large natural variability in storm

characteristics and the local meteorological environment that can

dwarf any cloud seeding effects, a large sample size of randomized

seeded and unseeded cases is required to obtain statistical signiﬁcance.

Even with large sample sizes, however, there could still be ambiguity

in understanding the physical mechanisms that were responsible for

any seeding effect the statistical analysis might have suggested.

The Queensland project was undertaken in the hopes that through

the use of physical measurements, such as those measured inside

clouds by aircraft and remotely-sensed observations from state-of-

the-art weather research radars, precipitation microphysical proc-

esses could be more directly observed. This could make it possible to

understand cause and effect seeding relationships, thus not having to

solely rely on statistical means that often generate controversy.

Steps in the hygroscopic seeding

conceptual model

1) Hygroscopic seeding produces larger cloud droplets near cloud base

2) Larger droplets near cloud base enhance the collision and coalescence

process and the subsequent production of drizzle (small raindrops)

3) Drizzle mixes through larger portions of the cloud so the seeding effect

propagates throughout the entire cloud

4) If the cloud vertically extends to temperatures colder than 0 °C, large

drizzle and raindrops would freeze preferentially before small cloud

droplets and become the primary embryos for graupel (small hail-like

ice particles), initiating more efﬁcient ice-based precipitation growth

processes

5) The more efﬁcient ice-based process may result in secondary ice

generation, which yields a natural ice seeding process

6) Additional loading of precipitation due to precipitation-sized drops forming

at lower levels in seeded clouds results in changes in updraft/downdraft

structures and can modify dynamic aspects of the storm that could

extend the lifetime of the cloud

Above from top to bottom: Figure 2 – Map of Southeast Queensland water

catchments (red) and location of primary radars used during the CSRP

(green); Figure 3 – Photo of the WXMOD seeding aircraft highlighting the

wing-mounted ﬂare racks (Courtesy Sarah Tessendorf/NCAR).

16 DECEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

AUSTRALASIA

THE QUEENSLAND EXAMPLE

Scientists from a group of different bodies conducted the feasibility

study for rainfall enhancement via cloud seeding during the summer

rainfall regime in Southeast Queensland in Australia (Figure 2). The

feasibility study, known as the Southeast Queensland Cloud Seeding

Research Programme (CSRP), included a range of measurement sys-

tems, some using novel technologies.

In situ cloud microphysical measurements in seeded and unseeded

clouds were taken with an instrumented research aircraft, and rand-

omized cloud seeding was performed by aircraft with seeding capa-

bilities (see Figure 3). The most unique aspect of this study was the

availability of a dual-polarization, dual-wavelength doppler weather

research radar (called CP2; Figure 4), in combination with other local

BOM doppler weather radars.

This is the ﬁrst cloud seeding experiment in the world that has had

a dual-polarisation, dual-wavelength radar available, which aids in

discriminating microphysical precipitation processes, in addition to

dual-doppler radar coverage that provides three-dimensional airﬂow

within the observed clouds. This is signiﬁcant in that it is the ﬁrst

time the potential exists to observe the evolution of microphysical

precipitation processes, such as precipitation particle type, number,

and size, within a seeded cloud while at the same time observing the

air ﬂow patterns within the cloud.

The speciﬁc aims of the Queensland CSRP are to:

U Understand the rainfall and weather patterns in the southeast Queensland

region to determine the frequency of clouds suitable for seeding.

U Collect detailed physical measurements of the cloud and precipitation

processes to infer how cloud seeding may be impacting those processes.

U Conduct a statistical randomized cloud seeding experiment to esti-

mate if cloud seeding is enhancing rainfall in the region.

The Queensland CSRP research is ongoing, however initial results

have been presented to the Queensland government and will be used

as guidance for planning future water resource management pro-

grammes, including assessing the potential of an operational cloud

seeding programme to enhance rainfall in the region.

The authors are Sarah A. Tessendorf and Roelof T.

Bruintjes, NCAR/RAL, P.O. Box 3000, Boulder, CO

80307, US. Email: saraht@ucar.edu

The Queensland Cloud Seeding Research Programme was

sponsored by the Queensland Government Department

of Environment and Resource Management through

the Queensland Climate Change Centre of Excellence.

Graphics for this article were produced by Kevin

Sampson, Cindy Halley-Gotway and Lara Ziady (NCAR)

IWP& DC

Research programme

Scientists working on the Southeast Queensland Cloud Seeding Research

Programme included representatives from:

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References

Foote, G.B., and R.T. Bruintjes (eds.), 2000: WMO International Workshop

on Hygroscopic Seeding: Experimental Results, Physical Processes,

and Research Needs (1999 : Mazatlán, México). World Meteorological

Organization, WMP Report No. 35, WMO/TD No. 1006, Geneva, 68 pp.

Above, left to right: Figure 4a – CP2 site infrastructure at Redbank Plains: Antenna and pedestal are within an inﬂated radome mounted over housing for

ofﬁce, storage and transceiver; Figure 4b – CP2 S-band and X-band antennae [Photos courtesy Scott Collis/CAWCR]

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PUMPED STORAGE

OSSIL fuel energy resources are limited and environmen-

tal regulations such as the Kyoto protocol have led to the

increased use of renewable energy resources such as wind,

solar, biomass and geothermal energy. In Germany, for

example, the installed power in wind farms in 2005 reached 19GW.

The declared target is to integrate 20% of installed renewable power

by the end of 2020. Other countries are following with similar devel-

opments. Due to the stochastic nature of wind energy, it is difficult

to plan and not possible to control the production of wind parks. In

order to guarantee the stability of a power network, production has

to be identical to the consumption. One way to control the power

balance in a network is to store the surplus electrical energy and, to

date, pumped storage power stations present the most efficient way

of storing large amounts of electrical energy at acceptable costs.

CONVENTIONA L PUMPED STORAGE

In conventional pumped storage power stations, a salient pole syn-

chronous motor-generator is coupled either to a separate pump and

Pumped storage projects are often the most

efﬁcient way of storing large amounts of

electrical energy at acceptable costs. Here,

Alexander Schwery and Thomas Kunz of

Alstom Hydro discuss the advantages of

variable speed energy production at pumped

storage schemes, using examples of planned

projects in Europe

Variable solutions

Figure 1 – Synchronous salient pole

motor-generatorwith two stage reversible Francis pump turbine

View of the Linth-Limmern project

WWW.WATERPOWERMAGAZINE.COM DECEMBER 2009 19

PUMPED STORAGE

turbine or to a reversible pump turbine. The arrangement can be

horizontal or vertical. Because the rotational direction of a separate

pump and turbine arrangement is the same in both operation modes,

this setup allows for a more rapid change from turbine to motor

operation mode or vice versa. It is however more complex and leads

to longer shaft arrangements resulting in higher costs, especially if the

power station is located in a cavern.

In contrast, solutions involving a reversible Francis turbine are

more compact. Figure 1 shows an example of a salient pole motor-

generator coupled to a reversible Francis turbine. In this case, the

hydraulic machine acts as a pump in one rotational direction and as a

turbine in the other rotational direction. Consequently, the unit has to

be stopped and accelerated for a change from turbine to pump mode

or vice versa. Depending on the hydraulic head, reversible Francis

turbines can be designed as single or double-stage arrangements. A

technical highlight of the double-stage pump turbine is that the power

can be regulated using synchronized guide vanes in both stages [6].

VARIABLE SPEED PUM PED STORAGE

In variable speed power stations static frequency converters are

used to vary the speed of the electrical machine. For installations

with a power lower than approximately 50MW, this can be real-

ized using conventional synchronous generators linked to the

grid by a static frequency converter. For larger units, this solu-

tion would be more difﬁcult to justify economically. For units

larger than 50MW, double fed induction machines with a static

frequency converter feeding the rotor are the preferred solution.

See [2] for a more in depth discussion of the two solutions.

The basic principle of the double fed induction machine con-

sists in the creation of a rotating ﬁeld on the rotor allowing the

machine to be operated within a certain speed range around the

synchronous speed. The relative difference in speed is called slip.

Usually, the slip range is within +- 10%. In ﬁrst realizations of

variable speed pumped storage projects, Cyclo converters were

used to create the rotating ﬁeld on the rotor. Cyclo converters

are direct converters and, consequently, absorb reactive power.

This power needs to be compensated by condensers or provid-

ed by the generator. Furthermore, the frequency range of such

converters is limited. They cannot be used to start the unit in

pump mode, which means an additional static frequency con-

verter needs to be used to start the unit. Cyclo converters using

Thyristors are a robust technology proven for many years. More

recently, improvements in the power ratings of IGBTs and IGCTs

allow for the construction of large voltage source inverters. This

kind of static frequency converter is used in other applications

like steel lamination production and can now be used instead of

Cyclo converters. These converters do not absorb reactive power.

Furthermore, they can be used to start the motor in pump mode.

For this purpose the stator is short-circuited and a rotating ﬁeld

of increasing frequency is injected into the rotor.

ADVANTAGES OF VARIABLE SPEE D

The main advantage of variable speed is that the power absorbed

in pumping mode can be varied over a certain range. Figure 2

shows a typical operation range in pump mode. It can be seen that

depending on the given head, the power absorbed from the net-

work can be varied by approximately 30%. This gives the opera-

tor of the power station the possibility to contribute to the grid

frequency regulation even in pump mode. In conventional pumped

storage plants frequency regulation is only possible in turbine

mode, which is economically interesting if the power demand is

high. If power demand and prices are low, however, it is preferable

to deliver the same service (frequency regulation) in pump mode to

the grid operator while ﬁlling the upper reservoir.

Since reversible Francis turbines are first designed as pumps, the

resulting operation area in turbine mode on the hill chart does not

coincide with the top of the hill. Consequently, the turbine cannot be

operated at the best possible efficiency. Modifying the speed of the

turbine allows for a shifting of the operation area closer to the top

of the hill chart resulting in higher efficiencies, especially in partial

load operation.

Large head variations make it difficult to find an optimal

design of the pump turbine for both operation modes. Using two-

speed motor-generators solved this difficulty in the past. These

are synchronous salient pole generators on which the number of

active poles on the rotor can be changed between two numbers.

Furthermore the stator is equipped with two separate windings,

each corresponding to one specific number of poles. See reference

[1] for more information about this technology. This solution is lim-

ited to a restricted set of speeds and consequently cannot be used in

all situations. Furthermore the electrical machine is more complex

and bigger. Taking this into account, variable speed technology is

more advantageous in today’s market.

Variable speed systems can respond more dynamically to changes

in power set point values. In conventional setups, power variation

Pump power [MW]

Head [m]

max

min

max

Above: Figure 2 – Operation range in pump mode (power) versus net head;

Right: Figure 3 – Retaining system of the rotor winding overhang using bolts

and auxiliary rim [8]

20 DECEMBER 2009 INTERNATIONAL WATER POWER & DAM CONSTRUCTION

PUMPED STORAGE

can be achieved only by changing the opening of

the guide vanes. This can only be done according

the deﬁned opening and closing laws in order to

avoid over or under pressures in the penstock and

the draft tube. Using variable speed units, the same

adaptations are done on the turbine side. During

the slow reaction time of the hydraulic system how-

ever, the power of the electrical machine can adapt

to the new set point value respecting electrical time

constants that are at least one order of magnitude

lower. The difference in power delivered to the grid

(according to the new set point) and the power deliv-

ered by the turbine results in a speed change that can

be corrected later on. In other words, the difference

in power is temporarily stored or provided by the

kinetic energy of the rotor. For more details on this

ﬂywheel effect refer to [3] and [7].

Finally, it is also possible to use the fact that the

speed of the unit can be changed in order to avoid critical zones in

pump operation. This results in a reduction of the required mini-

mum submergence of the pump turbine. Depending on the pow-

erhouse setup, this can have a positive effect on civil engineering

costs. It is important to note that this advantage cannot be exploit-

ed fully without negative impact on the possible power range, and

the potential efﬁciency improvement.

This variety of advantages and options show that it is more difﬁcult

to specify and deﬁne a variable-speed power station compared to a clas-

sical plant. Speed range, power range, starting time and submergence

are only some of the parameters that need to be optimized. An iterative

process between the ﬁnal customer and the OEMs, before the ﬁnaliza-

tion of the speciﬁcations, allows for more optimal solutions.

DESIGN OF DOUBLE FED IND UCTION

MOTOR-GENERATOR

The stator of a double fed induction machine is similar to the stators

of conventional hydro generators. Double fed induction machines are

larger in size, have a higher moment of inertia and smaller air gaps

compared to salient pole machines of the same speed and power. The

rotor design is quite different. Three main parts are completely differ-

ent compared to synchronous machines and are discussed here.

Slip rings

To feed the rotor with the three phase current system, at least one slip

ring per phase and usually one slip ring for the star point are needed.

The slip rings need to be designed for currents up to 8 kA depending

on power and design of the machine. Voltages can go up to 8 kV.

In normal operation, the frequency of the currents is low. At syn-

chronous speed, the currents become constant. During start up, the

frequency increases gradually and is equal to the grid frequency at the

end of the start up process. The brush system and the slip rings need

to be designed for these requirements. For a regular consumption of

the carbon brushes, air temperature and humidity need to be control-

led. Furthermore, it is advisable to separate the slip ring housing from

the cooling airﬂow of the motor-generator to avoid contamination by

carbon dust. Special attention has to be paid to an even distribution

of the currents on the different carbon brushes.

Winding overhang retaining system

The rotor winding overhang has to be supported to avoid deforma-

tion and damage if the machine speed comes close to runaway speed

during load rejection. Three basic concepts can be distinguished.

The ﬁrst concept uses a steel retaining cap similar to the design used

in Turbo generators (cylindrical rotor generators). The caps can be

shrunken and are dismountable. The steel cap needs to withstand high

mechanical forces and is made of forged non magnetic steel. For large

diameter machines, the manufacturing of such caps can be difﬁcult.

Furthermore the ring needs to be built in one piece. This can be incom-

patible with the transport limitations for underground stations.

For the second concept, the whole overhang is bandaged either by

a steel or synthetic wire or a synthetic foil. The application of the wire

or the foil needs special tools and a lot of experience. Dismantling of

the system without destruction of the bandage is generally not pos-

sible. Special attention needs to be paid to the cooling of the wind-

ing overhang for the ﬁrst two solutions, because the cooling airﬂow

cannot pass through the winding overhang in radial direction.

Above, left and right: Figure 4 – Test system for material and design validation of retaining bolts.

Below, left and right: Figure 5 – Test equipment and test specimen (after destructive testing) for the material validation of the rotor rim