THE HYDRAULIC MACHINERY OF
OBERVERMUNTWERK II
P. Meusburger, L. Werle
Abstract: In the course of the development work for the new
large scale hydro power plant of Illwerke, some different
machinery types and sizes were analysed. Both pumpturbines
as well as ternary units were investigated in detail.
The final machinery design ended up in two ternary units each
consisting of a pump with hydraulic torque converter, a motorgenerator and a separate turbine with a shifting clutch. Apart
from the maximum energy storage efficiency this variant also
offers the maximum operational flexibility with the shortest
switching times. Furthermore the operation mode of controllable
pump in hydraulic short circuit secures that the machine sets of
the Obervermuntwerk II can cover the full control band from 100% to +100% power and therefore can be used as a full
backup for the Kopswerk II units.
1 Introduction
The first design layout for the new pumped storage project was carried out in 2006,
two years before Kops II started in operation. At that time, the prevailing
circumstances of a supposedly limited energy transport system using the existing
110-kV high voltage transmission line, restricted the general layout with a maximum
capacity of 160 [MW]. In addition, the opinion prevailed that with the additional
capacity of 525 [MW] of Kops II, the volume of the market for balancing power and
frequency control could be well covered. Thus a shaft powerhouse with reversible
pumpturbines and a maximum capacity of 160 [MW] fulfilled the requirements at that
time.
In November 2008 Kops II started into full operation with all three machine sets. The
flexible concept of providing balancing energy of this power plant for the whole power
plant group of Illwerke has been fully confirmed by the experience since start of
operation. However, the suspected shift of retention of balancing energy from the
former largest pumped storage power plant of Illwerke “HPP Lünersee” to the new
Kops II has not occurred. The operation of the power plant Lünersee is largely
unaffected by Kops II and an expansion of the preserved balancing energy through
the power plant group “Obere Ill – Lünersee” took place.
It was found that the further development of renewable energy especially in Germany
requires more storage capacity and balancing energy for an efficient integration of
wind power and photovoltaics. To meet this demand it was decided to significantly
enlarge the design capacity of Obervermuntwerk II to at least 320 [MW]. Therefore
the capacity of the transmission line also had to be increased. The new powerhouse
will consequently be connected to the switchyard with a new 220-kV underground
cable.
2 Machinery Types
The Obervermuntwerk II uses the head of 243 – 311 [m] by connecting the two
existing reservoirs Silvretta and Vermunt with a new headrace system, a new
underground powerhouse and a new tailrace tunnel. In addition, the already existing
Obervermuntwerk I will be supplied from the headrace system by a new water
conduit.
To meet the demands of today’s electricity markets, the utmost operational flexibility
for the machinery is required. Beside phase shifting operation, both pump and turbine
mode as well as controllable pump mode in hydraulic short circuit or at least in mixed
operation was planned. The new power plant is designed for rapid switching times of
about 60-70 [s] from turbine to pump mode and vice versa. The regulation time of the
turbine guide vane will be around 30 [s].
2.1 Pumpturbines
At the very beginning of the project design the main aim was to increase the storage
capacity in the power plant group “Obere Ill – Lünersee”. Because of the lower
investment costs for the machinery different variants with pumpturbines were
analysed. A selection of the examined hydraulics is given in table 1.
Variant no.
HPU,opt [m]
QPU,opt [m³/s]
PPU [MW]
nPU [upm]
nq,opt,PU [upm]
kPU
1
286
92,3
280
272,7
37,67
637,08
2
286
50
151
375
38,13
644,80
3
286
49
147
428,57
43,14
729,51
4
286
45
140
428,57
41,34
699,10
5
286
43
133
500
47,14
797,28
6
286
41
125
500
46,03
778,52
7
286
35
110
500
42,53
719,30
8
286
25
76
500
35,95
607,92
9
286
24
73
600
42,27
714,77
HTU,rated [m]
QTU,rated [m³/s]
PTU [MW]
nTU [upm]
nq,rated,TU [upm]
kTU
280
123
310
272,7
44,18
739,35
280
65
162
375
44,17
739,09
280
65
163
428,57
50,48
844,67
280
61
152
428,57
48,90
818,27
280
56,5
140
500
54,91
918,77
280
51,8
129
500
52,57
879,72
280
47,5
120
500
50,34
842,42
280
33
83
500
41,96
702,16
280
33
82
600
50,35
842,59
D1 [m]
4,46
3,9
3,481
3,51
3,07
3,04
3,02
2,903
2,482
Table 1: Different pumpturbine layouts
Even though with higher speed one can cover the given net head with smaller
impeller diameters and thus smaller machinery resulting in remarkable cost savings
for the whole powerhouse, the mechanical stress expressed by the k-factor is limiting
the maximum power. As the minimum power of the machinery for Obervermuntwerk
II was given with 160 [MW] each, the rotational speed was therefore fixed with 428.57
[rpm].
The big disadvantage of all these machines is that the part load behaviour of
pumpturbines is very poor. Due to the hydraulic stimulation because of the wrong
incident flow at part load, the machines tend to have a strong vibration if the load
drops below approx. 40% of nominal load. To operate this machinery at deep part
load, additional stabilisation measures by blowing air into the housing of the
machines are necessary. An adaption of the hydraulic shape of the impeller, as can
be done with the turbines of ternary units, seems to be impossible with respect to the
utmost efficiency in pump operation.
2.2 Ternary Units
Compared to pumpturbines, ternary units offer some essential advantages, which
became crucial during the development work of the Obervermuntwerk II project.
Variant no.
HPU,opt [m]
QPU,opt [m³/s]
PPU [MW]
nPU [upm]
nq,opt,PU [upm]
kPU
1
HTU,rated [m]
QTU,rated [m³/s]
PTU [MW]
nTU [upm]
nq,rated,TU [upm]
kTU
D1 [m]
2
286
46
139
375
36,57
618,47
280
58
146
375
41,72
698,16
3,949
3
286
45
136
300
28,94
489,37
2,797
280
59
145
300
33,67
563,32
4,768
4
292
48
148
428,57
42,03
718,28
3,377
278
62,5
160
428,57
49,77
829,76
3,529
5
287,7
47
144
500
34,70
588,53
2,528
296
48
150
428,57
41,6
715,8
270
61
150
500
41,46
681,21
3,02
2,25
290
68,3
180
428,57
50,3
856,4
3,425
2,32
Table 2: Ternary unit designs
Table 2 shows some different layouts of the machinery for the ternary units of the
Obervermuntwerk II. Apart from the higher best point efficiencies of both pump and
turbine, resulting in a noteworthy improvement of the overall plant efficiency,
especially the part load behaviour of a ternary unit is much more convenient because
the turbine impeller can be designed independently from pump operation for this
demand. Furthermore ternary units can react much faster than pumpturbines to
variable load araising from the volatility in the electrical grid which is due to the
integration of windpower and photovoltaics.
As Illwerke particularly supply the German electricity market which is dominated by a
rapid growth of wind and solar power as pointed out by H. Schmöller (2014), the
tremendous increase of windpower and photovoltaics in Germany over the last years
has led to a change of requirements on the machinery of Obervermuntwerk II. Fast
reaction times and operational flexibility have become much more valuably compared
to the first machinery layouts at the very beginning of the project and thus the surplus
of investment costs of ternary units compared to pumpturbines can be compensated.
With ternary units operating in hydraulic short circuit one can expand the range of the
control band to negative power in pump operation.
Fig. 1: Power control band of a ternary set
Fig. 2: Power control band of a pumpturbines without part-load stabilisation
3 Power Control Band
Fig. 1 shows the possible power control range using ternary units for the
Obervermuntwerk II project compared to the power control band of pumpturbines
depicted in fig. 2. The yellow marked areas show the operating range where the
power of the plant can be regulated with respect to the demand of the electrical grid.
It is obvious that the control band of the ternary units which covers almost the whole
field of net head with just one single small gap (marked red in fig. 1) below net heads
of 250 [m] is much more beneficial than the one of pumpturbines showing a gap
around zero power over the whole net head range. Furthermore the area between 50% and -100% power (pump operation of both units) is not controllable using
pumpturbines otherwise a third unit would be installed.
A possibility to enlarge the power control range of pumpturbines is certainly to use a
variable speed drive. Indeed, with lower rotational speeds, the minimum load in
turbine operation can be reduced significantly but the possible pump power control
range strongly depends on the net head of the power plant and becomes smaller the
higher the net head. Therefore a solution with pumpturbines and variable speed
drives was rejected for the Obervermuntwerk II project as the power control band
would still have had unfavourable large gaps.
4 Shifting Time
Another essential precondition for the decision to realise ternary units in the
Obervermuntwerk II were the considerably lower shifting times between different
operation modes. Fig. 3 shows the presumed shifting times of a pumpturbine
following G. Penninger and F. Spitzer (2006).
Fig. 3: Shifting time of a pumpturbine
In contrast to ternary sets, the controlled pump operation mode is not possible with
pumpturbines thus depicted with dotted lines in fig. 3. However, a similar operation
can be achieved if the plant comprises of at least two units and if the pump power of
one unit is balanced with the turbine operation of the other machine. The balancing of
the power is then performed on the electrical section whereas with ternary units it can
be done at the mechanical level on the same shaft and therefore with higher
efficiency. This is because in this case the power losses of the generator can be
omitted twice.
Mainly when starting into pump direction, the pumpturbines lose much more time
compared to ternary units because they must be started with a depressed water level
in the draft tube using a start-up frequency converter. Improvements can certainly be
obtained using full size converters but they are not yet widely spread and for the
power of the Obervermuntwerk II units they are not actually known.
Fig. 4: Shifting time of a ternary unit
In contrast to pumpturbines, ternary units offer significantly shorter switching times as
shown in fig. 4 because the pump rotates in the same direction as the turbine and
they can be started using a hydraulic torque converter. For phase shifting mode one
just has to decouple the pump as well as the turbine impeller which can be done
much faster and easier than aerating the machine.
5 Conclusion
The higher efficiency of the machines and the operational flexibility has led to the
decision to realise ternary units for the Obervermuntwerk II. The fast switching time
between the different operation modes and the possibility to cover the full control
range from -100% until +100% load are the main advantages of these machineries
with respect to today’s electricity market requirements and additionally allow for the
use of the machines as a full back-up for the Kopswerk II units.
To avoid the requirement to empty the draft tube of the turbines in case of pump or
phase shifting operation mode, a shifting clutch is installed for the turbine. By
declutching the turbine impellers, the tailrace system can be simplified because
deaeration chambers can be omitted and the ventilation losses in these operation
modes are even reduced, which further increases the efficiency of the powerplant.
For a fast start-up of the pumps, hydraulic torque converters will be used. Compared
to the friction coupling, which was also examined in detail, the wear-free operation
and the well-known system of the hydraulic torque converters with numerous
comparable references constituted this decision.
Fig. 5: Cross section of the ternary machine set (sample picture from Voith)
To improve the behaviour of the machinery in part-load operation which is awaited in
a huge amount of time over the year in the Obervermuntwerk II, the volutes of both
the pump and the turbine will be embedded in concrete as shown in fig. 5.
References
[1]
[2]
H. Schmöller: Pumped storage power plant Obervermuntwerk II – economical
background. 18th International Seminar on Hydropower Plants. TU Wien 2014.
G. Penninger, F. Spitzer: New challenges for modern pump storage units. 14th
International Seminar on Hydropower Plants, ISBN 3-9501937-2-3. TU Wien
2006, p. 465 – 472.
Authors
Dr. Peter MEUSBURGER
Vorarlberger Illwerke AG
Engineering Services Maschinenbau
Anton-Ammann-Strasse 12, A-6773 Vandans, AUSTRIA
Phone: +43-5556-701-86260, FAX: +43-5556-701-17086260
E-mail: peter.meusburger@illwerke.at
Dr. Peter Meusburger graduated in Mechanical Engineering from the Technical
University of Graz, Austria. He then worked as an Assistant Professor at the
Department of Hydraulic Fluid Machinery at the Technical University of Graz where
he finished his doctoral studies in mechanical engineering in June 2006 with the
degree of a doctor of technical sciences. Since 2008 he has worked as an expert for
3D fluid hydraulics and waterhammer calculations at Vorarlberger Illwerke AG.
Dipl. Ing. Lucas WERLE
Vorarlberger Illwerke AG
Engineering Services Maschinenbau
Anton-Ammann-Strasse 12, A-6773 Vandans, AUSTRIA
Phone: +43-5556-701-86382, FAX: +43-5556-701-17086382
E-mail: lucas.werle@illwerke.at
Lucas Werle graduated in Mechanical Engineering from the Technical University of
Graz, Austria. Since 2009 he has worked as project engineer with a focus on FEM
and waterhammer calculations at Vorarlberger Illwerke AG.