Pressurized System

Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
In-Situ Measurement Campaign at the “Perućica” Hydropower
Plant in Montenegro - Part 2: Pressurized System
Danica Starinac1, Dragiša Žugić1, Zvonimir Predić2, Aleksandar Gajić3, Dušan Džopalić2
and Predrag Vojt1
1
Jaroslav Černi Institute for the Development of Water Resources, Jaroslava Černog 80, 11226
Pinosava, Belgrade, Serbia; E-mail: danica.starinac@jcerni.co.rs
2
A.P. Company, Koste Taušanovića 2, 11120 Belgrade; Serbia, E-mail: apcompany@sezampro.co.rs
3
University of Belgrade - Faculty of Mechanical Engineering, Kraljice Marije 16, 11120 Belgrade, Serbia;
E-mail: agajic@mas.bg.ac.rs
Abstract
The Hydraulic Department of the Jaroslav Černi Institute for the Development of Water Resources undertook
an important measurement campaign under the Perućica HPP Modernization and Revitalization Project.
The campaign included in-situ measurements (water levels, discharges, pressures, displacements, stresses
and vibrations) at different locations throughout the HPP system. Numerous steady-state and unsteadystate scenarios were tested, through continuous and simultaneous measurements at all measurement
points. This paper presents the measurements carried out in the pressurized system, starting from the
power intake structure and ending with the tailrace system. The applied measurement and data acquisition
methodology is explained, followed by results and conclusions. The measurement campaign provided
valuable data for the calibration of a mathematical model, as well as insight into the operation of particular
parts of the system.
Keywords: hydropower plant, in-situ measurements, measurement campaign, Perućica, pressurized system
Introduction
In June 2010, the Hydraulic Department of the
Jaroslav Černi Institute for the Development
of Water Resources undertook an important
measurement campaign under the Modernization
and Revitalization Project of the ‘‘Perućica’’
hydropower plant (HPP) (Starinac et al., 2011).
During the entire period of service, the HPP has
never been operated at the maximum power of 307
MW (total installed capacity), but only to 285 MW. The
average annual output (1960 to 1999) was 823 million
KWh, or about 63% of the potential annual capacity.
Many studies and analyses, such as in-situ
measurements,
mathematical
simulations,
observations, etc. (Djonin et al. 1984; Petrović and
Djonin, 1986) have been undertaken with the objective
to determine the origins of the problems and to identify
the measures best suited to the elimination, or at least
alleviation, of the problems. Based on the outcomes,
UDK: 621.311.21(497.16)
the operating power level was increased to 285 MW.
The main reasons for this limitation were inadmissible
mechanical vibrations of the turbine housing and load
fluctuations, especially in the tailrace system and
particularly during load change, load rejection and
shut down operations (Gajić et al. 1998).
The Perućica HPP Modernization and Revitalization
Project is aimed at increasing the operating capacity
up to the installed power level as a first step and,
after analyzing the possibilities for installing a new,
eighth generating unit, increasing the capacity even
further. For that purpose, it was necessary to carry
out detailed analyses of the entire system, including
mathematical modeling. For model calibration,
parameter values based on in-situ measurements
were required.
The measurement campaign included in-situ
recording of water levels, discharges, pressures,
displacements, stresses and vibrations, at different
locations throughout the Perućica HPP system, in
order to identify the origin of the existing problems and
3
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
to define the basis for rehabilitation. All measurements
were continuous and simultaneous, due to the
requirement to obtain real information on system
behavior under different conditions and to provide
adequate data for mathematical model calibration.
Considering different hydraulic characteristics, the
system could be divided into two parts – an open
channel system (upstream of the power intake ‘’Marin
Krst”) and a pressurized system (downstream of
the power intake). In this paper, the measurements
carried out in the pressurized system are presented
through an explanation of the applied methodology,
followed by results and conclusions.
Description of the System
The Perućica HPP is located on the Zeta River,
in the central part of Montenegro, near the Town
of Nikšić. The HPP has been designed for eight
generating units, but only seven were constructed in
the first phase, with a total installed capacity of 307
MW. The system makes use of the energy potential
of the Zeta River (catchment area 850 km², gross
head 550 m), between its upper course (the Upper
Zeta River) at Nikšić Polje and its lower course (the
Lower Zeta River) close to Glava Zete.
Hydropower plant configuration
The HPP has been planned and designed as a
high-head run-of-the-river hydropower plant, in
combination with three water storage reservoirs.
According to this configuration, the Zeta River (at
Nikšić Polje) was planned as the main supply source
for the Perućica HPP. The initial design called for
the impounding of the Zeta River by the Vrtac Dam,
for controlled release and use for power generation
in line with the power demand. In addition to the
Zeta River, plans called for the utilization of the
Moštanica River and the Opačica River, through
impoundment by the Krupac Dam and the Slano
Dam, respectively. This concept meant that both
reservoirs, Krupac and Slano, would contribute to
the power generation by releasing water into the
Vrtac Reservoir, as a regulating structure for the
Perućica HPP headwater.
From the Vrtac Dam, water is conveyed by a feeder
canal, Zeta I, to a compensation basin and the
power intake at Marin Krst. The compensation basin
has sufficient capacity to provide water during the
startup of the power plant or when additional units
are initiated. Similarly, the compensation basin
preserves water when the units are shut down.
Pressurized system
The pressurized system includes structures located
downstream of the power intake ’’Marin Krst’’: headrace
pressurized system, powerhouse and tailrace system
(which is analyzed in this paper even though it has the
characteristics of an open channel flow).
From the power intake, the headwater is conveyed to the
powerhouse and to each turbine through a pressurized
system (Figure 1). After passing the power intake,
the water enters a headrace tunnel (length 3,323 m,
diameter 4.8 m), which branches into three penstocks
at a trifurcation, at some distance downstream from a
surge tank. Each penstock is provided with a butterflytype emergency valve. The valves are housed in a
valve chamber “Povija” (Figures 2 and 3). The penstock
data are summarized in Table 1.
Figure 1: Longitudinal section through the power intake, headrace tunnel, surge tank and valve chamber
Figure 2: Longitudinal section through the surge tank, the lowest reach of the headrace tunnel,
trifurcation, valve chamber and penstock
4
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
Starinac et al.
Figure 3: Plan view of the lowest reach of the headrace tunnel, trifurcation, valve chamber and penstocks
Table 1: Penstock characteristics
Penstock
Unit
I
II
III
Diameter
[m]
2.2-1.8
2.2-2.1
2.65-2.5
Length
[m]
1,851
1,883
1,930
Discharge
[m³/s]
17.00
25.50
38.25
1, 2
3, 4, 5
6, 7, (8)
Generating units
At the entrance to the powerhouse (Figure 4), the
penstocks branch into manifolds for the generating
units. The powerhouse of the Perucica HPP
comprises seven (7) generating units. All seven
existing generating units consist of horizontal axis
generators. The turbines are of the Pelton type, with
two runners per generating unit, located on either
side of the generator. The characteristics of the
generating units are presented in Table 2.
Figure 4: Layout of the powerhouse, showing all eight units, tailrace tunnels and manifold cavern. Units 1
to 8 (right to left) with two-station sub-units of Unit 1
Table 2: Characteristics of generating units
Number of generating unit
1
2
3
4
5
6
7
8
Rated head (m)
526
Rated discharge (m³/s)
Rated capacity (MW)
8.5
8.5
8.5
8.5
8.5
12.75
12.75
12.75
38
38
38
38
38
58.5
58.5
58.5
No. of Pelton runners per unit
2
2
2
2
2
2
2
2
No. of nozzles per turbine
1
1
1
1
1
2
2
2
Each turbine has a separate turbine pit and a short
tailrace tunnel to the tailrace collector tunnel and
from there a tailrace canal to the confluence with
the Lower Zeta River.
and Un = 0.56" (for angle measurements). To
determine the vertical positions (heights), a Leica
DNA03 precision level was used. The accuracy of this
instrument is U = (0.286* L/2) mm, L in km.
Methodology and Equipment
Measurements at the power intake
Horizontal position and height measurements
In order to generate a unique coordinate system for
all measurements and to correlate these relative
measurements, a geodetic survey was conducted
to reference the measurements to the official
coordinate system of Montenegro.
The power intake is the interface between the open
channel system and the pressurized system. During
the measurement campaign, water levels upstream
and downstream of the trash rack were measured
(Figure 5). Vortex formation at the entrance to the
power intake was also recorded by video camera.
To determine the horizontal positions, a Leica TC 2003
total station was used. The precision of this instrument
is U = (1+1*10-6*L) mm (for distance measurements)
Water levels upstream of the trash rack were
measured using an Eijkelkamp diver, a device with
a pressure sensor and integrated data logger. The
5
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
device was placed inside a perforated pipe, next to
an existing level gauge (Figure 6).
Water levels downstream of the trash rack were
measured using a similar device, Solinst Levelogger
Gold Model 3001, located in the roller gate shaft of
the power intake.
Existing devices for water level measurement were
also used, together with a Datataker DT500 data
logger.
Vortex formation in front of the entrance to the power
intake was continuously recorded by video camera,
to determine any clogging or air retraction.
Figure 5: Power intake at Marin Krst, longitudinal section
Measurements at the surge tank
Water levels in the surge tank were measured
applying two methods: radar and pressure sensor.
The Endress+Hausers Micropilot M FMR240 radar
is a device for continuous, non-contact water level
measurement, with data acquired by Datataker
DT500. The radar sensor was installed above
the crest of the surge tank, suspended from the
structure braced to the fence of the surge tank top
gallery (Figure 7).
Figure 7: Measurement of surge tank water
levels by radar
Measurements at the penstocks
Pressures
Pressures were measured at the upstream ends
of the penstocks, at the Povija valve chamber, and
at the downstream ends of the penstocks, in the
powerhouse.
Four R.D.A. pressure probes (range 0-10 bar;
current output 4-20 mA) were installed in the
valve chamber: one at the downstream end of
6
Figure 6: Water level measurement
upstream of power intake
Besides the described method, water levels in the
surge tank were measured by a pressure sensor.
A Solinst Levelogger Gold Model 3001 was used,
which is a pressure sensor with integrated data
acquisition and storage. The probe was placed on
the bottom of a perforated metal housing (Figure
8) and submerged to an exact elevation. Since this
probe measures absolute pressure, the water levels
were determined upon appropriate correction for
barometric pressure.
Figure 8: Measurement of surge tank water
levels, pressure sensor housing
the headrace tunnel, before the trifurcation, at the
existing drainage pipe (Figure 9), and one at the
upstream end of each penstock, at the bypass pipes
(Figure 10). The probes were calibrated individually
in-situ, before installation, for values of 0-5 kp/cm2,
using a Deadweight Tester. The Deadweight Tester
(range 0-10 bar; accuracy ±1mbar), together with
a DT85G logger (sampling frequency 3.5 Hz), was
also used for continuous pressure recording during
steady state measurements.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
Starinac et al.
Figure 9: Pressure measurement in headrace
tunnel, before trifurcation
Figure 10: Pressure measurement at upstream end
of penstock
Pressures at the downstream ends of Penstocks
1 and 2 were measured in front of the pre-turbine
valves, at the entrance into auxiliary supply units 1
and 2, while pressures at the downstream end of
Penstock 3 were measured at the discharge valve,
just ahead of a bifurcation. During steady state
tests, pressures were measured using a deadweight
tester and transducers together, while in unsteady
state tests only transducers were used.
During unsteady state tests, pressure signals from
the downstream ends of Penstocks 1 and 2 were
downloaded from the HPP monitoring system and
the data recorded together with other signals from
Units 1 and 2, on a computer-based acquisition
system ED 2000.
One deadweight tester (Figure 11) was used
to measure pressures at the downstream ends
of Penstocks 1 and 2 during steady state tests.
Another deadweight tester was installed close to
the bifurcation and discharge valve on Penstock
3, and used for calibration of an HBM (Hottinger)
pressure probe used during unsteady state tests,
but also for steady state measurements. The data
were recorded by the Datataker DT800.
Discharge measurement
Penstock discharges were measured about 100 m from the
downstream end of the tunnel part of the penstock route, by three
ultrasound flow meters.
Figure 11: Deadweight tester in the
Povija valve chamber
Each flow meter was installed on the penstock with two pairs of
ultrasonic probes – a total of four at one measurement cross section
(Figures 12 and 13), in order to increase velocity measurement
accuracy. The instruments included Endress+Hausers Prosonic Flow
93W (clamp on) devices, which cover a velocity range of up to 15 m/s.
The logging equipment included a stand-alone DATATAKER DT800
(with data acquisition at 1 second intervals) and an Endress+Hausers
Memograph (with data acquisition at 60-second intervals).
Figure 12: Penstock discharge measurement point
Figure 13: Ultrasound probes installed on penstock
Displacement of Penstock 3
Displacement of Penstock 3 relative to the concrete block was measured at two locations: near Foundation
T5 (between T5 and T6), and at the expansion joint of Foundation T8.
Measurements at Foundation T5 included displacements of the penstock’s cross section in two perpendicular
directions. Inductive probes HBM W50TS (for Y direction, Figure 15) and HBM W50 (for X direction, Figure
14) were used, together with HBM KWS 2 channels (for signal conditioning). Data were acquired by a DT80
7
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
data logger. This device had an internal memory of 50 MB, from where the data could be transferred online
(using a GPRS router) or downloaded later (interface ports: Ethernet, USB, RS232). Both methods were
applied on instant request.
Figure 14: Measurement of displacement in X
direction at Foundation T5
Figure 15: Measurement of displacement in Y
direction at Foundation T5
Measurements at Foundation T8 included
displacements of the penstock’s cross section
in two perpendicular directions, as well as axial
displacement of expansion joint No. 8 at three points
along the circumference: 0°, 120° and 240° (Figure
16). The methodology was similar to that applied at
Foundation T5.
Stresses
Stresses at Penstock 3 manifolds and branches (both
static and dynamic) were measured using strain
gauges mounted on the surface of the penstock
at the manifolds and branches. The gauges were
connected to a data logger and readings were taken
automatically.
Figure 16: Measurement of axial displacement at
Foundation T8
Twenty Tokyo Sokki Kenkyujo strain gauges were
installed according to a previously defined layout
(Figure 17). The strain gauges were mounted
approximately at (as close as possible to) the
positions (points) where maximal stresses had been
observed during earlier measurements, so that the
results could be compared.
Based on the assumption that the directions of
principal stresses in a pressurized tube with a
circular cross section are known, the strain gauges
were positioned as biaxial rosette gauges with an
internal angle of 90°; one axis was parallel and
the other perpendicular to the penstock centerline
(Figure 18). Only at the most threatened points of
Branch A, triaxial rosette gauges with angles of 0°,
45° and 90°, marked as R1 and R2 (Figure 19), were
used. R1 and R2 were selected partly because of a
not-so-cylindrical contour of the manifold, but also
to verify the assumption regarding the directions
of the principal stresses. The results confirmed the
assumption.
8
Figure 17: Schematic representation of the
arrangement of strain gauges on Penstock 3
Each active strain gauge had a corresponding
compensation (passive) strain gauge with the same
characteristics, glued to a plate made of the same
material as the penstock. The plate was placed
beside the active strain gauge, paying attention
that the passive strain gauge would be exposed to
the same temperature and humidity conditions, but
not to any stress. In this way environmental effects
were compensated and annulled.
The principal stresses were calculated on the basis
of measured strain. A DataTaker DT800/3 was used
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
for data acquisition. The device was connected (via Ethernet port) to a computerized data acquisition
system in the powerhouse, providing online results of the stresses.
Figure 18: Measurement of stresses at Penstock 3
Measurements at the generating units
Measurements at the generating units included
pressures, needle and deflector positions, rotating
speed/overspeed, power at the generator outlet and
vibrations.
Pressures in front of spherical turbine valves and
pressures at turbine inlets were measured at each
generating unit (except A5), using transducers of
the existing HPP monitoring system. No sensor
was in place at Generating Unit A5, so an HBM
(Hottinger) pressure probe was installed. This probe
was calibrated in-situ, before installation, using a
deadweight tester. Unfortunately, since the valves in
the HPP were not set properly, measurement results
for Generating Unit 5 are missing.
Figure 19: Measurement of stresses using triaxial
rosette gauges
(galvanic isolation), and recorded by the data
acquisition system described below. An exception
was made at Generating Unit A6 because of the fact
that all four needle positions were represented by
only one signal, and both deflectors also by only one
signal. For that reason, four Vishay potentiometers
were installed to measure needle opening (Figure
20), and two Celesco probes to measure deflector
positions (Figure 21).
The rotating speed of the turbines was measured
at each unit using existing tachometers (0-200%;
4-20 mA (A1-A4, A6, A7), 0-20 mA (A5)). Obviously
a 100% range is better suited to rotating speed
measurements, but the 0-200% range was selected
in order to measure overspeed in transient regimes.
The position of the needles and deflectors were
observed by the existing HPP monitoring system.
The signals were transmitted through interfaces,
avoiding interference in the monitoring system
Instantaneous power measurement signals from the
existing HPP monitoring system were transmitted
using galvanic isolation interfaces and recorded
together with the other signals from the respective
unit.
Figure 20: Needle opening measurement at
Generating Unit 6
Figure 21: Deflector position measurement at
Generating Unit 6
All signals from the measurements at the generating units, except vibration, were recorded together using
three acquisition systems: ED2000 (signals from Generating Units 1 and 2, Figure 22), Datataker DT800/2
(signals from Generating Units 3 and 4, Figure 23), and Datataker DT800/1 (signals from Generating Units
5, 6 and 7, Figure 24).
9
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
PC notebook with CATMAN software, and was used
for data acquisition from Generating Units A5-A7.
Figure 22: Data acquisition system ED2000
Figure 25: Existing NI-085 sensors for vibration
measurement at Generating Units 1-4
Figure 23: Data acquisition system DT800/2
Figure 26: Installed NI-085 sensors for vibration
measurement at Generating Units 1-4
Water level measurements in the turbine pits
Figure 24: Data acquisition system DT800/1
During all steady state tests, vibrations were
measured on each generating unit using contactless
displacement sensors.
Sixteen (2 per turbine) Schenck NI-085 sensors
had already been in place (existing HPP monitoring
system) and 12 new sensors of the same type
were installed (for the measurement campaign) on
Generating Units A1-A4.
In the niches of 4b and 6b turbine pits, Druck
pressure sensors were installed on the bottom of
perforated pipes (Figure 27), to measure water
levels. Data were collected by a PC-based (Labtech
software) acquisition system ED 300.
It was difficult to install the sensors (Figure 28), but
even more difficult to maintain sensor safety and
position during the measurements, which explains
the incompleteness of the data. Because it was
impossible to better anchor and protect the sensors
in such conditions, huge water turbulence in the
turbine pits caused displacement, crashing into
the pipe wall and damaging the sensors before the
measurements were completed.
Schenck NI-083 sensors were installed on the
a-turbines of Generating Units A5, A6 and A7,
while 2 HBM sensors (A5), 2 Baluff sensors (A6)
and 2 Kaman sensors (A7) were installed on the
b-turbines of Generating Units A5, A6 and A7.
All contactless sensors were mounted in the same
way, to measure vibrations in two perpendicular
directions – horizontal and vertical.
Data were collected by two acquisition systems.
The first system, 16-channel ED-A2 with galvanic
isolated ports 0-20 V, was used for data acquisition
from Generating Units A1-A4.
The other consisted of two connected Spider8 (HBM)
systems, 3 RDA 2-channel signal conditioners and a
10
Figure 27: Water level measurement in turbine pit
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
The camera footage and electrical conductivity
data were recorded using a PC-based acquisition
system, while the data acquisition system for the
ultrasound level meter was a Fluke 289.
Figure 28: Installation of measurement equipment
in turbine pit
Measurements in the tailrace tunnel and canal
The measurement campaign included water levels in
the tailrace system and clogging in the tailrace tunnel.
Since access to the tailrace was not easy, existing
vertical aeration shafts were used (Figure 29).
Water levels were measured in three points: at the
upstream end, in the middle and at the downstream
end of the tailrace tunnel. At the first two points, the
existing aeration shafts were used for installation
of the measurement equipment (Figure 30). Huge
metal perforated pipes were mounted inside the
aeration shafts, and then the pressure sensors
inserted through them to the bottom of the tunnel.
Druck pressure sensors, calibrated in-situ, were
used at first, together with the ED300 data acquisition
system, used also for water levels in the turbine pits.
Similar to the turbine pits, huge water turbulence
caused significant vibrations of the protective tubes
and sensor displacement and damage before
the measurement campaign was completed.
These sensors were replaced by Eijkelkamp diver
pressure sensors. A Solinst Levelogger Gold M5
was installed at the downstream end of the tunnel.
All these instruments functioned as both pressure
sensors and loggers (integrated).
Figure 30: Water level measurement point
Figure 31: Measurement point for clogging
In-Situ Measurement Overview
Considering the main objectives, the measurement
campaign was divided into three groups of
measurements, based on the boundary conditions
in the open channel system and the HPP operating
regime in the pressurized system (Table 3).
Table 3: Measurement groups
Group
1
2
3
Figure 29: Measurement points in the tailrace
Clogging was observed by an ultrasound level
meter (for detecting distance up to the liquid foam
or water) and a video camera (for inspection inside
the tunnel), installed in separate aeration shafts.
Both measurement points were also equipped
with an electrical conductivity meter, placed at the
calotte of the tunnel to detect liquid foam or water.
Open channel
system
Unsteady
UP
boundary
(Upstream)
conditions
Steady
S (Steady)
boundary
conditions
Unsteady
U
downstream
(Unsteady)
boundary
condition
Mark
Pressurized
system
Steady state
– upstream
system testing
Steady state
regimes
Unsteady state
regimes
Steady state measurements: upstream
system testing
This measurement group was aimed at testing of
the system of open canals under both steady and
unsteady conditions, to provide relevant data for
mathematical model calibration. The measurements
are described in detail in Part 1 of this paper.
11
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
Starinac et al.
During all the tests, the flow conditions in
the pressurized system were steady and the
measurements were carried out only at the most
important points. Since these measurements
were not as important from a pressurized system
perspective, only a description of the tested
scenarios is provided (Table 4).
Table 4: Scenarios of the 1st measurement group (UP)
Scenario
Date
UP1
16.06.2010
UP2
17.06.2010
Time
start
end
07:00 13:00
13:00 19:00
19:00 22:00
07:00 13:00
13:00 19:00
19:00 22:00
HPP power
(MW) (%)
61
20
122
40
61
20
100
~30
180
60
100
~30
Measurements in steady state regimes
The measurements were made for six different
scenarios which included, for the most part, the
testing of high discharges and HPP operation at
high capacities.
This was partly caused by the hydrological
conditions (heavy precipitation had resulted in large
run-off and the discharges of the Zeta River were
unexpectedly elevated), such that the HPP had to
operate at high capacity in order to avoid excessive
water losses. The second reason was the argument
that system behavior during low discharges was
properly analyzed through measurements under
unsteady boundary conditions.
Upon examination of the steady state regimes, the
maximum power of the HPP was achieved at about
305 MW, while the water level in the compensation
basin was at 603.5 m.a.s.l. During this regime,
significant fluctuations were observed in the turbine
pits and the tailrace system, followed by water
jetting through the shafts between the turbine room
and the distribution facility of the power plant. For
the HPP operating at up to 290 MW, such events
were not observed. In all regimes, the tailrace tunnel
functioned as a conduit with a free water surface,
without any air pockets or clogging. As power
increased, losses along the flow conduits grew, until
the pressures in the penstocks dropped and the
maximum stresses at the C3 bifurcation fell from
294 MPa to 276.5 Mpa. However, when the power
increased beyond 280 MW, turbine vibration grew.
The measured shaft oscillations at the turbine beds
were within acceptable limits. For turbines 1A, 2A,
2B, 3A, 3B, 4A and 7B, vibration status corresponded
to Zone A according to ISO Standard 7919-5, and
for turbines 1B, 4B, 5B and 6B it corresponded to
Zone B. Turbines 5A, 6A and 7A were in Zone C.
Displacements of Penstock C3 in the zone of Blocks
T5 and T8 reached maximum values of up to ±2 mm
in the “x” and “y” directions, and up to ±15 mm in the
axial direction.
Based on measured water levels at the power
intake, pressure losses at the trash rack were found
to depend on discharge (Figure 32). The graph
shows a pressure loss at the trash rack of 1.8 m
during maximum discharge.
Table 5 shows the scenarios of this group, along
with the planned operating conditions of the HPP.
Each scenario lasted for two hours and allowed
steady state flow in the pressurized system to be
achieved.
Table 5: Steady state scenarios
Scenario
Date
S1
22.06.2010
S2
22.06.2010
S3
22.06.2010
S4
24.06.2010
S5
24.06.2010
S6
24.06.2010
*Planned values
Time
start
end
10:00 12:00
16:00 18:00
18:00 20:00
11:00 13:00
13:00 15:00
18:00 20:00
HPP power*
(MW) (%)
260
85
200
65
280
91
280
91
290
94
307
100
During the measurement campaign, water levels
in the compensation basin were generally between
603.0 and 603.5 m.a.s.l., and the trash rack on the
intake structure was regularly cleaned. Under such
conditions, the system functioned quite well in all
regimes.
12
Figure 32: Pressure losses at the trash rack of the
power intake as a function of discharge
Knowledge of the water levels in the surge tank
during steady state regimes allowed for the
estimation of pressure losses along the headrace
tunnel. The difference between the water level
downstream of the trash rack of the power intake
and the water level in the surge tank (Figure 33)
shows that the pressure loss in the headrace tunnel
reaches 6 m WC at maximum discharges.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
Starinac et al.
The results of water level measurements in the
tailrace system provided for the definition of the
losses along the tunnel as a function of discharge
(Figure 35). The water level at the upstream end
of the tunnel did not exceed 62.00 m.a.s.l., even
at maximum discharges. At the downstream end
of the tunnel, the highest water level at maximum
discharge was 61.6 m.a.s.l.
Figure 33: Pressure losses in the headrace tunnel
as a function of discharge
It was possible to estimate local losses at the
trifurcation based on the pressure differences
between the measurement points at the downstream
end of the headrace tunnel (in front of the trifurcation)
and the upstream end of each penstock, recorded
by a deadweight tester during steady state regimes.
Since the pressures were also measured at the
downstream ends of the penstocks, the pressure
losses in the penstocks could also be analyzed. The
clear situation at Penstock 3 and a direct pressure
measurement at the upstream and downstream ends
of this penstock yielded a very small dispersion of
points (Figure 34). For maximum capacity conditions,
pressure losses in Penstock 3 were about 10 m WC.
Figure 35: Differential water level in the tailrace
tunnel as a function of discharge
Measurements in unsteady state regimes
In the third group of measurements, unsteady flow
was caused by rapid changes in HPP operation,
while upstream boundary conditions were steady. In
this case, the most important measurement points
were those located in the pressurized system,
while the purpose of the measurements in the
open channel system was to provide the upstream
boundary condition for the pressurized system.
Figure 34: Pressure losses in Penstock 3 as a
function of discharge
These measurements were carried out for 9
different scenarios, presented in Table 6. The
duration of each scenario was approximately one
hour, including flow stabilization in the pressurized
system and subsequent transient flow.
Table 6: Unsteady state scenarios
HPP power*
Scenario
U1
U2
U3
U4
U5
U6
U7
U8
U9
*Planned values
Date
24.06.2010
25.06.2010
25.06.2010
25.06.2010
25.06.2010
26.06.2010
26.06.2010
26.06.2010
26.06.2010
Time
20:00
15:00
16:00
17:00
17:30
11:30
14:00
16:30
18:00
(MW)
start
305
266
235
195
305
245
285
305
80
Scenario
(%)
end
140
305
305
305
235
0
0
0
0
start
99
87
77
64
99
80
93
99
26
end
47
99
99
99
77
0
0
0
0
Normal power decrease
Normal power increase
Normal power increase
Normal power increase
Normal power decrease
Quick shutdown
Quick shutdown
Quick shutdown
Quick shutdown
13
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
In all unsteady state scenarios, the water level
fluctuations in the surge tank were within tolerance,
in the sense that the maximum water level was
below the top of the surge tank gallery (no overflow),
and also that the minimum water level was above
the critical level (no air suction). The maximum
water level of 622.5 m a.s.l. was achieved upon
load rejection (quick shutdown of HPP) at 305 MW
output (Scenario U8, Figure 36). The minimum
water level of 585 m a.s.l. was reached on the same
occasion (Scenario U8), but also during a normal
power increase from 190 to 305 MW (Scenario U4,
Figure 37).
Figure 36: Water level variation in surge tank,
Scenario U8
Figure 38: Pressures in the Povija valve chamber,
Scenario U4
Throughout the campaign, the water level in the
compensation basin did not descend below 603 m
a.s.l. This lead to the conclusion that no problem
occurs at Povija even at maximum operating
capacity if the water level upstream from the trash
rack is high enough. Maximum pressures reached
625 m.a.s.l. during load rejections at high operating
power levels, but also at low operating power
levels. However, pressures did not exceed those
which were observed earlier for the existing power
limitation of 285 MW.
The maximal allowed pressure in the penstocks
is defined as the value which is 10% greater than
the static pressure. The static pressure upstream
from the turbine nozzle for the maximal possible
water level at the intake structure of 618.0 m.a.s.l.
is 552.2 m WC (618.0-65.8), while the maximal
allowed pressure is defined as 1.1*552.2, or 607.42
m WC = 59.53 bar.
The maximal measured piezometric pressure in
Penstock C1 was pCImax = 55.85 bar, at Generating
Unit A1 it was (relative to the nozzle elevation)
p1max = 55.93 bar, and finally at Generating Unit A2
it was p2max = 56.0 bar.
Figure 37: Water level variation in surge tank,
Scenario U4
Pressure fluctuations in the Povija valve chamber
were also within tolerance limits. They were
analogous to those in the surge tank (both
measurement points had the same oscillation
frequency), but the minimal pressures in the valve
chamber during a normal power increase from 190
MW to 305 MW (Scenario U4) were lower, at 581.5
m.a.s.l. (Figure 38).
Since the pressures were not lower than the penstock
elevation at any point during the measurement
campaign, there was no risk of negative pressures
so the aeration valves were not operated. The
lowest measured pressure was 582 m.a.s.l., or
about 10 m above the level of the aeration valves
to the penstocks.
14
The maximal measured piezometric pressure in
Penstock C2 was pC2max = 56.0 bar, at Generating
Unit A3 it was p3max = 56.8 bar, and finally at
Generating Unit A5 it was p5max = 56.0 bar.
The maximal measured pressure in Penstock C3
was pC3max = 56.6 bar, at Generating Unit A6 it was
p6max = 57.3 bar, and at Generating Unit A7 it was
p7max = 56.6 bar.
An increase in the rotating speed of Generating
Unit A6 of Δn= 132% was above the allowed limit
(transient turbine overspeed 125%), which was
also the case in earlier tests. It will be necessary to
reduce the deflector closing time.
It would be useful to adjust the rotating speed and
deflector closing times at Generating Unit A5 as
well, where the measured transient overspeed was
Δn= 122.8%, and on Generating Unit A7 where it
was Δn= 126%.
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
It was noted that it is necessary to adjust the needle
closing speed (in order to synchronize operation of
the units). During scheduled overhaul, the needle
closing time needs to be adjusted to about 80 s,
from a fully open to a fully closed position. This
means that the closing time has to be extended at
Units A3 and A5 and the closing time at Unit A6 is
to be shortened accordingly. The needle closing
speed should be checked from time to time, in order
not to exceed recommended limits.
Displacements of Penstock C3 at the points of T5
and T8 blocks were within tolerance limits, even in
the transient regimes, with maximum values during
load rejection of up to 5 mm axially.
The displacements at Foundation T5 during the
entire campaign are shown in Figure 39. It is
apparent that significant displacements occurred
only during discharging and filling of the penstock.
The maximum displacement at T5, registered
before discharging of the penstock, was ±0.5 mm
in both directions. During discharging and filling
of the penstock, displacements in the x-direction
(transversely to the penstock axis) varied as follows:
first from 0 to +10 mm, than back, up to -30 mm and
then again to 0 mm.
During that period, the y-direction (vertical)
displacement sensor registered -25 mm, but this
was not an upward displacement since the penstock
was moving axially and thereby pulled the probe up
while bending its core. Upon filling, the penstock
went back to approximately its previous position,
and the probe came back to its zero position. Further
on during the campaign, the penstock dilated at T5
in both directions, but only slightly. The observed
displacement range was between 1 and 2 mm.
Figure 39: Displacement of Penstock 3 at
Foundation T5 during the measurement campaign
the case with T8-y which, beginning on June 19th,
continuously showed a value of about 10 mm. This
discrepancy was a consequence of the physical
movement of the probe’s zero position, which took
place on June 19th, 2010, after axial probe damage
at the expansion joint was rectified.
The damage to the probe at the 120o position
(broken) and the movement of the probe at 240o
for 5 mm from its bed were the consequences of
significant dilation of the expansion joint. The probes
were installed to measure within a range of ±55 mm.
The probe which was at the 0o position (the lowest
point of the expansion joint) was the only one that
kept measuring all the time since the displacements
did not exceed 55 mm. The probe at 120° (the left
probe in the flow direction) measured the greatest
displacement as it fell off and broke. The probe at
240° recorded a displacement of 55 mm plus 5 mm,
which was a dislocation from its bed, but it stayed
in place. During filling, the dilation itself had the
opposite direction, but the penstock did not stay in
its original position; instead, it moved back some
35 mm more, so the dilation trace on the penstock
was approximately 100 mm. Upon reinstallation of
the probes, the measurements were completed on
June 25th, 2010, and the results confirmed that the
displacements in the “x” and “y” directions were only
a couple of mm (Figure 40).
Axial displacements during normal operation were
approximately the same and did not exceed 20
mm in the direction of opening or closing, while in
transient regimes only axial displacements were
registered, and the greatest displacements, of 5
mm, occurred upon the first load rejection from 245
MW (Scenario U6, Figure 41).
Figure 40: Displacement of Penstock 3 at
Foundation T8 during the measurement campaign
The situation with T8 turned out to be a little
different, but only with regard to axial dilatation
at the location of the expansion joint, which was
significantly bigger. The displacements of the
penstock cross section relative to the concrete
block in the “x” and “y” directions were negligible,
as was the case at T5. The values measured by
sensor T8-x were around zero all the time, as was
15
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
the most critical points will increase. The maximum
stresses were registered during a quick shutdown
with the HPP operating at 285 MW (Scenario U7),
when the values of 302 and 312 MPa were recorded
(Figure 43). Rapid power variations also caused an
increase in the stresses at the manifold, but it was
not as significant (Figure 44).
Figure 41: Displacement of Penstock 3 at
Foundation T5, Scenario U6
The measured stresses at Penstock C3 of about
302 MPa were above tolerance. Measurement
point R1, where these stresses were recorded,
was located within the free part of Penstock C3,
upstream from the first branch, close to the place
where cracks had been observed in the welded
junction. In addition to this place, stresses as high
as 312 Mpa were observed at locations T7-T8.
These maximal stresses were recorded during
load rejections (quick shutdowns), when the HPP
operated at a total output of 285 MW and 245 MW.
Figure 43: Stresses at manifolds and branches of
Penstock 3 during unsteady state regimes – quick
shutdowns
The stresses on the manifold of Penstock C3, static
and dynamic state, are shown in Figures 42-44,
which support a certain penstock operating mode.
Different points on the manifolds exhibited different
stress levels. The highest stress was measured at
points T7-T8 (up to 294 MPa) and R1 (up to 278
MPa). A static state of stresses on the manifold, as a
consequence of static pressure within the penstock,
remained almost unchanged from the time of filling
of the penstock to the loading to 260 MW. Following
a further power increase, losses within the system
also grew, and the pressure at the manifold dropped
to 50.9 bar, while the stresses were reduced to
276.5 and 256.5 MPa (Figure 42).
Figure 44: Stresses at manifolds and branches
of Penstock 3 during unsteady state regimes –
normal power increase
No problems were noted in the tailrace tunnel even
upon load rejection at maximum power, but on that
occasion significant pressure fluctuations occurred
within the turbine pits, which were not a result of
water level variations.
Figure 42: Stresses at manifolds and branches of
Penstock 3 during steady state regimes
Based upon the above, it seems that more favorable
conditions at the manifold are provided during
operation at high capacities, but only if no load
rejection occurs. Should this happen, the stresses at
16
Upon quick shutdown of the HPP operating at
285 MW (Scenario U7), or that of Unit A4 at 35
MW, rapid pressure variations were from -0.75 to
+5.5 m WC, after which a longer-lasting negative
pressure with a peak of up to -1.5 m WC occurred
(Figure 45). These changes somehow influenced
probe behavior, since in the next experiment,
upon shutdown from a power level of 305 MW, the
respective probe showed illogical results, but rapid
pressure variations, both positive and negative
values, were present.
Starinac et al.
Water Research and Management, Vol. 2, No. 4 (2012) 3-17
An increase in the plant output up to the installed
power of 307 MW causes a certain increase in
vibrations and shifting of the 5A, 6A and 7A turbines
into Zone C, as well as higher pressure pulsation
within the turbine pits, followed by the ejection of a
mixture of water and air through the aeration shafts
located immediately downstream from the turbine
pits.
Acknowledgments
Figure 45: Water depths in Turbine Pit 4b,
Scenario U7
The initial idea behind measuring water levels in
the turbine pits was based on the assumption that
open flow conditions are present and that the value
of pressure (static head) in any floor point is defined
by the water depth (open flow conditions). However,
measurements showed that this was not the case
and that atmospheric pressure occurred in the
turbine pits only when the units were stopped. The
fact that water from the turbine pits did not flood the
powerhouse confirmed that the water level was in
the turbine pits, but the air pressure varied (some
mean value before load rejection, max. value at load
rejection and min. value after load rejection). That
is the reason why the measurement results are not
presented as absolute levels, but as water depths.
Apparently, the situation in A6B was even more
turbulent since the probe in this turbine pit had
stopped functioning earlier (Probe S3 at Turbine
Pit A6B operated until 24.06.2010 at 14:00 hrs
(Scenario S5), while at 19:00 hrs (in Scenario S6)
it was destroyed. Pressure pulsations were 3 to 4
times stronger, and average water levels were lower.
Conclusion
The measurement campaign was considered a
success, as it provided valuable information with
regard to the data required for both calibration of the
mathematical model and insight into the operation
of particular parts of the system. These facts should
be kept in mind while planning further upgrades.
Regardless of the maximum power at which the
HPP will operate, it is necessary to keep the water
levels in the settling basin (upstream from the
intake structure) above the critical level of 603 m
a.s.l. and to clean the trash rack regularly. It is also
necessary to resolve the problem of insufficient
strain capacity of the Penstock C3 bifurcation,
which is equally jeopardized even at an output of
285 MW; the regulators of some turbines should
also be calibrated to ensure tolerable overspeed at
all places.
We would especially like to thank the Electric Power
Industry of Montenegro for giving us the opportunity
to participate in the Project, and for providing
technical support during field measurements.
References
Gajić, A., Z. Predić (1998): Izvještaj o kompleksnom
ispitivanju HE "Perućica" [Report on
Comprehensive Testing of the Perućica HPP].
Faculty of Mechanical Engineering of the
University of Belgrade (FME), Belgrade.
Djonin, K. (1984): Izvještaj o ispitivanju odvodne
vade i pojava u turbinskim jamama HE
"Perućica" [Report on Tailrace-Outflow Channel
and Turbine Pit Phenomena Investigations
at the Perućica Hydropower Plant]. Jaroslav
Cerni Institute for the Development of Water
Resources, Belgrade, June 1984.
Petrović, N., K. Djonin (1986): Izvještaj o ispitivanju
ulazne gradjevine Marin Krst i spoja kanala Zeta
sa kompenzacionim bazenom i taložnicama
[Report on Model Investigations of the “Marin
Krst” Inlet Structure and Connection of the
Zeta Canal with Balancing Reservoir and
Sedimentation Tanks]. Jaroslav Cerni Institute
for the Development of Water Resources,
Belgrade, September 1986.
Starinac, D., Džopalić, D., Predić, Z., Gajić, A., Vojt,
P. and M. Dimitrijevic (2011): Final Report on Insitu Measurement Campaign at Perućica HPP,
Montenegro – Part 2: Pressurized system.
Jaroslav Cerni Institute for the Development
of Water Resources (JCI), Belgrade, October
2011.
17