reduced scale model test of pump-turbine...
TRANSCRIPT
6th IAHR International Meeting of the Workgroup on Cavitation and Dynamic Problems in Hydraulic
Machinery and Systems, September 9-11, 2015, Ljubljana, Slovenia
* Corresponding author: Nicolas Ruchonnet, Turbine Physics, Andritz Hydro SA, Rue des deux gares
6, 1800 Vevey, Switzerland, phone: +41 21 925 78 34, email: [email protected]
REDUCED SCALE MODEL TEST OF PUMP-TURBINE
TRANSITION
Nicolas RUCHONNET*
Turbine Physics, Andritz Hydro SA, Switzerland
Olivier BRAUN
Turbine Physics, Andritz Hydro SA, Switzerland
ABSTRACT Variable speed motor-generators based on full converter solutions offer new options for operation of
reversible pump-turbines, including fast transition from pump to turbine mode. In the context of
including such operation for providing ancillary services, the loads on rotating and stationary part as
well as pressure pulsations have to be considered to define feasible manoeuvers and for the estimation
of the lifetime of the components.
A pump-turbine reduced scale model test including transition is performed at the ANDRITZ HYDRO
laboratory in Linz. In addition to the standard model test instrumentation, dynamic pressure sensors
and strain gauges have been installed to monitor the dynamic loading of the different components
during the transition.
An innovative test rig configuration is used in order to reproduce prototype flow conditions during the
entire transition. The test program consists of transition from pump to turbine and vice-versa at
various speeds and guide vanes opening angles.
KEYWORDS Pump-Turbine, Measurement, Transition
1. INTRODUCTION
Variable speed motor-generators based on full converter solutions offer new options for
operation of reversible pump-turbines (PT) [1], including fast transition from pump to turbine
mode, where the pump is operating in pump-brake mode for a while. In the context of
including such operation for providing ancillary services, the loads on rotating and stationary
components as well as the pressure fluctuations have to be assessed [2].
In Francis turbine, off design flow conditions can be critical for the runner. High dynamic
stress on runner blades have been measured during start up and part load operation [3]. In PT,
the dynamic torque on the guide vanes is known to increase considerably in pump-brake
mode [4]. Other operating conditions have been studied thoroughly using experimental
methods and CFD simulation, see [5] for runaway, [6] for pump instability. But no data is
available to evaluate the effect of pump-turbine transition. The purpose of the present study is
to fill this lack of experimental knowledge. The collected data will be exploited in the
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
framework of the project HYPERBOLE to validate CFD simulation [7] and to optimize
transition of prototype pump turbine equipped with variable speed motor-generator.
Transition from pump to turbine and vice versa are performed with various guide vanes
opening and transition time on a pump-turbine model. Dynamic pressure sensors have been
installed to capture transient flow phenomena and various mechanical components have been
instrumented with strain gauges in order to evaluate the mechanical loading.
2. TEST OVERVIEW
The measurements have been carried out on the universal test rig at ANDRITZ Hydro GmbH
in May 2015. A PT model of specific speed NQE=0.17 (nSQ=207) with Zs=24 guide vanes and
Zr=7 runner blades was selected for the test.
A specific test rig configuration is used in order to perform the full transition with realistic
flow conditions. At prototype scale, the machine head is determined by the level difference
between upstream and downstream reservoir and is only marginally influenced by the losses
in the hydraulic circuit and the fluid acceleration. In the standard procedure at model scale, a
variable speed pump is used to deliver the desired head in turbine mode. In pump mode, a
valve is used to adjust the head. It is therefore impossible to perform a full transition from
pump to turbine using standard procedure, an alternative test rig configuration is necessary.
The selected configuration is presented in Fig. 1.
Fig. 1 Test rig configuration for pump-turbine transition
The PT is mounted in parallel with a pump and a diaphragm. The head, H, is proportional
with the discharge, Q, through the diaphragm according to the well-known relation.
2KQH (1)
The loss coefficient, K, of the diaphragm is selected in order to operate the pump in the stable
operating range and within the limits of the electric motor.
In pump mode, the discharge of both pump and PT flows through the diaphragm; the pump
speed is decreased in order to maintain the discharge through the diaphragm. In turbine mode,
part of pump discharge flows through the PT, the other part through the diaphragm; the pump
speed is increased in order to keep constant the discharge in the diaphragm.
The pump speed variation is triggered automatically with the turbine speed variation using
same transition time. Linear speed ramps are imposed to both PT and pump using variable
speed electric motor, see Fig. 2. Using this alternative test rig configuration, the transition as
seen from the PT is very close to the transition at prototype scale. The head variation is driven
by the fluid acceleration within the turbine.
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Fig. 2 Pump-turbine and pump speed transition
Measurements have been performed at constant guide vane opening on a large range of
transition time, guide vane opening, cavitation level and head. The present paper focus on test
performed with 20m head at prototype cavitation level (=0.25) for guide vanes
openings=5, 15, 25 deg. The PT is switched from pump to turbine mode in 8s, maintained
in turbine mode during 4s and then back to pump mode in 8s. In pump mode, the PT rotating
speed is n=-1195 rpm, in turbine mode n=1025 rpm. Transitions are repeated 6 times in order
to identify the stochastic effects. The measured transitions are summarized in Tab. 1and
presented in a nED-QED diagram in Fig. 3.
Guide vane
position
pump mode turbine mode Transition
time
Model
head
Cavitation
level
series
number
y nED QED nED QED t H
deg - - - - s mWC - -
5 -0.35 -0.11 0.3 0.07 8 20 0.25 1401-1406
15 -0.35 -0.24 0.3 0.20 8 20 0.25 1407-1412
25 -0.35 -0.26 0.3 0.29 8 20 0.25 1413-1418
Tab. 1Overview of measured transitions
Fig. 3 Overview of measured transitions in a nED QED diagram
3. INSTRUMENTATION
In addition to the stationary standard model test measurements, the instrumentation includes
dynamic pressure transducers at the unit inlet section (Pm01, Pm11), at the outlet section
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
(Pm12, Pm13), in one draft tube cross section with a 90 deg distribution (Pm02, Pm03, Pm04,
Pm05), near the rotor stator interface (Pm06, Pm07, Pm08) , in the chamber between the head
cover and the runner (Pm10) and in one runner channel (Pm16 near the inlet in turbine mode
at the channel center, Pm17 and Pm18 near the leading edge on pressure and suction side
respectively, Pm19 and Pm20 near the trailing edge on pressure and suction side
respectively). The definition of positions (inlet, leading edge …) corresponds to the turbine
definition. The positions of the pressure sensors are presented in Fig. 4 for the runner channel
and in Fig. 5 for the entire unit.
Guide vanes 3, 9, 15 and 21 are instrumented with strain gauges on the stem to measure the
dynamic torque, see positions in Fig. 5. The turbine shaft is instrumented with strain gauges to
record dynamic torque and axial thrust. Strain gauges are applied on the transition radius
between the blades and the crown; four strain gauges are distributed from the leading to the
trailing edge, see Fig. 4.
MID flow meter is used to record the transient discharge. Such instrument is capable of
capturing the flow variation in the frequency range of the tested transitions.
A camera with stroboscopic light (triggered with the turbine shaft rotating speed) is used to
observe the cavitation on the runner during the transitions.
The sampling frequency for the standard model test measurements including MID flowmeter
is 1000 Hz; the other signals are sampled at 5000 Hz. The blade passing frequency (BPF) is
120 / 139 Hz in turbine and pump mode respectively; the guide vanes passing frequency
(GVPF) is 410 / 478Hz in turbine and pump mode respectively.
Fig. 4 Pressure sensors (blue) and strain gauges (green) positions in the runner
Fig. 5 Pressure sensors (blue) and instrumented guide vanes torque (green) position in the unit
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
4. MEASUREMENT RESULTS
The time evolution of discharge, torque and head is presented in Fig. 6 for =15 (series
1407). The time signals have been smoothed using a low pass filter. The different operating
modes are identified with background colors, from 0 to 4.7s the machine operates in pump
mode, from 4.7 to 7.2s in pump-break mode, from 7.2 to 18.7s in turbine mode, from 18.7 to
22.1s in pump-break mode and from 22.1s in pump mode. The vertical black lines indicate the
transition of PT rotating speed. The maximum torque is reached during the transition from
turbine to pump at 22.4s and corresponds to 1.5 times the torque in pump mode. The time
evolution of head is similar to typical prototype transition.
Fig. 6 Time evolution of the normalized discharge (Qref=0.202m3/s), torque (Tref=65.2Nm) and head
(Href=20.0mWC)
The time evolution of head with =15 is presented in Fig. 7 for 6 series of measurement
(series 1407-1412). The initial times have been synchronized to compare the series. The
repeatability of the measurement is good; the pressure peaks (at 3s and 19s) have constant
amplitude for all measurements and the time evolution is also very similar. The deviation
from the average is small and is observed in steady state condition as well. Similar
repeatability is obtained for =5 and =15.
Fig. 7 Time evolution of pump-turbine head after synchronization, 6 measurements
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
In Fig. 8, the time evolution of pressure on the runner blade suction side near the leading edge
is presented for the series 1407 (=15). In Fig. 9 and Fig. 10 the pressure at the rotor-stator
interface in the stationary part and the guide vane torque are presented for the same series.
During the transitions, large amplitude of pressure and torque fluctuation is observed.
Maximum amplitude is reached in pump-break mode and the transition from turbine to pump
leads to higher amplitudes than the transition from pump to turbine. This effect is particularly
visible in the stationary part. From pump to turbine, low fluctuation is maintained until the
machine reaches the pump-brake mode (4.7s). In the transition from turbine to pump, the
large fluctuation generated in pump-brake mode (from18.7 to 22.1s) is maintained in pump
mode even after the PT reached its normal speed (23s).
Fig. 8 Time evolution of pressure on the runner blade suction side near the leading edge
Fig. 9 Time evolution of pressure at the rotor-stator interface from the stationary part
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
Fig. 10 Time evolution of guide vane torque
The spectrogram of pressure in the rotating part is presented for the same series (1407) in Fig.
11. In steady state conditions and during the first part of the transitions, the GVPF is clearly
visible (478 Hz in pump mode, 410 Hz in turbine mode). During the transition broad band
fluctuation is observed especially in pump-brake mode. In the stationary part near the rotor
stator interface, similar effects are observed; see Fig. 12 for pressure and Fig. 13 for guide
vane torque. The BPF (139Hz in pump mode, 120 Hz in turbine mode) and its harmonics are
clearly visible in steady state conditions and broad band fluctuation is observed during
transitions. In all spectrogram, a hysteresis effect during the transitions is visible. The
transition from pump to turbine is not symmetric with the transition from turbine to pump.
This effect is also observed on the global variables, flow inversion is faster from pump to
turbine than from turbine to pump, see Fig. 6. Similarly in term of fluctuation amplitude,
larger amplitudes are observed in the transition from turbine to pump than in the opposite
way, see Fig. 9.
Fig. 11 Spectrogram of pressure on the runner blade suction side near the leading edge
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Fig. 12 Spectrogram of pressure at the rotor-stator interface from the stationary part
Fig. 13 Spectrogram of guide vane torque
5. CONCLUSION
Measurements of PT transition at model scale have been presented. The selected alternative
test rig configuration with PT, pump and a diaphragm mounted in parallel offer the possibility
to measure full transition from pump to turbine and vice versa with conditions similar to
prototype conditions. The preliminary analysis of the collected data offer promising
perspectives. The hysteresis effect observed during the transitions highlights the importance
of transient measurement with respect to data collected in steady state conditions. Further
effort is necessary to finalize the post-processing of the results and perform the validation of
the CFD simulations.
6. ACKNOWLEDGEMENTS
The authors would like to thank their colleagues in Linz for performing the test rig
measurement. The research leading to the results published in this paper is part of the
HYPERBOLE research project, granted by the European Commission (ERC/FP7-
ENERGY2013-1-Grant 608532).
IAHR WG Meeting on Cavitation and Dynamic Problems in Hydraulic Machinery and Systems, Ljubljana 2015
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[3] Coutu A, Lauzon J, Monette C, Nennemann B, Huang X, Francis runner: cost of
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[4] DÖRFLER, Peter, SICK, Mirjam, et COUTU, André. Flow-Induced Pulsation and
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[5] HASMATUCHI, Vlad, FARHAT, Mohamed, ROTH, Steven, et al. Experimental
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[7] STENS, Christine, RIEDELBAUCH Stefan. CFD simulation of the flow through a
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