REDUCED SCALE MODEL TEST OF PUMP-TURBINE …iahrwg2015.si/files/papers/4_Hydraulic_transient_and_control... · ... Nicolas Ruchonnet, Turbine Physics, Andritz Hydro SA, Rue ... pump

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.


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


(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


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


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


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


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).


7. REFERENCES

[1] Hell J., Egretzberger M., Lechner A., Schürhuber R., Vaillant Y. Full size converter

solutions for pumpedstorage plants – a promising new technology. Hydro2012, Bilbao.

2012.

[2] Sick M, Oram O, Braun O, Nennemann B, Coutu A. Hydro projects delivering

regulating power: Technical challenges and cost of operation. Hydro2012, Innsbruck.

2013

[3] Coutu A, Lauzon J, Monette C, Nennemann B, Huang X, Francis runner: cost of

operation. 5th IAHR International Workshop on Cavitation and Dynamic Problems in

Hydraulic Machinery. Lausanne. 2013.

[4] DÖRFLER, Peter, SICK, Mirjam, et COUTU, André. Flow-Induced Pulsation and

Vibration in Hydroelectric Machinery: Engineer’s Guidebook for Planning, Design and

Troubleshooting. Springer Science & Business Media, 2012.

[5] HASMATUCHI, Vlad, FARHAT, Mohamed, ROTH, Steven, et al. Experimental

evidence of rotating stall in a pump-turbine at off-design conditions in generating mode.

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[6] BRAUN, Olivier. Part load flow in radial centrifugal pumps. Thèse de doctorat. École

Polytechnique Fédérale de Lausanne. 2009.

[7] STENS, Christine, RIEDELBAUCH Stefan. CFD simulation of the flow through a

pump turbine during a fast transition from pump to generating mode. 6th IAHR

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Documents

REDUCED SCALE MODEL TEST OF PUMP-TURBINE …iahrwg2015.si/files/papers/4_Hydraulic_transient_and_control... · ... Nicolas Ruchonnet, Turbine Physics, Andritz Hydro SA, Rue ... pump