Fig 1  Bajina Basta storage and dam. This storage is the lower reservoir for Bajina Basta pumped storage plant. The roof and the crane on it is at the bottom of the tailrace.

Pump - Turbine Specifications

Type:

Vertical shaft, single stage, single runner, Francis pump-turbine

Turbine:

Head: 497 - 600 m | | Rated head: 554 m

Output: 243 - 315 MW | | Rated output: 249 MW

Speed: 428.6 rpm

Specific speed: 73 m-kW

Pump:

Head: 532 - 621 m

Discharge: 37 - 51 m³/s

Speed: 428.6 rpm

Specific speed: 27 m- m³/s

Other data:

Runaway speed: 659 rpm

Transient speed rise after full load rejection: 45%

Maximal penstock pressure: 900 m

Submergence: 54 m

Mechanical Design

As a part time designing and consulting engineer at "Energoprojet", Belgrade Prof. S Pejovic had done mechanical design of "Bajina Basta" pumped storage power station.

Model Tests

The pump-turbines in Bajina Basta pumped storage operate with the head exceeding 600 m. This was the record high head. The series of model tests had been performed to verify the hydraulic characteristics. The figure shows the model test setup (TOSHIBA REVIW).

Four quadrant characteristics had so called unstable "S" form. The first explanation of the "S" form instability phenomena was published by Pejovic et al. Link: IAHR 1976 Leningrad S form characteristics.pdf

Fig 2 Model test setup at TGOSHIBS laboratory

Transient Analyses

Transient computer analyses had been based on four quadrant model test characteristics. For the firs time the "S" form instability had been discovered by computer simulations as Fig 3 suggested. The peaks of pressure fluctuations exceeded the 900 m design penstock pressure head. The philosophy of the governing and protective system had been changed to prevent a parallel runaway of the both units at full flow. The problem was solved if all protective devices properly responded in all critical transients. If not, pressure transient peaks would exceed the design limit of 900 m pressure head.

Water column separation and Reverse waterhammer

The draft tube water column separation had been analysed at the design phase. At my demand the manufacturer analysed and confirmed that pump-turbines submergence of 54 m would protect system from cavitation and water column separation in all pump, turbine and transients’ mode of operations.

In 2008, about 30 years later, editing and updating the second edition of the Guide to Hydropower Mechanical Design, ASME HPTC (American Society of Mechanical Engineering  Hydro Power Technical Committee), HCI Publications 1996 the water column separation in the Bajina Basta drat tubes has been noticed in the diagrams (Fig. 3 and 4). The calculated pressure in the draft tube was below the zero pressure; therefore dangerous water column separation could occur followed by severe rejoining in the draft tube and runner. To prevent a runaway, particularly the simultaneously runaway, the closing devices of the both pump-turbines, wicket gates and penstock valves are closing atomically by the control system. It is highly improbable the all four closing devices to fail to close but a careful maintenance must keep them operable at any and all the time.

Fig 3  Two units runaway;
guide vanes and inlet penstock valves are open;
both pump-turbines run at full runaway;
calculated zero pressure in the turbine draft tubes means water column separation

 

The water column separation in runaway has not been noticed by the team of engineers, I was the member of, and was not indicated by manufacturers and other experts involved in the design and construction of the Bajina Basta pumped storage plant. Later on additional analysis of the field test results depicted in Fig 4 shows the pressure jump for 4 bar up to 10 bar in the draft tube in the case of one turbine load rejection.

        

 

Fig. 4  Transient Calculations versus Field Test Results.

Low specific speed unstable “S” form pump-turbine four-quadrant curves is the source of severe transients even at one unit load rejection while other unit continues normal operation. Quick change of flow from turbine to pump direction results in sudden pressure jump and drop. Water column separation occurred in the draft tube and pressure jumped up to 10 bar when separated water columns rejoined. Recommendation: Each low specific speed pump-turbine should have its own penstock and tailrace. The data is from Bajina Basta pumped storage plant.

 

Pump-Turbines for Bajina Basta

The World wide five best companies had been carefully analyzed as potential manufacturers for Bajina Basta pump-turbines. Two of them had been selected for model tests. TOSHIBA's units are in operation.

Hydraulic Vibration and Stability Analyses

Hydraulic vibration and stability had been analyzed in frequency domain. In the "S" zone of four quadrant machine characteristics, system is unstable. Both wicket gates and penstock valves are constructed to prevent runaway in this dangerous unstable transient operating points.

Self Excited Vibrations

Self excited vibrations had been analyzed. System designed to prevent them.

Site Measurements

Measured data of transient regimes ware compared with the theoretical computer simulations. Agreement was good.

Measured machine properties confirmed the quality of the units in operation.

Back to Back Start in Pumping Mode of Operation

 

Vibrations of Turbine Units of Hydro Power Plant "Bajina Basta" during Back to Back Start in Pumping Mode of Operation the Pump-Turbines of Pumped Storage Power Plant "Bajina Basta" were analyzed in order to reduce excessive amplitudes of bearing displacement.

Bajina Basta Cross-section

 

Photo

Bajina Basta Power Plant (4x90 MW)
and
Pumped Storage Power Station Bajina Basta (2X300 MW)

      

 

Papers, Studies, Analyses, Troubleshooting, Simulations…

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