Draft cone tube pressure of pumped-storage power unit in load rejection test

Abstract: One of the key control parameters in hydraulic transient process of pumped-storage power unit is draft cone tube pressure, which should be within the design value. The design value is calculated by one dimension mathematical model of hydraulic transient theory with considering on certain pressure pulsation correction and calculation error. For a long time, there is non-negligible deviation between the calculated value by one dimension numerical simulation and the measured value of draft cone tube from on-site load rejection test. Henceforth, once engineers use measured values to check calculation results, they can not get reasonable explanation and evaluation between simulation results and measured values. In order to settle the difference between regulation guarantee calculation value and measured value, minimum pressure of draft cone tube was clarified from multiple perspectives in this paper, and Chinese standards or codes relating to hydraulic transient calculation were reviewed. The standard research indicates that for pumped storage unit, the minimum draft cone tube pressure should not below -0.08MPa in any circumstances. Measured draft cone tube pressure data of the rated load rejection tests performed in Hongping pumped storage power station during commissioning period was investigated. Firstly, field measuring conditions of draft cone tube pressure was thoroughly assessed and confirmed, and the validity of measured pressure was verified by using short-time-Fourier-transform to analyse the frequency characteristics. The frequencies of four draft cone tube pressure are similar, of which are low frequency component and rotor-stator frequency. The main frequencies are lower than first order eigen frequency of measuring pipes. The obtained data can be regarded as near wall measured pressure. Secondly, Savitzky-Golay filter was employed to separate trend and pulsation of measured draft cone tube pressure. The separated pressure trend can be deemed as the average section pressure of draft cone tube in one dimension numerical simulation, and the pressure pulsation represents the fluid complexity in the hydraulic transients. For pressure pulsations, relationship between maximum peak-to-peak values and time lengths were studied. For Hongping pumped-storage units, the maximum peak-to-peak values and time lengths of each unit are 0.489 MPa/0.46 s, 0.477 MPa/0.09 s, 0.532 MPa/0.66 s and 0.486 MPa/0.11 s, the average is 0.496 MPa/0.33 s. Consistency between pressure trends and one-dimension simulation results was verified. The verfication indicates that the guide vane closing principles are the same and pressure trends comply with the simulation results well. Thirdly, maximum peak-to-peak value of pressure pulsation was used to correct the one-dimension simulation results and trend items. The correction of draft cone tube has negligible errors with on-site measured values. Finally, the correction process was summarized for the hydraulic transient calculation of draft cone tube. The case study indicates that the pressure trend, which is consistent with one-dimension simulation result, can be effectively separated from measured draft cone tube pressure by using low-pass filter with cutoff frequency of 0.1-0.2 times rated rotational frequency; the maximum peak-to-peak value of pressure pulsation can be obtained by selecting data of 3-4 rotational periods; hydraulic transient calculation of draft cone tube can be effectively corrected and verified by superposing the maximum peak-to-peak value of measured pressure pulsation. The research provides effective support for regulation guarantee design and verification of pumped-storage power units.