Downstream Surge Tanks Research Articles

Study on the stability and ultra-low frequency oscillation suppression method of pumped storage power plant with dual units sharing one pipeline

This paper aims to investigate the stability of the pumped storage power plant (PSPP) with dual units sharing one pipeline (DUSOP) and the method of suppressing the ultra-low frequency oscillation (ULFO) characteristics. Firstly, four nonlinear models of pumped storage regulation system (PSRS) with DUSOP are established under the design scenarios of no surge tank, upstream surge tank (UST), downstream surge tank, and upstream and downstream dual surge tanks. Then the stability of the PSRS under different surge tank arrangements (STAs) in the long diversion system design scenarios (LDSDS) is comparatively analyzed. Finally, based on the concept of stability margin, the influence mechanism of the operating points on the ULFO characteristics is investigated in the LDSDS with UST, and the influence of the governor parameters, hydrodynamic parameters, and mechanical parameters on the damping ratio and frequency of the system is also analyzed. The results show that among STAs, the design scheme of arranging the UST is more reasonable for PSPP with LDSDS from the comprehensive consideration of the economy of the project and the stability of the system. For PSPP with UST, on the line of equal stability margin, increasing the proportional gain, reducing the unit inertia time constant, reducing the water flow inertia time constant, and appropriately increasing the area of the surge tank can increase the frequency and reduce the amplitude, which is conducive to the suppression of ULFO. In the stability domain range, increasing the proportional gain increases the frequency and the system stability increases and then decreases. Increasing the integral gain decreases the frequency and then increases the system stability. At the same time, the frequency of the unit can be changed by adjusting the parameters of the system to avoid resonance between the frequency of the unit and the frequency of the power grid and to ensure the stability and safety of the unit operation.

Stability behavior of load adjustment and primary frequency control of pumped storage power plant with upstream and downstream surge tanks

This paper studies the stability behavior of load adjustment and primary frequency control of pumped storage power plant (PSPP) with upstream and downstream surge tanks (UDST). Firstly, the model of PSPP with UDST under load adjustment and primary frequency control is established. The stable domains of PSPP with UDST under load adjustment and primary frequency control are determined. The influence of flow inertia and head losses of connecting pipes of UDST on stable domains is analyzed. The critical stable sectional areas (CSSAs) of UDST under load adjustment and primary frequency control are determined and analyzed. The results show that the stability of PSPP with UDST under load adjustment is usually better than that of PSPP with UDST under primary frequency control. Under load adjustment and primary frequency control, the flow inertia of connecting pipes of UDST is unfavorable for the stability of PSPP, while the head losses of connecting pipes of UDST have no impact on stability of PSPP. Under load adjustment, the CSSAs of UDST present a complex change rule in reference to different governor parameters. Under primary frequency control, the CSSAs of UDST present a monotonic change rule in reference to different governor parameters. With the rise of CSSAs of UDST, the stable domains under load adjustment and primary frequency control become larger.

Superposition control of extreme water levels in surge tanks of pumped storage power station with two turbines under combined operating conditions

This paper investigates the superposition control of extreme water levels (EWLs) in surge tanks of pumped storage power station (PSPS) with two turbines under combined operating conditions (COCs). Firstly, for PSPS with upstream and downstream surge tanks (UDST) and two turbines, the model and ten COCs are presented. The superposition control strategy of EWLs in UDST is designed. Secondly, the most favourable superposition time and the most unfavourable superposition time under COCs, i.e. superposition control moments, are determined. The concepts of initial gradient and superposition gradient are proposed to reveal the control mechanisms. Finally, the influences of tunnel parameters on superposition control moments are analyzed. The results show that tangential points for water level - flow velocity curves of initial operating condition and superposition operating condition are not the superposition control moments. For ten COCs, the most favourable superposition time is the moment of minimum superposition gradient, and the most unfavourable superposition time is contrary. With the increase of sectional area and decrease of length for tunnel, the superposition control moments decrease. Because the initial gradient and superposition gradient are mainly affected by tunnel parameters, the hydraulic coupling oscillations of UDST have no effect on superposition control moments.

Hydraulic coupling vibration characteristics and control of hydropower station with upstream and downstream surge tanks

This paper investigates the hydraulic coupling vibration characteristics and control of hydropower station with upstream and downstream surge tanks (UDST). The basic equations of hydropower station with UDST are established. The state equation and transfer function of hydropower station are derived. The hydraulic coupling vibration domain is proposed and determined. The hydraulic coupling vibration characteristics are analyzed based on vibration domain. The interference and resonance characteristics of UDST are revealed. The control method for avoiding hydraulic resonance is proposed. The results indicate that the hydraulic coupling vibration domain is related to the stable domain. In Domain Ⅰ and Domain Ⅱ, the dynamic response processes (DRPs) are superposed by three vibrations and two vibrations, respectively. In Domain Ⅱ, the coupling effect of the vibrations of upstream surge tank (UST) and downstream surge tank (DST) is strong and can change the frequencies of DRPs. The vibrations of UST and DST have the same frequency after coupling effect. The interference of UDST shows a beat phenomenon. The amplitudes of DRPs present a kind of persistent and periodic variation. The selections of sectional areas of UDST should avoid the hydraulic resonance domain.

Critical stable sectional area of downstream surge tank of hydropower plant with sloping ceiling tailrace tunnel

AbstractThis paper investigates the critical stable sectional area (CSSA) of downstream surge tank (DST) of hydropower plant with sloping ceiling tailrace tunnel (SCTT). Firstly, two types of nonlinear mathematical model of hydropower plant with SCTT during transient process are established. Then, the criterion of operation stability of hydropower plant is obtained based on nonlinear stability theory. Formulae for CSSAs of DST under two types of mathematical model are derived. Finally, the similarities, differences, and relationships of different CSSAs are clarified. The effect mechanisms of the assumption of steady power output of hydro‐turbine and the SCTT on CSSA of DST are revealed. The recommended use conditions of different CSSAs are explained. The results indicate that, under the assumption of steady power output of hydro‐turbine, the formula for CSSA of DST of hydropower plant with SCTT is analytical and has a simple expression. Under the complete model, the formula for CSSA of DST of hydropower plant with SCTT is not analytical and has a complicated expression. The former formula is suggested to be given the priority of use because of good precision and concise expression. SCTT can improve the stability of hydropower plant and decrease the CSSA of DST.

Influence of Bifurcation Pipe Position on Hydraulic Turbulence for Pumped Storage Power Station

There must be hydraulic turbulence transient in the pumped storage power station with one pipe corresponding to several units. This paper established a simulation model by using the method of characteristics for a pumped storage power station to analyse how the change of upstream and downstream bifurcation pipe position and different arrangements of downstream surge tank affect the hydraulic turbulence. The results indicate that unit output’s amplitude of power mode is smaller than that of frequency mode. And the upstream and downstream bifurcated pipes far away from the units can reduce the hydraulic turbulence of normal operating unit while the upstream without bifurcation pipe is better than the downstream without bifurcation pipe to control the output swing. Sharing the downstream surge tank can effectively reduce the maximum relative over output of the unit, while the surge tank placed in the downstream branch pipe will worsen turbulence process.

Analysis of Small Fluctuation Stability of Upstream and Downstream Double Surge chamber Diversion Power Generation System

Installation of upstream and downstream surge chamber is a significant and feasible measure for peak pressure control in the hydropower system with long water diversion and drainage tunnel. This paper presents a study on the fluctuation stability with such a measure. For this system, a complete mathematical model including diversion pipeline, upstream surge tank, hydraulic turbine and downstream surge tank was developed, calculating the hydraulic transition characteristics and analyzing the fluctuation stability with different surge tank cross areas. The calculation shows that, the hydropower system with upstream and downstream surge chamber, can also be stable even if the surge tank cross area is smaller than the calculated value. In practice, in the condition of meeting the stability requirement, multiple schemes should be calculated in order to find out the minimum areas to reduce project investment.

Setting condition of downstream surge tank of hydropower station with sloping ceiling tailrace tunnel

This paper aims to study the setting condition of downstream surge tank (SCDST) of hydropower station with sloping ceiling tailrace tunnel (SCTT). Firstly, the mathematical model of hydro-turbine governing system (HTGS) with SCTT is established and a third order nonlinear state equation is obtained. Then, the stability of HTGS with SCTT is analyzed based on Lyapunov's first method. SCDST based on stability of HTGS with SCTT is derived based on the stability criterion. Finally, the characteristics and influence of SCDST based on stability of HTGS with SCTT are analyzed. The difference and relation between SCDST based on stability of HTGS with SCTT and SCDST based on stability of HTGS with pressurized tailrace tunnel (PTT) are revealed by using comparative analysis. The results indicate that the complete SCDST based on stability of HTGS with SCTT is composed of several inequalities. One of the inequalities is the dominant inequality and determines the stable domain of HTGS and the allowable value of flow inertia. The flow inertia of penstock corresponding to the endpoint of inequality interval is the allowable value of flow inertia that the downstream surge tank is not necessary to set. The section shape of SCTT almost has no effect on the SCDST. From the view of SCDST for hydropower station with SCTT, it is reasonable that the slope of SCTT takes small value. The allowable value of flow inertia of penstock for hydropower station with SCTT is greater than that for hydropower station with PTT.

Dynamic performance analysis of hydro-turbine governing system considering combined effect of downstream surge tank and sloping ceiling tailrace tunnel

This paper aims to analyze the nonlinear dynamic characteristics of hydro-turbine governing system with downstream surge tank and sloping ceiling tailrace tunnel and reveal their combined effect mechanism on stability. First, the nonlinear mathematical model of hydro-turbine governing system is presented. Second, the Hopf bifurcation theory is adopted, and the stability performances of hydro-turbine governing system under different sectional areas of downstream surge tank are analyzed and verified. Finally, the combined effect of downstream surge tank and sloping ceiling tailrace tunnel on stability is investigated and the design methods are proposed. The results indicate that: The dynamic response process of hydro-turbine governing system shows an obvious characteristic of wave superposition. Two types of bifurcation lines reflect the critical stability characteristics of the two subsystems and constitute the boundaries of the stable domain. The sloping ceiling tailrace tunnel cannot make an obvious change of the stability performances of the subsystem of penstock - hydro-turbine unit, but it has significant effect on the stability performances of the subsystem of downstream surge tank - sloping ceiling tailrace tunnel. The larger aspect ratio or smaller ceiling slope gradient results in better stability of the subsystem of downstream surge tank - sloping ceiling tailrace tunnel.

Simulation of pump-turbine prototype fast mode transition for grid stability support

The paper explores the additional services that Full Size Frequency Converter, FSFC, solution can provide for the case of an existing pumped storage power plant of 2x210 MW, for which conversion from fixed speed to variable speed is investigated with a focus on fast mode transition. First, reduced scale model tests experiments of fast transition of Francis pump-turbine which have been performed at the ANDRITZ HYDRO Hydraulic Laboratory in Linz Austria are presented. The tests consist of linear speed transition from pump to turbine and vice versa performed with constant guide vane opening. Then existing pumped storage power plant with pump-turbine quasi homologous to the reduced scale model is modelled using the simulation software SIMSEN considering the reservoirs, penstocks, the two Francis pump-turbines, the two downstream surge tanks, and the tailrace tunnel. For the electrical part, an FSFC configuration is considered with a detailed electrical model. The transitions from turbine to pump and vice versa are simulated, and similarities between prototype simulation results and reduced scale model experiments are highlighted.

Benchmarking of Advanced Control Strategies for a Simulated Hydroelectric System

This paper analyses and develops the design of advanced control strategies for a typical hydroelectric plant during unsteady conditions, performed in the Matlab and Simulink environments. The hydraulic system consists of a high water head and a long penstock with upstream and downstream surge tanks, and is equipped with a Francis turbine. The nonlinear characteristics of hydraulic turbine and the inelastic water hammer effects were considered to calculate and simulate the hydraulic transients. With reference to the control solutions addressed in this work, the proposed methodologies rely on data–driven and model–based approaches applied to the system under monitoring. Extensive simulations and comparisons serve to determine the best solution for the development of the most effective, robust and reliable control tool when applied to the considered hydraulic system.

Study of the Time Response of a Simulated Hydroelectric System

This paper addresses the design of an advanced control strategy for a typical hydroelectric dynamic process, performed in the Matlab and Simulink environments. The hydraulic system consists of a high water head and a long penstock with upstream and downstream surge tanks, and is equipped with a Francis turbine. The nonlinear characteristics of hydraulic turbine and the inelastic water hammer effects were considered to calculate and simulate the hydraulic transients. With reference to the control solution, the proposed methodology relies on an adaptive control designed by means of the on-line identification of the system model under monitoring. Extensive simulations and comparison with respect to a classic hydraulic turbine speed PID regulator show the effectiveness of the proposed modelling and control tools.

Development of a reliable very high-head pump turbine considering challenging hydrodynamics

This article describes the development of a reversible single-stage pump turbine for a new power plant with a maximum pump head of roughly 750 m, a specific speed of nq = 34 and a ratio of maximum to minimum head including surge tank fluctuations of 1.57. At this plant the water systems of two existing hydro power plants are connected by means of additional water tunnels. Therefore the plant's water system features an upstream as well as a downstream surge tank. The hydraulic design of the pump turbine focuses on robust and stable operation under consideration of the complex hydrodynamics of the system of power plants while maintaining a very high level of pump and turbine efficiency and maximum operating flexibility. State-of-the-art numerical simulations of the flow behaviour at design and off-design conditions have been applied to predict the hydraulic behaviour during the design phase. These results are compared with experimental data from the model tests. From the agreement between simulation and experiment it is concluded that the application of modern simulation techniques in combination with design experience allows for successful developments also for projects within very challenging hydrodynamic boundary conditions.

Basic Modeling and Simulation Tool for Analysis of Hydraulic Transients in Hydroelectric Power Plants

In this paper, the simulation system of a typical hydroelectric power plant was developed in a MATLAB/Simulink-based software environment, which has a high water head and a long penstock with upstream and downstream surge tanks and is equipped with a Francis turbine. With MATLAB/Simulink, the models of the proposed simulation system are all modularized and visualized, and can be reused easily. No complex codes have been programmed and the proposed simulation system adapts varieties of hydroelectric power plants. The nonlinear characteristics of hydraulic turbine and then inelastic water hammer effect were considered to calculate and simulate hydraulic transients. Moreover, the influences of different parameters, such as hydraulic turbine speed governor proportional-integral-derivative (PID) gains, pressure water supply system, as well as surge tanks, were analyzed for the dynamic performance of hydraulic turbine regulating system. The digital simulations of a sudden full load rejection for an actual hydroelectric power plant in China were performed, and the results indicate that the proposed MATLAB/Simulink-based hydroelectric power plant simulation system is accurate and effective enough to represent and simulate hydroelectric power plant's nonlinear dynamics as well as to design hydraulic turbine speed control system. The proposed simulation system has also been proved to be useful for preliminary designs or assessments of hydropower projects.

Effects of Velocity Head and Momentum Exchange on Critical Stable Sectional Area of Downstream Throttled Surge Tank

The velocity head and momentum exchange at the T-junction between the surge tank with a riser and the tail water tunnel of hydroelectric powerplants have been introduced by using the Gardel’s head-loss formula. The effects of the velocity head and momentum exchange on the critical sectional area of the downstream tank have been analyzed. With Thoma’s assumption, the critical stable sectional area of the downstream throttled surge tank is derived theoretically. The study shows that the velocity head has an unfavorable effect on water level stability in the downstream surge tank while the effect of the momentum exchange is positive. These two effects counteract. Thus, the critical sectional area of a downstream throttled surge tank is close to that of a corresponding simple surge tank. However, if the size and shape of a riser are optimized, the area of the downstream surge tank may be further reduced. It is also demonstrated that a downstream throttled surge tank can reduce electricity loss and easily meet...

Operating stability of hydroelectric stations with downstream surge tanks

1. Hydraulic investigations of T-shaped forks made it possible to refine the character of head losses in the junction of the tank with the conduits. The use of the dependences obtained in calculating the stability of the regimes of a hydroelectric station just due to a more accurate consideration of the effect of the velocity head in the base of the surge tank permits substantiating a considerable decrease of the critical area of downstream surge tanks. 2. In calculations of the critical area of downstream surge tanks it is necessary to include in total head losses the velocity head in the outlet section of the outflow channel. In this case the value of the critical area decreases. 3. The proposed method of analyzing the stability of HES regimes with the use of the Routh algebraic criterion permits relating the adjustment parameters of the ARS, turbine characteristics, and power system among themselves to the area of the surge tank and solving the problem of stability in the complex. 4. At hydrostations with short outflow pressure conduits it is advisable to take small, up to 10% of the Thoma critical, cross-sectional areas of the surge pipe of a surge tank with an expansion chamber. This makes it possible, on one hand, to reduce the amount of works on the tank and, on the other, to provide stable regimes of the hydrostations while retaining the adjustment parameters of the ARS.