Stability Analysis of Different Regulation Modes of Hydropower Units

The dynamic characteristics of hydropower unit governing systems considerably influence the stability of hydropower units and the connected power system. The dynamic performances of hydropower units with power regulation mode (PRM) and opening regulation mode (ORM) are different. This paper establishes a detailed linear model of a hydropower unit based on the Phillips–Heffron model. The damping characteristic and stability of two regulation modes with different water inertia time constants TW were analyzed. ORM tended to provide negative damping, while PRM often provided positive damping in the major parts of the frequency range within the normal frequency oscillations when TW was large. Eigenvalue analysis illustrated that PRM has better stability than ORM. To validate the analysis, a simulation under two typical faults WAS conducted based on a nonlinear model of a hydropower unit. The simulation results illustrated that the responses of units with PRM are more stable in terms of important operating parameters, such as output power, rotor speed, and power angles. For hydropower units facing challenges in stable operation, PRM is recommended to obtain good dynamic stability.

Influence Mechanism of Geometric Characteristics of Water Conveyance System on Extreme Water Hammer during Load Rejection in Pumped Storage Plants

Pumped storage plants (PSPs) have achieved rapid development and deployment worldwide since the penetration of intermittent renewable energy sources (RES). Hydraulic transient analysis in the PSP, to obtain the control parameters such as extreme water hammer pressure, is vital to the safe design of water conveyance system. Empirically, simultaneous load rejection (SLR) is commonly accepted as the control condition for extreme water hammer, while it is not completely true for the PSP. Employing theoretical analysis and numerical simulation, this study systematically investigates the effects of geometric characteristics on the extreme water hammer, and reveals the mechanism leading to the maximum spiral case pressure (SCP) during a two-stage load rejection (TLR) process. The results indicate that the extreme water hammer pressure is closely related to geometric characteristics of the water conveyance system, performing the allocation of the water inertia time constant of the main and branch pipelines. When the water inertia time constant in the branch pipe is dominant (η1 > 0.24 for example), the maximum SCP will occur in TLR conditions rather than SLR. Moreover, the maximum SCP is almost the same, providing the water inertia time constants of both the main and branch pipelines are kept constant.

On the concept of sloped motion for free-floating wave energy converters

A free-floating wave energy converter (WEC) concept whose power take-off (PTO) system reacts against water inertia is investigated herein. The main focus is the impact of inclining the PTO direction on the system performance. The study is based on a numerical model whose formulation is first derived in detail. Hydrodynamics coefficients are obtained using the linear boundary element method package WAMIT. Verification of the model is provided prior to its use for a PTO parametric study and a multi-objective optimization based on a multi-linear regression method. It is found that inclining the direction of the PTO at around 50° to the vertical is highly beneficial for the WEC performance in that it provides a high capture width ratio over a broad region of the wave period range.

Effect Mechanism of Penstock on Stability and Regulation Quality of Turbine Regulating System

This paper studies the effect mechanism of water inertia and head loss of penstock on stability and regulation quality of turbine regulating system with surge tank or not and proposes the construction method of equivalent model of regulating system. Firstly, the complete linear mathematical model of regulating system is established. Then, the free oscillation equation and time response of the frequency that describe stability and regulation quality, respectively, are obtained. Finally, the effects of penstock are analysed by using stability region and response curves. The results indicate that the stability and regulation quality of system without surge tank are determined by time response of frequency which only depends on water hammer wave in penstock, while, for system with surge tank, the time response of frequency depending on water hammer wave in penstock and water-level fluctuation in surge tank jointly determines the stability and regulation quality. Water inertia of penstock mainly affects the stability and time response of frequency of system without surge tank as well as the stability and head wave of time response of frequency with surge tank. Head loss of penstock mainly affects the stability and tail wave of time response of frequency with surge tank.

Modeling of the Dynamic Response of a Pelton Turbine Hydroelectric Plant

The paper presents a detailed numerical model of the dynamics of a Pelton turbine installed in a hydroelectric plant. The model considers in detail the Pelton turbine with all the electromechanical subsystems, such as the main speed governor, the controller and the servoactuator of the turbine nozzle, and the electric generator. In particular it reproduces the effects of pipe elasticity in the penstock, the water inertia and the water compressibility on the turbine behaviour. The dynamics of the surge tank on low frequency pressure waves is also modeled together with the main governor speed loop and the position controllers of the nozzle needle actuators and of the hydraulic electrovalve. Model validation has been made by means of experimental data acquired during some starting tests after a partial revamping of a hydroelectric unit, which involved also the control system of the hydraulic actuators but not the nozzles. The model is used in order to identify the cause of the oscillations of the electric power mainly ascribed to the backlash of the nozzle needle system.