Turbine Speed Governor Parameters Validation in Islanded Production

The transient behavior of hydroelectric power plants is of high interest for ensuring stability of islanded electrical power networks. Therefore, it is suitable to determine and validate the set of parameters of a turbine speed governor using a realistic simulation model taking into account the hydraulic circuit, the rotating inertias, the electrical installations and the control systems. This paper presents the modeling of a 4x250 MW Francis turbine hydroelectric power plant taking into account the dynamics of all the aforementioned components. The transfer function of the turbine is determined through a time domain simulation using white noise excitation for the determination of an appropriate set of parameters. The performance of the regulator parameters are assessed using two different models: (i) a hydraulic model and (ii) a full hydroelectric model including the model of an islanded power network. The stabilization effect of Standard Power System Stabilizer (PSS) is evidenced.

Modeling and control of large salient-pole synchronous hydro generators and stability issues in isolated production mode

Basile Kawkabani, Christophe Nicolet, Alexander Schwery

This paper presents the modeling, simulation and analysis of the dynamic behavior of a hydroelectric power plant of 2x343 MVA including hydraulic system, electrical installations, rotating inertias and control systems. First, the modeling of the electrical and hydraulic systems is described. The electrical system is constituted by a salient-pole synchronous generator with its ST1A excitation system model and PSS2B power system stabilizer model. Then, the set of parameters of the turbine speed governor of the hydroelectric power plant is determined with a specific identification procedure to achieve stable operation for different cases as interconnected or isolated operation. The analysis of the stability of the power plant is investigated through time domain simulations for different sets of controllers’ parameters and for different cases (simulation of the only electric part and of the whole hydroelectric system).

Ieee2013

Etude du comportement dynamique des turbines Francis

A common phenomenon in Francis turbines is draft tube surge. In these fixed-bladed machines, a strong runner outlet swirl can develop under off-design conditions. The action of the diffuser, and in particular the draft tube elbow, on the rotating flow field induces synchronous pressure and discharge fluctuations. These low frequency disturbances are of special interest, because they can easily propagate throughout the whole hydraulic system. The dynamic response of the system to the natural excitations can inhibit the normal usage of the powerplant. If the natural excitation occurs at frequencies close to an eigenfrequency of the hydraulic system, an important amplification of the hydraulic fluctuations can be expected. Passive measures, such as air admission or minor design modifications, are commonly taken as a last resort to try to reduce these phenomena. This work explores an active control approach to alleviate the problem. It is shown that the hydroacoustic fluctuations in a hydraulic installation can strongly be reduced by "injecting" the inverse signal of the turbine's natural excitation. While the main theme is the application of the active control approach to improve the operation stability of Francis turbines, the scope of this study is larger and addresses the following scientific topics: Modeling of an hydroelectric installation The dynamic behavior of an installation is studied based on an one-dimensional model. A compilation of the basic modeling tools was done. Using these tools, the dynamic model of a hydraulic installation and an external hydroacoustical source is presented. It is used to explain how the overall hydroacoustic fluctuations can be reduced using a hydraulic exciter mounted in the wall of a draft tube's cone. Improving the operation stability by active control The hydraulic fluctuations associated with off-design operating conditions of Francis turbines are often very periodic and characterized by the presence of a dominant frequency component. The aim is to cancel out this component for which the amplitudes are often excessive. An active control system, composed of a hydraulic exciter and its controller, has been developed for a Francis turbine. The exciter employs a pulsed flow injection in the draft tube to generate an antiexcitation at a given frequency, which is then synchronized with the turbine's natural excitation. A self-tuning extremum control algorithm optimizes the operating parameters of the exciter to minimize the overall fluctuations in the hydraulic installation. Laboratory tests show the efficiency of the active control system. The amplitude of the pressure and discharge fluctuations at the dominant frequency component are reduced to background noise levels. Multiple configurations of the exciter have been tested. A spectral analysis of the pressure fluctuations is presented, the system's energy balance has been performed and the control algorithm of the system is analysed. The energy balance is very encouraging: the exciter system needed only about 1 percent of the hydraulic power of the Francis turbine. Prediction of the operation stability of turbines based on model tests Laboratory tests on a scale model, homologous to the turbine, are an indispensable step in the development of a hydroelectric powerplant. Unlike the static characteristics, the dynamic behavior of a turbine installation is difficult to predict. An original identification method of the dynamic characteristics of a model turbine is proposed. The method is verified on a theoretical case study and experimentally tested. It constitutes the basis of the prediction of the operation stability of the powerplant based on model tests.

EPFL2000

Methodology for Risk Assessment of Part Load Resonance in Francis Turbine Power Plant

Philippe Allenbach, François Avellan, Bob Greiveldinger, Christophe Nicolet, Jean-Jacques Simond

At low flow rate operation, Francis turbines feature a cavitating vortex rope in the draft tube resulting from the swirling flow of the runner outlet. The unsteady pressure field related to the precession of the vortex rope induces plane wave propagating in the entire hydraulic system. The frequency of the vortex rope precession being comprised between 0.2 and 0.4 times the turbine rotational speed, there is a risk of resonance between the hydraulic circuit, the synchronous machine and the turbine itself an acting as excitation source. This paper presents a systematic methodology for the assessment of the resonance risk for a given Francis turbine power plant. The test case investigated is a 1GW 4 Francis turbines power plant. The methodology is based on a transient simulation of the dynamic behavior of the whole power plant considering a 1D model of the hydraulic installation, comprising gallery, surge chamber, penstock, Francis turbine but also mechanical masses, synchronous machines, transformer, grid model, speed and voltage regulators. A stochastic excitation having energy uniformly distributed in the frequency range of interest is taken into account in the draft tube. As the vortex rope volume has a strong influence on the natural frequencies of the hydraulic system, the wave speed in the draft tube is considered as a parameter for the investigation. The transient simulation points out the key excitation frequencies and the draft tube wave speed producing resonance between the vortex rope excitation and the circuit and provide a good evaluation of the impact on power quality. The comparison with scale model tests results allows resonance risk assessment in the early stage of project pre-study.

2006

Etude du comportement dynamique des turbines Francis

EPFL2000