Contribution to Load-Frequency Regulation of a Hydropower Plant with Long Tail-Race Tunnel

In this paper, a hydroelectric power plant with long tail-race tunnel has been modelled for assessing its contribution to secondary regulation reserve. Cavitation problems, caused by the discharge conduit length, are expected downstream the turbine where low pressure appears during regulation manoeuvres. Therefore, governor's gains should be selected taking into account these phenomena. On the other hand, regulation services bidden by the plant operator should fulfil TSO (Transmission System Operator) quality requirements. A methodology for tuning governor PI gains is proposed and applied to a Hydro power plant in pre-design phase in northwest area of Spain. The PI gains adjustment proposed provides a proper plant response, according to some established indexes, while avoiding cavitation phenomena

A control system for low-head diversion run-of-river small hydro plants with pressure conduits considering the tailwater level variation

This paper presents a control system for low-head diversion run-of-river small hydro plants with pressure conduits. Since these hydropower plants usually have low or null water storage capacity, the water discharged through the turbines should be adapted to the possible extent to the natural river inflow. For this purpose, a control scheme aimed at maintaining a constant water level in the head pond is normally used in these cases. As an alternative, the option of maintaining a constant water level in the surge tank is studied in this paper. Furthermore, since in low-head hydro plants the tailwater level variation may represent a relatively important contribution to total head losses, it has been explicitly considered in the proposed control system. A small-perturbation stability analysis has been carried out in order to analyze the influence of the plant design and controller parameters in the plant dynamic response. Finally, in order to illustrate the applicability of the proposed control system, several simulations have been carried out using data gathered from a real hydro plan

Frequency control support of a wind-solar isolated system by a hydropower plant with long tail-race tunnel

Pumped storage hydro plants (PSHP) can provide adequate energy storage and frequency regulation capacities in isolated power systems having significant renewable energy resources. Due to its high wind and solar potential, several plans have been developed for La Palma Island in the Canary archipelago, aimed at increasing the penetration of these energy sources. In this paper, the performance of the frequency control of La Palma power system is assessed, when the demand is supplied by the available wind and solar generation with the support of a PSHP which has been predesigned for this purpose. The frequency regulation is provided exclusively by the PSHP. Due to topographic and environmental constraints, this plant has a long tail-race tunnel without a surge tank. In this configuration, the effects of pressure waves cannot be neglected and, therefore, usual recommendations for PID governor tuning provide poor performance. A PI governor tuning criterion is proposed for the hydro plant and compared with other criteria according to several performance indices. Several scenarios considering solar and wind energy penetration have been simulated to check the plant response using the proposed criterion. This tuning of the PI governor maintains La Palma system frequency within grid code requirements

Pond head level control in a run-of-river hydro power plant using fuzzy controller

The run-of-river hydro power plant usually have low or nil water storage capacity, and therefore an adequate control strategy is required to keep the water level constant in pond. This paper presents a novel technique based on TSK fuzzy controller to maintain the pond head constant. The performance is investigated over a wide range of hill curve of hydro turbine. The results are compared with PI controller as discussed in [1]

Simulation model of a variable-speed pumped-storage power plant in unstable operating conditions in pumping mode

This paper presents a dynamic simulation model of a laboratory-scale pumped-storage power plant (PSPP) operating in pumping mode with variable speed. The model considers the dynamic behavior of the conduits by means of an elastic water column approach, and synthetically generates both pressure and torque pulsations that reproduce the operation of the hydraulic machine in its instability region. The pressure and torque pulsations are generated each from a different set of sinusoidal functions. These functions were calibrated from the results of a CFD model, which was in turn validated from experimental data. Simulation model results match the numerical results of the CFD model with reasonable accuracy. The pump-turbine model (the functions used to generate pressure and torque pulsations inclusive) was up-scaled by hydraulic similarity according to the design parameters of a real PSPP and included in a dynamic simulation model of the said PSPP. Preliminary conclusions on the impact of unstable operation conditions on the penstock fatigue were obtained by means of a Monte Carlo simulation-based fatigue analysis

An adaptive control scheme for variable speed wind turbines providing frequency regulation in isolated power systems with thermal generation

The lack of synchronous inertia, associated with the relevant penetration of variable speed wind turbines (VSWTs) into isolated power systems, has increased their vulnerability to strong frequency deviations. In fact, the activation of load shedding schemes is a common practice when an incident occurs, i.e., the outage of a conventional unit. Under this framework, wind power plants should actively contribute to frequency stability and grid reliability. However, the contribution of VSWTs to frequency regulation involves several drawbacks related to their efficiency and equipment wear due to electrical power requirements, rotational speed changes, and subsequently, shaft torque oscillations. As a result, wind energy producers are not usually willing to offer such frequency regulation. In this paper, a new control technique is proposed to optimize the frequency response of wind power plants after a power imbalanced situation. The proposed frequency controller depends on different power system parameters through a linear regression to determine the contribution of wind power plants for each imbalance condition. As a consequence, VSWTs frequency contribution is estimated to minimize their mechanical and electrical efforts, thus reducing their equipment wear. A group of sixty supply-side and imbalance scenarios are simulated and analyzed. Results of the case study are compared to previous proposals. The proposed adaptive control reduces the máximum torque and rotational speed variations while at the same time maintaining similar values of the load shedding program. Extensive results and discussion are included in the paper.This work was partially supported by ‘Ministerio de Educación, Cultura y Deporte’ of Spain (ref. FPU16/04282) and by ‘Ministerio de Economía y Competitividad’, under the project “Value of pumped-hydro energy storage in isolated power systems with high wind power penetration” of the National Plan for Scientific and Technical Research and Innovation 2013–2016, grant number ENE2016-77951-R

Stability Analysis of a Run-of-River Diversion Hydropower Plant With Surge Tank And Spillway in the Head Pond

Run-of-river hydropower plants usually lack significant storage capacity; therefore, the more adequate control strategy would consist of keeping a constant water level at the intake pond in order to harness the maximum amount of energy from the river flow or to reduce the surface flooded in the head pond. In this paper, a standard PI control system of a run-of-river diversion hydropower plant with surge tank and a spillway in the head pond that evacuates part of the river flow plant is studied. A stability analysis based on the Routh-Hurwitz criterion is carried out and a practical criterion for tuning the gains of the PI controller is proposed. Conclusions about the head pond and surge tank areas are drawn from the stability analysis. Finally, this criterion is applied to a real hydropower plant in design state; the importance of considering the spillway dimensions and turbine characteristic curves for adequate tuning of the controller gains is highlighte

Dynamic response of hydro power plants for providing secondary regulation reserves considering elastic water column effects

In this paper, the dynamic response of a hydro power plant for providing secondary regulation reserve is studied in detail. Special emphasis is given to the elastic water column effects both in the penstock and the tailrace tunnel. For this purpose, a nonlinear model based on the analogy between mass and momentum conservation equations of a water conduit and those of wave propagation in transmission lines is used. The influence of the plant configuration and design parameters on the fulfilment of the Spanish Electrical System Operator requirements is analyse

Control de minicentrales hidroeléctricas fluyentes : modelado y estabilidad

Resumen de la tesis La implantación de minicentrales hidroeléctricas está experimentando un considerable impulso en los últimos años. En los países desarrollados las minicentrales permiten obtener energía en aquellas localizaciones donde una gran central no sería viable además de minimizar el impacto ambiental que produce la obra civil (presa, edificio de la central…). En los países en vías de desarrollo las minicentrales permiten la electrificación de zonas rurales alejadas de los grandes núcleos de población proporcionando un empuje decisivo para su crecimiento socioeconómico. La gran mayoría de minicentrales son fluyentes, es decir, carecen de un elemento almacenador suficientemente grande que les permita la regulación del caudal turbinado o de la energía producida. La pequeña potencia instalada (menos de 10 MW) tampoco les permite contribuir al mantenimiento de la frecuencia de la red, salvo que operen en isla, lo que ocurre en muy contadas ocasiones. Por tanto, en el caso de minicentrales fluyentes, es recomendable la operación de la central con el control de nivel en el azud de derivación o en la cámara de carga, porque posibilita la reducción de la superficie del embalse o de la cámara y permite combinar su uso hidroeléctrico con otros como puede ser el regadío. El primer objetivo de la presente tesis es la elaboración de un modelo matemático en el entorno de programación MATLAB, que simule la operación de una minicentral hidroeléctrica fluyente con control de nivel. Dicho modelo se aplica a las tres tipologías de minicentrales más comunes: a pie de presa, en derivación con canal en lámina libre y cámara de carga y en derivación con galería en presión y chimenea de equilibrio. Todos los modelos se implantan en una central de referencia y mediante simulaciones se comprueba su comportamiento dinámico. Merece especial mención el modelo de canal obtenido a partir de la linealización de las ecuaciones de Saint Venant y el posterior desarrollo de su matriz de transferencia que permite su conexión con los demás componentes del modelo. Una vez elaborados los modelos se procede al estudio de la estabilidad en pequeña perturbación de la central en condiciones normales de operación. Para ello, siguiendo las teoría de control clásico, se linealizan las ecuaciones que reflejan la dinámica de cada componente de la central dando lugar a su formulación canónica y a la matriz dinámica del sistema. A partir de dicha matriz y aplicando el criterio de estabilidad de Routh-Hurwitz se puede llegar a las siguientes conclusiones: Centrales en derivación con canal y centrales a pie de presa: Las dimensiones de la cámara de carga y del azud de derivación no influyen en la estabilidad de la central. Se mejora la estabilidad de la central conforme se turbina menor caudal. Centrales en derivación con galería y chimenea: El control de nivel resulta más estable que el control de frecuencia-potencia. La estabilidad empeora cuando se reduce el caudal turbinado. La superficie del azud y de la chimenea sí intervienen en la estabilidad de la central. El vertido del caudal ecológico entre el azud de toma y la descarga mejora la estabilidad de la central. Una vez estudiada la estabilidad de la minicentral es sus tres tipologías se propone un criterio heurístico que permite la sintonización de las ganancias del controlador PI que acciona el distribuidor del la turbina. La técnica del lugar de raíces, ampliamente utilizada en la teoría del control clásico, permite establecer una relación a priori entre las ganancias del controlador y las oscilaciones que aparecen en la respuesta de la central. Las ganancias propuestas permiten la minimización de la oscilación así como del tiempo de establecimiento de la respuesta, en función de las dimensiones de los principales componentes de la central así como de su punto de operación. De la aplicación del criterio se concluye: En la central con canal y cámara de carga la variación del punto de operación apenas modifica las ganancias, por lo que no se considera necesario el control adaptativo. En la central con galería y chimenea de equilibrio el ajuste del controlador para un punto de operación puede generar inestabilidades en otras circunstancias de funcionamiento, por lo que sí se aconseja el control adaptativo en este caso. En la central a pie de presa el criterio propuesto no es aplicable ya que exige una precisión excesiva en el sensor de nivel y una acción de control considerable. Por último, cabe añadir, que todo el trabajo realizado se enmarca dentro la fase de diseño y predimensionamiento de una minicentral. Con ello se pretende facilitar un diseño de la planta que garantice una respuesta adecuada en condiciones normales de operación. Abstract The implementation of hydroelectric small hydropower plants is experiencing a major expansion in the last years. In developed countries, small hydropower plants enable the production of energy in those locations where a great power station would not be feasible. Moreover, the environmental impact of the civil work they require is smaller than in conventional hydroelectric power stations (dam, buildings…). In developing countries, small hydropower plants stations allow the electrification of rural areas far away from great cities, promoting their socioeconomic growth. The great majority of small hydropower plants are run-of-river: they lack of a sufficient storage element for the regulation of the turbined volume or the produced energy. As they have a small installed capacity (less than 10 MW), they can not contribute to the maintenance of the network frequency, unless they operate isolated, this situation being unusual. Therefore, the best option to operate run-of-river small power plants is to keep control of the water level at intake basin or at the head pond, as this allows the reduction of the reservoir surface or the head pond and enables the combination of this hydroelectric use with others as irrigation. The first goal of this doctoral thesis is the design of a mathematical model using MATLAB, in order to simulate the operation of a run-of-river small hydropower plant with water level control. This model is then applied to the three main typologies of small hydropower plants: dam site located power plant, diversion plant with open channel and surge tank, and diversion plant with head-race conduit and surge tank. Each of the resulting models are implemented in a reference power station and, by means of simulations, its dynamic behaviour is tested. Special consideration should be paid to the channel model, obtained through linearization of Saint Venant equations. The corresponding transfer matrix is determined, allowing its connection with the other components of the model. Then a small perturbation stability analysis is carried out using the developed models. The equations representing the dynamic of each plant component are then linearized, in order to apply the classic control theory. The model equations are expressed in state space form and the dynamic matrix is determined. From this matrix and according to Routh-Hurwitz stability criterion, the following conclusions are reached: In diversion plants with open channel as well as in dam site located power plants: Plant stability is not affected by the dimensions of head pond and the intake basin. Plant stability is improved if turbined flow is reduced. Diversion plants with head-race conduit and surge tank: Level control is more stable than load frequency control. Stability gets worse when turbined flow is reduced. The surface areas of intake reservoir and surge tank do take part in the stability of the plant The spillage of the ecological flow between the intake reservoir and the draft tube enhances the plant stability. Once the stability the small hydropower plant in its three typologies has been studied, a heuristic criterion is proposed in order to tune the gains of the PI controller that modifies the wicket gates of the turbine. The root locus technique, widely used in classical control theory, allows the establishment of an a priori relation between the controller gains and the oscillations appearing in plant response. The proposed gains allow the minimisation of the oscillations and of the settling time, depending on the dimensions of main plant elements as well as on the operating point. The following conclusions are drawn from the application of the heuristic criterion: In plants with channel and head pond, the variation of the operating point hardly modifies the gains and therefore adaptive control is not necessary. In plants with head-race conduit and surge tank, the controller tuned with respect to a specific operating point may generate instabilities in other operating conditions, so in this case adaptive control is recommended. In dam site located power plants, the proposed criterion is no longer applicable, as it demands an excessive accuracy in the water level sensor as well as a considerable control action. Finally, it is necessary to remark that the whole work focuses on the stage of design and early dimensioning of a small hydropower plant. The aim is to contribute to a plant design that guarantees a proper response in normal operating conditions

Frequency Regulation of a Hybrid Wind-Hydro Power Plant in an Isolated Power System

Currently, some small islands with high wind potential are trying to reduce the environmental and economic impact of fossil fuels using its renewable resources. Nevertheless, the characteristics of these renewable resources negatively affect the quality of the electrical energy causing frequency disturbances, especially in isolated systems. In this study, the combined contribution to frequency regulation of variable speed wind turbines (VSWT) and a pump storage hydropower plant (PSHP) is analyzed. Different control strategies, using the kinetic energy stored in the VSWT, are studied: inertial, proportional, and their combination. In general, the gains of the VSWT controller for interconnected systems proposed in the literature are not adequate for isolated systems. Therefore, a methodology to adjust the controllers, based on exhaustive searches are proposed for each of the control strategies. The control strategies and the methodology have been applied to a hybrid wind-hydro power plant committed in El Hierro island in the Canary archipelago. At present, in this isolated power system, frequency regulation is only provided by the PSHP and diesel generators. The improvements in the quality of frequency regulation including the VSWT contribution have been proven based on simulating different events related to wind speed, or variations in the demanded power