Objective
The hydro-electric mini-plant of Ribeira da Praia has been developed to replace three old hydro-electric installations situated downstream in the part of the river that is short-circuited by the new installation.
This project is characterized by the following innovations :
- Penstock in cast iron which is particularly well adapted to the rough volcanic terrain and the seismic region.
- Double Pelton turbine arrangement allowing a more compact installation with ease of use and maintenance.
- Tele-surveillance, permitting the transmission of principal information to a PC (centrally installed HQ of "EDA") as well as the remote control of the installation.
The principle technical characteristics are the following :
* Nominal throughput : 580 l/s
* Head of water : 166,15 m
* Torque on shaft : 825 kW
* Turbine efficiency : 89.3%
This project has not known many major problems.
Once the problem of renegotiating the contract between EDA and CFG to take into account the civil engineering expenses between the date of the signing of the agreement (24th April 1986) and the date of the start of the detailed studies was overcome, the planning at this stage was able to be respected, thanks to :
- Absence of bad weather.
- The precise definition of the capabilities of the operators, thereby avoiding the interface problems inherent in this type of installation.
- The good execution of the works by the developer.
A particular mention must be made on the choice of the cast iron penstock, which adapted particularly well to the conditions on the island. Its installation does not require a specialised workforce and so could be done by civil engineers.
Referring to the provisional report, the plant has been modified to be realised finally, in full, and not in two stages as foreseen initially.
The project recorded a relatively important overrun of the civil engineering budget. This was caused by the building site conditions, which forced the civil engineers to make larger excavations than foreseen at the time of the provisional evaluation, taking into account the risk of caving in linked to the depth of the tunnels.
The water intake (which is equipped with an automatic grill cleaning system) is connected to an existing buffer reservoir of 5,000 m3 which also acts as a sand setllement reservoir. It is large enough to cover the peak load. The connection of the water intake to the plant consists of a 1997 m long penstock made mostly of cast iron. The lower 45 m is made of riveted steel.
The plant, which occupies an area of 120 m2, houses the double Pelton turbines which were specially developed for this project. The "Double Pelton" consists of two turbines with a simple jet system, placed on either side of the generator and on the same shaft. The plant also houses the electricity distribution box, the power transformer and the tele-surveillance equipment.
Functioning of the Plant :
- Water Flow.
The water flow is controlled by a "level adjusting" system which monitors the level of the water at the intake. This continuous level adjusting system works as follows :
- It is connected to an electronic device that delivers opening and closing pulses in relation to the water level (referring to the level that the operator has on his display) and also in relation to the range of the permitted operating levels controlled by the same device.
These open/close pulses depend on the settings of the buttons or potentiometers of the electronic control box, but may also depend on other factors (level differential, speed of decrease of water level, etc.).
This system also has a 'low water-level' switch contact which stops the plant immediately. This overrides all the other controls and comes into effect in the case of failure of the detection system due to e.g. lightening.
Peak-Time Operation:
Taking into account the water volume (in which 1 metre of water will run the turbines for more than one hour at full power), the following principles are applied :
- During the off-peak periods the water level is kept close to the overflow level. This allows all the water input to be utilised, except if it exceeds the maximum throughput of the turbine. - At the start of the peak periods, the turbine fully opens.
As long as the level is higher than the lowest permissible level in the reservoir, the turbine stays completely open. When there is enough water, it stays fully open until the end of the peak load. When the water level drops, the automatic regulation goes into service to restrict the throughput of the turbines in order to stabilise the water level at the pre-determined low position. This situation is maintained until the end of the peak period.
At the end of the peak period, and in the two previous cases, the turbine stops to let the water trench fill again.
Topic(s)
Data not availableCall for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
9500 Ponta Delgada
Portugal