PELTON CROSS-FLOW TURBINE-GENERATOR GROUP ON VERTICAL AXIS
Pelton wheel
The wheel buckets are in stainless steel of high mechanical
resistance and are obtained by precision casting, thus ensuring high accuracy
and good finishing. The buckets are fixed to the disk by TIG welding without
added material. The buckets size depends on the nozzles dimension. Generally,
the coupling of the turbine shaft with the generator shaft is directly made,
i.e. the wheel disk is directly splined to the generator shaft by means of a
pliers hub. The disk and the hub are in stainless steel. In some cases, the
coupling of the turbine shaft with the generator shaft is realised by means
of a revolution multiplier, which is made up of a couple of pulleys with a cogged
driving belt. All revolving parts are dynamically balanced.
Turbine case
The case consists of a welded assembly in sheet steel.
The upper part of the internal cylinder, the two flages and the external cylinder
form the distributor, i.e. the feeding chamber of the pressure water to the
nozzles. The internal cylinder has also the function of gathering and conveying
the water into the waste water trap. Furthermore, it fixes and supports the
turbine-generator group by means of brackets welded on the cylinder external
surface. A coupling ring, to be installed in the bedplate, is supplied to secure
the turbine case. The nozzles are pressed in special long-lasting plastic material,
and their maximum number is 6. The distributors for AC 4/38 and AC 4/75 series
are provided with 3 or 4 piston valves, respectively, for the manual flow regulation.
Feeding group
It connects the penstock to
the turbine case and consists of the introductor tube, on which the floodgate
for plant control is mounted, the pressure gauge connection, the pressure
gauge, and the pressure water side plug (by-pass).
Generator
Three-phase synchronous generators are used, from which
it is possible to draw single-phase energy. The rotating inductor is a 4-salient
pole type. These generators are characterised by the absence of brushes. The
exciting current is supplied by an auxiliary generator whose induced rotor is
mounted on the same shaft of the main generator. The excitation regulation is
voltamperometric by "Compound" system. The AC series plants are provided with
a single-phase synchronous generator with a 2-salient pole rotating inductor.
The ball bearings of all generators are "water tight", hence, they require no
lubrication. The generator is separated from the turbine by a special shield
preventing any contact with water.
ELECTRIC CONTROL BOARD
The electric control board consists of a sheet hanging cabinet in which the following devices are contained.
The meters of the single-phase control board are: a voltmeter, a digital frequency-meter, an ammeter indicating the total current distributed by the generator, and a second ammeter indicating the current drawn by the consumers. In the three-phase control board, there are six ammeters, 3 of them indicating the input current on each phase and the other 3 the current drawn by the consumers.
The protections of the single-phase control board consist of a three-pole circuit breaker, which protects the generator from possible short circuits on the consumer line, and an electronic voltage relay operating a warning buzzer and a visual signal in the event the voltage variation exceeds the preset limits. In the three-phase control board, there is a three-pole circuit breaker and three electronic voltage relays, each of them being connected between one phase and neutral.
The terminal boards are used for connection of the wires of the generator, the consumer line and the electronic regulator. Isolation and size of the terminals are suitable for the voltages and sections of the wires normally used.
ELECTRONIC REGULATOR
The electronic regulator is made up of the control unit and the ballast resistances. The regulator cares to keep the voltage and frequency stable, as the absorption of the energy produced by the turbine-generator group remains constant. In fact, this group always works at full load and the regulator dissipates the energy produced by the plant and not utilised by the consumers on some ballast reistances. The exceeding energy may be dissipated in air or water by means of largely dimensioned dissipating elements, automatically connected or disconnected by the electronic control unit of the regulator.
ELECTRICAL CHARACTERISTCS OF THE PLANT
The micro-hydroelectric plant supplies standard 220V single-phase and 220/380V three-phase AC electric energy, at the frequency of 50Hz and 60Hz. The plant global efficiency (i.e. the ratio between the generated electric power and the available hydraulic power) varies according to the distributed power and the conditions of use. However, the efficiency is always high compared to the involved powers.