DRIVE SYSTEM

In order to generate electrical power at a stable voltage and frequency, the drive system needs to transmit power from the turbine to the generator shaft in the required direction and at the required speed. Typical drive systems in micro-hydropower systems are as follows:

• Direct drive: A direct drive system is one in which the turbine shaft is connected directly to the generator shaft. Direct drive systems are used only for cases where the

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shaft speed of the generator shaft and the speed of the turbine are compatible. The advantages of this type of system are low maintenance, high efficiency and low cost.

• “V” or wedge belts and pulleys: This is the most common choice for micro-hydropower systems. Belts for this type of system are widely available because they are used extensively in all kinds of small industrial machinery.

• Timing belt and sprocket pulley: These drives are common on vehicle camshaft drives and use toothed belts and pulleys. They are efficient and clean-running and are especially worth considering for use in very small system drives (less than 3 kW) where efficiency is critical.

• Gearbox: Gearboxes are suitable for use with larger machines when belt drives would be too cumbersome and inefficient. Gearboxes have problems regarding specification, alignment, maintenance and cost, and this rules them out for micro-hydropower systems except where they are specified as part of a turbine-generator set.

3.7 GENRATOR

Generators transform mechanical energy into electrical energy. Although most early hydroelectric systems were of the direct current variety to match early commercial electrical systems, nowadays only three-phase alternating current generators are used in normal practice. Two types of current are produced by electrical generators, either alternating current (AC) or direct current (DC). In the case of AC the voltage cycles sinusoidally with time, from positive peak value to negative.

Because the voltage changes its sign the resulting current also continually reverses direction in a cyclic pattern. DC current flows in a single direction as the result of a steady voltage. DC is not usually used in modern power installations except for very low-powered systems of a few hundred watts or less.

Alternating voltage can be produced in a stationery coil or armature by a rotating magnetic field, but more usually a coil is rotated in a stationary magnetic field. The magnetic field can be produced either by a permanent magnet or by another coil (i.e., an electro-magnet) know as a field coil which is fed by direct current known as the excitation current. A generator supplying alternative current is described as an alternator to distinguish it from a machine designed to supply DC current which is known as a DC generator or dynamo. Current flows when a voltage difference is place across a conducting body. In AC circuits the magnitude and timing of the

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current cycle relative to the voltage cycle will depend on whether the conductivity body is resistance, inductive, capacitive or some combination of these elements.

3.7.1 SYNCHRONOUS GENRATOR

Synchronous generators equipped with a DC excitation system (rotating or static) associated with a voltage regulator, to provide voltage, frequency and phase angle control before the generator is connected the reactive energy required by the power system when the generator is tied into the grid. Synchronous generators are more expensive than asynchronous generators and are used in power systems where the output of the generator represents a substantial proportion of the power system load.

The synchronous generator is started before connecting it to the mains by the turbine rotation. By gradually accelerating the turbine the generator is synchronized with the mains, regulating the voltage, frequency and rotating sense, When the generator reaches a velocity close to synchronous, the exciter regulates its field coils current so the generator voltage is identical to the mains voltage. When the synchronous generator is connected to an isolated net, the voltage controller maintains a predefined constant voltage, independent of the load. If it is connected to the main supply, the controller maintains the reactive power at a predefined level.

3.7.2 ASYNCHRONOUS GENRATOR

Asynchronous generators are simple squirrel-cage induction motors with no possibility of voltage regulation and running at a speed directly related to system frequency. They draw their excitation current from the grid, absorbing reactive energy by their own magnetism. Adding a bank of capacitors can compensate for the absorbed reactive energy. They cannot generate when disconnected from the grid because are incapable of providing their own excitation current. Asynchronous generators are cheaper and are used in large grids where their output is an insignificant proportion of the power system load. Their efficiency is 2 to 4 per cent lower than the efficiency of synchronous generators over the entire operating range.

In general, when the power exceeds 5000 kVA a synchronous generator is installed.

An asynchronous generator needs to absorb a certain power from the three phase mains supply to ensure its magnetization even, if in theory, the generator can receive

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its reactive power from a separate source such as a bank of capacitors. The mains supply defines the frequency of the stator rotating flux and hence the synchronous speed above which the rotor shaft must be driven. On start-up, the turbine is accelerated up to 90-95% of the synchronous speed of the generator, when a velocity relay closes the main line switch. The generator passes immediately to absorbed by this dc generator amounts to 0.5% - 1.0% of the total generator power.

Nowadays a static exciter usually replaces the DC generator, but there are still many rotating exciters in operation.

Rotating exciters: The field coils of both the main generator and the exciter generator are usually mounted on the main shaft. In larger generators a pilot exciter is also used. The pilot exciter can be started from its residual magnetic field and it then supplies the exciting current to the main exciter, which in turn supplies the exciting current for the rotor of the generator. In such way the current regulation takes place in the smaller machine.

Brushless exciters: A small generator has its field coils on the stator and generates AC current in the rotor windings. A solid state rectifier rotates with the shaft, converting the AC output from the small generator into the DC which is the supplied to the rotating field coils of the main generator without the need of brushes. The voltage regulation is achieved by controlling the current in the field coils of the small generator.

Static exciters: The exciting current is taken out, via a transformer, from the output terminals of the main generator. This AC current is then rectified in a solid state rectifier and injected in the generator field coils. When the generator is started there is no current flowing through the generator field coils. The residual magnetic field, aided if needed by a battery, permits generation to start to be then stabilized when the voltage at the generator terminals reaches a preset value. This equipment is easy to maintain has a

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good efficiency and the response to the generator voltage oscillations is very good.