CHARLES ISIADINSO

TOTALLY ENCLOSED WATER TO AIR COOLED (TEWAC) GENERATOR

THE GENERATORIn a power plant, electricity is generated by the generator, which works on Faraday’s law of electromagnetic induction. A turbine (steam or gas) turns shaft to which a rotor is attached. The rotor spins in a gap in a part called the stator. The law of electromagnetic induction states that if a magnet is rotated close to a conductor (e.g. copper wire), a current is induced in the conductor. To vary the strength of the magnetic flux, the rotor is an electromagnet (a magnet with windings of a conductor around it) rotating inside the stator, which is a toothed metal ring shaped structure with coils of a conductor between the teeth. To start the process, current (excitation current) is passed into the conductor coils of the rotor; this creates a magnetic flux in the air gap between the rotor and stator. This magnetic flux then induces a flow of electrons (current) in the stator coil thus generating electricity. In some turbine systems, the generator is used to start the turbine. An exciter module, which contains the generator excitation system and start-up frequency converter (SFC) start system, starts the gas turbine by temporarily converting the generator into a motor, using the SFC. However, with GE’s 9E gas turbine, a separate staring motor is used. The generator is connected to the 9E after the exhaust system by a rigid metal shaft that runs through the entire turbine and into the generator, and is what the generator rotor is attached to. Rotation of the air compressor and turbine blades causes a rotation in the shaft, which then rotates the rotor in the stator, generating electricity.

The generator consists of a Stator consisting of a:1. STATOR CORE: is a silicon steel frame, to which field

magnets are attached.

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2. FIELD MAGNETS: Each magnet is made up of a pole core and shoe.

3. STATOR WINDING: Copper wire wound tightly round the pole core of the field magnet, make up the stator winding.

And a rotor consisting of:1. METAL CORE: The core is a cylindrical metal part that

houses the armature coils, which experiences a flow of electrons when rotated through the magnetic flux created by the field magnets’ pole coils.

2. COLLECTOR RINGS: or slip rings are used to transfer the electricity generated from the stator to the rotor.

3. WINDING SYSTEM: made up of high conductivity wedge-shaped copper segments, insulated from each other.

4. BEARINGS: Bearings are used to allow friction free movement of the armature shaft.

5. RADIAL-FLOW FANS: there are two fans positioned one at the commutator end, and the other at the armature end. Their primary function is to provide cooling air for the stator core and winding.

When in operation, the rotor is cooled by water, air or hydrogen. There are four types of generators (differentiated by their cooling systems):

1. Open-ventilated air cooled generator2. Totally enclosed water to air-cooled (TEWAC)

generator

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3. Hydrogen-cooled generators4. Water-cooled generators

1. OPEN-VENTILATED AIR COOLED (OV) GENERATOR:Outside air is drawn into the generator through a filter system to remove air contaminants, passed over the rotor, cooling it, and then discharged back into the surroundings.

2. TEWAC GENERATOR:

Unlike the open-ventilated generator, TEWACs are cooled using re-circulating air inside the enclosed generator, passed through air-to-water heat exchangers, effectively cooling the air. Air enter the generator through a filter system, this clean the air, removing contaminants. Filtered air then passes through silencers, for noise reduction. The stream is then directed over the rotor and then over frame-mounted heat exchangers, which extracts heat from the air, cooling it. Air from the heat exchangers is then directed through another set of silencers, and then out through an air outlet. Heat from the heat exchangers is transferred to water, which then flows out of the generator. At full load, TEWAC’s have efficiencies of

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approximately 98%. TEWAC’s are usually the chosen because of their reliability and simplicity.

THE EXCITERInduction generators need current to be present in the stator windings to work, the process of passing current through the windings, in order to starts the generator, is called excitation. In a power station, there is a separate unit (called an exciter) used to excite the generator. The exciter is also used to control the generator output. Varying the current supplier to the rotor, which varies the intensity of the magnetic flux, varies the current induced in the stator core field magnets.

There are two basic types of exciters, rotating and static exciters. The exciter maintains an output voltage by directly varying the amount of excitation as a function of the generator load. There are a further two types of rotating and static exciters, namely brushed and brushless excitation systems and shunt and series type excitation systems respectively. The main difference between the two rotating systems is the brushed system requires a slip ring, commutators and brushes and the brushless does not. This negatively affects the lifetime of the system as the DC voltage eventually wears out the slip ring. However, the brushless system are more complicated to design and, as a result, are more expensive. The difference between the static exciters is the shunt type operates with power from generator voltage only while the series type operates from generator voltage and current.

REFERENCE:

1. Kehlhofer, Rolf. Combined-cycle Gas & Steam Turbine Power Plants. Lilburn, GA: Fairmont, 1991. Print.

2. "Generators - Air-Cooled Generators | GE Energy." GE Energy. N.p., n.d. Web. [Accessed 17 June 2014].

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3. Petchers, Neil. Combined Heating, Cooling & Power Handbook: Technologies & Applications. Lilburn, GA: Fairmont, 2003. Print.