Construction of Generators:
The generator stator shall be shipped in parts to facilitate transportation and handling/assembly at site.
The stress in the rotating parts at runaway speed shall not exceed two thirds the yield stress of the material and at maximum short circuit shall not exceed half the yield stress of the material. Manufacturers will be required to submit the stress calculations for approval.
A damper winding will be specified to improve stability during fault conditions and to reduce voltages distortion during a single phase fault. A short circuit ratio of not less than 1 will be required to give good voltage regulation, stability and to complement the high speed excitation system.
Construction of Main Parts of Generator:
The design and construction of generators will follow conventional practices and detailed specifications will be covered by the proposed tender documents. The generator foundation design will consider 3 phase short-circuit torque and seismic loading in addition to weight of generator and other operating loads.
Main parts of a generator
The stator core will consist of magnetic steel punching with space between groups to provide radial ventilation ducts for cooling air. The stator winding will be braced to withstand a sudden three phase short circuit at the generator terminals, at the maximum excitation possible, for duration of 3 seconds without deformation.
The welded steel frame will be split into sections for safe transportation to site considering loading limits of roads and bridges. Complete circularity of stator shall be ensured.
The rotor rim will be carefully built up of magnetic steel plates to ensure a good mechanical balance. The field poles made up of magnetic sheet steel punching will be dove-tailed and keyed into the rim. There will be a combined thrust and guide bearing above the rotor and a guide bearing below the rotor.
The generators will have a closed circuit cooling system with air / water heat exchanger evenly spaced around the circumference of the stator frame. The maximum cooling water inlet temperature will be 30°C with a maximum air inlet temperature of 40°C at maximum load. This condition is to be maintained with 10% of heat exchanger tubes blocked. In addition, with one complete heat exchanger out of service, it will be possible to maintain a continuous output of 1.25 MVA without the air inlet temperature exceeding 40°C or the winding temperature exceeding 60°C.
Bearing Lubricating Oil
Each bearing will have a lubricating oil bath so arranged that lubrication surfaces will not be dependent on any external oil pumps or system. The oil will be cooled by water coolers immersed in the oil bath of each bearing.
A separate high pressure oil system will be provided for each unit for oil injection between the bearing surfaces of the thrust bearing to maintain the oil film during starting and stopping of the units.
Brakes and Jacks
A set of brakes will be provided for each generator, located below the rotor acting on the brake ring attached to the rotor rim. The brakes will be capable of bringing the unit to standstill from rated speed under manual control and, in not more than 5 minutes, from 30% rated speed to standstill under automatic stopping sequence control.
The brakes will be pneumatically operated and the compressed air system will have sufficient capacity for four complete brake operations without recharging. Provision will be made for jacking up the rotating parts under manual control but will be interlocked with automatic sequence system and the brakes.
Excitation System and Automatic Voltage Control
A fully controlled static excitation system will be specified as it is now a well proven modern practice and is in wide use world wide. The excitation system will of static type comprising of cast resin type excitation transformer, automatic voltage regulator, fully controlled rectification equipment, generator field circuit breaker with discharge resistor for rapid de-excitation, field flashing equipment and over-voltage protection.
Single phase dry type excitation transformers will step down the generator voltage to the required excitation voltage. The output will be taken from rectifier bridges controlled by a solid state automatic voltage regulator.
A follow up control is provided which ensures a bump less transfer from auto to manual channel. The excitation system will include all necessary protection and limiting devices. The generator terminal voltage is held within ±5% from no-load to full load at rated frequency. The manual channel will be equipped with a field current control. For field flashing, an additional supply from the DC system is required.
The system will allow widest possible operating range under various combination of active and reactive power loading of the generator as dictated by the grid system network. Power system stabilizer for good damping of rotor oscillation will be considered in detailed specifications.
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