In a thermal power plant heat is produced by burning of coal in boiler. This heat is used to generate steam which rotates the turbine. A turbine generator in thermal power plant is connected to the shaft of the turbine. The turbine provides the mechanical energy input to the generator which is converted into electricity.
The turbine generator in thermal power plant is completely enclosed for safe operation, uses hydrogen gas as the cooling medium. The ventilation system including the fans and gas coolers, is self-contained and completely enclosed to prevent dirt and moisture from entering into the generator. The Stator winding is directly cooled by demineralized water, and rotor winding are directly cooled by hydrogen. The stator core is also cooled by hydrogen.
The turbine generator frame is substantially cylindrical in shape and of welded gastight construction. The outer end shields at either end of the frame are also of gastight construction and support the generator bearings and shaft seals. The generator frame and end outer shields are of the explosion proof type.
Accessories of turbine generator in thermal power plant include a gas control device, a cooling water control device for controlling the gas temperature inside the generator as well as supervising and controlling device, and various instruments for hydrogen pressure, hydrogen purity, shaft sealing oil pressure, the temperature of the cooling system, the gas temperature inside the generator, and the temperatures of the windings. Stator winding cooling water control device for supervising and controlling the temperature, pressure flow and conductivity of the cooling water of stator winding is also included.
Components of Turbine Generator in Thermal Power Plant
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Stator Frame
The function of stator frame is to contain and support the stator core and winding and the rotating field, and also to provide multiple paths for circulating the cooling hydrogen gas throughout the generator.
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Stator Core
The function of stator core is to support stator winding. The stator core is made up of segmental, insulated punching of high-quality silicon steel to give minimum electrical loss.
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Stator Winding (Armature Winding)
The function of stator winding is to flow generated electrical current. The stator winding is formed by insulated bars assembled in the slots of the stator core, jointed at the ends to form coils, and connected to the proper phase belts by bus rings.
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Rotor Winding and Retaining Ring
The rotor core and retaining ring are brought in direct contact with each other, and to flow current in between in case of an unbalanced load or a sudden short-circuit fault. To reduce the effect of this current, the engagement between the retaining ring and rotor shaft is metallized for a reduced contact resistance.
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Outer End Shield
The outer end shield is built-in with main bearing and shaft sealing devices for supporting the rotor, and oil pipings for feeding oil to them. The outer end shield is divided into upper and lower halves at horizontal axis to facilitate disassembling and reassembling.
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Bearing Metal
The bearing metal is set in the bearing seat and the outer end shield. It is shaped elliptic to sustain the heavy weight of the rotor stably over a wide range of speed. The bearing metal is made of a high-rigidity, metal block (shell) whose inner walls are lined with babbitt metal. The bearing oil is subjected to temperature and pressure control by the lubricating oil system common to the turbine, and is supplied to the bearing metal. The outer surface of the bearing metal is spherically seated and ensures uniform contact with the bearing seat irrespective of the angle of the rotor shaft. A thermocouple is embedded right under the sliding surface of the metal block to monitor bearing metal temperature.
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Shaft Sealing Device
A shaft sealing device is installed inside the machine to come inside of the bearing, and is used to prevent hydrogen gas leaking from the machine through the shaft penetrant part.
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Hydrogen Gas Cooler
They use cooling water to remove heat from hydrogen gas that is heated while being circulated within the generator. The hydrogen gas cooler is composed of a heat exchange section having a bundle of copper alloy finned tubes, upper and lower tube sheets holding the upper and lower ends of the tube bundle, and upper and lower water boxes.
Heat exchange between the cooling water and hydrogen gas is carried out by the counter-flow system. Hot hydrogen gas enters the cooler from the cooling water drain side, and the cooled hydrogen gas leaves the cooler from the cooling water supply side.
That part of the hydrogen gas cooler which passes through the stator frame is totally sealed to prevent hydrogen gas leaking from the machine. At the lower part, a gland packing type seal is used which is slidable to absorb the expansion and contraction of the cooler due to temperature change.
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Water and Oil Detector
To protect the generator windings from water entering the casing as the result of a cooler leak, and from oil which could enter from the shaft seals because of imperfect functioning of the oil deflectors, a liquid detector is connected to the bottom of the generator casing. The detector operate the alarm system if more than 800cc of liquid should collect in the body of the detector
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Gas Temperature Alarm
To protect the generator windings from excessive temperatures resulting from failure of the water supply to one or more of the gas coolers, a high temperature alarm device is provided. It is located in the path of the gas leaving the coolers.
It should be adjusted to actuate an alarm at a temperature slightly above the maximum expected gas temperature for rated load at the hydrogen pressure which the generator normally operates.
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Gas Temperature Indicator
A temperature indicator is provided, indicating the temperature of the gas leaving the generator gas coolers.
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Machine Gas Pressure Regulator
The machine gas pressure regulator is adjustable to maintain the machine gas pressure constantly at any valve within the prescribed operating range.
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Hydrogen Gas Measuring Inst. Panel and Gen. Aux. Control Panel
These panels are required for supervising the operation of the generator in hydrogen.
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Pressure Switches
Pressure switches are provided at the H2 gas measuring ints. panel which operate the alarm system, to indicate low and high machine gas pressures.
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Gas Analyzers
A gas analyzer is provided on the gen. aux. control panel which indicates the percentage purity of the hydrogen in the generator.
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Control of Gas Flow Through Analyzer
A flowmeter is provided for measuring the rate of gas flow through the detector unit of the gas analyzer.
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Temporary Differential Fan Pressure Gauge
This instrument measures the differential pressure developed by the generator fans when operating in hydrogen. It must be shut off when operating in air. Since the pressure of the fans at constant speed is proportional to the gas density, the readings of the fan pressure gauge provide an independent check on the hydrogen purity in the generator. This gauge may be used, therefore, to indicate the generator purity if the analyzer for the casing gas should be out of service.
Safe Handling of Gas Control System of Turbine Generator in Thermal Power Plant
- As pure gas, hydrogen will not burn; however, when mixed with air it forms a highly explosive mixture. The flammability range for hydrogen at atmospheric pressure lies between 4.1 and 74.2% by volume in air. Hydrogen purity in the generator casing must always be maintained above the upper explosive limit, and should preferably not be allowed to fall below 90%.
- To avoid having an explosive hydrogen-air mixture in the generator at any time, either when the generator is being filled with hydrogen prior to being placed in service, or when hydrogen is being removed from the generator prior to opening the generator for inspection or repairs, an insert gas is used for purging out air or hydrogen initially within the generator casing. Carbon dioxide is ordinarily used as the purging agent.
- Carbon dioxide should be admitted to the generator until the percentage of carbon dioxide in the discharged gas is in excess of 70 percent.
- After purging air from the generator to obtain 70 percent carbon dioxide in air mixture in the discharge pipe, hydrogen may be admitted.
- Hydrogen should be admitted to the generator until the percentage amount of hydrogen in the gas mixture discharged is in excess of 95 percent.
- With a pressure between 0.1 and 0.3kgf/cm2g maintained in the casing, carbon dioxide should be admitted until a carbon dioxide concentration in the discharged gas in excess of 90 percent is obtained.
- After the hydrogen purging operation has been completed , and prior to work inside the generator or welding and cutting of connecting piping, the CO2 should be purged from the casing with dry air. After replacing the inside of generator by air, oxygen concentration inside the generator is measured and a safe thing is checked.
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