GENERATOR HYDROGEN COOLING

State the purpose of the Generator Hydrogen Cooling System Remove heat generated by friction, reactive power, I2R losses, eddy currents, and hysteresis Increase in electrical megawatt output which is limited by heat (Per Westinghouse: Rotor is limited to 110°C, and the Stator is limited to 96°C) Decrease the generator size Name the advantages and disadvantages of Hydrogen as a generator coolant ADVANTAGES: cheap, plentiful, low windage loss, good heat transfer, easily handled, and not corrosive DISADVANTAGES: explosive, invisible, and added fire protection and purging required Describe how the TCV for the hydrogen coolers is operated at Sequoyah Air operated Located TB, El. 662 (Cooler is located top of generator housing) TCV Controls RCW outlet temperature in order to maintain air outlet temp at 45°C Exciter is suitable for operation with air temperature from cooler up to 50°C Describe the operation of the generator leak detectors Detect liquid in the generator casing Elevation 706 of the Turbine Building Atmospheric drains are visually checked for liquid (Oil and / or Water) Name all equipment in the secondary reducing station Supply Pressure Gauge Supply pressure at the secondary reducing station Supply Pressure Switches LOW: 90 psig HI: 110 psig Both alarm on L-39 Pressure regulators Reduce pressure from 100 psig down to 75 psig H2 Integrators Take data daily (Mids) to determine H2 usage for that day CO2 Connections Provide for purging and venting ops at secondary reduction station Name all monitoring systems for machine gas which are located on L-39 panel, including alarms and setpoints ALARMS: Generator Hydrogen Purity High or Low High - 100% Low - 90% Generator Hydrogen Pressure High or Low High - 100 psig Low - 66 psig Hydrogen Supply Pressure Low 90 psig Generator Water Detector High 2" decreasing on the centerline of the top switch Hydrogen Temperature High 115°F INDICATORS: % Uncompensated Density 150 - 0% Hydrogen Purity (Red / Green scale) 100% H2 (Red) 100% CO2 (Green) Fan Pressure (Green Scale) 0 - 200" of Water Hydrogen Pressure (Red Scale) 0 - 100 psig Explain cold gas temperature and state its normal and maximum temperature Cold Gas Temperature is measured leaving the H2 Coolers: (normally 95 – 105°F) Normal: 35°C (95°F) Maximum: 46°C (114.8°F) Cold gas temperature in the generator is determined by the capacity of the hydrogen cooler to absorb heat and by the amount of heat to be removed. The difference between the average hydrogen temperature and the average water temperature depends upon the losses to be absorbed. When the load on the generator is increased without changing the gas pressure or the cooler water conditions, the temperature difference between the gas and the water must increase, with the result that the cold gas temperature goes up. At a given load, if the hydrogen pressure is increased, the temperature rise of the hydrogen gas in the generator is reduced as is the gas temperature drop through the cooler. On the other hand, if the gas pressure in the machine is varied as the load is increased correspondingly, there may be an increase in the cold gas temperature. Because of the increased mass flow of the hydrogen, however, the hot gas temperature will decrease and a satisfactory temperature will be maintained in the generator components. At all loads, the cold gas temperature should be maintained below its maximum value (46°C) by varying the cooler water flow Explain the operation of the hydrogen dryer 2 tower dryer One is in service and the other is reactivating 8 hour cycle 1st 4 hours: Desiccant is heater and water trap drains off the condensate 2nd 4 hours: Used to cool the desiccant bed State the information found on the generator capability curve and explain how the operator uses this information Curve is drawn so as to limit the hot spot temperatures in the stator and rotor windings and in the stator core to practical operating values and to limit the temperature differential across the insulation of the windings The curve relate the following parameters: Megawatt output Megavars Power Factor Machine Gas Pressure 0 pf (Over-Excited) - .9 pf (Over-Excited): limited by rotor winding temperature .9 pf (Over-Excited) - .95 pf (Under-Excited): limited by stator winding temperature .95 pf (Under-Excited) – 0 pf (Under-Excited): limited by stator core temperature Curve Explain how the generator is purged with CO2 and air During periods of generator maintenance it may be necessary to displace the gas in the generator because hydrogen can be explosive when mixed with air. CO2 gas is used to remove hydrogen which is also toxic. In order to fill the generator with hydrogen, the air must first be removed: CO22 is admitted through the bottom of the generator. Because it is heavier than air, the air will be forced to the top where it is vented off. With the generator now full of CO2, hydrogen may now be admitted. The hydrogen, being lighter than CO2, is introduced from the top of the generator. When a reading of 95% hydrogen is achieved at the bottom of the generator, normal operation may begin. Should maintenance become necessary inside the generator, the hydrogen must be removed. To do this: CO2 is admitted to the bottom of the generator, forcing the hydrogen out of the vent Air can then be used to remove the CO2 from the machine Explain how the hydrogen coolers are constructed and the hydrogen flow path through the coolers 4 horizontally mounted, double pass coolers mounted in the top of the generator Cooler vents and drains beneath the generator on elevation 706 Fans on each end draw air down shaft (both directions) and up the ends of the generator to the coolers. Through the coolers and down across the rotor. Briefly describe exciter cooling Consists of two cooling units per unit Cooled by RCW TCV on El. 662.5 maintaing air leaving the fan at 45°C Air circulates through the cooler above the PMG, around the PMG, through the shaft connected fan, around the bearing housing, around the Main Exciter and back to the cooler.