GENERATOR SYSTEMS
The following sections describe the excitation, cooling, and lubricating systems associated with the generator.
Excitation
The excitation system controls the output voltage and the reactive power (VAR) delivered by the generator. Brushless excitation relies on ac machines with diode bridges to convert the ac power to dc. The diode bridges can be stationary or rotating on the shaft. Self-excitation is also used by supplying power through a transformer connected to the terminals of the generator. The output of the transformer supplies either a diode or a thyristor bridge.
Hydrogen Cooling
The following are the ranges of air and hydrogen cooling in generators:
● Generators having a rating lower than 20 MW are air-cooled.
● Generators having a rating between 20 and 200 MW are either air- or hydrogen-cooled. However, the majority of generators having a rating over 100 MW are hydrogen-cooled.
● Generators having a rating over 200 MW are hydrogen-cooled.
Hydrogen has proved to be an excellent cooling medium for generators. Air-cooled generators having a rating higher than 100 MW had frequent forced outages and shorter lifetime due to poor cooling.
The hydrogen pressure inside the generator is 4 to 5 bar. It cools the stator and the rotor. The purity of the hydrogen should be maintained within specifications. Carbon dioxide and nitrogen have been used as a purging gas in generators. Note that carbon dioxide is not inert. It should not be stored in the generator for long periods.
The hydrogen cooling capability decreases with the pressure. If the hydrogen pressure cannot be maintained at the rated value, due to an operational problem, the rotor current and the capability of the machine should be reduced accordingly. The stator core temperature should be continuously monitored. The recommended temperature should not be exceeded.
Cooling of the Stator Conductors
The stator conductors of large generators are cooled by demineralized water to maximize the rating of the machine. The purity of the water should be maintained within strict limits. Its conductivity should be lower than 2 S/cm. It should also be kept free of oxygen to prevent the formation of oxides and the subsequent blockage of the water channels in the conductors. The coolant circuit should be free of ferrous materials to prevent the formation of magnetite. The water filters should be kept in a good condition.
Since the purpose of the water is to cool the stator bars, it is useful for the operator to understand the effects of the following conditions:
● Total or partial impairment of the coolant pumps
● Blockage of the water channels on the conductors due to either debris or the formation of oxide layer
● Reduction in coolant flow due to vapor (air or hydrogen) locks
● Hydrogen leakage into the water system
The manufacturer provides the limits for any of these conditions.
The temperature of the conductor bars will increase when the flow is reduced. If boiling occurs, the pressure will fluctuate. The boiling temperature is normally around 120°C because the water pressure is above atmospheric (the water is normally supplied from a raised header tank).
Hydrogen Seals
The hydrogen seals are supplied with oil for lubrication. A hydrodynamically generated oil film is established between the white metal of the seal and the mating rotor collar. Its thickness varies between 0.013 and 0.038 mm when the unit operates at normal speed. The film thickness decreases significantly when the rotational speed drops below 400 rpm. Thus, the faces of the seal will be poorly lubricated when the unit is on turning gear (barring phase) or when the speed is below 400 rpm. Therefore, it is necessary to main- tain the flatness and the surface finish of the seal faces within specifications to prevent seal failure.
The hydrogen seals have very small clearances. Any debris present in the oil could affect the performance of the seal if it is larger than the clearances. The situation is worsened when the unit is on turning gear (under barring conditions) due to thinning of the oil film. Thus, the seal oil filters should be able to remove particles down to at least 1 m to improve the integrity of the seal.
The seal should be able to move axially about 4 to 5 cm to accommodate the expansion and contraction of the shaft. Elastomers in the form of lip seal or O-ring are used in the hydrogen seal to provide oil and gas sealing as it moves axially. An increase in the white metal temperature or excessive leakage of oil or hydrogen is indicative of wear in the hydrogen seal. The contact face of the seal collar should not exceed 120°C. Since there is 4.5 to 6.0 mm of metal between the contact face of the seal and the thermocouple, the maximum operating temperature of the thermocouple should be limited to 100°C due to the 20 to 25°C temperature drop across the metal. This temperature drop will decrease due to wear. Thus, an allowance should be made if there is good reason to believe that wear has occurred.
The following are the three common operational problems that occur frequently on hydrogen seals:
PERFORMANCE AND OPERATION OF GENERATORS 15.3
● Hydrogen leaks
● Oil leaks into the generator
● High white metal temperatures
General advice to deal with these problems is difficult. However, the following actions have improved the condition of the seal in some applications:
● With high metal temperatures, increase the collar/seal gap or improve the cooling of the seal.
● With gas leakage, reduce the collar/seal gap.
● With oil leaks, reduce the differential pressure of the oil.