The use of gaseous hydrogen as a coolant is based on its low
density, high
specific heat, and the highest
thermal conductivity (at 0.168 W/(m·K)) of all gases; it is 7 to 10 times better at cooling than air. Another advantage of hydrogen is its easy detection by
hydrogen sensors. A hydrogen-cooled generator can be significantly smaller, and therefore less expensive, than an air-cooled one. For stator cooling, water can be used.
Helium with a thermal-conductivity of 0.142 W/(m·K) was considered as coolant as well; however, its high cost hinders its adoption despite its non-flammability. Generally, three cooling approaches are used. For generators up to 60 MW,
air cooling can be used. Between 60 and 450 MW hydrogen cooling is employed. For the highest power generators, up to 1800 MW, hydrogen and
water cooling is used; the rotor is hydrogen-cooled, while the stator
windings are made of hollow copper tubes cooled by water circulating through them. The generators produce
high voltage; the choice of voltage depends on the tradeoff between demands of
electrical insulation and handling high electric current. For generators up to 40 MVA, the voltage is 6.3 kV; large generators with power above 1000 MW generate voltages up to 27 kV; voltages between 2.3 and 30 kV are used depending on the size of the generator. The generated power is sent to a nearby
step-up transformer, where it is converted to the
electric power transmission line voltage (typically between 115 and 1200 kV). To control the
centrifugal forces at high rotational speeds, the rotor diameter typically does not exceed 1.25 meters; the required large size of the coils is achieved by their length and so the generator is mounted horizontally. Two-pole machines typically operate at 3000 rpm for 50 Hz and 3600 rpm for 60 Hz systems, half of that for four-pole machines. The turbogenerator also contains a smaller generator producing
direct current excitation power for the rotor coil. Older generators used
dynamos and
slip rings for DC injection to the rotor, but the moving mechanical contacts were subject to
wear. Modern generators have the excitation generator on the same shaft as the turbine and main generator; the
diodes needed are located directly on the rotor. The excitation current on larger generators can reach 10 kA. The amount of excitation power ranges between 0.5 and 3% of the generator output power. The rotor usually contains caps or cage made of nonmagnetic material; its role is to provide a low impedance path for
eddy currents which occur when the three phases of the generator are unevenly loaded. In such cases, eddy currents are generated in the rotor, and the resulting
Joule heating could in extreme cases destroy the generator. Hydrogen gas is circulated in a closed loop to remove
heat from the active parts then it is cooled by gas-to-water
heat exchangers on the
stator frame. The working pressure is up to 6
bar. An on-line
thermal conductivity detector (TCD) analyzer is used with three measuring ranges. The first range (80–100% H2) is to monitor the
hydrogen purity during normal operation. The second (0–100% H2) and third (0–100% CO2) measuring ranges allow safe opening of the turbines for maintenance. Hydrogen has very low
viscosity, a favorable property for reducing
drag losses in the rotor. These losses can be significant due to the rotor's high rotational speed. A reduction in the purity of the hydrogen coolant increases
windage losses in the turbine due to the associated increase in viscosity and drag. A drop of only a few percent in hydrogen purity can increase windage losses by hundreds of kilowatts in a large generator. ==Operation==