Generator Hydrogen Cooling System

Hydrogen is used for cooling in most large generators rather than air for several reasons.
1. Inherently better heat transfer characteristic (14 times).
2. Better the heat transfer with higher hydrogen pressure.
3. Less windage and friction losses than air.
4. Suppression of partial discharge with increased hydrogen pressure.
5. Significant increase of the breakdown voltage of generator components.

In additional to the hydrogen, a separate supply system is required for CO2 to purge the generator of hydrogen during filling and de-gassing. CO2 is used because it is inert and will not react with the hydrogen. If hydrogen in the generator were to be purged with air, this would encroach upon both the upper and lower explosive limit due to combustible nature of a hydrogen/oxygen mixture. Hydrogen at high purity will not support combustion above 90%, and at this level there is no danger of explosive since the explosive range of a hydrogen/oxygen mixture is 4 to 75% hydrogen in air.

To prevent the possibility of an explosive mixture when filling the generator with hydrogen for operation, air is first purged from the generator by CO2, and CO2 is then purged by hydrogen. When degassing the generator for shutdown, hydrogen is first displaced by CO2 and then CO2 is purged by air. This way no explosive mixture of hydrogen and oxygen can occur.


If hydrogen leaks occur, the pressure regulator admits additional hydrogen from the supply system until predetermined pressure is restored. There is always a certain amount of expected leakage into the seal oil, through minute leaks, permeation through the stator winding hoses and so forth, but most generator should capable of continuous operation below 500 cubic feet per day loss. If the loss increases to 1500 cubic feet per day, the source of leak should be investigated immediately and corrected.
Click here for calculation Generator H2 leak.

A hydrogen gas analyzer is usually present to monitor the hydrogen purity, which should be maintained above 97%. Dew point monitoring is sometimes provided to control the level of moisture inside the generator. The dew point is generally maintained below -10C and should not be allowed to rise above 0C.