The concepts of a hydrogen-cooled generator are very simple. As current flows in a conductor, heat is generated. A generator has a lot of conductors and a lot of current flowing through the conductors, generating a lot of heat. If that heat isn’t “removed” then the windings will be damaged (insulation “blisters”; conductors grow and elongate causing clearance and balance issues; etc.). In addition, in a synchronous generator (alternator), there are high currents flowing in the rotor windings, which also generates heat which must be “removed”.
Air can be used to cool a generator, by circulating it through the generator to absorb heat and then exhausting the air to another area outside the generator. A continuous flow of air from outside the generator, through the generator, to another area outside the generator will cool the generator and rotor. (The presumption is that the air entering the generator is cooler than the generator.)
Another way to cool the generator is to use hydrogen gas circulated through the generator and around the rotor to cool things. Hydrogen is seven to ten times better at transferring heat than air. That is, hydrogen is much, much better at absorbing heat and then at giving up that heat to another medium/area than air. This means that for the same size generator, if it’s cooled with hydrogen versus air that more current can flow in the stator and rotor windings which means that more power can be produced. Or, thinking about it a little differently, the same about of power can be produced with a smaller generator cooled with hydrogen than one cooled with air, which is the typical reason for using hydrogen cooling–to reduce the physical size (and cost) of the generator.
The “problem” with using hydrogen as a cooling medium is that it’s explosive when mixed with air and exposed to an ignition source. However, if the purity of the hydrogen is maintained at a very high level, meaning there is very little or no air in the generator casing to mix with the hydrogen, then even if there was a “spark” there won’t be an explosion.
So, to use hydrogen to cool a generator it’s necessary to prevent air from contaminating the high-purity hydrogen used to fill the generator casing after displacing the air. And that’s important, displacing as much of the air inside the generator casing before filling it with high-purity hydrogen.
The hydrogen gas inside the generator is usually at a pressure of approximately 2 barg (30 psig), which means that air cannot leak “into” the generator casing where the hydrogen is. In effect, this is the primary method of preventing air from getting into the casing and contaminating the hydrogen.
The hydrogen is circulated by fans on the ends of the generator rotor, and as it’s circulated around the generator it passes over coolers which have water circulating through them. The heat which is absorbed by the hydrogen gas as it passes through the generator and around the rotor is transferred to the water in the cooler. As the hydrogen exits the coolers, it’s recirculated back to the generator and rotor, in a continuous cycle.
Another important consideration is to keep the hydrogen from leaking out of the generator, mixing with air and causing an explosion- or fire hazard where it might leak out of the generator.
The generator rotor rotates where it passes through the end-shields and that is the area that must be sealed to keep the hydrogen in and not allow it to leak out. A hydrogen-cooled generator uses “seals” to keep the hydrogen gas inside the generator.
The hydrogen seals are on the two shaft “ends” that pass through the generator end-shields. Oil is typically used as the sealing medium, and is sprayed on the shaft around the entire circumference of the shaft. The “seal oil” is at a higher pressure than the hydrogen inside the generator casing. Some of the seal oil flows out of the seal area along the shaft to the “air” side of the generator and some of the oil flows out of the seal area along the shaft into the “hydrogen” side of the generator.
The oil that’s used as the seal oil is generally the same lubricating oil that’s used for the bearings. That oil is normally in contact with air when it’s in the lube oil tank and the bearing drains. So, air (in the form of small bubbles) can be entrained in the lube oil, and when sprayed on the generator shaft that air can be liberated from the oil that flows into the hydrogen side of the seal area. That air, if not “removed” somehow, can continue to collect inside the generator casing and reduce the purity of the hydrogen, and cause a safety concern.
So, because the hydrogen inside the generator casing is at a higher pressure than outside the generator casing, air can’t leak into the generator. And, because oil, which will have entrained air in it, is used as the sealing medium, the air released from the “seal oil” that flows into the hydrogen area can reduce the hydrogen purity if not removed. So, the primary source of air to reduce hydrogen purity (contaminate the hydrogen gas inside the generator) is air liberated from the oil used to keep the hydrogen inside the generator from leaking out along the shaft.
So, there is a system to remove the air which is liberated from the seal oil that flows into the hydrogen side of the generator seal. That system is typically called a “scavenging” system. A small amount of gas is allowed to be vented from the seal oil enlargement tank, where the entrained air liberated from the seal oil is hopefully contained. That vent is normally piped to a safe area of the atmosphere away from any ignition source since it will have hydrogen gas in it as well as air.
Because a small amount of gas (air and hydrogen) is continually being vented to atmosphere through the scavenging system, the pressure inside the generator would decrease if nothing else were done. However, there is a pressure regulator from a source of high-purity hydrogen that maintains the pressure by flowing a small amount of high-purity hydrogen into the generator casing to maintain the pressure.
There is usually a hydrogen purity monitoring system which can be used to detect changes in purity during operation. Again, the presumption is that the hydrogen purity is high to begin with (after the generator casing is purged of air and filled (“charged”) with hydrogen). But because of the air which can be liberated from the oil used as the sealing medium to keep the hydrogen from leaking out of the generator, it is necessary to have a means of monitoring the purity, usually at multiple points in the generator, to be sure that the purity is maintained to prevent a possible explosion or fire.
The scavenging flow rates must also be set and monitored to prevent excessive hydrogen consumption (to replace the hydrogen lost with the air that’s being vented to atmosphere).
The seal oil that flows into the hydrogen side of the seal area is usually directed to an area called and “enlargement tank” and that’s where it’s hoped the entrained air is liberated and “contained”, and vents from the enlargement tanks are directed through calibrated flow-meters to atmosphere (the scavenging system). The hydrogen purity monitors are usually capable of monitoring the purity of the gas in the top of the enlargement tanks that’s vented through the scavenging system.
The hydrogen purity monitor can also be set to monitor the purity inside the casing (which should be higher than the enlargement tanks, theoretically).
Usually, when the hydrogen purity monitor indicates the purity is decreasing, then it increases the scavenging flow-rate to try to increase the purity, but venting more “contaminated” gas to atmosphere and replacing it with high-purity hydrogen.
If the hydrogen purity drops below a certain level (around 80% or so, depending on manufacturers’ recommendations), then usually the generator and prime mover are stopped and then the generator is purged of hydrogen. (CO2 is usually used as the medium for purging air from the generator when filling with hydrogen, and also when purging the hydrogen from the generator.)
Presuming the hydrogen being used to maintain the pressure while scavenging is high purity, and because the casing pressure is usually two barg, the most likely way that purity can be decreased is from the air entrained in the seal oil. If the seal oil flow-rates (usually monitored with a single (unfortunately) flow meter) increase, then the air liberated from the seal oil will increase and that is the primary source of contamination and decreased purity. (This presumes that the purity monitors are working correctly.) So, it’s very, very important to monitor seal oil flow-rates to detect an increase which would usually result in a decrease in purity. But, with only a single seal oil flow-rate meter for seals at two ends of the generator, it’s difficult to detect which seal is degrading and consuming more oil which is liberating more air (but the decreased purity in a particular enlargement tank is the key ).