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Role Of Hydrogen In Generator Presentation Transcript
1.Role of hydrogen in generator
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".
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".
2.Air cooled generators
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.)
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.)
3.Why use H2 as cooling media
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.
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.
4.Risk with H2
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 "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.
5.Function of Generator Hydrogen Seal Oil
"Provide a seal between the generator housings and rotor shaft to maintain the pressurized hydrogen gas inside the generator. Also, provides a trap-vent system to prevent the release of hydrogen into the turbine generator lube oil system and building atmosphere."
"Provide a seal between the generator housings and rotor shaft to maintain the pressurized hydrogen gas inside the generator. Also, provides a trap-vent system to prevent the release of hydrogen into the turbine generator lube oil system and building atmosphere."
6.Why is oil for shaft seals vacuum treated?
So that a minimum amount of air and moisture will be released to the generator casing; thereby maintaining hydrogen purity at a satisfactory level without necessity of purging or adding hydrogen other than makeup.
So that a minimum amount of air and moisture will be released to the generator casing; thereby maintaining hydrogen purity at a satisfactory level without necessity of purging or adding hydrogen other than makeup.
7.THRI 108/44: Capability (Class B Temps. With 99°C Stator)
8.Generator Cooling
Stator is indirectly Hydrogen cooled .
Rotor has direct radial gas cooling.
Stator core is radially gas cooled.
CW flow to gas-coolers opens on load.
Cold gas minimum temp specified.
To minimize stresses in machine with load variations, CW flow is regulated automatically.
Stator is indirectly Hydrogen cooled .
Rotor has direct radial gas cooling.
Stator core is radially gas cooled.
CW flow to gas-coolers opens on load.
Cold gas minimum temp specified.
To minimize stresses in machine with load variations, CW flow is regulated automatically.
9. THRI : Ventilation Scheme
10.Shaft Seals - 2
Ring relief oil pressure on air side reduces friction to ring movement and to transient vibrations. Rings freely adjust in operation both radially and axially.
Forces on seal ring
with ring relief oil
Ring relief oil pressure on air side reduces friction to ring movement and to transient vibrations. Rings freely adjust in operation both radially and axially.
Forces on seal ring
with ring relief oil
11.H2 Shaft Seal
12.THRI-210 Seal Oil Temp Rises
13.Single Flow Seal Oil System
14.Ring Seal Oil System - 2
Seal oil / H2 differential press.indicator, gives alarm for low differential.
Falling seal oil press.switch gives alarm
High oil level in H2 side pre-chambers.
Low level in seal-oil storage tank alarm.
Low level in intermediate oil tank alarm to prevent escape of H2 to vacuum tank or outside.
Max./Min.oil level in vacuum tank alarm
Seal oil / H2 differential press.indicator, gives alarm for low differential.
Falling seal oil press.switch gives alarm
High oil level in H2 side pre-chambers.
Low level in seal-oil storage tank alarm.
Low level in intermediate oil tank alarm to prevent escape of H2 to vacuum tank or outside.
Max./Min.oil level in vacuum tank alarm
15.Shaft seal oil vacuum (level) and hydrogen purity.
ABSOLUTE PRESSURE of 1.0" Hg or less in the vacuum tank results in hydrogen purity of 97% or higher in generator.
ABSOLUTE PRESSURE of 1.0" Hg or less in the vacuum tank results in hydrogen purity of 97% or higher in generator.
16.Function of recirc seal oil pump
Recirculates oil in vacuum tank through recirc spray header to aid in air and moisture removal from oil and to reduce foaming in vacuum storage tank.
NOTE: Recirc seal oil pump driven by Main Seal Oil Pump motor.
Recirculates oil in vacuum tank through recirc spray header to aid in air and moisture removal from oil and to reduce foaming in vacuum storage tank.
NOTE: Recirc seal oil pump driven by Main Seal Oil Pump motor.
17.Besides main seal oil pump and emergency seal oil pump, what other method is there to supply oil to the seals?
Can be supplied directly from bearing header. However, this can only provide sealing for machine gas pressure up to about 8 psig.
Can be supplied directly from bearing header. However, this can only provide sealing for machine gas pressure up to about 8 psig.
18.How is excess discharge pressure prevented?
Pressure relief valves on the discharge side of each seal oil pump relieves back to seal oil system.
Emergency seal oil pump relief valve is set at 80 psig and returns oil to pump suction line.
Main seal oil pump relief valve is set at 90 psig and returns oil to vacuum tank via recirc seal oil pump discharge line.
Pressure relief valves on the discharge side of each seal oil pump relieves back to seal oil system.
Emergency seal oil pump relief valve is set at 80 psig and returns oil to pump suction line.
Main seal oil pump relief valve is set at 90 psig and returns oil to vacuum tank via recirc seal oil pump discharge line.
19.Very large generators have their casing filled with hydrogen to reduce windage losses from the huge rotating generator rotor. In order to contain the extremely volatile hydrogen in the casing‚ pumps are used to pump oil into the generator shaft labyrinth seals at a pressure higher than the hydrogen. This keeps the generator seals pressurized containing the hydrogen.
20.The oil gravity drains to a tank located about 2 meters above the oil pumps. As a result‚ the oil pump inlet pressure is extremely low. Three screw pumps are used for their high reliability and ability to operate at extremely low suction pressures as well as their ability to operate at higher speeds than comparable flow gear pumps. Other systems integrate the seal oil system into the lube oil tank and use centrifugal pump.
21.Each hydrogen cooled generator is served by a main and auxiliary seal oil pump driven by an AC motor. There is also an emergency DC motor driven pump to provide the critical oil in the event of a turbine trip where AC power is not available. This allows the generator to safely coast down with out leaking the hydrogen gas.
22.To keep the hydrogen inside the generator, various places in the generator are required to seal against hydrogen leakage to atmosphere. The seal oil it self actually a portion of the lube oil, diverted from the lube oil system. It is then fed to a separate system of its own with pump, motor, hydrogen detraining or vacuum degassing equipment, and controls to regulate the pressure and flow.
23.The seal oil pressure at hydrogen seals is maintained generally about 15 psi above the hydrogen pressure to stop hydrogen from leaking past the seals. Once of the critical component of the seal oil system is the hydrogen degasifying plant. The most common method of removing entrained hydrogen and other gases is to vacuum treat the seal oil before supplying it to the seals. This is generally done in the main seal oil supply tank. As the oil is pulled into the storage tank under vacuum through a spray nozzle, the seal is broken up into the fine spray. This allows the removal of dissolved gases. In addition there is often re-circulate oil back to the tank through a series of spray nozzles for continuous gas removal.
24.After passing through he generator shaft seals, the oil goes through the detraining sections before it returns to the bearing oil drain. As a safety feature there is often a dc motor driven emergency seal oil pump provided. This motor will start automatically on loss of oil pressure from the main seal oil pump. This is to ensure that the generator can be shut down without risk to personnel or the equipment.
25.How to avoid H2 leakage
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 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.
26.How to seal H2
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
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
27.How to remove air from seal oil
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.
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.
28.System of air removal
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
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
29.Purity of H2
30.Enlargement tank
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.
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.
31.Limit of h2 purity
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.)
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.)
32.Seal construction
seal" is actually a brass ring with an inner diameter slightly larger than the outer diameter of the alternator rotor. It is held tightly around the shaft with springs. Oil is admitted through ports and grooves in the seal rings and "sprayed" onto the shaft. The brass rings and holding springs can wear or even fail over time. Dirty oil can also cause wear of the seal rings. The generator rotor shaft, as all shafts in journal bearing, moves as it is accelerated and even axially with the application of torque and electrical forces (though axial movement is generally very small, as is the radial movement). Unattended high vibrations of the generator shaft can also cause problems, as can induced voltages that are not properly grounded (through the shaft grounding brush). Also, the seal oil pressure regulators do require maintenance and adjustment and can be the source if increased seal oil flow rates.
seal" is actually a brass ring with an inner diameter slightly larger than the outer diameter of the alternator rotor. It is held tightly around the shaft with springs. Oil is admitted through ports and grooves in the seal rings and "sprayed" onto the shaft. The brass rings and holding springs can wear or even fail over time. Dirty oil can also cause wear of the seal rings. The generator rotor shaft, as all shafts in journal bearing, moves as it is accelerated and even axially with the application of torque and electrical forces (though axial movement is generally very small, as is the radial movement). Unattended high vibrations of the generator shaft can also cause problems, as can induced voltages that are not properly grounded (through the shaft grounding brush). Also, the seal oil pressure regulators do require maintenance and adjustment and can be the source if increased seal oil flow rates.
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