Introduction to the Generator II

Following experimentation with various alternative cooling media including helium and hydrogen, around 1937, hydrogen became the cooling medium of preference in larger machines.

Hydrogen gas has a higher thermal conductivity and heat transfer coefficient than air but a much lower density, (7.00:1 1.35:1 and 0.07:1 against air in each case) therefore it acts as a much better heat transfer medium than air when applied to cooling the internal components of the generator.

In addition, pressurised hydrogen is a better electrical insulator than air, and hydrogen is not an oxidising agent.

Hydrogen is readily obtained in unlimited quantities, is inert, non-explosive and will not support combustion when mixed with air in concentrations less than 4% and greater than 70%.

Increases in generator output of 20% were gained by the replacement of air with hydrogen at a pressure slightly higher than atmospheric pressure 3 kPa (to prevent air ingress into the frame). As the density of the hydrogen was increased through increasing the pressure of the gas, further gains in heat transfer and related generator output were made. Typical hydrogen pressure in modern day generators vary from 200 to 450 kPa. 

Hydrogen Gas Safety

In considering hydrogen as a cooling medium there was a concern for hydrogen fires, a concern, which air and helium did not share.

It was considered, however, that as long as adequate safety precautions were maintained, hydrogen could be used effectively in an enclosed cooling system.

Hydrogen Gas forms an explosive mixture with air in concentrations between 5% and 70% Hydrogen.

The intensity of the explosion caused by ignited hydrogen/air mixture varies according to the percentage of hydrogen in the air/hydrogen mixture having zero values at 0% and 70% Hydrogen.

The actual pressure of the gas mixture also affects the intensity of an explosion should one occur. 

To ensure that a condition does not occur that will create a possible explosion and to reduce the intensity of the explosion, should one occur, the following precautions must be carried out:

Components of the Hydrogen Gas System 

The components of the Hydrogen Gas System include the following:

Hydrogen Gas Supply 

A makeup supply of hydrogen gas must be available to the hydrogen cooling system to allow the pressure and purity of the hydrogen gas to be maintained while ever the generator is in service and/or filled with hydrogen.

Makeup supply is normally taken from storage banks of compressed hydrogen bottles connected to supply manifolds.

The storage bottles themselves may be replenished by an external supplier or refilled from hydrogen generating plant located on site.

The supply pressure from the hydrogen bottles to the alternator frame may be automatically regulated by a manually set pressure regulating valve or, alternatively, by a reducing valve provided in parallel with the pressure regulating valve.

The pressure of the storage bank and the pressure downstream of the pressure reducing station are normally monitored.

Hydrogen Gas Dryer

The dewpoint of hydrogen within a generator is normally maintained in the order of –25° C in order to prevent internal moisture generated corrosion and failure. This would normally necessitate the inclusion of a hydrogen dryer as part of the hydrogen system.

A number of types of dryer are available in the industry, heatless, regenerative dryers, desiccant dryers, and refrigerant dryers. Any one of these could find an application as a hydrogen dryer, however, because water freezes and can no longer be removed from the system at temperatures below 0° C, refrigerant fryers are not normally used.

Within the generator frame, the hydrogen circulating fans create areas of low and high pressure at their suction and discharge respectively.

By locating the hydrogen dryer supply and return pipework between the high and low pressure sections of the generator frame a gas flow is induced through the dryer whenever the alternator shaft is rotating at synchronous speed and the shaft mounted circulating fans are in operation.

Unless a dedicated circulating fan is incorporated into the dryer circuit, drying cannot take place with the generator out of service. 

Hydrogen Gas Pressure and Purity Monitoring

The cooling quality of hydrogen gas is proportional to its density and therefore the pressure it is under within the generator frame.

Reduction of pressure results in a reduction in cooling capacity. The pressure of the gas is therefore constantly monitored to ensure that optimum gas density is maintained within the frame.

Maintaining hydrogen gas purity is of paramount importance due to its loss of performance as a cooling medium and its likelihood of falling into an explosive range if the loss of purity is caused by its mixture with oxygen.

The purity of the hydrogen gas within the generator frame is normally monitored by passing a sample of the gas through an analyser.

As carbon dioxide is used as an interfacing agent when gassing up and degassing the generator, it is most common for the analyser to have several functions and to be able to monitor hydrogen purity and the concentrations of hydrogen and carbon dioxide in air.

The suction side of the analyser can therefore be connected to either of two suction lines, one taken from the upper portion of the generator and used for hydrogen purity analysis during hydrogen filling ,the other, taken from the bottom section of the generator and used for analysis of the gas purity while CO₂ is being used as an interfacing medium between hydrogen and air.

During normal operation the lower detection point should be used as lower purity hydrogen will tend to be more dense than high purity hydrogen and a reduction in purity should be evident first in the lower portion of the frame.

Although different limits may be placed on purity at different power station sites, hydrogen purity should be maintained above 93%.

During normal operation of the generator the small amount of seal oil migrating into the generator frame may carry some entrained air and moisture with it. This air and moisture will be released from the seal oil as it migrates to the defoam tank and will eventually cause a deterioration in hydrogen purity. In order to maintain hydrogen purity it may be necessary to bleed off a portion of the gas while making up the gas volume with pure hydrogen from the cylinder bank.

In some installations a constant, regulated bleed is maintained through bleed lines directed to atmosphere through a flow regulating valve, an oil mist separator and a flowmeter.

Figure 7:  Hydrogen Bleed Station Showing Regulating Valves, Flowmeters and Oil Mist Separators

Carbon Dioxide Supply

Carbon Dioxide is used as an inert interfacing agent when filling the generator frame with hydrogen or air.

The Carbon Dioxide Supply usually consists of banks of storage cylinders attached to a supply manifold. The storage cylinders may be replenished by an external supplier or by a bulk carbon dioxide facility within the station site.

It should be noted that the gas discharge pipework temperature can be below freezing point and contact between skin and the pipework may result in the skin freezing to the pipework causing frostbite injury to the affected area. Appropriate safety precautions should be taken during operation of this equipment.

During purging the introduction of carbon dioxide may be allowed to cease once the carbon dioxide reaches a purity <75%. The detection of carbon dioxide purity must be taken from the upper sampling point in the generator frame. 

Hydrogen Gas Cooling Circuit

The hydrogen cooling circuit includes paths through the stator and rotor windings (see previous diagram of air cooled circulation paths) where heat is gained and a path through the hydrogen coolers where heat is dissipated. To ensure a positive flow through the circuit two fan impellers are mounted on the generator rotor shaft and each draws a portion of the hydrogen from the cooler and passes it through the generator windings and back to the windings. This circuit creates a high and low pressure area within the generator frame while ever the machine is operating at rated speed.

The hydrogen dryer takes advantage of the two different pressure zones to establish a hydrogen flow through the drying chamber.

Banks of hydrogen coolers are normally provided at the side or top of the generator frame. The cooling medium is normally high quality demineralised water from a closed cooling water system.

Contamination of the Generator frame can occur due to a failure of the hydrogen coolers, causing a water leak into the frame, or failure of the hydrogen shaft seal allowing oil ingress into the frame. In either case leakage must be detected. Detection points are normally tapped into the generator frame at each end of the generator adjacent to the seals. These detectors should be regularly checked and the quantity and type of liquid found in them should be logged.

System Monitoring and Alarms

The following parameters associated with the hydrogen gas system would normally be monitored: