In the Beginning
Thomas Edison began to profit from the flow of electrons from electric dynamos at the Pearl Street Station in New York City in 1882. The commercially practical transformer was developed in 1885. Nikola Tesla, with the help of others, developed the induction motor in 1888, which in turn brought about the need for the generation of three-phase electricity. And thus, the electric utility industry was born.
The Need for Cooling
Early electric dynamos (generators) were air-cooled. As machines became increasingly larger, the use of air as a cooling medium became inadequate. Larger machines produced more heat and required more cooling. These larger machines also introduced other new complications. Windage losses and noise increased, and filters required a significant amount of maintenance. Eventually it became very difficult to remove all the heat created by the losses.
Helium was considered for cooling but its relative specific heat is only five times better than air. High cost was another factor against helium. Helium is not in abundance and can cost up to five times more than other more readily available elements.
Hydrogen has a relative specific heat [specific heat is the amount of heat, as measured in joules or calories, required to raise the temperature of one gram of substance by one degree Celsius] that is 14 times better than air and one-fourth the relative density of air. In addition, pressurized hydrogen is a better insulator than air and it is not an oxidizing agent.
Hydrogen does pose fire hazards that are not a concern with air or helium. However, hydrogen does not support combustion in a nearly pure state (>90%). With proper operation, fire hazards are almost completely eliminated (as shown by the operating records of utilities in the industry).
Around 1937, hydrogen replaced air as the primary element used to remove heat from stator windings, rotor windings and stator core iron in larger generators.
The Challenges of Hydrogen
Presently, most generators (larger than 150 mw) are cooled with hydrogen gas. The generator frames are designed to hold 99% pure hydrogen gas under pressures up to 90 psig. The gas is forced through the cooler housing and generator components by fans mounted on the generator rotor.
Moisture
Gas-to-water heat exchangers for hydrogen cooling are mounted on the side or top of the generator frame. In its delivered form, hydrogen gas has a dew point as low as -60°F. The hydrogen dew point should be maintained at less than +32°F in this type of equipment. If hydrogen moisture levels increase beyond this point, problems could occur when the machine is shut down. If there is moisture on the rings, the water will cause the retaining rings to crack. It’s not uncommon to have operating generators that are cooled by hydrogen with dew points of -15°F or lower.
Seal Oil System
A seal oil system for the generator was developed to keep the hydrogen from leaking along the rotor shaft (Figure 1). The seal ring floats on the shaft and does not rotate. It receives makeup oil from the bearing lubricating oil system.
Seal oil enters the seal housing at about 5 psig above the machine hydrogen gas pressure. The seal oil flows in both directions along the shaft. Most oil (air-oil) flows toward the bearing (zero psig) and some oil (hydrogen-oil) flows toward the hydrogen (90 psig) thus blocking the flow of hydrogen along the shaft. When the air-oil mixes with the bearing lubricating oil, some hydrogen gas becomes entrained in the hydrogen-oil. In most generator manufacturer’s systems, lubricating oil and hydrogen seal oil are combined; in others the oils are separated with minor interaction between the two.
Water in lubrication oil causes a plethora of problems. The amount of moisture allowed in the lubricating oil of a typical turbine/generator is specified by ASTM D95 as 2000 ppm (0.2%). Most users would like to see moisture concentration below 1000 ppm. Lubricating oil moisture affects hydrogen dew point. This 1000 ppm lubricating oil is at risk of mixing with the seal oil where it can come in contact with the hydrogen gas. Hydrogen gas at -60°F dew point is hygroscopic and looks like a dry sponge in the presence of oil with 1000 ppm water. In service, the hydrogen gas dew point usually is maintained between -15°F and +32°F depending on the following:
- The generator manufacturer,
- The condition of the seal oil system clearances and operation,
- The size of the generator,
- The hydrogen gas pressure, and
- A few other operating conditions.
Experience has proven that water in the hydrogen seal oil is the most common source of moisture for hydrogen dew point deterioration. Seal oil moisture of less than 50 ppm is necessary to maintain hydrogen dew points below +15°F, as required. Experience further suggests that seal oil moisture above 50 ppm will elevate the hydrogen dew point, and just 250 ppm of water will increase the dew point above acceptable levels.
Retaining Rings
Around 1960, generator manufacturers began using a non-magnetic, stainless steel material, ASTM A289, Class B, commonly known as 18Mn-5Cr, for the generator rotor retaining rings. The rings retain or hold the rotor copper winding in place on the rotor. If the retaining ring were not in place, the winding would be flung from the rotor due to the centrifugal force caused by the rotor speed (typically 3600 rpm). In the body of the rotor, the windings are held in place by wedges in the winding slots.
In Europe during the 1980s, there were a number of in-service failures by retaining rings. The reason for the failures was identified as “stress corrosion cracking” caused by the presence of condensed water on the highly stressed retaining rings. Manufacturers urged users to replace the rings with a different ring material, ASTM A289, Class C, known as 18Mn-18Cr. A unit outage for replacing retaining rings can last three to four weeks naturally, and a retaining ring failure could destroy the generator. Utilities became very sensitive to moisture in the hydrogen gas. Most utilities established a dew point upper limit of +32°F. Dew points higher than this might allow moisture in the hydrogen gas to condensate on the retaining rings when the unit is out of service.
A Solution
The major source of water in the turbine/generator lubricating oil is water from poor steam seals on the turbine. Turbine/generator lubricating oil and the hydrogen seal oil should be routinely analyzed for moisture to maintain it at levels in the range of 10 ppm (0.001%). If the seal oil moisture content rises above 250 ppm, the dew point of the hydrogen may exceed acceptable limits. It is necessary to find the source of the seal oil moisture and eliminate it in order to avoid potential problems.
References
1. Material from General Electric’s Large Steam Turbine-Generator Seminar, October 1982.
2. Gonzalez, Baldwin, Stein, & Nilsson, Monitoring and Diagnosis of Turbine-Driven Generators, Prentice Hall, Englewood Cliffs, New Jersey, 1995.
3. Sheets, W.J., Electric Generators, Volume 1, EPRI Power Plant Electrical Reference Series, EL-5036, EPRI, Palo Alto, California, December 1991.
4. NMAC Lubrication Guide (Rev 2) Table 4-6, EPRI February 1995.
5. ASTM D95-83 Standard Test Methods for Water in Petroleum Products and Bituminous Materials by Distillation.
6. ASTM A289/A289M-97 Standard Specification for Alloy Steel Forgings for Nonmagnetic Retaining Rings for Generators.
7. General Electric, Technical Information Letter, TIL-1101; Asea Brown Boveri, Service Information Letter, SIL-TG950814; Siemens/ Westinghouse, Operation & Maintenance Memo, OMM-068.
Notes from the files of J.R. Michalec, AEP Pro Serv, Inc., Graphic consultant: Don Kemmerling of AEP Pro Serv, Inc.
