By Douglas J. Smith, IEng, Senior Editor
During operation power plant turbine generators produce large amounts of heat, and unless it is dissipated, the generators are unable to operate at maximum efficiency. Prolonged excessive heat can also lead to a reduction in the life of the generator. Although air is used as the cooling medium in smaller units, larger generators typically use hydrogen for cooling, although, technical developments over the last few years are now allowing manufacturers to supply air-cooled generators up to 500 MVA.
Hydrogen has many advantages over air including high heat capacity and very low viscosity. Hydrogen cooled generators also have less windage/friction loss than air-cooled generators. Windage, which is caused by friction between the hydrogen and the rotor, can account for 30-40 percent loss in the efficiency of a generator.
Unless electric generators have adequate cooling, catastrophic failures and very expensive repair costs can result. An example of this was the failure of a generator at PacifiCorp’s Hunter Unit 1 in Utah in November 2000. Not only did they have the expense of a complete rebuild it also was very costly to replace the lost power. According to the utility, from the time the unit went off line to it returning to service in May 2001, the cost for purchasing replacement power was $270.1 million.
Improving the operating efficiency and reducing the maintenance costs of generators is critical in today’s competitive environment. As a result utilities are now starting to incorporate real-time online monitoring and diagnostic systems for the generators. According to Environment One Corporation, for example, early warning of generator overheating can mean the difference between a brief shutdown versus a major overhaul that could take months of costly downtime.
Monitoring for Purity
As the purity of the hydrogen used for cooling the generators drops the windage losses increase. This translates directly into higher electricity production costs, Figure 1. An eight percent drop in efficiency of an 800 MW unit would increase the cost of producing electricity by almost $4,000/day.
Two major U.S. generating companies are using an Environmental One Corporation microprocessor controlled generator gas analyzer (GGA) to continuously analyze hydrogen in their generators, Figure 2. The analyzer monitors the purity of the hydrogen during normal operations and during startup of the generator when it is being purged with CO2. During the purging process, the analyzer monitors the amount of hydrogen in the CO2 until the correct purity level of 98 percent is achieved. Although not all power plants have continuous monitoring systems they do monitor the purity of the hydrogen on a regular basis.
One generating companies, with 37 plants in operation or under construction, has installed 30 GGAs over the last four years on their gas turbines. According to the company, the analyzers are used in real-time to monitor and control the purity of the hydrogen. At present the purity of the hydrogen used in the company’s steam turbine driven generators is controlled manually. However, they do have plans to install automatic generator gas analyzers on the steam turbine generators.
Although the GGA can be used on its own, it is generally installed in parallel with other monitoring systems such as hydrogen temperature, MW output and VARs that give early warning of generator overheating. In addition, generator gas dryers are also typically installed. These gas dryers continually remove moisture and other contaminants from the hydrogen during generator operation.
Figure 2. Gas monitoring station. Photograph courtesy of Environmental One Corporation.
Another manufacturer, Yokogawa Corporation, has developed a vibrating element method for measuring hydrogen purity, Figure 3. In operation the device measures the vibration of a cylinder, surrounded by a sample of the hydrogen used in the generator, to determine its purity. According to Yokogawa, the vibrating element type monitoring system does not require reference air, is resistant to oil mist and vibration, and is not affected by changes in ambient air temperature.
Two years ago, Southern Company’s Georgia Power Yates power plant began to switch from thermal conductivity analyzers to vibrating element type analyzers to measure hydrogen purity. According to the plant’s operators the accuracy of the old thermal conductivity analyzers was suspect, unreliable and difficult to calibrate. Today the new vibrating element type analyzers take less time to calibrate than it takes to pick up the test equipment and walk out to the analyzer, says one of the plant’s technicians.
Another power plant that has recently changed to vibrating element type analyzers for measuring the purity of hydrogen in their generators is Watson Cogeneration Company. Watson Cogen has fitted two vibrating element type analyzers on each of four 90 MVA GE generators. Although only one analyzer was sufficient Watson wanted a second redundant system. Philip Gray P.E., a Watson staff engineer, states that the turbine generator is too expensive not to have a redundant backup system.
On-site Hydrogen Generation
When a generator is in operation hydrogen is continually being lost and must be replaced on a regular basis. In general electric utilities use hydrogen delivered to the site in cylinders or shipped in bulk by tanker truck. With bulk storage the hydrogen is transferred from the tanker to a pressurized holding tank.
Figure 3. Monitoring for hydrogen purity. Photograph courtesy of Yokogawa Corporation of America.
Although on-site hydrogen generation technologies have been available for many years it has only been utilized by remote and inaccessible power plants. However, since 9/11 there has been some concern raised in safety of delivering bulk hydrogen to power plants in large cities. One technology now being utilized by some electric utilities is electrolysis which only requires deionized water and electricity to produce the hydrogen, and there is no need for a compressor.
Although an electrolysis hydrogen generator could supply all of a power plant’s hydrogen needs in most instances, it is typically only used for makeup. Hydrogen used for purging and filling a generator after maintenance is generally supplied from an outside contractor. A GE Frame 7 gas turbine generator, for example, requires approximately 21 cubic feet of hydrogen per hour for makeup. However, to refill the generator requires 7,500 cubic feet of hydrogen.
Golden Valley Electric Association’s 25 MW coal-fired Healy power plant, located on the Nenana River near Mt. McKinley, Alaska installed a Proton Energy Systems “Hogen” hydrogen generator on Unit 1 in 1999. Prior to the installation of the hydrogen generator, the Healy plant used 275 to 600 cylinders per year for makeup. The hydrogen was delivered by truck from Fairbanks 110 miles from the plant.
With a cost of $60 for a 200 cubic feet cylinder the plant was spending $18,000 to $36,000 per year for hydrogen. In addition, the manpower for unloading the delivery trucks and handling of the hydrogen cylinders onsite was costing the plant $27,000 per year. On average the plant spent $90 per 100 cubic feet of hydrogen.
According to Paul Morgan, construction manager, under normal operations the Hogen system is able to supply all of the plant’s hydrogen makeup requirements. However, if time permits during an outage, the system can supply sufficient hydrogen for purging and filling the generator as well. Nonetheless, the Healy plant still stores 10 cylinders of hydrogen for filling the generator after a shutdown or for emergency use.
Since being put into service the only maintenance that has been required on the hydrogen generator is regular cleaning of the filter. This takes only 15 minutes to complete, says Morgan. In addition, because there is sufficient hydrogen stored in the piping there is no need to use hydrogen from the cylinders when the filters are cleaned. With less hydrogen now being stored onsite, the risk from its storage has been drastically reduced, says Morgan.
Originally the hydrogen cooling system was monitored manually. However, the plant has since connected it to the plant’s distributed control system (DCS). The DCS monitors and trends H2, O2, CO2 and the humidity of the hydrogen. This now allows the operators a little more time to rectify a problem before it goes into an alarm condition, says Morgan.
Another plant owner that has installed a Hogen hydrogen generator is Minnesota Power and Light. Under a test program to determine the feasibility of generating hydrogen on-site the utility has installed a hydrogen generator on Unit 1 at their Boswell plant. Boswell has four coal-fired units with a total capacity of 1,000 MW. Unit 1, with a capacity of 70 MW, went into service in the late 1950s.
According to Brad Hill, operations superintendent for business Units 1 and 2, the major attribute of the hydrogen generator is it produces hydrogen with a purity of 99.9 percent. This is extremely pure and is almost unheard of when compared with purchasing it in bulk or in cylinders, Hill says.
Since being put into service on January 21, 2002, the hydrogen generator has been out of service for about 3 days for the replacement of a failed power supply. However, Hill says that if it had been required they could have returned it to service sooner. Other than this outage, the system has operated at 100 percent capacity.
Although Hill has not yet calculated any cost savings for the hydrogen generator, he does believe that it is able to produce the hydrogen at less cost than buying it in bulk. Hill is expecting a three-year payback for the unit.
Deionized water to operate the hydrogen generator is supplied by Unit 1. Figure 4 shows a cross section of a hydrogen generator and its electrode assembly. Although the hydrogen produced by the unit is supplied to the main header of Unit 1 it can be cross-connected into the headers of the other three units.
Generally the Boswell plant has sufficient hydrogen on-site but there have been times when they needed to purchase hydrogen in an emergency. With the installation of the hydrogen generator this has been almost eliminated, says Hill.
For the foreseeable future Boswell will continue to supplement the hydrogen from the Hogen systems, and purchasing hydrogen in bulk for use when filling and purging the electric generators. However, Hill believes that if they were to install another two hydrogen generators they could supply sufficient hydrogen for makeup during normal operation of the generator.
Generator On-line Monitoring
In addition to monitoring the cooling of generators there is still a need to monitor their operation. Generator condition monitoring systems are available to monitor core overheating. The circulating currents between the generator laminations cause overheating of the core and early detection can reduce the damage from thermal degradation.
High concentrations of thermal decomposition sub-micron particles are produced whenever any material in the generator is subjected to excessive temperatures. When these particles are present in the hydrogen they can be measured and monitored.
Environmental One’s generator monitoring system uses the generator fans to circulate the hydrogen cooling gas through an ionization chamber. The chamber consists of an ionizing section and an ion collecting section. In the collecting section some of the sub-micron particles attach to an electrode that in turn produces a current. After amplification the current is displayed on a meter and/or a recorder.
Should the system determine that there is an excessive amount of particles present in the hydrogen a microprocessor initiates an alarm. Simultaneously, a fixed amount of the hydrogen flow is automatically passed through a sampling system where the particles are removed for laboratory analysis. Analysis of the particles can determine the source of the particles.
According to Barry Reichman, electric systems engineer, Public Service Electric and Gas, his company has installed generator condition monitoring systems (core monitors) on all of their hydrogen-cooled generators. These systems continuously monitor the presence of sub-micron particles in the hydrogen gas.
If particles are detected, the system triggers an alarm, but does not trip the unit. At the same time, the system automatically samples the hydrogen. The presence of sub-micron particles is indicative of an insulation problem, says Reichman. However, the only way to remove the particles in the hydrogen is by purging the system.
Although in the early days there has been problems with oil mist getting into the instruments, and giving false alarms, the manufacturers have made modifications to minimize this problem. In Reichman’s opinion the generator monitoring systems they use are very reliable and he and his company are firm believers in on-line generator monitoring.
Optimizing generator cooling and utilizing on-line monitoring systems allow power plants to maximize productivity, reduce maintenance costs and extend the life of the generators. Adequately cooling and maintaining generators is critical if catastrophic failures and possible loss of life is to be avoided.
Fitting a Square Peg in a Round Hole
Thermal overloading of high voltage insulation in early 2002 destroyed the generator rotor at the Loy Yang coal-fired power plant in Australia. The original rotor’s stator windings were water-cooled and due to poor water treatment the hollow conductors eventually became plugged. This resulted in overheating of the insulation and the destruction of the stator winding and part of the stator core.
Faced with the prospect of an extended 12-14 month outage to repair the damaged generator the power plant decided to look at the possibility of finding a replacement generator. A contract was eventually awarded to Siemens Power Generation.
The challenge for Siemens was to come up with a replacement generator that could be adapted to the Loy Yang plant in regards to capacity, cabling and pipe connections. After looking at various options Siemens located a generator at the decommissioned Moorburg plant in Germany. Although the generator was shorter and heavier and not the same type as the original generator, Siemens still determined that it could be modified for the Loy Yang plant.
In order for the generator to fit into the existing foundation, the feet were modified. Similarly, to accommodate the difference in length in the axial direction, Seimens manufactured and installed an adaptor at the coupling joining the generator to the LP turbine. Because the replacement generator was hydrogen cooled, the stator winding cooling water supply was also removed.
Generator installation at Loy Yang proceeded smoothly, in part because the original generator seal oil and hydrogen systems were re-useable and required very little modification. According to Siemens, the unit returned to commercial operation within four months of the disaster, saving at least 10 months in downtime.