Hydrogen system safety

Matt McMasters, Duke Energy

McMasters’ presentation focused on safety concerns associated with use of hydrogen to cool large generators. Hydrogen cooling has been used since 1938 and there have been some dramatic and costly fire events over the years—a few involving personnel injury and death. These events generally have been associated with a failure to use proper caution and procedures during operation and maintenance.

The speaker began by pointing out that a 12-pack of standard hydrogen gas cylinders is equivalent to about 420 lb of TNT and a tube trailer is equivalent to about 5600 lb of TNT. Hydrogen is odorless, explosive in the range of about 4% to 74% mixture with air, auto ignites at about 1050F, ignitable by static electricity, and burns with a nearly invisible flame.

These characteristics mean that safety considerations are top priority and would include maintaining flammable/combustible materials at least 25 ft from hydrogen systems, adequate ventilation, and extreme caution in attempting to extinguish.

A fatality occurred at a Duke plant in 2011 and, in response, the company focused on several corrective actions—including confined-space program revisions, standardization of confined- space monitors, improving procedures for purging generators and auxiliary systems (that is, dryers, core monitors, purity analyzers, etc), developing procedures for handling incoming hydrogen supply, standardization of painting and labeling of hydrogen systems, purchasing dedicated hand-held hydrogen leak detectors for system purging/leak checking, and use of intrinsically safe tooling around hydrogen systems.

In performing this review, the following industry standards were referenced:

• OSHA Standard 1910.103, Hydrogen.

• ASME B31.12, Hydrogen piping and pipelines.

• ASME B31.1, Power piping (for hydrogen systems designed and installed before the adoption of B31.12).

• NFPA 2, Hydrogen technologies code.

• NFPA 55, Compressed gases and cryogenic fluids code.

• CGA G-5.5, Hydrogen vent systems.

• OEM design specifications. Access presentation

Protective relaying: Stator and rotor

Rogerio Scharlach, Schweitzer Engineering Laboratories

The ground protection relay systems for stator and field windings recommended in the current version of IEEE Standards C37.101 and 102 are both deficient. Example: For stators, relay 59GN, will not detect grounds in the bottom 5% to 10% of the stator winding.

For fields, the standards say that a single ground is a concern because of the possible exposure to a dangerous failure in the event of a simultaneous second ground. But a single ground, if caused by a broken conductor, can be extremely dangerous itself. Scharlach addressed this misinformation and described relay systems that eliminate the associated exposure to extreme generator damage.

He began his presentation showing the common stator ground-detection relay system in Fig 1, where relay 59GN is connected in parallel to relay RG. This relay system leaves undetected grounds in the low-voltage end of the winding. Examples are shown in Fig 2. Four such recent failures in a two-year period involved repair costs of close to $500 million. Full winding protection can be obtained by using an injection relay, 64S. But the more common update involves leaving 59GN in place and adding a 64S relay (Fig 3).

Field-winding ground detection systems typically use AC or DC voltage injection. These systems will detect winding grounds reliably provided they are installed correctly—specifically, connected to the correct polarity.

Broken conductor or other types of single grounds are somewhat common—unfortunately. A striking example of a single ground failure is shown in Fig 4. Left-hand photo is of a single ground resulting from the shorting out the two largest coils on one pole. The companion photo shows the resulting burn damage to the retaining-ring forging caused by the flow of current bypassing the coils. (This ring was believed to be close to catastrophic failure.)

Fig 5 shows damage to the forging at one ground point of a double ground. In this case, the damage at each ground point was minor and easily repaired. Given the danger associated with the misinformation in these two IEEE standards, both are currently under revision. Access presentation

Generator condition monitor

Steve Kilmartin, EnvironmentOne

The generator condition monitor (GCM) was introduced in the 1960s to detect hot-spots in generators caused by various malfunctions. The device is extremely sensitive and will detect very minor locations, as shown in the left-hand and center photos in Fig 6. In addition, it detects the onset of major damage (Fig 6, right).

Because of the inherent sensitivity of the instrument, early versions of the GCM were subject to false positive failure alarms, thereby gaining a reputation for unreliability. Versions since the late 1980s have eliminated these problems and are considered quite reliable.

Two versions of the GCM are manufactured. The GCM-X version is for hydrogen-cooled generators and operates with an ion chamber for signal generation. The GCM-A is for air-cooled generators and uses a cloud chamber for signal generation. The operating principles of both were explained by Kilmartin using illustrations.

In parallel with the GCM, thermal particulate paints are available in several classes. By applying the paints to generator components at critical locations, a developing hot spot can be identified quickly upon receipt of a GCM alarm. Access presentation

Modern stator-core design/robotic core stacking

Dave Charlton, PE, and Justin England, Siemens Energy

Charlton and England described current approaches to stator core design and ventilation arrangements. Robotic equipment is used to stack the core laminations in sections and the speakers showed a descriptive video of this complicated but highly efficient process. These sections are then placed within an inner frame to make up the stator core assembly.

Photos then were shown of Siemens’ core isolation system which has proven effective.

Global vacuum pressure impregnation (GVPI) is used for the stator windings on all but the largest units. This process greatly simplifies winding assembly; however, GVPI has gained a reputation for lack of consistent reliability. In answer to a question, the speakers said Siemens has used the GVPI process on these sizes of generators since the early 1980s and have had no significant service issues relating to the process.

Among these hundreds of generators there have been rare GVPI windings requiring replacement because of failures unrelated to GVPI. Winding replacement of a GVPI stator winding is difficult, costly, and calendar-time consuming. But overall, Siemens believes the advantages override the potential disadvantages. Access presentation

Improvements in modern generator design

Tony Arrao and John Yagielski, GE

The speakers reviewed the differences in design philosophy of GE’s new modular generator compared to its historic design approach—for example, standard centerline height, coolers in the fan inlet bay, no high-voltage bushing box. A standard cross section is used today with rating increase accomplished by increasing the core length. Core ventilation is radially outward for the entire core.

Long-time proven components and systems are used throughout the new design—for example, slot wedging system, core spring isolation system, Micapal stator-bar insulation, TetraLoc™ stator end-winding support system.

On the rotor, the inboard radial stud is moved outward beyond the retaining ring to allow ready access for maintenance. Tilting-pad bearings are standard.

The company’s stator leak monitoring system is standard on water-cooled windings. Access presentation