This article is based on a 2005 technical paper written by Clyde V. Maughan, P.E., of Maughan Engineering Consultants titled: “Carbon-brush collector maintenance on turbine-generators.”
Carbon-brush collectors have historically been an operations/maintenance concern on steam turbine-generators. Although a relatively a small component of the generator, it requires routine maintenance of brushes, brush holders and collector rings to provide reliable operation of the generator. Without this maintenance, serious service problems can occur, the worst of which are forced outages and collector/rotor shaft damage due to flashovers. While the collector system inspection and maintenance effort is relatively minor, it is sometimes overlooked, done inadequately or is difficult to perform on older brush riggings with non-removable brush holders.
It is now possible to change brushes installed in single holders on live equipment, without service interruption, using a cartridge style plug-in brush holder. This type of brush holder allows quick and easy change-out
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This article will discuss collector performance and root causes of collector ring and brush holder rigging performance issues, such as collector ring marking and brush sparking, both which can lead to forced outages. It will provide guidance on identifying and correcting collector operating conditions before serious problems develop. In addition, information is included on the retrofit of fixed brush holders with removable cartridge style, single pocket, plug-in brush holders.
Carbon-brush collectors transfer a controlled amount of DC current from a static exciter to a rotating generator field. It has successfully done so for more than 100 years, with many improvements in configuration and carbon brush/collector ring materials during that time. However, carbon-brush collectors do require the ongoing attention of operations and maintenance personnel.
This work is of a relatively minor nature, but unfortunately, the collector operates at high energy voltages, typically from 100-700 volts DC, in a very noisy environment and in proximity to a rotating shaft. This is why there are significant safety concerns for operations and maintenance personnel to perform the needed inspections and on-line service of the brush holders, brushes and collector rings. In addition, older non-removable and/or helical-coil brush holders make the required correct maintenance very difficult. As a result, collectors are sometimes overlooked, and failures occur. These failures can be severe, and the resulting forced outages or generator collector/shaft damage might be very costly and lengthy in time to repair.
Carbon-brush collectors will give high reliability service, provided recommendations of the Original Equipment Manufacturers (OEMs) and brush manufacturers are followed. This article will summarize general practices, operating and maintenance procedures for collectors. Information provided by the generator OEM should be the primary reference source of information for operations and maintenance personnel, since they detail specific maintenance recommendations for the collector system.
Several types of multi-brush magazine brush holders have been developed that improve the ease and safety of performing the brush maintenance mentioned above. But single brush holders have not advanced in the same way until recently, with the introduction of single magazine-style brush holders, such as in this article.
Basic principles of collector operation
In the power plant, generator performance is monitored by many devices. Even with state-of-the-art instrumentation, several of the more important generator deterioration mechanisms are not monitored at all. The collector falls in the latter category. Many sites monitor collector cooling air temperature increase, but the most important collector performance parameters are not monitored by control-room instrumentation. As a result, the condition of the collector must be monitored by looking directly at the collector brushholders and brushes themselves. This unique situation is often cited as a primary reason that collectors are sometimes neglected.
