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Grounding brush discharge monitoring

In recognition of the possibility of static charge build up in condensing steam turbines, API 612 (2005) specifies that grounding brushes be installed. The electrical flow to ground through these brushes  be monitored and useful information can be extracted.

This article carries excerpts from the paper, “Babbitted bearing health assessment” by John K Whalen of John Crane, Thomas D Hess of Chestnut Run, Jim Allen of Nova Chemicals and Jack Craighton of Schneider Electric.

Grounding brushes take current from the rotor to ground so that a charge does not build up on the rotor to the point where it discharges to ground though the best path possible – which is usually the closest point between the rotor and stator which is usually (hopefully) the point of minimum film thickness in a bearing. Typically this point of minimum film thickness is found in the active thrust bearing (as will be shown later). Shaft grounding brushes serve two purposes. The brushes are able to transmit modest amounts of stray current to ground for prevention of arc damage through parts of the machine (especially the bearings). The brushes also permit measurement of the shaft voltage and current, which allows assessment of the electromagnetic condition of the machine.

There are two types of electrical currents in rotating machines: electrostatic and electromagnetic. Electrostatic currents are primarily generated by impinging particles or droplet atomization (such as in wet stages of steam turbines) and can generally be handled with one grounding brush per shaft. Electrostatic currents usually cause minor damage, such as frosting of bearing babbitt, and the progress of damage is relatively slow. Electromagnetic currents are usually due to residual magnetism and/or stray currents created by electrical machines, such as generators, motors, and exciters. Events such as machinery rubs, improper welding, or lightning strikes can induce residual magnetism in the machine parts and cause the rotating machine to become a generator.

Electromagnetic currents can be extremely destructive. Severe damage can occur rapidly because of high current density. Damage such as burned areas and welding of components are possible. Monitoring to determine shaft current level and changes is performed using a shaft voltage current monitor (VCM). The VCM measures voltage and current through the grounding brushes. Ideally, the output of the VCM should be connected to a recording device or plant data historian so that trends of the voltage and current can be used to detect changes in shaft electrical properties.

If readings increase slowly, it could be an indication of deterioration of the electrical machinery (motor, generator, or exciter) or possible progressive self-magnetization of the machine. Sudden increases in current may indicate that self-magnetization has occurred, possibly due to the reasons previously mentioned.

If readings from the VCM decrease to zero, the grounding brush has most likely lost contact due to wear, or some other part of the brush circuit has opened. More detailed analysis of the shaft currents to determine cause of excessive shaft currents can be performed using an oscilloscope.



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