Page 13 - GuardII+ Series 4208 Platform SM User Manual

GuardII+ SM Card Operation

4. GuardII+ SM Card Operation

During the normal operation of rotating machines, AC and DC voltages can be induced in
the shaft. If voltages are high enough, shaft currents can reach levels capable of causing
bearing failures. The main sources of shaft voltages are:
• Potential applied to the shaft as result of rotor winding ground fault or spikes caused
by excitation system (spikes up to 200 V). The introduction of static exciters and more
recently variable frequency inverter driven motors, has introduced a new source of
shaft voltages, not present in older, more traditional designs. Due to the relatively high
frequency of the excitation system spikes and their fast rise time, the normal methods
such as insulating bearing pedestals, and thin oil film between bearings and shaft, are
often not effective in preventing damage. Bearing insulation, in the presence of fast
rise time voltage pulses, acts as a capacitance of very low impedance and leads to
current flow to ground, resulting in possible damage to the bearings and the shaft.
Tests conducted on a 150 MW generator at two different loads indicated that at lower
loads excitation system voltage spikes are of much higher amplitudes, compared to
those at full load. Although shaft voltages created by static exciters may not be so
harmful with generator operating at full load, operation of conventional generators at
lower loads is today more frequent than before, due to introduction of non-controllable
energy sources, such as solar panels and wind turbines.

• Asymmetry of magnetic fields caused by design, manufacturing details, rotor winding
shorted turns or by large stator core faults (which can induce up to 150 Vac). Magnetic
asymmetries can be caused by design, less than perfect manufacturing or because of
faults during operation of rotating machines. Different measures are taken at each
manufacturing and assembly step to minimize potential for creation of shaft voltages,
such as staggering of stator laminations, avoiding core splits, correct rotor position and
alignment, prevention of asymmetrical loads and monitoring of rotor and stator faults.
The asymmetrical magnetic field can induce a voltage on the rotor, that in turn may
break down the bearing oil film or lead to high currents through shaft grounding
brushes or bearing pedestal insulation.
• Flux generated by magnetized shaft, compressor, turbine or generator parts. Given
that most of the parts surrounding the machine are made of ferromagnetic materials,
undesirable permanent magnetization is possible. This could lead to creation of an
axial flux in the shaft (due to the large cross section conductor) that will induce very
large shaft currents. Care should be taken during any welding work in vicinity of
rotating parts, to avoid unintentional magnetization.

• Electrostatic effects caused by charged turbine steam or lubricants. Electrostatic
charges in low pressure turbines are generated mostly by the friction between blades
of the turbine and wet steam. The shaft potential can be raised as high as 130 V and
in absence of dedicated grounding, the only limiting factor preventing even higher
voltages is discharge because of oil film breakdown, leading to pitting of the bearings
and shaft.

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