Wind Turbine
HealthIndicators
Cumulative Impulse
Cumulative Impulse is a set of measurements that detectand trend the passage of debris particles through theplanetary stage of a wind turbine gearbox.
The planetary stage of a wind turbine gearbox has
traditionally been a difficult section to monitor using
conventional vibration-based techniques. The sun gear,planet gears, and planet bearings have to transmit defectsignals through multiple parts and connections to arriveat the outer fixed ring gear where the accelerometer ismounted. This relatively complex signal path can significantly
attenuate vibration signals. In addition, during operation,
the planets move about the sun, constantly changing
the location of the teeth engagement and changing thevibration transmission path. This motion tends to change(modulate) the amplitude of the defect signals.
When damage begins to occur in the planetary stage,
whether on the ring gear, the planet gears, the planet
bearings, or the sun gear, metallic debris will be shed fromthese parts. Usually in wind turbines, the planetary stage
components are arranged in a vertical plane, so debris shedfrom one location will tend to fall down into other parts of theplanetary stage mesh.

The debris particle causes a vibration impulse on the gearbox
casing, followed by a decaying vibration at the gearbox
casing natural frequency (Figure 1). When a debris particle
falls into the mesh, it can become lodged in the ring, in thesun, or in one of the planets. When a planet moves aroundthe ring or the sun rotates, eventually the gear will move toa position where the debris particle gets trapped between
meshing teeth. When this happens, if the debris particle islarge enough, it will tend to force apart the two mating gears.
This produces an impulse that is transmitted through theentire planetary gear set to the outer ring gear The ring geartransmits the impulse directly to the outer case of the gearbox
where the planetary stage accelerometer is mounted. The
result is an impulse of the gearbox casing followed by adecaying vibration at the structural natural frequency.
This characteristic impulse-response signal is detectedby the 3701 monitor and is reported as three variables. The
monitor studies the planetary accelerometer waveform inreal time, looking for impulse response events. When it finds
one, it checks to see if it is above a detection threshold; if it
is, it increments a counter, measures the amplitude of theimpulse, and calculates the number of impulse events thathave occurred in the past hour.

PlanetRing

Sun

PlanetPlanetFigure 1. Planetary
gear arrangement
showing debris thatcauses a detectable
impulse vibration.

The monitor reports this information in three variables:
Cumulative Impulse Count, Cumulative Impulse Energy, andCumulative Impulse Rate.
• Cumulative Impulse Count is the most fundamental ofthe measurements. Each time a new impulse is detected,
the count is increased by one. Trending this measure overtime reveals how many particles above the detection
threshold have appeared in the planetary stage mesh. On
this trend line, an area of steeper slope indicates that a
relatively large number of debris particles were detected
in a relatively short time. Shallow slopes indicate periodsof relatively low debris particle detection. As the planetarystage progresses toward failure, the number of debrisparticle detections should increase more rapidly.
• Cumulative Impulse Energy is a measure of both thenumber and the amplitude of the accelerometer impulse
signals. The signal amplitude is related to the size ofthe particle that has been trapped in the mesh. Larger
particles will produce larger impulse amplitudes. These
amplitudes are accumulated to produce Cumulative
Impulse Energy. When this measure is trended over time,
the slope is a measure of both the number and size ofparticles that have appeared in the planetary mesh.
This gives us an idea of how bad the damage is in theplanetary stage. Like the previous variable, a steep slope
indicates a period of frequent and/or large particles, whilea shallow slope indicates a period of relatively infrequent
and/or small debris particles.

Copyright 2020 Baker Hughes Company. All rights reserved.
BHCS19503 (02/2020)• Cumulative Impulse Rate measures the number ofimpulse events that have occurred over the past hour. It
is a plot of the slope of the Cumulative Impulse Count. A
high Cumulative Impulse Rate indicates that a relativelylarge number of debris particles have been detectedin the mesh in the past hour. A low rate (or even zerorate) indicates a low (or zero) rate of particle detection.
Cumulative Impulse Rate is well suited for alarming sothat operators can be alerted to sudden changes in debris
particle detection rates.
To reduce the chance of false alarms, the CumulativeImpulse system only detects debris particles that produce a
signal above a minimum threshold. Thus, it is possible that
relatively small particles will not be counted. In the earlieststages of deterioration, the planetary stage may shedparticles too small to be detected but that may be detectedby other means, for example, inductive oil debris detectors
or filter inspection. As damage progresses, larger particles
will be generated, the Cumulative Impulse system detection
threshold will be exceeded, and the Cumulative Impulse
variables will begin to show changes.

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