Shaft Ground Monitoring (SGM) Troubleshooting

Please note, the ground current test points are connected to the output of the current sensor located at the grounding location so the monitoring system must be powered on to take measurements at the ground current test points.  

To measure current flowing to ground from the shaft, place the positive and negative probes of a scope or hand-held meter on the “Current +” and “Current –“ test points, respectively. Refer to the following table to convert the resulting voltage reading to the associated ground current value.

To read the shaft voltage, place the positive probe of the hand-held device on the Shaft Voltage or Exciter Shaft Voltage test point, and place the negative probe on the Signal Ground test point. The resulting voltage or waveform read at this point represents the shaft voltage at the associated metering rope location. The Shaft Voltage test points do not require power, so valid measurements can be taken even if the monitoring system is powered off.

The wear indicator test points are connected to a powered, resistance-based wear indicator circuit, so the monitoring system must be powered on to take measurements at the wear indicator test points.

To take measurements at the wear indicator test points, place the positive probe of a scope or hand-held meter on the desired rope wear indicator test point, and place the negative probe on the “Signal Ground” test point. Refer to the following table for interpretation of measurements:

Rope Fault Troubleshooting Recommendations

Using a voltmeter set to measure resistance, measure between the wear indicator contact and the outer rope contact on the underside of the rope grip to confirm whether or not the rope needs replacement. See the image below:

• If 500 Ohms or greater resistance is measured, the rope does not require replacement.

• If less than 500 Ohms is measured, the rope requires replacement, even if it may not look like it.

If the above recommendation does not resolve the issue, contact Cutsforth Support.

Every generator has its own distinct set of normal operating conditions. Normal conditions on one generator may be alarming on another, even if the two units are of the same type. Plant personnel responsible for monitoring the systems should be familiar with normal average and peak levels for each unit.

It is also advantageous to periodically view the waveforms on the Generator Field Monitoring System for each generator and watch for signatures that are markedly different from what has been viewed in the past.

A change in a unit’s waveform output can be a powerful indicator that something requires attention within the generator or the grounding system.  

The conditions listed in this topic may be fault indications. This is not a comprehensive list, but rather it is intended to draw attention to scenarios that can warrant further investigation.

If a significant step down in grounding current takes place in conjunction with an increase in shaft voltage this could indicate a potential loss of grounding and should be investigated immediately. Inspect the grounding assembly, grounding rope, and shaft surface for potential deterioration of the grounding rope-to-shaft connection.

If you notice an unusual step down in grounding current and shaft voltage occurs, it may be a significant event:

When grounding current and voltage are markedly lower than what is normally seen on the generator and no correlating operating condition exists to explain it, the grounding hardware should be evaluated to ensure that the generator shaft is grounded.

Baseline measurements are crucial for identifying events of concern. Baseline readings should be known for generator online status, turning gear status, offline status, and at differing load levels for comparisons when questions arise.

Typically, shaft voltages and grounding currents on the generator increase or decrease in amplitude along with generator output levels and other operating conditions. Soon after installing, you should capture what normal readings are for your generator at different loads. Knowing the difference between your generator's grounding system's readings at lowest and highest generator outputs helps you to fine tune your monitoring system's thresholds so that you are not triggering snapshots and alarms at levels that are potentially just normal periods of greater demand.

"If shaft cleaning does not resolve the higher levels while taking the above into consideration, share the waveforms you are seeing with Cutsforth so we can compare against our library of waveforms which may indicate other possible problems occurring in the generator that are being detected by the grounding monitoring waveforms.

Trending values on the generator can be a powerful diagnostic tool. If your challenge is poor rope-to-shaft contact tracking, the trend will help to know if steps at remediation are effective or not before alarming takes place. If your waveforms are showing, for example, that there are shorts in the stator winding insulation, trending the amplitude and frequency of the problem signature over time and at various unit loads can help give insight as to whether the condition is getting worse and at what load does the condition first appear now vs. time past when the condition was first identified.

If you have questions concerning the integrity of shaft voltage or current measurements:

If you have concerns:

Cutsforth can then help you connect with professionals experienced in shaft diagnostic and root cause analysis.