The Shaft Ground Assembly (SGA) contains two ropes, depicted in the figure below. Both ropes provide grounding. The grounding rope (A) acts as the primary direct connection between the generator shaft and the unit case, and the metering rope (B) connects the shaft to the unit case through a small resistor which facilitates measuring shaft voltage while also providing a secondary ground connection. The metering rope also provides temporary primary grounding when the grounding rope is being replaced or is otherwise disconnected from the shaft temporarily.
The Assurance Monitoring System provides easy access to test points for diagnostics and preventative maintenance. Careful monitoring can reveal indications that are possible fault conditions.
The Assurance Monitoring System is equipped with an OLED display that shows live shaft voltage and ground current measurements as well as rope status information. This information is split into three different screens as outlined in the following table. There is a push button adjacent to the display that can be pressed to cycle through the various screens.
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Screen 1 |
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Screen 2 |
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Screen 3 |
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ELECTRICAL DANGER
Only qualified electrical personnel should take measurements at the remote test points. For any of the test points on the faceplate, use a hand-held voltmeter, oscilloscope, clamp-on ammeter, or other appropriate testing device. Always follow proper electrical safety procedures.
Test points in the monitoring system connect directly to the following sensor locations:
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“Shaft Voltage” is connected directly to the SGA’s meter rope on the shaft.
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The negative test points are connected to the unit case grounding location and provide the ground reference for test point measurements.
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Each wear indicator test point is connected to a resistance-based circuit that will detect when the rope is 50% worn through (or higher).
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Ground current + and – are connected to the output of the ground current sensor which the main grounding wire connects to.
Note: In order to measure ground current and rope status at the test points, monitoring must be powered on. All shaft voltage readings are direct connections and can be taken regardless of whether the monitoring system is powered on or off.
To measure current flowing to ground from the shaft, place your meter/oscilloscope probes on the ground current test points. Refer to the following table to convert the resulting voltage reading to the associated ground-current value.
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Meter Reading (V) |
Shaft-to-Ground Current (A) (–100 mV/Amp scale) |
|---|---|
|
3.15 |
-15 |
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2.65 |
-10 |
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2.15 |
-5 |
|
1.65 |
0 |
|
1.15 |
5 |
|
0.65 |
10 |
|
0.15 |
15 |
To read the shaft voltage, place your meter/oscilloscope probes on the shaft voltage test points. 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.
Cutsforth Shaft Grounding Assembly ropes come equipped with an insulated conductor threaded into the center of the grounding and metering ropes. If the rope has not worn out to its center point, the conductor insulation will be intact and there will be no electrical contact between the wear indicator conductor and the outer portion of the ground rope. When the conductor insulation has worn through, the Assurance Monitoring System will indicate that a rope fault has occurred. A rope fault can also be verified at the test points located on the faceplate of the monitoring system. To check for rope faults, place your meter probes on the wear indicator test points for the rope in question, and use the following guide.
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Rope Fault Condition |
Test Point Reading |
|---|---|
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No Rope Fault |
5 V DC ± 1 V |
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Rope Fault |
0 V DC ± 1 V |
Each wear indicator test point is also equipped with its own LED that will flash when a rope fault is present or has occurred within the past 10 seconds. When no rope fault is present, the LED will be illuminated with a solid (i.e., not flashing) light.
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:
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If 500 Ohms or greater resistance is measured, the rope does not require replacement.
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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 the normal average and peak voltage and current levels for each unit. It is also recommended to periodically view the waveforms from the monitoring system’s test points 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 either something within the generator may require attention or the grounding system needs maintenance.
The conditions listed in this section may be fault indications. This is not a comprehensive list; 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 an unusual reduction in grounding current and shaft voltage occurs, it may be that a significant event had taken place. Has something else contacted the shaft or has a bearing lost insulation, allowing alternate paths to ground? Is the rope-to-shaft contact of sufficient integrity to allow current to flow freely when voltage is present? 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, it is important to capture what normal readings are for the generator at different loads. Knowing the difference between the generator's grounding system's readings at lowest and highest generator outputs helps when it’s time to fine tune the monitoring system's thresholds to avoid triggering snapshots and alarms at levels that are potentially just normal periods of greater demand. If taking the above into consideration does not resolve the higher levels, share the waveforms you are seeing with Cutsforth to compare them against our library of waveforms; they may indicate other 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 there 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 waveforms show, 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 to whether the condition is getting worse and at what load the condition first appears, comparing now vs. the point when the condition was first identified.