General Troubleshooting Techniques

When the first EMSA scans are taken, look at the frequency scan for humps or spikes of higher-level frequencies. Record the higher readings for future comparison.

For the permanently installed units, the Power Spectrum of the frequency bands are captured and trended in InsightCM. The overall EMSA spectrum is saved on a company/IT determined frequency for comparison.

For periodic monitoring, capture the peak values and frequencies for comparison as more data is captured for trending purposes.

Currently, neither method automatically captures the demodulated time-domain waveforms. However, the demodulated time-domain waveforms should be captured for all the areas of interest to compare with similar data in the future.

In the demodulated data from the areas of interest, sometimes the same patterns are seen at different frequencies. However, there may be some changes to the patterns that may be of interest:

These indications may help indicate to which frequencies the defect is mostly attributed.

NI-9770 card trends and periodic measurements have different levels of concern. The NI-9770 card uses the Power Spectrum, which is more sensitive to developing problems and can more easily detect issues that have not developed signals stronger than the local radio stations. The periodic measurement process uses the peak scan voltage.

The NI-9770 card has the following levels of concern for the areas of interest:

Recently, a generator was above 10,000 uV for about five years. A step change in level from 13,000 uV to 16,000 uV (Power Spectrum: 30 MHz – 100 MHz) was noted approximately three months before the unit tripped on stator ground. The issue was found in the Isolated Phase Bus: some cracked insulators and significant dirt contamination on other insulators.

For the periodic Instrument (Agilent E-7402), peak values of the spectrum are:

If very high values occur when the unit is at high loads, it may be worthwhile to take another scan at low load values. Signal strengths decreasing significantly at the low load values indicates loose components in the monitored equipment. Loose wedge systems or loose end winding ties will be moving more at high loads and cause the discharge signals to increase.

In other cases, the unit is tight and does not show different EMI signatures with load.

Note the area of frequency to determine the location of the loose component.

One of the strengths of EMI Monitoring is the ability to trend the data over time. The signal strength usually increases long before the defect causes the plant to trip. However, this is not always the case, especially if the defect deteriorates rapidly or is caused by some rapid phenomena such as water or rain intrusion.

Comparison of the spectrum over time or trending the Power Spectrum values over time usually gives adequate time to develop an inspection and repair plan.

It is also good to do before and after maintenance comparisons.

Example 1: The demodulated waveforms in the lower frequency band showed that the excitation system might have a loose connection. Although these values were still low, the unit had a Not in Demand period, and the new air-cooled power rectifiers were examined. Loose hardware was found on the disconnect switch associated with rectifiers. The snapshot below shows the before and after maintenance scans.

Example 2: When the motor was sent out for cleaning, it was found to have salt contamination. The snapshot below shows the before and after maintenance scans.

Example 3: Four major water clip leaks were found and repaired. The unit had a wet bar that was dried after the repairs were completed.

InsightCM has the capability of trending features extracted from the EMI overall scan.

For Generators, the trends for the Power Spectrum values of the overall scan and four sub-bands are used.

The sub-bands are compared to the total power band to help validate the defect regions of interest. In the snapshot below, the upper frequency band contributed the most to the total power in band values and the defects were found in the Isolated Phase Bus area.

For Transformers, only the Power Spectrum for the total power in band (full scan) is used. There is not data or case studies to make any conclusions about defect location based upon frequency range yet. However, research is currently being performed to try developing a similar categorization as the generator area.

For Motors, only scans from 30 kHz to 30 MHz (sometimes only 10 MHz) are used. The upper frequencies have never detected any motor issues, which makes sense because motors do not have an Isolated Phase bus structure. For induction motors, the lower frequency band does not have much relevance either because they do not have an excitation system. The scans may be appropriate for Synchronous motors. Although there is not much data associated with synchronous motors, they are similar to generators, so those areas are still monitored. The scan below shows an induction motor with severely contaminated motor windings.

Comparing sister units can be useful in troubleshooting, but it is essential to make sure that the comparisons are valid.

Both the feature values and the spectrum signatures can be compared.

However, many other things can affect the signatures when comparing like machines.

To help localize EMI sources, use an EMI sniffer or ultrasonic acoustic monitor.

An EMI sniffer is a broad band radio with two antenna and three different ranges. It has a gain dial that allows desensitization enough to aid location of the source. It is especially useful for bearing electrolysis issues, brush rigging issues, and shaft ground connection issues.

The Acoustic ear, used by PdM technicians for many years for bearing issues, can also be used for PD detection and for localizing the source of EMI signatures. It is especially useful for connection issues on the high side bushing of large GSU transformers.

Transformers have multiple winding arrangements available: delta-delta, delta-WYE, WYE-delta, two winding, three winding, etc.).

For the most part, the ground cable coming from one of the WYE terminations is used, which can lead to more noise from the system if the WYE is from the switchyard side of the unit, like most Generator Step-up Transformers (GSU). Comparison between a switchyard side WYE location with a plant side WYE location is be very difficult and not recommended.

If trying to compare like units care, be sure CT locations are the same.

Frame and case grounds are used on many Combustion Turbine units, motors, and transformers. They use the shielding effects of the frames and the casing structure to absorb the EMSA signal and port it through the ground conductor that the HFCT is around.

Because the industry does not have a standard for grounding a unit, there are many different grounding configurations. Even sister units at the same facility can have different grounding configurations.

When comparing like units, make the location of the HFCT as similar as possible (same end (exciter vs. turbine), etc.). In some cases, multiple components (generator output bus and generator frame) are tied to the same ground conductor.