I am seeing lots and lots of reports by testing companies which mention limits and declare asset has no issue if limit is not violated say by 0.1 or 0.001. Its a very broad generalization and lack depth guys. Be careful. Limits from maintenance standards do help but it takes much more than that to evaluate what’s going on in your asset. I will describe this in below examples:

  1. I saw one report for transformer bushing testing. Bushing tan delta results was 0.43% and report read bushing tan delta is within limit of IEEE and NETA mentioned limits. Well, true. Case closed. Power Plant will test these bushings after 3 to 5 years. And they have a declaration certificate from a Multinational Service provider, “All is well”. It so happens that the SAT results which was never asked by this vendor had bushing tan delta of 0.26%. Now thats a huge change. Bushing needs close monitoring in 1 to 2 years perhaps as tan delta is close to being doubled. Risk level is high. Thermography, PD, DGA in that order is required to evaluate further. So this require more than limits to be looked at. Trending. We have seen 500 KV bushing failures start country wide cascaded tripping starting from Guddu transformer. Reason: bushing results were not evaluated carefully.
  2. In another plant maintenance report on motor, vibration was noted as 4.3 mm/s and mentioned within limit. Yes, it is within limit of ISO standard for that practical motor application and alarm was set at 5 mm/s. But if a closer look at data has been taken, it would have been obvious that vibration levels have increased from 2.6 mm/s to 4.3 mm/s in less than a quarter. A fault is developing and was missed in maintenance report and FFT was not carried out.
  3. A DGA report says all gas values are within limits of IEC and hence everything is ok. Did not take into account the fact that some of the gases were not even present in last DGA results or were much lower in values. Something is happening inside transformer and now it will appear in next years report when the values will be well past the IEC limits. You have to find and ascertain rate of generation of gases to make sure there is no incipient fault in your transformer.

So if you are doing condition monitoring and/or diagnostics, ask for previous results, trend the values. Ask for maintenance history as limits change if asset is already repaired once, or its key component was replaced recently. e.g., dehydration and degassing will not help in comparing with older results. So know what you are doing, know what client has been doing. Ask for data, convert it into useful information. ACTION PLAN with clear cut indication on what to do is what client want.

Using fancy equipment, generating automated results and preparing Colorful reports with IEC/IEEE standards does not matter If client does not know where to look at the fault and how to make the equipment run reliably.

Published By:
Engr Irfan Akhtar, PE, ME, BE