Transformer / Cable testing

[mikehughes8]

I am hoping that somebody in this forum is familiar with substation maintenance and testing particularly as it applies to cables and transformers. I have been struggling to determine what advantages AC Power Factor testing has over the DC Insulation resistance test. The insulation resistance test measures leakage current through a systems insulation and returns a test value usually in the MegaOhm range. The returned value is simply a funtion of the applied test voltage divided by current returned to the test set.The power factor test returns a dissapation factor which is determined by the ratio of resistive current (Ir) to capicitive charging current (Ic). A dissipation factor below 5% in most systems represents a acceptable value.
Whichever ever test I choose to apply I answer the same question. Is the insulation good or bad? So why is one test better than the other? There must be a reason why someone is willing to pay $40,000 for a power factor test set when they can get an Insulation Resistance test set for $5000 and achieve the same result.

Any clarification would be appreciated.

 

[zog]

Well first off, lets talk about cables, DC hipot testing as not allowed by NETA, IEE, or the ICEA for ervice aged MV cables, it has been proven to be a destructive test, mostly in XPLE insulation.

A DC test is very misleading, really it is no better than a go-no-go test, tell you if the cable is good or bad, if your test voltage causes dielectric breakdown, cable fails, the end.

Tan Delta (Similar to the Pf test you described) is a condition assessment test, it can find voids, water trees, moisture in insulation and give you a good evaluation of the condition of the cable (Or bushing, insulation, etc).

 

[mikehughes8]

I realize that the Hi pot test is destructive and I don't use it. Hipot testing involves voltages up to 3 times the rated voltage of the specimen under test. However, the DC insulation test (megger) usually involves voltages at or near the rated voltage of the specimen being tested. The destructive nature of the megger is probably overstated.
If I use the NETA standards as a guide and my test reading exceeds the recommended value then I can assume that my cable is serviceable. Over time my test readings may diminish. So the megger works well for trending. Once I get a reading less than the recommended value replacement of the cable may be the way to go.
The PF test will also return a value that I will compare with a recommended value. It either meets the standard or it doesn't. Therefore in my mind they are both go / no go tests.
The only advantage I see with the PF test is the fact that it is non-destructive. Whether or not it detects air voids or moisture is irrelevant to me. I have no way of knowing what it actually detected. I only care about the quantitative measurement the test set returns.

 

[zog]

I realize that the Hi pot test is destructive and I don't use it. Hipot testing involves voltages up to 3 times the rated voltage of the specimen under test. However, the DC insulation test (megger) usually involves voltages at or near the rated voltage of the specimen being tested. The destructive nature of the megger is probably overstated.
If I use the NETA standards as a guide and my test reading exceeds the recommended value then I can assume that my cable is serviceable. Over time my test readings may diminish. So the megger works well for trending. Once I get a reading less than the recommended value replacement of the cable may be the way to go.
The PF test will also return a value that I will compare with a recommended value. It either meets the standard or it doesn't. Therefore in my mind they are both go / no go tests.
The only advantage I see with the PF test is the fact that it is non-destructive. Whether or not it detects air voids or moisture is irrelevant to me. I have no way of knowing what it actually detected. I only care about the quantitative measurement the test set returns.

Now you are comparing apples to oranges, a megger test is nnot an overpotential test, it simply measures the Insulation Resistance value, not the dielectric strength of the insulating system. In fact there are 2 types of cable tests as defined by IEEE Megger is type 1 , Tan Delta (What you call Pf) is type 2, so is PD and VLF

a) Type 1 Field Tests are intended to detect defects in the insulation of a cable system in order to improve the service reliability after the defective part is removed and appropriate repairs are performed. These tests are usually achieved by application of moderately increased voltages across the
insulation for a prescribed duration. Such tests may be categorized as “pass/fail” or “go/no go.”
b) Type 2 Field Tests are intended to provide indications that the insulation system has deteriorated, hence, are “diagnostic.” Some of these tests will show the overall condition of a cable system, and others will indicate the locations of discrete defects that may become the sites of future service
failures. Both varieties of such tests are usually performed by means of moderately increased voltages applied for relatively short duration, or by means of low voltages.

That is from IEEE 400, the very 1st paragraph of the standard, you may want to read it.

 

[mikehughes8]

I appreciate that explanation. I don't have acess to the IEEE right now but I will read it once I get back to work. I knew there was an important difference and was just trying to get to the bottom of it.