No, but for certain types of insulation systems, some do consider it as a type test only and not a routine insulation test. There are some schools of thought that consider BIL testing to age the insulation to a certain degree.
Some 50-60 years back, transformer lightning impulse testing was considered as a destructive test or design test as the failure detection techniques were not reliable enough. Many times transformer declared as passed impulse test, failed in service soon after commissioning. When I entered industry 45 years back, many of the transformer specifications from utilities had a clause: impulse test shall not be done on the transformers under contract, but a impulse type test certificate shall be furnished for test done on a unit of similar design, supplied to some one else! After the introduction of neutral current monitoring during testing ( late 1940’s) impulse test failure detection became more reliable. After passing through Polaroid camera era , today impulse wave form recording and comparisons have gone completely digital with appropriate soft ware. Another major advance was in the computation and checking of impulse voltage distribution in transformer windings using computers . These advances in design and testing made impulse testing a very reliable quality assurance test and today standards reflect that.
As per IEC and IEEE standards, Lightning Impulse Test is a routine test for all transformers with HV voltage more than 72.5 kV. Below that voltage rating, it is a design or special test.
In US, a special lighting impulse test was introduced as routine test for distribution transformers( early 1950’s) and continuing even today for Distribution transformers for overhead applications and pad mounted transformers ( C57.138-1998 Recommended Practice for Routine Impulse Test for distribution transformers)
Since this test is performed in better controlled environment than similar test done in the pass, I am not sure if this test can not longer considered as destructive test
Impulse tests including the BIL, over stress the transformer insulation. Even if the transformer survive the test, it is expected some life reduction.
Gentlemen, let me disagree with you.Lightning impulse test is not a destructive test and repetition of it will not reduce insulation life.None of the dilelectric tests are overstressing insulation.It is only checking whether the insulation can withstand the margin of additional stress over normal working stress as required by standards.It is designed for these overvoltages.
Quote
IEC 60076-3 Clause 9.0 Repeated dielectric Tests:
For transformers which have already been in service and have been refurbished or serviced,
dielectric tests according to 7.2, 7.3 and 7.4 shall be repeated at test levels of 80 % of the
original values, unless otherwise agreed upon, and provided that the internal insulation has not
been modified. Long-duration AC induced tests (ACLD) according to 12.4 shall always be
repeated at 100 % test level.
NOTE The partial discharge criteria should be discussed between the purchaser and supplier depending on theextent of the repair.
Repetition of tests required to prove that new transformers, having been factory tested to 7.2,
7.3 and 7.4, continue to meet the requirements of this standard is always performed at 100 %
of test level.
Unquote
As per IEC, the sequence of impulse testing is as below:
1- Reduced Full wave 50-70% of test level
1- Full wave at 100%
1- Reduced chopped Wave at 50 -70% of test level
2-Chopped wave Test at 110 % of test level
2- Full Wave Test at 100 % test level.
If Impulse testing will reduce insulation life, standards would not have asked for so many applications. Nature or location of Dielectric stressing from tests are different .That is why you may see a transformer failed in impulse test may with stand AC high voltage test. A unit failed in impulse chopped wave test may withstand impulse full wave test.
prc, I disagree with you. Destructive or non-destructive test is defined by only one criteria - if the test fails is there a insulation breakdown happened? If a transformer fails on impulse test there must be an insulation breakdown or in other words breakdown is the only criteria to measure it pass or not, however, if a unit fails on megger or PD there is no breakdown required.
pwrtran-In normal terminology a destructive test is one where we go on increasing the voltage or stress till the specimen breaks down.Usual BIL test is the withstand voltage test and not breakdown voltage test that we may be doing as a development test.Please see
If I can add my 2 cents, the point of an impulse test is to ensure that the transformer can withstand a temporary overvoltage condition caused by lightning or switching while in service. If you consider the impulse test to be destructive, you are essentially saying that you expect the transformer to fail if you put it in service and lightning strikes nearby, or someone is a little careless in their switching. I, for one, do not anticipate replacing my transformers this often.
Additionally, due to issues with a transformer being built, I asked our engineering group to look into how many times a transformer bushing could be impulse tested safely. They replied back that the industry does not have a limit on impulse testing. This indicates to me that the industry considers impulse testing to be non-destructive.
I think we're confusing the term destructive testing (which means the sample is purposefully destroyed during the test) with the idea that BIL testing can age certain types of insulation. In the HV, oil-filled instrument transformer world, it has long been held by most that routine impulse testing shortens the life of a unit, specifically current transformers and inductive voltage transformers. Traditionally, impulse testing has been listed as a type test and not a routine test in most standards.
In fact, some utilities and standards contain a type test called an "aging" test, which consists of subjecting the sample to repeated impulse tests (300 is a typical amount) and then measuring the partial discharge performance after the impulse sequence. Most recognize that the PD performance will be worse after such a test sequence and less stringent levels are set (as compared to the normal PD routine test). It's a very difficult test to pass normally, as the PD performance reduction after testing is always very difficult to predict. However, in my opinion, it does prove that impulse testing does age an oil/paper, capacitively-graded insulation system.
Hi Mesutphen,
Insulator is classified as self-restoring insulation. Transformer winding on the other hand is a non-self restoring insulation. Therefore it not fully recovery its insulation properties after a disruptive discharge by applying an overvoltage. The following quote from an IEEE publication may be appropiate for this discussion:
…….. this is a very important aspect since all faults involving nonself-restoring insulation are DESTRUCTIVE in nature and hence have to be necessarily rectified. In case of failure of a transformer during an impulse test, manufacturers have to untank the winding in order to locate the fault……. REFERENCE: Power Engineering Society Winter Meeting, 2000. IEEE “Fault location in power transformers during impulse tests” by Vanaja, R. & Udayakumar, K.
Hi prc, your link explains the concept that in none electrical engineering field. It most suitable for mechanical, structural and material engineering. A good example for it is a car crash test. However, in power electrical engineering especially in high voltage test technology, destructive considers whether the test can lead a possible dielectric breakdown or the test conditions to certain degree that will add to dielectric degradation.
To better explain, the concept of "polarization" is very important. Most structure of gas molecules are symmetrical like O2,N2,CO2 and SF6. They are in non-polarized nature and thus considered insulation gas. Under a certain degree of electric field they become momentarily ionized and lose their dielectric characters as a result a breakdown could happen. After the energy dissipated gas molecules de-ionized or neutralized and self-restore its insulation nature. This type of breakdown is called "electrical breakdown", like lightning breaks down air space.
However, in liquid and solid dielectric materials, molecules are in polarized nature but they are scatter to all directions, in overall or in bulk they are still non-polarized. Applying electric field to stress them up, molecules of solid or liquid dielectric material start to line up and you will see capacitive leakage current whether thru the bulk martial or along the surface, and here is the megger test come across. At this point if the electric field is removed then molecules become messy again and in bulk wise it still non-polarized. Therefore solid or liquid dielectric material have some self-restoring nature and that is why megger is not consider a destructive.
Further increasing the field stress pass a certain threshold the molecules will be twisted too much and they cannot restore even the electric field is removed. This type of polarization is permanent. At this point breakdowns may not happen immediately as the polarization path may not form completely, but the damage has been made permanently and as the result the insulation has been weakened. That is why impulse test is destructive. Most breakdown in solid dielectric materials is the combination of "thermal breakdown" and "mechanical and chemical breakdown", like aging, overloading and excessive temperature rise, moisture, chemical reaction etc. Pure electrical breakdowns seldom happen.
Dissipation factor or tangent delta are not destructive because the material under test only stress up to a low level that enough to cause the capacitive/resistive leakage current to be detectable but far away to cause the permanent polarization. PD is not destructive because it only stress up the gas molecules in the voids of the solid or liquid dielectrics not the material itself. Once the field source is removed materials go back to their previous characteristic.
mesutphen - we are talking all about the probabilities, the probability for numbers of lightning strikes per year, numbers of strikes to the power lines or stations, chance of lightning strikes are not protected by skywires and LA's, poor design and poor grounding, accumulated apparatus aging or damaging etc, all probabilities multiply together makes you feel transformers do not fail very often, but they do fail under lightning.
scottf,It is true that for EHV Instrument transformers Lighning Impulse test is a type test. This is not because LI test is destructive or due to loss of life for insulation.Due to the peculiar nature of CT, impulse test is not critical( less or nil L and hence no voltage swing as in case of transformers).But repeated impulse test is done for EHV CTs ( 400kV and above) to simulate the fast transients on opening a Disconnector in the system. Such 300 numbers impulse test is a destructive test as after these, CT is not sent for service in the grid.But today LI test is a routine test for transformers above 72.5 Kv and bushings above 132 kV. Some years back LI test was routine only for voltage class 400 kV and above.The reason LI test was looked upon with suspicion in the past was not due to its destructive nature, but due to the difficulty in detecting partial failures that went undetected.I remember veterans writing about 40 years back in IEEE &IEE that impulse test for transformers is no longer a design or type test but a quality assurance test as the type of stresses coming from impulse test cannot be simulated from other dielectric tests.
I will be doing impulse test on transformers at the specified test level to see that insulation with stands that voltage.Of course all test levels have a probability of breakdown. I remember a reputed firm setting up their internal standard that when 100 transformers are tested at a test level probability of failure for the insulation structure shall be less than 1.But when a new insulation structure is to be checked for safety margin, I will go on increase the voltage till break down occurs.Then it is a destructive test as I have no intention to reuse the specimen.
I am told that electrical engineers arrived at the twice the working voltage as test voltage from civil engineering. They normally test beams or other structures for twice the normal load though break down load may be still more.
prc, I disagree with your assessment that impulse testing of CTs does not age the insulation.
If impulse testing doesn't age the insulation, why do you view the 300 shot test as destructive?
Until IEEEC57.13 -.5 was introduced, impulse testing was never a routine test for CTs or VTs. With that addition, it is now a batch test, which I still disagree with. Most of the manufactures resisted the addition due to the concern over aging and the additional costs for performing the tests as a batch test. Luckily very few customers actually call for or reference the .5 addendum, so it doesn't really come up that much.
