Results of February, 2007 Survey October, _ Yes, but only down to 4-69, 34.5, 4.16 kv, 200 BIL (18 %)

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1 Question 1. Should lightning impulse tests be made on all power transformers, regardless of voltage ratings? 25_ Yes (64 %) 7_ Yes, but only down to 4-69, 34.5, 4.16 kv, 200 BIL (18 %) 5_ No (13 %) 2_ Abstain (5 %) 39_ Total (33 % of 118 surveys sent out) Yes, All transformers may be subjected to fast impulse wave such as lightning impulses. The insulation withstand for high frequency impulses shall be demonstrated by specific tests. None of the other routine tests do stress the insulation as a lightning impulse. (Pierre Riffon) Yes, Testing units down to 69 kv may also be acceptable if 34.5 kv is too contentious. (Sue McNelly) Yes, It is a bit difficult to answer this question because the scope of the Power Transformer is fairly general. There certainly should be some exceptions I.E. the LV s of the furnace and rectifier transformers. (Joe Foldi) Yes, QC impulse only. (Gylfi Olafsson) Yes. One reason that impulse tests on Class I transformers is not a routine test is that as a design or other test it can be added as an optional test and a standard fee applied. In the old days impulse was always an added test and was included as such in purchase orders for a fee. When we made Class I and Class II categories we made it a routine test on the Class II units and the price was included in the bids. (I hope!) Another consideration for Class I impulse tests not being a routine test is that a lot of the Class I manufacturers deal with different time constraints with tests and the volume of units. Additional impulse tests would probably slow down the flow of units in the factory. If we want to make impulse a routine test on Class I units we should probably try to make a special routine impulse test for the Power units that resembles the Distribution units. No chopped waves. (Joe Melanson) Yes. Principally I consider the impulse voltage test as the most efficient test to prove the capability of transformers to withstand overvoltages in service, not only from the angle of a proper design but also concerning the quality of the production. In addition, overvoltages in medium and/or low voltage grids can be very serious, depending on protective measures and local conditions. On the other hand there may be objections because of economical considerations, but in the end our efforts should be to obtain a good and reliable product. I am not so familiar with distribution networks in the US. I do not see a necessity to extend the impulse test to transformers which are installed in cable networks only. (Gustav Preininger) Page 1

2 Yes. Lightning induced voltage that may appear at the transformer terminals is determined from the characteristics of the system. As it would be impractical to design each transformer to withstand these voltages at various locations on the system. Transformers exposed to lightning impulse have to be designed to withstand one of a number of standard impulse tests. The standard impulse (BIL) for the transformer is usually selected so that the insulation breakdown level is 15 to 20 percent greater than what the impulse voltages can reach or be limited to across the surge arrester, arrester lead length, and arrester ground lead. (John Rossetti) Yes. What is a power transformer vs a distribution transformer? (Bob Ganser) Yes, but only down to 69 kv. Additionally, the definition of distribution and power transformer needs to be updated. Table 12 of C should to be reviewed since it defines distribution as 500 kva an below and power above 500 kva. Other standards for distribution transformers cover ratings up to 2500 kva (C Pad Mounted, C Underground, C Pad Mounted). It is proposed to define power transformers as transformers with ratings above 2500 kva. It also is proposed to include for Class I power transformers an impulse test equivalent to the routine quality test defined for distribution transformers (C , clause 10.4). For reference, IEC60345 Loading Guide for Oil Immersed Power Transformers, defines: Distribution: up to 2500 kva Medium Power: up to 100 MVA Large Power: above 100 MVA. (Eduardo Gómez-Hennig) Yes, but only down to 34 kv. Insulation dimensions at lower voltages are generally established by the mechanical and cooling requirements rather than electrical stresses (margins are greater at lower voltage classes). (Barry Beaster) Yes. The Impulse Test as specified now, that is including chopped waves, should be a design test, a required test on all new designs and that a new format routine and simplified test should be utilized as a QC test on all other transformer. (Art Molden) No. Lightning impulse is a vague term. Suggest Full Wave Impulse Tests be used and then my answer would be Yes. (Jerry Corkran) No. Assume this refers to Routine tests. Routine tests are quality tests. If a user wishes to ask for routine impulse test because of concerns for quality then he is able to do so (at a cost). Impulse testing is relevant only where the system/circuit is subject to impulse strikes, or significant transferred impulse voltages. (Alan Darwin) No. Many of my customers, and some manufacturers that I am familiar with, specify required test levels by terminal BIL rating, rather than voltage class. For example, some requirements call for impulse tests on all terminals rated 350 kv BIL or higher. This type of requirement is independent of whether the unit is Class I or II, or distribution, or power, and therefore, in my opinion, is preferred. In any event, I don t believe impulse tests should be mandatory for all power transformers. (Bruce Forsyth) Abstain. Caution should be taken with the logistic aspects: 1) Longer delivery time; 2) Bottle neck in manufacturers test field limiting the global industry capacity; and 3) Cost increase. Besides that, impulse level rate to the operation voltage are quite higher for the design test of these equipments. (Roberto Asano) Page 2

3 Question 2. Should the scope of Class II power transformers be extended down to include transformers with high voltage terminals of 69 kv? 24_ Yes (62 %) 12_ No (31 %) 3_ Abstain (8 %) 39_ Total 69 kv Transformers are not (as least within our company) considered to be transmission level. (Pierre Riffon) Yes. I would even like to see it down to 34.5 kv for medium size units such as 28 MVA. We purchase these occasionally and would consider them just as important as our normal 69kV or 115 kv units. (Sue McNelly) Yes. I think the Transformers Committee may need to define what is meant by a Class I and Class II power transformer. The 1987 edition of C stated in paragraph 5.10 Insulation Levels, "(1) Class I power transformers shall include power transformers with high-voltage windings of 69kV and below. (2) Class II power transformers shall include power transformers with high-voltage windings from 115 kv through 765 kv." Prior to this in the 1968 edition of C no mention is made of Class I and II power transformers. Example: (1) Class I power transformers shall include power transformers with distribution BIL levels rated 10 MVA and below with a sealed-tank fluid preservation system. (2) Class II power transformers shall include power transformers with power BIL levels rated above 10 MVA and tank(s) designed for full vacuum filling in the field. (John Rossetti) Yes, if the majority votes "No" for question 1 (Gustav Preininger) Yes. The differentiation in class I and II should be discarded from the standard. Insulation levels should be chosen according to the occurring stress in the power grid for all transformers in the same way. (Peter Heinzig) No. I think there is confusion about when a transformer is considered a Class I or Class II transformer. C (Section 5.10 Insulation Levels) does specify the break as you said in your that contained this message (69 kv and below and 115 kv and above), but Phil Hopkinson s proposed new test tables redefine the break as 72.5 kv and below for Class I and > 72.5 kv as Class II. I actually like this proposal, since it does not leave any gap in voltage rating that could result in an undefined transformer. (Jeff Britton) No. I am of the impression that the industry experience with the present treatment in the testing of transformers rated 69 kv has been quite satisfactory. (Steve Snyder) Page 3

4 No. Systems above 69 kv generally have graded neutrals or earthed neutrals. Thus extending Class II will require changes to low frequency tests as well. Better for user to ask for routine tests if feels strongly about it, rather than default to routine impulse tests. (Alan Darwin) No. I would be interested in a discussion outlining the problems that 69 kv transformers may have that the present dielectric test levels do not discover. (Barry Beaster) Page 4

5 Question 3. If answer to Question 2 is yes, should the voltage be extended to a voltage lower than 69 kv? 9_ Yes, to 3-All, , 15, kv (38 %) 13_ No (54 %) 2_ Abstain (8 %) 24_ Total Yes, to 34.5 kv. I am flexible on this value. It may however mean more significant revisions to the standards for which we may not have enough time or resources. (Devki Sharma) Yes. I think that the testing requirements for Class II power transformers should be the test equipments for all Power transformers. Then we could eliminate the distinction between I and II. (Don Platts) No, I do not believe that e.g. a PD test would be necessary in that range. (Gustav Preininger) Page 5

6 Question 4. Should neutral terminals on all power transformers be impulse tested on a routine basis? 20_ Yes (51 %) 8_ Yes, but only down to 4-69, 34, 15, kv, 350 BIL (20 %) 9_ No (23 %) 2_ Abstain (5 %) 39_ Total Yes. Power frequency voltage withstand test is generally sufficient to prove the quality of the insulation. In reality, the lightning impulse applied on the line side terminal is smoothed by the winding itself and the resulting waveshape appearing on the neutral terminal is something which looks like a switching surge (front time around 100 µs). In that case, the power frequency test seems to be enough. (Pierre Riffon) Yes, Same comment applies as in 1. (Joe Foldi) Yes, but only down to 69 kv or lower, if the neutrals are not solidly grounded in service. (Gustav Preininger) Yes, but only when the test in the HV terminals are also required on a routine basis. (Roberto Asano) Yes, provided they are brought out of the tank. (Steve Snyder) Yes. I see many cases where tap windings, series transformers, etc. would get no dielectric testing if not for the impulse test (transient as opposed to HiPot). In these cases the impulse in my opinion is valuable. (Bob Ganser) Yes, but only down to 69 kv. Neutral terminal should always be tested if a test for the line terminals is specified. (Eduardo Gómez-Hennig) Yes, but only down to 34 kv. Neutrals that are not solidily ground should be impulsed tested. (Barry Beaster) Yes, as a design test. I agree with the argument that if a terminal has an insulation level assigned to it, it should be tested to verify that it has that withstand capability. (Art Molden) No. This should only apply to a transformer neutral tied to a low impedance ground. Such as a multi grounded neutral system. Neutrals that are "floating" or connected through an impedance, resistor or reactor, should be impulse tested. (John Rossetti) Page 6

7 No. The neutral only requires testing if its grounding is such that the neutral could be raised in voltage during impulse on another terminal, i.e. not grounded directly or grounded through a low impedance. There is a clause that calls for an impulse test on the neutral. C clause states that When lightning impulse tests are required on line terminals, the neutral terminals rated 200 kv BIL and above shall be lightning impulse tested. This would imply that such a neutral is not normally grounded because the BIL is 200 kv or above. (Alan Darwin) Page 7

8 Question 5. The test voltage values and other criteria for the front-of-wave (steep front) tests be given in: 11_ The main body of C and C (27 %) 15_ The annex of C and C (37 %) 3_ The main body of C57.98, Impulse Guide (7 %) 8_ Should not be given anywhere (20 %) 1_ Other (2 %) 3_ Abstain (7 %) 41_ Total Standards are not intended to cover more than 90% of the market. Such front-ofwave tests are exceptionally required and to my opinion, fall in the special cases area. (Pierre Riffon) Main body of C In general, compulsory requirements should go in the main body of the standard. Compulsory requirements cannot go in a guide, and will be less likely to be observed in an annex. (Jeff Britton) Main body of C They are already included in C for Class I transformers. I believe that they need to include the values for the Class II transformers in as well. There is no place for them in C Including them in C57.98 is a good suggestion, just to keep from having to go between Standards when looking at the information. There are still users that call for the tests and there are no legal values to use. Manufacturers have to go back to the old standards for the information. (editorial comment: not very logical in my opinion, by definition there are no values.) (Joe Melanson) Main body of C If it still remains as a test then it should be clearly specified. (John Lackey) Main body of C and C If steep waves are even going to be considered as a possible test, the values should be listed in C and C (Bob Ganser) Annex of C and C It is not recognized in the current standards and therefore it should be in annex. Some users are still using the guidance and should be retained. (Bipin Patel) Annex of C and C If this is a non-recommended test, and the information is being presented for historical continuity, and as a common reference for those users who still specify it, then it should not be a part of the main body of either or So I think it would HAVE to go in an annex. This is similar to what we Page 8

9 have tried to do in Insul Life Sub where requirements for a very specific design of transformer is not added to the main body, but relegated to an annex. (Don Platts) Annex of C and C There are a lot of old transformers which has been tested in that with front of wave tests. The values should be available. But it should be clear mentioned that this test is not a standard test any more. (Peter Heinzig) Annex of C and C The information needs to be available for those who have special needs. FOW should not be used on a routine basis. (Fred Elliott) Annex of C and C This question does not let you pick an option and provide comment (upon clicking a choice it doesn't place an "X"). I vote for the annex of C &C Placing it in the guide implies that it's a required test; eventhough it shouldn't be that way generally users treat the guide as a standard. (Bipan Patel) Should not be given anywhere. I would prefer chopped wave tests with an equipment allowing fast chopping. This would probably also be helpful in case of vacuum breakers. Front of wave tests always inhere risks and most of the factories may not have the proper equipment for an exact measurement. (Gustav Preininger) Should not be given anywhere. I think is better to provide a higher BIL level (like next value in the table) instead of front of wave. (Juan Luis Thierry) Should not be given anywhere. Front of wave test is only specified by a very small number of users. This test should be removed from the standards. If a user definitely requires the test to be considerd (because of the specialty of his network conditions), he should include in his specification the test requirements. (Eduardo Gómez-Hennig) Should not be given anywhere. Maybe in an Annex to C (Bertrand Poulin) Other. As an annex of C , not in C (Art Molden) Abstain. If approved, one may indicate its purpose as special test. (Roberto Asano) Page 9

10 Question 6. Should the present information pertaining to dielectric testing, as presently given in C and C , be reorganized so as to be presented in a more orderly, less confusing fashion? 29_ Yes (74 %) 7_ No (18 %) 3_ Abstain (8 %) 39_ Total Yes. I will join the WG to do this. (Don Platts) Yes. If we look to IEC, there is one document which deals with the dielectric tests and test levels and procedures. I propose to reorganize both standards accordingly i.e. one document where test procedures and test levels are included. (Peter Heinzig) Probably yes. I use these so much that I have probably overcome any hurdles due to lack of organization. The part that is confusing to me is the definition of a Power Transformer and a Distribution Transformer. I always ask for the higher test levels in C when in doubt. (Bob Ganser) Yes, but it will be difficult. (Alan Darwin) Yes. If we only have one class then the information would not be so confusing. (Dennis Marlow) Yes. A fundamentally confusing issue is that the various categories that are used in the standards are not all defined. Class I and Class II are defined by terminal voltage, but I am not aware of a definition for distribution or power. All 4 categories are used in various parts of the standards. It is not clear to me that we need 4 different categories, but I don t believe it is practical to treat small power units that are built in batch quantities the same as the one-of-a-kind large power units that are built. If the distribution and power product groups can be defined, and if their requirements are fundamentally different, then I believe the effort to re-organize the standard would be worth-while. (Bruce Forsyth) Yes. I agree that it is confusing and it should not be so. The information should be more readily obtainable. (Art Molden) No. Seems adequate (Pierre Riffon) No. At this time I think the focus should be on getting Phil s new proposed test tables complete and into the standard. Since the debate on these tables has been going on ever since I started attending meetings in 2002, and as far as I know they re still not done, I m not sure that a further major reorganization of the information is an achievable task. I ve been working on a similar reorganization of IEEE Standard 4 with the HVTT Suncommittee within PSIM for over ten years now, and we re just now getting to the stage of Page 10

11 final editing work. A major rewrite of a large standard document is, as you say in your , a monumental task. I think it s better right now just to get the tables finished to where everybody is happy with them, and get them into the standard for use by the industry. (Jeff Britton) No. It looks good to me the way it is. (Juan Luis Thierry) No. You may want to wait till Phil's work is completed to see if there is a need to reorganize the info in the two standards mentioned. (Bipan Patel) Page 11