Short-Circuit Apparent Power of System Survey Comments

Transcription

1 WG Item 87 Short-Circuit Apparent Power of System Survey Comments Again, the values given in Table 18 are totally unrealistic of system conditions. I do not know any systems for which the short-circuit current exceed 63 ka (except for only few cases). Moreover, you will not find on the market, any HV circuit-breakers having a rated short-circuitcurrent exceeding 80 ka except for some very special MV generator circuit-breakers (not exceeding 36 kv). For me, specifying system fault current exceeding 63 ka is misleading since this is more or less impossible to get it for most of the substations. Specifying too high system fault current may lead to overdesign the transformer regarding its ability to withstand the forces causes by the short-circuit-current, mainly for high MVA and low impedance transformers. See attached document (Table below) giving practical examples of resulting overdesign. If these values were there in order to get a kind of safety margin, it will be more adequate to specify a realistic value of the system short-circuit current (I propose an unified value of 63 ka for all system voltages) but to calculate the total fault impedance (system + transformer) at an overvoltage of 105% or 110%. For me, this is more logic than to specify values such as 160 ka fault current at 145 kv... Hoping this is clarifying my comment. Transformer rating Short-circuit current on primary using system fault current of the actual Table 18 of C Short-circuit current on primary using a system fault current of 63 ka Fault current overrating Mechanical forces overrating 100 MVA, 145 kv, 3φ, 10% 300 MVA, 145 kv, 3φ, 10% 100 MVA, 145 kv, 3φ, 20% 300 MVA, 145 kv, 3φ, 20% 300 MVA, 242 kv, 3φ, 15% 600 MVA, 242 kv, 3φ, 15% 300 MVA, 242 kv, 3φ, 25% 600 MVA, 242 kv, 3φ, 25% 550 MVA, 800 kv, 1φ, 12% 550 MVA, 800 kv, 1φ, 20% (ka rms sym.) (ka rms sym.) (%) (%) 3,89 3,75 3,7 7,6 11,12 10,05 10,7 22,4 1,97 1,93 2,1 4,2 5,76 5,46 5,5 11,4 4,60 4,44 3,7 7,3 8,87 8,29 7,0 14,5 2,80 2,74 2,2 4,6 5,48 5,25 4,4 9,0 8,82 8,57 2,9 5,9 5,54 5,44 1,8 3,7

2 1) Obviously, there are a great many who support added design margins via the infinite bus and/or 110% prefault voltage approach. For a fair segment of the transformer market, this relates to satisfying the requirement with a combination of special configurations and materials. It's only then a matter of cost optimization and which is ultimately included in the quoted price of the transformer. There are those other segments, as Pierre as identified, who will be faced with a difficult challenge. There is another set of users, probably not well represented at the committee, who seek the most realistic solution at the minimum of expenses. For this segment, realistic system impedance values may be important in getting approvals for projects that will still provide reliable transformer designs based upon their specific applications. I am only commenting on feedback received from the field sales persons asking how these can engineers specify their needs at the most economical levels. From a manufacturers viewpoint, once the requirements are specified, there will be an engineering solution developed. For that certain segment, today the published system impedance values may not satisfy their need. 2) I assume the system fault capacity values in Table 18 are symmetrical and not asymmetrical since there is reference to the X0/X1 ratio. Symmetrical values on our 765kV system are below 40kA. I will check further on the lower voltages. Maximum system Voltage (kv) Below 48.3 Table 18 Proposed Revisions (AEP) System fault capacity (Existing Values) (ka rms) (MVA) AEP Projected System Fault Capacity (ka rms) (MVA) ) Duke Energy Carolinas has parts of its system with fault currents in excess of 63 ka. We have made some adjustments to mitigate this using neutral reactors, etc. but some levels remain >63 ka. We have had to use some 80 ka breakers at 230 kv. We have some new generation coming

3 on to replace some older plants and sometimes the location of these new plants change the system fault current availability. This same statement would apply to possible future new nuclear plants planned in the next decade. I believe that research continues into breakers above 80 ka which tells me the problem is not going away. We call for infinite bus and 110% pre-fault voltage for calculated thru-fault designs. The majority of transformers fail on our system due to thru-fault or dielectric failures due to change in dimensions/clearances due to thru-fault so we have made the business decision to seek this level of margin. I agree with Robert Thompson s comment that some conservative value should appear in the absence of a specified system impedance to protect less experienced users. 4) David s comments also apply to the BGE system. I also agree that the conservatism should be maintained in the standard for less experienced users. 5) Steve: I have to agree with Dave especially about one thing : It is a good operating procedure to have all transformers designed for short circuit with 110% pre-fault of the rated tap voltage applied. This simply results in about 110% of the current and 121% of the normal forces. This has been in the CSA CAN3 specification since 1979 The system fault MVA in the USA and Canada is typically a lot higher than in Europe and other parts of the world. I do believe that we still should use the anticipated maximum system fault MVA for the installed locations. If individual users wish to specify infinite bus, that is their option. I have seen many large autotransformers with low impedances which were nearly impossible to design with infinite bus. This also holds true for many Windfarm Y-Y-delta transformers which may be paralleled with similar transformers. In those cases a single line to ground fault on the LV could result in excessively high forces unless realistic LV system impedances were used or designed with artificially high TV impedances. This discussion is getting a little out of the original scope but many of the points brought up need to be tabled for future consideration in the standard. I think the present Xo/X1 ratio of 1 in C version is indicative of modern grounding practices and will all ensure little more margins in all transformers as the systems continue to grow. 6) After reading many of these replies, I don't have anything different to add. I too believe that the standard should set a minimum default requirement. I don't see any problem with retaining the present values.

4 7) We, SCE is already facing over 70kA in several substation and preparing for 80kA. I don't agree with the statement of not exceeding 63kA. In my view, transformers should be designed for infinite bus. We should review IEC In general, the IEC standard is in line with Table 18. Also, we should review IEEEC57.109, since the table 18 was used for transformer through fault current duration. 8) Our specifications typically call for an assumption of infinite bus for calculated thru-fault design for all types of transformer we purchase- GSUs, Distribution and Auto-banks. At the same time, we specific high impedances for both auto banks and distribution transformers. The transformers we purchase can be used generally anywhere on our system with the exception of GSUs. We are in favor of retaining Table 18 for providing some guidance to less experienced users and designers as well. 9) Although I am now retired from Entergy I can confidently state that Entergy always preferred using an infinite bus for units installed on their transmission system. Generator main transformers and other transformers associated with auxiliary power could, if practical, be designed with generator impedances in mind. 10) Our preference here at APS is also to have a unit that can operate in any system/location as units do get moved around. It's obvious that the easiest thing to do is to use infinite bus and we simply treat the "overdesign" as extra design margins. For certain large units we did have to resort to system impedance. Therefore the idea of eliminating system impedance in general and including system impedance in special occasions sounds good to me. 11) My comment was only that at Xcel Energy, we assume an infinite bus for through fault calculations on our TRs. 12) With regard to the question of system short-circuit current, we previously queried the table 18 with respect to the 765 kv entry back in 2007, as you can see from the (s) copied below. Looking at Pierre's comments, I must agree with him that the maximum short-circuit current in the table is too high and I think his proposal is reasonable. One of my colleagues has queried something in the latest C :2006. It refers to the fault capacity table (table 18). As you can see below, there appears to be an anomoly that has existed from at least the 2000 version. Forget his first question, but does his second query come under your PCS C WG, or does it belong with the Dielectric Test SC? I also find his comment at the end of his of interest. Do you have access to the IEC standard?

5 If there is an error with the entry in Table 18 for the 765 KV entry, then can we get it corrected in time for the next revision (i.e. before the December deadline)? The fault capacity table in both the 2000 version (table 16) and 2006 version (table 18) give values for the system fault capacity which, in all but the last case, are the product of the nominal system voltage (not actually given in the table but elsewhere in the documents) and fault current given in the table. My questions are: 1) Why is it necessary to give both the current and MVA? (This is actually rhetorical since, if both hadn't been given, I'd have assumed the fault capacity at the highest system voltage and not the nominal value.) 2) Why don't the values for the 765/800kV levels follow this same "logic"? Or have I done something silly? I also question the difference between C and what IEC calls "Current North American Practice". 13) I agree that the fault capacities in the table seem excessive and 63 ka does seem like an appropriate limit as only a few HV systems ever get higher than that. I think the Montreal and Toronto areas are over 63 ka on their 245 kv systems. We usually refer to the CSA standard for transformer purchases and the fault levels defined there are similar to the existing ANSI levels but not exactly the same. We work to an ultimate level for stations recorded in an informal table established many years ago where the maximum levels are as follows:

6 145 kv 33 ka 245 kv 63 ka 362 kv 32 ka 550 kv 40 ka I don't think the change of Xo/X1 from 2 to 1 is a good idea. A very strong system with a weak zero sequence source will force more current through a a grounded winding than would otherwise occur for an equivalent system with a strong zero sequence because the transformer becomes the main zero sequence current source when the system is weak in the zero sequence. The current can go above what the transformer alone would carry. This may be the reason for the apparently excessive source strength in the standards. My understanding is that transformer failures on through faults have been found more common than expected across the industry suggesting that all the transformer designers don't have a good handle on the design required to withstand through fault forces. It might be better to retain some extra design margin. The excessive fault levels defined don't make very much difference to the through fault that is restrained by the transformer impedance. 14) I know of several locations on our 115 kv and 230 kv systems where the available fault current is approaching 60 kaic. There are 80 ka breakers available for 230 kv. That doesn't mean that max fault currents are 80 ka. However it probably does mean they are above 63 ka. For 500 kv and above, 80 ka would be a fairly good sized source and probably not realistic. This probably doesn't help. Someone from the switchgear committee that's involved with planning and breaker duty might give some insight. As a utility user, Southern Company's applications are typically distribution, transmission (network autos), and generating. The impedances for these applications are relatively high. A utility user's general specification will typically call for infinite source impedance as ours does. In this case, it is a non-issue. And in most cases, it is a non-issue. For the specical application that requires system impedance, I would expect the user (or consultant) to provide the system impedance for their particular application. The table is there to protect the user and manufacturer when the user forgets or doesn't know to provide the source impedance for a special application that requires this information. 15) Please get info from large power users and manufacturers. Delta Star uses infinite bus for all transformers including autos. The table might make sense for very large autos with very low impedance but I do not know realistic system values. 16) On the surface, it seems that Pierre's request is correct and that changes should be made. But we are all part of the transformers committee, and the specification of power system short-circuit MVA is not a transformer issue, but a power system question. So I think you should consider asking the appropriate committee in the IEEE to give us some inputs on this question. Maybe the reason that you did not get much feedback from the working group on this question is that the members are primarily transformer experts and not power system analysis experts. 17) In real "transformer" world situations, the "overdesign" is NOT as significant for most power transformers with "standard" or higher impedances. For the rare cases that Pierre referenced, such as large autotransformers with low impedances, the user is generally aware of the cost

7 increase associated with fault duty of the transformer and works with manufacturers to add the "realistic" system impedance to limit the calculated fault currents. My preference is to eliminate the system impedances so that the transformers are designed to be strong enough to withstand faults in any system. We can add provisions to address the rare cases where a user and manufacturer can include the system impedance to reduce the calculated fault currents. 18) I concur with Mark and Jin s comments. As representing users, my specifications typically call for an assumption of infinite bus for calculated thru-fault design. If a system impedance is to be included, the user should provide the number. If a standard system impedance is to be assumed, it should be conservative in the absence of a number based on actual system characteristics.

8 The following three (3) responses resulted from an appeal to the PES SCC (Standards Coordinating Committee) earlier this year : 1) I am writing you concerning the first document. It says: "Most of the values given are not realistic and are leading to unnecessary over designs. Except for very rare cases, the short-circuit current never exceeds 63 ka r.m.s. and this should be the ultimate standardized value. Specifying values which are higher than that lead to uneconomical designs." This might be true for transformers but definitely it is more complicated in case of AVRs design. It seem that it is difficult to design AVR devices that meet all grid code requirements when the short-circuit power is too small (less than ~6 MVA). I do not know if that matches with the North American practice but this is the case in a few European systems. 2) I am an electrical engineer for the Bureau of Reclamation and I do all sorts system studies for many of our plants, including arc flash studies, fault current studies, equipment rating studies, etc. I was passed along an regarding IEEE C and the suggestion to put a ceiling on the short-circuit current a new transformer is required to withstand, which asked that responses be sent to you. The proposed ceiling is for transformer short-circuit withstand capability is 63 ka rms. In studies I have done for our facilities, I have indeed run into scenarios where the transformer could see greater than 63 ka rms during a fault condition on the low voltage bus. Also, I am concerned that because the system can change over the years resulting in fault current increases. So while an existing installation may have fault currents just below 63 ka, future changes could result in fault currents above 63 ka. Changing the standard to have a ceiling of 63 ka maximum short-circuit withstand capability for new transformers would cause more work on our end as we would have to look into this parameter for every transformer and likely specify a higher fault current for many of our new installations. So, while this may save the manufacturer some money since they don t have to build the transformer as robust, it costs us more time and money to have to put specific values into our spec instead of just referencing the standard. I am curious how much money the manufacturer would save by using this 63 ka ceiling. The examples provided showed an average of 9% mechanical overrating and 7% fault current overrating. Is this saving the manufacturer only 7-9% of their cost? I am also wondering which of these examples could actually see greater than 63 ka fault current, and therefore do actually need to be built for greater than 63 ka. Lastly, the existing standard gives the option of using actual system values rather than those listed Table 18. The systems represented in the table essentially provide a worst-case scenario, which is an infinite bus. If the manufacturer used actual system values, which he could likely get from the customer, he could probably reduce the fault current the transformer needs to withstand, which would save him money without changing the standard.

9 Therefore, I do not see the need to place a ceiling on the fault current withstand capabilities in the standard. This would cause difficulties on the customer end as we would need to do a more in-depth study of each transformer for short-circuit withstand, rather than just relying on the standard. I hope this is helpful. Please let me know if you have any questions. I attended the IEEE Transformer Committee meeting in Houston in March (it was the first one I have attended) and will try to attend in Toronto in the Fall or San Diego in the Spring. Thank you for this opportunity to contribute. 3) We had a request for assistance to provide fault level for various voltage levels. Below are the 2010 numbers for the voltages on the Southern Electric System. From the SCS 2010 Base case the maximum fault currents at the follow voltage levels are: Voltage (kv) SLG (amps) 3-Ph (amps) ,508 40, ,330 61, ,932 19, ,580 57, ,808 8,479 If you have any questions, please let me know.