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% impedance of power transf ormer
from the electrical engineering department...
% impedance of power transformer Posted by ramesh on 5 August, 2008 - 1:12 am
Please elaborate on % impedance of power transformer.
Posted by Ananymous on 6 August, 2008 - 11:28 pm
"The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage." "It is also the percentage of the normal terminal voltage at on side required to circulate full-load current under short circuit conditions on other side." The impedance of a transformer has a major effect on system fault levels. It determines the maximum value of current that will flow under fault conditions. It is easy to calculate the maximum current that a transformer can deliver under symmetrical fault conditions. By way of example, consider a 2 MVA transformer with an impedance of 5%. The maximum fault level available on the secondary side is: 2 MVA x 100/5 = 40 MVA and from this figure the equivalent primary and secondary fault currents can be calculated. A transformer with a lower impedance will lead to a higher fault level (and vice versa). The figure calculated above is a maximum. In practice, the actual fault level will be reduced by the source impedance, the impedance of cables and overhead lines between the transformer and the fault, and the fault impedance itself.
Posted by N.S . Narayanan on 3 November, 2008 - 1:52 am
My client has stipulated that the PERCENTAGE IMPEDANCE VOLTAGE for the 60MVA - 132/66KV Power Transformer shall be MORE THAN 20%. My questions are as follows: [1] If the Percentage Impedance is more, the FAULT LEVEL CURRENT will less. Thus it will be useful. [2] But are there any ADVERSE EFFECTS if the PERCENTAGE IMPEDANCE is more? [3] Is there any ceiling for the PERCENTAGE IMPEDANCE VALUE? If yes, how much it should be?
1 out of 1 members thought this post was helpful... Posted by Phil Corso
control.com/thread/1026248874
on 5 November, 2008 - 3:09 pm
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Responding to Points [2] & [3] in Narayanan's 03-Nov-08 (01:52) query: Typically a transformer's impedance is sufficient to withstand maximum fault exposure. Also, for most manufacturers, 20% is considered an upper limit. However, voltage-regulation will suffer and non-normal or additional taps may be be required as compensation. Beyond 20%, physical constraints can result in a model far removed from good engineering practice. In my opinion, it is far more prudent to install an impedance in the transformer's primary supply circuit than choose a one-of-a-kind transformer design. Regards, Phil Corso (
[email protected])
Posted by Dilip on 28 June, 2011 - 3:27 pm
@Phil, i am purchasing a transformer for Submerged Arc Furnace. Rating 45 MVA,33KV Primary, 170-300 v secondary ,, cooling OFWF. On Load tao changer on 33 KV side with 32 tap positions. Bidders are quoting impedance in range 6%-8%. What should be my requirement to ask for more impedance on less? pl elaborate with calculations. You may send mail on mel_em-at-rediffmail.com
Posted by Phil Corso
on 28 June, 2011 - 6:46 pm
Dilip... for the assistance you seek, please contact me off-list at: cepsicon[at]AOL[dot]com Regards, Phil Corso
Posted by Phil Corso
on 28 June, 2011 - 6:57 pm
Dilip... further to my earlier reply, I like to know why you never responded to Control.Com Thread #: http://www.control.com/thread/1257876382 "Parallel Operation of Transformers" Phil Corso
Posted by S dhir B. Pednekar on 1 April, 2012 - 1:09 pm
> Responding to Points [2] & [3] in Narayanan's 03-Nov-08 (01:52) query: control.com/thread/1026248874
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> Typically a transformer's impedance is sufficient to withstand maximum fault exposure. Also, for most manufacturers, > 20% is considered an upper limit. However, voltage-regulation will suffer and non-normal or additional taps may be > be required as compensation. Beyond 20%, physical constraints can result in a model far removed from good engineering practice. > In my opinion, it is far more prudent to install an impedance in the transformer's primary supply circuit > than choose a one-of-a-kind transformer design. > Regards, Phil Corso (
[email protected]) Phil's answer is totally right answer.
Posted by Randhir S hinmarh
on 12 November, 2008 - 8:30 pm
Hello dear, 1. If percentage impedance of transformer increase, then losses will increase and efficiency will decrease. 2. Your 20% figure is abnormal to me. Z% is a trade off between economics and performance.
Posted by Phil Corso
on 13 November, 2008 - 12:52 pm
Randhir, 1) While a higher impedance will result in an increase in percent regulation, it doesn't necessarily increase losses. 2) 10% is usually the preferred maximum limit, but manufacturers can go higher. 3) I agree % Z is a trade off between economics and performance, but aren't most designs? Regards, Phil Corso (
[email protected])
Posted by bishnu
on 20 October, 2010 - 6:42 am
Hello, I would like to include the transformer tap (real value) in the load flow and optimisation. For this, i need the realistic data of resistance and reactance of a transformer (5 MVA, 33/0.4 kV). The rating of a transformer could be higher as well. Next, how can i incorporate the transformer data(resistance and reactance) with the transmission line between two buses? bishnu (getbishnu100 [at] yahoo.com)
control.com/thread/1026248874
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Posted by pinaki chatterjee on 28 S eptember, 2012 - 3:59 am
>[3] Is there any ceiling for the >PERCENTAGE IMPEDANCE VALUE? If yes, how >much it should be? the value should be as per ISS 20 %
Posted by aj on 26 December, 2011 - 1:57 am
this % impedance is calculated at 50 hz , what will be impedance at 5hz. 10 hz will it be linear i.r for 5hz divide by 10 and 10hz divide by 5 and so on. As R is not affected due to frequency change only L will be changed.
Posted by Phil Corso
on 26 December, 2011 - 7:31 pm
AJ... the impedance formula is: %Z = SQRT[ (%R)^2 + (%X)^2 ] at 50Hz. And, as you deduced, only the X term is affected by frequency. Thus, the %Z formula, for any frequency, f, is: %Z = SQRT [ (%R)^2 + (%X x f/50)^2 ] Please note the formula is for typical power transformers! It ignores frequency-sensitive elements found in low-impedance transformer designs, high-efficiency transformers, non-linear core-effects, harmonics due to saturation, interwinding capacitancs, and winding-to-core capacitances. Regards, Phil Corso
Posted by Jeffrey Alden Olson on 10 January, 2012 - 7:31 am
when recording audio, is the voltage AC or DC? and does that matter when i am Trying to figure Impedance? To tell you the truth, I am still VERY lost, and confused.. If i have a voltometer and I would like to measure whether the salvaged transformers I have are Step-Up, Step-Down, or even for isolation (As you can see, i know very little of what the internet has provided me with) ** I am also quite young. Please any help will do. j.alden.olson [at] gmail [dot] com
Posted by shiv on 30 March, 2012 - 3:34 am
Hello, I am a trainee. I need to simulate the power system in a software called power world simulator. control.com/thread/1026248874
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For that, I need R and X specifications of transformer and transmission line. In transformer being used here at my workplace, %impedence volt is provided. Its given like 6.25% +/- IS TOL. What does it mean? How can I derive R & X values from this. Load Loss is also provided. For the following ratings, I need R & X values 1) 1000kva 2) 6300KVA 3) 1600KVA 4)630KVA 5) 400KVA For Transmission line, XLPE cable is used. Where to find its R & X values? In general, what are the values for it?
Posted by Phil Corso
on 7 August, 2008 - 12:45 am
Responding to Ramesh's 5-Aug-08 (00:12) request... following is a simplified discussion: A transformer's impedance (also referred to as impedance-voltage) is equal to the voltage, in % of its ratedvoltage, that when applied to the primary-winding of a transformer will cause rated-current to flow in its shorted secondary-winding. For example, consider a transformer rated 500kVA, 6kV/400V, and 4.0% impedance. It means that when 240V is applied to the primary-winding, then the rated current, 720A, will flow in the shorted secondary. To elaborate, consider a two-winding transformer having an equal number of turns in its primary and secondary windings. The effective transformer resistance is the sum of the primary and secondary resistances. Similarly, the effective transformer reactance is the sum of the primary and secondary leakage reactances. Finally, the effective transformer impedance is the vector addition of its effective resistance and reactance. If additional information is required, let me know! Regards, Phil Corso (
[email protected])
Posted by Phil Corao
on 20 October, 2010 - 12:10 pm
Bishnu... your question is a good one. Although, the effect taps have on load-flow and fault-duty studies is seldom considered it is clear tap-changers can influence accuracy of those studies. To give you an idea of the variation that influences the theoretical effects, following are several design factors that must be taken into account: a) core or shell construction; b) 3 or 5-limb magnetic structure; c) location of tap, i.e., terminal-end or neutral-end of a wye-winding ; d) tap-changer steps that are symmetrical about the nominal rating or skewed to either the low higher voltage; and e) others! All is not lost however. Most system planners or designers use either of two approaches to "adjusting" the reactance component of the transformer's nominal impedance: A) proportional to the step-interval; or B) proportional to the square of the step-interval. If you would like additional detail contact me off-forum. Regards, Phil Corso (cepsicon [at] aol [dot] com) control.com/thread/1026248874
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Posted by George Mattam on 12 January, 2011 - 2:29 pm
If a lower than standard impedance voltage is required for a power transformer while designing, how is it achieved?
1 out of 1 members thought this post was helpful... Posted by Phil Corso
on 12 January, 2011 - 10:08 pm
George... a large number of factors influence a transformer's impedance. Key factors are core material selection such as permeability, flux density, laminate thickness, and coating. Others are conductor characteristics (resistance, shape, Ampacity, and insulation.) Still others are core geometry such construction-type (core or shell,) number of limbs or legs (3 or 5) limb shape (circular, square, hexagonal, or octagonal) coil arrangement on the core-limb, and others. Fortunately, there are a number of manufactures that will gladly provide cost increments based on the desired deviation from industrial standards. In closing, if you are contemplating purchase of a transformer having an off-norm impedance to compensate for power supply inadequacies, or to mitigate harmonic effects, there are alternatives, as well as restraints, that should be considered. One such concern is the impact on short-circuit duty. On the other hand, if basic knowledge is your goal, then I was glad to help. Regards, Phil Corso
Posted by Ronnie on 8 February, 2011 - 4:29 am
I'm using 45/55 MVA ONAN/ONAF transformer. Suppose I'm using an impedance of 12.5% for the purpose of calculation...then this impedance which corresponds to 55MVA would be different for transformer operation at 45 MVA? Please advice. Also advice if there's any IEC code which states sth related to this problem.
Posted by Phil Corso
on 8 February, 2011 - 8:38 am
Ronnie... Unless shown differently on the transformer's nameplate, the impedance is based on its OA (45 MVA) rating. Furthermore, impedance is not altered for operation at other capacities. However, tap setting does affect reactance. Regards, Phil Corso (cepsicon [at] aol [dot] com)
Posted by Phil Level
control.com/thread/1026248874
on 28 June, 2011 - 12:44 pm
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Phil Corso wrote: > ...if you are contemplating purchase of a transformer having an > off-norm impedance to compensate for power supply inadequacies, or to > mitigate harmonic effects, there are alternatives, as well as restraints, > that should be considered. One such concern is the impact on short-circuit duty... I am considering three 20/110kV step-up transformers rated at 30, 45 and 55 MVA. The 30MVA transformer substation 110kV busbar is connected to a distribution network via 19km of 630mm² trefoil XLPE. The 45MVA is connected to the same 110kV busbar for a 5km remote generator. The 55MVA is connected to the same 110kV busbar via 13km of 150mm² trefoil cable. Impedances are expected to be less than 10%, I need to do some calculations for this but I'm actually interested in mitigating harmonic effects and also in dealing with charging currents for such long underground HV lines.
Posted by Phil Corso
on 28 June, 2011 - 2:17 pm
Phil Level...; to avoid mis-interpretation of your data, please forward a simple SLD (Single-LineDiagram) or hand-drawn sketch to me at: cepsicon[at]AOL[dot]com Regards, Phil Corso
Posted by mustafa on 15 February, 2012 - 2:49 pm
dear Mr Corso i am asking why it was called percentage impedance (%Z) while it is % of voltages? regards ...
Posted by Phil Corso
on 15 February, 2012 - 10:19 pm
Mustafa... your question is really an excellent one. In fact, %Z (impedance) and %V (% of rated primaryvoltage to produce rated secondary-current with secondary shorted) are not exactly equal! But, approximations during testing are negligible in the interest of practicality! Let's start with the definition of "Exact"" impedance. Impedance is the total opposition to AC current flow in an electrical apparatus, thus 'involved' with losses. But, in order to simplify calculations, testing parameters are carried out to just a few significant figures. For example consider the "Open-Circuit" test. Impedance voltage is usually lower than full-load impedance voltage. Thus, excitation losses for the part-voltage test are just a few percent less than when the test is carried out at full-voltage Now consider the "Short-Circuit" test. If the primary-current is at rated voltage, but exciting-current ignored, control.com/thread/1026248874
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then secondary-current is also at its rated value. I hope the above is adequate for your purposes. If not, let me know. Regards, Phil Corso
Posted by S unil on 16 February, 2012 - 9:31 am
I need help from you guys, We have two transformers (20MVA, 10% IMPEDANCE, 16KA fault amaps) feeding to 6.6KV switchgear (single bus). Now there is an issue with CT knee point voltage, before the consultant has designed the C.T as per transformer fault calculation. but now new setting has been issued by considering B/C close condition (2 transformer in parallel) and the incomer CT's for REF protection are to be replaced with New setting with Knee point voltage double then before. Now my question is how does the CT sizing for the incomers matter with the B/C Open or close condition. I feel the sizing for the CT's should be changed only for out going feeders not for incoming side. but I want to know whether I am right or wrong, please give your thoughts. Regards Sunil Kumar
Posted by Phil Corso
on 16 February, 2012 - 1:46 pm
Sunil... location and ratios of the phase and neutral CTs used for the REF scheme is very important. Can you provide a simple Single-Line-Diagram (SLD) or a hand-drawn sketch of system? Regards, Phil Corso (cepsicon[at]AOL[dot]com)
Posted by Phil Corso
on 18 February, 2012 - 11:09 pm
Sunil... based on the SLD you forwarded to me, your instinct that a CT change out is unwarranted, is correct! Regards, Phil Corso
Posted by Jacky on 20 February, 2011 - 4:36 am
i'm doing my uni's project. About open-circuit test for a transformer, why does increasing the input voltage, Xm (reactance i suppose) will decrease? is there an explanation behind it? and also, why is Rc constant throughout the experiment? You can email me at jackyk1988 [at] hotmail.com
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Posted by Phil Corso
on 20 February, 2011 - 7:05 pm
Jacky... (a) Reur Xm comment: I don't quite understand your comment that, as test voltage increases, Xm decreases. Can you provide a calculation illustrating your observation? (b) Reur Rc comment: Because Rc represents the winding resistance, as well as the core loss. Regards, Phil Corso Your use of this site is subject to the terms and conditions set forth under Legal Notices and the Privacy Policy. Please read those terms and conditions carefully. Subject to the rights expressly reserved to others under Legal Notices, the content of this site and the compilation thereof is © 1999-2013 Nerds in Control, LLC. All rights reserved. Users of this site are benefiting from open source technologies, including PHP, MySQL and Apache. Be happy. Fortune No man in the world has more courage than the man who can stop after eating one peanut. -- Channing Pollock
