Please elaborate on % impedance of power transformer.

 

"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.

 

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 rated-voltage, 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])

 

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?

 

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])

 

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.

 

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])

 

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)

 

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)

 

If a lower than standard impedance voltage is required for a power transformer while designing, how is it achieved?

 

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

 

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.

 

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)

 

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

 

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

 

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.

 

Phil Level...; to avoid mis-interpretation of your data, please forward a simple SLD (Single-Line-Diagram) or hand-drawn sketch to me at:

cepsicon[at]AOL[dot]com

Regards, Phil Corso

 

@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

 

Dilip... for the assistance you seek, please contact me off-list at:

cepsicon[at]AOL[dot]com

Regards, Phil Corso

 

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