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Trfr %Z and load current

Trfr %Z and load current

Trfr %Z and load current

(OP)
Hi

If I recall correctly the %z of a trfr is the sum of the primary and secondary winding leakage reactances (resistances ignored). The %z is usually expressed at rated apparent power (MVA) of the trfr.

1) Does %z change much from no load to say 2 x FLA? I cannot see how it does as changes in load current does not change the core flux (flux produced by secondary load current is countered by flux due to load current component in primary current).

2) Does the %z change with tapchanging? If so why and how much variation can one expect with tapchanging (generally speaking)?

I am primarily interested in core type constructions.

Any feedback, comments will be much appreciated.

Thanks.


 

RE: Trfr %Z and load current

Transformer impedance will be affected by the load on the unit as temperature changes.  The transformer impedance will also be affected by tap changes, as you are changing the amount of winding you are using.  I cannot comment effects from the core due to the loading, but maybe others here can.

Mark

RE: Trfr %Z and load current

Also %Z does include R, as eluded to by marks1080.

Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.

RE: Trfr %Z and load current

Sorry for the wrong info Veritas.  Alehman, thank you for the correction!  So Veritas thoughts in his first question are correct in regards to load on the unit?

Mark

RE: Trfr %Z and load current

Sorry, I first read Alehman's post as 'does not.'  Through me for a loop, but I am not one of the more experienced users of the forum.   

RE: Trfr %Z and load current

(OP)
Hi

I'm not sure I quite follow the last few threads. I agree that %Z = %R + j%X. But usually the R is way less than then X and can be neglected most of the time unless accurate transient studies are required where the X/R ratio is important.

I cannot see how the load can change the impedance since the X is a functions of the leakage reactance and R a function of the conductor used (this does not change with loading).

I can appreciate that the %Z may change with tap as the number of windings used to couple primary to secondary changes. But I am wondering if there is a rule of thumb as to how much change one can anticipate with say a 5-tap offload trfr compared to say an onload 25-tap trfr?

Thanks.

 

RE: Trfr %Z and load current

R is a function of temperature.  More load will result in a higher temperature.  If you are working with a power transformer than it is important to determine the effects of temperature at different pre-loading intervals to determine the units LTR ratings.  

RE: Trfr %Z and load current

1) Since the R component is negligible ( except for distribution transformers),for all practical purposes,we can say Z in ohms remains the same with overloading.But as percentage impedance,it will vary as per % of  overloading.

2) Percentage impedance with tap changing will vary depending on the  geometrical positioning of regulating winding with respect to other windings.Variation is due to the change in leakage flux pattern. The extent of  impedance variation and nature of variation ( ie + or - )will differ for various tapping arrangements ( position of regulating winding,tapping position in winding, whether two winding or auto connection) You can refer to some of the text books for details or to the following references
-Methods and means of voltage regulation of large Auto Trfs- CIGRE SC 12 WG by Dr B Heller Electra  No29 ,1973 Pages 11-27
- Taps by R Shankar,Presentation at IEEE Committee meeting ,Las Vegas, October,2004

RE: Trfr %Z and load current

Quote:

But as percentage impedance,it will vary as per % of  overloading.
I think this can be confusing.  If, as is normally the case, Z% is given as a percentage of the base impedance (calculated with rated MVA), then Z% does not vary with load.  Z% as a percentage of the base impedance is equivalent to the percent of nominal voltage across the transformer with shorted windings and nominal current flowing (which is how it is calculated by test).

  

RE: Trfr %Z and load current

Agree with jghrist, the Z% does not change(with R ignored). X=2*Pi*f*L.  When frequency is fixed, say 60Hz, the only variable is inductance L, and L depends on the material of the core, the winding conductor and the construction of the winding in terms of diameters,number of turns,spacing of the turns.  Once the transformer has been made, inductance L is determined and X is determined.

RE: Trfr %Z and load current

I had said that Impedance value as Ohms will not change with overloading.But % impedance will go up.

Without going in to details,when we want to send 100 % current in shorted secondary,we have to apply % impedance of rated voltage .ie with 10 % impedance and 220 kV transformer,voltage to be applied is 22kV. If you want to get 150% current in secondary, 22KV has to be enhanced by 50 % as leakage impedance remains the same.This voltage when expressed as a % of 220 Kv is 15 %.

RE: Trfr %Z and load current

(OP)
prc

What you say is true, but it is really applicable only in the testing realm. Even then, I would be careful to allocate different %Z values for the same trfrs because it can be confusing especially if one is not informed of the context.

I agree with jghrist and pwrtran that one should rather not go there. The %Z is classically expressed at rated power and that is the value used for calcs, studies, etc. The ohmic impedance is essentially constant over the load current range if one ignores R and variation of R with temp (which is a pretty good approximation since increse in R with temp generally will not exceed 10% of total impedance).

But I agree in theory you are correct, %Z if calculated based on load will vary.

Kind regards.

 

RE: Trfr %Z and load current

It sounds like short circuit testing for available short circuit ratings, regulation, and a changing KVA base are all mixed up here.

%imp Volts is DEFINED at full load current, not 150% of full load current. Even if a transformer is overloaded 50%, that does not change the definition of %imp voltage.
Further, although the resistance may be a small part of the impedance, it may contribute to a much more significant portion of the voltage drop or regulation with normal loading or overloading than the inductance, particularly with a resistive load. The I2R drop is in phase with the transformer voltage. Even though the I2X drop may be numerically larger than the I2R drop, the I2X drop is at a 90 degree angle and may have less effect on the terminal voltage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Trfr %Z and load current

I have a transformer 20 MVA with 10 % impedance. Iam continuously overloading ( may be adding some fans ) it to 25 MVA. When I do  system SC calculations etc, I have to consider this as a 25 MVA unit with 12.5 % impedance or 20 MVA with 10 %impedance.

RE: Trfr %Z and load current

1) %Z has no practical significance during normal operation or even during some overload conditions. The load impedance is much greater than transformer impedance during loaded conditions. So I am not sure what the question is. %Z is becomes important only during short circuit conditions, when load is shorted out. It is only determined by testing as stated before. Does the actual Z varies due to construction? I may, but it may be complex to calculate to testing is the way.

What is more important during normal load conditions is the voltage regulation, which is something different.

2) Yes, %Z does change with tap changing due to different turn ratios. But I believe (not sure), the %Z is again determined by testing and posted on nameplate by mfrs if requested.


 

Rafiq Bulsara
http://www.srengineersct.com

RE: Trfr %Z and load current

Oh, %Z does become important if transformers are paralleled during normal conditions and even the tap positions, as even a small variation can have a large effect on the circulating current which are only impeded by the internal impedance of the transformers in parallel.

Rafiq Bulsara
http://www.srengineersct.com

RE: Trfr %Z and load current

By definition, the % impedance voltage is expressed at base rated KVA. If you declare that the base KVA has changed from 20,000 KVA to 25,000 KVA, the % impedance voltage will change in the same ratio. This is a way of expressing the impedance. The actual impedance in ohms and the available short circuit current do not change.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Trfr %Z and load current

Quote:

I have a transformer 20 MVA with 10 % impedance. Iam continuously overloading ( may be adding some fans ) it to 25 MVA. When I do  system SC calculations etc, I have to consider this as a 25 MVA unit with 12.5 % impedance or 20 MVA with 10 %impedance.
If you load a 20 MVA transformer to 25 MVA, it is still rated at 20 MVA and the impedance is at 20 MVA.  I can conceive of no earthly reason to complicate your SC calculations by changing the transformer rating to match the load.
 

RE: Trfr %Z and load current

" Yes, %Z does change with tap changing due to different turn ratios. But I believe (not sure), the %Z is again determined by testing and posted on nameplate by mfrs if requested. "

No, rbulsara.% Impedance is changing not due to change of turns ratio.Manufacturers can make units with  very little change of impedance with tap changing by selecting tap winding position. Manufacturers do calculate % Z at extreme tappings and check compliance by testing.

RE: Trfr %Z and load current

%Z at all taps is normally tested, at least for power transformers.  I've never seen this on a nameplate, however.

One example for a 12 MVA, 100-12.47 kV transformer:

Tap A 8.84% (105 kV)
Tap B 8.84% (102.5 kV)
Tap C 8.87% (100 kV)
Tap D 8.97% (97.5 kV)
Tap E 9.12% (95 kV)

On another transformer of the same manufacturer, 15 MVA, 100-12.47 kV:

Tap A  9.18%
Tap B  9.10%
Tap C  9.08%
Tap D  9.13%
Tap E  9.22%

Notice that %Z is higher at both higher and lower taps than at neutral for the second transformer but not for the first.  

As indicated by prc, the variation depends on a lot of things.
 

RE: Trfr %Z and load current

jghrist
In your first example I can see there was some measurement error.  However, in the second example are the taps on primary? Does the xfmr has LTC on the secondary? If not, what situation can result increased Z% at both ends?   
 

RE: Trfr %Z and load current

Quote:

In your first example I can see there was some measurement error.  However, in the second example are the taps on primary? Does the xfmr has LTC on the secondary? If not, what situation can result increased Z% at both ends?
How can you tell there was measurement error?  Taps are on primary with no LTC.  As prc indicated,

Quote:

Percentage impedance with tap changing will vary depending on the  geometrical positioning of regulating winding with respect to other windings.Variation is due to the change in leakage flux pattern. The extent of  impedance variation and nature of variation ( ie + or - )will differ for various tapping arrangements ( position of regulating winding,tapping position in winding, whether two winding or auto connection)
and

Quote:

.% Impedance is changing not due to change of turns ratio.Manufacturers can make units with  very little change of impedance with tap changing by selecting tap winding position.
There is no simple relation between tap and %Z.
 

RE: Trfr %Z and load current

jghrist
Your Tap A and Tap B has same Z%. When on Tap A, the primary has more turns than Tap B. Inductive impedance must have polarities either positive or negative and there isn't a neutral inductive impedance.  When you have more turns you have more impedance, it will be either added or subtracted to the main winding in the case of regulation winding has a selector switch. In auto connect it just simply increase or decrease.  

Of cause manufacture can make different transformers with different turns ratio and different voltage ratings but have same Z%.  compare Z% with different transformers does not explain.  Once a transformer has been manufactured, speaking to itself, the impedance is determined and it does vary with tap changing since the turn ratio changed.

Z% makes some confusing because it is not a vector.  Z or X is different story.
 

RE: Trfr %Z and load current

pwrtran,
Why doesn't the second transformer bother you as well.  The impedance at the 95 kV tap is higher than at the 100 kV tap.

Do you think the manufacturer made errors in these measurements also?
 

RE: Trfr %Z and load current

jghrist
I have some our transformer impedance test results attached that show linearity of the impedance. In addition, when doing transformer site specific loss allocation calculation one step is to interpolate the impedance for intermediate taps if the principal and extreme taps impedance values are known.  

I don't understand your second transformer test data and that is why I asked you, if you could explain the reason it will be appreciated.  

Again, X or Z should be linear given the transformer is not over excitation.  I wonder if you have the impedance voltage test result for the second transformer to see if they have a linear trend.  However, Z% is different story. Did the manufacture did some base conversion when the fixed tap changed?

 

RE: Trfr %Z and load current

I can't explain why, other that for the reasons given by prc, but I checked four other transformer test reports (3- Kuhlman, 1- GE) and they all had lower impedance at neutral tap than at the highest or lowest tap, similar to the second transformer in my previous post.  These were all primary taps.

I assume that the %Z at all taps was based on putting rated current through the transformer with secondary windings shorted and dividing the voltage by rated voltage.  It could be that the current used was the rated current for each tap and that the impedance was based on the measured voltage divided by the tap voltage.  The rated current is inversely proportional to the tap voltage.

RE: Trfr %Z and load current

I bet you the manufacture changed the voltage base for each tap's Z% calculation.  When doing so, you will find a "V" impedance trend with the center tap the lowest.  Having said that, we can say:
1) Z% is NOT a tested result, it is a calculated result
2) the impedance voltage value measured during the short circuit test for each tap is a true reflection of the transformer Z or X, but not Z%
3) some questions raised - what voltage base should apply to Z% calculation? Should only rated voltage be used or should vary with the taps? If different tap voltages are used should manufacture also different current and thus the MVA base accordingly?
 

RE: Trfr %Z and load current

Quote:

the impedance voltage value measured during the short circuit test for each tap is a true reflection of the transformer Z or X, but not Z%
I don't understand the difference.  Percent impedance is equal to the percent impedance voltage.  Percent impedance voltage is calculated from test data by adjusting for temperature.

See ANSI/IEEE Std C57.12.90

RE: Trfr %Z and load current

Quote (jghrist):

I don't understand the difference.  Percent impedance is equal to the percent impedance voltage...
The difference is the "Percentage". Measured values are the impedance voltage, when divided by the base voltage it becomes to the "Percentage Impedance Voltage" and that is equal to Z%.  So Z% is a calculated value.  Attached is another test report I have found, the transformer is made by Pauwels.  The numbers in the Green, Red and Blue box would very well explain the difference and why a "V" shaped Z% obtained.

Hope it could help.

RE: Trfr %Z and load current

Quote:

The rated current is inversely proportional to the tap voltage.
I don't quite agree with that one. If the secondary winding is rated for 1000 Amps, going to a lower tap won't magically make the winding able to pass 1050 Amps without overheating

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Trfr %Z and load current

Not rated current, but base current.  At least in the ANSI world transformer impedances are all based on the lowest, self cooled, rating and the transformer can be capable of considerably more capacity with all cooling turned on.  The IEC world starts out at the maximum rating so there is less wiggle room above base rating.

RE: Trfr %Z and load current

Quote:

I don't quite agree with that one. If the secondary winding is rated for 1000 Amps, going to a lower tap won't magically make the winding able to pass 1050 Amps without overheating
I meant rated primary current.  For a given rated VA, if you increase V, you decrease A.
 

RE: Trfr %Z and load current

pwrtran, impedance voltage is a measured value ( of course ,transformer designers are capable of calculating the same too!)ie the voltage to be applied at any tap to create rated current of the corresponding tap in the shorted secondary winding.MVA at all taps is same ie the rated MVA.
% Z at that tap is the impedance voltage expressed as a percetage of the rated voltage of that particular tap.You can see that Pauwels test report also, it was done the same way.

We will get V curve, flat line, drooping line, rising line when we plot %z against the tap numbers.This is a special topic for transformer designers and they know the charactristics for different winding arrangements, tapping positions and range.

RE: Trfr %Z and load current

Quote:

The numbers in the Green, Red and Blue box would very well explain the difference and why a "V" shaped Z% obtained.
I agree.  If you calculate ohmic impedance it is nearly linear with tap voltage.  210.7, 136.7, 95.8 ohms (referred to primary) for 255.5, 215.5, 175.5 kV respectively.

How would you model a transformer on an off tap in a fault current analysis?  Would you change the base voltages to match the voltage ratio at the tap used or would you keep system base voltages the same and adjust the impedance?  Changing the base voltages would be a major pain.  It wouldn't be just one voltage if you had other transformers connected to the same primary system that didn't change taps.  What is the proper way to adjust the impedance if you keep the base voltage the same?
 

RE: Trfr %Z and load current

Hi folks
Let get back to the original topic of this thread.
1) Z% does not vary with the load
2) Z is obtained directly from sc test, Z% is a calculated value relative to the base voltage used
3) Z% should be defined at rated voltage and base MVA rating, so overload a transformer does not change the Z%, and no load doesn't change it either
4) manufactures interpret Z% in different way as you can see some use rated voltage (or principal nameplate voltage) as the base voltage whereas others use each tap voltage
5) "How would you model a transformer on an off tap in a fault current analysis?"
- it depends. If you use software tool you may select transformer tap affect the nominal voltage if you really want to. If you are doing a hand calculation you always use one base voltage for each voltage level, then you can apply 1.05pu on the power sending end i.e. or vice versa.
6) transformer taps do change not only Z but also Z%.  However, it is not critical when sizing the equipment or even coordination. When fault further away from the transformer, the influence of the tap on Z or Z% is negligible.  For a close-in fault, how much different tween a 10kA fault and a 11kA fault?

 

RE: Trfr %Z and load current

Let me take an example: Consider a 220/132 Kv core type  auto transformer with 20 % tappings on HV (series winding)for HV variation.
When I put regulating winding  physically in between Series and shunt winding, I will get the same %Z at all taps( or the variation in %Z  is negligible)But the ohmic value of Z will vary drastically  with the tap changing.We call it constant percentage impedance variation.

When I put the regulating winding innermost ie reg-common-series winding,the %Z will vary drastically with tap changing,but ohmic impedance will be almost same at all tappings.This is called constant ohmic impedance tap variation.

RE: Trfr %Z and load current

Thanks prc for the comments on auto xfmr, good point.  However, how much the difference it will give you if a fault  let's say 1 km, 2km and 5km downstream.  We set our relay minimum pickup <50% of the end-of-line 3ph and L-G fault and tweak the time dial so that it will trip the feeder less than 1.5s.  I would say the line impedance and the the bus tie  play more role than the xfmr taps. So for most engineering practice, use constant Z% is sufficient.  I only slit hair and calculate the Z% for a certain tap (most frequent operating tap)when doing the transformer loss allocation for wholesale revenue metering work.

RE: Trfr %Z and load current

pwrtran-The impedance variation with tap changing becomes relevant and critical when parallel operation is required.As can be seen from the given data in this thread,%z can be substantial between different units of same rating,but different designs.

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