Dry Transformer Percent Impedance Definition

Home / Technical Articles / Transformers / Dry Transformer Percent Impedance Definition

Introduction

The percent impedance is the percent voltage required to circulate rated current flow through one transformer winding when another winding is short-circuited at the rated voltage tap at rated frequency.

%Z is related to the short circuit capacity of the transformer during short circuit conditions.

For a two winding transformer with a 5% impedance, it would require 5% input voltage applied on the high voltage winding to draw 100% rated current on the secondary winding when the secondary winding is short-circuited.

If 100% rated voltage is applied to the high voltage winding, approximately 20X rated current would flow in the secondary winding when the secondary winding is short-circuited.

Impedance Levels

 Based kVA Minimum Impedance, % 0 – 150 Manufacturer’s standard 151 – 300 4 301 – 600 5 601 – 2,500 6 2,501 – 5,000 6.5 5,001 – 7,500 7.5 7,501 – 10,000 8.5 Above 10,000 9.5

Important Notes

1. The impedance of a two-winding transformer shall not vary from the guaranteed value by more that ± 7.5%
2. The impedance of a transformer having three or more windings or having zig-zag windings shall not vary from the guaranteed value by more than ± 10%
3. The impedance of an auto-transformer shall not vary from the guaranteed value by more than ± 10%
4. The difference of impedances between transformers of the same design shall not exceed 10% of the guaranteed values
5. Differences of impedance between auto-transformers of the same design shall not exceed 10% of the guaranteed values

Impedance vs. Percent Impedance

Impedance is defined, in the Standard Handbook for Electrical Engineers, as “the apparent resistance of an alternating current circuit or path… the vector sum of the resistance and reactance of the path”. Impedance may be comprised of resistance, capacitive reactance and inductive reactance, and is expressed in ohms.

From the perspective of a load, the total input impedance may include the impedance of the upstream generator, transformer, line reactor and conductors.

The power system impedance is useful for estimating the available short circuit current.

Sample calculations for a three phase transformer rated 500kVA, 4160:480, 60Hz, 6% impedance:
Transformer reactance Xt = (kV2/MVA) x %Z/100 = (0.482 / 0.5) x 0.06 = 0.027648 ohms
Approximate available short circuit current = 480/(1.732 x 0.027648) = 10,023.7 amps

Effective Percent Impedance

Effective impedance is the relative impedance of a reactor or transformer under actual operating conditions. Since smaller (kVA) loads have higher impedance and thus draw lower current than larger (kVA) loads, the internal ohms of a reactor or transformer represent a smaller percentage of the load impedance for a small (kVA) load than for a large load.

The value in ohms will cause a lower voltage drop when less than rated reactor or transformer current is flowing. If the load is only one half the rated current, then the voltage drop across the impedance will be onehalf of the rated voltage drop.

Sample calculations for a three phase transformer rated 500kVA, 4160:480, 60Hz, 6% impedance:

Transformer reactance Xt = (kV2/MVA) x %Z/100 = (0.482 / 0.5) x 0.06 = 0.027648 ohms
Rated secondary current = 500,000 / (480 x 1.732) = 601.4 amps
Actual Load current = 300 amps
Voltage drop at actual load = 300 x 1.732 x 0.027648 = 14.36 volts (14.36 / 480 = 0.0299, or 3% of 480 volts)
Effective percent impedance = 6% x (300 / 601.4) = 2.99%

Transformer Percentage Impedance (VIDEO)

Cant see this video? Click here to watch it on Youtube.

Resource: Substation Comissioning Course – Dry Type Transformer

SEARCH: Articles, software & guides

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

1. Azhar Ali
Aug 10, 2015

Hi, Can you tell me if I want to model this impedance per phase of three phase transformer, how can I do this. Is this impedance is included in all three phases or there should be division with something to represent it equivalent in single phase. Thanks

2. RAZZAQUE AHMED
May 10, 2014

I need Genrator impidence formula / calculation
Prefer if any software is available for this.

3. apolinar
Mar 09, 2014

can impedance of dry type transformer will be adjusted as per our project requirement. for example 15kVA dry type transformer manufacturer std. is 3.3 % and our design requirement is 6.4%.it is possible to increase the impedance?

4. Khairul
Jan 29, 2014

Hi all,
I’ve one question. I’ve done site acceptance test for dry-type transformer that we just bought from one of the manufacturer. Upon doing the test, one of the test, leakage reactance was failed where the value is not the same as FAT value.. During FAT, we got 5.98 where by during SAT we got 7.38. we used doble M5400 to conduct this test at site. Could you help me to enlighten this problem.

5. Than Dai
Jun 15, 2013

My transformer is not OLTC.

6. Than Dai
Jun 15, 2013

Dry type is so noisy.I have 2MVA,11/0.415kv.I would like to get secondary voltage 0.4kv.It have -or + 2 tap.If i change ,how could I get disadvantages?

7. tigerdon
Oct 03, 2012

OLTC can be implemented for Dry Type Transformers? if not than why? it yes than how? plz guide

• Edvard
Oct 03, 2012

As far as I know there are dry type transformers with on-load tap changers available. It’s done with vacuum box-structure with some automatic voltage controller. OLTC can be controlled on site or from a some remote location.