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
- The impedance of a two-winding transformer shall not vary from the guaranteed value by more that ± 7.5%
- 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%
- The impedance of an auto-transformer shall not vary from the guaranteed value by more than ± 10%
- The difference of impedances between transformers of the same design shall not exceed 10% of the guaranteed values
- 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.
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)
Resource: Substation Comissioning Course – Dry Type Transformer
Related electrical guides & articles
Dear Sir,
I would like to know what standard is applied in establishing the acceptable criteria of transformer impedance.
Thank you.
How should impedance be measured when additional bus work is provided to integrate adjacent switchgear in the project? Should the secondary short circuit be as short as possible or should it include the additional bus?
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
I need Genrator impidence formula / calculation
Prefer if any software is available for this.
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?
No need 6.4%for this capacity…if more %of impedance affects real power…better to go with manufacturers standard
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.
My transformer is not OLTC.
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?
OLTC can be implemented for Dry Type Transformers? if not than why? it yes than how? plz guide
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.