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Dry Transformer Percent Impedance Definition

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Dry Transformer Percent Impedance Definition
Dry Transformer Percent Impedance Definition (on photo dry type transformer by Engineering company B&S, Ukraine)

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 kVAMinimum Impedance, %
0 – 150Manufacturer’s standard
151 – 3004
301 – 6005
601 – 2,5006
2,501 – 5,0006.5
5,001 – 7,5007.5
7,501 – 10,0008.5
Above 10,0009.5

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

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Edvard Csanyi

Edvard - 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