## 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.

For a two winding transformer with a * 5% impedance*, it would require

*applied on the high voltage winding to draw 100% rated current on the secondary winding when the secondary winding is short-circuited.*

**5% input voltage**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* X_{t} = (kV^{2}/MVA) x %Z/100 = (0.48^{2} / 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* X_{t} = (kV^{2}/MVA) x %Z/100 = (0.48^{2} / 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