**Document**

**Name**

# The Voltech Handbook of Transformer Testing

**Description**

1 Transformer Basics |

2 Available Tests |

2.1 CTY - Continuity |

2.2 R - Winding Resistance |

2.3 RLS or RLP - Equivalent Series or Parallel R |

2.4 LS_LP - Primary Inductance |

2.5 LSB_LPB - Inductance With Bias Current |

2.6 QL - Q factor |

2.7 D - Dissipation Factor |

2.8 LL - Leakage Inductance |

2.9 C - Inter-winding Capacitance |

2.10 TR - Turns Ratio and Phasing |

2.11 TRL - Turns Ratio by Inductance |

2.12 Z_ZB - Impedance_Impedance with Bias |

2.13 R2 - DC Resistance Match |

2.14 L2 - Inductance Match |

2.15 C2 - Capacitance Match |

2.16 GBAL - General Longitudinal Balance |

2.17 LBAL - Longitudinal Balance |

2.18 ILOS - Insertion Loss |

2.19 RESP - Frequency Response |

2.20 RLOS - Return Loss |

2.21 ANGL - Impedance Phase Angle |

2.22 PHAS - Inter-winding Phase Test |

2.23 TRIM - Trimming Adjustment |

2.24 OUT - Output To User Port |

2.25 IR - Insulation Resistance |

2.26 HPDC - DC HI-POT |

2.27 HPAC - AC HI-POT |

2.28 SURG - Surge Stress Test |

2.29 STRW - Stress Watts |

2.30 MAGI - Magnetizing Current |

2.31 VOC - Open Circuit Voltage |

2.32 WATX - Wattage (External Source) |

2.33 STRX - Stress Watts (External Source) |

2.34 MAGX - Magnetizing Current (Ext. Source) |

2.35 VOCX - O.C. Voltage (External Source) |

2.36 LVOC - Low Voltage Open Circuit |

2.37 ILK - Leakage Current |

2.38 LSBX - Inductance with External Bias (Series) |

2.39 LPBX - Inductance with External Bias (Parallel) |

2.40 ZBX - Impedance with External Bias |

2.41 ACRT - AC HI-POT Ramp |

2.42 DCAT - DC HI-POT Ramp |

2.43 ACVB - AC Voltage Break Down |

2.44 DCVB - DC Voltage Break Down |

2.45 WATT - Wattage |

2.0 Available Tests On The AT Series |

3 Examples of Different Transformer Types |

## 2.28 SURG - Surge Stress Test

### Where Used

This test may be used to highlight a short-circuit between adjacent turns in a winding. It is applicable to any transformer, but is particularly suitable for transformers with a large number of turns using very fine wire.

For such wire, the enamel coating is very thin, and there is a danger that it will be scratched, giving rise to exposed copper. In some cases, the scratch does not immediately cause a shorted turn, but will leave a weak spot which may eventually fail.

By applying a higher than normal voltage across the winding, any weakness in wire insulation will be encouraged to fail.

### Measurement Method

Each SURG test can be programmed to consist of a number of impulses. For each impulse, the AT will charge an internal capacitor to the high voltage specified. This stored charge will then be suddenly discharged into the winding-under-test, and the resulting transient voltage will be analysed. The product from the discharge will be a sinusoidal wave with decaying amplitude.

Figure 44

Where t_{s} = The time of releasing the impulse

V_{P} = The peak voltage just after switch-on

At the start of the surge test, the AT performs an initial run to compensate for the effect of the capacitance of the transformer winding.

Without this compensation, the peak voltage would be reduced by charge sharing between the winding capacitance and the reservoir capacitance within the AT, and would not be the value you require.

Therefore, the full test sequence is a follows: -

Preliminary Impulse: - The value of V_{P} is measured, and the
starting conditions changed to compensate for the charge sharing effects.

Impulse #1: The value of VP is again checked. If it is as specified for the test, this becomes the first impulse of the sequence, and the transient is analysed. (If not it is treated as a second preliminary impulse, and impulse #1 is repeated.)

Repeated impulses, up to the number programmed for the test.

Impulse #n: - The value of VP is re-checked, and the transient analysed on each impulse of the sequence.

### Transient Analysis

During the decay phase after the impulse has been fired, the AT measures both the voltage amplitude along the transient, and the time of decay.

A good transformer will have a clean and sustained transient, with a long decay period. A transformer with a shorted turn will have a heavily damped response, with a shorter decay period.

The calculation performed is to calculate the area underneath the graphical plot of the decaying transient. (For the calculation used, both negative peaks and positive peaks add to the total area.)

The area, measured in Volts-seconds, is much smaller for the faulty winding with a shorted turn.

### Specifying the Test Limits.

It is very difficult to predict the Volts-seconds area under the curve
from theoretical calculations.

The recommended method is to use the Measure Mode (see Chapters 3 & 6)
to obtain some values.

The procedure is as follows: -

Measure the area on a known good transformer; let this result be area A_{G}.

Wrap an additional single turn round the core, short the two ends
together, and re-measure the area; let this result be area A_{F}.

Set the limits as follows:

Max Area = 3A_{G}/2

Min Area = (A_{G} + A_{F})/2

Remember that these limits are taken from only one transformer, and may
need to be revised after more have been tested.