Tettex 2293 - Application Hints; Core Magnetization; Winding Short-Circuits

Magnetic Balance
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13.6 Application hints
13.6.1 Core Magnetization
Magnetic Balance results are sensitive to Transformer Core Magnetization. To get reproducible
results, make sure to properly demagnetize the transformer core (directly) before performing a
magnetic balance test:
In a lot of cases, the built-in demagnetization function of the WA2293 will deliver good result.
In some cases, especially for larger power transformer (>50MVA), it can be necessary to use a
High Voltage AC power supply to properly demagnetize the transformer. Usually 110% of the
nominal winding voltage is necessary.
The following example shows an YNyn0 DryType Transformer in two cases:
The upper results show a proper measurement (green box)  Balance is nearly 50%/50% on the
middle phase.
The lower results show a measurement with a magnetized transformer core (red box)  Balance
is bad on the middle leg (26.3%/72.8%).
13.6.2 Winding short-circuits
A single turn fault can cause high magnetic imbalance. Also the excitation current can be greatly
influenced. The following example shows an YNyn0 DryType Transformer in two cases:
The upper results show a proper measurement (green box)  Balance is nearly 50%/50% on the
middle phase.
The lower results show a measurement with a single turn short-circuit on phase A of the high
voltage winding (red box)  Balance is very bad on the middle leg (8.66%/94.0%).