Some of these conditions are convenient and some are mandatory. The convenient conditions are: Same voltage Ratio and Turns Ratio, Same Percentage Impedance, Same KVA Rating, Same Position of Tap changer. The mandatory conditions are: Same Phase Angle Shift, Same Polarity, Same Phase Sequence and Same Frequency. When the convenient conditions are not met paralleled operation is possible but not optimal.
The transformer core can saturate due to, High Voltage, Low Frequency or a combination of both. Protection element V/Hz (ANSI 24) can be used to protect the transformer against over excitation as this can cause damage due to overheating / excessive core loss. This protection element is commonly used on generator step-up transformers.
In 3 leg three phase transformer, do we get some phase voltage magnitude and angle change due to un-symmetrical reluctance seen by every different phases? The phase at the middle leg is not distorted because fluxes come from both rear legs equally in magnitude. But voltage and angle of rear legs change somewhat because the other two phases' flux come to rear legs in different magnitudes. So this distorts both magnitude and angle of voltage.
What will happen when we back-charge 220 kV / 20 kV, 500 MVA Generator transformer through 75 MVA, 220/132 kV Grid Auto Transformer. Whether the 75 MVA Grid Auto Transformer can successfully back-charge the 500 MVA Generator transformer? If not, what can be the optimum transformer size to back-charge the 500 MVA Generator transformer.
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