Biased low stage
The current differential protection needs to be biased because the possible
appearance of a differential current can be due to something else than an actual
fault in the transformer (or generator).
In the case of transformer protection, a false differential current can be caused by:
• CT errors
• Varying tap changer positions (if not automatically compensated)
• Transformer no-load current
• Transformer inrush currents
• Transformer overexcitation in overvoltage
• Underfrequency situations
• CT saturation at high currents passing through the transformer
The differential current caused by CT errors or tap changer positions increases at
the same percent ratio as the load current.
In the protection of generators, the false differential current can be caused by
various factors.
• CT errors
• CT saturation at high currents passing through the generator
1MRS757644 H
Protection functions
620 series
Technical Manual
527
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