ABB 615 Series ANSI, Техническое руководство

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 345
Technical Manual
Waveform blocking
The restrained differential can always be blocked with waveform blocking but it cannot be
disabled with the Restraint mode parameter. This algorithm has two parts. The first part is
intended for external faults while the second is intended for inrush situations. The
algorithm has criteria for a low current period during inrush where also the differential
current (not derivative) is checked.
Restrained differential (low set)
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:
• CT errors
• CT saturation at high currents passing through the generator.