ABB RELION 670 Series Technical Manual page 133

Generator protection

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1MRK 502 066-UUS B

The unrestrained (that is, non-stabilized, "instantaneous") part of the differential protection is

used for very high differential currents, where it should be beyond any doubt, that the fault is

internal. This settable limit is constant and not proportional to the bias current. Neither harmonic,

nor any other restrain is applied to this limit, which is therefore allowed to trip the power

transformer instantaneously.

The restrained (stabilized) part of the differential protection compares the calculated

fundamental differential (operating) currents and the bias (restrain) current, by applying them to

the operate - restrain characteristic. Practically, the magnitudes of the individual fundamental

frequency differential currents are compared with an adaptive limit. This limit is adaptive because

it is dependent on the bias (that is, restrain) current magnitude. This limit is called the operate -

restrain characteristic. It is represented by a double-slope, double-breakpoint characteristic, as

shown in figure 36. The restrained characteristic is determined by the following 5 settings:

IdMin (Sensitivity in section 1, multiple of trans. HV side rated current set under the parameter

RatedCurrentW1 )

EndSection1 (End of section 1, as multiple of transformer HV side rated current set under the

EndSection2 (End of section 2, as multiple of transformer HV side rated current set under the

SlopeSection2 (Slope in section 2, as multiple of transformer HV side rated current set under

the parameter

SlopeSection3 (Slope in section 2, as multiple of transformer HV side rated current set under

the parameter

The restrained characteristic in figure

SlopeSection2

SlopeSection3

Technical manual

RatedCurrentW1 )

is defined by the settings:

Differential protection

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