ABB REG650 Technical Manual page 102

Generator protection

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Differential protection

sequence current contributions, respectively, defined by expression shown in

equation 23. It performs a directional comparison between these two phasors. Taking

into account the phase rotation transformation the relative phase displacement between

the two negative sequence current phasors is calculated. In case of three-winding

power transformers, a little more complex algorithm is applied, with two directional

tests. The overall directional characteristic of the internal/external fault discriminator is

shown in figure 33, where the directional characteristic is defined by two setting

If one or the

currents is too

low, then no

is done, and

IEC05000188 V3 EN

Operating characteristic of the internal/external fault discriminator

In order to perform directional comparison of the two phasors their magnitudes must

be high enough so that one can be sure that they are due to a fault. On the other hand,

in order to guarantee a good sensitivity of the internal/external fault discriminator, the

value of this minimum limit must not be too high. Note that, in order to enhance

stability at higher fault currents, the relatively very low threshold value IminNegSeq is

dynamically increased at currents higher than normal currents: if the bias current is

higher than 110% of IBase current, then 10% of the bias current is added to the

IminNegSeq. Only if magnitudes of both negative sequence current contributions are

Internal/external

fault boundary

en05000188-3-en.vsd

1MRK 502 043-UUS B

Technical Manual