ABB RELION REX640 Technical Manual page 668

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Protection functions

The operation of LREFPNDF can be described using a module diagram. All the

modules in the diagram are explained in the next sections.

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Functional module diagram

Earth-fault detector

The operation is based on comparing the amplitude and the phase difference between

the sum of the fundamental frequency component of the phase currents (ΣI, residual

current) and the fundamental frequency component of the neutral current (Io) flowing

in the conductor between the transformer or generator's neutral point and earth. The

differential current is calculated as the absolute value of the difference between the

residual current, that is, the sum of the fundamental frequency components of the

phase currents I_A, I_B and I_C, and the neutral current. The directional differential

current ID_COSPHI is the product of the differential current and cosφ. The value is

available in the monitored data view.

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An earth fault occurring in the protected area, that is, between the phase CTs and the

neutral connection CT, causes a differential current. The directions, that is, the phase

difference of the residual current and the neutral current, are considered in the

operation criteria to maintain selectivity. A correct value for CT connection type is

determined by the connection polarities of the current transformer.

The current transformer ratio mismatch between the phase current

transformer and neutral current transformer (residual current in the

Residual current

Phase difference between the residual and neutral currents

Neutral current

1MRS759142 C

(Equation 124)

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