ABB RELION REX640 Technical Manual page 750

Protection and control

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Protection functions

Figure 422: 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.

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

is determined by the connection polarities of the current transformer.

During an earth fault in the protected area, the currents ΣI and Io are directed

towards the protected area. The factor cosφ is 1 when the phase difference of the

residual current and the neutral current is 180 degrees, that is, when the currents

are in opposite direction at the earth faults within the protected area. Similarly,

ID_COSPHI is specified to be 0 when the phase difference between the residual

current and the neutral current is less than 90 degrees in situations where there is

Residual current

Phase difference between the residual and neutral currents

Neutral current

The current transformer ratio mismatch between the phase current

transformer and neutral current transformer (residual current in the

analog input settings) is taken into account by the function with the

properly set analog input setting values.

1MRS759142 F

(Equation 177)

CT connection type

Technical Manual