ABB 615 Series ANSI, Technical Manual

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1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 373
Technical Manual
4.6.3.2 Function block
Figure 176: Function block
4.6.3.3 Functionality
The restrained restricted low-impendence ground-fault protection 87LOZREF for a two
winding transformer is based on the numerically restrainedrestrained differential current
principle. No external stabilizing resistor or non-linear resistor are required.
The fundamental components of the currents are used for calculating the residual current
of the phase currents, the neutral current, differential currents and stabilizing currents. The
operating characteristics are according to the definite time.
87LOZREF contains a blocking functionality. The neutral-current second harmonic is
used for blocking during the transformer inrush situation. It is also possible to block
function outputs, timers or the function itself, if desired.
4.6.3.4 Operation principle
The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are Enable and Disable.
The operation of the restrained restricted low impedance ground-fault protection can be
described by using a module diagram. All the modules in the diagram are explained in the
next sections.
Figure 177: Functional module diagram
Ground-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 (
Σ I, residual current)