Siemens 7SA511 manual Polygonal Impedance Fault Detection Optional, Phase-Phase

7SA511 Line Protection Relay Method of Operation

phase pickup without ground fault detection. In ungrounded system, the multi-phase loop is always selected in the case of a single-phase pickup without ground fault detection.

The faulty phases are indicated (see Table A-1). The overcurrent fault detector is reset when the pickup value is below 95%.

Table A-1.Single-Phase Overcurrent Fault Detection

Fault Detection

Module

L1

L2

L3

L1

L2

L3

L1

L2

L3

Ground Fault

Detection

no no no

no no no

yes yes yes

Parameter (1705)

1PH FAULTS

PHASE-PHASE

(Uph - Uph)

PHASE-EARTH *

(Uph - E)

irrelevant

Resultant Loop

L3-L1

L1-L2

L2-L3

L1-E

L2-E

L3-E

L1-E

L2-E

L3-E

Annunciation

L1

L2

L3

L1

L2

L3

L1, E

L2, E

L3, E

* Only applicable for grounded systems.

A.2.3 Polygonal Impedance Fault Detection (Optional)

Polygonal impedance fault detection is a loop-dedicated fault detection procedure. Either the three phase-to-phase loops (without ground fault) or the three phase-to-ground loops (with ground fault) are monitored depending on the result of the ground fault detection. For measurement of a loop impedance, at least one of the assigned phase currents as well as the difference current decisive for the loop must have exceeded the minimum operating current value Iph>.

The impedances are calculated separately for resistance, R, and reactance, X, in cyclic time intervals and compared with the set values of any of the six line loops for which the current conditions are met. A measured value step change monitor is used to synchronize the measurement window on occurrence of a fault. The calculation procedure is the same as that described for distance measurement.

The polygonal impedance fault detector utilizes a special “dog bone” characteristic (Figure A-3) to provide secure pickup for faults without pickup on load. The X-intersections X+A and X-Aare decisive for the fault reach in the forward and reverse directions respectively. The R-intersections can be set differently for multi-phase loops (RA1) and phase-to-ground loops (RA1E). It is therefore possible, for example, to permit a larger fault resistance tolerance for ground faults (bold dotted lines in Figure A-3). Extension of the polygon in the R-direction is only valid when a ground fault has been detected as described in the previous section.

In order to guarantee the criteria for distinction between load operation and a short-circuit, especially in the case of long, heavily loaded lines, the characteristics can be set dependent on the

A-10 July 27, 1995