Siemens 7SA511 manual Figure A-6. Phase-to-Ground Short Circuit Loop

7SA511 Line Protection Relay Reference A

L1

L2

IL3	ZL
	L3
VL3-E
IE	ZE
	E

Figure A-6. Phase-to-Ground Short Circuit Loop

A.3.2 Fault Loop Determination in Grounded Systems

In power systems with grounded neutrals, each contact of a phase with ground is a short circuit that must be interrupted immediately by the nearest protective device. Fault detection occurs in the faulted loop.

With double faults, pickup normally occurs in two phases and ground (overcurrent detection) or for two phase-to-ground loops (impedance detection). In this case, it is possible to set the relay so that preference is given to one of the fault loops. Only the phase-to-phase loop, the loop of the leading phase-to-ground, or the loop of the lagging phase-to-ground will be selected.

Note: In grounded power systems, the relay must be connected to three wye-connected VTs with grounded primary common and to three CTs. The common connection for the CTs should be connected to the ground current input of the relay. If the current input of the relay is fed from other current sources, such as from the common connection of the CT set of a parallel line for parallel line compensation, the relay calculates the ground fault from the sum of the three phase currents. If the VTs have residual voltage windings, these should be connected in broken- delta to the voltage input to enable the relay to monitor the VTs’ secondary circuits.

Table A-6 and Table A-7 show the measurement quantities used for distance measurement in grounded systems.

July 27, 1995 A-17