Directional Earth Fault Polarity Check (67N) - Siemens Argus 7SR21 Technical Manual

7SR210 & 7SR220 Commissioning & Maintenance Guide
2.2

Directional Earth Fault Polarity Check (67N)

Derived Earth Fault, Measured Earth Fault and Sensitive Earth Fault elements can be set as directional. These
are polarised from residual voltage, calculated from the 3 phase voltage inputs or the 3Vo input depending on the
Phase Voltage Config setting in the CT/VT Config menu.
The relay Char Angle setting is the Characteristic Phase angle of the fault impedance i.e. the phase angle of the
fault current with respect to the voltage driving the current. The earth fault functions are polarised from the
residual voltage which is in anti-phase with the fault voltage for a single-phase to earth fault. Care is required
when testing by secondary injection with regard to current and voltage polarity.
To simulate an earth fault on a relay with 3 phase-phase or 3 phase-neutral connected voltage inputs, defined by
the Phase Voltage Config setting of Van,Vbn,Vcn or Va,Vb,Vc, proceed as follows. Balanced 3P voltage should
first be applied, then the phase-neutral voltage magnitude on the faulted phase should be reduced in magnitude
with no change in phase angle to produce Vres and simulate the fault. The fault current, on the faulted phase
only, should be set at the MTA with respect to the phase-neutral voltage on the faulted phase, e.g. for a relay
setting of -15º, set the phase current to lag the ph-n voltage by 15º.
Alternatively, a single phase voltage source can be used in the above test. The polarity of this voltage, applied to
the faulted phase-neutral alone, must be reversed to produce the same residual voltage (Vres) phase direction as
that produced by the 3P voltage simulation described above.
For the Phase Voltage Config of Vab, Vbc, Vo, the single phase voltage applied to the Vo input is used as the
polarising quantity. The inversion is once again required since this input is designed to measure the residual
voltage directly, as produced by an 'open delta VT' arrangement. The current must be set at the MTA with respect
to the inversion of this voltage. e.g. for a relay setting of -15º, the phase current must lag the (Vo+180º) voltage by
15º, i.e. if Vo is set at 180º, set Iph at -15º.
If the Pickup of one directional Earth Fault element is mapped to an LED, this can be used to check directional
boundaries for pickup and drop-off as the current phase angle is increased and decreased. Note that the Derived
Earth Fault, Measured Earth Fault and Sensitive Earth Fault have separate directional settings and must be
tested individually.
Figure 2.2-1 Directional Earth Fault Boundary System Angles
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