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For indication of a phase distance trip the following conditions have to occur:
·
For a three phase fault, the RT (inner) blinder and no OSB (out of step block) and the 3
selector and any of the phase ground units have to have operated.
·
For a phase to phase fault, only the phase to phase unit needs to operate.
·
For a phase to phase to ground fault, either the phase to phase unit or the phase to ground units have to operate.
·
For a single phase to ground fault, any of the phase to ground units has to operate.
The phase fault detection is supervised by I
The phase to ground fault detection is supervised by I
directional unit FDOG.
The three phase fault detection is supervised by the phase directional units FDOP and the load restriction blinder, RT.
Each zone has its own timer to coordinate with relays further away for step distance relaying. Separate phase and
ground timers are provided.
Single Phase to Ground
Single phase to ground fault detection is accomplished by 3 non-directional phase units, A, B and C. The operating and
reference quantities are:
Z
=
−
+
0
L
OP
V
I
XG
X
=
+
+
REF
j
V
I
XG
X
V
= V
, V
or V
XG
AG
BG
CG
I
= I
, I
or I
X
A
B
C
I
= zero sequence relay current
0
Z
, Z
= positive and zero sequence line impedance
1L
0L
Z
= forward zone reach setting in secondary ohms
FG
Z
= reverse reach setting in secondary ohms
RG
The unit will produce an output when the operating quantity leads the reference quantity.
Measuring Elements and Operational Logic
, the low set phase overcurrent unit.
L
−
Z
⋅
⋅
1
L
I
Z
0
FG
Z
1
L
−
Z
Z
⋅
⋅
0
L
1
L
I
Z
0
RG
Z
1
L
ABB REL 356 Current Differential Protection
, the medium set ground overcurrent unit and the ground
OM
F
F output of the phase
4-9
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