Woodward HighPROTEC - Page 831

Protective Elements
Typically, the impedance reach setting for such local backup protection is chosen as the smaller value out of the
values resulting from the following criteria:
• Set impedance reach to 120% of unit transformer to get enough sensitivity by faults inside transformer and
bus:
|Z
a
| = 1.2 ⋅ X
TG
⋅ Z
B,secondary
= 1.2 ⋅ 0.11607 ⋅ 17.56 Ω = 2.45 Ω
φ
a
= 90°
• 80% of zone 1 reach setting of the line distance relay on the shortest transmission line to get coordination
with the connected transmission line. Then the impedance reach setting for distance protection zone 1 is
equal to the unit transformer impedance plus 80% of the zone 1 line distance protection setting. Assuming
the line distance protection reach on the shortest line is set at 80% of the line, then the impedance reach is
calculated from the following:
(X
TG
+ 0.8 ⋅ (0.8 ⋅ Z
SLG1
)) ⋅ Z
B,secondary
= (j0.11607 + 0.8 ⋅ (0.8 ⋅ (0.02196 + j0.23219))) ⋅ 17.56 Ω
= (0.25 + j4.65) Ω
∣Z
b
∣=
√
0.25
2
+4.65
2
=4.6 Ω
φ
b
=tan
−1
(
4.65
0.25
)=87°
The setting parameter »Mho Pos.Imp.Reach« is set to the minimum of |Z
a
| and |Z
b
|, the angle is selected as 90°
(i.e. the resistive part is neglected) and no offset impedance is needed:
• »Mho Pos.Imp.Reach« = 2.5 Ω
• »Mho Pos.Imp.Angle« = 90°
• »Mho Offs.Imp.Reach« = 0 Ω
• »Mho Offs.Imp.Angle« = 0°
The time delay should be set greater than the tripping time of zone 1 of the shortest line distance protection (0.1 s)
plus the tripping time of the used breaker failure function (0.1 s), as well as the CB opening time (e.g. 0.1 s). With an
additional security margin of 0.1 s the resulting time setting is:
• »Trip delay time« = 0.4 s
This time delay gives the primary protection functions (generator differential 87G, transformer differential 87T and
overall differential 87GT as well as bus protection) enough time to trip before the operation of generator phase
distance protection function Z[1].
831 MCDGV4 DOK-HB-MCDGV4-2E