Toshiba GR200 Series Instruction Manual page 64

Special characteristic in ZS
(ii)
Overreach countermeasure
If we consider ZS, when a phase-to-phase fault occurs, the setting of the ZS reactance element
tends to be less than the fault impedance. For example, Figure 2.1-20 shows the relationship
between the phase-to-phase impedances in a three-phase system when a phase-to-phase BC
fault arises across Bus H. Here, ZS is not required to operate because the fault lies outside of
the protection zone. However, ZS may operate because the impedance of the phase-to-phase
impedance AB is located inside the setting of the reactance element (X
phase-to-phase protection element AB, which is not connected with the faulted phase, may
operate although the other elements do not operate.
X
F: Fault location
S: Infinite bus system
O: Location of Protective relay
Z : Fault impedance seen from S
L1
Z : Fault impedance of phase-to-phase bc in the fault
bc
Figure 2.1-20 Impedance seen from ZS on occurrence of phase-to-phase BC fault
The previous phenomenon is referred to as overreaching of the leading phase in distance
protection terminology. A countermeasure is necessary to deal with this phenomenon. The
countermeasure is to utilize a Zmin element as shown in section 2.1.7. It is generally
recognised as being the result of a phase-to-phase fault impedance which is smaller than the
normal impedance.In the above instance, the element Zmin-BC of the faulted phase would
operate but the Zmin-AB and Zmin-CA elements of the healthy phases would not operate, so
that the unnecessary overreaching operation of ZS-AB can be blocked.
Z
ca
1
Bus G
O
ZS
- 43 -
jX
Z
bc
o
60°
60°
Operation zone of X
S
element
I
a
Z
bc
Z
L1
6F2S1915 (0.46)
). This means that the
1
Z
ab
R
1
Bus H
F
GRZ200 (Soft 031 & 032)