Toshiba GRZ100 B Series, Instruction Manual

Page: 21 / 447

⎯
20
⎯
6 F 2 S 0 8 4 6
2.3 Multi-Terminal Line Protection
2.3.1 Increased Use of Multi-Terminal Lines
The number of multi-terminal transmission lines has increased in recent years, mainly for
economic reasons. For example, connecting three substations through three-terminal transmission
lines can reduce the construction cost considerably compared to connecting substations through
individual lines. On the other hand, from the standpoint of protection, multi-terminal lines cause
various difficulties. Taking an example of a three-terminal line, these problems are illustrated
below.
2.3.2 Protection Problems on Three-Terminal Application
2.3.2.1 Underreach in the Case of an Internal Fault Further than the Branch Point
In the three-terminal line shown in Figure 2.3.2.1, if a phase fault occurs near terminal C, the fault
current flows in from both terminal A and terminal B and the voltages at terminal A and terminal
B are influenced by the current from one another, have represented by the following equations:
VA = IA
×
(Z1 + Z3) + IB
×
Z3.....................................(2-15)
VB = IB
×
(Z2 + Z3) + IA
×
Z3 .....................................(2-16)
where, VA: Voltage at terminal A
VB: Voltage at terminal B
IA: Current at terminal A
IB: Current at terminal B
Z1: Impedance from terminal A to branch point
Z2: Impedance from terminal B to branch point
Z3: Impedance from fault point to branch point
From equations (2-15) and (2-16), impedance ZA and impedance ZB seen from the relay at
terminal A and terminal B are given by the following equations:
ZA = VA/IA = (Z1 + Z3) + Z3
×
IB/IA .........................(2-17)
ZB = VB/IB = (Z2 + Z3) + Z3
×
IA/IB..........................(2-18)
From equation (2-17), the impedance seen from the relay at terminal A is greater than the actual
impedance (Z1 + Z3) up to the fault point by (Z3
×
IB/IA). That is, if the current infeed from local
terminal A is large its influence is small, but if the current infeed from local terminal B is large the
relay sees the fault point much further than the actual distance.
Z3 Z1
Terminal A
Terminal C
Terminal B
Z2
Figure 2.3.2.1 Three-terminal line