GE L90

10-50 L90 LINE CURRENT DIFFERENTIAL SYSTEM – INSTRUCTION MANUAL
SINGLE-POLE TRIPPING CHAPTER 10: THEORY OF OPERATION
10
10.5.6 Cross-country fault example
Assume a single pole operation application where L90 relays are used to protect a two-terminal line, (terminals T1 and T2)
using phase and ground distance zone 1, 2, and 3 elements in a permissive over-reaching transfer trip scheme. The
performance of the system with one, two, and four-bit communications channels is outlined for a mid-line phase A-to-
ground fault and a co-incident phase B-to-ground fault just behind terminal T2. Assume also that the reclosers are enabled
and reset.
At T1 the following protection elements pick up:
• Ground Distance Zone 1, 2, and 3 for an AG fault
• Ground Distance Zone 2 and 3 for a BG fault
• Phase Distance Zone 2 and 3 for an AB fault
At T1, the phase selector determines the fault is type ABG. This response is independent from the distance elements; the
Phase Selector sees two forward faults.
At T2, the following protection elements pick up:
• Ground Distance Zone 1, 2, and 3 for an AG fault
At T2 the phase selector determines the fault is type AG. The reverse BG fault is likely to be ignored.
If a one-bit channel is used, terminal T1 trips three poles but terminal T2 trips phase A only, which is undesirable. See the
table.
Table 10-34: One-bit channel tripping
If a two-bit channel is used, both terminals trip phase A only, which is desired. See the following table.
Table 10-35: Two-bit channel tripping
If a four-bit channel is used, both terminals trip phase A only, which is desired. See the following table.
Table 10-36: Four-bit channel tripping
Terminal Remote data Local data
Bit pattern received Remote
determination of
fault type
Local determination
of fault type
Trip output
RX1
T1 1 Any MULTI-P (ABG) Trip Three Phases
T2 1 Any AG Trip Phase A
Terminal Remote data Local data
Bit pattern received Remote
determination of
fault type
Local determination
of fault type
Trip output
RX1 RX2
T1 1 0 AG ABG Trip Phase A
T2 1 1 ABG AG Trip Phase A
Terminal Remote data Local data
Bit pattern received Remote
determination Of
fault type
Local determination
of fault type
Trip output
RX1 RX2 RX3 RX4
T1 0 0 0 1 AG ABG Trip Phase A
T2 1 0 0 0 ABG AG Trip Phase A