Pilot Scheme / Phase Selector Coordination; Cross-Country Fault Example - GE C60 Instruction Manual

CHAPTER 8: THEORY OF OPERATION
Table 8-22: Echo table for four-bit channels
Local determination of fault type
AG
BG
CG
AB, ABG, BC, BCG, CA, CAG, 3P, Unrecognized
For the directional comparison unblocking scheme, the echo is performed in the same manner as the Hybrid POTT
echo, but by applying additionally the following logic for both the Rx (received) and LOG (loss-of-guard) for each
channel: ECHO = LOG
N

8.2.5 Pilot scheme / phase selector coordination

The pilot schemes use the Phase Selector for local fault type identification. The latter can fail to respond to certain fault
scenarios. Examples are simultaneous forward and reverse fault, simultaneous SLG and LL fault involving different phases
(for example AG and BC), or two simultaneous faults in the same direction but at very different locations. The Phase
Selector is optimized to either indicate correctly the forward fault or to assert the Void flag. For example, a combination of
AG and BC is not a valid fault type; it is two different simultaneous faults and as such cannot be described by any single
fault pattern, therefore, the Phase Selector asserts the Void flag.
The VOID phase selection combined with a local trip request (such as high-set directional overcurrent) results in a three-
pole trip as per the Trip Output logic.
The Pilot Schemes, however, try to recover more information from the distance elements. Each scheme uses a forward
looking, either underreaching or overreaching, distance zone. A given Pilot Scheme analyzes this zone for fault type
identification if the Phase Selector asserts its Void flag: the DUTT scheme uses Z1; all the other schemes use Z2. The
schemes analyze all six fault loops of the zone to determine the fault type.
For example, simultaneous forward AG and reverse BG faults can result in the Void indication. The POTT scheme would
analyze the Z2 response. As only the AG element is picked up, the local phase selection is determined as AG. This is a
correct indication.
Depending on the number of bits used for communications, the accuracy of the overall response can be further improved
as illustrated in the next section.
This enhanced operation of the pilot-aided schemes is the reason to use a short pilot scheme priority time when setting the
Trip Output logic. The timer forces the scheme to wait for a decision from the pilot scheme for a short period of time before
accepting any local trip request. The advantage, however, materializes only if more than one-bit communications
channels are used, and is important only on parallel lines or when the application requires maximum accuracy of single-
pole tripping. In other cases, it is not recommended to delay the local trip decision.

8.2.6 Cross-country fault example

Assume a single pole operation application where C60 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.
C60 BREAKER PROTECTION SYSTEM – INSTRUCTION MANUAL
Echoed bits
TX1 TX2
1 0
0 1
0 0
Send back as received
AND RX .
N N
TX3 TX4
0 0
0 0
1 0
SINGLE-POLE TRIPPING
8-17
8