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2. 1
INTRODUCTION
Both the REL 301/302 relay systems detect faults in three zones of, phase and ground
distance Zones1 and 2 are forward set; Zone3 can be set to forward or reverse. The
fault locator can be set to indicate fault distance in miles or kilometers. REL 302 has
a separate pilot Zone (Section 2-5).
The R-X Diagram, shown in Figure 2-1, shows a composite of characteristics available
with REL 301/302. Zone1 phase and ground settings are chosen to provide substan-
tial coverage of the protected line without overreaching the next bus. A setting of 80%
of the line impedance is typical. Faults occurring within the reach of the Zone1 mea-
surement cause direct tripping without regard to any action occurring at the remote
terminal.
Zone2 settings are chosen to assure that faults occurring on the next bus are detected.
Settings are chosen (independent of the Zone1 settings), generally to be 120 to 150%
of the line impedance. Any fault occurring on the protected line will be detected by this
Zone2 measurement (within the fault resistance and current limitations of the relaying
system). Zone2 tripping occurs after time delay (T2).
The Zone3 measurement is directional, and may be chosen to respond to forward or
reverse faults. The reverse sensing option is used in conjunction with the T3 trip func-
tion, chosen to coordinate with adjacent terminal Zone2 timing. The forward sensing
option produces time delayed backup to other devices sensing forward faults.
Blinder measurements (B1, B2, B3, B4) are available for out-of-step sensing. The inner
blinder also restricts the trip reach of each of the 3-phase fault measuring units.
2. 2
LINE MEASUREMENT TECHNIQUES
Line measurement techniques applied to each Zone include:
• Single-Phase-To-Ground fault detection
• 3-Phase fault detection
• Phase-to-Phase fault detection
• Phase-to-Phase-to-Ground fault detection
2.2.1
Single-Phase-to-Ground Fault
Single-phase-to-ground fault detection (Figure 2-2) is accomplished by three self-po-
larized phase-ground units (ph-A, ph-B, ph-C). Equations (1) and (2) below are for Op-
erate and Reference quantities, respectively. The unit will produce output when the
Operate quantity leads the Reference quantity in phase angle. The Reference quantity
has been modified from earlier versions, in order to produce a fixed mho characteristic
which will have better performance when applied to resistance grounded systems. The
"forward" reach applies to the operate (TRIP) direction of the unit, while the "reverse"
reach applies to the restraint direction and is used primarily to define the overall size
of the characteristic and the amount of each along the R-axis. (In the case of the re-
versible Zone3, the "forward" reach actually is in the reverse line direction, since that
(10/94)
Section 2. FUNCTIONAL DESCRIPTION
I.L. 40-386.3
2-1
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