Schweitzer Engineering Laboratories SEL-787, Instruction Manual, Page: 202 / 564

5.14
SEL-787 Relay Instruction Manual Date Code 20081022
Metering and Monitoring
Through-Fault Event Monitoring
There are only four settings to set the though-fault event monitor, all under the
Through Fault category in Global Settings (SET G Command) (see
Table 5.10 ). Enable the through fault element by setting the SEL
OGIC
equation
ETHRFLT for the conditions that you what the element to run. Use Setting
THFLTD to select the winding that you want the element to use when
calculating the through-fault current. Switch S1 in
Figure 5.18 selects OFF or
one of windings 1 or 2. Set the through-fault alarm pickup (THFLTPU) to the
desired value, and enter the transformer percentage impedance at the XFMRZ
setting.
NOTE:
When you change the
ETHRFLT setting, the relay also clears
the data and records, i.e., it has the
same effect as the
TFE C or TFE R
command.
On the basis that mechanical stress takes effect only at high current values, the
through-fault element runs only if the selected phase current is greater than
4.75 times full load current. Allowing a hysteresis of 0.25 of full load, the
through-fault element resets when the current falls below 4.5 times the full
load current.
Figure 5.18 shows a functional diagram of the through-fault element for the
A-phase of Winding 1 (THFLTD = 1), the B-phase and C-phase elements
have identical diagrams. When SEL
OGIC
equation ETHRFLT asserts and the
A-phase current exceeds 4.75 times the transformer full-load current, Enable
asserts and the 1-minute Timer starts. When Enable asserts, the following
occurs:
➤
The thermal element advances the A-phase fault counter by
1 count.
➤
The thermal element advances the total fault counter by
1 count.
➤
The thermal element records the time when the fault starts
(rising edge of Enable).
➤
The process to determine the maximum through-fault current
for the fault duration starts.
➤
The integration process starts, whereby the element sums the
values (
Equation 5.3 ) calculated each processing interval (1/4
of a power system cycle).
Table 5.10 Through-Fault Element Settings
Setting Description Range
THFLTD Identifies the winding whose phase
currents will be used in the thermal element OFF, 1, 2
ETHRFLT Enable Through-Fault Monitor (SEL
OGIC
Equation)
SEL
OGIC
Equation
THFLTPU Through-Fault Alarm Pickup 50.0–900.0 %
XFMRZ Percentage Transformer Impedance 2.0–40.0 %