Sel SEL-787 - Table 5.10 Through-Fault Element Settings

5.14
SEL-787 Relay Instruction Manual Date Code 20150130
Metering and Monitoring
Through-Fault Event Monitoring
Equation 5.3 show the three equations that the element uses to evaluate the
thermal curve each processing interval. Note that the calculated currents are in
primary values. To convert the secondary current to primary current, the element
multiplies the secondary current by the CT ratio of the particular winding.
Equation 5.1
Equation 5.2
Equation 5.3
There are only four settings to set the through-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 want the element to run. The default setting for
ETHRFLT := NOT TRXFMR disables Through-Fault Monitor for all
transformer internal faults (Relay Word Bit TRFXMR := 1).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.
NOTE: The MVA setting must be set
to the transformer MVA rating when
the THFLTD setting is set to 1 or 2.
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.
where:
I = Measured current
S = Transformer MVA rating (MVA)
kV
LL
= Line-to-line voltage (kV)
Z
PU
= Transformer impedance (per unit)
K = 1250 if (4.5  I
PU
0.5 • I
MAX_PU
)
or
2 • (I
MAX_PU
)
2
if I
PU
0.5 • I
MAX_PU
I
PU
=
I3kV
LL

1000 S
----------------------------------
I
MAX_PU
=
1
Z
PU
----------
tI
PU
 =
K
I
PU

2
----------------
Table 5.10 Through-Fault Element Settings
Setting Prompt Setting Range Setting Name := Factory Default
THR FLT WDG OFF, 1, 2 THFLTD := OFF
ENABLE THR FLT SEL
OGIC ETHRFLT := NOT TRXFMR
THR FLT ALARM PU 50.0–900.0 % THFLTPU := 100.0
XFMR IMPEDANCE 2.0–40.0 % XFMRZ := 10.0