Sel 2414 - Page 106

5.38
SEL-2414 Transformer Monitor Instruction Manual Date Code 20130214
Metering and Monitoring
Monitoring
Equation 5.23
Equation 5.24
See Table 5.18 for the through-fault element settings. Enable the element by
setting the SEL
OGIC control equation ETHRFLT for the conditions under which
you want the element to run. Set the through-fault alarm pickup (THFLTPU) to
the desired value, and enter the transformer percentage impedance (in percent) at
the TRFRZ setting.
On the basis that mechanical stress takes effect only at high current values, the
through-fault element runs only if the 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 5.27 shows a functional diagram of the through-fault element for the
A-phase, the B-phase and C-phase elements having identical diagrams. When
SEL
OGIC control equation ETHRFLT is asserted and the A-phase current
exceeds 4.75 times the transformer full-load current, Enable asserts and the
10-cycle timer starts. When Enable asserts, the following occur:
➤ 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 calculated each processing interval (1/4 of a power system
cycle)
where:
I
MAG
= 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
MAG_PU
≤ 0.5 • I
MAX_PU
2 • (I
MAX_PU
)
2
if I
MAG_PU
> 0.5 • I
MAX_PU
Table 5.18 Default Transformer Constants
Setting Description Range
ETHFLTM Enable Through-Fault Monitoring
(Y,N)
ETHRFLT Enable Through-Fault Monitor (SEL
OGIC control
equation). Deassert during testing.
SELOGIC control
equation
THFLTPU Through-Fault Alarm Pickup 50.0–900.0%
TRFRZ Percentage Transformer Impedance 2.0–40.0%
I
MAX_PU
=
100
Z
PU
----------
tI
MAG_PU
() =
K
I
MAG_PU
()
2
-------------------------------