GE P743 - Earth Fault Protection; EF Time Delay Characteristics; External Fault Detection by High-Set Overcurrent;Earth Fault Element; Zero Sequence Current Supervision

P74x/EN OP/
Na7
Operation
(OP) 5
-
26
MiCOM P741, P742
, P743
OP
The definite time can be set (as defined in IEC) to zero. Range 0 to 100 seconds in steps of
0.01 seconds.
The Inverse Reset characteristics are dependent upon the selected IEEE/US IDMT curve as
shown in the table below.
All inverse reset curves conform to the following formula:
(
)








−
×






=
α
IsI
1
tr
7
TD
t
Reset
Where: t
Reset
= Reset time
tr = Constant
Ι = Measured current
Ι
S
= Current threshold setting
α = Cconstant
TD = Time Dial Setting (Same setting as that employed by IDMT curve)
IEEE/US IDMT Curve description Standard tr Constant
α Constant
Moderately Inverse IEEE 4.85 2
Very Inverse IEEE 21.6 2
Extremely Inverse IEEE 29.1 2
Inverse US-C08 5.95 2
Short Time Inverse US-C02 2.261 2
Inverse Reset Characteristics
1.5 Earth Fault Protection
1.5.1 EF time delay characteristics
The earth-fault measuring elements for EF and SEF are followed by an independently
selectable time delay. These time delays are identical to those of the Phase Overcurrent
time delay. The reset time delay is the same as the Phase overcurrent reset time.
1.5.2 External Fault Detection by High-Set Overcurrent or Earth Fault Element
An ultra high-speed detection is carried out by each of the peripheral units (P742 and P743)
and can generate a blocking signal from the moment of the first sample at 0.42 ms.
In this scenario de-saturation may not occur until after the scheme has eliminated the
saturation condition for the external fault.
This function can be activated independently for phase faults (Ι>2) and for Earth Faults
(Ι
N
>2).
1.5.3 Supervision
1.5.4 Zero Sequence Current (Ι
O
) Supervision.
The four current inputs (A, B, C, N) of the Peripheral Units are used to verify that the
calculated zero sequence current is within the correct range for CT supervision purposes.
This then provides continuous supervision of the relay's measuring chain (internal CTs, ADC,
etc…).
The residual current 3Ι
o
is derived from the three phases Ι
a
+ Ι
b
+ Ι
c
and compared to the
measured value of Ι
N
from the neutral CT input.
|3
Ι
O
- Ι
N
|