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Two step definite and inverse time
delayed phase overcurrent protection
(TOC2)
If inverse time characteristics are used with equal current and time setting for all phase
current protections in the system the selectivity is assured as long as there are more than
two bays carrying fault current to each substation. Sometimes this is however impossi-
ble due to the fault current distribution between the different lines.
If definite time characteristic is used the co-ordination between the different phase over-
current line protections is done by means of current setting.
As the phase overcurrent protection often is used as a back-up protection of lines, where
a distance protection is the main protection, relatively long operation times are accept-
able for the phase overcurrent protection.
The following formulas are valid for the inverse time characteristic:
7DEOH
)RUPXODV IRU WKH LQYHUVH WLPH FKDUDFWHULVWLF
&KDUDFWHULVWLF
Normal inverse
Very inverse
Extremely inverse
RI inverse
where:
I
denotes (measured current)/ I>Inv and
k
is a time multiplier factor, settable in the range of 0.05 to 1.10.
6HWWLQJ RI UHOD\ RSHUDWLQJ FXUUHQW
If Is is the primary setting operating value of the function, then the secondary setting
current (Is
) is:
SEC
7LPH GHOD\ V
,
0 14
-------------------- - k ⋅
t
=
,
0 02
I
1
–
,
13 5
------------ - k ⋅
t
=
I 1
–
80
------------ - k ⋅
t
=
2
I
–
1
1
--------------------------------------- --------------- k ⋅
t
=
,
〈
(
,
) I ⁄
〉
0 339
–
0 236
&KDSWHU
&XUUHQW
(Equation 41)
(Equation 42)
(Equation 43)
(Equation 44)
97
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