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I
+ I
+ I
=0
1F
2F
0F
− Z
Z
I
I
= 0
2F
2F
0F
0F
− E
− Z
E
= Z
I
1A
1B
1F
1F
where,
E
, E
: power source voltage
1A
1B
Z
: positive sequence impedance
1
Z
: negative sequence impedance
2
Z
: zero sequence impedance
0
From the equations (1), (2) and (3), the following equations are derived.
Z
+ Z
2
0
I
=
1F
Z
Z
+ Z
Z
+ Z
1
2
1
0
−Z
0
I
=
2F
Z
Z
+ Z
Z
+ Z
1
2
1
0
−Z
2
I
=
0F
Z
Z
+ Z
Z
+ Z
1
2
1
0
The magnitude of the fault current depends on the overall system impedance, difference in phase
angle and magnitude between the power source voltages behind both ends.
Broken conductor protection element BCD detects series faults by measuring the ratio of negative
to positive phase sequence currents (I
impedance of the system:
I
|I
|
Z
2F
2F
0
=
| =
I
|I
Z
+ Z
1F
1F
2
0
The ratio is higher than 0.5 in a system when the zero sequence impedance is larger than the
negative sequence impedance. It will approach 1.0 in a high-impedance earthed or a one-end
earthed system.
The characteristic of BCD element is shown in Figure 2.8.1.2 to obtain the stable operation.
I
2
0.01×In
0
0.04×In
(1)
(2)
I
(3)
2F
2F
− E
(E
)
1A
1B
Z
2
0
− E
(E
)
1A
1B
Z
2
0
− E
(E
)
1A
1B
Z
2
0
/ I
). This ratio is given with negative and zero sequence
2F
1F
I
1
Figure 2.8.1.2 BCD Element Characteristic
⎯ 40 ⎯
| ≥ BCD
|I
|/|I
2
1
setting
| ≥ 0.04×In
|I
1
| ≥ 0.01×In
|I
2
In: rated current
6 F 2 S 0 8 2 8
&
BCD
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