Areva MiCom P120, Техническое руководство

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Application Guide P12x/EN AP/A96
MiCOM P120/P121/P122/P123
Page 47/80
21. BROKEN CONDUCTOR DETECTION (P122 & P123 ONLY)
Most of the faults that affect a power system occur between one phase and the earth or
between two phases and the earth. These faults are shunt faults and are caused by
lightning discharges and other overvoltages generating flashovers. They may also arise from
birds on overhead lines or mechanical damage on underground cables, etc.
Such faults lead the current to increase appreciably and therefore they can easily be
detected in most applications. Open circuit faults are a different type of faults that can
happen in electrical networks. These faults can be caused by broken conductors, blown
fuses or misoperation of a pole of a circuit-breaker.
Series faults will not lead to an increase in phase current and therefore they can not be
easily detected by common overcurrent relays. However, this type of faults produce an
unbalance that creates negative phase sequence current, which can be detected.
The use of negative phase sequence overcurrent is then recommended to detect such faulty
conditions. However, on lightly loaded lines, the value of the negative sequence current
caused by a faulty condition may be very close to, or even inferior, to the full load steady
state unbalance generated by CT errors, load unbalance, etc. As a consequence, a negative
sequence protection element would not work for low level of loads.
As a solution, MiCOM P122 and P123 have a protection element that measures the ratio
between the negative and the positive phase sequence current (I2/I1). Using this ratio
instead of the measure of I2 only, the relay will be able to detect a faulty condition
independently on the level of load on the network, since the ratio is approximately constant
with variations in load current. It is then possible to get a more sensitive setting.
NOTE: the Broken conductor function is inhibited if the current value flowing
in each one of the three phases is inferior to 10% of the nominal
current.
Setting Guidelines
On single point earthed power systems, there is a low zero sequence current flow and the
ratio I2/I1 that flows is close to100%. On multiple earthed power systems, (assuming that the
impedances in each sequence network are equals), the ratio I2/I1 will be equal to 50%.
It is possible to calculate the ratio of I2/I1 corresponding to various system impedances,
according to the following equations:
I
1F
=
E
g
(Z
2
+Z
0
)
Z
1
Z
2
+Z
1
Z
0
+Z
2
Z
0
I
2F
=
–E
g
Z
0
Z
1
Z
2
+Z
1
Z
0
+Z
2
Z
0
Where:
E
g
= System Voltage
Z
0
= Zero sequence impedance
Z
1
= Positive sequence impedance
Z
2
= Negative sequence impedance
Therefore:
I
2F
I
1F
=
Z
0
Z
0
+Z
2