44x/EN AP/Hb6
-100 MiCOM P40 Agile
Another type of unbalanced fault which can occur on the system is the series or open circuit
fault. These can arise from broken conductors, maloperation of single phase switchgear, or
the operation of fuses. Series faults will not cause an increase in phase current on the
system and hence are not readily detectable by standard overcurrent relays. However, they
will produce an unbalance and a resultant level of negative phase sequence current, which
can be detected.
It is possible to apply a negative phase sequence overcurrent relay to detect the above
condition. However, on a lightly loaded line, the negative sequence current resulting from a
series fault condition may be very close to, or less than, the full load steady state unbalance
arising from CT errors, load unbalance etc. A negative sequence element therefore would
not operate at low load levels.
The relay incorporates an element which measures the ratio of negative to positive phase
sequence current (I
2
/I
1
). This will be affected to a lesser extent than the measurement of
negative sequence current alone, since the ratio is approximately constant with variations in
load current. Therefore, a more sensitive setting may be achieved.
The following table shows the relay menu for the Broken Conductor protection, including the
available setting ranges and factory defaults:
4.6.1 Setting guidelines
In the case of a single point earthed power system, there will be little zero sequence current
flow and the ratio of I2/I1 that flows in the protected circuit will approach 100%. In the case of
a multiple earthed power system (assuming equal impedances in each sequence network),
the ratio I2/I1 will be 50%.
It is possible to calculate the ratio of I2/I1 that will occur for varying system impedances, by
referring to the following equations:
I
1F
=
( )
g2 0
12 10 20
E ZZ
ZZ ZZ ZZ
+
++
I
2F
=
Where:
E
g
= System Voltage
Z
0
= Zero sequence impedance
Z
1
= Positive sequence impedance
Z
2
= Negative sequence impedance
Therefore:
=
It follows that, for an open circuit in a particular part of the system, I2/I1 can be determined
from the ratio of zero sequence to negative sequence impedance. It must be noted however,
that this ratio may vary depending upon the fault location. It is desirable therefore to apply as
sensitive a setting as possible. In practice, this minimum setting is governed by the levels of
standing negative phase sequence current present on the system. This can be determined
from a system study, or by making use of the relay measurement facilities at the
commissioning stage. If the latter method is adopted, it is important to take the
measurements during maximum system load conditions, to ensure that all single phase
loads are accounted for.
Note that a minimum value of 8% negative phase sequence current is required for
successful relay operation.
Since sensitive settings have been employed, it can be expected that the element will
operate for any unbalance condition occurring on the system (for example, during a single
pole autoreclose cycle). Hence, a long time-delay is necessary to ensure co-ordination with
other protective devices. A 60 second time-delay setting may be typical.
To Next Page
Loading...
