Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-45
AP
2.15 Broken conductor detection
The majority of faults on a power system occur between one phase and ground or two
phases and ground. These are known as shunt faults and arise from lightning discharges
and other overvoltages which initiate flashovers. Alternatively, they may arise from other
causes such as birds on overhead lines or mechanical damage to cables etc. Such faults
result in an appreciable increase in current and hence in the majority of applications are
easily detectable.
Another type of unbalanced fault that can occur on the system is the series or open circuit
fault. These can arise from broken conductors, maloperation of single phase switchgear, or
single-phasing of fuses. Series faults will not cause an increase in phase current on the
system and hence are not readily detectable by standard protection. 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.
2.15.1 Setting guidelines
For a
broken conductor affecting a single point earthed power system, there will be little zero
sequence current flow and the ratio of 2/1 that flows in the protected circuit will approach
100%. In the case of a multiple earthed power system (assuming equal impedance’s in each
sequence network), the ratio 2/1 will be 50%.
In practice, the levels of standing negative phase sequence current present on the system
govern this minimum setting. 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: 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 auto-reclose 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.
The example following information was recorded by the relay during commissioning;
full load = 500 A
2 = 50 A
therefore the quiescent 2/1 ratio is given by;
2/1 = 50/500 = 0.1
To allow for tolerances and load variations a setting of 20% of this value may be typical:
Therefore set 2/1 = 0.2
In a double circuit (parallel line) application, using a 40% setting will ensure that the broken
conductor protection will operate only for the circuit that is affected. Setting 0.4 results in no
pick-up for the parallel healthy circuit.
Set 2/1 Time Delay = 60 s to allow adequate time for short circuit fault clearance by time
delayed protections.
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