Broken Conductor Protection; Broken Conductor Protection Implementation; Broken Conductor Protection Logic; Application Notes - GE MiCOM P40 Agile Technical Manual

Chapter 6 - Current Protection Functions
13

BROKEN CONDUCTOR PROTECTION

One type of unbalanced fault is the 'Series' or 'Open Circuit' fault. This type of fault can arise from, among other
things, broken conductors. Series faults do not cause an increase in phase current and so cannot be detected by
overcurrent protection. However, they do produce an imbalance, resulting in negative phase sequence current,
which can be detected.
It is possible to apply a negative phase sequence overcurrent element to detect broken conductors. 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 imbalance arising from CT errors and load imbalances, making it very difficult
to distinguish. A regular negative sequence element would therefore not work at low load levels. To overcome this,
the device incorporates a special Broken Conductor protection element.
The Broken Conductor element measures the ratio of negative to positive phase sequence current (I2/I1). This ratio
is approximately constant with variations in load current, therefore making it more sensitive to series faults than
standard negative sequence protection.
13.1

BROKEN CONDUCTOR PROTECTION IMPLEMENTATION

Broken Conductor protection is implemented in the BROKEN CONDUCTOR column of the relevant settings group.
This column contains the settings to enable the function, for the pickup threshold and the time delay.
13.2

BROKEN CONDUCTOR PROTECTION LOGIC

The ratio of I /I is calculated and compared with the threshold setting. If the threshold is exceeded, the delay
2 1
timer is initiated. The CTS block signal is used to block the operation of the delay timer.
I2/I1
I 2/I1 Setting
I2
Low current
CTS Block
V00609

Figure 58: Broken conductor logic

13.3

APPLICATION NOTES

13.3.1

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 I /I
2 1
power system (assuming equal impedance's in each sequence network), the ratio I
In practise, 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 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.
126
that flows in the protected circuit will approach 100%. In the case of a multiple earthed
&
/I
2
BrokenLine Start
Broken Line Trip
will be 50%.
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