Areva MiCOM P441, Technical Manual

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P44x/EN AP/E33 Application Notes
Page 80/220 MiCOM P441/P442 & P444
2.16 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 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. Hence, a more sensitive setting may be achieved.
2.16.1 Setting Guidelines
The sequence network connection diagram for an open circuit fault is detailed in Figure 1.
From this, it can be seen that when a conductor open circuit occurs, current from the positive
sequence network will be series injected into the negative and zero sequence networks
across the break.
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
=
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