AUTOMATIC CONTROL FUNCTIONS
6.1 Trip Commands
This menu is used to assign the trip of the protection and the automatic control function to
the relay 1. See the P12y/EN FT (User Guide).
The relay 1 is usually used for the trip of the CB and the logic output is used to start all the
functionality relevant to the CB control.
6.2 Latch relays
Sometimes it occurs to memorise trips or alarms.
By this menu it is possible the latching of the relay from 1 to 8 the relevant relays to the 79
function must not be assigned latched.
6.3 Broken Conductor Detection (P126 & P127)
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 flashover. Alternatively, they may arise from other
causes such as birds on overhead lines or mechanical damage to cables etc.
Such faults induce an appreciable current increase and are easily detectable in most
applications.
Another type of unbalanced system condition is the series or open circuit fault. This fault can
arise from broken conductors, mal-operation of single-phase switchgear, or the operation of
fuses.
Series faults will not induce an increase in phase current on the system and hence are not
easily detectable by standard overcurrent protection elements available with common relays.
However, they will produce an unbalance and an important level of negative phase
sequence current, which can be detected.
It is possible to apply a negative phase sequence overcurrent relay to detect a series fault
condition as described. 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 protection
element therefore would not operate at low load levels.
The MiCOM P126 and P127 relays incorporate a protection element, which measures the
ratio of negative to positive phase sequence current (I2/I1). This protection element 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.
6.3.1 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 impedance in each sequence network),
the ratio I2/I1 will be 50%.
The setting for the broken conductor protection element is described in Technical Data and
the menu is described in User Guide.
