GE Agile P153, Техническое руководство, Страница: 162 / 268

6 Protection Functions P50 Agile P153
6-12 P153/EN M/B
The thermal state measurement is made available in the MEASUREMENTS menu.
The thermal state can be reset from the HMI panel by enabling the Thermal Reset setting to ‘Yes’
under RECORD CONTROL menu.
2.6 Broken Conductor
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 IEDs. 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.
2.6.1 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.
Two stages are supported - BC Alarm & BC Trip. The BC Trip function is not part of General trip. Both
stages can be configured independently.
2.6.2 Broken Conductor Protection Logic
The ratio of I
2
/I
1
is calculated and compared with the threshold. If the threshold is exceeded, the delay
timer is initiated.
Note: Broken conductor is activated only if current in any phase is more than 0.1 amp .
2.7 Undercurrent /Loss of Power Protection Function
The relay includes undercurrent elements that can be used to provide additional functions to prevent
damage to the power system. This function allows typical applications such as loss of load. The
undercurrent protection function is available only if the auxiliary contact of the CB status is connected
to the relay, for example, when CB (52A) contact of CB is high.
2.8 Circuit Breaker Fail Protection
When a fault occurs, one or more protection devices will operate and issue a trip command to the
relevant circuit breakers. Operation of the circuit breaker is essential to isolate the fault and prevent, or
at least limit, damage to the power system. For transmission and sub-transmission systems, slow fault
clearance can also threaten system stability.
For these reasons, it is common practise to install Circuit Breaker Failure protection (CBF). CBF
protection monitors the circuit breaker and establishes whether it has opened within a reasonable