GE P50 Agile P253, Техническое руководство

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6 Protection Functions
P50 Agile P253
6-20 P253/EN M/C
Each column contains an identical set of elements, whereby the EARTH FAULT 1 (IN1) column is
used for earth fault current that is measured directly from the system, whilst the EARTH FAULT 2
(IN2) column contains cells, which operate from a residual current quantity that is derived internally
from the summation of the three-phase currents.
The product provides three stages of Earth Fault protection with independent time delay
characteristics, for each EARTH FAULT column.
Stages 1, 2 and 3 provide a choice of operate and reset characteristics, where you can select
between:
•
A range of standard IDMT (Inverse Definite Minimum Time) curves
•
DT (Definite Time)
For the IN1 column, this is achieved using the cells:
•
IN1>(n) Function for the overcurrent operate characteristics
•
IN1>(n) Reset Char for the overcurrent reset characteristic
For the IN2 column, this is achieved using the cells:
•
IN2>(n) Function for the overcurrent operate characteristics
•
IN2>(n) Reset Char for the overcurrent reset characteristic
where (n) is the number of the stage.
Stages 1, 2 and 3 provide a Timer Hold facility. This is configured using the cells IN1>(n) tRESET for
Earth Fault 1 and IN2>(n) tRESET for Earth Fault 2.
The fact that both EF1 and EF2 elements may be enabled at the same time leads to a number of
application advantages.
2.7.2 Earth Fault Protection Logic
The Earth Fault current is compared with a set threshold ( IN1>(n) Current Set ) for each stage. If it
exceeds this threshold a Start signal is triggered, provided there are no blocks.
Earth Fault protection can follow the same IDMT characteristics as described in the Overcurrent
Protection Principles section. Please refer to this section for details of IDMT characteristics.
The description also applies to the Earth Fault 2 element (IN2).
The Earth Fault protection trip signal can be blocked by:
•
The Second Harmonic blocking function which is for all three phases. The blocking of Earth
Fault 1 is activated by setting the IN1>(n) 2H Blocking cell to ‘Enabled’ and blocking of Earth
Fault 2 is activated by setting the IN2>(n) 2H Blocking cell to ‘Enabled’, where (n) is the
number of the stage.
•
User defined blocking logic using ‘IO Mask’ functionality.
2.8 Sensitive Earth Fault Protection
With some earth faults the fault current flowing to earth is limited by either intentional resistance (as is
the case with some HV systems) or unintentional resistance. For example, in very dry conditions and
where the substrate is high resistance, such as sand or rock.
To provide protection in such cases it is necessary to provide an earth fault protection system with a
setting that is considerably lower than for normal line protection. Such sensitivity cannot be provided
with conventional CTs, therefore SEF would normally be fed from a core balance current transformer
(CBCT) mounted around the three phases of the feeder cable. Also a special measurement class SEF
transformer should be used in the IED.
With SEF protection, settings as low as 0.2% can be used.