GE Agile P50 Series, Technical Manual

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P50 Agile P153 6 Protection Functions
P153/EN M/B 6-7
•
Line to Line to Earth (accounting for approximately 5% of all faults)
•
Line to Line to Line (accounting for approximately 2% of all faults)
Although not as common as earth faults (single line to earth), phase faults are typically more severe.
An example of a phase fault is where a fallen tree branch bridges two or more phases of an overhead
line.
2.2.1
Phase Overcurrent Protection Implementation
Phase Overcurrent Protection is implemented in the OVERCURRENT column of the relevant settings
group.
The product provides three stages of three-phase overcurrent protection with independent time delay
characteristics. All settings apply to all three phases but are independent for each of the three stages.
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)
This is achieved using the cells
•
I> (n) Function for the overcurrent operate characteristic
•
I> (n) Reset Char for the overcurrent reset characteristic
where (n) is the number of the stage.
The IDMT-capable stages, (1, 2 and 3) also provide a Timer Hold facility. This is configured using the
cells
I> (n ) tRESET , where (n) is the number of the stage. Timer Hold facility is applicable for both IEC
and IEEE curves. IEEE & US curves have IDMT reset curve option in addition to DT Phase
Overcurrent protection logic.
Phase Overcurrent modules are level detectors that detect when the current magnitude exceeds a set
threshold. When this happens, the Phase Overcurrent module in question issues a signal to produce
the
Start signal. This Start signal is applied to the IDMT/DT timer module. It is also made available
directly to the user for use in the logic functions.
There are three Phase Overcurrent Modules, one for each phase. The three Start signals from each
of these phases and common Start signal for each stage of phase overcurrent function is available for
customer use.
The outputs of the IDMT/DT timer modules are the trip signals which are used to drive the tripping
output relay. The three
Trip signals from each of the phases and common Trip signal for each stage
of phase overcurrent function is available for customer use.
The Phase overcurrent trip signal can be blocked by:
•
The Second Harmonic blocking function which is for all three phases. The blocking is
activated by setting the
I> (n) 2H Blocking cell to ‘Enabled’, where (n) is the number of the
stage.
•
User defined blocking logic using ‘IO Mask’ functionality.
The Phase Overcurrent threshold setting can be influenced by the Cold Load Pickup (CLP) function, if
this functionality is available and used.
2.3 Negative Sequence Overcurrent Protection
When applying standard phase overcurrent protection, the overcurrent elements must be set
significantly higher than the maximum load current, thereby limiting the element’s sensitivity. Most
protection schemes also use an earth fault element operating from residual current, which improves
sensitivity for earth faults.