Setting Guidelines (Directional Element) - GE MiCOM P40 Agile Technical Manual

P14D
This example is for a device feeding a LV switchboard and makes the following assumptions:
CT Ratio = 500/1
●
● Full load current of circuit = 450A
Slowest downstream protection = 100A Fuse
●
The current setting on the device must account for both the maximum load current and the reset ratio, therefore:
I> must be greater than: 450/drop-off = 450/0.95 = 474A.
The device allows the current settings to be applied in either primary or secondary quantities. This is done by
setting the Setting Values cell of the CONFIGURATION column. When this cell is set to primary, all phase
overcurrent setting values are scaled by the programmed CT ratio.
In this example, assuming primary currents are to be used, the ratio should be programmed as 500/1.
The required setting is therefore 0.95A in terms of secondary current or 475A in terms of primary.
A suitable time delayed characteristic will now need to be chosen. When co-ordinating with downstream fuses, the
applied characteristic should be closely matched to the fuse characteristic. Therefore, assuming IDMT co-
ordination is to be used, an Extremely Inverse (EI) characteristic would normally be chosen. This is found under the
I>1 Function cell as IEC E Inverse.
Finally, a suitable time multiplier setting (TMS) must be calculated and entered in cell I>1 TMS.
3.5.4

SETTING GUIDELINES (DIRECTIONAL ELEMENT)

The applied current settings for directional overcurrent devices are dependent upon the application in question. In
a parallel feeder arrangement, load current is always flowing in the non-operate direction. Hence, the current
setting may be less than the full load rating of the circuit.
You need to observe some setting constraints when applying directional overcurrent protection at the receiving-
end of parallel feeders. These minimum safe settings are designed to ensure that there is no possibility of
undesired tripping during clearance of a source fault. For a linear system load, these settings are as follows:
We recommend the following settings to ensure that there is no possibility of malaoperation:
Parallel plain feeders: Set to 50% pre-fault load current
●
● Parallel transformer feeders: Set to 87% pre-fault load current
In a ring main application, the load current can flow in either direction. The current setting must be above the
maximum load current.
The required characteristic angle settings for directional devices depend on the application. We recommend the
following settings:
Plain feeders, or applications with an earthing point behind the device location, should use a +30° RCA
●
setting
Transformer feeders, or applications with a zero sequence source in front of the device location, should use
●
a +45° RCA setting
Although it is possible to set the RCA to exactly match the system fault angle, we recommend that you adhere to
the above guidelines, as these settings provide satisfactory performance and stability under a wide range of
system conditions.
P14D-TM-EN-8
Chapter 6 - Current Protection Functions
87