GE MiCOM P40 Agile, Technical Manual, Page: 187 / 962

Note:
Mho characteristics have an inherent tendency to avoid unwanted overreaching, making them very desirable for long line
protection.
6.6 DIRECTIONAL ELEMENT FOR DISTANCE PROTECTION
Distance zones are directionalized by the Delta decision. For Delta directional decisions, the relay characteristic
angle (RCA) settings must be based on the average line impedance angle for a fault anywhere internal or
external to the line. Typically, the Dir Char Angle is set to 60°, as it is not essential for this setting to be precise.
When a fault occurs, the delta current is never close to the characteristic boundary, so an approximate setting is
good enough.
The 60° angle is associated with mainly inductive sources and suits most applications. However, in series
compensated line applications where the capacitor is physically located behind the line voltage transformer, the
Delta directional characteristic angle needs adjusting. In such applications the capacitor is included in the
equivalent source impedance. Then the overall source impedance seen by the protection becomes predominantly
capacitive if the inductance of the normally strong source is less than the capacitor value. In this case, the
calculated operating angle during an internal fault may not fall within the default 60° Delta directional line
operating boundary. This could lead to an incorrect (reverse) directional decision. A zero degree shift is most
suitable for such a fault. However, the constraining factor is the case of external faults for which the source is
always inductive regardless of the degree of compensation and for which the 60° shift is most appropriate. To
ensure correct, reliable and fast operation for both fault locations in the case of predominantly capacitive source,
we recommend a Dir Char Angle setting of 30°.
6.7 FILTERING SETUP
A number of filters and features are provided to help avoid false tripping during conditions that can be challenging
to distance protection.
6.7.1 DISTANCE DIGITAL FILTER
In most applications, we recommend setting the Digital Filter setting to
Standard
. This ensures that the
protection provides fast, sub-cycle tripping. In certain rare cases, such as where lines are immediately adjacent to
High Voltage DC (HVDC) transmission, the current and voltage inputs may be severely distorted under fault
conditions. The resulting non-fundamental harmonics could affect the reach point accuracy of the protection. To
prevent the protection being affected, you should select the
Special Applics
setting to enable the special
applications filter.
Note:
When using the Special Applications filter the instantaneous operating time is increased by about a quarter of a power
frequency cycle.
6.7.2 SETTING UP CVTS
CVTs with Passive Suppression of Ferroresonance
Set CVT Filters to
Passive
for any type 2 CVT (those with an anti-resonance design). You need to apply an SIR
cut-off setting, above which the protection operation is deliberately slowed by a quarter of a cycle. A typical SIR
setting is 30, below which the protection trips sub-cycle, and if the infeed is weak the CVT filter adapts to slow the
protection and prevent transient overreach.
CVTs with Active Suppression of Ferroresonance
Set CVT Filters to
Active
for any type 1 CVT.
P446SV Chapter 7 - Distance Protection
P446SV-TM-EN-1 155