P54x/EN AP/La4 Application Notes
(AP) 6-20
MiCOM P543, P544, P545 & P546
AP
2.3.9 Phase fault zone settings (Distance option only)
Each zone has two additional settings that are not accessible in the Simple set mode. These
settings are:
A tilt angle on the top line of any quadrilateral set for phase faults;
A minimum current sensitivity setting.
By factory defaults, the Top Line of quadrilateral characteristics is not fixed as a horizontal
reactance line. To account for phase angle tolerances in the line CT, VT and relay itself, the
line is tilted downwards, at a “droop” of –3
o
. This tilt down helps to prevent zone 1
overreach.
The fixed Tilt setting on the phase elements may also be used to compensate for overreach
effects when prefault heavy load export was flowing. In such cases, fault arc resistance will
be phase shifted on the impedance polar plot, tilting down towards the resistive axis (i.e. not
appearing to be fully resistive in nature). For long lines with heavy power flow, the Zone 1
top line might be tilted downwards within the range –5 to –15
o
, mimicking the phase shift of
the resistance.
Note: A minus angle is used to set a downwards tilt gradient, and a positive
angle to tilt upwards.
Note: mho characteristics have an inherent tendency to avoid unwanted
overreaching, making them very desirable for long line protection, and
one of the reasons for their inclusion within the MiCOM P54x relay.
The current Sensitivity setting for each zone is used to set the minimum current that must
be flowing in each of the faulted phases before a trip can occur. It is recommended to leave
these settings at their default. The exception is where the relay is made more insensitive to
match the lesser sensitivity of older relays existing on the power system, or to grade with the
pickup setting of any ground overcurrent protection for tee-off circuits.
2.3.10 Distance directional principle and setup (Distance option only)
2.3.11
Delta directional - selection of RCA (Distance option only)
Dista
nce zones are directionalized by the delta decision. For delta directional decisions, the
RCA settings must be based on the average source + line impedance angle for a fault
anywhere internal or external to the line. Typically, the Delta Char Angle is set to 60
o
, as it
is not essential for this setting to be precise. When a fault occurs, the delta current will never
be close to the characteristic boundary, so an approximate setting is good enough.
The 60° angle is associated with mainly inductive sources and will work perfectly well for
most applications. However, in series compensated line applications where the capacitor is
physically located behind the line VT the Delta directional characteristic angle needs
adjusting. In such applications the capacitor is included in the equivalent source impedance
and the overall source impedance as seen by the relay will become predominantly capacitive
if the inductance of the source (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 and that could potentially lead to an
incorrect (reverse) directional decision. A zero degree shift will be most suitable for such a
fault, but 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 case of
predominantly capacitive source, a Delta Char Angle setting of 30° is strongly
recommended.
To Next Page
Loading...
