RFL 9300 RFL Electronics Inc.
August 9, 2001 4 - 5 (973) 334-3100
4.2.2 GND BIAS - GROUND BIAS
Parameter #21 (GND BIAS) controls ground bias. It can be set anywhere from 1.0 to 2.25 am-
peres, in 0.25-amp steps. The error budget components for ground bias are the same as previ-
ously listed for phase bias (0.25 amperes for false differential current, 0.5 amperes for analog
and digital resolution errors, and 0.25 ampere for a safety factor.)
Fortunately, most tapped load transformers are delta-connected on the high-side and grounded-
wye-connected on the low-side. This blocks the flow of zero-sequence current, and permits a
sensitive setting for the ground bias, even for lines with heavy tapped loads.
The same method used to calculate the net line charging current is used to calculate the ground
bias calibration. Of course, this calculation is now based on zero-sequence
capacitive current.
4.2.3 BIAS CTRL - BIAS CONTROL
Parameter #22 (BIAS CTL) is used for Bias Control. It can either be enabled or disabled. When
enabled, the phase bias and ground bias levels are boosted for a preset interval (See paragraph
4.2.5).
Bias control is enabled to adapt the RFL 9300 to the characteristics of many applications. For
example, underground cables and long overhead lines may have a large capacitive inrush cur-
rent when first energized (See paragraph 4.2.19). This inrush appears as false differential to the
RFL 9300. The inrush current may be rich in harmonic content.
High harmonics tend to prevail during line charging, because the limiting impedance is net ca-
pacitive. Even so, there may be certain applications where the characteristic of the inrush cur-
rent may look to the RFL 9300 like a low-grade internal fault. (An example of this would be a
very long line energized from a weak source.)
To prevent tripping in these cases, bias control should be enabled. This boosts the phase bias
and ground bias levels by either 1.5 or 3.0 amperes for a preset interval. (The next setting con-
trols this interval.)
Because of the safety margin and line charging current accommodation built into the phase bias
and ground bias settings, bias control should not be needed in almost all applications. Unless it
is needed, it should be disabled.
4.2.4 GND DLY - GROUND DELAY
Parameter #23 (GND DLY) controls the RFL 9300's ground trip delay function. It can be set to
either ENABLE or DISABLE.
As explained in the discussion of tapped loads in Section 2 of this manual, some tap transform-
ers will allow 3I
0
to flow from the high-side to low-side faults. For these applications, this pa-
rameter may have to be enabled.
Whenever this parameter is enabled, all 3I
0
trips are delayed by the time delay that was set by
the GROUND DELAY parameter (50 ms to 30 seconds).
4.2.4.1 3IO TRIP ALGORITHM
When the RFL 9300 relay is configured for single-pole operation the GROUND DELAY parame-
ter takes an additional role. In certain situations, the ground bias may be set to be much more
sensitive than the phase bias. In the event that the 3IO controller calculates a trip, all three
phase controllers will be commanded to trip in the event that none of them issues a trip signal by
the time the GROUND DELAY timer expires. GROUND DELAY must be enabled in order to en-
able the 3IO trip algorithm. If the GROUND DELAY function is not enabled, trips calculated by
the 3IO controller will have no effect in RFL 9300 single-pole trip applications.
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