Sel SEL-787 - Page 110

4.24
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
Current TAP
The relay uses a standard equation to set TAPn, based on settings entered for
the particular winding (n denotes the winding number.)
where:
The relay calculates TAPn with the following limitations:
➤ The tap settings are within the range 0.1 • I
NOMn
and
6.2 • I
NOMn
➤ The ratio of the highest (TAPn/INOMn) to the lowest
(TAPn/INOMn) is less than or equal to 7.5, where n =1, 2
Restrained Element Operating Current Pickup
The O87P setting range is 0.1 to 1.0; we suggest an O87P setting of 0.2 to 0.3.
The setting must be at a minimum for increased sensitivity but high enough to
avoid operation because of steady-state errors such as unmonitored station
service loads, transformer excitation current, and relay measuring error at very
low current levels. The setting must also yield an operating current greater
than or equal to a minimum of 0.1 • INOMn/TAPn, where n =1, 2.
Restraint Slope Percentage
The purpose of the percentage restraint characteristic is to allow the relay to
differentiate between differential current from an internal fault versus
differential current during normal or external fault conditions. You must select
slope characteristic settings that balance security and dependability. To do
this, it is helpful to determine what slope ratio is characteristic of normal
conditions (slope must exceed that for security) and what slope ratio is
characteristic of an internal fault (the slope must be below that for
dependability). In the case of the SEL-787 Relay, the slope ratio for a bolted
internal fault is 100%.
The sources of differential current for external faults fall into three categories:
➤ Differential current that is not proportional to the current flow
through the zone (steady state).
➤ Differential current that is proportional to current flow through
the zone (proportional).
➤ Differential current that is transient in nature (transient).
SLP1 should be set above normal steady-state and proportional errors. SLP2 is
used to accommodate transient errors. The following is a list of typical sources
of error that must be considered.
➤ Excitation current (typically 1 to 4%)
➤ CT accuracy (typically less than 3% in the nominal range)
➤ No-Load Tap Changer (NLTC) (typically ±5%)
C = 1 if WnCT setting = Y (wye-connected CTs)
C = if WnCT setting = D (delta-connected CTs)
MVA = maximum power transformer capacity setting (must be
the same for all TAPn calculations)
VWDGn = winding line-to-line voltage setting, in kV
CTRn = current transformer ratio setting
TAPn
MVA 1000
3VWDGCTRn
------------------------------------------------------
C=
3