4.25
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
➤ Load Tap-Changer (LTC) (typically ±10%)
➤ Relay accuracy (±5% or ±0.02 • I
NOM
, whichever is largest.)
We recognize that the excitation current of the transformer is not proportional
to load flow. However, a conservative approach would include it as a
proportional error.
CTs create both steady-state and transient errors, which can result in false
differential current. IEEE Standard Requirements for Instrument
Transformers, IEEE Standard C57.13-1993 specifies that a relay-accuracy CT
must be 3 percent accurate at rated current and 10 percent accurate at 20 times
rated current when ZB is the standard burden. It is important to note that the
rated current specified in the standard is a symmetrical sinusoidal waveform
(it does not have a transient DC component). Because the burden is usually
designed to be much smaller than the standard burden, the error current will
likely be much less than 3 percent for current flow at low multiples of the
nominal rating of the CTs.
The errors can be added to determine the amount of error that the SLP1
characteristic must accommodate for normal system conditions. At that point,
use the following equation and add margin to determine SLP1 and determine
the minimum limit of the allowable slope ratio.
where:
The variable restraint characteristic provided by SLP2 at high multiples of
TAP for a through fault accommodates transient CT error. SLP2 can be set
fairly high without jeopardizing sensitivity for low-grade partial winding
faults. The CTs should be evaluated for the likelihood of going into saturation
for a through fault, and SLP2 should be adjusted accordingly. Another
consideration for selecting the SLP2 setting is that the effectiveness of the
variable percentage depends on SLP1 and IRS1, which determine the starting
point of SLP2. If SLP1 is set very low, a higher SLP2 may be warranted.
Unrestrained Element Current Pickup
The instantaneous unrestrained current element is intended to react quickly to
very heavy current levels that clearly indicate an internal fault. Set the pickup
level (U87P) to approximately 8 to 10 times TAP. The unrestrained differential
element only responds to the fundamental frequency component of the
differential operating current. It is not affected by the SLP1, SLP2, IRS1,
PCT2, or PCT5 settings. Thus, it must be set high enough so as not to react to
large inrush currents.
Note that the U87P must be set lower than the minimum of
31 • INOMn/TAPn, where n =1, 2.
Second-Harmonic Blocking
Transformer simulations show that magnetizing inrush current usually yields
more than 30 percent of IF2/IF1 in the first cycle of the inrush. A setting of
15 percent usually provides a margin for security. However, some types of
transformers, or the presence within the differential zone of equipment that
SLP1
MIN
= slope ratio that will just accommodate Err with no margin
Err = amount of error expected in normal operation
k = AVERAGE restraint scaling factor (1 for the SEL-787)
SLP1
MIN
%
Err%
200 Err%–k
------------------------------------------
100=
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