Schweitzer Engineering Laboratories SEL-700G Series, Инструкция по эксплуатации

Страница: 131 / 824

4.25
Date Code 20170814 Instruction Manual SEL-700G Relay
Protection and Logic Functions
Group Settings (SET Command)
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-700G1 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%)
➤
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:
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-700G)
SLP1
M IN
%
Err%
200 Err% –
 
k

------------------------------------------
 
 
100

=