Date Code 20011112
Line Current Differential, Distance, Out-of-Step, Overcurrent,
3-11
Voltage, Synchronism Check, and Frequency Elements
SEL-311L Instruction Manual
During an external unbalanced fault, both terminals see nearly identical negative-sequence fault
current. If CTs at one line terminal saturate, the effect on the Alpha plane ratio can be predicted,
because relays at both line terminals measure negative-sequence fault current. For a severely
saturated CT, the measured current magnitude might decrease to as little as 1/3 the unsaturated
current. This moves the Alpha plane ratio from 1 to as low as 1/3 or as high as 3. Setting
87LR = 6 as recommended above enables the restraint region to contain even severe CT
saturation.
Severe CT saturation can also cause an angle error as large as 40 degrees as shown in Figure 3.5.
A large 2 ms asymmetrical channel delay produces 22 degrees of data alignment error, which
might add to the CT saturation induced angle error. In that case, the Alpha plane angle can be in
error by as much as 62 degrees. The default 87LANG setting of 195 degrees allows 35 degrees
of margin for other sources of error.
Similar considerations for the phase elements produce similar results. An external fault with
simultaneous CT saturation and a large 2 ms channel delay asymmetry can produce up to 300%
error in phase current ratio magnitude, and 62 degrees of phase current ratio angle error. The
default restraint region easily contains these cases, even if they occur simultaneously.
This setting approach favors security. However, given the approach of setting the line current
differential elements to detect internal faults even for extreme system conditions, the SEL-311L
Relay line current differential elements also give excellent dependability.
SEL-5601 Produces Alpha Plane Plots
Use the SEL-5601 Analytic Assistant to visualize SEL-311L Relay event reports on the Alpha
plane. Retrieve differential event reports using the
CEV L
or
EVE C
commands. The SEL-5601
reads compressed SEL-311L Relay event reports and produces Alpha plane plots for phase
current, negative-sequence current, or zero-sequence current. For example, Figure 3.9 shows an
SEL-5601 screen capture of the Phase A Alpha plane plot for an external fault that produced CT
saturation at the local terminal but not at the remote terminal. The saturated and unsaturated
current waveforms are shown in Figure 3.5.
Summary of Contents for SEL-311L
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