ABB RELION RET670 Applications Manual page 227

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
The transient part has an angular frequency b and is damped out with the time-constant
α.
The difference in performance of fault currents for a three-phase short circuit at the end
of a typical 500 km long 500 kV line is presented in figure 97.
The short circuit current on a non-compensated line is lower in magnitude, but
comprises at the beginning only a transient DC component, which diminishes
completely in approximately 120ms. The final magnitude of the fault current on
compensated line is higher due to the decreased apparent impedance of a line (60%
compensation degree has been considered for a particular case), but the low frequency
oscillation is also obvious. The increase of fault current immediately after the fault
incidence (on figure 97 at approximately 21ms) is much slower than on non-
compensated line. This occurs due to the energy stored in capacitor before the fault.
10
5
0 0.02 0.04
5
10
IEC06000610 V1 EN-US
Figure 97: Short circuit currents for the fault at the end of 500 km long 500 kV line
without and with SC
Location of instrument transformers
Location of instrument transformers relative to the line end series capacitors plays an
important role regarding the dependability and security of a complete protection
scheme. It is on the other hand necessary to point out the particular dependence of
those protection schemes, which need for their operation information on voltage in IED
point.
Protection schemes with their operating principle depending on current measurement
only, like line current differential protection are relatively independent on CT location.
0.06 0.08 0.1 0.12
t[ms ]
Section 8
Impedance protection
0.14 0.16 0.18 0.2
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