Fault Level And X/R For A Phase Through-Fault - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
·
The CT core was induced with a one Tesla of remnant flux prior to the fault being applied.
·
The fault inception point was set at zero degrees, which produces the largest dc offset in the primary
fault current and the highest dc transient flux requirement in the CT core. Most short circuit faults occur
at between forty-five and ninety degrees.
On solidly earthed systems the earth-fault level can exceed the phase fault level by up to a factor of 1.2. However
the X/R of the earth-fault will always be less than the three-phase fault as the return path via the earth/sheath is
mainly resistive. This will reduce any dc offsets in the primary fault current for an earth-fault. It is therefore it is
sufficient to consider three-phase faults only.
The above figures demonstrate the feeder impedance reduces the CT minimum requirements as the feeder
length increases. The Feeder reactance and resistance will become more dominant as the feeder length
increases. This is shown graphically for the 132 kV cable feeder used in the example.
External Fault Level and X/R decreasing with circuit length
50000
40000
30000
20000
10000
0
0 1 2 3 4
The above shows the X/R ratio of feeder through-fault current is less than half the source X/R when the cable
exceeds only 3 km's in length. The fault level also reduces with increasing length and may allow the use of an
instantaneous high set over current element for longer feeders. This may be set to provide fast tripping for close
up faults, if say a flexible earth clamp was inadvertently left connected when the circuit is energised.
5.3.2

Fault Level and X/R for a Phase Through-fault

Combinations of X/R and Fault level will rarely exceed a maximum of 1000 on any power system. This is because
a high X/R will tend to reduce the fault level. System X/R and fault level therefore have an inverse relationship. In
the above example the source reactance (Xs) and resistance (Rs) are calculated and additional feeder reactance
(X ) and resistance (R ) are added as the feeder length increases, to arrive at the profile shown above.
F F
Therefore using the Maximum Breaking capacity of the CB and maximum system X/R together is not technically
valid as the both of these values cannot occur at the same time. If one of these parameters is at a maximum the
other tends towards its minimum value. This is why a limit is imposed on the product of these two parameters.
These limits are the maximum practical case possible.
The circuit type also affects the CT requirements. Cables have much lower X/R ratios than over head lines and
therefore tend to dominate as feeder length increases.
Unrestricted Page 28 of 63
Typical 132kV Cable Feeder
5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
Length of Cable Feeder (km's)
Figure 5.3.1-1 Fault Level and X/R reducing with feeder length
Three Phase External Fault Level
for 30kA Source
Three Phase Fault Level For a 40kA
Source
X/R Ratio of External Three Phase
Fault for a Source X/R of 30
X/R of Three Phase External Fault
for a Source X/R of 50
©2018 Siemens Protection Devices Limited
60
50
40
30
20
10
0