Section 5: Determining Current Transformer Requirements; Step 1 - Determine The Bias Break Point Setting; Step 2 - Determine The Fault Level And X/R Ratio Of A Through-Fault - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide

Section 5: Determining Current Transformer Requirements

There are four parameters that must be established before the minimum CT knee-point voltage can be specified
for a particular circuit. This assumes the Bias Slope 2 setting is set to its default of 150%. For all applications of
the relay this setting should be set to 150%.
·
Bias Breakpoint Setting.
·
Maximum Three-phase Fault Through-fault Level.
·
X/R of the through-fault.
·
Estimation of the total CT secondary resistance
The process of specifying the CT knee-point voltage required is done in three steps:
5.1

Step 1 - Determine the Bias Break Point Setting

As discussed above the Bias Breakpoint setting is established by examining the earth-fault sensitivity required to
detect the minimum internal earth-fault. The relay settings are selected so that the relay measures this fault to be
in the operate region. This setting should be set as low as possible to lower CT requirements and add stability for
through-faults.
As a general guideline, cable feeders used on power systems with solidly earthed neutrals allow for lower Bias
Breakpoint Setting of 0.5 to 1 xIn to be selected. Cable and all overhead line feeders that are resistively earthed
may require a setting of 1.0 to 2.0 xIn, in order to detect the minimum earth-fault, as some load bias will also be
measured during the fault. The Bias Break Point setting should therefore be set as low as possible, but should be
set to attempt to allow tripping of the minimum earth-fault on the feeder. This compromise between lowered CT
requirements for through phase faults and detection of low level internal earth-faults with load bias dictate the best
setting to adopt.
5.2 Step 2 - Determine the Fault Level and X/R Ratio of a
Through-fault
This maximum level for a three-phase through-fault can be calculated if the source and feeder primary
resistance (R) and reactance (X) values are known. Sometimes only the source fault level at the busbars will be
known. The system primary time-constant L/R, [inductance (L) / resistance (R)], can also be used to calculate the
source reactance ratio (X/R), as the time-constant (L/R) = (2π x f x L) / R. Time constant is L/R, typically 45 ms.
The maximum through-fault and maximum X/R ratio cannot occur simultaneously as one counter acts the other.
Therefore it is not technically sound to use both the circuit-breaker breaking capacity and maximum system X/R
simultaneously when calculating the CT requirements. If the source X/R is at a maximum the external fault level
will tend towards a minimum.
The above Transient Current Multiple (TCM) limits are the extremes taken from the system data contained in
international power system standards. The above figures can be used for all circuits, except for circuits where the
feeder protected by the relays is fed from a busbar source with several directly connected (i.e. no step up
transformer) generators, such as at 11kV. In this case the source TCM may exceed the above limits, and such
circuit will need careful consideration for the CT's requirements.
For example at the 132kV busbar, the source X/R is considered to be 50 and the circuit breaker has a fault
current breaking capacity of 40kA, this produces a TCM of 2000. This value is not practical for a through-fault on
any power system, so the practical maximum limit of 700 is imposed.
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System Voltage (kV)
33 and Below
66
132
275
400
Source - Transient Current
Multiple(TCM)
X/R x Fault Level (kA)
500
600
700
900
1000
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