Siemens 7SR18 Solkor, Manual

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7SR18 Applications Guide
Unrestricted ©2018 Siemens Protection Devices Limited Page 41 of 63
Some typical Examples of Cable Charging Currents are given in the Table below:
Voltage kV Charging current (A) per km
3.3 0.2 to 0.7
6.6 0.5 to 1.6
11 0.7 to 2.4
22 1.1 to 3.2
33 1.3 to 3.5
66 4 to 7.5
132 5 to 11
220 10 to 20
400 15 to 30
The above figures are for single cables only. Where two cables per phase are used the feeder charging current
will double. The highest charging current figures at the top end of the range are for the largest cross-sectional
area single core cables and for small diameter three core cables at the bottom of the table. If the charging current
is not known the top figure in the range may be used with confidence, as it will tend to overestimate the feeder
charging current and set the relay to a more stable differential setting.
The above table should only been used as a worst case estimate. For optimum relay settings the differential
setting to select should be based on a multiple on the true charging current or susceptance of the cable.
7.2 Plain Poly Phase Cable Feeders
This type of cable is usually used at 33 kV and below. The reactance of these cables tends to be low as the
phase currents tend to cancel in each cable. The X/R of the external fault to use in the CT formula will tend
towards the cable X/R if the cable exceeds 2 km to 3 km. This assists in reducing the CT requirements.
7.3 Single Phase Cable Feeders
The major difference between this type of circuit and poly-phase cable circuits, is that the transient and steady
state charging current will be higher. The charging current will rise with rated voltage and the length of the circuit.
The variable setting relay is recommended for this type of circuit, as it offers the flexibility to cope with a variable
level of charging current. The P/F differential setting must be set above the charging current on the feeder. The
waveform recorder in the relay can be used to assess the magnitude of charging current. Several Cables per
phase may be used and this may increase overall charging current by a multiple of a single cable.
The P/F Differential should be set as per the recommendations in 6.1.
The cable X/R will reduce the external fault X/R to a significant extent is the cable is more than a few kilometres in
length.
7.4 Overhead Line Feeder
The relay is suitable for protecting circuits of this type. The settings can be set more sensitively than for cable
feeders as charging current is much lower. The P/F differential setting may need to be set towards the lower
settings of 10% to 20% to cover arc resistance and/or resistively earthed neutral. The Slope 1 Setting should also
be set to 20% and the Bias Break Point may need to be set to 1.0, 1.5 or 2.0 to allow for load current flow during
the internal earth-fault. The Bias Break Point will usually have to be set higher for overhead lines, than for cable
circuits, to allow for load bias during a high resistance earth-fault on the feeder.