Section 7: Differential Protection Settings For Feeder Circuits; Capacitive Charging Current - Cable And Hybrid Feeders - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
Section 7: Differential Protection Settings For Feeder
Circuits
The relay provides protection for two ended sub-transmission and distribution feeders. It combines current
differential feeder protection with back-up over current and earth-fault protection suitable for these feeders. There
are various primary circuit types each requiring specific considerations.
These include: -
·
Plain poly-phase Tri-foil cable Feeders.
·
Phase segregated, single phase cables Feeders.
·
Overhead line feeders.
·
A Feeder with a mixture of the above.
Each of these configurations is discussed below:
Special consideration is also given to earth-fault protection provided by the relay for different network designs with
respect to the method of grounding. The feeder charging current will have the most impact on the minimum
differential setting that may be chosen.
7.1 Capacitive Charging Current – Cable and Hybrid Feeders
Significant Charging currents are to be expected on cable or hybrid (a mix of cable and over head line) feeder
circuits. Pure overhead line feeders will not have significant charging currents and will not generally affect the
lowest differential setting that may be chosen. The cable charging current will increase linearly with circuit length.
The capacitive charging current is at leading power factor to the feeder load current and has the affect of causing
a phase and magnitude difference to arise between the current measured at each end of the feeder. This normal
steady state difference in currents will have an impact on the minimum differential settings that may be used.
The relay must be set with a P/F Differential setting that must be minimum multiple of the steady state charging
current. The steady state charging current may be calculated from the cable data and the circuit length. This
multiple of charging current is necessary to accommodate transient charging current, steady state charging
current and rises in phase to neutral voltages during system faults.
Typical figures are in terms of a multiple of the steady state feeder charging current (Ic).
Feeder
Type and
System
Earth
Method
Solidly
Earthed
Resistance
Earthed
Reactance
Earthed
Isolated
A significant transient charging current will flow a cable feeder is first energised. The relay digital filtering is
designed to remove almost all of this transient current. The frequency of this current tends to be a high multiple of
power system frequency. Therefore the steady state charging current only need to be considered when selecting
the differential setting.
The revision of the relay software installed, may be found moving to the top of the menu structure to display the
relay identifier and holding the [Cancel] and [Test/Reset] pushbuttons depressed or by selecting [Relay]
[Information][Get System Information] in Reydisp Evolution. If the setting file for the relay is saved, open the .set
file in Reydisp Evolution, clicking on the Info (i) tab at the top right hand corner of the "Settings Editor" window.
The Software Revision should now be displayed in the "Settings Source Information" window.
These above figures include provision for transient increases in healthy phase charging current during external
earth-faults on non-effectively earthed power systems.
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Pure
Cable
Feeder
2.5 x Ic
4 x Ic
5 x Ic
5 x Ic
Hybrid
Feeder
(OHL+Cable)
4 x Ic
4 x Ic
5 x Ic
5 x Ic
©2018 Siemens Protection Devices Limited
Pure OHL
Feeder
4 x Ic
4 x Ic
5 x Ic
5 x Ic