7PG21 Solkor Rf Applications Guide
4
5/15Kv Isolation Voltage
Any electrical current which flows in a path parallel to the pilot cable will cause a voltage to be induced along the
pilot cable. This voltage can become significant for large values of current, long lengths of parallel path and higher
mutual coupling factors caused by poor screening or close proximity of current paths. This voltage can lead to
flashover inside of the relay case from the circuits connected to the pilots to the relay case and local ground. The
problem can be worsened by ground voltage shift between the two substations at the feeder ends due to earth
fault current. Earth shift voltage is often ignored in cable power systems because of the high percentage of the
earth fault current which returns through the cable sheath and armouring, however with overhead line systems the
earth shift voltage can be as significant as the induced longitudinal voltage.
The Solkor R/Rf relay will withstand 5kV rms voltage. This can be increased to 15kV by the addition of an
isolation transformer.
5kV isolation is usually acceptable for 11kV cable distribution systems where zero sequence currents are
relatively low and protected feeder lengths and therefore parallel runs are relatively short. For higher voltages
where longer feeder lengths are common15kV insulation may be required but 5kV may be acceptable if fault
levels are low or feeder lengths are short.
The pilot cores should be allowed to ‘float’ with neither core earthed at either end. Capacitive coupling to the local
ground along the cable length will ensure that voltage at either end will cause the pilot voltage to remain
symmetrical to the ground voltage such that the withstand requirement at each end is approximately half of the
longitudinal induced voltage.
Induced voltage is proportional to parallel length, maximum parallel current and the coupling or screening factor
between the pilot and the current path. This can be very difficult to assess accurately by calculation and cannot
generally be measured.
The maximum current is generally accepted as the EARTH fault level for an out of zone fault. Although a phase-
phase or 3-phase fault may have a higher fault current, the fault current for these faults will return locally in a
parallel path in the opposite direction i.e. in the other phase(s). With an earth fault, the return path may be distant
or non-parallel with the pilot such that the net current which couples to the pilot can be considered maximum for
the earth fault. The through fault current level is used in combination with the total feeder length as a worst case
scenario because although an internal fault may have a greater fault current, the parallel path will be shorter by
definition.
5
Pilot Cables
The above considerations of insulation and balance between cores, it is evident that pilot cables for use with pilot
wire current differential feeder protection are required to have special consideration when long lengths and high
fault currents are involved. It is also apparent that the effects are not easily analysed or modelled and thus in-
service experience is the most reliable basis in deciding which types of pilot will be satisfactory.
The UK has vast experience of the use of pilot wire differential feeder protection and the UK supply industry
specification on multipair cables, ESI Standard 09-6 is therefore particularly applicable as a reference for pilot
wire requirements.
It should be noted that the voltage between cores in the pilots is limited by the non-linear resistors which are
connected across the summation transformers in the Solkor relays at the ends. Also note that any induced
voltage will be at an equal level per unit length in all cores and screen. Thus it is possible to use pilots with 500v
grade insulation between cores and core to screen. The 5 or 15kV insulation requirement exists only between
‘internal cores and screen’ to the local earth. Similar considerations should be observed at any cable terminations
where standard 500v terminals can be used but the whole terminal block should be mounted on an insulating
baseplate to comply with insulation requirements to the local ground. Terminals should be shrouded and clearly
marked since during a system fault (included a fault on any parallel feeder, not only the protected circuit) the
induced voltage may pose a serious risk to health. Inside of the protection panel, the insulation to local earth and
segregation of wiring for health and safety purposes may be more easily achieved by the use of separate cable
trunking which can be routed independently and clearly marked rather than by the use of special cabling inside of
the panel. Special precautions will be required when terminating or handling pilot connections.
Pilot inter-core capacitance has the effect of shunting the relays in the current balance scheme. As the
capacitance increases a point is reached where the shunt impedance has a significant effect on the relay settings.
This produces a maximum limit for pilot capacitance which can be used with the relay. With the relays in the
Solkor R connection mode the pilot capacitance maximum limit is 2.5
μ
F and with the Solkor Rf connection mode
this limit is 0.8
μ
F. These limits can be increased for the Solkor Rf mode by the use of transformer tappings if the
©2010 Siemens Protection Devices Limited
Chapter 3 Page 10 of 15
