Handbook of Electrical Installation Practice - BeKnowledge

Fig. 22.11 Solkor R pilot wire protection.
Protective Systems 613
Although Solkor R is in essence a current balance scheme, a current-to-voltage
conversion is made by the summation transformer and pilot voltage limiting nonlinear
resistance. This voltage is then used to drive current around the pilot circuit
which is fixed in ohmic value by having pilot padding resistors. The diodes in this
pilot loop ensure that the relays are always at the negative end of the voltage divider
created by the fixed resistor and the pilot resistance. By making the fixed resistor
1750W and padding the pilots to 1000 W the relays have a negative bias which is
effective because they are positively polarised by diodes connected in series with
them. The important factor to note is the absolute and instant application of the
bias. In biased relays generally the smoothing sometimes required (e.g. when biasing
is arranged within magnetic circuits) causes delay in the establishment of the bias
and hence the requirement to use a relatively slow measuring relay to avoid
transient instability.
Solkor Rf system
This protection is a development of the Solkor R range but utilises a double output
on summation transformer to produce a sine wave current flowing through the pilots
composed of alternative positive and negative pulses. Solkor R has half wave unidirectional
pulses current in the pilots. The advantage of the Rf system is that for
internal faults the protection operation is faster. Also, the effective a.c. pilot current
enables interposing transformers to be inserted in the pilot circuit. These transformers
are used to provide 15kV barriers between the pilot and relay circuits for
those conditions where the possibility exists of high voltages being induced into the
pilots.
Figure 22.12 shows the Solkor Rf connections.
Solkor N system
This is a modern numeric protection which utilises either RS485 twisted pair cable
(up to 1.2km) or optical fibre cable (up to 49km) for the end-to-end signalling
channel. Because it exploits the processing power of modern microprocessor chips,