Patent GB-1141524-A (GB)
Title: Information link for high voltage power lines
Inventor: BREWSTER ARTHUR EDWARD
IPC: G08C23/04
Filed: 1966-03-22
Abstract: 1,141,524. Transistor switching circuits; modulating light; signalling over power lines. STANDARD TELEPHONES & CABLES Ltd. 22 March, 1966. No.12555/66. Headings H3T, H4F and H4R. [Also in Division G1] The current or voltage on a high-voltage line is detected and its value transmitted to a receiver at earth potential over a light path which can also be used in the transmission of a further intelligence signal (speech or code) from the location at earth potential to the high-voltage line for transmission of the intelligence along the line. In one arrangement, Fig. 3, a transmitter 18. 23 receives signals representing the current or voltage on the line, the signals being picked up by transformer or capacitative means e.g. a transformer comprising a coil 16 clamped round the cable which acts as the primary of the transformer. Alternatively, a number of Hall effect sensors (25), Fig. 6 (not shown), are mounted in series around the cable, to cancel out unwanted field effects and enable operation down to D.C. The light pulses emitted by the transmitter follow a free-space path through the atmosphere to a receiver 13 at ground level. The transmitter is transistorized and comprises a pulse width modulator 18, Fig. 3, feeding an infra-red-emitting diode or a laser, 23. The circuit, Fig. 4, includes a pre-amplifier for the signal picked off the line, supplying a longtailed pair of transistors which also receive a signal from a sawtooth waveform generator, producing a train of width modulated square pulses which are amplified before application to the transmitter diode. A feature of the pulse amplifier is that a diode D 1  is provided between input and output to protect against saturation of the final (power) transistor T 5 . If amplitude modulation is used a detector located in the light path can provide some negative feedback to the transmitter to compensate for non- linearities in light emission. A. lens system may be used to focus the beam on to the receiving diode 13 which is connected into the receiver circuit. Fig. 5 (not shown) comprising a demodulator, amplifier and low-pass filter 19. The signal representative of the current or voltage value is then applied to a cathode-ray oscilloscope 21. There is provision for speech communication along the high-voltage line between locations at earth potential. A telephone handset 20 is coupled to a modulator 22 controlling a transducer 24 emitting light over the same path as used in the system 16-13, the intelligence being detected at 14 and impressed on the line by the transformer 16. Besides sensing the line condition the system 16-13 is used to pick up speech signals on the line and feed them to the handpiece 20. The frequency used in the intelligence link is 10 or 100 times that of the 50 c/s. of the frequency of the high voltage in the line. A linesman can thus read the oscilloscope 21 and send information as to the line condition to a remote control station. A coding unit located at earth potential can also be to examine the line condition and retransmit it to the remote station in code form, the frequency used being outside the speech band or speech and code being transmitted alternately over the intelligence link. Instead of the transformer 16 and electronic transmitter 18, 23, other devices may be used to detect line condition and intelligence signals and transmit them over the light path, e.g. electro-optical devices operating by the Kerr or Pockels effects, or devices using the Faraday or Kerr magneto-optical effects. In using the Kerr magneto-optical effect, Fig. 7(a), a transformer core is clamped round the high-voltage cable. By means of a passsage (H) through the core light can impinge on and be reflected from the polished north pole, its polarization being a function of the current in the cable.