3High Frequency Power Plants Telephony (HFPPT)

Transmission equipment had been created based on the principle known from the high-frequency carrier telephone systems. It was used with amplitude modulation or frequency modulation. Domestic Standard CSN 33 4640 permitted, based on an exemption from the Telecommunications Act, to implement service transmission for energy purposes in the band of 30-750 kHz and the width of each channel of 2.5 kHz and 4 kHz. At the same time, the recommendation specified the parameters of splitters and safety requirements.

Coupling devices for connecting communication equipment to power lines must provide several basic parameters. First of all, they must exhibit high attenuation for the frequency of 50 Hz, and the smallest possible attenuation for the telecommunications band. It must contain components that ensure safe handling, not only up to the nominal voltage and phase, but also at the overvoltage or short-cut conditions. It should also contain components for signal routing (i.e. there should be huge attenuation for the direction to a power transformer and small attenuation in the direction to a line).

Exemplary schematic arrangement of the most widely used interphase capacitive coupling of high-frequency devices to high-voltage power lines is shown in the following figure.

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Schematic arrangement of most widely used interphase capacitive coupling high-frequency devices to high-voltage power lines.

Coupling capacitors for outdoor use (of the order of nF) are connected to a coupling filter and create a bandpass filter tuned to the telecommunications signal band. A glow is used as internal overvoltage protection. Coupling filter also provides impedance match of the coupling elements with the impedance of a high-frequency symmetrical cable, which is brought to a transmission device.

Signal routing towards high-voltage line or from the line towards transmission facilities provide high-frequency band stop filters. These filters are tuned to the transmitted high-frequency band around the carrier and thus prevent the loss of signal in the transformer substation, and reduce crosstalk to other lines. The technologically most challenging part of the filter is a choke that must be dimensioned for the short circuit current for a given phase, which is of the order to hundreds of amperes. Safety functions are ensured by lightning arresters, working even in the case of failure of coupling capacitors. When handling with a coupling device, appropriate working conditions are ensured by earthing.

Although implementation of narrow-band high-frequency systems was very demanding from the technological and economical perspective, their deployment was very purposeful. High-frequency systems were used not only for telephony, but also for data transmission, telex and telemetry. Individual high-frequency lines in the Czech Republic were gradually interconnected to create a vast and robust network, used for the control of the operation of a power system. At the end of the last century, therefore, there was a nationwide dispatching network, connecting all power plants and important switching stations.

A new component of remote external energy network - fibre optic cable incorporated into the protective steel rope in outdoor high-voltage and very-high voltage links, opened door for a technically and economically advantageous transmission path with variety of broadband transmission options. We deal with the replacement of original copper wires (R, AlFe) by specialty combined grounding cables (CGC), in which telecommunication optical cables are combined with the conductive wires. Optical cables are for broadband telecommunication channels used in long-distance telecommunications transmission technology.