ABB ETL600, Instruction Manual

Page: 33 / 382

ETL600 1KHW001489-EN
ABB Switzerland Ltd
3. STRUCTURE AND FUNCTION
3.1. Introduction
Transmission lines exhibit low attenuation in the carrier frequency
range between 20 and 500 kHz. They are therefore a good means of
communicating information over medium to long distances (20 to
100 km, or 100 to 500 km respectively). The maximum range of a PLC
communications channel operating at approximately 80 kHz or less
can reach up to 800 km, which cannot be matched by other means of
communication that power companies have at their disposal (cables,
optical fibres, pilot wires, normal radio or point-to-point radio) without
repeaters or repeater stations.
By installing appropriate coupling devices and line traps in the power
stations and substations, communications channels can be provided,
which exhibit
•
extremely high mechanical rigidity and high reliability of the
interconnecting lines.
•
lines and terminal equipment, which belong to and is permanently
under the control of the power utility.
•
low, relatively constant attenuation and moderate long-duration
noise level (corona) under normal atmospheric conditions.
•
breakers and load-break isolators.
The system includes means to combat burst noise, which minimize the
possibility of false signals or tripping. In spite of the additional cost
represented by the coupling devices and line traps, especially at very
high voltage levels, the overall cost of a PLC communications system
is relatively low compared with other techniques and the cost relation
becomes even more favorable with longer distances. These are the
two main reasons why many power utilities prefer PLC for power
system communication.
The PLC equipment is capable of transmitting information for a mixture
of various services such as (refer to Fig. 3-1)
•
telephony,
•
teleoperation,
•
data,
•
teleprotection
simultaneously by combining all information into one single signal,
which occupies a predefined bandwidth in the
R
adio
F
requency
(subsequently = RF) frequency range from typically 40 to 500 kHz,
suitable for transmission via a power line. The amount of information
which can be transmitted is limited by the bandwidth of the RF signal,
which is typically 4 kHz or a multiple thereof. This is called the nominal
bandwidth B
n
of the PLC equipment. For the backwards channel, a
distinct frequency band also of size B
n
is used. This band is usually
adjacent to the one of the forward channel, refer to Fig. 3-2a.
Structure and Function November 2005 3-1