User manual

PQM-702, PQM-703 Operating Manual
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Since the introduction of such signals to the power supply may have negative consequences
for some devices, similarly to the effect of harmonics or interharmonics, EN 50160 standard defined
limits for the 3-second mean values of such signals, as shown in Fig. 33. During the measurement,
99% of average 3-second control signals values must be below the specified limit.
Low frequency signals (up to 3 kHz) are used for switching on/off the loads, filters and protection
devices. One application is to control the street lighting or (in some countries) remote controlling of
HVAC devices. Often, this kind of signals are used for customers using two types of energy tariff
(e.g. when using a cheaper night tariff, the energy supplier automatically disables selected loads).
This type of communication is usually unidirectional. Due to the low attenuation features of the
distribution network at this frequency range (attenuation increases with increasing frequency), com-
munication using this method allows users to achieve the greatest range of transmission (even
hundreds of kilometres). During the transmission control signal is transmitted in several packages
and repeated at specified intervals. The period during which the signal is active may be quite long,
e.g. for 2 seconds signal is on and for 2 seconds it is off - this sequence is repeated several times.
There are cases when this type of transmission results in flicker. An example of this type of trans-
mission is shown in Fig. 32.
Fig. 32. An example of low-frequency signal transmission.
Higher transmission frequencies (and hence, higher bit rates) are typical for PLCC communica-
tion. This type of communication uses modulation of amplitude or carrier frequency (or other mod-
ulation method). Modern methods use complex algorithms to process signals in order to achieve
the highest resistance to interference and highest bit rate (transmission speed). PLCC transmission
continuously gains popularity and its application range increases. The communication between net-
work points may be bidirectional. The concept of so-called smart grid is based on PLCC, which is
one of the main methods of communication between energy meters and central points. The main
application areas include: telemetry, optimization of power consumption, remote control of loads.
Attenuation of the distribution network limits the maximum transmission range. Maximum range may
reach a few km, while there is a strong correlation between the type of modulation, bit rate and
achieved distances.
At the same time, standardization works are in progress to use of higher frequencies (above
148.5 kHz to tens of MHz) for the purpose of short-distance data transmission.