Environmental Engineering Reference
In-Depth Information
4.5.4 Harmonics
The nonsinusoidal current
i
s
in Figure 4.25 may cause problems for the AC supply network.
In particular, transformers and cables in the network will experience additional heating.
Despite this, the circuit shown in Figure 4.23 and variants with the same problem are very
widely used, because they are cheap. TVs and computers are the biggest offenders, mainly
because there are so many of them. Indeed, in areas where the load is dominated by TVs and
computers, it is common for the customer's voltage waveform
v
c
in Figure 4.25 to be
fl at-
topped
, due to the nonsinusoidal voltage drop in the supply impedance. Plotting instantaneous
supply current
i
s
against voltage
v
c
would show a highly nonlinear relationship. Appliances
such as TVs and computers are sometimes called nonlinear loads.
Another way of describing the problem is by considering the harmonic content of the
waveforms which can be obtained through Fourier analysis. For example, the supply current
i
s
in Figure 4.25 has a very large third harmonic component. This is a particular problem in
three-phase systems (in just about every large power system in the world) because the third
harmonics in the neutral conductor do not cancel (Appendix). Thus, the normal assumption
that the neutral conductor carries zero current in a balanced three-phase system no longer
holds. In general, all harmonic currents cause undesirable heating in the transformers and
cables of the supply system. In a bid to prevent excessive harmonic currents in distribution
networks, electricity utilities have instigated the introduction of regulatory standards that
set maximum permissible levels for the harmonic currents that individual appliances may
cause.
In general, all power electronic converters cause some harmonic currents in the AC network
to which they are connected. Converters, used in large high voltage DC (HVDC) transmission
systems are particularly prone to this. Indeed, much of the engineering design of such con-
verters concentrates on reducing the harmonic currents to an acceptable level. It is therefore
understandable that electricity utilities have expressed concern regarding the suggestion that,
in future, large numbers of inverters will be used to connect PV and other renewable energy
sources to distribution networks.
Fortunately, the converters used in modern renewable energy systems use techniques that
reduce the low order harmonics to negligible levels. In the context of a network supplying a
typical collection of TVs and computers, etc., the harmonic contribution from renewable
energy systems can be expected to be negligible.
4.5.5 The Thyristor Bridge Converter
Replacing the diodes in the bridge of Figure 4.22 with thyristors, as shown in Figure 4.26,
allows the DC voltage to be controlled. In many applications, the anode-cathode current is
that of a line in an AC circuit, and thus, switching-off occurs when the line current naturally
reaches zero. The transfer of current from one conducting device to another previously non-
conducting device is called
commutation
. Thyristors are solely suited in converter applica-
tions where the commutation process is carried out by the AC supply. Such circuits are known
as
line commutated
converters.
Note that the load now includes an inductance, which is typical of practical applications
of thyristor bridges. This inductance is usually large enough to ensure that the current
i
dc
is
