Page 227 - Power Electronic Control in Electrical Systems

//SYS21/F:/PEC/REVISES_10-11-01/075065126-CH006.3D ± 215 ± [177±262/86] 17.11.2001 10:23AM

Power electronic control in electrical systems 215

shown in Figure 6.39. Clearly, as described above, four DC bus sources are used and
this time more clamping diodes are required.
For the five-level half-bridge leg shown in Figure 6.39, there exist switch pairs that
require complementary control signals. These pairs are: (S a1 , S a5 ), (S a2 , S a6 ), (S a3 ,
S a7 ), and (S a4 , S a8 ). Moreover, the various switches do not have the same switching
frequency and as the number of levels increases such a problem becomes more of a
drawback.
The states of the switching devices of a five-level single-phase half-bridge VSC as
shown in Figure 6.39 are summarized in Table 6.7.
One significant drawback of the NPC topology is the unequal distribution of
switching losses among the switches and also the unequal load distribution among
the various capacitors. This problem becomes more serious as the number of levels of
the converter increases.

6.3.6 Other multilevel converter topologies
So far many power electronics based circuits have been presented which are capable
of generating more than two-level voltage waveforms. There is however another
converter topology, which is also capable of producing multilevel voltage waveforms.
This topology contains two legs per phase as shown in the single-phase version in
Figure 6.40. Each phase leg consists of two legs similar to the half-bridge explained
earlier and shown in Figure 6.23. Each leg is controlled independently and with a
specific phase-shift is able to generate a three-level voltage waveform between the
phase point A and the mid-point of the DC bus O. In order to be able to add the
waveforms generated by the two legs, an inductor-based configuration is used as
shown in Figure 6.40.
For square-wave operation, the voltage waveform between the point A 1 and the
DC bus mid-point is a two-level waveform taking values between V dc /2 and V dc /2.
The same applies for the voltage waveform between the other point of the phase leg
A 2 and the point O. These two signals are phase-shifted accordingly and are drawn in
Figures 6.41(a) and (b). The potential of point A referred to the point O is the sum of
the two waveforms v A1O , and v A2O . This voltage waveform is shown in Figure 6.41(c).
It is a three-level waveform taking values of V dc , 0 and V dc . Finally, the voltage
across the inductor, that is the potential difference between the two points A 1 and A 2 ,
is illustrated in Figure 6.41(d). Similar arrangements can be used for the other two-
phase legs to build a three-phase converter. Furthermore, more legs per phase can be
used and more inductor arrangements can be used to sum even more voltage wave-
forms so that higher numbers of levels for the phase voltage waveform can be
generated. This depends upon the application of course. Such arrangements can also
offer opportunities to cancel more harmonics with appropriate phase-shifting.
Another circuit to obtain multilevel systems is of course the combination of the
NPC converter and the arrangement with inductors to add voltage waveforms. The
converter then is three-level with respect to one-phase leg, and becomes a five-level
one with respect to the phase. Such a five-level circuit based on inductor summing
and the NPC converter is shown in Figure 6.42. In this case appropriate phase-shifted
PWM techniques can be used to take advantage of the topology and position the first
significant harmonics of the resultant output voltage waveforms to higher frequencies.