Environmental Engineering Reference
In-Depth Information
+
DC
power
source
Grid
Direction of power
Figure 4.28
Line commutated thyristor inverter
Note that the inductance on the DC side is still required. It causes the direct current feeding
the inverter bridge to be pretty much constant (from one cycle to the next), which leads to
this circuit being called a
current - fed
or
current - source inverter
.
The thyristor inverter shown in Figure 4.28 has several shortcomings:
1. The current waveform on the AC side is almost a square-wave (see Figure 4.27 ) and
therefore has very high harmonic content.
2. The fundamental component of the square-wave alternating current is out of phase and
lagging the AC voltage. The circuit consumes reactive power from the grid.
3. The circuit requires connection to the grid (or a synchronous generator) in order for the
currents to commutate. Therefore, the circuit cannot be used as a general purpose stand-
alone AC supply.
Nonetheless, thyristor inverters and controlled rectifi ers, based on the above concepts, were
the foundation of the power electronics industry during the 1960s and 1970s. Thyristor invert-
ers were used in early variable speed wind turbines and grid-connected PV systems. The
shortcomings of the converter based on thyristors are all but removed by the use of transistor
switching devices.
4.5.6 The Transistor Bridge
During the 1980s, self-commutated power semiconductor devices (BJTs, MOSFETs, IGBTs,
GTOs and IGCTs) started to become commercially viable, which led to the development of
self - commutated inverters
an example of which is shown in Figure 4.29. In a self- commutated
inverter, the switching, both on and off, of the main power semiconductor devices is under
control. Thus, a self-commutated inverter does not need a grid connection in order to operate
and can be used to create a standalone AC power supply, the frequency of which is determined
by the transistor fi ring control circuit.
Note that the inductance on the DC side is no longer required. This inverter is fed with
a voltage that is pretty much constant (from one cycle to the next), which leads to it being
called a
voltage - fed
or
voltage - source inverter
. On the other hand, for reasons explained
later, an inductance is required on the AC side if the inverter is to be connected to the
grid.
