Environmental Engineering Reference
In-Depth Information
i
dc
i
s
v
dc
v
s
Supply
Load
Figure 4.26
Circuit of a single -phase thyristor bridge
v
s
t
Average
v
dc
α
i
s
Figure 4.27
Waveforms in a thyristor bridge, while rectifying
nearly constant, at least in the short-term from one cycle to the next. In the longer term, it
will vary to match the average level of
v
dc
.
A control circuit (not shown in Figure 4.26) applies fi ring pulses to the thyristor gate con-
nections. The thyristors are fi red in diagonal pairs. The fi red pair take over the conduction of
the current, which reduces the current in the other pair to zero and switches them off. Control
of the fi ring angle
provides a means of controlling the average DC voltage. Figure 4.27
shows the situation when
α
= 50 °. The shaded areas, above and below the average DC
voltage, are equal (because, in the steady state, the average voltage across the inductance
must be zero). At
α
were
increased past 90 °, the average DC voltage would theoretically become negative. This is an
impossible operating condition in the circuit of Figure 4.26, because the direct current would
be required to reverse direction, i.e. fl ow from cathode to anode through the thyristors.
However, if the load resistor were to be replaced by a DC source, whose voltage was larger
than the average negative voltage of the converter, the DC current direction will be main-
tained, power will be transferred from the DC source to the AC side and the bridge will
operate as an
inverter
. Rotating the circuit diagram in Figure 4.26 through 180 ° puts the
positive DC rail back at the top and gives the normal way of drawing an inverter, with the
power fl owing from left to right as in Figure 4.28.
α
= 90 °, the average DC voltage would be reduced to zero. If
α
