Environmental Engineering Reference
In-Depth Information
inverter arrangement known from controlled d.c. drives. The average d.c voltage is
adjustable by using phase control, dependent on trigger delay angle
α
.Byfiringthe
thyristor at angle
, commutation is initiated, and the relevant valve takes over load
current, until at the next commutation the thyristor is extinguished when the current
decreases below the holding current. The circuit as shown is capable of working both
as a rectifier supplying d.c. loads from the three-phase grid (0
<
α
α
<
π
/
2t) and as an
a.c inverter supplying power from the d.c. to the a.c. side (
α
max
), where
α
max
is the maximum permissible trigger delay angle with respect to the stability
limit, in practice
π
/
2
<
α
<
α
max
= 150
◦
...
160
◦
.
The average d.c. voltage for fully controlled bridge circuit, with
U
2
=
U
UV
the
secondary line-to-line voltage, is:
U
di
0
=
3
√
2
π
U
di
α
=
U
di
0
cos
α
where
·
U
2
= 1
,
35
·
U
2
(4.1)
= 0.
Assuming ideal conditions, i.e. neglecting commutation delay and a very large
d.c. side inductance, the current waveform will be composed of 120
◦
blocks, see
Fig. 4.3. The transformer secondary side current reflects the 120
◦
blocks. The same
current blocks appear on the primary side in the case of YY-connection (as shown
in the figure), whereas in the case of
Note that the case of a diode bridge rectifier is included with
α
Δ
Y-connection the terminal current shows a
different step-function.
With
I
d
the (flat) d.c. current according to Fig. 4.3, the following related quanti-
ties are:
I
v
=
1
-
average valve current
3
·
I
d
= 0
,
33
·
I
d
I
2
=
2
-
r.m.s. line current
3
·
I
d
= 0
,
816
·
I
d
(4.2)
I
2
,
1
=
√
6
π
-
fundamental line current, r.m.s.
·
I
d
= 0
,
78
·
I
d
Obviously the currents contain harmonics, of which in the B6 circuit the promi-
nent order numbers are 6, 12 on d.c. side and 5,7,11,13 on the a.c. side.
Figure 4.3 is a simplified picture assuming sinusoidal a.c. voltage, flat d.c. cur-
rent (no harmonics) and immediate commutation. More realistic is the picture in
Fig. 4.4, where commutation delay expressed by an overlap (commutation) angle
u
is taken into account [IEC60146]. Figure parts (a) and (b) is related to rectifier and
a.c. inverter operation, respectively. For the sake of simpler visual representation
the waveforms belong to a three-pulse M3 circuit, and not to a B6 circuit as in the
former figure.
Presuming an inductance
L
C
per phase in the commutation circuit, a decrease of
the average voltage can be described by using a fictitious series resistance
R
x
on the
d.c. side. Equation (4.1) is to be replaced by:
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