Electric power conversion systems – Current conversion – With means to introduce or eliminate frequency components
Reexamination Certificate
2001-05-07
2002-05-07
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
With means to introduce or eliminate frequency components
C363S067000
Reexamination Certificate
active
06385064
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
The field of the invention is AC to DC converter systems and more specifically a blocking reactor including three cores for blocking harmonic currents in a nine-phase converter system.
Rectifiers are used to rectify AC voltages and generate DC voltages across DC buses. A typical rectifier includes a switch-based bridge including two switches for each AC voltage phase which are each linked to the DC buses. The switches are alternately opened and closed in a timed fashion that, as the name implies, causes rectification of the AC voltage. As well known in the energy industry the global standard for AC power distribution is three-phase and therefore three-phase rectifier bridges are relatively common.
When designing a rectifier configuration there are three main considerations including cost, AC line current harmonics and DC bus ripple. With respect to AC current harmonics, when an AC phase is linked to a rectifier and rectifier switches are switched, the switching action is known to cause harmonics on the AC lines. AC line harmonics caused by one rectifier distort the AC voltages provided to other commonly linked loads and therefore should generally be limited to the extent possible. In fact, specific applications may require that large rectifier equipment be restricted in the AC harmonics that the equipment produces.
With respect to DC link ripple, rectifier switching typically generates ripple on the DC bus. With respect to cost, as with most hardware intensive configurations, cost can be minimized by using a reduced number of system components and using relatively inexpensive components where possible.
The most common and available type AC to DC converter is a three-phase rectifier system including six semiconductor switches arranged to form a converter that links three AC input lines to positive and negative DC buses where the voltage on the input lines is spaced by 120 electrical degrees. This type of six-switch converter system exhibits relatively high DC output voltage ripple at a frequency that is six times the AC line frequency. For example, where the line frequency is 60 Hertz, the ripple is typically 360 Hertz. Converters that include six switches are generally referred to as six-pulse rectifiers.
It is well known in AC to DC rectification that AC current harmonics and DC ripple may be improved by increasing the number of AC phases that are rectified where the AC phases are phase-shifted from each other. For example, by rectifying nine-phase AC current instead of three-phase currents, harmonics and ripple are reduced appreciably. To rectify nine phase currents the industry most solutions employ three three-phase rectifiers, each of the three rectifiers including six switches arranged to form a bridge between each of three of the AC supply lines and DC rectifier outputs. The outputs can be linked in several different fashions to provide one positive DC bus and one negative DC bus as described in more detail below. Three rectifier configurations that include a total of 18 switches are generally referred to as 18 pulse rectifiers.
As the global standard for AC power distribution is three-phase, a mechanism for converting three-phase current to nine-phase current is necessary prior to rectification via any 18-pulse rectifier. To this end the industry has devised several different three to nine-phase transformer configurations. An exemplary three to nine-phase transformer and rectifier configuration is illustrated in
FIG. 1
including a transformer
100
, and first, second and third rectifiers
120
,
140
and
160
, respectively, that link three AC supply lines
122
,
124
and
126
to positive and negative DC buses
128
and
180
, respectively. Transformer
100
receives three 120 degree phase shifted AC currents I
A
, I
B
and I
C
on input lines
122
,
124
and
126
and provides nine AC output currents I
1
through I
9
on nine AC output lines (not numbered) where the output currents include three currents I
4
-I
6
that are in phase with the input currents, three currents I
1
-I
3
that lag the input currents by 20 degrees and three currents I
7
-I
8
that lead the input currents by 20 degrees.
Currents I
1
through I
3
, currents I
4
through I
6
and currents I
7
through I
9
are provided to rectifiers
120
,
140
and
160
, respectively. The outputs of rectifiers
120
,
140
and
160
are linked together in parallel. The rectifier input currents I
1
-I
9
are summed together to produce a primary current I
A
through I
C
having reduced harmonics. Because the pre-rectified voltages V
1
-V
3
, V
4
-V
6
and V
7
-V
9
are spaced out 20 degrees, their rectified DC voltages fill each other's valleys and hence produce an 18 times fundamental frequency ripple that is relatively smoother when compared to six-switch configurations.
In theory 18 pulse systems like the one illustrated in
FIG. 1
have the advantage that each rectifier needs only include components having a power rating corresponding to one third the overall DC output power rating. Thus, in theory 18-pulse rectifier switches in parallel linked configurations can be one third the size of switches required for six pulse rectifiers.
In reality, however, for two reasons the rectifier components have to be greater than the theoretical one-third rated DC size. First, due to manufacturing limitations, slight magnitude differences occur in most cases among the rectifier input voltages. These slight voltage magnitude differences produce slight DC voltage differences at each of the separate rectifier outputs. For example, DC output voltage variance among rectifier outputs is often within the range of 0 to 2 volts.
Converter systems are typically constructed for very low impedance to provide a stiff voltage source to a load. For this reason the slight differences in DC voltage, although small in most cases, cause the rectifier with highest output DC voltage to carry much more DC load current when compared with the current carried by the other rectifiers.
Second, referring again to
FIG. 1
, in a typical application the three-phase power source would be linked to many loads like the one illustrated and each of those loads would cause some degree of harmonic distortion on supply lines
122
,
124
and
126
. As known in the industry, the rectified DC voltage for a single three-phase bridge with pre-existing 5
th
and 7
th
harmonics is:
V
d
c
=
3
3
2
π
V
1
(
1
-
1
5
V
5
V
1
cos
φ
5
-
1
7
V
7
V
1
cos
φ
7
)
Eq
.
1
with
V
A
=V
1
sin &ohgr;t+V
5
sin(5&ohgr;t+&phgr;
5
)+V
7
sin(7&ohgr;t+&phgr;
7
) Eq. 2
V
B
=
V
1
sin
(
ω
t
-
2
π
3
)
+
V
5
sin
(
5
ω
t
+
φ
5
+
2
π
3
)
+
V
7
sin
(
7
ω
t
+
φ
7
-
2
π
3
)
Eq
.
3
V
C
=
V
1
sin
(
ω
t
+
2
π
3
)
+
V
5
sin
(
5
ω
t
+
φ
5
-
2
π
3
)
+
V
7
sin
(
7
ω
t
+
φ
7
+
2
π
3
)
Eq
.
4
Equation 1 indicates that both the magnitude and angle of the harmonic voltages influence the DC voltage. As obvious from
FIG. 1
, the rectifier input voltages V
1
-V
3
, V
4
-V
6
and V
7
-V
9
are spaced out 20 degrees. Thus the values of the harmonic angles (see Equations 1 through 4) for each rectifier
12
,
14
and
16
, are changed causing the rectified DC voltages from each rectifier to be different. Thus, the pre-existing harmonics also contribute to current unbalance among different rectifiers.
In order to avoid such unbalance problem, one solution is to connect all three bridges in series, instead of in parallel. Referring again to
FIG. 1
, this ty
Buchholz Brian R.
Guskov Nickolay N.
Hachey Bruce A.
Maslowski Walt A.
Skibinski Gary L.
Han Jessica
Rockwell Technologies LLC
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