Invertor

Details Invertor Invertor

1998-08-21

2001-01-02

Han, Jessica (Department: 2838)

Electric power conversion systems

Current conversion

Having plural converters for single conversion

C363S040000, C363S043000

Utility Patent

active

06169676

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is concerned with the field of power electronics. It relates to an invertor comprising a plurality of invertor bridges, which operate in parallel on the same DC voltage intermediate circuit and whose output voltages are summed via summation means, the invertor bridges each being driven with pulse duration modulation according to a carrier signal, and the carrier signals of the individual invertor bridges having a constant phase difference between one another, and the summation means having a center tap, which is grounded via a ground connection.
2. Discussion of Background
In order to connect electricity supply systems having a different number of phases and/or AC voltage frequency, such as e.g. between a 50 Hz three-phase power supply system and a single-phase 16 ⅔ Hz railway grid, use is increasingly being made of solid-state couplings and railway power converters which are equipped with power semiconductors and are often designed as converters having a DC voltage intermediate circuit. In accordance with
FIG. 1
, such a railway power converter
10
comprises, for example, a (thyristor-equipped) converter
13
which draws the three-phase current from the three-phase power supply system
11
via a transformer
12
and converts it into a direct current, a DC voltage intermediate circuit
14
for smoothing and/or buffer-storage, and an invertor
15
which converts the direct current back into an alternating current at the desired frequency and feeds it into the railway grid
16
.
In the invertor
15
, use is usually made of one or more invertor bridges, operating in parallel, with switchable valves (e.g. GTOs), which are driven with pulse duration modulation and approximate the desired sinusoidal output voltage by a sequence of duration-modulated square-wave pulses of alternating polarity. A triangular-waveform auxiliary control voltage is usually used in this case for the pulse duration modulation. Details about the driving can be found for example in an offprint (No. 9608-1000-0) from the applicant “Vollstatische 100-MW-Frequenzkupplung Bremen” [Solid-state 100 MW frequency coupling Bremen]. If a plurality of invertor bridges are operated in parallel, the output voltages are summed. A reduction in the harmonic content is achieved by driving the individual invertor bridges via the auxiliary control voltages in a phase-shifted manner.
An example of the structure of an invertor
15
is represented in FIG.
2
. The invertor
15
of this example comprises
8
invertor bridges B
1
, . . . ,B
8
which operate in parallel and, with a respective capacitor C
1
, . . . ,C
8
in parallel at the input, are connected to the input lines
17
,
18
coming from the DC voltage intermediate circuit
14
. A transformer
19
is provided for the purpose of summing the output voltages of the invertor bridges B
1
, . . . ,B
8
, which transformer contains a winding pair comprising a primary winding P
1
, . . . ,P
8
and a secondary winding S
1
, . . . ,S
8
for each of the invertor bridges B
1
, . . .,B
8
. The outputs of the invertor bridges B
1
, . . . ,B
8
are respectively connected to the corresponding primary windings P
1
, . . . ,P
8
; the secondary windings S
1
, . . . ,S
8
are connected in series. The summed output signal is available on the output lines
20
,
21
. In order to suppress harmonics, the transformer
19
may additionally be equipped with tertiary windings T
1
, . . . ,T
8
, which are connected in series and are damped by a corresponding filter circuit
25
(in this respect see, for example, EP-B1-0 149 169). Examples of duration-modulated and phase-shifted pulse trains for the invertor bridges B
1
, . . . ,B
8
are represented in FIG.
3
. Summation of the individual pulse trains in the transformer
19
produces therefrom the resultant summation voltage u
Bi
in FIG.
4
.
Problems with the type of invertor illustrated in
FIG. 2
arise if—as is necessary in the case of some railway grids—the transformer
19
of the invertor
15
is grounded at a center tap
23
by a ground connection
24
via a resistor
22
(or else without a resistor, that is to say in “hard” fashion) (see FIG.
2
). These problems may be illustrated with reference to the equivalent circuit diagrams represented in
FIGS. 5
to
8
: The invertor, which operates as a voltage source converter (
V
oltage
S
ource
C
onverter, VSC), can be described in principle (
FIG. 5
) by a voltage source
26
having the voltage u
Bi
which drives a corresponding current i
Bi
through a circuit formed by the impedances
27
,
28
and
29
. The impedances
27
and
28
with the values z
1
and Z
2
, respectively, represent the transformer
19
, and the impedance
29
with the value Z
3
represents the filter circuit
25
. The railway grid
16
can be described in the equivalent circuit diagram by the impedance
30
(Z
4
) and the voltage source
31
.
As a result of the grounding (via the resistor
22
) at the center tap
23
of the transformer
19
, the equivalent circuit diagram of the VSC from
FIG. 5
can be converted into an equivalent circuit diagram in accordance with FIG.
6
. The voltage source
26
is in this case divided into two voltage sources
32
and
33
having the partial voltages u
Bi,a
and u
Bi,b,
where:
u
Bi
=u
Bi,a
−u
Bi,b
  (1)
The impedances
27
and
28
of the transformer
19
are now divided in
FIG. 6
into impedances
34
and
39
and, respectively,
35
and
40
, in each case having half the original impedance value, namely z
1
/2 and z
2
/2. The impedance
29
with the value Z
3
is preserved while the impedance
30
and the voltage source
31
of the railway grid
16
are likewise divided into the impedances
36
and
41
(in each case having the value Z
4
/2) and, respectively, voltages sources
42
and
43
. The grounding via the center tap
23
of the transformer
19
is represented by the resistor
37
having the value R
E
in
FIG. 6. A
corresponding resistor
38
having the value R
E,r
describes the total remote grounding resistance of the railway grid
16
.
According to the concept of modal decomposition, the equivalent circuit diagram of
FIG. 6
can be decomposed into two superposed subsystems, namely into the common-mode system and the differential-mode system. The two superposed systems can then be treated separately from one another and the resultant currents and voltages simply added at the end of the analysis in order to obtain the real physical quantities. The equivalent circuit diagram in the common-mode system for the upper half of the VSC is represented in FIG.
7
. In addition to the already known impedances
34
,
35
and
36
, the circuit contains the resistors
45
and
46
, which each amount to twice the grounding resistors
37
and
38
, respectively. The voltage source
44
outputs a voltage u
BI,CM
which drives a current i
Bi,CM
through the circuit. The equivalent circuit diagram in the differential-mode system for the upper half of the VSC is illustrated in FIG.
8
. In addition to the already known impedances
34
,
35
and
36
, the impedance
48
is present here as well, which impedance corresponds to half the impedance
29
and is characteristic of the filter circuit
25
. The voltage source
47
outputs a voltage u
Bi,D
which drives a current i
Bi,D
through the circuit.
The following relationship emerges for the voltages and currents:
u
Bi,a
=u
Bi,CM
+u
Bi,D
  (2)
 
u
Bi,b
=u
Bi,CM
−u
Bi,D
, and also  (3)
i
Bi
=i
Bi,CM
+i
Bi,D
and  (4)
i
E
=2
* i
Bi,CM
.  (5)
is immediately evident from
FIGS. 5
to
8
and equations (1) to (5) that the common-mode voltage u
Bi,CM
is undesirable because it drives a common-mode current i
Bi,CM
which can flow back only through the grounding resistors
37
and
38
. The level of the common-mode current i
Bi,CM
is primarily limited by the impedances z
1
and z
2
of the transformer
19
. The common-mode current i
Bi,CM
ha

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