Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2003-03-13
2004-02-24
Vu, Bao Q. (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S098000
Reexamination Certificate
active
06697274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an open-loop and closed-loop control method for a self-commutating three-point converter that is fed from a DC voltage intermediate circuit and has active clamped switches. Typically, such three-point converters have two series-connected main switches/inverse diodes between each DC voltage connection and each load connection, with the common junction point of the two inner main switches forming the load connection, and with an active clamped switch with an inverse diode being connected between each common junction point of an inner main switch and an outer main switch and the center tap of the DC voltage intermediate circuit, as a result of which two possible paths are formed for connecting a load connection to the center tap. The invention further relates to an apparatus for accomplishing the method. Converters such as these may be used both as self-commutated rectifiers and as self-commutated inverters. They are primarily used in medium-power and high-power electrical drives.
The topology of the self-commuted, diode-clamped three-point converter on the DC intermediate circuit (three-point NPC converter) is generally known. It is also used industrially for fields of application such as high-power industrial or traction drives (medium-voltage drives). In this case, insulated gate bipolar transistor (IGBT) modules with an integrated inverse diode are used as main switches. For reasons of modularity, simplification of the mechanical construction, or else in order to ensure that the blocking voltage is shared uniformly when semiconductor components are connected in series in converters such as these, IGBT modules are also frequently installed as NPC switches (referred to in the following text as active NPC switches or active neutral-point clamped switches, or active clamped switches) instead of neutral-point clamped diodes (NPC diodes). These IGBTs are in this case either placed in the “off” state by short-circuiting the gate-emitter path, or else are operated in the active range in order to control the blocking voltage distribution, while the integrated inverse diode carries out the function of the NPC diode.
FIG. 1
shows a generally known, self-commutated three-point converter, fitted with NPC switches, of this type on the DC intermediate circuit, or three-point NPC converter, for short. An outer main switch T
1
U, T
1
V, or T
1
W—referred to in general form by T1 in the following text—and an inner main switch T
2
U, T
2
V, or T
2
W—also referred to in general form in the following text as T
2
—are respectively connected in series between the positive DC voltage connection and the three load connections, with a respective inverse diode D
1
U, D
1
V, or D
1
W—also referred to in general form in the following text as D1—being connected back-to-back in parallel with each outer main switch T
1
U or T
1
V or T
1
W, respectively, and a respective inverse diode D
2
U, D
2
V, or D
2
W—also referred to in general form in the following text as D2—being connected back-to-back in parallel with each respective inner main switch T
2
U, T
2
V, or T
2
W.
A respective outer main switch T
4
U, T
4
V, or T
4
W—also referred to in general form in the following text as T4—and an inner respective main switch T
3
U, T
3
V, or T
3
W—also referred to in general form in the following text as T3—are connected in series between the negative DC voltage connection and the three load connections, with a respective inverse diode D
4
U, D
4
V, or D
4
W—also referred to in general form in the following text as D4—being connected back-to-back in parallel with each respective outer main switch T
4
U, T
4
V, or T
4
W, and a respective inverse diode D
3
U, D
3
V, or D
3
W—also referred to in general form in the following text as D3—being connected back-to-back in parallel with each respective inner main switch T
3
U, T
3
V, or T
3
W. The load-side phase currents (load currents) are annotated i
phU
, i
phV
, and i
phW
.
The common junction point of T
1
U, D
1
U, T
2
U, and D
2
U is connected via an active NPC switch T
5
U with a back-to-back parallel-connected inverse diode D
5
U to the center tap of the DC intermediate circuit. The common junction point of T
1
V, D
1
V, T
2
V, and D
2
V is connected in the same way via an active NPC switch T
5
V with a back-to-back parallel-connected inverse diode D
5
V to the center tap of the DC intermediate circuit. In the same way, the common junction point of T
1
W, D
1
W, T
2
W, and D
2
W is connected via an active NPC switch T
5
W with a back-to-back parallel-connected inverse diode D
5
W to the center tap of the DC voltage intermediate circuit. The active NPC switches T
5
U, T
5
V, T
5
W are also referred to in general form in the following text as T5. The inverse diodes D
5
U, D
5
V, D
5
W are also referred to in the following text as D5.
The center tap is connected via two capacitors with the same capacitance to the two DC voltage connections. The voltage across each of the capacitors is V
dc
/2 (half the intermediate circuit voltage).
The common junction point of T
3
U, D
3
U, T
4
U, and D
4
U is connected via an active NPC switch T
6
U with a back-to-back parallel-connected inverse diode D
6
U to the center tap of the DC voltage intermediate circuit. The common junction point of T
3
V, D
3
V, T
4
V, and D
4
V is connected in the same way via an active NPC switch T
6
V with a back-to-back parallel-connected inverse diode D
6
V to the center tap of the DC voltage intermediate circuit. In the same way, the common junction point of T
3
W, D
3
W, T
4
W, and D
4
W is connected via an active NPC switch T
6
W with a back-to-back parallel-connected inverse diode D
6
W to the center tap of the DC voltage intermediate circuit. The active NPC switches T
6
U, T
6
V, T
6
W are also referred to in general form in the following text as T6. The inverse diodes D
6
U, D
6
V, D
6
W are also referred to in the following text as D6.
An investigation into diode-clamped three-point NPC converters with sinusoidal modulation shows that the thermal configuration of these converters is governed by four critical operating points, which are quoted in the following Table I. At each of these four critical operating points, the phase current (load current) and hence the output power from the converter is limited by the maximum permissible losses in those power semiconductors which are most heavily loaded at this critical operating point. All the other semiconductors reach only a lower boundary layer temperature at the respective critical operating points. Since the maximum losses and the maximum boundary layer temperatures of the individual semiconductors reach comparable values at the operating points that are critical for them, all the components must be replaced by larger components if the output power of the converter is to be increased.
An additional critical operating point when using converters in electrical drive systems, particularly those with synchronous machines, is the starting or stopping of the drive. This situation is characterized by a very low output frequency from the converter, down to zero Hertz, and a low modulation level M. The phase current (load current) is in this case limited by the losses in the NPC diodes, which corresponds to case 2 in Table I below. Due to the low output frequency, one phase may be loaded with the peak value of the load current for a certain time period, this being sufficient to reach the thermally steady state. The achievable load current is thus reduced considerably in comparison to operation at high output frequencies. Although this problem can be minimized by reducing the switching frequency while stopping, a reduction in the load current with respect to the rated current whilst stopping cannot be avoided in conventional medium-voltage drives. Applications such as hot and cold rolling mills typically demand 200% load torque and hence twice the load current when the drive is being stopped, however. In consequence, satisfaction of this condition leads in a disadvantageous m
Bernet Steffen
Brückner Thomas
ABB Research Ltd.
Vu Bao Q.
LandOfFree
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