Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2002-01-17
2003-08-05
Berhane, Adolf Deneke (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S137000
Reexamination Certificate
active
06603675
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention is occupied with the problem to convert direct voltage into three-phase alternating voltage and conversely in all types of applications, such as in plants for transmission of electric power and for connecting three-phase electric machines to DC voltage loads and sources. Any ranges of voltages, currents and powers are conceivable.
The invention relates more particularly to such an apparatus comprising a VSC-converter (VSC=Voltage Source Converter) having a direct voltage intermediate link with a positive and a negative pole and at least one phase leg interconnecting the two poles and having at least two current valves connected in series, each current valve having at least one semiconductor device of turn-off type and a rectifying member connected in antiparallel therewith, the apparatus further comprising a transformer with two opposite ends of a first winding thereof connected to an output each of the VSC-converter and with a second winding thereof connected to an arrangement adapted to form voltage pulses for forming an alternating phase voltage, the apparatus also comprising a unit adapted to control the VSC-converter and said arrangement for obtaining said voltage conversion, and a method for converting direct voltage into alternating voltage and conversely according to the preamble of the appended independent method claim.
It is pointed out that “first winding” and “second winding” are to be interpreted as a primary and a secondary winding of a transformer used for voltage transformation, although it is here not indicated which one is which.
“Rectifying member” is here and in the entire disclosure, including the appended claims, to be interpreted broadly, and it may be any member with ability to take a voltage and block current in at least one direction therethrough, and it does not have to be a diode, but it could for example also be controllable, such as a thyristor (see for example
FIG. 3
of this disclosure). Furthermore, the rectifying member and the semiconductor device may also be integrated in one single semiconductor device or switching device. This means for the VSC-converter a semiconductor device with reverse conducting property, such as a MOSFET with an inherent “body diode”.
An apparatus of this type may be used for converting direct voltage into alternating voltage and conversely in applications where it is important to obtain a galvanic isolation between the direct voltage side and the alternating voltage side. Furthermore, it is possible to obtain a voltage with variable frequency and amplitude on the alternating voltage side, a bilateral power flow and voltage as well as current transformation by an apparatus of this type.
A known such apparatus comprises a transformer that operates at the alternating voltage side frequency, which generally means a low frequency and thereby a heavy and bulky transformer. This results in a considerably lower efficiency of the transformer and thereby of the apparatus than would the transformer be able to operate at higher frequencies.
An apparatus of this type enabling operation of the transformer at higher frequencies than the alternating voltage frequency is known through “Power loss reduction techniques for three-phase high frequency link DC-AC converter” 24
th
Annual IEEE Power Electronics Specialists Conference, PESC '93 Record. pp. 663-668, 1993 by I. Yamato and N. Tokunaga, and through “High-frequency link DC/AC converter with suppressed voltage clamp circuits-naturally commutated phase angle control with self turnoff devices”, IEEE Transactions on Industry Applications, vol. 32, No. 2, pp. 293-300, March-April 1996 by M. Matsui, M. Nagai, M. Mochizuki and A. Nabae, the latter being shown in the appended FIG.
1
. The reference numerals used there are as follows: direct voltage intermediate link
1
, voltage source converter
2
, transformer
3
, arrangement
4
, direct voltage side
5
and alternating voltage side
6
. The arrangement on the alternating voltage side of the transformer is here a cycloconverter operating with natural commutation and converting the high frequency alternating voltage from the voltage source converter into an alternating voltage of the desired frequency. However, the voltage source converter still operates with forced commutation and hard switching resulting in comparatively high stresses on the semiconductor devices of the current valves resulting in comparatively high switching losses. Furthermore, the current valves of the voltage source converter are in the apparatus according to the latter reference controlled by a control unit
7
according to a method resulting in square voltage pulses with no zero-voltage interval thereby indirectly increasing the content of harmonics in the alternating voltage side output voltage. Besides the fact that the power lost in the form of heat results in considerable costs the semiconductor devices of the current valves have to either be dimensioned to be able to withstand high thermal stresses and thereby be costly or a lower frequency of the VSC-converter has to be applied resulting in a more bulky transformer and a degraded curve shape for the alternating current curves.
It is also known to utilize so called soft switching for reducing switching losses in apparatuses for converting direct voltage into alternating voltage and conversely, and these concepts generally incorporate additional semiconductor devices that do not take part in the power conversion itself. These additional (auxiliary) semiconductor devices and the control circuitry associated therewith add to the costs and complexity of such an apparatus. Moreover, they often involve a derating of the main semiconductor devices in the current valves either in terms of the maximum current or voltage.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an apparatus of the type defined in the introduction having improved properties with respect to such apparatuses already known.
This object is according to the invention obtained by providing such an apparatus, in which the VSC-converter comprises at least one snubber capacitor connected to said current valves thereof, in which said arrangement comprises a direct converter having at least three phase legs connected through the opposite ends thereof to opposite ends of said second winding of the transformer and having at least two current valves connected in series, each of these current valves being able to conduct current and block voltage in both directions and to turn on by gate control, and in which the midpoints of said phase legs of the direct converter are provided with phase outputs for forming a terminal for said alternating phase voltage between these phase outputs.
The use of at least one such snubber capacitor in an apparatus of this type including a VSC-converter, a transformer and a direct converter results in a possibility to obtain soft switching of the semiconductor devices in the VSC-converter. This capacitor/these capacitors will be used as energy storing means and be discharged and recharged when changing the switching state of the VSC-converter remarkably reducing the voltage derivatives when the valves are switched and the direct converter commutating the current gives rise to further advantages with respect to switching losses and stresses for the semiconductor devices and rectifying members, and the former may also be turned on at zero-voltage and low current derivatives. The rectifying members, e.g. diodes, may be turned on at low voltage derivatives and turned off at zero-voltage and at low current derivatives. In the direct converter no hard turn-off capability is needed, but the valves may very well turn off at a current zero-crossing similar to the turn-off process in a conventional thyristor converter. Accordingly, the losses may be reduced in an apparatus of this type with respect to such apparatuses already known and thereby costs be saved. Less costly semiconductor devices may also be used thanks to the reduced therma
ABB AB
Berhane Adolf Deneke
Dykema Gossett PLLC
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