Electric power conversion systems – Current conversion – Having plural converters for single conversion
Reexamination Certificate
2002-03-22
2004-01-20
Sherry, Michael (Department: 2838)
Electric power conversion systems
Current conversion
Having plural converters for single conversion
C363S037000, C363S124000, C307S082000
Reexamination Certificate
active
06680856
ABSTRACT:
BACKGROUND
The invention relates to a power converter circuit arrangement for use as a transformer between a generator with a dynamically variable output and a medium-voltage power grid. Such dynamically variable outputs occur, for example, in wind energy devices, where the generated power depends on the wind velocity. Typically, the generated currents are fed into power grids of up to tens of kV voltage at frequencies of 50 Hz or 60 Hz.
In the prior art, applications such as wind energy devices, where the current produced by the generators is temporally variable, consist of the following technologies:
For power outputs of up to 1 MW, generators with voltages of up to 690 V are used. This means that the voltage in the intermediate circuit, or in the DC connection to the power converter, lies at about 1100 V. In the associated power converter circuits, IGBTs (insulated gate bipolar transistors) are often used as power switches for a voltage of 1200 V or 1700 V. However, these intermediate circuit voltages of 1100 V are too low for higher power outputs. Since transmission losses increase as the square of the current, higher voltages, which reduce the current, reduce losses. The lower voltages of 1100 V or so result in excessive losses, for example in power lines.
For that reason, medium-voltage generators of the standardized voltage classes of 2.2 kV, 3.3 kV, 4.16 kV and 6.3 kV are used in power generation with outputs starting at about 1 MW. The resulting high intermediate circuit voltages require high-voltage power switches such as IGBTs or IGCTs (integrated gate commutated thyristors). However, these high-voltage versions have the disadvantage that power losses are higher by a factor of 3 to 10 in comparison with the standard versions.
The generators used for the above mentioned technologies are asynchronous machines. However, these robust generators require four-quadrant transformers, such as the examples described in DE 198 32 225 A1 and DE 198 32 226 A1, since for excitation, these generators need an input current which is fed in and regulated via the four-quadrant transformer.
The prior art also includes forms of power generation with synchronous machines as generators. For example, in the forms of power generation known here, the rectified output voltages of several generators are connected to a joint power converter circuit. In these, DC voltages generated by means of transformers and downstream rectifiers are used in connection with power converters, which are in the range of 100 kV, since they have low conduction losses. A large number of series-connected high-voltage IGBTs or IGCTs are used as power switches. However, the high intermediate circuit voltage has the disadvantage that such voltages result in relatively slow switching times on the order of about 1 &mgr;s, with transient voltage variation values on the order of about 100 kV/&mgr;s. To prevent such high-voltage variation values from destroying the coils in the generator and transformer, additional passive LC elements must be integrated as filters.
All these requirements increase the technical complexity and also increase the cost of such power generation systems. Furthermore, the above power converter circuits are not flexibly adaptable to generators of variable voltage and conductor classes.
A further disadvantage in using only one power converter circuit for several generators of dynamic output is the fact that, due to their constant excitation, they do not work at identical speeds and therefore do not provide identical output voltages. For that reason, various kinds of regulating mechanisms are required all of which contribute to higher losses in power generation.
OBJECTS AND SUMMARY
It is an object of the present invention to introduce a power converter circuit that generates a high-quality line voltage and works reliably even at low outputs and with a high degree of efficiency across the entire output range, which feeds the temporally variable output of a power-generating medium-voltage generator into a high-voltage grid, whereby the variable output is based on the variable speed of the generator, which leads directly to a variable output voltage and also to a variable output frequency.
It is a further object of the invention to provide a power converter circuit arrangement that is easily adaptable to variable generator outputs as well as being able to tolerate the failure of individual power switches, whereby such failure does not lead to the failure of or reduction in the power generation output.
These objects are achieved by means of the following substantial components:
a medium-voltage generator, preferably a permanently excited synchronous machine;
a rectifier circuit arrangement for transforming the AC voltage produced in the generator into a medium range DC voltage;
a DC connection in the medium-voltage range from the rectifier circuit arrangement to a power converter configured as a cascaded series-connected arrangement of power converter cells according to the invention;
a high-voltage transformer for transforming the output voltages of the power converter cells into the voltage required for the power grid;
and a master control unit.
Thanks to its temporally dynamic variable power output, the generator has an AC voltage U
gen
between 0V and a few tens of kV. The frequency of the generated voltage varies between 0 Hz and a maximum value of F
max
, which is determined by the type of generator. The generated output also fluctuates between 0W and W
max
, whereby W
max
should preferably be at least 3MW.
From this, the directly downstream rectifier circuit arrangement produces a DC voltage between 0V and approximately 1.35×U
gen
. The components of this rectifier circuit arrangement preferably consist of prior-art diodes or thyristors. Thyristors have the advantage that they can decouple the DC transmission lines from the generator in case of a malfunction.
The downstream DC voltage line connects the rectifier circuit arrangement with the power converter. In accordance with the inventive configuration, the latter consists of a cascaded series-connected arrangement of “power converter cells” according to the invention, whereby each of these power converter cells consists of the following:
a bridging switch for bridging or deactivating the power converter cell;
an input diode;
at least one intermediate circuit capacitor;
an at least single-phase bridge circuit, preferably a 3-phase bridge circuit, for each phase consisting of one power switch in TOP and one in BOT position, each with at least one parallel-connected free-running diode. Each of the power switches in turn preferably consists of a parallel-connection of several power transistors, preferably standard-type IGBTs, since these ensure fewer losses during operation than is the case with high-voltage versions;
one line inductor per phase;
one primary coil of the medium-voltage transformer per phase. The minimum number of power converter cells for a power converter of a given maximum output is determined directly by dividing the maximum DC voltage produced by the generator with a downstream rectifier circuit by the maximum intermediate circuit voltage of a power converter cell. If the number of power converter cells is greater than that minimum, this leads to a redundancy of the power converter, the use of which will be described below.
The inventive use of the power converter cells is described below. The minimum output required for efficient power generation is determined by the medium circuit voltage of an individual power converter cell.
In case of an interim circuit voltage that is lower than or equal to the maximum intermediate circuit voltage U
z,max
of a power converter cell, the master control uses only a random power converter cell, while all other power converter cells are bridged by means of their bridging switch and are therefore not active. Now, the entire circuit arrangement works like a 3-phase bridge circuit with an input rectifier according to prior art.
The master control is a master circuit with an input of data of
Darby & Darby
Laxton Gary L.
Semikron Elektronik GmbH
Sherry Michael
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