Electric power conversion systems – Current conversion – Including automatic or integral protection means
Reexamination Certificate
2000-04-20
2001-03-20
Nguyen, Matthew (Department: 2838)
Electric power conversion systems
Current conversion
Including automatic or integral protection means
C363S137000
Reexamination Certificate
active
06205040
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to an auxiliary resonant commutated pole (ARCP) three-point or multipoint converter and, in particular, to soft-switching multipoint converter topologies for high-power converters.
Hard-switching multipoint converters, as have been proposed, for example, by A. Nabae et al. in the publication “A New Neutral Point Clamped PWM Inverter”, in “Transactions of the IEEE Industrial Applications Society”, Vol. 1A-17, No. 5, 1981, are used in the high-power field for controlling three-phase drives and, in power transmission systems, for gateways and compensation. The multipoint converter concept has been proven, in particular at high voltage levels for which the maximum reverse voltage of an individual active semiconductor device that is now available is inadequate.
At present, GTO switches with inverse diodes are used in multipoint converters with a voltage intermediate circuit in the high power field. In this configuration, the maximum current gradients di/dt and the voltage gradients du/dt that occur have to be limited by passive limiter networks, in order to avoid destruction of the active semiconductor devices. Such networks often have high losses, and contribute significantly to converter complexity and converter costs.
The maximum achievable switching frequency in these high-power converters is limited by the switching losses that occur in the semiconductor and by the minimum switching and recovery times of the semiconductor components. Since the switching frequency has a direct influence on the quality of the electrical input and output variables, and thus on the overall system configuration, the achievable switching frequency is a major quality criterion for a converter.
Progress in power-semiconductor development is now allowing converters to be operated with a considerably greater di/dt and du/dt, and this has resulted in the limiter networks becoming considerably smaller, or even being dispensed with. The present achievable switching frequency is thus now governed essentially only by the maximum permissible semiconductor losses.
Various soft-switching converter topologies that allow the switching losses to be reduced have been proposed in order to increase the maximum switching frequency for converters in the low and medium power ranges. In particular, the “Auxiliary Resonant Commutated Pole” (ARCP) principle for two-point converters, proposed in U.S. Pat. No. 5,047,913 by R. De Doncker et. al, is highly suitable for reducing switching losses. In such an ARCP converter, a snubber capacitor is connected electrically in parallel with each main switch.
Furthermore, an auxiliary circuit is proposed, which contains an auxiliary switch that is electrically connected in series with a resonant inductance, and which connects the neutral point of a DC intermediate-circuit capacitor to one output connection of the converter phase.
In addition to the drastic reduction in switching losses, the ARCP principle also allows the maximum du/dt and di/dt to be controlled which, apart from the opportunity to use critical semiconductor switches, also results in a reduction in the load on the end turns in three-phase motors.
Possible ways to extend the ARCP principle to three-point converters with neutral point clamp (NPC) diodes have been proposed by Cho et al. at the IEEE PESC Conference 1996, German Patent DE 195 36 470 by Dr A. Mertens and M. Bruckmann and by D. G. Rouaud et. al. in U.S. Pat. No. 5,684,688. In these solutions, the converter output is once again connected to at least one resonant inductance, which may be connected independently, via at least two bi-directional switches, to the two voltage neutral points of the two converter levels in the three-point converter. The difference in the topologies is the way in which the snubber capacitances for the four main switches are disposed. The number and configuration of the snubber capacitances have been varied considerably in an attempt to solve the problem of asymmetric charging movement between the upper and the lower converter level during commutation. However, it has not yet been possible to find the ideal situation, which guarantees maximum main switch load relief and a uniform capacitor load, that is to say a parallel circuit containing exactly equal snubber capacitances as close as possible to the respective main switch.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a auxiliary resonant commutated pole three-point or multipoint converter that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which a snubber network is the same for each main switch, and which can be disposed directly in parallel with each main switch.
With the foregoing and other objects in view there is provided, in accordance with the invention, a multipoint converter, containing:
at least four main switches per converter phase disposed in series defining a first series circuit, the main switches are to be connected electrically in parallel between DC voltage rails formed of a positive DC voltage rail and a negative DC voltage rail;
snubber capacitances, one of the snubber capacitances disposed in parallel with each of the main switches and the snubber capacitances define junction points between adjacent pairs of the snubber capacitances;
at least two converter levels having voltage neutral points disposed between the DC voltage rails, a junction point formed between each adjacent pair of the converter levels and the voltage neutral points of the converter levels are accessible;
a second series circuit formed of at least one resonant inductance and independently controllable bidirectional auxiliary switches;
an output of the converter phase connected to the voltage neutral points of the converter levels via the second series circuit, the output is further connected to one of the junction points formed by the snubber capacitances; and
charge control switches connected between the junction points of the snubber capacitances which are not connected to the output and the junction point formed between each adjacent pair of the converter levels.
The configuration for the three-point converter is obtained by the configuration of the plurality of converter phases. The converter phases are each connected electrically in parallel with a DC voltage intermediate circuit formed of a series circuit formed by two capacitors. The converter phases have an upper and a lower converter level each having two main switches, and whose junction points are each linked by a NPC diode to the junction point between the two capacitors in the DC voltage intermediate circuit.
Based on the hard-switching three-point converter topology with NPC diodes, the soft-switching ARCP three-point converter topology with charge control switches can be achieved by the below listed modifications and extensions.
a). In each converter level, a series circuit containing two capacitances or alternative voltage sources have to be provided in order to ensure that a voltage neutral point is available for that converter level.
b). The output of the converter phase must be connected through a series circuit containing a resonant inductance and independently controllable bidirectional auxiliary switches to the voltage neutral points in each converter level. In this case, one connection of one auxiliary switch is in each case connected to one voltage neutral point of one converter level, and one connection of the resonant inductance is connected to the output of the converter phase.
c). A snubber capacitance must be fitted in parallel with each main switch.
d). A controllable auxiliary switch must be connected back-to-back with each NPC diode.
A low-loss ARCP converter with charge control switches is thus obtained, which combines the advantages of a three-point converter or multipoint converter with those of an ARCP converter and in which it is possible to achieve maximum switching load relief. Control of the charge of the snubber capacitances via the main switches is
ABB Patent GmbH
Greenberg Laurence A.
Lerner Herbert L.
Nguyen Matthew
Stemer Werner H.
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