Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1998-02-24
1998-12-08
Berhane, Adolf
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 34, H02M 545
Patent
active
058479430
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention is directed to a method for controlling a four-dimensional vector for a controlled system by means of a discrete-value control element with a limited switching frequency. The present invention is also directed to a device for implementing this method.
BACKGROUND INFORMATION
Asynchronous machines that are supplied from current source PWM (pulse-width modulation) inverters are especially useful for high-speed, high-power drives, that is, those drives that operate in accordance with more than 1 MW. These types of converters consist of an impulse-commutated GTO inverter on the load side, while a line-commutated thyristor converter is used on the line side.
The use of shut-off valves even on the line side makes it possible to build a high-quality, steady-state, and dynamic power control element that produces only minor disturbances on the line side. Nevertheless, the only controllable valves currently available in the specified power range are GTO thyristors. Taking into account the losses produced in the control element, the possible pulse frequency is therefore limited to a few 100 Hz (f.sub.p .ltoreq.300 Hz). Due to its low damping, an LC resonant circuit causes considerable problems during transient processes, especially on the line side. An automatic control method with high dynamic characteristics is needed for the line-side impulse-commutated converter in order to actively damp the resonant filter circuit.
If the nominal fundamental frequency of the controlled system is f.sub.m =50 Hz, the ratio between the switching frequency and the nominal fundamental frequency is therefore only f.sub.p /f.sub.m .ltoreq.6.
So-called optimized pulse patterns are used with these types of low frequency-response ratios (f.sub.p /f.sub.m) in order to minimize the harmonic component in the periodic quantity of the controlled system. The low-pass effect of the controlled system, which is generally present, is not by itself sufficient for this purpose.
Nevertheless, optimizing the switching times with the technical capabilities available today can be done only off-line and only for steady-state operation, due to the complexity of the mathematical relationships. For this purpose, certain idealized assumptions must be made, such as an ideally smoothed direct current or an ideally smoothed direct voltage in the link circuit and the lack of a harmonic component in the "inner" source voltage and/or source current of the manipulated variable.
From a control point of view, modulation using an optimized pulse pattern is therefore a controlled method of determining the switching time with what are in principle poor dynamic characteristics when very fast reference value and disturbance changes occur. Since high-power, impulse-commutated load-side converters do not generally have any extreme requirements regarding the dynamic commutation and disturbance characteristics, modulation with an optimized pulse pattern has been used extensively for this purpose.
Nevertheless, a very different problem arises when using impulse-commutated high-power converters as line-side converters. Because of switching operations on the supply line, short interruptions, etc., the occurrence of fast, transient disturbances must practically always be expected.
High-speed control of the control element on the line side makes it possible, for example, to considerably increase plant availability without having to greatly overdimension the control element.
The dynamic requirements of line-side converter control increase even further due to the ever greater need to have the impulse-commutated converter behave in a more "line-friendly" manner through the use of higher-order filters. The intrinsic damping effect of the LC filters that are used is generally very minor, and, as a result, highly dynamic converter control is required in order to achieve effective, active vibration damping.
If the LC filter is also a functional and indispensable part of a control concept, which is the case, for example, with impulse-commutat
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Berhane Adolf
Siemens Aktiengesesellschaft
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