Starting in the early sixties, resonant power converters have been developed in order to overcome the restrictions caused by switching losses in power semiconductors. Due to the presence of a resonant circuit, values for the di/dt applied to the semiconductors can be limited. Consequently, switching losses are reduced, and higher operating frequencies can be reached. Initially, resonant converters were used 111ainly t.o generate high-frequency AC power from a DC source. Addition of a rectifier makes it possible to supply DC power to a load. However, with this addition the control of the converter becomes more complex. The topology of the power circuit or the DC-DC resonant converter can easily be extended to cater to multiphase applications. Another extension, phase-staggering, makes it possible to operate several converter modules in parallel. In this way the maximum power of a converter can be raised substantially, while at the same time increasing the effective inverter frequency. In multiphase applications, the resonant. L - C circuit is time shared by all phases. The control circuit. needs to ascertain that every phase is serviced adequately, while maintaining the integrity of the power circuit. A problem arises here because the voltages applied to the resonant circuit, which are taken from different phases for subsequent cycles of the resonant current, do -contrary to the single-phase DC-DC converter- not necessarily balance over time. This may cause the oscillation in the resonant circuit to die out, or worse, to become unbounded, leading to destruction of the components of the power circuit. The integrity of the power circuit can be assured if maximum voltages and currents in the resonant circuit, which lies at the very heart of the converter, can be kept. below a predefined maximum. An important measure for both the maximum voltage and the maximum current. in the resonant tank is the peak capacitor voltage (Vcpeak). Control of the value of Vcpeak implies control of the maximum voltages and currents applied to every element of the power circuit. The structure of a simplified, idealized model of the power circuit suggests a way to control the value of Vcpeak, independent of the magnitude of the phase voltages which are used. Under classic current. control, paralleled converter modules tend to become synchronized. The purpose of phase-staggering control is to counteract this behaviour, in order to obtain equal load-sharing between the paralleled modules and a higher effective inverter frequency. For this purpose, a prediction of the total charge displaced by every current pulse is computed, and injected in the current controller. Before construction of a prototype converter equipped with the new control system, the design of the converter hardware and the operation of the control algorithm need to be checked by means of simulations. For use in the simulations simple models are derived. Parameters for these models can be obtained either from easy to perform experiments, or from manufacturer's data sheets. The actual simulations are performed using several dedicated computer programs. Two converter systems are constructed and tested. The first converter is a 15 k\V system, converting power from the three-phase 380 V utility grid to a three-phase output. Measurements show that the system features high efficiency over a large section of the operating area, and very fast control. With the experience gained from the 15 kW setup, another system was constructed for 6 k\V operation. This system was conceived especially to demonstrate the possibility of phase-staggering control. Measurements on this converter are shown to illustrate a variety of features, the most important being the operation of the Vcpeak controller, and phase-staggering control under AC conditions.
|Qualification||Doctor of Philosophy|
|Award date||21 Nov 1992|
|Place of Publication||Delft|
|Publication status||Published - 1992|