【文章內(nèi)容簡介】 of freedom exist which enable sinusoidal input and sinusoidal output currents. However, in this context it is believed that the most advantageous control of the inverters is to control the primary converter to operate in squarewave current mode. Here, the switches in the CSC are turned on and off only once per fundamental period of the input and output current respectively. In square wave current mode, the switches in the primary converter may either be , or a series connection of an IGBT and a diode. Unlike the primary converter, the secondary converter has to operate at a high switching frequency, but the switched current is only a small fraction of the total load current. Figure 7 illustrates the current waveform for the primary converter, the secondary converter, is, and the total load current il. In order to achieve full control of the current to/from the backtoback PWMVSI, the DClink voltage is boosted to a level above the grid voltage. As mentioned, the control of the tandem converter is treated in only a few papers. However, the independent control of the CSC and the backtoback PWMVSI are both well established, (Mutschler amp。 Meinhardt, 1998)。 (Nikolic amp。 Jeftenic, 1998)。 (Salo amp。 Tuusa, 1997)。 (Salo amp。 Tuusa, 1999). Figure 7. Current waveform for the primary converter, ip, the secondary converter, is, and the total load current il. Advantages in the use of the Tandem Converter The investigation of new converter topologies is monly justified by the search for higher converter efficiency. Advantages of the tandem converter are the low switching frequency of the primary converter, and the low level of the switched current in the secondary converter. It is stated that the switching losses of a tandem inverter may be reduced by 70%, (Trzynadlowski et al., 1998a) in parison with those of an equivalent VSI, and even though the conduction losses are higher for the tandem converter, the overall converter efficiency may be increased. Compared to the CSI, the voltage across the terminals of the tandem converter contains no voltage spikes since the DClink capacitor of the secondary converter is always connected between each pair of input and output lines (Trzynadlowski et al., 1998b). Concerning the dynamic properties, (Trzynadlowski et al., 1998a) states that the overall performance of the tandem converter is superior to both the CSC and the VSI. This is because current magnitude mands are handled by the voltage source converter, while phaseshift current mands are handled by the current source converter (Zhang et al., 1998b). Besides the main function, which is to pensate the current distortion introduced by the primary converter, the secondary converter may also act like an active resistor, providing damping of the primary inverter in light load conditions (Zhang et al., 1998b). Disadvantages of using the Tandem Converter An inherent obstacle to applying the tandem converter is the high number of ponents and sensors required. This increases the costs and plexity of both hardware and software. The plexity is justified by the redundancy of the system (Trzynadlowski et al., 1998a), however the system is only truly redundant if a reduction in power capability and performance is acceptable. Since the voltage across the generator terminals is set by the secondary inverter, the voltage stresses at the converter are high. Therefore the demands on the output filter are parable to those when applying the backtoback PWMVSI. In the system shown in Figure 38, a problem for the tandem converter in parison with the backtoback PWMVSI is the reduced generator voltage. By applying the CSI as the primary converter, only % of the grid voltage can be utilized. This means that the generator currents (and also the current through the switches) for the tandem converter must be higher in order to achieve the same power. Matrix converter Ideally, the matrix converter should be an all silicon solution with no passive ponents in the power circuit. The ideal conventional matrix converter topology is shown in Figure 8. Figure