Siebel
Dr.-Ing. Henrik Siebel
"Netzseitiger Wechselrichter mit quasiresonanter Topologie und hoher Leistungsdichte" (2001)
The present work presents a new concept of an active inverter for the feeding of a DC-load out of a three-phase mains. The main objective is the design of a topology with a high efficiency at a high switching frequency. These features lead to a compact mechanical design with integrated filter components and line chokes in the same housing of the power electronics.
The topology presented in this work features a considerable reduction of the switching losses. A quasiresonant commutation unit has been added to an IGBT inverter bridge. For a short time this unit reduces the voltage of the DC-link to zero so that the IGBTs can be switched on with practically no losses. Capacitors connected parallely to each IGBT enable their soft switching.
Since the reverse recovery charge in the diodes causes a considerable amount of the switching losses in a hard-switched topology this work proposes a topology with a limited rate of fall of the currents in the diodes reducing in this way the losses during the commutation. The switching of the diodes at zero current reduces the reverse current and the switching losses of the diodes. It is also remarkable that all power semiconductors of the proposed quasiresonant topology are softly switched.
A principal requirement for the operation of the front end inverter is a total harmonic distortion of the line currents as low as possible. Besides the DC-link voltage has to be stabilized. Both demands can be fulfilled by the classical cascaded control. In this conventional control scheme the controller for the DC-link voltage delivers the reference value for the current controller. The control of the three-phase system is carried out in an orthogonal frame of coordinates. For this purpose the measured values of line current and line voltage have to be transformed into a system rotating with the corresponding frequency of the mains. The orientation angle gamma necessary for the transformation of coordinates is calculated out of the measured voltages.
For the calculation of the asymmetrical PWM patterns the particularities of the resonant topology are taken into account. Therefore a modified space vector modulation is also proposed in this work. It is synchronized to the polarity of the phase currents in a way that only two of the six bridge IGBTs have to be switched in the same cycle. Hence the switching losses are further reduced.
Measurements in the laboratory confirmed the excellent characteristics of the presented topology. For this purpose three laboratory samples were built. The first one was designed only for the rectifier operation. In a second design the topology was extended for the regenerative operation. The third design was optimized for a high power density (2,4 kW/l and 3 kW/kg) by using standard power modules.
The digital control of the system was realized by using a commercial DSP (Digital Signal Processor) featuring a great flexibility during the test phase and the commissioning. The control scheme includes an adaptive algorithm able to reject periodical disturbances by means of a look-ahead learning strategy. Since the reference value of the currents are derived from the line voltage its disturbances result into undesired distortions of the currents. In order to enhance the current quality while keeping a fast behavior of the voltage control a non-linear controller with cubical characteristic was implemented. Thus the harmonics of the line currents could be reduced to a minimal amount obtaining a THD of 1,4%.
The active front end was designed for the operation on mains of different voltages and frequencies. Due to the voltage control the load connected to the supply can relay on a constant power regardless of variations and tolerances of the voltage in the mains.
Further practical tests confirmed the capability of the control to detect and manage line disturbances like under voltage, unbalance and phase failure.
The topology presented in this work features a considerable reduction of the switching losses. A quasiresonant commutation unit has been added to an IGBT inverter bridge. For a short time this unit reduces the voltage of the DC-link to zero so that the IGBTs can be switched on with practically no losses. Capacitors connected parallely to each IGBT enable their soft switching.
Since the reverse recovery charge in the diodes causes a considerable amount of the switching losses in a hard-switched topology this work proposes a topology with a limited rate of fall of the currents in the diodes reducing in this way the losses during the commutation. The switching of the diodes at zero current reduces the reverse current and the switching losses of the diodes. It is also remarkable that all power semiconductors of the proposed quasiresonant topology are softly switched.
A principal requirement for the operation of the front end inverter is a total harmonic distortion of the line currents as low as possible. Besides the DC-link voltage has to be stabilized. Both demands can be fulfilled by the classical cascaded control. In this conventional control scheme the controller for the DC-link voltage delivers the reference value for the current controller. The control of the three-phase system is carried out in an orthogonal frame of coordinates. For this purpose the measured values of line current and line voltage have to be transformed into a system rotating with the corresponding frequency of the mains. The orientation angle gamma necessary for the transformation of coordinates is calculated out of the measured voltages.
For the calculation of the asymmetrical PWM patterns the particularities of the resonant topology are taken into account. Therefore a modified space vector modulation is also proposed in this work. It is synchronized to the polarity of the phase currents in a way that only two of the six bridge IGBTs have to be switched in the same cycle. Hence the switching losses are further reduced.
Measurements in the laboratory confirmed the excellent characteristics of the presented topology. For this purpose three laboratory samples were built. The first one was designed only for the rectifier operation. In a second design the topology was extended for the regenerative operation. The third design was optimized for a high power density (2,4 kW/l and 3 kW/kg) by using standard power modules.
The digital control of the system was realized by using a commercial DSP (Digital Signal Processor) featuring a great flexibility during the test phase and the commissioning. The control scheme includes an adaptive algorithm able to reject periodical disturbances by means of a look-ahead learning strategy. Since the reference value of the currents are derived from the line voltage its disturbances result into undesired distortions of the currents. In order to enhance the current quality while keeping a fast behavior of the voltage control a non-linear controller with cubical characteristic was implemented. Thus the harmonics of the line currents could be reduced to a minimal amount obtaining a THD of 1,4%.
The active front end was designed for the operation on mains of different voltages and frequencies. Due to the voltage control the load connected to the supply can relay on a constant power regardless of variations and tolerances of the voltage in the mains.
Further practical tests confirmed the capability of the control to detect and manage line disturbances like under voltage, unbalance and phase failure.