Research On Key Aspects In Z-source Converter

The traditional power converter consists of two type topologies: Voltage source (VS) and Current source (CS). It is used in different occasions, and there exist some limitations and drawbacks in traditional power converter: 1) the voltage source converter can be destroyed by shoot-through states results from EMI, while current source converter has the same problem hurted by open-cuicuit; 2) the VSR and CSI is a boost converter, and CSR amd VSI has a buck characteristic, it does not achieve a buck/boost feature. The recently developed Z-source converter has some special characteristics due to the extra topology.This dissertation focuses on the points (advantages and problems) which appeared in the practical applications. It concludes the advantages and presents the methods for exists problems.The proposed Z-source converter achieved some merits, such as buck/boost voltage and improved reliability of the inverter without adding any other circuits concerning the X-type Z-source network. On the contrary, it affects the dynamic response. In order to reseach the dynamic response of the Z-source converter, a small-signal modeling of Z-source network, which based on Z-source inverter, has been presented, each transfer function has been analyzed in detail. The control-to-Z-source capacitor voltage transfer function has non-minimum phase characteristics, and has imfluenced by Z-source network component values heavyly. A linear PID controller for Z-source capacitor and A fuzzy PID controller for dc-link voltage has been designed, and gained good performance (fast dynamic response, small steady state errors).In the practical applications, the weight, volume, dynamic response and adaptability to load plays important roles in evaluate the converter. The Z-source network inductor increases converter's weight, volume, and decreases its dynamic response and adaptability. Select small Z-source inductor can improve all fetures but it: 1) results in dc-link voltage aberrance, 2) increase current stress. The light-load and low power factor also can arouse dc-link voltage aberrance, compelled the dc-link voltage out of control, and deteriorate inverter output ac voltage. A bi-derectional Z-source inverter has been proposed. The presented bi-derectional Z-source inverter can eliminate the dc-link voltage aberrance with small Z-source network inductor at light-load condition, and obtain the high-performance output ac voltage. The system dynamic response is improved due to reduced small Z-source inductor, and the adaptability for load is ameliorated.The application of adjustable-speed drives (ASD) in commercial and industrial facilities is increasing due to improved efficiency, energy saving, and process control. Voltage sags can interrupt an ASD system, thus shutting down critical loads and processes. The Z-source inverter ASD system can provide ride-through during the voltage sags without any additional circuits. Concerning the ASD's light-load condition, a bi-derectional Z-source inverter ASD system has been proposed, which avoid the abnormal operation mode shown in chapter 2. A "partly PAM/PWM" control method has been presented, which increases the modulation index M and reduces the iron loss of the induction motor. In order to decrease the dc-link voltage ripple, a feedforward controller has designed, the effects of the input voltage ripple (especially the 6-pulse voltage ripple caused by front-end diode rectifier) have been eliminated, and a good performance output ac voltage is obtained.In the micro-grid and distributed generation system, the output dc voltage of the micro-source (fuel-cell, or photovoltaic) is wide-range changed, in order to decrease the KVA requirement of the inverter, a dc-dc boost converter or Z-source inverter can be used. The "output power-output voltage curve" and "output voltage-output current curve" of the photovoltaic array have affected strongly by environment values (irradiance level and panels' tempreture), it is necessary to track continuously the MPP in order to maximize the power output from a PV system, for a given set of operating conditions. This dissertation proposes two control methods for both MPPT and grid-connectted inverter. 1. two-stage control: using modulation index to achieve constant dc-link peak voltage, regulating shoot-through duty cycle to fulfill the PV MPPT control. 2. one-stage control: employing unified space vector for control both PV MPPT control and grid-connected controller. A guideline for designing both MPPT controller and grid-connected controller is depicted. With aforementioned analysis, a MPPT controller based on "perturb & observe" has been designed and employed in Z-source inverter PV system, the experimental results verified the theory analysis.Three different inverter system topologies are to be investigated: the conventional PWM inverter, the dc/dc boosted cascaded PWM inverter, and Z-source inverter. Concerning the active switching devices and system reliability, the Z-source inverter has prominent superiority: a. removed a dc-link active switching device, its drive and protection circuits, minimized the cost and improved the system reliability; b. shoot-through can no longer destroy the inverter. From dc link passive components requirement and CPSR, the Z-source inverter and the dc/dc boosted PWM inverter have almost the same values.The boost type PWM rectifier has been increasingly employed since it offers the possibility of a low distortion line current with unity power factor for any load condition. Another advantage is its capability for nearly instantaneous reversal of power flow. Unfortunately, it does not provide buck/boost feature, and can be destroyed by shoot-through. A novel ZVS (ZCS) Z-source rectifier has been presented, which has buck/boost feature and permit shoot-through. A ZVS (ZCS) of the rectifier switching devices has been achieved with selecting felicitous time to add the shoot-throgh interval. It improves the active switching devices working environment, and decreases power loss.From the generalized Z-source converter, a new three-phase ac-ac Z-source converter is proposed. The proposed three-phase ac-ac Z-source converter can keep the output voltage steady by operating at buck or boost mode. Therefore, it has the capability to overcome the voltage sag or voltage surge which may damage the equipments.