Research On The Key Technologies For High-efficiency Utilization Of Photovoltaic Generators

At present, the barrier to the large-scale application of Photovoltaic grid-connected generator is its economic performances in developing countries. In addition to improving the photo-electric transformation efficiency, it is one of the most feasible schemes to develop the high efficiency inverter topology and utilization scheme of electric energy. This thesis is meant to research on the high effieciency transformerless grid-connected inverters (TLGCIs) and the distributed DC supply system (DC-Microgrid).TLGCIs are characterized with higher efficiency, lower bulk, weight and cost, etc., its technology issues have been discussed in this thesis. Firstly, the design process of two-stage TLGCI, including the topologies of DC converter and grid-connected inverter, maximum power point tracking (MPPT), DC bus voltage controller, grid current controller, has been presented. For the convenience of system optimized and control, the double switch Buck-Boost converer with interleaving switching strategy is proposed as pre-stage DC converter. Characterized with lowest inductance, transferring power directly in partial, and simple control, this converer has achieved high performance with low cost. Through TLGCI prototype testing and analyzing, it showed that the leakage current and low impedance filter are keys in TLGCIs for PV application.While no transformer is used in TLGCI, the galvanic connection appears between the PV arrays and the grid. When energized by a fluctuating potential, the stray capacitance to the earth formed by the surface of the PV array may lead to the occurrence of leakage currents. A full-scale leakage current analytical model for TLGCI and the rules to eliminat common-mode source at switching frequency have been developed, and the rules have been adopted in full-bridge and half-bridge TLGCI respectively. An optimized full-bridge structure with two additional switches and capacitor divider is proposed in this thesis, which guarantees that freewheeling path is clamped to half input voltage in freewheeling period, and, the high frequency common-mode voltage has been avoided in unipolar SPWM full-bridge inverter and the output current flows through only three switches in power processing period. In addition, the clamping branch makes the voltage stress of the added switches be equal to half input voltage. The total losses of power devices in several existing topologies and proposed topology (named as oH5) are fairly calculated. Finally, the common-mode performance of these topologies is compared by a universal prototype inverter rated at 1kW. In half-bridge TLGCI, the leakage current suppressing performance of conventional topology structure is not perfect due to the different construction mechanism of parasitic parameters of bridge legs. Some feasible compensation strategies, named as'Filter branch offset solution','Parasitic branch offset solution'and'Full-offset solution', have been proposed. The compensation effectiveness of these strategies is verified by theoretic analysis and circuit test.The high reliability is an important index of TLGCI. Unfortunately, the bridge-type TLGCI also suffer from the shoot-through problem, which is a major killer of the reliability. Based on neutral point clamped three level inverter (NPCTLI) and dual Buck half bridge inverter (DBHBI), a novel Split-Inductor-Neutral-Point -Clamped three level inverter (SI-NPCTLI) with variable hysteresis band fixed-frequency control is proposed in this thesis. There are no leakage current and shoot-through problems in the proposed inverter. With the above control method, single-phase SI-NPCTLI can be extended to three-phase four-line grid-connected inverter structure. Similarly, the oH5 has been improved with above methodology.Third-order LCL filter characterized with low impedance and high harmonic attenuation performance, is suitable for TLGCI in PV applications. Base on the literatures about LCL filter design, a design program for LCL filter has been presented with trade-off means. In order to enhance the damping capability of the LCL filter, the state variable feedback to advance damping capability has been analyzed systemically. A novel damping method with differential feedback of grid-side inductor voltage has been proposed, and a parameter design procedure with proposed damping strategy and proportional plus resonant (PR) controller has been discussed detailedly.Finally, the thesis pushed primary research on high efficiency utilization framework of surtainable energy resource, a distributed generation system with DC-bus, namely DC-Microgrid, has been introduced. Following the introduction of system framework and key techniques, four converter topologies have been discussed in detail. multi-single-phase bi-directional AC/DC converter with line-transformer has been employed to connect DC-bus to the grid; a three channels transformerless bi-directional DC/DC converter has been adopted to connect DC-bus to energy storage equipment; For transferring energy between the two DC buses, a current-voltage-fed bidirectional DC-DC converter has been proposed, which refers to a current-fed inverter at low voltage side and a voltage-fed inverter at high voltage side, can realize ZVS for the switches with the use of phase-shift (PS) technology. In order to avoid the current-fed switches suffer from high voltage spike and high circulating conduction loss, a novel ZVS bidirectional DC-DC converter with phase-shift plus PWM (PSP) control scheme is proposed in this thesis, Thanks to the active clamping branch and PSP technology, the converter can realize ZVS for all switches in a wide range of load variation while input or output voltage varies; A improved double switch Buck-Boost converter with an auxiliary winding and two diode has been as interface to connect PV array with DC-bus. Above interface converters lay a solid foundation for DC-Microgrid investigation.