A Three-port Converter Based Distributed DC Grid-connected PV System with Autonomous Out pur Voltage-Sharing Control
Abstract: A distributed dc grid-connected photovoltaic (PV) generation configuration based on hybrid-connected three-port converters (TPCs) and its control strategy are proposed in this paper. Distributed maximum power point tracking (MPPT) and autonomous voltage sharing control are achieved with the proposed configuration and control strategy. The proposed system consists of multiple modular PWM plus phase-shift (PPS) controlled TPCs, which feature soft-switching and low voltage stress. The input-port of each TPC is connected to an independent PV energy source to achieve individual MPPT, and the output-ports of these TPCs are connected in series to interface with a high-voltage (HV) dc bus, while the bidirectional-ports are in parallel to build a low-voltage (LV) dc bus. The mismatch power of input sources can be transferred through the LV dc bus among these TPCs, and the power and voltage balancing at the high-voltage output side can be realized. By regulating the voltage reference of a bidirectional-port in a linear relationship with output voltage, output voltage sharing is realized with only the module’s own voltage and current being sensed. Fully modular design is achieved. System stability with the proposed control strategy is revealed by using
the Routh-Hurwitz criterion. Simulation and experimental results are provided to verify the effectiveness and advantages of the proposed configuration and control strategy. Existing system: The large-scale integration of PV energy into grid causes challenges to optimize the employment of these renewable energy sources. Presently distribution systems are mainly based on ac technology. Cascaded multilevel converters have been used to connect large-scale PV generation systems to the medium-voltage (MV) AC distribution network, due to the advantages of modularity, low device rating, low harmonics, and high ac voltage capability, etc. However ,the ac grid-connection approach features multi-stage power conversion, which results in reduced system efficiency. The dc grid has been considered to have the features of smaller loss, higher transfer capacity, long distance transmission capability, better stability and controllability, more convenient to supply the dc load. Proposed system: A significant challenge to connect PV arrays to the MVDC distribution network is the efficient high step-up voltage conversion, which is necessary because the output voltages of PV arrays are much lower than the high-voltage (HV) dc bus voltages in the MVDC distribution network. Currently, the research on dc gridconnected PV system is mainly concerning the dc micro grid application with low voltage (<1kV) dc bus, while there are only a few researches on PV connecting to MVDC distribution network . The differential power processing (DPP) and the cascaded dc-dc solutions are good candidates when doing MPPT of a PV system with the dc output voltage varying from dozens to hundreds volts . The DPPs only process the differential power, which brings the benefits of the low converter power rating and low power losses. Some distributed control strategies of DPPs have been proposed , which make the DPP solution benefits higher system modularity and well-suited for long sub module strings. By increasing the number of the PV modules, DPP solution could extend . Advantages:
Cascaded multilevel converters have been used to connect large-scale PV generation systems to the medium-voltage (MV) AC distribution network, due to the advantages of modularity, low device rating, low harmonics, and high ac voltage capability .However , the ac grid-connection approach features multi-stage power conversion, which results in reduced system efficiency. The dc grid has been considered to have the features of smaller loss, higher transfer capacity, long distance transmission capability, better stability and controllability, more convenient to supply the dc load, etc.. Considering the dc output nature of PV arrays, the MVDC distribution network with a 10kV or higher bus voltage, is accommodate large capacity renewable generation system integration with the advantages of reduced power conversion losses. Disadvantages:
The differential power processing (DPP) and the cascaded dc-dc solutions are good candidates when doing MPPT of a PV system with the dc output voltage varying from dozens to hundreds volts . The DPPs only process the differential power, which brings the benefits of the low converter power rating and low power losses. Some distributed control strategies of DPPs have been proposed , which make the DPP solution benefits higher system modularity and well-suited for long sub module strings. By increasing the number of the PV modules, DPP solution could extend to medium-voltage application. However, since the string PV modules are directly connected to the 10kV or higher dc-bus, high-voltage insulation and safety requirements would be the challenges .Additional isolated converters might be the challenges . Modules: Differential power processing : A significant challenge to connect PV arrays to the MVDC distribution network is the efficient high step-up voltage conversion, which is necessary because the output voltages of PV arrays are much lower than the high-voltage (HV) dc bus
voltages in the MVDC distribution network. Currently, the research on dc gridconnected PV system is mainly concerning the dc micro grid application with low voltage (<1kV) dc bus, while there are only a few researches on PV connecting to MVDC distribution network . The differential power processing (DPP) and the cascaded dc-dc solutions are good candidates when doing MPPT of a PV system with the dc output voltage varying from dozens to hundreds volts . The DPPs only process the differential power, which brings the benefits of the low converter power rating and low power losses. Some distributed control strategies of DPPs have been proposed , which make the DPP solution benefits higher system modularity and well-suited for long sub module strings. By increasing the number of the PV modules, DPP solution could extend to medium-voltage application. However, since the string PV modules are directly connected to the 10kV or higher dc-bus, high-voltage insulation and safety requirements would be the challenges. Additional isolated converters might be challenges . Modular multilevel converter : The two-stage dc-dc modular multilevel converter (MMC) technology is shown in is capable of connecting PV arrays to a10kV or higher bus voltage. The first-stage dc-dc converter is used for individual MPPT and ensures the isolation between PV arrays and grid. The second-stage half-bridge cells are in a cascaded connection with an output filter inductor to connect to the HV dc bus. The average output voltage of each power unit would vary and differ from others when individual MPPT is realized. Still the same dc-link voltages can be obtained with the duty cycle being varied, as the average output voltage of the half-bridge choppers is DĂ&#x2014;Vdc_i. The equal voltage stress on every power unit can be obtained. However when large different MPPs occur, large different duty cycles happen, which will result in large variable voltages being applied to the inductor and then lead to high inductor current stress and conduction losses. Meanwhile, two-stage power conversion is used and system efficiency is hurt. Moreover, the half. Configuration description and analysis of the proposed TPC-based system : The configuration of the proposed system based on modular TPCs is illustrated .The input-port of each TPC is connected to aPV energy source , and the output-ports of these TPCs in one cluster are connected in series to interface with a
HV dc bus. There can be multiple clusters of PV generation system connected to the HV dc bus. The bidirectional-ports of these TPCs in one cluster are connected in parallel to build a LV dc bus. It should be noted that there is not load connected to this dc bus .Individual MPPT control can be realized ,because each PV array is regulated by different TPC. The bidirectional-port of these TPCs can absorb power from or supply power to the LV dc bus if it is regulated to realize voltage sharing among output-ports of these TPCs. Specifically ,a TPC with higher input PV power can supply some power to the LV dc bus ,and this part of power can be fed to the TPC with lower PV input power and then transferred to its output-port. Thus, the output power on output-ports of these TPCs can be equal to each other, no matter the input power from the PV source of each TPC is equal or not, and output voltage sharing can be achieved simultaneously. Failure of PV energy source : Thanks to the mismatch power processing ability, the proposed system can maintain stable operation even under the condition of the failure of some PV energy sources. Take a two-TPC-based system ,for example ,and assume PV2 is disconnected from the system. In other words, there will be no power input form the input-port of TPC2. According to the power-flow paths as shown in Fig. 3(b), part of the input power from PV1 will bet transferred to the bidirectional-port of TPC2 and then delivered to the output-port. As a result, the output voltage sharing performance can still be ensured even with the failure of the PV energy source, and no overvoltage problem will be induced.