| If photovoltaic(PV)system is connected to the middle and low voltage distribution network,the intermittence and fluctuation of its output power will weaken the safe and stable performances of the distribution network system.Moreover,with permeability increasing of PV system,the transient negative problems such as voltage fluctuation and short-circuit current of the connected power system will become prominent.Therefore,the complexity and severity of PV system's transient problems make the transient problems analysis of distribution network and the study of related control strategies become the academic community's focus and difficulty.By consulting,analyzing and summarizing a large number of literatures and on the basis of the previous work,the steady-state voltage regulation method of PV system and the transient characteristics of microgrid and of PV system are mainly studied,which startes from the study of stable/transient voltage regulation control strategy of photovoltaic power generation system.In addition,the voltage regulation scheme of PV system and the transient response mechanism of microgrid are discussed.Finally,a static voltage regulation method of the reactive power of the inverter Q(P,U)is proposed to solve the voltage problem caused by its own access into power system.Hybrid coordinated control strategy based on adaptive non-mppt algorithm of PV system,hybrid control algorithm of energy storage grid-connected inverter and flux coupling current limiter(FCL)are used to improve transient performance of PV system and microgrid.The main contents and achievements of the thesis are as follows:(1)In view of the over limit problem of the parallel network steady-state voltage caused by the distributed PV access,From two aspects of regional coordinated control and local control,this paper proposes a distributed photovoltaic access distribution network reactive voltage control system.Through coordinated control of multiple regional distributed photovoltaic and local control of a single distributed photovoltaic under poor communication conditions,the effective regulation of reactive voltage is realized.(2)In order to realize the voltage regulation function of two-stage photovoltaic power generation system and improve the ability to deal with voltage transient problems,a hybrid algorithm for PV system is proposed.According to the eight predicted switching states and system value function,the strategy can quickly follow the reference instruction input by the inverter control system through the current model predictive control.And in the case of symmetric and asymmetric voltage drop,the grid-connected inverter can output the set symmetrical current,and solve the problem of ac side over current.Based on model current prediction control,the inverter grid-connected current can quickly follow the reference instruction and the symmetrical current can be always obtained to solve AC side overcurrent problem in the case of symmetric and asymmetric faults.Based on the voltage sag of the point of common coupling(PCC)and the adaptive Non-MPPT algorithm to adjust Boost converter's duty ratio,the output power of PV array can be reduced,so that DC-link overvoltage can be inhibited in the failure process which is caused by the power imbalance between AC and DC side of the grid-connected inverter.By introducing DC-link voltage feedback,the second harmonic component of DC-link voltage can be removed in the asymmetric fault condition.(3)In order to further improve the transient performance of PV system's connected-grid voltage,microgrid is constructed through the energy storage system.Then,under the parallel and isolated island operation mode,the PV system can be kept in grid-connected operation state.And for improving the dynamic performance of energy storage system in microgrid,a hybrid control algorithm based on energy storage system is proposed.Based on the hierarchical control structure of inverter,the two degree of freedom control principle and the current model predictive control are combined.Thereinto,together with current model prediction control,the current inner loop of control layer can eliminate PI controller and PWM regulator.In the voltage outer loop of control layer,the principle of two degrees of freedom is adopted to construct the inverse model of the controlled object.And based on the current model predictive control of inner current loop,the unit transfer function of the control layer is realized.Then,the dynamic full compensation for harmful disturbances can be realized,which are contained in the voltage and current signals.For the application layer of ES system,when microgrid is island operation,its frequency and voltage are used as feedback quantities and introduced into PI controller of ES control system to modified ES traditional droop control,so that the ES system can be improved to realize the no-deviation regulation for microgrid's voltage and frequency.And because the control layer can be used as the shared layer of ES system,during the switching process of microgrid operation mode,the transient surge current and bus voltage distortion rate of microgrid can be significantly suppressed.(4)In order to comprehensively improve the dynamic performance of PV system when dealing with voltage transient problem,a coordinated voltage control strategy for microgrid is proposed to improve the low voltage ride through capability of PV system.When the the voltage drop of microgrid's PCC point is caused by external power grid failure,the hybrid coordinated control strategy can quickly suppress the impingement current and dc side overvoltage of PV inverter through the non-mppt operation mode,the improved droop control of energy storage unit and FCL.Then,the low voltage ride through capability of PV system is improved and the voltage performance of microgrid can be improved comprehensively. |
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