Research On Operational Control Of Single-Phase Grid-Connected Inverter

With the continual global energy crisis and environmental pollution,the power generation techniques associated with renewable energy resources such as wind and solar have experienced rapid development.Distributed generation systems(DGSs),which act as important supplements of traditional large-scale power plants,have attracted worldwide attention.As a special kind of DGS,the microgrid,which can effectively reduce the negative impacts of distributed renewable energy resources(DERs)on the utility,is a promising method to connect DERs to the utility.However,traditional grid-connected inverters in a microgrid only inject pure active current to grid,which has many technique limitations and cannot meet the strict requirements of the modern electricity market.In this dissertation,some critical techniques related to the operational control of single-phase grid-connected photovoltaic(PV)inverters in a microgrid have been detailly and deeply studied,and some novel conclusions are draw as follows.1.To improve the stability,dynamic performance,and harmonics rejection capability of single-phase grid synchronization,some feasible methods have been proposed.Firstly,a decoupled structure is developed for improving the dynamic response and stability of the traditional second order generalized integrator(SOGI)based phase locked loop(PLL)(SOGI-PLL).In addition,to ensure high precision and fast transient performance of single-phase PLLs under distorted grid voltages,a new orthogonal generator(OSG)is designed for reducing the time delay introduced by the OSG stage.Although the PLL technique is widely used in grid-connected power generation systems,it still suffers from some disadvantages.On the one hand,to reject high-frequency noise and high-order harmonics,the bandwidth of PLLs cannot be high,which limits the dynamic response.Moreover,an extra filter should be inserted in/pre the PLL loop for rejecting the possible low-order harmonics,which further degrades the response speed.On the other hand,the frequency and phase of the grid voltage are estimated within the same loop so that they interact with each other,thus reducing stability margins.To avoid these issues,an open-loop grid synchronization method is proposed for distorted grid voltages in this dissertation.The proposed method can provide superior dynamic performance and strong rejection capability on harmonic components in the measured grid voltage,while suffering from no stability issues.2.An improved deadbeat control,which can well adapt to parameter uncertainties of grid-connected inverters,is designed for the grid-connected current control of single-phase grid-tied PV inverters.Deadbeat control is of fast dynamic performance,but it is sensitive to parameter uncertainties of grid-connected inverters.Thus,the mathematical model of the current tracking error by using the deadbeat control is established firstly.Based on this model,the corresponding compensators are designed for eliminating the current tracking errors due to the mismatched modeling of the output filter inductor.Finally,the improved deadbeat control can ensure the tight tracking control of the grid-connected current reference.3.A novel low voltage ride through(LVRT)strategy,which is independent of the detection of grid voltage sag faults,is built for two-stage grid-connected PV inverters.At present,conventional LVRT strategies are mostly dependent on the detection signals of grid voltage sag faults.However,the inevitable time delay in the voltage detection process is adverse for LVRT,especially in cases of drastic and deep voltage drops.Moreover,the mode switching operations in traditional LVRT approaches may cause large transient voltage/current spikes.In addition,solar irradiance variations during the period of LVRT are seldom considered in previous LVRT strategies.Hence,a new LVRT strategy designed based on the inherent relationship between the dc-bus voltage and the grid feeding active power is employed for two-stage single-phase grid-connected PV inverters.Different from traditional LVRT methods,the proposed one does not need to detect the grid voltage sag fault in terms of the realization of LVRT,and also has no mode switching operations.Furthermore,the proposed technique can guarantee the reliability and stability of the power system in cases where solar irradiance varies during the LVRT period.4.To assist the implementation of single-phase multi-functional grid-connected PV inverters,the reactive current detection schemes for linear and nonlinear loads are firstly studied,followed by the compensation strategies for the required power quality improvement.To make grid-tied PV inverters be capable of generating and compensating reactive and harmonic components of local loads,a fast reactive current detection method suitable for linear loads is developed for making the reactive current compensation timelier.Besides,the detection methods of reactive and total harmonic components in nonlinear loads are also discussed for implementing the reactive and harmonic current compensation of single-phase grid-connected PV inverters.Finally,to strategically utilize the limited capacity margin of each single PV inverter,an objective-oriented optimal compensation scheme is presented.5.To implement the operational control of single-phase grid-connected PV inverters in both the islanded mode and grid-tied mode,a feasible universal control strategy is developed in this work.With the proposed universal control strategy,the single-phase PV inverter does not need to reconfigure its control system when switching between the islanded mode and grid-connected mode,thus can achieve the seamless transfer.In the grid-connected operating mode,the proposed universal controller can allow single-phase PV inverters to inject the maximal power of PV arrays into the utility for maximizing the power generation profit or provide constant active and reactive power output according to the power dispatched command from the utility.In the islanded operating mode,the single-phase PV inverter can adjust its output power according to the available power of PV arrays for ensuring the power balance,thus can provide continual electricity for local loads.In addition,the universal controller can also allow multi single-phase PV inverters connected in parallel to share the consuming power of local loads in islanded mode.