Study On Oscillation Characteristics And Control Methods Of Large-scale Photovoltaic Plant Connected To Weak Power Grid

In order to relieve the energy pressure caused by the exhaustion of fossil energy,renewable energy has received more and more attention and development.In addition to the intermittent characteristics of large-scale photovoltaic(PV)plants affected by the irradiance,they are usually connected to the grid through long distributed transmission lines which impedance cannot be ignored.Hence the stable operation of the gridconnected photovoltaic power plants faces the practical challenge of connecting to the weak grid.It is necessary to analyze the generation mechanism of the stability problems and design corresponding control methods in order to ensure the stable operation when PV plants are connected to the weak grid,which includes the oscillation instability,harmonic amplification,and the bus voltage out of limitation.Therefore,in this paper,a series of stability problems of large-scale PV plants connected to weak grids are studied,which are introduced in detail in the following parts:(1)The precise dynamic aggregated model is established.The structure-preservation based aggregated model with comprehensive equivalent parameters for large-scale PV system is proposed in this paper.A complete two-stage PV system model is established to analyze the dynamics of the system.Then,the aggregation method is obtained by comparing the dynamic equations of the detailed model with the aggregated model,which is based on the energy relationship in the PV system.Furthermore,four different case studies are considered including the aggregation of identical and different ten parallelconnected PV units both under the same irradiance condition,and the aggregation of different ten parallel-connected PV units under different irradiance and weak grid scenarios,where the aggregation models are obtained through the proposed equivalent modeling method.Finally,the effectiveness of the proposed aggregation method is verified by the simulation results from PSCAD/EMTDC platform,and the consistency between the aggregated model and the detailed model is confirmed under different disturbances of irradiance variation,and continuous symmetric and asymmetric grid faults.(2)In order to obtain the impedance characteristics of the photovoltaic(PV)inverter and reveal potential stability issues of the PV inverter connected to weak grid,a complete impedance model of the two-stage PV inverter is established in this paper,which includes the PV arrays,front-end Boost converter,and rear-end inverter with output LCL filter.The impedance modeling of the PV inverter is derived at different interface of dc-link and point of common coupling(PCC),where the dc impedance characteristic at dc-link and d-q axis impedance feature at ac side are both investigated.It is worth noting that the dc output impedance of front-end circuit behaves as a first-order low-pass filter and it is mainly influenced by the dc-link capacitor.Moreover,a significant feature is found that a reduced integral gain in the outer voltage loop may cause a negative resistor and capacitive impedance in d-axis impedance component.In addition,based on the impedance analysis method,the comprehensive stability issues of the grid-connected PV inverter in case of weak grid are evaluated by different division of source-load subsystems,and it demonstrates that the 162 Hz and 30 Hz oscillation are induced by the decreased integral gain of the outer voltage loop,and the increased bandwidth of PLL,respectively.The stability margin is deteriorated with the increasing output capacity.Finally,the impedance model and the stability analysis are confirmed by the simulation results implemented in PSCAD/EMTDC platform.(3)In order to further analyze the harmonic resonance characteristics of the PV plant connected to the weak grid,the passive impedance network is established by using the impedance model in the positive and negative sequence coordinate.Through the series resonance analysis of impedance network,harmonic voltage amplification coefficient at PCC is obtained,and the effects of different photovoltaic inverter parameters,different reactive power compensation capacities and different lengths of distributed transmission lines on the harmonic amplification are evaluated in detail.In addition,based on the impedance network and the harmonic amplification coefficient,the harmonic amplification of PV inverter at specified background harmonics is assessed.A harmonic current controller based on harmonic resonance and active damping feedback control is proposed to enhance the stability of PV inverter under the condition of the background harmonic.And the effectiveness of the harmonic resonance control strategy is verified by the simulation results under the condition of symmetrical and asymmetrical background harmonic of the grid.(4)Considering the reactive power compensation devices are required to support the PCC voltage in the PV plants.The basic structure and control method of star connected H-bridge cascade Static Var Generation(SVG)are introduced in this paper.A simulation model of 8MVar SVG with 8 H-bridges per phase is established in the PSCAD platform,and the constant reactive power control mode and constant bus voltage control mode of SVG are realized respectively.It was found that when the constant reactive power control is adopted,the output capacity of SVG is stable,but the capacitance voltage of H-bridge has sinusoidal fluctuation with double-line frequency.When using constant bus voltage control,SVG will output different reactive capacity with the change of bus voltage,but the double-line frequency voltage fluctuation of H bridge capacitance is obviously reduced.Considering that both SVG and PV inverter have reactive power output ability,a three-layer compensation strategy for reactive power coordination control of PV plant is formulated according to the reactive-voltage sag curve,and the reactive power compensation is rationally distributed to different device to meet the requirements of bus voltage protection.A simulation model of large-sacle PV plant with 50 MW rated capacity is built in the PSCAD platform to verify the effectiveness of the reactive coordinated control strategy under different conditions of steady state and grid fault.