Transient Stability Analysis Of Power System With PV Generation Based On The Averaged Converter Model

Transient stability is essential for power system planning,designing,operation and recovery in case of contingency.With the increasing scale of power grid and integration of new resources,the issue of transient stability appears more complicated.The large-scale photovoltaic power station exhibts special characteritics comparing with the traditional synchronous machines.It is important for the improvement of transient stability to accurately describe the dynamics and to disclose the mutual impact between the photovoltaic power station and the synchronous machines.In this paper,the transient behaviors of grid-connected photovoltaic station and its influence on transient stability are analyzed.The energy function for the grid-connected photovoltaic multi-machine power system is proposed and is used in transient stability analysis by the potential energy boundary surface method(PEBS).First,the photovoltaic power station model is established based on the averaged converter model,including the photovoltaic cell array,boost converter,the grid-connection inverter,and the corresponding control units to implement maximum power point tracking and inverter control strategy.The approach of representing large photovoltaic power station by lower rating units is proposed.Second,the multi-machine power system model with photovoltaic power generation is built up.The change of parameters is analyzed when synchronous machines are connected in parallel to supply the power.The transient behaviors of grid-connected photovoltaic station is then investigated.The calculation results on a single machine system and a three machine system suggest that,the photovoltaic power station exhibits fast dynamic characteristics,which indirectly influences the transient behaviors of the synchronous generators,by affecting their output power responses.Then,based on the concept of equivalent load,the potential energy due to photovo ltaic power generation is expressed by the voltage of the grid-connection point and the power injected into it.The energy function of the power system with photovoltaic power generation is proposed,which overcomes the difficulty brought by the complicate structure of the photovoltaic power plant.A sufficient condition is proposed for the monotonic decreasing property of the energy function after fault-clearing.The procedure of determining the potential energy due to photovoltaic power generation is presented.Finally,taking the results obtained by the time domain simulation as reference,using the proposed energy function,the transient stability is analyzed by the PEBS method.The calculation results obtained on a three-machine-nine-bus system and a ten-machine-39-bus system show that,the constructed energy function monotonically decreases along the post-fault trajectory,and thus the qualification of the proposed energy function for direct stability analysis is demonstrated.The results also show that,comparing with the time domain simulation,in most contingencies,the PEBS method is fesasible and effective for mult-machine system transient stability analysis.