Investigation On Impacts Of Large-scale PV Integration On Low Frequency Oscillation And Its Control Strategies

With the increasingly serious energy crisis and environmental pollution,to develop new energy with the characteristic of being clean and renewable has become the consensus of the world.However,the gradual increasing penetration of new energy represented by wind energy and solar energy in power grid will bring profound impacts on security and stability of power system due to its intermittence and randomicity.In particular,the zero-inertial grid-connected photovoltaic(PV)generation will affect the low frequency oscillation stability of power system.In this paper,impacts of large-scale PV integration on power system low frequency oscillation stability are comprehensively investigated based on the study case of multi-machine power system integrated with large-scale PV generation.On this basis,a wide-area damping control scheme is proposed for the low frequency oscillation suppression of PV.The main contents of this paper are as follows:1)The linearization analysis model of multi-machine power system with grid-connected PV is established.Based on detailed PV dynamic model including PV array,inverter and its control block combined with the mathematical models of synchronous generators and network,the state equation and linearized model of multi-machine power system with PV integration are deduced and built by detailed coordinate transformation and linear transformation.2)Impacts of different points of common coupling and P V penetration on power system oscillation modes are investigated with two typical analysis scenarios taken into consideration:keeping rotational inertia changed and unchanged.The two-area four-machine system is selected as the study case and modal analysis is conducted based on its linearized model,which is then validated by Prony analysis.From the simulation results of various scenarios,it can be concluded that grid-connected PV power plant produces no additional oscillation modes.The different impacts of grid-connected PV on oscillation modes are associated with points of common coupling locations and capacity of PV.Moreover,the impacts of grid-connected PV on inter-area oscillation modes in the scenario of keeping rotational inertia unchanged are verified by sensitivity analysis method.Finally,contributions of PSS to damp oscillations and impacts of PV transmission distance on power system damping characteristics are simulated and analyzed.3)A scheme of wide-area damping controller(WADC)based on reactive power modulation of PV is proposed.Unlike current active modulation schemes,reactive power modulation does not require the PV power plant to drop out of operation.In the respect of feedback signals selection for WADC,a two-step method is proposed to obtain the input signal for WADC with optimal performance from candidate signals considering synthetically the controllability/observability and control costs.In the respect of system modeling,pseudorandom correlation identification method is employed to obtain the open-loop transfer function,avoiding the difficulty in accurate linearized modeling of complex power grid.In the respect of controller design,pole placement method and multi-objective robust control method are proposed respectively.Finally,a two-area four-machine system integrated with large-scale PV power plant is established in DIgSILENT/PowerFactory.Results of time domain simulations demonstrate that designed supplementary controllers can both significantly improve damping properties of inter-area oscillation mode.Compared with pole placement method,the designed multi-objective controller exhibits better robustness due to its good damping performance under multiple disturbances.