Study On The Application Of Generalized Short Circuit Rat In In Multi-Infeed Photovoltaic Systems

Vigorously developing renewable energy generation is an effective way to tackle the challenges of energy crisis and environmental pollution.High proportion of renewable energy integration has become one of the basic characteristics of future power system.With the massive access of converters,the dynamic properties of power system have undergone significant changes,and the power electronic features have become increasingly apparent.High penetration of electronic inverters with low inertia and under-damping inevitably enlarges the equivalent AC grid impedance and weakens the AC grid,which makes the interactions between power-electronic-based devices(PEDs)and AC grid and the interactions among PEDs more complex.As a result,the risk of oscillation issues arises or becomes more critical in a weak AC grid.Therefore,to accurately and quantitatively evaluate the strength of AC grid and the interactions of PEDs is in urgent demand to guarantee the safe and stable operation of power system.Given the above backgrounds,this dissertation focuses on the definition,analysis and application of generalized short circuit ratio(gSCR)for multiple power electronic based devices infeed systems(MPEIS).The detailed contents of the dissertation are as follows:1)The mathematical model of single power electronic based devices infeed systems(SPEIS)is formulated via Jacobian transfer matrix.And the relationship between the SCR and small signal stability margin of the grid-connected system is particularly analyzed.Similarly,establish a mathematical model for MPEIS,and discuss analysis methods and sufficient conditions for the equivalent decoupling of MPEIS.Based on the closed-loop characteristic equation of multi-infeed system,the MPEIS is decomposed into independent single-infeed systems through matrix transformation.In this regard,the concept of gSCR derived from SCR is proposed for the small signal stability evaluation of MPEIS,which can be used to measure the strength of AC systems.2)The gSCR under rated operating conditions is extended to operation generalized short circuit ratio(OgSCR)under general operating conditions to evaluate the small signal stability margin when the PEDs output arbitrarily power.In addition,the analysis on sensitivity to gSCR and OgSCR is conducted using modal analysis theory,and an approach to identifying power grid vulnerability based on gSCR is presented3)Aiming at the planning issue of how to optimize the allocation of grid-connected capacity of each feeding point when total capacity of photovoltaic devices(or fields)accessed to the grid is fixed,this dissertation proposes a capacity optimization method for multi-infeed photovoltaic systems by utilizing equal incremental gSCR criterion.The Lagrange multiplier technique is applied to prove that this method can accomplish the largest small signal stability margin of multi-infeed photovoltaic systems.And take three-infeed PV system as calculation case to verify the feasibility and effectiveness of the proposed capacity optimization method4)Aiming at the operation issue of how to adjust the power output of each photovoltaic power plant when total active power reduction of clustering photovoltaic plants accessed to the grid is given,the dissertation proposes an optimal dispatching strategy for power reduction of multi-infeed photovoltaic systems based on OgSCR.In detail,drawn on the gradient descent method,this dissertation establishes a stepping optimal dispatching model,and solves it through linear programming theory.And take three-infeed PV system as calculation case to verify the feasibility and effectiveness of the proposed active power reduction dispatching method.