Analysis And Enhancement Of Small Signal Synchronization Stability For Renewable-Integrated Power Systems

With the continuous increase in penetration of renewable energy resources in power system,such as wind power and photovoltaics,power system is undergoing a rapid transformation with a significant trend or key feature,i.e.,high penetration of renewable energy and high penetration of power electronic devices.The power electronic devices(i.e.,power converters)have significantly changed the dynamics of power system dominated by synchronous machines and transformed the AC grids into weak grids.The phase-locked loop(PLL)is widely used in power converters for synchronization with AC grids.It has been pointed out that PLL can give rise to small signal synchronization instabilities under weak grid conditions due to the strong interaction among different devices,devices and the power network.For renewable-integrated power system,revealing the small signal synchronization stability mechanism of grid-connected converters system(GCS),clarifying the interaction between the devices and the grid,and increasing the stability margin of renewable integrated power system is the key to improving the consumption level of renewable energy.This dissertation aims at filling these gaps by proposing a stability analysis method based on generalized-impedance and generalized short circuit ratio(g SCR).The main contributions of the dissertation are summarized as follows.1.The applicability of the GISC and its corresponding mechanism for the PLL-induced synchronization instability in GCS has been studied.The impedance model of GCS is usually a multiinput and multi-output(MIMO)model.It is inconvenient to give a physical explanation of the oscillation mechanism and quantify the stability margin based on generalized Nyquist criterion analysis.The stability analysis of GCS can be transformed into a stability analysis of a singleinput and single-output(SISO)system via both SISC and GISC.Then the oscillation mechanism can be explained from the perspective of circuit resonance.However,the existing research lacks applicability evaluation for the two methods.To end this,the mathematical principle of the SISO system obtained from the two impedance models is first analyzed.Then,the process is uniformly illustrated based on the primal-dual complex circuit.Secondly,it is found that SISC is inaccurate when neglecting the couplings in impedance model.Although the couplings in SISC is manageable with schur complement method,the open-loop transfer function of SISC may have unstable poles,resulting in irrationality to explain the oscillation mechanism with a circuit.This problem can be generally avoided by using GISC.Furthermore,to evaluate the applicability of impedance criterion,the index,condition number is proposed to quantify the influence of parameter uncertainty on the stability margin.Our research indicates that GISC shows preferable applicability for PLL-induced synchronization instabilities,such as sub-/super-synchronous oscillations.2.For the small signal synchronization stability in single renewable-integrated power system,we reveal the influence mechanism of the control loop dynamics on stability based on GISC.First,the impedance model of PLL-based GCS in polar coordinate considering DC voltage control or constant active power control is constructed.And on this basis,the equivalent SISO system corresponding to the GISC is derived.Then,it is proved that the open-loop transfer function of GISC under the two outer loop control does not have unstable poles and the condition number is small,proving satisfactory applicability of GISC.Finally,through our analysis base on GISC,it is clearly illustrated why the stability of GCS weakens with the decrease of the grid strength and how PLL interacts with DC voltage loop and reactive power loop.And the impact of the control parameters of the converter is summarized.3.For the small signal synchronization stability in multi renewable-integrated power system,analysis methods have been proposed to quantify the stability margin and identify the key devices that dominate the synchronization instability in renewable-integrated power system.First,the impedance model established above is extended into a multi-device system.Then,We found that a homogeneous multi-device system in which the converters' dynamics are similar can be decoupled into several single GCS.So our analysis method and conclusion for the single GCS based on GISC introduced in first two chapters can be transplanted into multi-device system.Furthermore,we show how to quantify the stability margin of the system and identify the dominant devices that cause instability based on g SCR method.As for the heterogeneous multi-device system,it is proved that the small signal stability margin can also be assessed based on g SCR.Moreover,we can transform it to an equivalent complex circuit due to the special structure of converters and network admittance model in polar coordinates.Then the small signal instability of the system is equivalent to the resonance of its equivalent complex circuit.And the critical PLL device can be located by analyzed the modal power in complex circuit,which facilitates parameters tuning.4.To improve the small signal synchronization stability margin of renewable-integrated power system,we demonstrate that placing grid-forming converters can enhance the stability of PLLintegrated power systems.First,we model the system which describes how the grid-forming converters interact with PLL-based converters via the power network.Then,based on matrix perturbation theory,we explicitly demonstrate that the placement of grid-forming converters is equivalent to increasing the power grid strength(characterized by g SCR)and thus improving the small signal stability of PLL-based converters.Based on our findings on how grid-forming converters affect the small signal stability of PLL-integrated power system,we investigate the optimal locations to place grid-forming converters by increasing the smallest eigenvalue of the weighted and Kronreduced Laplacian matrix of the power network.Our analysis potentially lays the foundation for understanding the interaction between PLL-based(i.e.,grid-following)converters and grid-forming converters,and coordinating their placements in future converter-dominated power systems.