| Energy is recognized as the guarantee of the human beings,which has been the unnecessary resource.Fossil fuel source is gradually exhausted as the primiary energy of the natural source.Power energy is considered as an important constitute of the energy.Besides,as the environment polluted,the renewable energy is urgently to be exploited as the new energy,e.g.,wind power generation and photovoltaic power generation,etc.The renewable and sustainable energy has been penetrated into power systems via power electronic converters,which significantly changes the operation characteristics and architecture of conventional power systems.As the battle of AC power and DC power between Thomas Edison and Nicola Tesla,the AC power has taken up the larger market share as well as broader network architecture of power industry.However,as the level of voltage gradually improved as well as the power electroni cs technology advanced,the advantage of DC power has been gradually appeared.Over AC power,the reactive power as well as the synchronized stability issue has not emerged in DC fields.Especially in the fields of power energy transmission featuring high voltage level,great capacity,and long distance,DC power has the uncomparable advantage.However,because of the characteristics of multi-time scale and rapidness of power electronics,the dynamic interaction between multiple power converters as well as the dynamic influence between power converter and DC networks is unavoidable,which threatens the stable and secure operation as well as development of VSC-HVDC system and other flexible DC systems.Aiming at low voltage flexible DC system,high voltage VSC-HVDC system,as well as VSC-MTDC schemes,and modular multi-level converter-based HVDC system,the stability analysis is developed on DC-side.Besides,the dynamic interaction between power converter and DC network as well as dynamic interaction between various converters are analyzed by small signal models.The internal mechanism of oscillation of DC voltage is clearly clarified.The main contributed works are organized as follows:(1)A virtual synchronous generator(VSG)control scheme which emulates t he working principle of synchronous generator is put forward for application in flexible DC system.Specifically,the control loop is cascaded between voltage outer loop and current inner-loop to enhance the inertia and damping of the whole DC network to suppress oscillation of DC voltage,and improve the stability and robustness of the system.The small signal model of flexible DC system is built.By means of impedance analysis,impedance ratio criterion,eigenvalue analysis,robustness performance analysis,and robust stability analysis,the mechanism of suppressing oscillation of DC voltage is clearly clarified.It illustrates that the proposed cascaded VSG control can acquire better parameter-adaptivity and parameter-robustness,which is independent on s ystem parameters.The proposed VSG control strategy is cascaded between outer voltage loop and current inner loop,which does not alter the original control structure and parameters with superior architecture robustness.Moreover,it is applicable for various oerpation conditions with good parameter-adaptivity and parameter-robustness since the operation ranges is broad,and it is not sensitive to system original parameters.Finally,the feasibility and effectiveness of the proposed VSG control strategy is well demonstrated by simulations as well as Star-Sim hardware-in-the-loop(HIL)experimental results.(2)As for VSC-HVDC scheme,a virtual inertia control strategy applied in DC voltage-controlled converter has been put forward,which provides a damping channel for DC voltage suppression and dampen the fluctuation of DC voltage.Aiming at the built small signal model of VSC-HVDC system with passivity-analysis,impedance ratio criterion,etc.,the poorly-damped modes and impedanceresonance interaction is clearly clarified from various perspectives with conventional dual-loop control structure.After the cascaded VSG control is introduced,damping performance and inertia is significantly improved and oscillation of DC voltage is well suppressed.The introduced virtual inertia control dynamics is essentially as the same as energy conservation and active power balance on DC side.Hence,the essence of adopting virtual inertia contributes to improving the transient response of DC voltage by emulating the inertia.From dynamic motion equation of virtual inertia control,it can be known that the greater virtual inertia results in much slower rate of change of DC voltage(Ro Co V),i.e.,much slower and smoother change of DC voltage.Greater virtual damping coefficien t results in smaller dynamic deviation and smaller DC voltage nadirs.The introduction of virtual inertia strengthens damping and inertia of the system on DC-side.Inversely,the larger Ro Co V means that virtual inertia can provide larger support of virtual inertia power to implement better damping effect and inertia support effect.(3)Based on average model,the smallsignaldynamic model of MMC and DC network is built to quantitatively analyze the dynamic interaction between MMC and DC network.By means of frequency response analysis,impedance frequency characteristics is obtained.The effectivenss and correctness of the proposed impedance model have been verified by the method of frequency impedance scanning.Besides,the impact of active damping control strategy on system stability is analyzed.Simulations and theoretical analysis show that the stability and damping performance is evidently strengthened after introduction of active damping control method.(4)Based on superposition theory,the mathematic al model of the system is built for analysis of dynamic interaction among multiple power converters.By means of partitioning the respective excitation source on dynamic response of DC networks.The response of DC voltage of a certain converter can be obta ined by superposition of multiple excitation source s.It is quantitatively analyzed that the inspired oscillation of DC voltage is non-sinusoidal and time-variant when various branches of DC lines are unsymmetrical.Simulations show that the potential oscillation of DC voltage exists in VSC-MTDC system.And in some special operation conditions,the poorly-damped oscillation of DC voltage has a large risk of occurring when the DC line parameters are determined. |
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