| The power grid in China is faced with a new situation of high renewable energy penetration,large-capacity trans-regional power transmission,regional grid interconnection and ultra-high voltage AC/DC hybrid interconnection.Power system interconnections bring primary economic and technical advantages,such as load peak and valley balance,large-scale renewable energy accommodation,spinning reserve share and maintenance schedule coordination,which can be realized by synchronous manner and asynchronous manner conventionally.Asynchronous interconnection is unlimited in size from technical viewpoints,but it is constrained by economic aspects,i.e.,transmission facility cost and transmission loss.The synchronous interconnection is constrained not only by transmission facility cost and transmission loss,but by technical aspects,which is necessary to be further studied.This paper is to study the reasonable size of synchronous power systems from the viewpoints of synchronizing support effect disappearance constraint,the frequency stability constraint of receiving grids,the minimum oscillation frequency constraint of low frequency oscillations,grid side damping control of synchronously interconnected grids as well as AC/DC hybrid structure constraint and DC segmentation optimization.The main work is organized as follows:(1)The synchronizing support effect disappearance constraint is studied.The definition and calculation method of synchronizing support effect is studied based on theoretical analysis.The synchronizing power coefficient is the index to depict synchronizing support effect,and the synchronizing support effect disappearance criterion is thus derived,i.e.,remaining relative rotor angle between two arbitrary synchronous generators within 90°.The concept of direct synchronism and indirect synchronism is proposed.Based on perturbed trajectory analysis,the overall transient process after power deficit including initial synchronous power support,forward swing stability and backward swing stability is compared between the direct synchronism and indirect synchronism.The results indicate that synchronizing power coefficient turns negative in indirect synchronism,which leads to negative initial power support and deteriorating the transient overall swing stability,and is thus prone to losing synchronism.(2)The reasonable size of receiving grids with renewable energy under frequency stability constraint is studied.The size of receiving grids can be represented by critical dc infeed ratio.First,taking into consideration the renewable energy penetration and dc infeed ratio,the traditional frequency deviation index is modified.Second,the detailed electromagnetic model is established,and the influence of the gain coefficient in emulation inertia controllers,the installed capacity of wind farms and the hydro power ratio on critical single dc infeed ratio are studied.The results indicate that due to the water hammer effect,the hydro power ratio deteriorates the critical single dc infeed ratio;the installed capacity of wind farms helps to improve the inertia support effect of wind farm emulating inertia on the system;and equipped with reasonable gain coefficient,the wind farms can provides well inertia level.(3)The low frequency oscillation constraint on the reasonable size of synchronous power systems is studied.The analytic relationship between source side damping and oscillatory frequency is obtained using theoretical derivation,and the damping effect of the unregulated generator,fast excitation system and PSS are analysed respectively.The unregulated generator itself provides positive damping torque in low frequency spectrum,and the value increases when frequency is lower.On the contrary,the excitation system presents negative damping torque,and the lower frequency the worse damping torque is.PSS helps to improve damping effect,but the damping effect is limited in the very low frequency spectrum.The theoretical analysis and practical operating experience both indicate that the total damping of generator with fast excitation and PSS tends to be poor or even negative when the oscillatory frequency is lower than about 0.3 Hz,and the very low frequency oscillation is thus difficult to suppress,deteriorating the secure operation of power systems.Based on two-area interconnected system,the dominant factors influencing low frequency oscillation of power systems are studied.(4)The grid side damping control technique for low frequency oscillation of synchronously interconnected grid is studied.Base on FACTS devices,a novel auxiliary damping controller design method is proposed:First,the residue ratio index is used to choose a robust input signal;second,the test signal method is used to identify system modes and tune parameters;then,the maximum gain method is used to coordinate the source and grid side control effect.The FACTS capacity is distributed to improve both transient stability and small signal stability.The method is verified in Xilinguole League Base to Shandong ultra high voltage transmission system.(5)The AC/DC hybrid structure constraint and the general principle for DC segmentation optimization is proposed.First,the advantage as well as the stability constraint are analysed for AC/DC hybrid structure.Next,AC transmission corridor strength index,AC/DC transmission corridor strength ratio index and AC transmission corridor efficiency index are proposed from the aspects of power system security and transmission economy efficiency,which is to represent the AC transmission capacity,strong DC weak AC degree and AC corridor efficiency quantitatively.Then,the general principle for DC segmentation is proposed.The static calculation and transient simulation are carried out in South China Grid in 2030,which verifies the proposed principle.Finally,the quadrangle criteria to determine the reasonable size of AC/DC synchronous grids are concluded. |
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