| A large AC/DC hybrid power grid with the greatest scale and the highest voltage level in the world has been built in China,showing the characteristics of high DC proportion,huge transmission capacity,and strong coupling between AC and DC systems.The commutation failure(CF)of line commutated converter-high voltage direct current(LCC-HVDC)caused by AC faults has become a typical cascading failure,resulting in that the large capacity DC transmission power greatly fluctuates in a short time,even be blocked,which is a key factor affecting the transient stability of the power systems.In order to improve the safe and stable operation level of AC/DC hybrid power systems and avoid the occurrence of blackouts,the theoretical analysis and research about the interaction mechanisms between the CF and the system transient stability,the system transient stability assessment approaches considering the CF and the system transient stability control strategies with CF involved are carried out in this paper.The following results have been achieved:(1)The interaction mechanisms between the CF and system transient stability are revealed.Firstly,the model of the sending-end power system and the DC power model of the CF are established,where the ramp function is utilized to characterize the DC power recovery process after the CF.Next,the DC power flow model is established,and the analytical expression of the relative rotor angle of the sending-end power system with respect to DC power is deduced theoretically,thus the impact mechanism of the CF and its DC power recovery speed on the system transient stability is obtained.Meanwhile,the mathematical relationship between the control parameters of the voltage dependent current order limiter(VDCOL)and commutation failure prevention(CFPREV)and the DC power recovery speed is deduced theoretically.Secondly,the impact mechanism of the transient stability of the receiving-end power system on the occurrence of continuous commutation failure(CCF)during the recovery process is revealed by modeling the receiving-end power system as synchronous generators.The presented interaction mechanisms provide the theoretical basis for the control strategy to further improve the system transient stability after the occurrence of the CF.(2)A system transient stability margin assessment approach considering the CF is presented.Firstly,the network structure-preserving model of the AC/DC hybrid power system is established,where synchronous generators adopt the sixth-order model,and the DC systems adopt the electromagnetic model which could reflect the dynamic process of the CF.Secondly,the system transient energy function(TEF)satisfying the conditions as the qualified TEF is deduced theoretically,and the TEF based transient stability criteria are further put forward.Combined with the bisection method,the quick and precise evaluation of the system transient stability margin considering the CF is realized.Finally,the simulation results show that the assessment precision of the system transient stability margin and the efficiency of the system dynamic security assessment are improved by the proposed theoretical approach.(3)A system transient instability trendency identification approach considering CCF is proposed.First of all,aiming at the problem that the existing maximum Lyapunov exponent(MLE)algorithm is not suitable for non-smooth systems,a switching compensation model is established by theoretically analyzing the stretching or shrinking of adjacent trajectories passing through the switching surface.Secondly,combined with the qr decomposition based Jacobian matrix algorithm,a switching compensation based MLE algorithm is presented.According to the mapping relationship between the phase trajectory of the switching system and its MLE,a switching system’s MLE based transient stability criteria are further put forward.Finally,by modeling the dynamic process of CCF caused by multiple disturbances as the switching process,the early identification of system transient instability considering CCF is achieved without using the rotor angle and post-fault equilibrium point,which provides favorable conditions for the system to take reasonable and effective transient stability control measures.(4)A system transient stability control strategy considering CF is put forward.At the level of converter station,based on the impact mechanism of the transient stability of the receiving-end system on the CCF,a DC power recovery speed control strategy is presented,and the control enabling and exiting criteria are given,which not only effectively prevents the occurrence of CCF,but also improves the transient stability of the system by by improving the power recovery characteristics.At the level of system,the analytical expressions of the needed control quantity and control enabling time,inertia and other system parameters are derived based on the slope change characteristics during the first swing of the phase trajectory.The mathematical relationships between the provided control quantity and the number of DC line participating in power support and generator-tripping at sending-end are established.Based on participation sequence,the emergency coordinated control of DC power support and generator-tripping at sending-end is achieved,which ensures the safe and stable operation of the system after DC blocking.Meanwhile,the conservatism of the emergency control strategy is improved and its control cost is further reduced. |
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