| The global energy structure is being constantly adjusted while new energy resources are gradually penetrating into the large power grid,which brings many challenges to the stable and safe operation of power grid.The access of wind power makes the traditional protection scheme of distribution network appear many discomforts.This paper proposes a new "sufficient" current differential protection scheme in the condition of weak synchronous communication of distribution network,which can effectively balance the economy and efficiency of protection.The principle of "sufficient" protection under the condition of weak synchronous communication in distribution network is expounded.The "sufficient" principle of relay protection makes full use of the limited state information and electrical quantity information under the "Weak synchronization condition" of lack of communication device to extract the fault characteristics of distribution network to the greatest extent and constitute the fault determination conditions with these sufficient characteristics.The main characteristic of the "sufficient" principle is that the fault characteristics are not the necessary and sufficient conditions for faults judgment,and the sensitivity and speed of some uncommon faults are sacrificed in exchange for the high economical efficiency and sensitivity of most common fault judgment.The main characteristics of "sufficient" protection are adequacy,adaptability,optimality and the performance of discrete inverse time limit.This paper discusses the sufficient characteristic of fault current in the distribution network with wind power.The basic operating principle and structural characteristics of doubly-fed wind turbine are analyzed in this paper.Combined with the low voltage control mode of doublyfed wind turbine,the main characteristics of the fault current amplitude and harmonic current frequency are analyzed.It shows that the current amplitude of the system side are much larger than that of the wind power side and the short-circuit current contains a frequency component closely related to the slip ratio and abundant second and third harmonics.These characteristics can be regarded as sufficient characteristics of fault judgment.A specific sufficient differential protection scheme is proposed.Based on the current amplitude characteristic,a sufficient current amplitude differential protection criterion is constructed.The spectrum index to measure the difference of current spectrum between two ends of a line is constituted by the ratio of the amplitude of a specific harmonic component in a certain range of fault current to the fundamental frequency.Then the current amplitude ratio differential protection criterion with spectrum index and the new type of differential protection criterion using virtual angle of spectrum index are proposed.The new protection criterion can get better sensitivity and reliability in many uncommon conditions to make up for the deficiency of criteria only using current amplitude.The frame composition and basic meaning of the comprehensive sufficient protection scheme are proposed that can realize the complementarity of fault discrimination ability and enlarge the size of fault set covered by the sufficient protection criteria.Finally,a MATLAB/Simulink model of distribution network with doubly-fed wind turbine is built.Numerous simulation examples are designed to verify the sensitivity and reliability of the protection scheme in different fault conditions.Besides,the resistance capability to transition resistance and to synchronization error are analyzed.Then the simulation results fully verify the theoretical analysis above and the protection scheme has practical application significance in distribution network with wind power. |
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