Study On Impact Mechanism Of Wind Power Integration On Protective Relaying And New Sufficient Criterion Protection Schemes

The significant distinction between wind turbine generator(WTG)and synchronous generator(SG)in construction and operation principle results in total difference in their transient characteristics during fault.The traditional relay protection designed for SG will be affected after wind power integrating into system.For the purpose of promoting the healthy and rapid development of wind power,it is an extreme emergency to solve the new relay-protection problems.In this thesis,doubly fed induction generator(DFIG)is chosen as the research object.The influence mechanisms of wind power integration to relay protection are revealed and new schemes based on sufficient criterion are proposed.The transient characteristics of WTG are the foundation of the influence of wind power integration research and new protection scheme development.The concept of calculated sequence impedances(CSIs)of DFIG is presented.Analytic expressions of CSIs under crowbar protection and converter control are deduced respectively.It is observed that positive sequence calculated impedance has an evident transient change process while negative sequence calculated impedance doesn't.The amplitude of the former is larger and fluctuant while the latter is smaller,and the steady phase angle of the former is between-90°and-1800° for crowbar protection or between 0° and 180° for converter control while the latter is between 30° and 90°.Conclusions are drawn by researches on frequency characteristics during fault of DFIGs from the perspectives of electromagnetic transient and converter-control process.The rotating frequency inter-harmonic,the initial amplitude of which is high while will attenuate to zero in one and a half circle,will occur in stator current since the rotor flux cannot change abruptly in crowbar protection condition.Second and third harmonics,the size of which are concerned with the parameters of PI controller,appear when decaying de and negative sequence components circulate in the converter during external fault,because of the asymmetry of converter's d-q axis control loop.The influence mechanisms of wind power integration to relay protection are analyzed from four aspects,which are collector line protection,pilot protection,distance protection and distribution network current protection.It is indicated that the sensitivity and selectivity of traditional protection are not met due to the restriction of objective condition of network environment in collector lines and distribution network with wind power integrated.The optimal conditions of directional relays,phase selection elements and power frequency distance relays using fault component cannot be met under the influence of CSIs of wind farm,which results in the malfunction or rejection of protection.The wind power system tends to be opaque to relay protection due to its complicated transient characteristics so that the traditional protections emphasizing equally on reliability,selectivity,speed and sensitivity become inapplicatable.For this,a new sufficient criterion protection scheme is proposed.The sufficient criterion protection responds to sufficient fault characteristics and is with the features of sufficiency,adaptivity,optimization and time-correlation which is an efficient scheme to deal with the non-transparence of protection object and is extremely adaptive to wind power system.A sufficient collector line protection scheme for the purpose of avoiding cascading trip-off of WTGs is presented.The fault collector line is distinguished firstly based on the weak-infeed property of WTG in this scheme.Then a risk factor of trip-off of WTG is calculated by different functions according to its relay operation margins,and the trip-off risk of wind farm is assessed using classification analysis method.At last the trip-off risk assessment module is added in the protection logic.Due to the consideration of WTG in non-fault collector lines,this scheme can reduce the trip-off probability of WTG and the number of trip-off WTGs.A sufficient directional comparison pilot protection scheme based on CSIs characteristics is proposed.To deal with the problem that traditional pilot protection is inadaptable to wind power system,a sufficient fault component directional element reflecting the inequality of impedances in each side of transmission line is designed.Sufficient criteria based on the difference of CSIs and the variation of amplitude and angle of positive sequence calculated impedance are figured out.The new sufficient directional comparison pilot protection scheme is constructed,which enhances the adaptive capacity to transient variation characteristics of wind farm impedance.A new phase selection scheme based on difference compensation of CSIs is put forward.This scheme can compensate the differences of positive,negative and zero sequence current distribution coefficients by computing CSIs in real time.Specific to traditional superimposed current phase selection and fault-sequence component phase selection,two improved phase selection methos are built,which are adaptive to the CSIs characteristics variation of wind farm and applies to wind power system where the system side is strong while the wind farm side is weak.A sufficient distance relay using power frequency fault components is proposed based on phase identification principle.The relay fixes the angle-variable comprehensive impedance of wind farm to the direction of setting impedance,through which the relative location of setting and fault point can always be reflected by the relative value of operating and fault voltage.Compared with traditional scheme,the sufficient distance relay has a same ability of enduring fault resistance,but it can improve the sensitivity of internal faults and avoid mal-operation of external faults.A new sufficient distribution network protection scheme is proposed based on the spectrum difference.A spectrum index decided by the amplitude of several special harmonic components of short circuit current is used,and fault location is determined by the comparation of spectrum indexs in two ends of line.The difference of spectrum indexs in two sides is used as operation component,and the harmonic decay characteristic is introduced in restraint component.In this way,protection can tolerate a synchronization error of a fundamental frequency period.A complementary criterion specific to rotating frequency component with high amplitude and quick attenuation is also proposed.This new scheme is very suitable for distribution system with poor communication and voltage information shortage.The wind power system model is constructed on the MATLAB/Simulink platform.The transient characteristic of DFIG and the validity of each new sufficient criterion protection scheme are simulated.Extensive simulation results demonstrate that the application of new sufficient criterion protection scheme to wind power system significantly improves the performance of traditional relay protections.