Study On Interaction Mechanism Of Uncertain Weak-feed Sources In Wind Farms And Relay Protection

Large number of weak-feed sources,represented by DFIG-based(Doubly fed Induction Generator)wind turbines,are connected to the power grid,which has brought great challenges to relay protection.In this dissertation,the variable-speed constant-frequency DFIG-based weak-feed sources are taken as the research object.Based on the systematic analysis of the DFIG characteristics and the influence of the DFIGs connected to grid on relay protection,a new protection idea guided by the idea of the sufficient protection is put forward.Therefore,a protection scheme suitable for the wind power grid-connected system is constructed,which provides a solid line for the grid security and provides a reliable guarantee for the healthy development of renewable energy.The equivalent model and fault characteristics of the DFIG-based weak-feed sources are the basis for the development and application of relay protection principles.According to the difference between the two types of fault ride-through strategies,the crowbar protection and converter control,the expressions of the positive sequence-and negative sequence-fault current of the DFIG-based weak-feed source under the two LVRT strategies are derived respectively.It is clear that the fault current has typical characteristics of uncertainty.The conclusions are as follows: the equivalent model of the DFIG-based weak-feed source in the positive sequence loop has polymorphism,and can be impedance,voltage source or current source;the equivalent model in the negative sequence loop is constant impedance related to rotor speed.The fault current uncertainty,week-feed characteristics and the polymorphism of equivalent models will have an impact on the traditional relay protection,especially current protection.The virtual impedance of the DFIG-based weak-feed source as a whole is studied and the characteristics of the virtual impedance and its influence on the traditional directional elements are further studied.The virtual impedance of the weak-feed source is calculated arccoding to the positive and negative sequence fault components.It is found that the negative sequence virtual impedance are the same as the actual negative sequence impedance of the weak-feed source.However,the positive sequence virtual impedance is quite different from the actual positive sequence impedance.The specific performance is as follows: the negative sequence virtual impedance has resistive inductance characteristics and the phase angle is in the range of 45°?90°;the positive sequence virtual impedance is different due to the LVRT strategies.It has a negative impedance characteristic when the crowbar protection is put,and the phase angle fluctuates greatly.In addition,the steady phase angle of the positive sequence virtual impedance is in the range of-90°?-270°.When the converter works,the phase angle can be in any interval.The influence of the weak-feed source virtual impedance characteristic on the performance of various directional elements has also been investigated.It is concluded that the positive-sequence directional element and the power-directional element may malfunction,the negative-sequence directional element may have reduced sensitivity,and the zero-sequence directional element will not be affected.The frequency characteristics of the DFIG-based weak-feed source output current are studied from the two aspects of fault and disturbance.During the fault,the main frequency component among the harmonics of the weak-feed source when the crowbar protection is put is the rotor speed frequency component,caused by the rotor-side attenuated DC excitation.In addition,the attenuation speed is very fast.When the converter works,the harmonics of the weak-feed source are mainly the second harmonic and the third harmonic.The second harmonic is generated by the stator-side DC component and the third harmonic is generated by the negative sequence component coupling through the controller.Based on the method of small disturbance modeling,it is concluded that there is a coupling relationship between the equivalent positive-sequence system and the equivalent negative-sequence system.The coupling degree becomes larger as the difference in the equivalent impedance of the d and q axes increases.The coupling between sequence components is the root of the multiple frequency response of system with DFIG-based weak-feed sources.The idea of sufficient protection is proposed to solve the current challenges of relay protection.In case of simple communication,unavailable information,complexity of DFIG-based weak-feed sources,etc,traditional protection based on the " four characteristics " is becoming increasingly difficult to achieve.Therefore,the idea of sufficient protection is proposed to deal with these dilemmas.The most notable feature of the idea is based on fault features rather than fault types.In addition,it has the characteristics of preference,adequacy,quasi-asynchrony,etc.In this dissertation,the feasibility of applying the protection idea to the collection line current protection,the collection line direction elements,and the differential protection of distribution networks is analyzed.It is concluded that the protection idea has obvious advantages in wind power systems with missing information and opaque information.Based on the known sufficient condition of fuse characteristics,a sufficient inverse time protection scheme for collection lines is proposed.First,the problems of staged current protection on collection lines is analyzed.The stage I current protection needs to exceed the action time of the downstream fuse,which worsens of the rapid anction speed of the current protection.What's more,the multi-supply characteristics of wind farms cause the selectivity problem of the stage II current protection between fault collection line and non-fault collection line.Based on the known characteristic curves of different fuses,the minimum sensitivity curve can be get according to each fuse curve in different sections.According to the minimum sensitivity curve and the appropriate coordination time,the sufficient-type inverse time-lapse action curve for upstream collection line protection is fitted by the least square method.This solution can ensure the selectivity of current protection among collection lines,and can effectively improve the rapidity of current protection,achieving a balance between rapid fault removal and the system's need for tough power sources and ensuring the safety of the wind farm.Based on the sufficient fault characteristics included in the fault component voltage,a method for fault collection line selection using the optimized strategies is proposed.Firstly,the fractal theory is introduced in detail.Second,according to the calculation results of the multi-fractal spectrum of the fault component voltage,the prominent degree of the transient and steady-state characteristics contained in the fault signal is quantitatively described.Finally,a corresponding protection scheme is proposed.Three methods for discriminating fault direction are proposed.The steady-state method based on the weak-feed characteristic and the transient method based on the polarity characteristic are both aimed at making full use of the characteristics of the fault line.They both have simple and reliable advantages under the condition of satisfying sufficient fault characteristics.On the other hand,the gray correlation method is proposed when the transient and steady state judgments are fuzzy.Through the comprehensive analysis of the fault characteristics of each line,the grey correlation method overcomes the limitations based solely on the transient,steady state characteristics and the insufficient local information characteristics.Furthermore,the method improves the reliability of fault direction judgment for collection lines.Based on the analysis that the sensitivity of the scalar product differential protection cannot be balanced with the ability to resist synchronization errors,a sufficient differential protection with a concave function of braking angle and amplitude ratio is proposed.When the amplitude ratio of the current on both sides is large,the blocking angle increases significantly with the increase of the amplitude ratio,and it is aimed at improvin the ability to resist synchronization errors.In contrast,when the amplitude ratio of the current on both sides is relatively small,the blocking angle increases slowly and very little with the increase of the amplitude ratio,and it is aimed at improving the protection sensitivity.Furthermore,by taking advantadge of the complex frequency signals of wind power sources,it is proposed that the protection criterion is transformed to be suitable for the full frequency domain.Therefore,the sensitivity in the fault zone is further improved.At the same time,the amplitude and phase angle calculated by the vector composed of the sampling sequence allow the sampling time window to be adjusted arbitrarily.It has better flexibility.It is proposed to use the pre-fault current as a reference to calculate the phase angle difference of the fault current on both sides.It enables differential protection to operate under a quasi-asynchronous mechanism,which significantly improves the ability to resist synchronization errors.It is more suitable for distribution networks with weak communications.Based on the MATLAB / Simulink platform,simulation models are established to verify the current characteristics,impedance characteristics,frequency characteristics,sufficient inverse time protection scheme,fault direction discrimination scheme,and sufficient differential protection scheme of the DFIG-based weak-feed source.Simulation results show that the proposed protection schemes can adapt to the power grid with wind power.