Study On Principle Of High-Speed Protection For Extra-High-voltage Transmission Line Based On Time-Frequency Analysis

With the construction of strong smart grid in china, many large-scale new energy power plants connect to grid network, high-voltage direct current (HVDC) transmission lines in grid are increasing gradually, flexible alternating current transmission systems (FACTS) are widly used in network. The development of the grid has put forward higher requirements on the selection, sensitivity and reliability, instancy of the transmission line protection. On the other hand, high speed fault isolation is required to reduce the damage on the system equipments due to electrodynamic and thermal stresses generated by short-circuit current. Furthermore, through reducing the fault clearance time, transmission line’s transmission capacity can be increased and the stability of the power system can be greatly improved. Therefore, the research on the fast, reliable and sensitive protection principle is of great significance to guarantee the safe and stable operation of power system. Based on this, in this thesis, the transient travelling wave based ultra-high-speed protection and fast distance protection based on differential equation algorithm is studied.At first, the S transform is applied to obtain the time domain distribution of the fault generated travelling wave, and the travelling wave based unit pilot protection, in which the transfer function of transmission line is taken into consideration, is proposed. In order to accurately describe the travelling wave’s propagation process and its attenuation law in transmission line, the equivalent two-port network of the lossy uniform transmission line is used to set up the lumped circuit of faulted network, and the mathematical relationship of initial backward traveling waves on both ends of the faulted line is deduced. The proposed algorithm determines whether a fault is internal or external to the protected zone by the ratio of initial backward traveling waves on both ends of the line. In principle, the method is immune from the adverse effects of factors such as the error of line model, circuit parameter uncertainty, etc. The simulation data from PSCAD/EMTDC is applied to verify and test the proposed algorithm, the simulation results show that the algorithm can reliably discriminate between internal fault and external fault.At second, on the basis of the definition of apparent surge impedance, two kinds of traveling wave directional relay are present. One is the apparent surge impedance based direction relay. The criteria to determine the fault direction, which is referred to as double mho circle criteria, is designed. The criteria consist of forward mho circle which is set according to the busbar network surge impedance and the reverse mho circle which is set according to the forward line surge impedance. The fault direction is determined by the location of the apparent surge impedance in the double mho circles. The apparent surge impedance seen by the relay locates in the forward mho circle when a forward fault occurred and in the reverse mho circle when a reverse fault occurred. The other is the apparent surge impedance angle based direction relay. If forward fault occurs, the apparent surge impedance angle is greater than 0.5π(rad); if reverse fault occurs, the apparent surge impedance angle is close to zero. Based on the value of apparent impedance surge angle, the forward and reverse criterion can be designed. Quantities of simulation results show that the proposed two kinds of directional relay can discriminate a fault as forward or reverse direction rapidly and reliably, and their performance can endure the influences of various factors, including different fault inception angles, fault distances, fault resistances.At third, a scheme of distance protection, in which Hilbert-Huang transform (HHT) is used to decompose high-frequency components of the results from the differential equation algorithm (DEA), is presented. The results from DEA constitute the time sequences. Through empirical mode decomposition (EMD), high frequency harmonics of the time sequences are separated out, and the residue which has monotonic trend is obtained. The residue is used to evaluate the impedance of the faulted line. Simulation results show that the proposed scheme can quickly evaluate the impedance of the faulted line with high accuracy.At last, two methods based on the change rate of fundamental current phasor are suggested. The one is the method selecting faulty phase and the other one is the method identifying lightning disturbance. When fault occurs, the change rate of fundamental current phasor of faulty phase, which is affected by the decaying DC component and the fundamental frequency fault component, increases rapidly, and that of non-faulty phase is small. If grouding fault occurs, the change rate of fundamental current phasor of zero sequence current is affected by the decaying DC component and the fundamental frequency fault component and increases rapidly; otherwise its value is near zero. In according to the characteristics of the change rate of fundamental current phasor under various conditions, the change rate of fundamental current phasor based method, which can rapidly select faulty phase, is presented. Under the condition of transmission line fault caused by short-circuit or lightning stroke, the additional current component contain the decaying DC component and the fundamental frequency fault component which causes the change rate of fundamental current phasor to increase rapidly. The additional current component contains high frequency components and the change rate of the fundamental current phasor is small when transmission line is suffered lightning stroke without causing fault. In according to the characteristics of the additional current components generated by fault and lightning stroke, the change rate of fundamental current phasor based method, which can rapidly identify lightning disturbance, is presented. The EMTDC simulation results show that the proposed methods are effective and reliable.To further verify whether the protection algorithm conform to the requirements of the specifications by using the methods of dynamic test or onsite testing, the floating-point digital signal processor TMS320C6748 based hardware system of experiment equipment is designed, a real-time operating system kernel SYS/BIOS based system software architecture is introduced, and the architecture of the application software is designed. Furthermore, the critical program and algorithm is tested.