Multi-time-scale Transient Characteristics Analysis And Protection Principle Research Of AC Transmission Lines

With the widespread application of new energy power generation technology in power systems and large-scale interconnection of the power grid,the transmission capacity of transmission lines is increasing,and the voltage level is getting higher and higher.Therefore,a higher fault removal time for relay protection is proposed.Claim.The traditional power frequency protection is to filter out the electromagnetic transient harmonics after the fault through the delay link,so that the power frequency is used for fault identification,but the electromagnetic transient process of the ultra-high voltage transmission line is longer,which makes the power frequency protection Failure to remove faults quickly.On the other hand,a large number of power electronic devices and inverter-type power supplies are connected to the power grid.After a fault occurs,they will quickly adjust the response,resulting in changes in the power electromotive force parameters and transmission path parameters that reflect the fault,making the power frequency amount The correct performance of the protection is reduced.In addition to the power frequency component,a large number of transient components are included in the UHV transmission line after a fault occurs.These transient components contain rich fault information.If the characteristics of these transient components are used for line protection,the protection can be greatly improved.Accuracy and quick-action.In this paper,a simulation model of a 500kV AC transmission system is built in PSCAD/EMTDC,and the influence of the initial waveform characteristics and bus boundary conditions on the transient components of the fault is analyzed.This article first theoretically analyzes the fold and reflection coefficient expressions of different buses and the propagation process of the fault traveling wave on the transmission line.The effects of the bus boundary conditions on the transient components of the fault are summarized.The bus has higher frequency components.Strong attenuation,and the lower frequency components hardly attenuate when transmitted through the bus.The multi-time scale analysis of the transient process of transmission lines in PSCAD.A large number of simulation results prove that the data transient characteristics within 3 ms after the fault are the most obvious.Because the line boundary has a strong attenuation effect on high-frequency components,when a fault occurs inside or outside the transmission line,the transient component of the fault detected by the protection installation place is very different.Based on this feature,faults in the area and outside are proposed.Identify new scenarios.This paper proposes to use the synchronous squeeze wavelet transform to process and analyze the fault information,which can obtain signals in any desired frequency band.Compared with the existing discrete wavelet transform,the frequency division accuracy is greatly improved,and the spectral aliasing phenomenon is avoided.Select the wavelet base.Simulation results show that the protection scheme is not affected by fault conditions,and can effectively discriminate between internal and external faults.For AC transmission systems with multiple outgoing lines,this paper proposes the use of double-ended quantities for fault identification.For non-faulty lines,the transient component of the fault detected at the protection installation is transmitted through the attenuation of the bus,and the energy value in the high frequency band is greatly reduced,which is much lower than that of the faulty line.Use this feature to form a protection scheme,compare the high-frequency energy values of each outgoing line on the same bus,and select the fault line.It can be seen from the simulation results that the fault identification scheme based on fault transient components proposed in this paper has strong applicability.Under different fault types,different fault locations,different transition resistances,and different initial phase angles of the fault,etc.Both can quickly and accurately identify faulty lines.