| The proposal of the carbon emission peak and carbon neutrality target has promoted the accelerated construction of new power system and the rapid development of new energy generation.The distribution network is an important component of new power system,responsible for distributing electricity and providing reliable power supply for users,and new energy is mostly connected to distribution network in a distributed manner.With the large-scale integration of new energy generation into distribution network,the application of power electronics technology,direct current(DC)distribution technology,digital technology,and intelligent technology,the distribution network is undergoing profound changes and rapidly transitioning from traditional distribution network to new distribution network.The new distribution network can effectively increase the capacity of new energy consumption,reduce energy costs and improve energy utilization efficiency,which is the inevitable trend of distribution network development.However,the structure of new distribution network is complex and diverse,including DC distribution network,alternating current(AC)distribution network,and AC/DC hybrid distribution network,and the power supply includes a high proportion of distributed new energy.This leads to significant differences in the operating principles,fault electrical quantity characteristics,and signal types with traditional AC system.Traditional AC protection principles are not suitable for direct application to new distribution network,so there is an urgent need to research the protection principles for new distribution network in order to improve operational safety and power supply reliability of new distribution network.In this paper,the reference voltage defined based on measured voltage,measured current and line model.The protection method applicable to new distribution network is studied based on the waveform and amplitude relationship between the measured voltage and the reference voltage at different locations of faults.The main work of the paper is as follows:(1)A protection principle based on reference voltage for longitudinal comparison is proposed according to the characteristics of fault voltage and current in new distribution network.Firstly,the typical structures of new distribution network are introduced and the structures of flexible DC distribution network,AC distribution network with DG and AC/DC hybrid distribution network studied in this paper are identified.Secondly,the fault characteristics of each distribution network under this structure are analyzed separately using simulation methods.Finally,on this basis,the principle of longitudinal comparison protection based on the reference voltage is proposed.The corresponding longitudinal comparison logic is designed,and the selection of reference point location and line model is analyzed.The protection principle is basically independent of the power supply type and operation mode,and is applicable for various structural forms of new distribution network.(2)For flexible DC distribution network.a longitudinal comparison protection method based on the quadratic integration value of reference voltage is proposed,according to the protection principle of reference voltage,and based on the difference between the positive and negative value of reference voltage amplitude of flexible DC distribution line in case of internal and external faults.Firstly,the fault characteristics at different fault stages of the converter submodule are analyzed,and the relationship between the positive and negative polarity of the reference voltage and the internal and external faults is derived.This pattern is found that the positive polarity of the reference voltage corresponds to the faults outside the protection range,and the negative polarity of the reference voltage corresponds to the faults inside the protection range.Secondly,on the basis of this correspondence,a protection method based on the reference voltage secondary integral value is proposed,and the reliability of the protection criterion is improved by calculating the secondary integral value.The delay of the protection criterion and the action time of the circuit breaker are analyzed.Finally,the fault simulation model of flexible DC distribution network is built by using PSCAD/EMTDC,which verifies that the protection method has high reliability and good applicability and has good resistance to transition resistance,and has good response ability to the calculation error caused by distributed capacitance and noise interference.In addition,this protection method is capable of operating within 1 ms at the earliest and can be applied to the whole fault process.(3)For AC distribution network with DG.a longitudinal comparison protection method based on the similarity of the reference voltage waveform is proposed,according to the protection principle of reference voltage,and based on the difference between the measured voltage and the reference voltage waveform similarity of the AC distribution line with DG in case of internal and external faults.Firstly,the impact of DG access to the distribution network on protection is analyzed in terms of both the type of DGs and the location of DG access.Secondly,the waveform relationship between the measured voltage and the reference voltage is analyzed and derived for internal faults and external faults.This pattern is found that the measured voltage is similar to the reference voltage waveform for faults outside the protection range,and the measured voltage is not similar to the reference voltage waveform for faults inside the protection range.Thirdly.based on this correspondence,a longitudinal comparison protection method based on the similarity of the reference voltage waveform is proposed,which uses the cosine similarity to represent the waveform similarity.Finally,the fault simulation model of AC distribution network with DG is built by using PSCAD/EMTDC,which verifies that the protection method has high reliability and good applicability and has certain resistance to transition resistance,and is not affected by the unmeasurable branch load and noise interference.(4)For the AC/DC hybrid distribution network,the longitudinal comparison protection method based on the similarity of the reference voltage waveform is adopted,according to the analysis of the adaptability of the reference voltage based protection method of DC distribution network and AC distribution network in AC/DC hybrid distribution network.Firstly,the fault coupling characteristics of AC side and DC side faults in AC/DC hybrid distribution network are analyzed.Secondly,the adaptability of reference voltage based protection methods for DC distribution network and AC distribution network with DG is analyzed.It is found that the longitudinal comparison protection method based on the similarity of reference voltage waveforms is fully suitable for AC/DC hybrid distribution network.Finally,the fault simulation model of AC/DC hybrid distribution network is built by using PSCAD/EMTDC,which verifies that the longitudinal comparison protection method based on the similarity of reference voltage waveform has good reliability and applicability in AC/DC hybrid distribution network. |
PDF Full Text Request