| The flexible DC transmission system can independently adjust reactive power and active power,and its transmission method is flexible,which can be applied in fields such as flexible transmission,asynchronous networking,and large-scale new energy grid connection.Due to the characteristics of small inertia and low damping in flexible DC power grids,the short-circuit current rapidly increases after line faults,requiring flexible DC protection to respond within 3 ms.The protection algorithm based on transient characteristics has become the mainstream of high-speed flexible and direct protection.However,the high-frequency signals contained in lightning current have a significant impact on transient protection,and may even cause misoperation.Therefore,the research on lightning interference identification for flexible DC transmission lines is particularly important,and accurate identification of lightning interference can improve the reliability of protection.Based on this,this article focuses on the identification of lightning interference on flexible DC transmission lines,mainly including the following work:(1)Propose a lightning interference recognition method based on the characteristics of voltage traveling wave heads.Firstly,using the principle of refraction and reflection of traveling waves,the transient voltage traveling waves of lightning strike lines are analyzed and the following characteristics are obtained:when the lightning strike position is different,the polarity of the voltage zero mode first traveling wave head has different characteristics,that is,the polarity of the wave head at the lightning strike wire position is negative,and the polarity of the wave head at the lightning strike tower top position is positive;When there is lightning interference,the reflection of the line boundary does not change the polarity of the wave head.When there is a lightning fault,the reflection of the fault point changes the polarity of the wave head.At the same time,the grid method is used to derive the time interval between the arrival of voltage traveling wave heads at the line boundary,and the characteristics of the number of wave heads under lightning interference and lightning faults are sorted out.Then,combining the polarity and quantity characteristics of voltage traveling wave heads,a lightning interference recognition principle based on voltage traveling wave head features is proposed,and wavelet modulus maximum decomposition is used to extract voltage traveling wave heads.Finally,simulation validation was conducted under different lightning strike distances,lightning current models,and line parameters.The validation results showed that the above method was accurate and effective.Using data within a 1.7 ms time window after protection startup,ultra fast discrimination was achieved between lightning faults within 75%of the entire line range and lightning interference within 50%of the line range.The rest of the lightning disturbance cases could be quickly identified using data from a 2.3 ms data window after protection startup,Meet the requirements for quick action of flexible and straight system protection.(2)Propose a lightning interference recognition method based on short-term window energy product.Firstly,analyze the energy process in the transmission line after lightning strikes.Under the interference of lightning strikes,the structure of the transmission line remains unchanged,and the amplitude fluctuation of voltage and current in the transmission line is relatively small,resulting in slow dissipation of lightning current energy;In the case of a lightning strike fault,the insulator string is broken down,and the fault grounding point is added to the line.The lightning current energy is quickly released through the grounding point.The analysis results show that during lightning interference,the energy transformation is small,while during faults,the energy transformation is large.Then,the energy characteristics in the line are used to identify lightning interference,and the energy value within the 2 ms window after the lightning strike is calculated.Considering the impact of the lightning strike position(lightning conductor and lightning tower top)on energy,an adaptive threshold value is set based on the lightning strike position,and a lightning interference recognition scheme based on the short-term window energy product is designed.Finally,the simulation verification results indicate that this recognition method can accurately identify lightning interference,lightning single pole faults,and lightning bipolar faults,and is not affected by lightning location,lightning distance,lightning current model,line parameters,and noise.(3)Propose a comprehensive recognition method for lightning interference by integrating the characteristics of voltage traveling wave waveform and the short-term window energy product.Firstly,analyze the potential insensitivity of the two schemes in extreme situations.Then,in response to the above insensitive situation,the complementarity of the two schemes was studied,and the concept of trust weight was defined using the proximity between the short-term window energy value and the threshold value of Scheme 2.A comprehensive lightning interference recognition scheme combining Scheme 1 and Scheme 2 was proposed.Finally,through simulation verification,the results show that the recognition scheme based on method fusion can accurately identify lightning interference.This comprehensive recognition method can eliminate the insensitive area of the original single scheme,improve the accuracy of lightning interference recognition in extreme cases,and comprehensively classify lightning types and lightning positions. |
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