Research On Resonance Suppression Mechanism And Application Of High-order Damped Filter

With the successive completion and commissioning of high voltage(UHV)AC/DC transmission projects,China has formed the largest AC/DC hybrid power grid with the highest voltage level in the world.However,nonlinear devices such as converters and high-power DC loads in AC/DC hybrid power grid generate a large number of harmonic currents.If the frequency matches the resonance points formed by the interaction of harmonic impedances of multiple devices in the network,serious resonance accidents will occur,resulting in grid voltage oscillation,and even equipment explosion and direct current oscillation flow blocking and other serious consequences.At present,passive filter is the main way of resonance suppression in power grid.The traditional design method can control the harmonics at single or multiple tuning frequencies,and can achieve effective resonance suppression in the power grid with only a few characteristic subharmonics.However,in the AC / DC hybrid power grid with abundant harmonic frequency and multiple resonance points,the traditional design can not avoid the resonance risk at the non tuning frequency.Therefore,it is necessary to conduct in-depth research on the wide frequency domain resonance suppression technology.Resonance is caused by the interaction of capacitive and inductive harmonic impedances in the power grid,and the damage it causes to the system mainly depends on the harmonic level and harmonic intensity at the resonance point.Therefore,how to design a filter so that it can always reduce the harmonic level or resonance intensity at the resonance point when responding to changes in the network harmonic impedance is the key issue and technological difficulty of wide frequency domain resonance suppression.To tackle this issue,this paper proposes a resonant-free design method considering the uncertainty of system frequency response.Its core is: if the filter can control the resonance intensity of all potential resonance points in the worst operating conditions within the threshold range,it can avoid the resonance risk caused by the dynamic changes of network harmonic characteristics.In this paper,a lot of theoretical and experimental work is carried out around the topology implementation,parameter selection and application verification,and some achievements are acquired which mainly include:1)This paper proposes a resonant-free design method considering the uncertainty of system frequency response.The quantitative index of resonance intensity is established,and then it is deduced that the filter should have broadband damping characteristics in order to limit the resonance intensity of the system in the worst condition within the acceptable range.The research shows that the high-order damped filter has broadband damping characteristics.Based on this situation,the parameter optimization design methods of high-order damped filter under two scenarios of unknown and partially known system harmonic impedance are proposed.The differences in cost,power loss,resonance suppression performance and robustness between the two methods are compared.Further,the simulation verification is carried out based on actual cases,and a high-order damped filter design guideline suitable for different system conditions and resonance suppression requirements are summarized,which can provide reference for practical application It provides reference for parameter design of wide frequency resonance suppression damped filter in practical engineering,and provides guidance for resonance suppression of AC/DC hybrid power grid.2)To tackle the issue of high active power loss of third-order damped filter,this paper presents a double topology third-order damped filter.Firstly,the resonance mechanism of capacitor in high voltage transmission network is analyzed,and the defects of traditional single tuned filter in resonance suppression are pointed out.Secondly,aiming at the issue of large active power loss of the third-order damped filter,the topology of the third-order damped filter is proposed.The core idea is to add a breaker so that the damping unit can be switched on or off depending on whether resonance occurs.The design of and control mechanism for the proposed third-order damped filter are presented.Finally,the performances of the proposed filter in terms of harmonic mitigation,loading,switching transient and cost are verified for an actual capacitor application case.The results indicate that the proposed filter significantly mitigates the power loss and improves the filtering performance of the system.3)To tackle the issue of the non-characteristic harmonic resonance in LCCHVDC terminal,this paper presents a double topology damped high pass filter.Firstly,the case study on a non-characteristic harmonic resonance incident that occurred in a real-life LCC-HVDC terminal is carried out.It is revealed that the improper design of the HVDC filter bank leads to the resonance at the 5th harmonic.Secondly,two existing solutions are then investigated.One is to add additional noncharacteristic harmonic filters,and the other is to use damped high pass filters.The results indicate the latter has superior performance but induces considerable power loss.In view of this,this paper further improves the performance of the damped high pass filter by adding an additional breaker in series with the damping unit.The breaker is designed to disconnect the damping unit when the resonance condition does not occur.Furthermore,a retrofitting-based design method is proposed to reduce the cost of resonance-mitigation.Finally,based on the simulation verification of the established converter station model,the results show that the proposed double topology damped high pass filter can satisfactorily eliminate the harmonic resonance and meanwhile reduce the power loss.The transient and cost analysis indicates that the additional switch is not a big concern.4)To tackle the issue of the complementary resonance in AC/DC system,a resonant suppression scheme based on damped high-pass filters is proposed.Firstly,analyze the mechanism of the complementary resonance,and deduce the conditions that the suppression method based on AC filter needs to meet: the low-order harmonic impedance of the filter has weak inductive characteristic.Secondly,it is found that the low order harmonic damping can be used to reduce the peak impedance of its low order parallel resonance point,so that the low order harmonic impedance of the damped high-pass filter presents weak inductive characteristic.Then,the design method and calculation flow of damping filter parameters are given.Finally,a model is built to simulate the complementary resonance scenarios and compare different resonance suppression schemes.The result shows that the proposed complementary resonance suppression scheme based on damped high pass filter can simultaneously filter the AC side characteristic subharmonics and suppress the complementary resonance,and has no significant adverse effect on the performance of other filters and system operation reliability,so it can be used to solve the problem of complementary resonance in AC/DC system.This paper proposed a resonant-free design method considering the uncertainty of system frequency response,which can ensure that no serious harmonic amplification accident occurs even the harmonic impedance characteristics of the power grid change.The research results further enrich and improve the existing theory and application system of passive resonance suppression,which has important scientific significance for actively promoting the development of power resonance suppression.