Research On Deep Limitation And Fast Energy Absorption For Short-Circuit Fault Current In HVDC Grid

The development of high voltage level of dc grid and the interconnected structure leads to the rapid-expanding scale of dc grid.The increasing fault current with rapidly rising rate of change puts pressure on dc circuit breaker,and may even exceed its maximum breaking capacity.Therefore,it is necessary to deeply limit fault current and quickly dissipate the short-circuit energy to reduce the electrical stress for the dc circuit breaker and to realize the fast interruption of fault current.Current limitation and energy absorption are considered separately in most of the existing fault interruption strategies.The fault current is limited mainly by the method of increasing impedance on the dc grid side,i.e.,current-limiting reactor.The short-circuit energy is dissipated by inserting arresters in the dc circuit breaker to add reverse voltage.However,due to the deep physical coupling between the fault current limitation and the energy absorption process,the deep limitation process of fault current will inevitably reduce the energy absorption speed of the arresters,and may lead to the increase of the overall energy absorption of the system,which will put huge pressure on the energy absorption of the arresters.It can be seen that the existing methods do not fully consider the complex energy coupling relationship in the fault current interruption process,and it is difficult to realize the deep limitation of fault current and fast absorption of short-circuit energy at the same time.Therefore,the thesis deals with the contradictory of deep limitation of fault current and fast absorption of short-circuit energy.An adaptive-impedance tracking and matching method for current-limiting reactor and an active energy absorption method for dc circuit breaker are proposed.Then,the current-limiting reactor and the dc circuit breaker are bidirectionally deep-coupled by constructing an energy-flow coupling channel to realize deep limitation of fault current and fast absorption of short-circuit energy at the same time.:(1)The influence rules of circuit basic parameters on fault current limitation and energy absorption process are revealed.Based on the equivalent complex frequency domain model of dc grid,the mapping relationship of model-matrix is established.The basic-cut-set-matrix-based equations are established to build the mathematic models for current limitation and absorption process of short-circuit fault.The sensitivity index of fault current to basic circuit parameters is extracted,and the influence rules of circuit basic parameters on fault current interruption and limitation are revealed.(2)A hybrid current-limiting reactor topology and its adaptive-impedance tracking and matching method are presented.Based on the demand for loop impedance based on fault interruption dynamic performance,the asymmetry impedance regulation criterion for current-limiting reactor is established.An adaptive-impedance tracking and matching method for hybrid current-limiting reactor is proposed.It can adaptively increase the equivalent impedance in the fault current limiting stage and reduce the equivalent impedance in the energy absorption process,realizing the deep limitation for fault current in dc grids.The practicability and effectiveness of the fault current deep limitation are verified by high voltage simulation test and low voltage test circuit.(3)An active energy absorption method for dc circuit breaker and bi-directionally deep coupling are proposed.Based on the two basic topologies of dc circuit breakers,the coordination method of the bottom parameters and upper control of the branches are deeply analyzed.The active-tracking and sorting control strategy of hybrid dc circuit breakers and the continuously active absorption topology of mechanical dc circuit breakers based on spark-gap bridge are propsoed respectively.By deconstructing the coupling mechanism of short-circuit between energy stored equipments,an uncontrollable power electronic device is used to construct an energy flow coupling channel,and the current-limiting reactor and dc circuit breaker are deeply coupled by the energy flow coupling channel,realizing deep limitation of fualt current and fast disspation of short-circuit energy at the same time.(4)A comprehensive strategy of fast interruption for short-circuit fault in dc grid is proposed.The transient characteristics of single-terminal modal voltage for typical line faults are analyzed to propose a fast fault detection scheme based on waveform similarity.On the premise of fast fault detection scheme,a comprehensive strategy of fast interruption for short-circuit fault in dc grid is formed by combining deep current limitation with active energy absorption.The short-circuit tests in dc grid show that: the short-circuit faults in dc grid can be comprehensively and quickly detected in 1 ms,realizing a quick start for the comprehensive strategy;the limiting rate of fault current of hybrid current-limiting reactor is increased by 16.4% contrast to conventional current-limiting reactor;compared with conventional dc circuit breakers,the short-circuit energy reducing rates of the two active energy absorption methods are41.4% and 46.1%,respectively;compared with the conventional fault interruption strategy,the limiting rate of fault current is increased by 47.4%,the energy acceleration absorption ratio is up to 48.5%,and the fault interruption acceleration ratio is up to40.8%.It is verified that the comprehensive strategy of fast interruption for short-circuit fault in dc grid realizes the deep limitation of fault current and the fast absorption of short-circuit energy through the bi-directionally deep coupling betwen hybrid current-limiting reactor and active energy absorption of dc circuit breaker.