Research On Active Arc Suppression And Line Selection Technology In Medium Voltage Distribution Network

The operation mode of distribution network is complicated and changeable,and the single line-to-ground fault occurs frequently,which seriously threaten the safety of power equipment and users.The medium voltage distribution network in China mainly uses arc suppression coil to compensate the grounding current to elimate the instantaneous fault arc and prevent its development into a permanent fault.However,because of the industrial pollution,insulation aging and the wide application of nonlinear elements such as power electronic equipment,the defect that arc suppression coil can only compensate the reactive fundamental wave component of the fault current is gradually exposed.Due to the flexibility of the control of power electronic converter,active arc suppression technology can fully compensate all frequency components of the fault arc,so it has become a hot research direction of instantaneous fault arc suppression technology in distribution network.Due to the influence of fault phase identification accuracy,line voltage drop and inverter control performance,the active arc suppression technology still needs further improvement.In addition,when a permanent grounding fault occurs,the application of active arc suppression technology will change the fault characteristics,which will lead to the decline of the accuracy of fault line selection and location.Therefore,this paper carries out in-depth research on the active arc suppression and fault line selection technology of single line-to-ground fault for medium voltage distribution network.The main research results are as follows:(1)A fault phase identification method based on sector division of voltage coordinante system is proposed.Aiming at the problem that the traditional method is easy to misselect the fault phase when the grounding fault with high resistance occurs at asymmetric distribution network,the influence of three-phase asymmetry on the zero sequence voltage trajectory is analyzed.According to the amplitude and phase characteristics of three-phase voltages in distribution network under different zero-sequence voltage phases,the voltage coordinate system is divided into 6 different sectors and corresponding phase selection criteria are set.The reasonable division of sectors elimates the influence of three phase asymmetry of the line-to-ground admittance and avoids the influence of phase measurement error at the boundary of viltage sectors.The method can improve the accuracy of single line-to-ground fault identification in the distribution network and lay a good foundation for the application of active arc suppression technology.(2)An active arc suppression method and its control strategy considering line voltage drop are proposed.Considering the factors such as fault distance,transition resistance and load size,the influence of line voltage drop between fault phase bus and fault point on the effect of active arc suppression is analyzed.The reference voltage of grounding fault with high resistance and grounding fault with low resistance are set as zero and the calculated value of line voltage drop.The influence of line voltage drop between the busbar and fault point on the arc elimination effect is eliminated.In the specific implementation,the double closed-loop control strategy based on quasi-PR controller series first-order inertial link is designed,which makes the output voltage of inverter track reference value quickly and accurately to suppress the fault point voltage yo zero.The method can realize full compensation for the active power,reactive power and harmonic components of the fault arc and effectively suppress the fault arc.(3)A fault line selection and location method for permanent fault based on zero sequence characteristic parameters is proposed.By analyzing the fault characteristics of distribution network in the case of active arc suppression,the zero sequence characteristic parameters of each fault indicator are defined.The zero sequence characteristic parameters of each indicator are calculated by calculating the change of zero sequence voltage and current when different currents are injected relative to the pre-fault.Therefore,the position relationship between each indicator and fault point can be judged while eliminating the influence of three phase asymmetrical of the line-to-ground fault,so that the fault line and fault section can be selected.In addition,the method also has strong adaptability to the complex topology structure of distribution network.