Investigation On Synergy And Arc Control Characteristics Of Series-connected Vacuum Arcs In Multi-break VCBs

With the rapid increasing demand of the smart grid and the environmental protection,the intelligent,reliable and clean electric power system equipment has been attracting more and more attention.High voltage circuit breakers are the important power transmission and transformation equipments,which are used to protect the safety and reliability of the power system.SF6 circuit breakers are widely used in the high voltage above 110kV.However,the environmental problems such as greenhouse effect exhaust pollution and the high liquefaction temperature limit its application.Vacuum circuit breakers(VCBs)are widely used in the voltage below 72.5kV because of its excellent arc extinguishing ability and environmental protection.However,it is difficult to extend VCBs to high voltage level be limited by the saturation effect.The multi-break VCBs are composed of series-connected short vacuum gaps,which take advantage of the insulation and arc extinguishing ability of the short gaps.It is the possible solution and development trend to substitute for SF6 circuit breakers in future.Nowadays,there is little deep investigations on the synergy characteristic among the vacuum arc effects,current zero characteristic,post arc characteristic,post arc voltage distribution and the dynamic insulation of series-connected VCBs.In addition,the arc control characteristic and methods of multi-break VCBs is not clear.In the paper,the synergy and the arc control characteristic of series-connected vacuum arcs are investigated and ascertained in order to find the proper synergy control and the magnetic arc control method.The equivalent circuit of the static and dynamic voltage distribution(VD)in multi-break VCBs is proposed to describe the mechanism,which considers the influence of the equivalent capacitance parameters,post arc conductance,post arc charge on the VD.The controlled post arc current source is introduced to describe the influence of the post arc charge on the VD.The mechanism of the self-sharing VD in the condition of asynchronous cooperation and series-connected different vacuum interrupters is obtained by analysis of the static electric field and experimental research.The above mechanism is that the self equivalent capacitance of the upper side compensates the influence of the stray capacitance of the upper side.The new post arc current measure equipment(PACME)based on the vacuum arc commutation and arc blow is proposed and designed by theoretical analysis and experimental study.The prototype of the PACME is set up and tested in the synthetic test circuit.The duration which the main current absolutely flow through the shunt resistor is more than 20μs and the peak current of the shunt resistor is below 500A below the measuring range of the high-precision current sensor,which meets the requirement of the post arc current measure.The bandwidth and accuracy of the PACME is 15MHz and 0.3%respectively.Based on the PACME and the high-precision RC divider,the post arc current(PAC)and the transient recovery voltage(TRV)can be measured and then the PAC,post arc charge and post arc conductance are obtained.The results indicate that the post arc charge can represent the residual plasma in current zero and the scatter of the post arc characteristic is caused by the difference of the final position of the last cathode spot.According to the above study,the model of vacuum arc interruption of double-beak VCBs is established to analyze the post arc TRV voltage distribution and dynamic insulation of different combinations with radial magnetic field(RMF)and axial magnetic field(AMF)contacts VCBs connected in series.The test platform of double-break VCBs in different combinations is set up.The influence of the post arc charge and the equivalent capacitance on the dynamic voltage distribution and the insulation is investigated.The results indicate that the optimal VD and breaking capacity of the combination when the upper side is RMF vacuum interrupters while the lower side is AMF vacuum interrupters.The post arc dynamic VD is depended on the post arc charge and the equivalent capacitance.The asynchronous operation of the double-break VCBs can utilize the influence of the arcing parameters on the post arc charge,the mechanism of the dynamic VD and the dynamic insulation characteristic to realize the perfect synergy of the dynamic VD and dynamic insulation.So the largest breaking capacity can be obtained.In order to investigate the influence of the magnetic arc control on the vacuum arc development and the post arc characteristic,the interaction among the arc memory effects in arcing time,residual plasma in current zero,the TRV voltage distribution and dynamic insulation in post arc phase is studied in the test platform of AMF arc control in multi-break VCBs.The pulse width and applied moment can be adjusted by the externally applied AMF system.The influence laws of the AMF magnitude,applied moment and pulse width on the vacuum arc development and post arc characteristic is researched in detail.The different combinations of externally applied AMF are applied to the double-break VCBs to analyze the effect of the AMF arc control on the VD.The mechanism of the AMF arc control of double-break VCBs is that the optimal AMF arc control reduces the unbalanced PAC’s charge injected into the grading capacitors which generates the unbalanced voltage.The optimal AMF arc control of multi-break VCBs is obtained.In addition,there exists the magnetic arc blow phenomenon caused by the interactive magnetic field in multi-break VCBs which can be also called bias magnetic field(BMF).The influence of magnetic arc blow on the vacuum arc development,post arc characteristic and the post arc VD is obtained by theoretical analysis and the test.The results indicate that direction of the vacuum arc is affected by the vacuum arc model,magnitude of the magnetic field and the current.The vacuum arc moves in Ampere’s direction and the post arc charge increase obviously because of the mental vapor sputtering when BMF is more than 200mT.At the same time,the metal vapor sputtering is disadvantage to the dynamic insulation.The investigation on the magnetic arc blow supplies the basis for utilizing or shielding magnetic arc low caused by BMF.In a word,the synergy and arc control characteristic of series-connected vacuum arcs in multi-break VCBs is ascertained and the synergy control method is proposed.The optimal AMF arc control method is obtained.Therefore,all of the above supplies the theoretical base for increasing the dynamic insulation,breaking capacity and technological development in high voltage,large capacity area.The paper has some valuable reference and it can guide the application of multi-break VCBs.