高电压技术 教学课件 作者 吴广宁9.31.ppt

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1、9.3切除空载变压器过电压9.3Removalofno-loadtransformerover-voltage正常运行时,空载变压器表现为一励磁电感。切除空载变压器就是开断一个小容量电感负荷，会在变压器和断路器上出现很高的过电压。开断并联电抗器、电动机等，也属于切断感性小电流的情况。Duringthenormaloperation,no-loadtransformeractsasanexcitationinductor.Removalofno-loadtransformerisbreakingas

2、mall-capacityinductanceload,andahighover-voltagewillappearontransformersandcircuitbreakers.Breakingshuntreactor,motor,arealsobelongtothesituationofcuttingoffasmallemotionalcurrent.9.3.1发展过程9.3.1Development9.3.2影响因素与限制措施9.3.2Influencingfactorsandrestri

3、ctivemeasures本节内容Contentsinthissection变压器和断路器上出现过电压的原因是变压器的空载电流过零前就被断路器强制熄弧而切断，导致全部电磁能量转化为电场能量而使电压升高。Thereasonwhyover-voltageappearsontransformerandcircuitbreakeristhatbeforetransformerno-loadcurrentcrosseszero,ithasbeenshutoffbycircuitbreaker’mandato

4、ryextinction.Alltheelectromagneticenergyischangingintoelectricenergywhichincreasesvoltage.9.3.1发展过程9.3.1Development研究表明：切断100A以上的交流电流时，开关触头间的电弧通常是在工频电流自然过零时熄灭的；当所切除的电流很小时（变压器的空载电流非常小，只有几安到几十安），电弧往往提前熄灭，亦即电流会在过零之前就被强行切断，即所谓的截流现象。Itisdemonstratedthatwhe

5、ncuttingoffcurrentismorethan100AAC,thearcusuallyextinguisheswhenfrequencycurrentisoverzeronaturallybetweencontacts.Whenthecurrenttoberemovedisverysmall(thetransformerno-loadcurrentisverysmall,onlyafewANtoscoresAN),thearcoftenextinguishesbeforehand.Tha

6、tistosaythecurrentswillbeforciblycutoffbeforetheyareoverzero,thatis,thephenomenonoftheso-calledclosure.可用图9-8所示的简化等值电路来说明这种过电压的发展过程。图中为变压器的激磁电感，为变压器绕组及连接线的对地电容。Figure9-8showsthesimplifiedequivalentcircuittoillustratethedevelopmentofover-voltage.Inthef

7、igureforthetransformermagnetizinginductance,forthetransformerwindingandconnectingtheline-to-groundcapacitance.图9-8切除空载变压器的理想等值电路Fig9-8Theidealequivalentcircuitofremovalofno-loadtransformer假如空载电流时发生截断（即由突然降到零），此时电源电压为，则切断瞬间在电感和电容中所储存的能量分别为:Iftheno-load

8、currentoftruncationoccurs(thatissuddenlyreducestozero),atthistimethesupplyvoltageis,thenthemomentwhencutofftheenergystoredintheinductorandcapacitor,whichareshowedbelow:(9-6)此后即在、构成的振荡回路中发生电磁振荡，在某一瞬间，全部电磁能量均变为电场能量，这时电容上出现最大电压Thereafterelectroma