引起工频过电压的原因是（    ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69490' and id < 4501403 and id<>'4501403' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69490' and id > 4501403 and id<>'4501403' order by id asc limit 5