电网中的工频过电压一般是由（    ）引起的。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69517' and id < 4503679 and id<>'4503679' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69517' and id > 4503679 and id<>'4503679' order by id asc limit 5