在电网高频保护中，长时间发送高频信号的作用是()。（2.0）

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69423' and id < 4495250 and id<>'4495250' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69423' and id > 4495250 and id<>'4495250' order by id asc limit 5