当发电设备发生偶然性事故时，为维持系统正常供电，频率不受影响，所设置的备用是(  )。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69512' and id < 4503266 and id<>'4503266' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69512' and id > 4503266 and id<>'4503266' order by id asc limit 5