500kV变压器后备保护主要有（   ）。

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SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69396' and id < 4492321 and id<>'4492321' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku_ where sourceid='69396' and id > 4492321 and id<>'4492321' order by id asc limit 5