对于三相变压器绕组，无论采用哪种接法，一、二次侧线电动势的相位差总是( )的倍数。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='30222' and id < 2444814 and id<>'2444814' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='30222' and id > 2444814 and id<>'2444814' order by id asc limit 5