Chương 10-Quá điện áp thao tác.ppt

V vëy sau khi ãng tiõp ióm chýnh xut hiön qu

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V× vËy sau khi ®ãng tiÕp ®iÓm chÝnh xu¸t hiÖn qu¸ ®iÖn ¸p (®êng 5) víi biªn ®é b»ng thµnh phÇn tù do x¸c ®Þnh bëi hiÖu ®iÖn ¸p gi÷a hai ®êng 3 vµ 4 vµo thêi ®iÓm tríc khi ®ãng (giai ®o¹n 2 cña qu¸ tr×nh qu¸ ®é). Giai ®o¹n nµy sÏ lµm xuÊt hiÖn qu¸ ®iÖn ¸p.
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07/29/18 Tõ ®©y suy ra lµ ®Ó h¹n chÕ qu¸ ®iÖn ¸p khi ®ãng ®êng d©y kh«ng t¶i cÇn sö dông ®iÖn trë son cã trÞ sè kh«ng lín h¬n tæng trë sãng cña ®êng d©y. §iÖn trë nµy còng cã hiÖu qu¶ h¹n chÕ qu¸ ®iÖn ¸p khi c¾t ng¾n m¹ch kh«ng ®èi xøng hoÆc chÕ ®é kh«ng ®ång bé, bëi v× qu¸ tr×nh ®ãng vµ c¾t trong hai trêng hîp nµy t¬ng tù nhau.
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07/29/18 C¾t ®êng d©y kh«ng t¶i V× vËy nhiÖm vô cña ®iÖn trë son kh«ng ph¶i dËp ®îc thµnh phÇn dao ®éng tù do khi hå quang ch¸y l¹i, mµ lµ viÖc gi¶m ®iÖn ¸p phôc håi ®Õn trÞ sè kh«ng g©y ch¸y l¹i cña hå quang. NÕu ®êng d©y kh«ng cã kh¸ng, cã thÓ thay thÕ mét c¸ch gÇn ®óng b»ng ®iÖn dung tËp trung ( Z =X ®v =X C ). §iÖn ¸p trªn ®êng d©y sau khi t¸ch tiÕp ®iÓm chÝnh C R t C C C C C e X R R X R arctg t X R X E U . 2 2 2 2 2 cos Trêng hîp tèi u khi c¾t ®êng d©y kh«ng t¶i lµ khi kh«ng xuÊt hiÖn hå quang ch¸y l¹i Cã thÓ ®¹t ®îc ®iÒu nµy nÕu chän R gÇn b»ng m« dun tæng trë ®Çu vµo cña ® êng d©y, tøc lµ cÇn t¹o ®iÒu kiÖn dao ®éng phi chu kú. Cã thÓ bá qua ¶nh hëng cña ®iÖn c¶m bªn trong cña nguån, vµ do ®ã còng kh«ng cÇn tÝnh ®iÖn ¸p do hiÖu øng ®iÖn dung cña ®êng d©y
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07/29/18 Qu¸ tr×nh qu¸ ®é khi c¾t ®êng d©y kh«ng t¶i : a) s¬ ®å ®êng d©y; b) ®êng cong ®iÖn ¸p víi R=3X ®v; 1 - thµnh phÇn cìng bøc; 2 - thµnh phÇn tù do 3-®iÖn ¸p x¸c lËp; 4- søc ®iÖn ®éng cña nguån HiÖu gi÷a ®iÖn ¸p tæng (®êng 3) vµ søc ®iÖn ®éng cña nguån (®êng 4) chÝnh lµ ®iÖn ¸p kh«i phôc trªn c¸c tiÕp ®iÓm chÝnh cña m¸y c¾t. vµo thêi ®iÓn A, khi dßng ®iÖn ®êng d©y qua trÞ sè kh«ng, ®iÖn ¸p cña ®êng d©y cã gi¸ trÞ lín nhÊt, tiÕp ®iÓm phô ®îc t¸ch ra, trªn ®êng d©y cßn ®iÖn ¸p kh«ng ®æi. PhÇn g¹ch chÐo b»ng nÐt ®Ëm lµ ®iÖn ¸p kh«i phôc gi÷a c¸c tiÕp ®iÓm chÝnh, cßn phÇn g¹ch b»ng nÐt ®øt lµ ®iÖn ¸p xuÊt hiªn trªn tiÕp ®iÓm phô cña m¸y c¾t.
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07/29/18 NÕu t¨ng trÞ sã ®iÖn trë son R, ®iÖn ¸p phôc håi trªn c¸c tiÕp ®iÓm chÝnh t¨ng nhng l¹i lµm gi¶m ®iÖn ¸p trªn tiÕp ®iÓm phô. BiÖn ph¸p h¹n chÕ qu¸ ®iÖn ¸p cã hiÖu qu¶ khi ®ãng ®êng d©y hoÆc tù ®éng ®ãng l¹i lµ ®iÒu khiÓn thêi ®iÓm ®ãng m¸y c¾t. TrÞ sè qu¸ ®iÖn ¸p phô thuéc vµo thêi ®iÓm ®ãng c¾t ®· ®îc xem xÐt ë phÇn ®Çu ch¬ng. Nã
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