khái niệm về quá trình quá độ điện từ

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  1. 1. Tc Gi: L Vn t Chng 1: KHI NIM V QU TRNH QU IN T I. KHI NIM CHUNG Ch ca h thng in thay i t ngt s lm pht sinh qu trnh qu in t, trong qu trnh pht sinh do ngn mch l nguy him nht. tnh chn cc thit b in v bo v rle cn phi xt n qu trnh qu khi: - ngn mch. - ngn mch km theo t dy. - ct ngn mch bng my ct in. Khi xy ra ngn mch, tng tr ca h thng in gim, lm dng in tng ln, in p gim xung. Nu khng nhanh chng c lp im ngn mch th h thng s chuyn sang ch ngn mch duy tr (xc lp). T lc xy ra ngn mch cho n khi ct n ra, trong h thng in xy ra qu trnh qu lm thay i dng v p. Dng trong qu trnh qu thng gm 2 thnh phn: chu k v khng chu k. Trng hp h thng c ng dy truyn ti in p t 330 KV tr ln th trong dng ngn mch ngoi thnh phn tn s c bn cn cc thnh phn sng hi bc cao. Nu ng dy c t b dc s c thm thnh phn sng hi bc thp. Nhim v ca mn hc ngn mch l nghin cu din tin ca qu trnh ngn mch trong h thng in, ng thi xt n cc phng php thc dng tnh ton ngn mch. II. CC NH NGHA C BN Ngn mch: l mt loi s c xy ra trong h thng in do hin tng chm chp gia cc pha khng thuc ch lm vic bnh thng. - Trong h thng c trung tnh ni t (hay 4 dy) chm chp mt pha hay nhiu pha vi t (hay vi dy trung tnh) cng c gi l ngn mch. - Trong h thng c trung tnh cch in hay ni t qua thit b b, hin tng chm chp mt pha vi t c gi l chm t. Dng chm t ch yu l do in dung cc pha vi t. Ngn mch gin tip: l ngn mch qua mt in tr trung gian, gm in tr do h quang in v in tr ca cc phn t khc trn ng i ca dng in t pha ny n pha khc hoc t pha n t. in tr h quang in thay i theo thi gian, thng rt phc tp v kh xc nh chnh xc. Theo thc nghim: trong : I - dng ngn mch [A] Ngun: Http://bkeps.com
  2. 2. Tc Gi: L Vn t l - chiu di h quang in [m] Ngn mch trc tip: l ngn mch qua mt in tr trung gian rt b, c th b qua (cn c gi l ngn mch kim loi). Ngn mch i xng: l dng ngn mch vn duy tr c h thng dng, p 3 pha tnh trng i xng. Ngn mch khng i xng: l dng ngn mch lm cho h thng dng, p 3 pha mt i xng. - Khng i xng ngang: khi s c xy ra ti mt im, m tng tr cc pha ti im nh nhau. - Khng i xng dc: khi s c xy ra m tng tr cc pha ti mt im khng nh nhau. S c phc tp: l hin tng xut hin nhiu dng ngn mch khng i xng ngang, dc trong h thng in. V d: t dy km theo chm t, chm t hai pha ti hai im khc nhau trong h thng c trung tnh cch t. Bng 1.1: K hiu v xc xut xy ra cc dng ngn mch DNG NGN MCH HNH V QUY C K HIU XC SUT XY RA % 3 pha N (3) 5 2 pha N (2) 10 2 pha-t N (1,1) 20 1 pha N (1) 65 III. NGUYN NHN V HU QU CA NGN MCH III.1. Nguyn nhn: - Cch in ca cc thit b gi ci, h hng. - Qu in p. - Cc ngu nhin khc, thao tc nhm hoc do c d tnh trc... Ngun: Http://bkeps.com
  3. 3. Tc Gi: L Vn t III.2. Hu qu: - Pht nng: dng ngn mch rt ln so vi dng nh mc lm cho cc phn t c dng ngn mch i qua nng qu mc cho php d vi mt thi gian rt ngn. - Tng lc in ng: ng lc in t gia cc dy dn c gi tr ln thi gian u ca ngn mch c th ph hng thit b. - in p gim v mt i xng: lm nh hng n ph ti, in p gim 30 n 40% trong vng mt giy lm ng c in c th ngng quay, sn xut nh tr, c th lm hng sn phm. - Gy nhiu i vi ng dy thng tin gn do dng th t khng sinh ra khi ngn mch chm t. - Gy mt n nh: khi khng cch ly kp thi phn t b ngn mch, h thng c th mt n nh v tan r, y l hu qu trm trng nht. IV. MC CH TNH TON NGN MCH V YU CU I VI CHNG: Khi thit k v vn hnh cc h thng in, nhm gii quyt nhiu vn k thut yu cu tin hnh hng lot cc tnh ton s b, trong c tnh ton ngn mch. Tnh ton ngn mch thng l nhng tnh ton dng, p lc xy ra ngn mch ti mt s im hay mt s nhnh ca s ang xt. Ty thuc mc ch tnh ton m cc i lng trn c th c tnh mt thi im no hay din bin ca chng trong sut c qu trnh qu . Nhng tnh ton nh vy cn thit gii quyt cc vn sau: - So snh, nh gi, chn la s ni in. - Chn cc kh c, dy dn, thit b in. - Thit k v chnh nh cc loi bo v. - Nghin cu ph ti, phn tch s c, xc nh phn b dng... Trong h thng in phc tp, vic tnh ton ngn mch mt cch chnh xc rt kh khn. Do vy ty thuc yu cu tnh ton m trong thc t thng dng cc phng php thc nghim, gn ng vi cc iu kin u khc nhau tnh ton ngn mch. Chng hn tnh chn my ct in, theo iu kin lm vic ca n khi ngn mch cn phi xc nh dng ngn mch ln nht c th c. Mun vy, ngi ta gi thit rng ngn mch xy ra lc h thng in c s lng my pht lm vic nhiu nht, dng ngn mch gy nn dng ln nht, ngn mch l trc tip, ngn mch xy ra ngay ti u cc my ct ... gii quyt cc vn lin quan n vic chn la v chnh nh thit b bo v rle thng phi tm dng ngn mch nh nht. Lc y tt nhin cn phi s dng nhng iu kin tnh ton hon ton khc vi nhng iu kin nu trn. Ngun: Http://bkeps.com
  4. 4. Tc Gi: L Vn t CHNG 2:CC CH DN KHI TNH TON NGN MCH I. Nhng gi thit c bn: Khi xy ra ngn mch s cn bng cng sut t in, c in b ph hoi, trong h thng in ng thi xy ra nhiu yu t lm cc thng s bin thin mnh v nh hng tng h nhau. Nu k n tt c nhng yu t nh hng, th vic tnh ton ngn mch s rt kh khn. Do , trong thc t ngi ta a ra nhng gi thit nhm n gin ha vn c th tnh ton. Mi phng php tnh ton ngn mch u c nhng gi thit ring ca n. y ta ch nu ra cc gi thit c bn chung cho vic tnh ton ngn mch. 1. Mch t khng bo ha: gi thit ny s lm cho phng php phn tch v tnh ton ngn mch n gin rt nhiu, v mch in tr thnh tuyn tnh v c th dng nguyn l xp chng phn tch qu trnh. 2. B qua dng in t ha ca my bin p: ngoi tr trng hp my bin p 3 pha 3 tr ni Yo/Yo. 3. H thng in 3 pha l i xng: s mt i xng ch xy ra i vi tng phn t ring bit khi n b h hng hoc do c c d tnh. 4. B qua dung dn ca ng dy: gi thit ny khng gy sai s ln, ngoi tr trng hp tnh ton ng dy cao p ti in i cc xa th mi xt n dung dn ca ng dy. 5. B qua in tr tc dng: ngha l s tnh ton c tnh cht thun khng. Gi thit ny dng c khi ngn mch xy ra cc b phn in p cao, ngoi tr khi bt buc phi xt n in tr ca h quang in ti ch ngn mch hoc khi tnh ton ngn mch trn ng dy cp di hay ng dy trn khng tit din b. Ngoi ra lc tnh hng s thi gian tt dn ca dng in khng chu k cng cn phi tnh n in tr tc dng. 6. Xt n ph ti mt cch gn ng: ty thuc giai on cn xt trong qu trnh qu c th xem gn ng tt c ph ti nh l mt tng tr khng i tp trung ti mt nt chung. 7. Cc my pht in ng b khng c dao ng cng sut: ngha l gc lch pha gia sc in ng ca cc my pht in gi nguyn khng i trong qu trnh ngn mch. Nu gc lch pha gia sc in ng ca cc my pht in tng ln th dng trong nhnh s c gim xung, s dng gi thit ny s lm cho vic tnh ton n gin hn v tr s dng in ti ch ngn mch l ln nht. Gi thit ny khng gy sai s ln, nht l khi tnh ton trong giai on u ca qu trnh qu (0,1 0,2 sec). II. H n v tng i: Bt k mt i lng vt l no cng c th biu din trong h n v c tn hoc trong h n v tng i. Tr s trong n v tng i ca mt i lng vt l no l t s gia n vi mt i lng vt l khc cng th nguyn c chn lm n v o lng. i lng vt l chn lm n v o lng c gi i lng c bn. Nh vy, mun biu din cc i lng trong n v tng i trc ht cn chn cc i Ngun: Http://bkeps.com
  5. 5. Tc Gi: L Vn t lng c bn. Khi tnh ton i vi h thng in 3 pha ngi ta dng cc i lng c bn sau: S cb : cng sut c bn 3 pha. U cb : in p dy c bn. I cb : dng in c bn. Z cb : tng tr pha c bn. t cb : thi gian c bn. cb : tc gc c bn. Xt v ngha vt l, cc i lng c bn ny c lin h vi nhau qua cc biu thc sau: S cb = U cb . I cb (2.1) (2.2) (2.3) Do ta ch c th chn ty mt s i lng c bn, cc i lng c bn cn li c tnh t cc biu thc trn. Thng thng chn trc S cb , U cb v cb . Khi chn cc i lng c bn th cc i lng trong n v tng i c tnh t cc i lng thc nh sau: E *(cb) c l E tng i c bn (tc l sc in ng E trong h n v tng i vi lng c bn l U cb ). Sau ny khi ngha r rng v s dng quen thuc th c th b du (*) v (cb). MT S TNH CHT CA H N V TNG I: 1) Cc i lng c bn dng lm n v o lng cho cc i lng ton phn cng ng thi dng cho cc thnh phn ca chng. V d: S cb dng lm n v o lng chung cho S, P, Q; Z cb - cho Z, R, X. 2) Trong n v tng i in p pha v in p dy bng nhau, cng sut 3 pha v cng sut 1 pha cng bng nhau. 3) Mt i lng thc c th c gi tr trong n v tng i khc nhau ty thuc vo lng c bn v ngc li cng mt gi tr trong n v tng i c th tng ng vi nhiu i lng thc khc nhau. 4) Thng tham s ca cc thit b c cho trong n v tng i vi lng c bn l Ngun: Http://bkeps.com
  6. 6. Tc Gi: L Vn t nh mc ca chng (S m , U m , I m ). Lc : 5) i lng trong n v tng i c th c biu din theo phn trm, v d nh khng in, my bin p... TNH I I LNG TRONG H N V TNG I: Mt i lng trong n v tng i l A *(cb1) vi lng c bn l A cb1 c th tnh i thnh A *(cb2) tng ng vi lng c bn l A cb2 theo biu thc sau: A t = A *(cb1) * A cb1 = A *(cb2) * A cb2 V d, cho E *(cb1) , Z *(cb1) ng vi cc lng c bn (S cb1 , U cb1 , I cb1 ) cn tnh i sang h n v tng i ng vi cc lng c bn (S cb2 , U cb2 , I cb2 ): Nu tnh i cc tham s ng vi lng nh mc (S m , U m , I m ) thnh gi tr ng vi lng c bn (S cb , U cb , I cb ) th: Khi chn U cb = U m ta c cc biu thc n gin sau: CHN CC I LNG C BN: Thc t tr s nh mc ca cc thit b cng mt cp in p cng khng ging nhau. Tuy nhin, s khc nhau khng nhiu (trong khong 10%), v d in p nh mc ca my pht in l 11KV, my bin p - 10,5KV, khng in - 10KV. Do trong tnh ton gn ng ta c th xem in p nh mc U m ca cc thit b cng mt cp in p l nh nhau v bng gi tr trung bnh U tb ca cp in p . Theo qui c c cc U tb sau [KV]: Ngun: Http://bkeps.com
  7. 7. Tc Gi: L Vn t 500; 330; 230; 154; 115; 37; 20; 15,75; 13,8; 10,5; 6,3; 3,15; 0,525 Khi tnh ton gn ng ngi ta chn U cb = U m = U tb , ring i vi khng in nn tnh chnh xc vi lng nh mc ca n v gi tr in khng ca khng in chim phn ln trong in khng tng ca s , nht l i vi nhng trng hp khng in lm vic in p khc vi cp in p nh mc ca n (v d, khng in 10KV lm vic cp 6KV). Ni chung cc i lng c bn nn chn sao cho vic tnh ton tr nn n gin, tin li. i vi S cb nn chn nhng s trn (chng hn nh 100, 200, 1000MVA,...) hoc i khi chn bng tng cng sut nh mc ca s . Trong h n v tng i, mt i lng vt l ny cng c th biu din bng mt i lng vt l khc c cng tr s tng i. V d nu chn b lm lng c bn th khi *(b) = 1 ta c: III. Cch thnh lp s thay th: S thay th l s cho php th cc mch lin h nhau bi t trng bng mt mch in tng ng bng cch qui i tham s ca cc phn t cc cp in p khc nhau v mt cp c chn lm c s. Cc tham s ca s thay th c th xc nh trong h n v c tn hoc h n v tng i, ng thi c th tnh gn ng hoc tnh chnh xc. III.1. Qui i chnh xc trong h n v c tn: Hnh 2.1 : S mng in c nhiu cp in p Xt mng in c nhiu cp in p khc nhau (hnh 2.1) c ni vi nhau bng n my bin p c t s bin p k 1 , k 2 , ...... k n . Chn mt o