giao trinh thuy dien

7/28/2019 Giao Trinh Thuy Dien

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L Vn t, ng quc thngng vn dng

Nh my

thy in

H ni 12 - 2005


2/125

M u

I. . Cng ngh nng lng v vai tr ca thy nngin nng cn c to ra t cc dng nng lng khc tim tng trong t nhin

nh cng ngh bin i nng lng. Chng hn, nhit nng tim tng trong cc

dng nhin liu (than , du m, kh t ...) c gii phng qua phn ng chy,

bin i thnh c nng v cui cng thnh in nng cc nh my nhit in.

C nng ca dng nc (sng, sui, thy triu ...) c bin thnh in nng cc

nh my thy in. Ti cc nh my in nguyn t, nng lng gii phng t

phn ng ht nhn (ca cc nguyn t c nguyn t lng ln - U235) cng cbin thnh in nng qua cc qu trnh bin i nhit c in t. Ngoi cc

cng ngh quan trng ni trn nhng cng ngh nng lng micng ang c

nghin cu p dng nh: nng lng mt tri, nng lng a nhit, nng lng

gi, nng lng sinh khi, sinh kh ...

Lch s pht trin cho thy t l cc dng nng lng t nhin c bin i khai

thc khng ging nhau v c s bin ng mnh theo thi gian (hnh 1).

Vo nhng nm 50 ca th k trc, tuyt i a s in nng c sn sut ra l cc nh my nhit in (trn 90%). Tuy nhin theo thi gian t l in nng do cc

nh my nhit in pht ra c xu hng gim dn, thu in tng dn v c s pht

trin nhanh ca phn in nng do cc nh my in nguyn t sn xut. iu ny

c th gii thch c bi s cn dn ca cc loi nhin liu v nhu cu ng dng

n vo cc lnh vc kinh t khc ngy cng c gi tr hn. (V d s dng than ,

du m, kh t nh l nguyn liu sn xut cht do, t nhn to, vt liu mi ...).

Trong khi k thut xy dng v

khai thc thu nng li c nhng b

c thay ivt bc, cho php lp t nhng t my cng sut ln, p p ngn sng xy

dng nhng NMT khng l ( NMT Tam Hip, xy dng trn sng Dng T

(TQ) vi tng cng sut 26 x 700 MW = 18200 MW l mt v d) lm cho gi

thnh xy dng (tnh trn mt n v cng sut lp my) ngy cng gim.

Tnh chung trn th gii, sn lng in ca cc NMT sn sut ra hin ang cung

cp cho trn 1/3 tng sn lng in tiu th. Mt s nc c t l thu in ln


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nh: Canaa (93%), Phn Lan (91%), Nauy (99%), Thu S (99,5%). Vit Nam,

theo con s thng k nm 1997 t l thu in chim 65% tng sn lng in

nng sn xut v tiu th ton quc.

II. Vi nt v pht trin thu in Vit Nam

Tnh n nm 1954 (sau khng chin chng Php) sn lng in nng Vit nam

ni chung v thu in ni ring hu nh cha c g. Tuy nhin, min Bc c

cc NMT T Sa, Na Ngn, Bn Thch; min Nam c NMT a Nhim (cng sut

4 x 40 = 160 MW). Nm 1971 do LX c gip xy dng khnh thnh NMT

Thc B vi cng sut 3 x 36 = 108 MW. Lc ny tng cng sut ca cc NMN

min Bc vn cha vt qu 100 MW, ngha l thu nng gi t l quan trng

trong HT Vit Nam t nhng ngy u xy dng. Nm 1992 NMT Ho Bnh

xy dng v lp t xong n t my cui cng vi tng cng sut ln ti 1920

MW (8 x 240 MW). Vo thi im ny tng cng sut tiu th ca HT min Bc

lc cc i ch vo khong 1100 MW, do ng dy siu cao p 500 KV c

xy dng gp rt ti in nng tha ca NMT Ho Bnh vo min Nam (nhit

in min Bc lc ny cn c cc nh my Ph Li 440 MW, Ninh Bnh 100

MW). Tip sau NMT Ho Bnh, p ng nhu cu tng trng nhanh ca ph

ti in, ln lt nhiu NMT ln c xy dng v a vo vn hnh. C th

nhn thy rng (xem bng) HT Vit Nam c mt tim nng thu in kh ln. T

l thu nng chim cao trong tng sn lng in nng ton quc. Thu in, c

th pht trin khp c 3 min. Min Bc c th xy dng nhng NMT ln do

nhng dng sng ln, dc cao. Min Trung c nhiu thu in nh (sng c

dc ln, nhng lu lng li b), min Nam c kh nng xy dng mt s NMT

cng sut trung bnh ( dc cc dng sng thng khng ln).

Qu kh (cho n hin ti) thu in Vit Nam c mt lch s pht trin mnh

m, tim nng thu in Vit Nam cn rt di do, tng lai thu in Vit Nam

s cn pht trin v chim v tr quan trng trong HT Vit Nam.


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Bng 1. Cc NMT ang vn hnh

Sn lng in nnghng nm (GWh)

Nh my

thy inTnh Cng

sut t(MW)

Chiucao p(m)

Chiucao

ctnc(m)

Nmavo

vnhnh

thit k thc t

Nchp tcxy

dng

Gi

xy

dng(106

USD)

Da Nhim Lam Dong -

Ninh Thuan

4 x 40 38 741 1964 1 055 809 Japon 50

Thac Ba Yen Bai 3 x 36 45 30 1972 416 450 URSS 110

Hoa Binh Hoa Binh 8 x 240 128 88 1984 8 100 7 200 URSS 1 500

Tri An Dong Nai 4 x 100 40 50 1988 1 760 1 400 URSS 400

Vinh Son Binh Dinh 2 x 33 37 + 40 612 1994 228 252 France 70

Thac Mo Binh Phuoc 2 x 75 46 90 1995 640 600 Ukraine 150

Song Hinh Phu Yen 2 x 75 43 148 1999 378 320 Sude 100

Ham Thuan Lam Dong -

Binh Thuan

2 x 150 93,5 250 2001 1 017 800 Japon 300

Dami Lam Dong -Binh Thuan

2 x 87,5 72 143 2001 590 500 Japon 180

Yali Gia Lai -Kontum

4 x 180 7 190 2001 3 650 3 300 UkraineRussie

700

Bng 2. Cc NMT ang xy dng

Giai on 2002 - 2005

NMT Cng sut t Khi cng Vo vn hnh Ghi ch

1 Se San 3 300 2002 2005

2 Thc M 50 2005 m rng

3 Cn n 72 2005 BOTGiai on 2006 - 2010

4 Tuyn Quang 342 2002 2006-2007

5 i Ninh 300 2003 2006-2007

6 Ro Qun 70 2003 2007

7 A Vng I 170 2003 2007

8 Pleikrong 110 2003 2007

9 Ban La 300 2004 2008-2009

10 ng Nai 3-4 510 2005 2009-2010

11 Bun Kup 280 2003 2008-200912 Sng Ba H 250 2005 2009-2010

13 Sng Tranh 2 135 2005 2009-2010

14 KonTum Thng 220 2006 2010

15 Sng Con 2 69 2006 2009-2010

16 Bn Chac-Hui Qung 740 2006 2009-2011

17 c Rinh 97 2006 2009-2010

18 Ea Krong 65 2004 2008-2009


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Bng 3. Cc NMT ln trn th gii

Itaip Brazil/Paraguay 1984/1991/2003 14000 MW 93.4 TW-hours

Guri Venezuela 1986 10200 MW 46 TW-hours

Grand Coulee hoa K 1942/1980 6809 MW 22.6 TW-hours

Sayano Shushenskaya Nga 1983 6721 MW 23.6 TW-hours

Robert-Bourassa Canada 1981 5616 MW

Thc Churchill Canada 1971 5429 MW 35 TW-hours

Yaciret Argentina/Paraguay 1998 4050 MW 19.1 TW-hours

Iron Gates Rumani/Serbia 1970 2280 MW 11.3 TW-hours

Aswan Ai Cp 1970 2100 MW

p Tam Hip, Trung Quc. Pht in ln u thng By 2003, d kin honthnh 2009, 18200 MW

Cc nc c cng sut thuin ln nht

Canada, 341312 GWh (66954 MW lp t) Hoa K, 319484 GWh (79511 MW lp t) Brazil, 285603 GWh (57517 MW lp t) Trung Quc, 204300 GWh (65000 MW lp t) Nga, 169700 GWh (46100 MW lp t) (2005) Na Uy, 121824 GWh (27528 MW lp t)


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Chng I

Khi nim v s dng nng lng dng nc v nh my

thu in

1.1Nng lng ca dng nc v kh nng s dngMt dng chy t nhin c s mt ct dc nh trn hnh v (hnh 1.1), trn

xt 2 mt ct ngang I-I v II-II ( cc v tr bt k). Ti mt ct I-I, gi chiu cao

mc nc l z1, vn tc trung bnh ca dng chy l v1, p sut trong lng nc l

p1. K hiu tng t cc thng s cho mt ct II-II.

Xt mt khi nc W ang chuyn ng ti I-I. Theo l thuyt ng lc hc cht

lng, nng lng cha trong khi nc W ny c th tnh theo cng thc sau:

kgmwg

vpzE ,

2

2

111

11

++=

Trong : z1 - cao ca mt ct I-I;

- trng lng ring ca nc (1000 kg/m3);

Hnh 1.1 S tnh ton tim nngca dng sng

HH

I

I

II

II

z1, v1, p1

z2, v2, p2

W

0


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v1, p1 - vn tc v p sut trung bnh ca dng chy ti mt ct I-I;

g = 9,81 m/sec2 - gia tc ri t do bi trng trng.

1 - h s hiu chnh k n s phn b khng u ca dng chy trn

mt ct. Nu vn tc nhnhau trn ton b din tch mt ct th 1 = 1. Ngc li,

cn ly gi tr khc i, tnh theo cng thc:

,3

3

v

du= vi : - din tch mt ct ngang ca dng chy, u l

vn tc ti vi phn din tch d, v l vn tc trung bnh. Thng thng c th coign ng 1.

Th tch nc W tnh bng m3.

Trong l thuyt thu kh ng hc, thnh phn trong ngoc c gi l ct p, c

th nguyn l m. N bao gm thnh phn ct p thu tnh (z +p/) v ct p thu

ng v2/2g. V tr s, ct p bng nng lng cha trong mt n v trng lng

nc di dng th nng (tng ng vi ct p thu tnh) v ng nng (ct p

thu ng). Vi dng chy l tng, khng tn hao v ch chu tc dng lc trng

trng th ct p ti mi v tr mt ct u bng nhau v nng lng cha trong

khi nc W s khng i khi chuyn ng. i vi dng chy ca cc dng sng

nng lng lun lun b tn tht (do c mt phn bin thnh cng bo mn t ,

vn chuyn ph sa ...). V th khi chuyn ng n mt ct II-II, nng lng cha

trong khi nc W ch cn li l :

1

2

222

222

Ewg

vpzE


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khi cng do khi nc W sinh ra khi chuyn ng t I-I n II-II c th tnh

theo cng thc gn ng:

( ) WHWzzE ..21 = . (1-1)

By gi nu xt khi nc W c th tch bng tng lng nc ca dng sng chy

qua mt ct trong thi gian 1 sec (ngha l ng bng tr s lu lng nc Q ca

dng chy), ng thi xt khong cch t I-I n II-II l ton b chiu di ca

dng sng th E l s cng ca c dng sng sinh ra trong 1 giy ( hay cng

chnh l cng sut ca n, k hiu l N), ta c th vit li :

skgmQHQzzN /,..)( 21 == .

Nu i sang tnh bng kW (1kW = 102 kgm/s) th cng thc tr thnh:

N = 9,81.H.Q , kW . (1-2)

Cng thc trn thng c dng nh gi tr lng thu nng ca dng sng.

Tr s tnh c chnh l cng sut tng cng m dng sng c th sinh ra c

(cn c gi l tim nng ca dng sng). D thy, tr s ny khc rt xa vi tim

nng thu in, bi con ngi ch c th tn dng c mt phn nng lng

nhng on nh H ca dng sng.Cng t cc cng thc tnh nng lng ca dng chy nh nu trn (cng thc

1-1) ta cn nhn thy rng nng, lng tim tng trong mi dng sng c phn

b tri khp theo chiu di dng chy. Mt on ngn bt k ca dng sng u c

cha mt nng lng, tnh c theo (1-1). Tuy nhin mt phn b khng u,

on c dc cng ln th mt nng lng tp trung cng cao (do H ln).

cng chnh l nhng v tr thun li xy dng NMT.

Ngo

i ra, da theo (1-1) v

(1-2) ta cng c th trc tip

a ra cng thc tnhcng sut cho nh my thu in:

NT = 9,81. .Q.H0 (kW). (1-3)Trong : H0 - chnh lch mc nc pha trc v pha sau NMT, cn gi l

ct nc hnh hc ca nh my;

Q - lu lng nc chy qua NMT ;

- hiu sut chung ca NMT;


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Vi vic xt n hiu sut bin i nng lng nu trn, cng sut tnhtheo (1-3) cho NMT chnh l cng sut in pht ra ca cc my pht. C th xc

nh gn ng hiu sut : = T.F ; y T - l hiu sut ca tua bin nc k c n tn tht nng lng

ng ng; F - hiu sut ca my pht;Vi T = (0,88 - 0,91), F = (0.95 - 0.98), khi thit k tnh gn ng c th

ly 0,86. Khi thay vo (1-3) ta c :NT 8,3.Q.H0 (kW). (1-4)

Cng thc trn thng c dng nh gi s b cng sut ca NMT khi thitk.

Thc ra khi xc nh cng sut NMT theo (1-3) hoc (1-4) ta b qua c chnh

lch p sut v vn tc dng chy trc v sau NMT (tnh ca vo v ra ca

thit b thu nng), bi H0 mi ch xt n chnh lch ct p thu tnh. Khi xt y

cc tn tht ct nc ca NMT cn tnh theo cng thc:

g

vvppzzH

2

2

22

2

1121

21

+

+= ,

cn cng sut NMT khi l:

N = 9,81..Q.H (kW)s ph thuc phc tp vo nhiu yu t hn (H khng cn l hng s).

1.2Cc loi nh my thu inT cng thc tnh cng sut ca NMT c th thy rng vic to ra chnh lch

mc nc H0 l iu kin tin quyt cho vic xy dng NMT. Chnh v th cc

phng n khc nhau to ra chnh lch mc nc cng ng thi l iu kin phn

loi cc nh my thu in.

1. Nh my thu in kiu pBng cch xy dng cc p chn ngang sng c th lm cho mc nc trc

p dng cao to ra ct nc H0 xy dng NMT. Khi ta c NMT kiu

p. p cng cao th cng sut ca NMT c th nhn c cng ln. Tuy nhin

chiu cao p cn phi c la chn k lng theo cc iu kin kinh t - k


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thut v hng lot nhng yu t an ton khc (lin quan n mi trng, di dn,

quc phng...). Mt khc khi xy dng p cao nc dng ln c th lm ngp

nhng khu vc quan trng (ng dn, th trn c, hm m cha khai thc...). Nhiu

trng hp, y li l yu t chnh hn ch chiu cao ca p. Ni chung NMT

kiu p thng c th xy dng thun li nhng ni dng chy c dc ln,

chy ngang qua thung lng ca nhng qu i. Trong trng hp ny, vng ngp

nc d hn ch c trong khu vc thung lng, trong khi ch cn xy dng cc

p ngn ni gia cc qu i chn dng sng.

Nh my thu in kiu p c cc u im sau:

- C th to ra nhng NMT cng sut rt ln, do c kh nng tn dng cton b lu lng ca dng sng;

Ho

pH cha

NM

Vn n nc

p

Ni i

Sn c

NM

Hnh 1.2. S NMT kiu p

I

I

II

II

H


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- C h cha nc, m h cha l mt cng c ht hiu qu iu tit nc vvn hnh ti u NMT, iu tit l, phc v ti tiu v nhiu li ch khc.

Cc nhc im chnh ca NMT kiu p:

- Vn u t ln, thi gian xy dng lu;- Vng ngp nc c th nh hng nhiu n sinh thi mi trng (di dn, thay

i kh hu).

Nh my thu in kiu p l loi ph bin nht xy dng trong thc t (trn

th gii cng nh trong nc). nc ta cc NMT kiu p cng l loi ch yu:

Ho Bnh, Thc B, Tr An, Thc M, ...

Cn ni thm l, c th xy dng ni tip rt nhiu NMT kiu p trn cng mt

dng sng. Trong trng hp nhvy hiu qu iu tit vn hnh phi hp ca cc

NMT s tt hn khi ch c mt NMT (xem chng 4).

2. Nh my thu in kiu knh dnCt nc ca NMT cn c th to ra c bng vic s dng cc knh dn.

Trng hp chung nht knh bao gm 2 phn: phn u c xy dng di dng

cc knh dn h (cn gi l knh dn khng p). Phn ny c nhim v dn nc

t ni m dng chy c mc nc cao n ni m dng chy c mc nc thp (v

tr xy dng NMT) nhng gi nguyn mc nc (knh c dc rt nh). Phn

cui l cc ng dn kn (cn c gi l knh dn c p). Phn ny c nhim v

a nc t trn cao xung thp chy tuabin.

Do dng chy trong ng kn bo ton c ct p thy tnh nn ct nc ca

NMT c th c tnh nh t mc nc cui knh dn h (pha trn ng dn kn)

n mc nc pha sau NMT. D thy, ct nc ca NMT kiu knh dn c th

rt ln nu ngun nc ly c xut pht t v tr cao. Cng cn ni thm l s

cu trc ca NMT kiu knh dn nu trn nhm minh ha nguyn l chung

(trong knh gm 2 phn). Thc t khng nht thit phi c phn knh dn h.

Phn ny ch c to ra khi c th (vi a hnh cho php) v em li hiu qu

kinh t (c vn u t nh hn ng dn kn). C trng hp phn knh dn h


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c thay th bng hm dn nc (hnh 1.4). p lc nc trong hm c th ln

hn p sut kh quyn (do c dc), nhng thng khng ln.

D thy, u im ni bt ca NMT knh dn l vn u t nh, cng sut n

nh (t ph thuc vo mc nc). a hnh thch hp cho NMT knh dn l vng

ni i, ni c cc dng sng (sui) dc chy t trn cao xung. Cng c khi l ni

c 2 con sng chy cnh nhau vi mc nc chnh lch ln. Trong trng hp ny

knh c xy dng dn nc t dng sng c mc nc cao sang dng sng c

Ho

H

Knh dn

ng dn kn

B iu tit

NMSng cn

p thp

p thp

Knh dnB iu tit

NM

Sng cn

Hnh 1.3. S NMT knh dn


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mc nc thp. Nh my c xy dng gn b sng c mc nc thp v x nc

vo dng sng ny.

Nhc im chnh ca NMT kiu knh dn l khng c h cha nc, do

khng c kh nng iu tit nc v iu chnh cng sut. Khc phc nhc im

ny trong nhiu trng hp, vi a hnh cho php, ngi ta xy dng cc h nhn

to cc v tr cao (cui ca cc knh dn h). Tuy nhin khi vn u t tng

cng ca cng trnh li tng ln.

H0NM

Thp iu p

Hm dn nc

Ca nhn nc

ng dn kn

Hnh 1.4. S NMT knh dn (c hm dn nc)

NM

Hm dn nc

Sng cn

p thp

Thp iu pCa nhn nc


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Mt v d in hnh cho NMT knh dn Vit Nam l thy in a Nhim. Nh

my ny c xy dng t nhng nm 60 (ca th k trc). Knh gm 2 ng kn

di gn 3 km dn nc t thung lng trn nh cao nguyn Lt (h a Nhim)

xung vng t thp to ra ct nc ti 1020 m. Nh ct nc rt ln nn ch vi

lu lng nh cng sut nh my rt ng k.

3. Nh my thy in kiu hn hp v nguyn tc chung nng cao cng sut

cho cc nh my thy in

Vi nhng a hnh thch hp, bng vic kt hp xy dng dp vi knh dn, c th

to ra NMT c cng sut ln, vn u t li nh. S in hnh ca NMT

kiu hn hp nh trn hnh (1.5). H xy dng c v tr cao trong khi c v tr

thp hn b tr nh my (ni t tua-bin nc v my pht). Tn dng chnh

p

NM

H'0

H0

ng dn kn

H cha

Hnh 1.5. Nh my thy in kiu hn hp


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lch cao pha di p c th nng cng sut nh my ln ng k trong khi ch

cn u t thm dn ng dn nc t trn cao xung thp. ng dn kn bo ton

ct p nn ct nc vn c tnh t mt thong ca h (trn cao) n mc nc h

lu pha sau nh my. Ngoi u im v kinh t, nh my cn t hp c cc u

im ca NMT kiu p v kiu knh dn nh: c ct nc cao, cng sut n

nh, c kh nng iu tit nh h cha.

Nh my thy in Yaly v nh my thy in Hui Qung (cha xy dng) l

nhng v d rt in hnh ca NMT kiu hn hp Vit Nam. H cha nc ca

NMT Yaly c cao 515 m (so vi mc nc bin). Ngi ta xy dng mt

ng hm dn nc t h xung v tr thp (305 m) di ti hn 4km cung cp

nc cho nh my (ni lp t cc tua-bin v my pht). p v h to ra ct nc

khng ln (15m) nhng c ngha tp trung lu lng nc. Hm kn tch ly ct

nc rt ln (thm gn 200m), nh th cng sut ca NMT c nng ln rt cao

(720 MW).

T cu trc ca cc NMT nu (gp ph bin nht trong thc t) d thy c

nguyn tc chung nng cao cng sut cho cc NMT: l cc bin php tp

trung lu lng v tp trung dc (ct nc). H cha cho php tp trung lu

lng (c th ca nhiu dng chy) cn p v knh dn tp trung ct nc. Thc

cht l tp trung cc lng nc v v tr cao so vi ni c th xy dng nh my

(cng thp cng tt).

Trong phng n NMT kiu p (hnh 1.1) nng lng lc u ca khc sng t

I-I n II-II phn b gn nh ng u theo chiu di ( dc lng sng gn nh

nhau). Nh c p dng nc ln cao, dc dng chy trc p gim nhiu (ch

cn li bng H), ng nng dng chy cng gim nhiu do tit din h m rng

(bin thnh th nng). Khi vn tc nc chy trong h nh th H cng nh,

khi gn nh ton b dc ca dng chy tp trung v phn cui (t trc p

cho n mt ct II-II), ni khc i, nng lng dng chy c tp trung n pha

trc p. Vi phng n s dng knh dn, dc ca knh lm thay i mt

phn b nng lng ca dng chy. Phn u ca knh c dc rt nh nng

lng tp trung vo cui.


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Cng cn ni thm l, do ng kn bo ton c ct p nn ch cn tp trung nng

lng dng chy n trc cc ng dn kn, ton b nng lng s c nhn

u ra. Li dng c im ny ngi ta c th chn cc v tr thun li tp trung

nng lng, v d cc h cha, b cha nc trn cao. tp trung lu lng, c

th s dng cc p thp chn v i hng dng chy, cng c th dng bm.

Hnh (1.6) minh ha phng n tp trung ct nc bng bm. Thc cht phng n

ny l tp trung lu lng nc c ct nc H1 vo v tr c ct nc H2 so vi nh

my. Bm cn s dng nng lng in nng cao ct nc trc khi tp trung

vo h cha, nhng b li c th tn dng c ngun nng lng ca cc dng

chy nm trn a hnh khng thun li dn nc n nh my.

4. Mt vi nh my thy in dng khc

a. Nh my thy in thy triu

Ti nhng vng b bin c mc nc thy triu ln xung chnh lch ln ( >7m ),

li dng dng nc chy vo v chy ra cc ca vnh (t nhin hoc nhn to),

ngi ta xy dng cc nh my thy in (NM thy triu). V bn cht, thy

H1

H2

B cha nc

NM

Trm bm

Hnh 1.6. Tp trung nc bng bm


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triu l hin tng nc bin dng ln h xung theo lc hp dn ca mt trng.

Theo chu k thng (m lch), ti nhng khu vc mt trng i ngang qua mc nc

bin dng ln, h xung mt ln. C nhng vng b bin mc nc thy triu

ln xung chnh lch rt ln nh vng Fandi (Canaa) 19m, Ty Bc M 10m,

Arcgentina 11m, mt s vng bin Anh, Php ti 14m . Ti mt s khu vc b

bin thuc LX c cng c mc thy triu chnh lch ln nhvnh Pengin (13,4m),

Tugur v Megen (10m) ... c th xy dng c nhng NM thy triu cng sut

rt ln. Vit Nam thy triu (6-8)m xut hin cc khu vc bin Mng Ci, H

Tin. Tim nng l thuyt ca nng lng thy triu trn tri t rt ln (2,5 ln

tim nng ca tng cc dng sng). V th NM thy triu l i tng nghin

cu v xy dng rt nhiu ni.

Hnh (1.7) m t cu trc v hot ng ca NM thy triu chy n. Cng trnh

chnh ca nh my vn l p ngn nc ti ca vnh. C h thng cng iu

khin ct nc. H thng dn nc vo tua-bin ca ca NM thy triu thng c

Hmin

Hmin

Hmin

Hmin

Z m

P MW

t

t

Hnh 1.7. Nh my thy in thy triu

a) b)

Mc nc bin

Mc nc vnh

t1 t2 t3 t4 t5 t6 t7

H0

Vnh

Vnh

Bin

Bin

Cng

NM


18/125

cc van mt chiu m bo cho nc chy qua tua-bin theo mt chiu duy nht

trong khi nc vo ra t bin n vnh i chiu theo chu k. iu khin ct nc

bng h thng cng theo thi gian ln xung ca thy triu (tng ng vi ngy

m lch) l phng thc vn hnh c bn ca NM thy triu. Cn la chn ng

thi im chuyn i phng thc vn hnh (hnh 1.7,b):

t1 - ng cng, khng chy my (ch n ct nc Hmin) ;

t2 - ng cng, chy my (pht cng sut thit k) ;

t3 - M cng, ngng my (nc vnh h xung bng mc nc bin);

t4 - ng cng, khng chy my (ch n ct nc Hmin) ;

t5 - ng cng, chy my (pht cng sut thit k);

t6 - M cng, ngng my (nc vnh dng ln bng mc nc bin);

t7 - Lp li nhgiai on t1.

Do phi c cc thi gian ch ct nc t n quy nh vn hnh nn cng sut

ca NM thy triu pht khng lin tc v nh my cn c ni lm vic vi h

thng. Ngoi ra, tua-bin ca NM thy triu cn cn c ch to c bit

thch hp lm vic vi ct nc thp, nh th khong thi gian lm vic trong chu

k (ng vi H Hmin ) c th ko di.Vnh thng c th tch rt ln, nc cung cp cho nhiu t my ng thi vn

hnh. V th cng sut tng ca cc NM thy triu c th rt ln, hng nm cung

cp cho h thng mt lng in nng ng k.

c in nng pht lin tc, ngi ta cng to ra NM thy triu lm vic theo

nhiu phn khc nhau ca h (vnh). Tuy nhin khi in nng tng nhn c

lun thp hn ca nh my lm vic theo chu k n.

Trn th gii nhiu NM thy triu c xy dng vi cng sut rt ln. NMthy triu Rance (Php) vi cng sut 400MW, hng nm cung cp ti 500.000

kWh in nng. LX c, sau khi xy dng th nghim NM thy triu u tin

400kW vo nm 1968 ( vng bin Baenxova) xy dng hng lot NM thy

triu cng sut ln vo nhng nm 1981-1985. in hnh l cc NM thy triu

Lumb 300 MW, Mezen v Tugur (10.000MW), Pengin 30.000 MW. Vng vnh


19/125

Fandi ca Canada c cc nh my Kamberland 1000MW, Kouwid 4000MW.

Anh c nh my in thy triu Severn 4000MW.

b. Nh my thy in tch nng

y l kiu NMT khng s dng nng lng ca dng sng m nhim v ca n

ch l bin i 2 chiu: in nng ca HT thnh c nng ca nc v ngc li.

V khng s dng nng lng ca dng sng nn v tr xy dng NMT tch nng

thng c chn nhng ni c v tr cao thun li xy dng c h, bn cnh

khu vc thp lun c nc (dng sng, m nc hoc b bin) c th bm

nc ln h v thot nc cho nhm my. u tin cc v tr gn cc trung tm ph

ti gim tn tht cho li. i khi c th kt hp xy dng vi NMT thng

(kiu hn hp) nhng dng sng nh nhng li c h cao, dung tch rt ln

pht trin thm cc t my lm vic theo kiu tch nng.

Hnh (1.8) m t NMT tch nng c s ging nh NMT kiu hn hp. S

khc bit ch yu y l trong nh my va c tua-bin nc va c bm. Tua-bin

quay my pht in khi cn pht in nng vo HT, cn bm s dng in

H0

Hnh 1.8. Nh my thy in tch nng

NM

H cha nc Thp iu p


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li bm nc ln h. Ngi ta cng ch to kiu tua-bin nc c bit, c th

lm vic c ch bm (my pht cng lm vic c ch ng c ng

b).

V ch lm vic, NMT tch nng hot ng theo gi trong ngy. Vo nhng

gi cao im ca ph ti h thng, NMT tch nng s dng nc ca h chy

tua-bin, quay my pht in pht in vo h thng cn vo nhng gi thp

im ca ph ti - nh my s dng in li chy bm a nc ln h.

Hiu qu hot ng ca NMT tch nng rt ln. Nh kh nng iu chnh cng

sut nhanh trong phm vi rng (t m sang dng) m HT lm vic kinh t hn:

cc NMN c lm vic n nh trong phm v cng sut c hiu sut cao, khng

phi ng ct l (do khng mt chi ph khi ng), tn tht in nng trn li

cng gim (theo ch vn hnh ti u). Ni chung cc hiu qu trn (tnh bng

tin) thng ln hn rt nhiu ln so vi chi ph tn tht nng lng (do 2 ln bin

i) chnh bn thn NMT tch nng. C th hiu n gin hiu qu ca NMT

tch nng nh l kh nng ct nh v b y biu ph ti tng h thng. Nh

th khi vn hnh ti u, chi ph vn hnh chung ca ton h thng gim nhiu, c

bit trong trng hp c mt t l ln cc NMN lm vic vi hiu sut thay i(khi iu chnh cng sut). Khi trong HT c nhiu NM khng mong mun iu

chnh cng sut v khng c kh nng gim thp cng sut (N than, NM

nguyn t) th vai tr ca cc NMT tch nng cng quan trng hn.

Mt im ng ch l tn tht nng lng qua 2 ln bin i NMT tch nng

kh ln (ti gn 30%) v ph thuc nhiu vo ct nc. Ct nc cng cao th t l

tn tht cng t (hiu sut bin i ln hn), do v tr c h cao s thun li cho

vic la chn xy dng NMT tch nng.Trn th gii ang c mt s lng ln cc NMT tch nng vn hnh. Hu nh

cc HT ln c t l ng k cc nh my nhit in v in nguyn t u c

nghin cu xy dng NMT tch nng. cc HT thuc LX c nhiu NMT tch

nng c xy dng ngay ngoi cc thnh ph ln (cung cp thng n trung tm

ph ti) nh Kiev (285 MW), Matxcva 1200 MW, Litva 1600 MW, Leningrad

(1300 MW), Trung (2000 MW), nhep (2200 MW), Kanhep (3600 MW).


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Php hng chc NMT tch nng c pht trin ng thi vi qu trnh xy dng

cc NM nguyn t nhm m bo nhu cu iu chnh cng sut v tch ly in

nng. Ti Vit Nam cc d n xy dng NMT tch nng bt u c nghin

cu. Kh khn ch yu l phi la chn c cc v tr thch hp (mi m bo

tnh kinh t v hiu sut lm vic).

1.3. cc cng trnh v thit b chnh ca nh my thy in

Tuy c cc phng n khc nhau xy dng NMT, nhng ni chung trong mi

cng trnh thy in u c cc thnh phn chnh sau:

1) Cng trnh ct nc (p, knh dn, bm ...);

2) Cng trnh iu tit nc (h cha, b iu tit, cng...);

3) Thit b bin i nng lng (tua-bin, my pht, ng c, bm);

4) Thit b phn phi in (my bin p, my ct in, dao cch ly, thanh dn,

thanh gp ...);

5) Cc cng trnh ph (ng dn nc vo tuabin, thit b chn rc, cng trnh x l,

p trn, cng trnh x ct, ...).

Ngoi ra cn c th c cc cng trnh khai thc li ch tng hp ngun nc: u

tu, ng c i, cng ly nc ...

Cn c vo a hnh c th, cn phi nghin cu nhng cch b tr hp l cho cc

cng trnh trn. iu kin t nhin thng mun hnh mun v, cch b tr khc

nhau cn ty thuc nhiu vo chiu cao ct nc cng nh kt cu ca p (p

t, p b tng, p ...). Trn hnh 1.9 th hin cch b tr ca mt s

NMT kiu p. Khi p thp (di 25m), c kt cu b tng ngi ta thng b

tr ta nh NMT (ni t tua-bin, my pht in) lin vi p, pha h lu (sau

p). Trong trng hp ny, tng ca nh my cng ng thi chu lc nh mtphn ca p. Nc c a trc tip vo tua-bin theo h thng ng dn xuyn

qua thnh p (hnh 1-9,a).


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Cng vi kt cu b tng nhng ct nc cao hn (trn 25m) th cn phi b trring ta nh NMT tch ri khi p, v tr thp pha h lu. Cch b tr nh

vy s thun li cho vic tng cng bn ca p bng cch thay i hnh dng

2 m bn ca n (hnh 1.9,b). Trong trng hp ny a nc vo tua-bin cn

xy dng thm h thng ng dn vi ca nhn v thp iu p (khi ct nc ln,

ng dn nc i xa). Vi p c kt cu li t hoc th v tr ta nh NMT

lun c b tr pha sau. l v hnh dng p trong trng hp ny thng

2

5

b)

6

3

4

12

3

a)

3

46

7

8

9

c)

5

d)

10

11

4

5

7

9

3

Hnh 1.9. B tr cng trnh NMT kiu p

1- u tu; 2-p trn; 3-NMT; 4-Ca ly nc5-p b tng; 6-ng dn nc vo tua-bin;7-p t hoc ; 8- Li x trn; 9-Thp iup; 10-Thp ly nc x; 11-ng hm x nc.


23/125

c m rng v 2 pha, ng dn nc ko di, c ca nhn nc v ging iu p

t ring (hnh 1.9,c,d).

Cc trng hp ct nc rt cao thng thuc v NMT kiu knh dn. B tr cc

cng trnh trn mt bng thng nh hnh 1.3 v hnh 1.4. Ni chung, do a hnh

thc t ti cc khu vc xy dng NMT mun hnh mun v nn cch b tr rt adng. Cc v d trn ch minh ha mt s nguyn tc chung. Khng bao gi gp 2

NMT c b tr hon ton nhnhau. Hnh 1.10 l v d c th b tr mt bng ca

NMT Tr An. to nn h cha cn xy dng mt s cng trnh p v ngn

nc. p trn c xy dng tch ri hn vi p chnh. Tn dng cc vng nc

su lm h cha ngi ta to ra 2 khu vc h chnh, h ph thng vi nhau bi

knh dn (nhm ngn bi lng lng h).

Hnh 1.10. B tr p NMT Tr An

1- NMT; 2- Ca nhn nc; 3- Knh dn ra4- p Sui Rp; 5- p lng sng; 6- p trn;7- Knh ni h chnh v h ph; 8,9- chn .

Sng . Nai123

4

5

6

7

8

9


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1.4. vn s dng tng hp cc li ch ca ngun nc

Khi xy dng mt cng trnh thy in ngoi li ch v in nng cn rt cn phi

ch n cc li ch tng hp khc ca ngun nc c th phi hp khai thc

ti a hiu qu theo nhiu mc ch: phc v ti tiu, chng l lt, cung cp nc

ngt, pht trin thy sn, du lch ... ng thi cng phi ht sc quan tm n cc

tc ng gy nh hng xu ti mi trng (t nhin, x hi), p dng cc bin

php cn thit gim thiu tc hi xung n mc an ton.

Trc ht, li dng mc nc c dng cao trong h, c th xy dng h thng

nng giang phc v ti tiu (nu xung quanh nh my c khu vc hn hn, khng

pht trin c nng nghip). Nhiu trng hp thc t gp, li ch ny c

ngha kinh t ln. Mt v d in hnh thng nu cho NMT xy dng Ai Cp

(vi p Aswan ngn sng Nin). Nh xy dng nh my ny m mt din tch ln

t ai kh cn Ai Cp v Xung c ti nc. Nn nng nghip pht trin

di lu nhanh chng thu hi vn u tcho c nh my.

Mt li ch khc, lun c coi l gn lin vi vic xy dng cc NMT kiu p

l kh nng iu tiu l bng h. Vo ma l, nc dn nhiu xung h lu,

nu ma l ko di th nguy c ngp lt rt ln. Khi vic gi nc li trong h

pha thng ngun l bin php hu hiu trnh c s c (v ) gy ngp lt

pha h lu. Ngy nay ngi ta cn rt nhn mnh n nhu cu cung cp nc

ngt: nc l ti nguyn qu ca mi quc gia, do dn s v nhu cu pht trin

nng nghip tng ln nhiu. H cha ca cc NMT cn l cc kho d tr nc,

t c th truyn dn v phn phi tit kim cho mi nhu cu (gim mt mt do

chy ngm, bc hi), c bit nhm cung cp nc sch cho cc thnh ph ln.

Ngoi cc li ch trn, vic xy dng cc NMT cn to iu kin cho pht trindu lch, nui trng thy sn, pht trin vn ha x hi cc khu vc min ni ...

Tri ngc vi cc li ch, mt ni dung quan trng khc cng cn c quan tm

lin quan n vic xy dng NMT l cc tc ng i vi mi trng. Vng

ngp nc ca h c th chim din tch ln, trn c t ai nng nghip, khu

dn c, cc cng trnh vn ha c ngha lch s ... dn n cc mt mt v mt

lng vn u t ln cn ph thm i vi cng trnh thy in (cho n b di dn,


25/125

bo tn cc di tch ...). Ngoi ra tc ng ny cn nh hng n mi trng x

hi, n cuc sng bnh thng ca hng chc ngn dn.

Trong nhng mc khc nhau, xy dng NMT c th nh hng n mi

trng t nhin, kh hu (ty theo a hnh c th). Chng hn, vn tc dng chy

thay i c th gy bi lng pha h lu, ca sng. Mc nc thay i lm nh

hng n m v kh hu cc khu vc ven sng. Nhng phng n chn nc,

i dng, nhm tp trung lu lng v nng cao ct nc (hnh 1.11) i khi gy ra

hn hn kh cn cho c nhng khu vc rng ln. Trong nhng trng hp nhvy,

thng cch khc phc l phi thay i li ton b phng n nh my (gim cng

sut hoc ngng hn) tr li iu kin t nhin ban u cho dng sng.

Tuy nhin, khi lp d n cng nh thit k nu qu thn trng phng, s xy ra

cc tnh hung tng t cng c th dn n nhng thit hi ng tic. V d cng

phng n trn nhng dng sng b chn ch chy qua vng ni i (khng dn c)

th vic p dng phng n tp trung nc s em li hiu qu ln m khng c

hu qu no v mi trng. Nu b qua phng n ny cng sut ca NMT b

gim i ng k. Thc hin tnh ton t m cc tc ng mi trng, nh gi ng

Sng cn

Vng t khKnh dn

H tp trung nc

NMT

p ngn

Bin

Hnh 1.11


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mc cc nh hng bt li, p dng mi bin php gim thiu tc hi xung n

mc an ton, trong khi ch n cc phng n khai thc ti a li ch in nng

l ht sc cn thit.

Trong mt s trng hp cn phi ch c n cc nh hng gin tip i vi

nhng cng trnh c sn xung quanh NMT. Chng hn, nh hng lm tng cp

ng t cc b (cn gi l ng t th cp) do xy dng NMT. Hin tng ny

thng xy ra i vi cc NMT ln, kiu p. Khi nc ca h nng trn mt

t, khi bt u tch nc a nh my vo vn hnh, c th lm tng cp ng t

(lu di sau hoc trong mt thi gian). Hin tng ny rt cn c quan tm

i vi khu vc thng xuyn c ng t v c nhng cng trnh ln c xy

dng trc khi c NMT.

1.5. Cc c im ca nh my thy in

So vi nh my nhit in, NMT c nhng c im ring ng ch nhsau:

1. Nh my thy in s dng ngun nng lng v tn ca thin nhin.

Nh bit, NMT s dng nng lng ca cc dng nc t nhin bin i

thnh in nng (tr thy in tch nng), l ngun nng lng v tn, gn lin

vi s tn ti vnh vin ca cc dng sng, dng sui, thy triu ... Nu khng xy

dng NMT, hoc xy dng chm th nng lng ca dng sng cng lin tc mt

i, bin thnh cc dng nng lng v ch khc (bin thnh cng bo mn t ,

vn chuyn ph xa, bin thnh nhit nng ta vo mi trng ...). Nh vy u tin

pht trin thy in cng ng ngha vi tn dng nng lng t nhin, tit kim

c cc dng nhin liu c th phc v cho nhiu mc ch.

Trn mt kha cnh khc, c im ny cn th hin u th vt tri cn c k

thm cho cc d n thy in khi nghin cu, so snh cc phng n quy hochpht trin ngun. Chng hn, sau (20 - 25) nm (l thi gian tnh ton cho cc d

n) cc nh my nhit in ni chung khng cn s dng c na (cn kit ngun

nhin liu ti ch, khng cn ph hp v tr cho ngun nhin liu mi v.v...) v d

n kt thc. i vi NMT sau thi gian trn s vn cn l mt nh my lm vic

bnh thng do lun c duy tu, sa cha ln v hin i ha t my. Cc dng

sng vn tip tc cung cp u nng lng cho nh my hot ng. Cc cng trnh


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b tng, t c tui th ko di hn rt nhiu so vi thi hn tnh ton. Cng v

c im ny, cn phi ch nhiu hn n cc li ch di lu ca d n thy in.

Mt v tr c nhiu tim nng thy in m xy dng cng trnh vi quy m nh

(do thiu vn u tchng hn) s khng cn kh nng pht trin thm sau ny.

2. Nh my thy in c chi ph vn hnh thp, vn hnh n gin, d dng

thc hin t ng ha.

Nhin liu cn s dng cc nh my nhit in thng chim phn t l ln trong

chi ph vn hnh. Khng c phn chi ph ny lm cho NMT c gi thnh in

nng sn xut (gi in nng ni b) rt thp. So vi nh my nhit in, NMT

khng c phn l, gia cng nhin liu, gim nhiu c cng sc vn hnh. L hi

l b phn vn hnh phc tp trong nh my nhit in, tua-bin hi lm vic vi p

sut hi ln, nhit cao, tc quay nhanh, gy cng thng cho ngi phc v.

Vn hnh NMT cn nh nhnh hn do iu kin mi trng tt, thc hin t

ng ha c hu ht cc b phn ... S ngi phc v NMT thng t hn

khong 8-10 ln so vi nh my nhit in. NMT Bratxkaia 4500MW (LX c)

ch bin ch 320 ngi, nh my Kratxno-iaxkaia 5000 MW cng ch c 350 ngi

phc v. Trong khi cc nh my nhit in cng cng sut ( Nga) cn bin chn 8000 ngi. Nh my thy in Thc B theo bin ch lc u ch c 72 ngi

(k c 13 ngi lm cng tc hnh chnh).

3. NMT c vn u t ln thi gian xy dng lu.

B li vi u im v chi ph vn hnh thp, NMT li c nhc im ln l

vn u t cao, thi gian xy xng ko di. Thng vn u t cao tp trung vo

cc cng trnh ct nc v iu tit (xy dng p, h cha, knh, ng dn kn ...).

Nhng cng trnh ny thng i hi mt khi lng ln btng, st thp. Vic sanlp mt bng, khi su lng h trc khi xy dng p i hi rt nhiu thi gian

cng sc cha k n nhng cng trnh phi xy dng lin quan trc nhng

giao thng, di dn, xy dng khu cho ngi lao ng,... Sut vn u t cho

NMT (tnh cho 1 n v cng sut t) thng gp t 5 n 10 ln ca nh my

nhit in (tr nhng NMT c a hnh xy dng ht sc thun li). Thi gian

xy dng thng ko di t 5 n 10 nm. Nhng NMT ln c th phi xy dng


28/125

ko di hng vi chc nm. NMT Ha Bnh khi c cng xy dng nm 1979,

sau 9 nm mi a vo vn hnh t my u tin (1988), v khnh thnh vi y

8 t my nm 1994. Vi k thut hin i NMT Tam Hip ca Trung Quc

(18200 MW) bt u c xy dng 1994, d kin khnh thnh nm 2009. Nh

my thy in Sn La ca Vit Nam c khi cng xy dng ngy 2 thng 12

nm 2005, d kin pht in t my u tin vo nm 2012. Vn u t ln thi

gian xy dng li lu (chm hon vn) l mt c im lm kh khn pht trin

thy in i vi cc nc c vn u thn hp.

4. Xy dng NMT c th em li nhiu li ch tng hp ca ngun nc

nhng ng thi cng c th c nhng nh hng bt li v sinh thi mi

trng(xem mc 1.4).

5. Ngun nc cung cp cho cc NMT (t cc dng chy t nhin) thay i

theo thi gian (ph thuc kh hu, thi tit).

y cng l mt c im khc bit so vi nhit in, rt cn c quan tm khi

thit k vn hnh NMT. Lng nc chy v NMT trong ma kh (ma cn)

thng nh hn rt nhiu so vi ma ma (ma l). Lng nc tng hng nm

chy qua NMT cng khng ging nhau (c tnh ngu nhin). Tn dng th tchh cha nhm lm tng kh nng iu tit cho NMT (tch nc ma l s

dng trong ma cn) l mt trong nhng mc tiu c bn khi vch cc phng n

thy in (nhng ng thi phi ch n nh hng mi trng). Mt khc, do

c im ny trong quy hoch pht trin ngun in cn phi tnh n gii hn v

lng nc theo ma ca cc NMT. Trong HT c t l cng sut thy in cng

ln th d phng phi cng cao mi m bo c an ninh nng lng.

Trong vn hnh (khi cng sut NMT v dung tch h cha xc nh) vic lpra cc phng n iu tit ti u dng chy (di hn, ngn hn) c ngha rt quan

trng trong vic nng cao hiu qu kinh t cho NMT ni ring v ton h thng

ni chung.


29/125

Chng 2

Cc c trng ca dng chy t nhin

2.1. biu thy vn

S liu thy vn c trng nht ca cc dng chy t nhin (dng sng, dng sui)

l chui cc s liu quan st v thng k lu lng nc. Cc trm quan trc

thy vn t trn nhng v tr khc nhau ca dng sng c th ghi tr s lu lng

nc o hng ngy trong nhiu nm lin tc (ghi theo th t thi gian). Da trn c

s chui s liu thy vn ny ngi ta thit lp cc loi s liu c trng khc nhau

cho dng sng. c c cc c trng tin cy cc s liu phi c quan st

lin tc t 50 nm tr ln. Chui s liu thy vn thng c ghi li di dng

bng s hoc th.

D thy c trng chu k ca chui s liu thy vn bi cc dng chy t nhin

ph thuc thi tit, kh hu. Theo vng quay ca tri t xung quanh mt tri, kh

hu thi tit c tnh chu k nm. Ngoi ra do nh hng phc tp ca a hnh, lc

hp dn ca mt trng chui thy vn i khi cn c tnh chu k nhiu nm.

1.Biu thy vn.

Biu thy vn biu th din bin ca lu lng nc trn sng theo chu k nm.

Biu c xy dng trn c s chui s liu thy vn theo cc phng php xc

xut thng k, dng in hnh nh trn hnh 2.2. Cc s liu xy dng biu (khi

Q m3/s

t

nm

1 2 3 4 5

Hnh 2.1. Chui s liu thy vn


30/125

cho di dng bng s) c th c tnh l lu lng nc trung bnh ngy, trung

bnh 5 ngy, trung bnh tun, c khi l trung bnh thng. Mi dng sng c th xy

dng mt biu thy vn in hnh (khi dng chy tng i ng u), mt vi

biu (cho cc nm nc to, nm nc trung bnh, nm nc nh) hoc cho mt

vi nm c th.

D thy, biu thy vn trong mt chng mc no th hin tnh quy lut ca

dng chy. Tnh chu k v n nh ca dng chy cho php xy dng cc biu

thy vn in hnh cho dng sng. Biu thy vn v th c ngha ng dng ln

trong cc tnh ton thit k v vn hnh NMT. Tuy nhin, do c tnh thay i

ngu nhien ca dng chy cc s liu c tnh theo biu thy vn ch c

ngha trung bnh xc sut. Gi tr thc t lu lng nc ca dng sng thi

im tnh ton c th no c th khc so vi gi tr xc nh c. Cng

chnh v th cc tnh ton v thy nng lun phi ch n cc phng n dphng, nhm m bo tin cy cung cp in i vi h thng.

2. 2. Cc i lng trung bnh

Biu thy vn l s liu y nht c trng cho mi dng sng, tuy nhin

ngi ta cn a ra cc nhng i lng c ngha trung bnh. Chng c s

dng trong cc trng hp khc nhau rt thun tin.

1. Lu lng nc trung bnh nm.

Hnh 2.2. Biu thy vn ca dng sng

Q m3/s

thng

I II III IV V VI VII VIII IX X XI XII


31/125

],/[ 31 smT

Q

Q

T

ii

namtb

=

=

trong : Qi - tr s lu lng nc trung bnh ca ngy th i;

T- Thi gian trong nm tnh bng ngy (T=365 hoc 366).

2. Lng nc trung bnh nm:

W = Qtb nm . 31,54.106 [m3].

y, 31,54 106 chnh l thi gian ca mt nm tnh bng sec.

C th hiu W chnh l tng lng nc ca dng sng chy qua nh my thy

in t u nm n cui nm. cc NMT ngi ta cn hay gi l lng ncv trong nm. Nm nc v nhiu th NMT c ma, ngc li l mt ma thy

in.

3. Lng nc trung bnh nhiu nm:

];[... 321

0m

n

WWWW n

+++=

Trong : n - s nm thng k s liu;

Wi - lng nc trung bnh ca nm th i.4. Lu lng nc trung bnh nhiu nm:

],/[10.54,31

3

6

01

0sm

W

n

Q

Q

n

i

inamtb

==

=

c ngha tin cy cho Q0 v W0, s nm tnh ton n phi ln (n 50nm).5. H s lch dng:

H s lch dng ca nm th i:

.00

Q

Q

W

Wk

inamtbi

i ==

D thy, ki > 1 - nm i l nm nc to;

ki < 1 - nm i l nm nc nh;

ki = 1 - nm i l nm nc trung bnh;

6. n nh dng:


32/125

c trng bin dng ca dng chy trn sng c nh gi bng n nh dng

tnh theo cng thc sau:

,1

)1(1

2

=

=

n

k

C

n

i

i

v

cn khng i xng:

.)2)(1(

)1(

3

1

3

v

n

i

i

sCnn

kn

C

=

=

Khi dng sng n nh tuyt i (ki = 1) th Cv =0. Khi ki > 1 hoc ki


33/125

khc, ng cong tn xut m bo nc c trng cho tin cy cung cp nc

theo yu cu ca dng chy.

C 2 tr s lu lng nc Qs xc nh theo ng cong tn sut m bo nc

c s dng nhiu nht, l cc lu lng nc ng vi tn xut 90% v lu

lng nc 50% (cng c k hiu l Q90% v Q50%). Tr s lu lng nc 90%

thng c dng xc nh tr s cng sut m bo, l tr s cng sut ca

NMT c tin cy pht in tng ng vi cng sut nhit in. C th tnh

theo cng thc:

Nb = 9,81..H.Q90%.

Tr s ny c dng trong cc tnh ton lun chng so snh phng n xy dng

ngun gia thy in v nhit in.

Tr s lu lng nc 50% c th c s dng nh gi s b sn lng in

nng ca NMT (c h cha nc) khi lun chng hiu qu kinh t ca nh my

lm vic trong h thng.

2.4. ng cong thi gian m bo nc

C ngha tng t nh ng cong tn sut m bo nc ngi ta xy dng

ng cong thi gian m bo nc (hnh 2.4,b). Mt im trn dng cong ny

xc nh trn trc honh khong thi gian trong nm m dng chy m bo c

n

Qs Qs

p%0 1009050pa

Qa

Q50%

Q90%

Hnh 2.3. ng cong tn sut m bo nc

(a)

a) b)


34/125

lu lng nc ca n khng nh hn gi tr xc nh c trn trc tung. Nhvy

ng cong thi gian m bo nc cng c trng cho tin cy cung cp nc

theo yu cu. Lu lng yu cu cung cp cng ln th cng c ngn thi gian

trong nm c th m bo nc (ch trong ma l).

ng cong thi gian m bo nc cng l mt s liu cn thit, thng c

dng tnh ton quy hoch sa cha cc t my.

2.5. ng cong ly tch biu din v tnh ton dng chy ngi ta a ra khi nim ng cong ly

tch. ng cong ly tch l quan h gia tng lng nc ca dng chy tnh t

mt gc thi gian no n thi im c xt vi khong thi gian tri qua.

V ton hc, nu k hiu tng lng nc ca dng chy tnh t thi im 0 n

thi im t l W th c th vit:

.)()(0

=

t

dttQtW

Q(t) - l lu lng nc ca dng chy;

Nh vy, ng cong ly tch l mt cch biu din khc v s bin thin lng

nc ca dng chy theo thi gian. i vi dng sng, ngoi cch biu din thy

vn qua chui lu lng ngi ta cng hay s dng ng cong ly tch. ng

cong ly tch cn c s dng nhmt cng c tnh ton iu tit dng chy cho

NMT (chng 4).

Q m3/s

t1 2 3 n

Hnh 2.4

t1 t2 t3 tn n

tttt n

+++=

...21

Q m3/s

T0 ta

Qa

a) b)


35/125

1. ng cong ly tch trong h ta xin gc

Mt trong nhng c im ca ng cong ly tch khi biu din trong h ta

vung gc l lun lun tng theo thi gian. V th ng cong nhanh chng i ra

khi phm vi t giy, rt kh biu din v s dng (nu chn t l xch nh th

chnh xc tnh ton km). khc phc kh khn ny ngi ta biu din ng

cong ly tch sang h ta xin gc (gi tt l h ta xin).

Thc cht, vic biu din ng cong ly tch W(t) trong h ta xin gc tng

ng vi vic biu din ng cong W'(t) trong h to vung gc thng

thng, vi :

==t

tQtWtQdttQtW0 00

.)(.)()(' .

Trong : Q0 - l mt hng s ty (c th nguyn lu lng nc). Q0 thng

c chn bng tr s lu lng nc trung bnh (c th lm trn) ca dng chy

trong chu k bin thin (mt nm).

Hnh 2.5

W m3 W' m3

W1

W1

t1

t1

A

B

C

Q0t

- Q0ttg =WT/T=Q0

WT

T

T

t

t

a) b)


36/125

Hnh 2.5. th hin tng quan so snh gia ng cong ly tch biu din trong h

ta vung gc v h ta xin. ng thng tng ng vi hm -Q0.t chnh l

trc honh xin gc ca h ta xin. Nh vy h ta xin c ba trc biu

din: trc thng ng biu din tng lng nc chy (tnh bng m3), trc thi gian

nm ngang v trc honh xin gc. Trong h ta xin rt d biu din v tnh

ton vi ng cong ly tch v n ch l ng cong dao ng hai bn trc nm

ngang.

Trong h ta xin (hnh 2.5,b) tng lng nc chy t t=0 n t1 no cng

bng khong cch t ng cong n trc honh xin gc ( di AC). tnh tr

s ny ta c th dng t im A trn ng cong theo hng song song vi trc

honh xin gc n trc tung (im W1). Tht vy (ly t l di tnh theo m3),

ta c:

AC = AB + BC = W'(t1) + Q0.t1 = W1 .

Mt khc cng t quan h trn c th vit:

W1 = AB + Q0.t1 ,

ngha l ta c th tnh lng nc tng n t1 bng cch xc nh khong cch AB

t ng cong n trc nm ngang (c k du), sau cng vi lng Q0. t1 lun

tnh c. Nhn xt ny cng cho thy, trong h ta xin, ng cong ly tch

c th biu din rt di theo thi gian (khng cn v trc honh xin gc). tnh

lng nc tng n mt thi im t bt k cng ch cn o khong cch t ng

cong n trc nm ngang (sau cng vi tr s Q0.t).

2. c im ca ng cong ly tch (trong h ta xin)

S rt c li khi n mt s c im ca ng cong ly tch.

- Khi lu lng nc ca dng chy khng i: Q(t) = Q = const , ng cong lytch l ng thng:

tQQtQdtQtWt

).(.)('0

0

0==

Tronng trng hp ny,

+ nu Q > Q0 : ng thng dc ln;

+ nu Q < Q0 : ng thng dc xung;


37/125

+ nu Q = Q0 : ng thng nm ngang .

- Trong trng hp chung lu lng nc Q(t) thay i, ta vn c nhng tnh cht

tng t. Chng hn, xt nghing ca tip tuyn vi ng cong ly tch ti thi

im t, ta c:

+ nu Q(t) > Q0 : tip tuyn dc ln;

+ nu Q(t) < Q0 : tip tuyn dc xung;

+ nu Q(t) = Q0 : tip tuyn nm ngang ;

+ nu Q(t) = 0 : tip tuyn song song vi trc honh xin gc.

l v nghing ca tip tuyn ti im t bt k t l vi o hm ca hm s

biu din ng cong (trong h ta vung gc):

{ }0

0

0)(.)(

)('QtQtQdttQ

dt

d

dt

tdWtg

t

=== .

Hnh 2.6

Q m3/s

Q(t1)

Q(t2)=Q0

Q(t3)

0' Q=0

0

W m3

t1 t2 t3 t4 t5 t6

t

- Q0.t

0


38/125

Da vo nghing ca tip tuyn c th thy ngay t hnh 2.6, lu lng nc ti

cc thi im : Q(t1) > Q0, Q(t2) = Q0, Q(t3) < Q0. Giai on t 0 n t2 v t t4

n t5 l ma nc to cn cc giai on t t2 n t4 v t5 n t6 l ma nc nh

(Q(t)


39/125

- C th lm trn ng gp khc thnh ng cong ly tch cn xy dng (tng

ng vi php ni suy lu lng nc trong mi on).

Hnh 2.7 minh ha ng cong ly tch v theo s liu lu lng nc trung bnh

thng, cho trong bng chui thy vn 1 nm.

Bng 2.1. Chui lu lng nc theo biu thy vn (12 thng)

t 1 2 3 4 5 11 12

Qs Q1 Q2 Q3 Q4 ... ... ... Q11 Q12

Q m3/s

Q1

Q2

Q3Q0

Q12

Q4

O

O' 0

W' m3

1 2 3 4 5 6 7 8 9 10 11 12 t

-Q0t

0

Hnh 2.7


40/125

Chng 3

H cha v ct nc ca nh my thy in

H cha nc cc NMT c nha rt quan trng trong vic iu tit dng

chy. Nh kh nng tch nc vo h, s dng nc vo nhng thi gian hp l m

nh my c ch vn hnh ti u em li hiu qu kinh t cao. Ngoi ra h cha

nc cc NMT cn c ngha khai thc li ch tng hp ngun nc vi

nhng mc ch khc nhau nh : phc v ti tiu, chng l lt, cung cp nc

ngt ... c th tnh ton em li hiu qu ti a theo cc mc ch nu cn

phi bit rt r cc s liu c tr

ng cho h. Cc s liu v

c tnh h

c quytnh bi iu kin a hnh cng nh chiu cao cc cng trnh p v b h. Hy

xt cc c trng c bn ca h qua s hnh 3.1,a.

3.1. cc mc nc v th tch h

1. Mc nc dng bnh thng (MNDBT)

Mc nc dng bnh thng l mc nc cao nht trong h tnh pha thng lu

ca p, khi nh my thy in c th vn hnh lu di, m bo cc ch tiu

Hnh 3.1. Cc mc nc v c tnh th tch h

Z m

Vm3V1 V2 V30

b)

Z1

Z2

Z3

Vhi Vhi

a)

MNDBT

MNDCB

MNDC

Bi lng

Vhi

Vdt

MNTL


41/125

kinh t-k thut quy nh. Mc nc dng bnh thng l thng s c bn nht,

c la chn t giai on u tin ca lp d n (chng 6) trn c s cc s liu

v a hnh, a cht v thy vn ca dng chy. Ngoi cc ch tiu kinh t-k

thut MNDBT cn phi c la chn tnh n cc li ch tng hp, nh hng

mi trng v hng lot cc yu t an ton khc (din tch ngp lt, mc bin gii

quc gia, an ninh qun s, iu kin mt nc ca h...). MNDBT ca NMT Sn

La (215m) c lun chng, la chn ko di hng chc nm ( xut lc u l

265m), trong yu t di dn v qun s tr thnh vn quyt nh ch yu

giai on cui.

Theo nh ngha, c th hiu MNDBT l mc nc gii hn trn c php dng

ln trong h trong cc iu kin bnh thng (ko di ty ). Nu dng cao lu di

mc nc ln trn MNDBT th cc ch tiu k thut khng m bo (h s an ton

bn vng gim xung so vi tnh ton, nhiu iu kin an ton khng m bo,

gy thit hi kinh t ma mng ...).

2. Mc nc cht(MNC)

Mc n

c cht l

mc n

c thp nht trong h tnh pha th

ng l

u ca p, khi nh my thy in c th vn hnh lu di, m bo cc ch tiu kinh t-k

thut quy nh. Cng c th hiu MNC l gii hn di ca mc nc vn hnh

trong cc iu kin lm vic bnh thng ca nh my. Nu vn hnh ty tin

xung di MNC th s rt lng ph nc bi cng lu lng nc Q cng sut

pht ra rrt nh, in nng nhn c t. Mt khc, MNC c tnh theo cc

iu kin lm vic an ton cho tua bin. Nu vn hnh ct nc thp, dng chy

qua tua bin c th dng chuyn ng ri (xut hin cc bt kh) lm mn cnh

tua bin (cn gi l hin tng xm thc ca nc), gy rung ng v pht nng

trc quay.

3. Mc nc dng cng bc (MNDCB)

Mc nc dng cng bc (cn gi l mc nc gia cng) l mc nc cao nht

trong h tnh pha thng lu ca dp, khi NMT c th lm vic ngn hn,

tm thi trong cc iu kin ca thin tai, l lt. Mc nc dng cng bc nm


42/125

cao hn MNDBT khng nhiu (thng ch mt vi mt), tuy nhin do mt h m

rng nn th tch nc tng thm ng k. Vo ma l, nh th tch ny mt lng

nc ln ca dng chy c th gi li trong h (khng cn x trn qua p) lm

gim nguy c ngp lt pha h lu. Cng cn ni thm l, trong iu kin vn hnh

ngn hn, vic dng nc ln cao hn MNDBT (trong mt gii hn no ) vn

m bo c mc an ton cn thit. l do c nhng yu t lm gim thp

nguy c ph v p. Chng hn, coi xc sut xut hin ng t cp cao trong thi

gian ngn c th b qua th h s an ton c th c xc nh vi cp ng t

thp hn, cng khng cn tnh n nh hng ca hin tng ngm nc lm yu

b p (nu vn hnh lu di trn MNDBT, nc c th ngm xa, lm gim

cng ca t). Xt n nhng kh nng ny ngi ta c gng nng MNDCB ln

cng cao cng tt (trong phm vi c th) phc v mc ch chng l lt. NMT

Ha Bnh c MNDBT l 115 m, MNDCB ban u c tnh 117 m (hin ti nng

ln mc 118 m).

4. Dung tch hu ch ca h

Dung tch hu ch ca h l phn th tch h nm gia MNDBT v MNC (vng

Vhi trn hnh v). V ngha dung tch hu ch ca h chnh l phn th tch c th

s dng iu tit dng chy. Dung tch hu ch c th nh hn nhiu so vi

dung tch tng cng ca h bi trong iu kin bnh thng khng c vn hnh

mc nc bn trn MNDBT cng nhbn di MNC. Dung tch hu ch l thng

s quan trng nht ca h cha nc, n c s dng trong hu ht cc tnh ton

thit k v vn hnh NMT.

5. Dung tch d tr

Dung tch d tr l phn th tch h nm gia MNDBT v MNDCB. V ngha sdng dung tch d tr chnh l phn th tch h c php s dng thm trong iu

kin thin tai, l lt.

6. Dung tch phng l v mc nc trc l

Dung tch phng l (Vphl) l phn dung tch h dng cho mc ch iu tit nhm

hn ch thin tai, l lt. Da trn c s cc s liu d bo kh tng thy vn (d

bo nh l) hng nm cn tin hnh tnh ton xc nh dung tch phng l cn


43/125

thit (bi ton iu tit l). Yu cu ca bi ton l m bo an ton cho bn thn

cng trnh thy in v trnh c ngp lt pha h lu trong thi gian xut hin

nh l. Dung tch phng l c tnh t MNDCB xung n mt mc nc gi l

mc nc trc l (MNTL). Dung tch phng l thng ln hn dung tch d tr

nn MNTL c th nm di MNDBT v c phn th tch dng chung gia mc

ch iu tit l v iu chnh cng sut vn hnh. Khi , iu tit l c th nh

hng n sn lng in nng (gim thp). Dung tch d tr cng ln th iu tit

l cng t nh hng n sn xut in nng.

7. Dung tch cht

Dung tch cht l phn th tch h nm di MNC. Phn th tch ny khng tham

gia vo qu trnh iu tit nhng cng rt cn thit (dung tch ti thiu) m bo

mi sinh lng h v cc vn khc.

8. Dung tch bi lng

Dung tch bi lng l phn dung tch h ng vi cao trnh bi lng. y chnh l

phn dung tch h sn sng lm nhim v lu tr lng bn ct lng ng t dng

chy m bo cho h cha lm vic bnh thng. C th sau thi gian di phn ny

b dng cao ln chim phn trn. Khi cn c cc bin php k thut ci to

lng h (no vt, x lng trong ma l).

3.2. cc ng cong c tnh h

1. c tnh th tch

L ng cong quan h gia th tch nc c trong h vi mc nc thng lu.

Quan h ton hc thng biu din c tnh th tch dng V = f(Z), tuy nhin

ng cong c tnh thng c v vi trc thng ng biu din mc nc

thng lu Z (hnh 3.1,b). Thc t, ng cong c xy dng di dng bng s

(c th tim cn gii tch khi s dng) da vo cc phng php khc nhau: m

hnh ha, o c, tnh ton. M hnh ha lng h v cc cng trnh u mi giai

on u c th cho php xy dng ng cong c tnh th tch, dng ngay

vo nhiu tnh ton lun chng kinh t k thut. Da vo ng cong c tnh th

tch c th bit ngay lng nc ang c trong h khi bit mc nc thng lu.


44/125


45/125

Trong cc tnh ton iu tit, rt thng hay xc nh lng nc (th tch) nm

gia hai mc nc Z1 n Z2 ca h. Khi , theo ng cong din tch, V1-2

bng tr s din tch phn gch cho trn hnh 3.2,b, vi cc cng thc gn ng:

))((3

1

))((2

1

12212121

122121

ZZFFFFVhay

ZZFFV

++

+

(3.2)

Cch xy dng v c im ca ng cong c tnh din tch cng tng t nh

c tnh th tch (c c tnh ng theo lu lng nc Q).

C th ni c tnh th tch v din tch l cc c trng quan trng nht ca h

bi n cha hu ht cc thng tin cn thit.

3. c tnh mc nc h lu

Mc nc h lu c nh hng n chiu cao ct nc ca NMT. Ngoi ra phc

v cho nhiu mc ch tnh ton khc rt cn bit s thay i mc nc h lu. V

tr mt ct sng xc nh tnh ton mc nc h lu thng c chn ti ca

knh x ra ca NMT.

Hnh 3.3. c tnh mc nc h lu

Zhl m

Q m3/s


46/125

Ty theo cu to a hnh b tr NMT m lu lng nc x c hay khng c nh

hng n mc nc h lu. Nu NMT c b tr theo kiu sau p hay kiu

lng sng th lu lng tnh ton l tng lu lng nc s dng Q v lu lng x

Qx v lu lng nc x qua p trn cng tp trung ngay ti ca knh x ra ca

NMT. Khi c tnh c dng: Zhl = f(Q+Qx); Nu v tr knh x ra ca NMT

nm cch xa p (kiu knh dn) th quan h ch xt n lu lng nc Q vn

hnh qua tua bin NMT.

3.3. ct nc tnh ton ca nh my thy in

Nh xt trong chng 1, khi tnh ton s b cng sut ca NMT c th s

dng ct nc tnh ton gn ng cho nh my nh l chnh lch mc nc dng

chy trc v sau NMT: H0 = Z1 - Z2 .

Z1

Z2

H H0

AB

C

CA B

g

vA

A2

2

g

vC

C2

2

g

vB

B2

2

ZA

ZB

ZC

pA/ pB/

pC/

Hnh 3.4


47/125

Thc cht khi l suy ra tr s cng sut ca NMT da vo chnh lch nng

lng ca khc sng t A n C. Mi mt mt nng lng sau khi bin t c nng

sang in nng c nhp vo hiu sut chung ca nh my. Hn na coi gn

ng b qua c phn chnh lch nng lng gy ra do thay i vn tc v p sut

dng chy. Khi thc hin cc tnh ton i hi chnh xc cao (c bit cc tnh

ton vn hnh) cn xc nh y hn cc yu t nh hng n ct nc. Trong

trng hp ny ct nc tnh ton ca NMT (cn gi l ct nc lm vic) c

xc nh l hiu s ct p ca dng chy pha trc v pha sau tuabin, tch ring

phn tn tht knh dn v ng ng dn nc trn cc cng trnh a nc vo

ta nh NMT (ni t cc t my). Theo s trn hnh 3.4 ct nc ton phn

(tnh t pha trc dn pha sau NMT) s l:

CA

CCC

C

AAA

AfulEE

g

vpZ

g

vpZH =++++= )

2()

2(

22

.

Cn ct nc tnh ton hay ct nc lm vic c tnh l:

CB

CCC

C

BBB

B EEg

vpZ

g

vpZH =++++= )

2()

2(

22

.

Nh vy ct nc tnh ton ca NMT chnh bng chnh lch c nng tnh cho

1m3 nc ca dng chy trc khi vo v sau khi ra khi tuabin. Ta cng c :

H = Hful - HK

Trong , HK tn tht ct p trn on AB (ca knh v ng dn).

T hnh v ta c :

g

v

g

vH

g

vZ

g

vZEEH CCAACCAA

CAful22

)2

()2

(22

0

2

2

2

1

+=++== .

Hay c th vit :

HHHHHHdK

==00

. (3.3)

vig

v

g

vH AACCd

22

22

= l phn ng nng chnh lch ca dng chy pha trc

v pha sau NMT. Thng thng nc ra khi tua bin c tc ln hn tc


48/125

dng chy trc p (rt nh), do Hd > 0 v th hin nhmt phn nng lng

b mt mt.

Tnh vi lu lng nc Q lm vic, chnh lch c nng ca dng chy trc v

sau khi i qua tua bin (trong 1 sec):

N = .Q.H [kgm/s] = 9,81.Q.H [kW].

Cng sut truyn cho phn quay ca tuabin (vi hiu sut T):

NTB = 9,81.T .Q.H [kW].

Sau khi bin i qua my pht in (hiu sut F) ta c cng sut in nhn u

cc my pht : NT = 9,81FTQH = 9,81QH [kW].

D nhn thy rng, ct nc tnh ton ph thuc phc tp vo ch lm vic ca

nh my. Theo (3.3) ta c:

H(Q,Qx) = Z1(Q)-Z2(Q+Qx)-H(Q).

Hnh 3.5 minh ha s bin thin ct nc tnh ton theo Q (khi lu lng dng

sng Qs khng thay i, cn lu lng x Qx = 0).

Cc im gy trn c tnh th hin s thay i t bin ca tn tht ct p khi

thay i s t my lm vic. Vi cng lu lng nc vn hnh tng, khi a thm

Z m

Q m3

/s

Z1

Z2

Hnh 3.5

H m

Q m3

/s

H0=Z1-Z2

H=H0-H

H

Q1 Q2


49/125

t my vo vn hnh, tc dng chy trn cc ng dn song song s gim i,

tn tht ng dn gim t ngt. S thay i phi tuyn ca c tnh ct nc lm

cho bi ton iu it tr nn phc tp. Khi mc nc h t thay i v coi tn tht

ct p l nh (thng ch chim t 2% n 2,5%) bi ton c gii vi ct nc

c nh s n gin hn nhiu.

3.4. phng trnh cn bng nc ca h cha

Khi tnh ton iu tit dng chy cn phi bit c trng thi lm vic ca h,

nh s thay i mc nc, th tch nc cn li trong h... D thy, th tch nc

trong h ph thuc vo tng quan gia cc lng nc chy vo v chy ra khi

h, ng thi c quan h mt thit vi mc nc thng lu (theo cc ng cong

c tnh). Phng trnh thit lp mi quan h gia cc i lng trn gi l phng

trnh cn bng nc ca h cha. Theo nguyn l cn bng nc (ng cho mt

dung tch cha nc bt k), tng cc lng nc chy vo h trong khong thi

gian t xc nh phi cn bng vi tng cc lng nc chy ra khi h cng vi

th tch nc gi li trong h (cng trong t). Vi h cha, trong trng hp

chung (xt n mi lng nc c th), ta c th vit nhsau:

Ws = W+Wx +Wr +Wtl +V. (3.4)

Trong phng trnh trn:

Ws - th tch nc t dng sng chy vo h;

W- th tch nc s dng (chy ra khi h qua tua bin ca NMT);

Wx - th tch nc x (trn qua p);

Wr - th tch nc mt mt do r r qua cc cng trnh p v lng h, k c

lng nc bc hi;

Wtl - th tch nc dng cho mc ch thy li (ti, tiu, cung cp nc ngt);

V - th tch nc gi li trong h.

Tt c cc lng nc trn u c xt chy trong khong thi gian t; Khi xt

khong t ngn (lu lng nc coi nh khng i ta c dng vit khc gn

ng ca (3.4):

V = Qst - (Q+ Qx+ Qr + Qtl )t. (3.5)


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C th hiu cc lu lng nc trong (3.5) c tnh vi tr s trung bnh trong t.

Nu chia c hai v ca phng trnh (3.4) cho t v ly gii hn khi t 0 ta c

phng trnh cn bng cc tr s lu lng nc nhsau:

dt

dVQQQQQ

tlrxs++++= ; (3.6)

S hng cui nhn c do p dng nh ngha o hm i vi hm V(t). Phng

trnh ng vi mi thi im t tnh lu lng nc v th tch nc trong h.

Mt khc, xt V l hm hp ca t qua Z(t) ta c:

dt

dZZF

dt

dZ

Z

V

dt

dV)(=

= .

Thay vo (3.6) ta c:

tlrxs QQQQQdt

dZZF =)( (3.7)

Phng trnh (3.7) l dng ca phng trnh cn bng nc ca h trong trng

hp chung nht. Tuy nhin, thc t ngi ta hay s dng cc dng gn ng, n

gin hn. Trc ht c th tnh tng ng lu lng nc Qr v Qtl vo Qs. l

v cc lu lng nc ny cng thng c c lng (d bo) v cho di dngchui s liu nhbiu Qs ca dng sng. Khi c th tnh trc:

Q's = Qs - Qr - Qtl ,

thay vo (3.7) ta c:

xsQQQ

dt

dZZF =

')( . (3.8)

Cc tnh ton theo lu lng nc trung bnh, thng p dng (3.5), vi cch tnh

gn ng th tch V theo mc n

c thi im u v

cui khong t nh

ccbiu thc (3.2).

Phng trnh cn bng nc ca h l mt trong nhng m t ton hc c bn

trong bi ton iu tit dng chy.

3.5. xc nh th tch nc trong h theo ng cong ly tch

Nu v trn cng mt th ng cong ly nc chy vo h v ng cong ly

tch nc chy ra khi h th c th xc nh c ngay th tch nc c trong h


51/125

(ti mi thi im ): chnh bng khong cch gia 2 ng cong. D hiu c

iu ny bi tnh t gc thi gian n mt thi im bt k th tng khi nc ca

dng chy (chy vo hoc ra khi h) bng khong cch khong cch t ng

cong l tch n trc honh xin gc. Trong khi , chnh lch khi nc gi li

trong h :

V = WV - WR ,

chnh l chnh lch gia 2 khong cch, nn phi bng khong cch gia 2 ng

cong (hnh 3.6).

Khi V < Vhi th khong cch gia 2 ng cong cng l th tch nc ang c

trong h (cha y). Nu V > Vhi th khong cch bng Vhi + Qx (h y nc v

c x trn).

Ngi ta cng t bi ton xc nh dung tch h cha sao cho c kh nng san

bng hon ton dng chy, ngha l mt th tch h ln ti thiu m c th s dng

nc u n quanh nm (lun bng lu lng nc trung bnh). Hy xt ng

cong ly tch nc chy vo h ca mt dng sng no . Do tnh cht chu k ca

dng chy, ta c th chn im u v cui ca mt nm tnh ton l ty , gi s

chn l thi im c lu lng nc trung bnh. Vi cch chn cho nm c th ny

tip tuyn ti im u v cui vi ng cong s trng vi trc nm ngang. Nh

Hnh 3.6

W m3

WV

WR

V

VH V'H V"H a

b

ta0

T

t


52/125

vy nu vn hnh vi lu lng nc khng i (bng Qtb) th trc nm ngang

cng trng vi ng cong ly tch nc chy ra khi h. Th tch nc trong h

khi chnh bng khong cch gia ng cong ly tch vi trc nm ngang. Th

tch nc gi li trong h s ln nht vo thi im ta xc nh c bng cch v

tip tuyn nm ngang tip xc pha trn vi ng cong ly tch nc chy vo h.

Khong cch ab xc nh th tch h cha ln cho php san bng hon ton

dng chy. D thy, t 0 n ta tch nc vo h, t ta n T s dng nc t h.

Mi nm c lu lng nc trung bnh v ng cong ly tch khc nhau, cn tnh

cho nhiu nm chn ra th tch ln nht cn thit cho iu tit san bng ho n

ton dng chy (xem hnh v).

Th tch ln ca h m bo san bng hon ton dng chy, xc nh nh trn

thng c tr s kh ln (chim t 25% - 40% lng nc trung bnh nm ca

dng chy). Thc t cc h thng c tr s nh hn nhiu, do ch c kh nng

iu tit hn ch.


53/125

Chng 4

iu tit dng chy

4.1. ngha v phn loi iu tit

1. iu tit dng chy theo nhu cu iu chnh cng sut NMT

Ch thay i ca dng chy t nhin thng khng ph hp vi nhu cu cung

cp in. Chng hn, xt trong chu k mt nm th lu lng nc trn sng thay

i rt mnh, tng ng vi biu thy vn (hnh 4.1,a), trong khi in nng

tiu th hnh thng ca ph ti h thng li rt t thay i. Nh vy vo ma l

lng nc v qu nhiu, khng th s dng ht theo cng sut vn hnh cc t

my, cn ma cn lu lng nc ca dng chy qu t khng p ng cho nhu

cu pht in. Xt trong phm vi ngn hn, v d mt ngy m, th lu lng

nc hu nh cha thay i, trong khi biu cng sut ph ti tng h thng

li thay i rt ln theo ca kp lm vic ca cc nh my x nghip v nhu cu in

sinh hot (hnh 2.2,b).

Vn t ra l c th tn dng ti a kh nng tch nc ca h iu tit li dng

chy nhm p ng tt nht cho nhu cu pht in (cha ni n cc loi iu tit

khc). H cng ln th hiu qu iu tit cng cao.

Hnh 4.1

QS

Pt

a) b)

QS

Pt

ng

y

t

gi

t24365

P,Q

P,Q


54/125

Mc tiu chung ca bi ton iu tit dng chy l nhm em li hiu qu kinh t

cao nht, xt trong phm vi ton h thng. Tuy nhin, ty theo iu kin ngi ta

thc hin cc bi ton khc nhau, mc tiu c th ca mi trng hp c th khng

ging nhau, do ch khai thc c mt phng din nht nh ca hiu qu kinh t.

V d iu tit sn lng in nng c nm ca NMT t c cao nht. Khi

hiu qu kinh t ca vic iu tit l tn dng c ti a ngun nng lng r

tin t dng sng. Trng hp iu tit, vn hnh NMT lm cho biu pht

cng sut ca cc NMN hp l, hiu sut t cao, th hiu qu kinh t li l gim

chi ph nhin liu cho cc NMN. Mc tiu ca cc loi iu tit phc v li ch

tng hp (iu tit l, ti tiu, cp nc sinh hot ...) cn hon ton khc na.

2. Phn loi iu tit

a. Theo thi gian, ngi ta chia ra cc loi iu tit sau:

- iu tit ngy;

- iu tit tun;

- iu tit ma;

- iu tit nm;

- iu tit nhiu nm;

Vic phn loi theo thi gian l nhm xc nh khong thi gian thc hin tnh

ton iu tit. Vi nhng khong thi gian khc nhau, m t ton hc v hm mc

tiu ca bi ton c th hon ton khc nhau. l v kh nng thc hin iu tit

v hiu qu em li khng ging nhau.

Trong phm vi mt ngy m, lu lng ca dng chy hu nh khng thay i

trong khi ph ti h thng li thay i nhiu, ch c th t bi ton iu chnh

cng sut NMT (cng c ngha l thay i lu lng dng chy) sao cho h

thng c li nht. Khng c kh nng lm tng thm sn lng in nng cho

NMT bi lng nc khng i. Hn na, bi ton cng ch c th thc hin khi

h vi, cn kh nng iu chnh mc nc trong h (ngoi ma l). Thc ra bi

ton iu tit nc cho NMT trong trng hp ny nm trong bi ton chung ca

h thng: phn b ti u cng sut cho cc NM vi HT hn hp thy nhit

in. Tuy nhin, xt ring cc NMT ngi ta cng thng p dng cc phng


55/125

php ring n gin hn (chng 5). Kt qu ca bi ton iu tit ngy (d thc

hin theo phng php no) cng u c dng chung l iu chnh cng sut

NMT theo phn nh ca biu ph ti tng h thng (hnh 4.2). Khi cc

NMN c kh nng lm vic vi biu pht bng phng hn.

iu tit tun cng c ngha tng t nh iu tit ngy. Lu lng nc trn

sng trong phm vi mt tun cng vn c coi nhkhng i. Cc ngy lm vic

(t th 2 n th su) in nng tiu th nhiu hn ngy th su v th by. iu

tit tun thc cht l thay i tr s lu lng nc trung bnh cc ngy trong tun

NT

t

24 ht1 t2Q

t

24 ht1 t2Z

t

24 ht1 t2

Ntb

N

Qtb

Ztb

Q

Z

a)

b)

c)

Hnh 4.2

NT

t

Q

t

7 ngyZ

t

7 ngy

Ntb

N

Qtb

Ztb

Q

Z

b)

c)

Hnh 4.3

a)

7 ngy


56/125

p ng s tng in nng s dng cc ngy lm vic (u tun) v gim in

nng trong cc ngy ngh cui tun. Trn hnh 4.3,a,b,c minh ha s thay i tr s

trung bnh ngy v cng sut, lu lng v mc nc trong h. ng mc nc

v bng nt t trn hnh 4.3,c biu th s thay i mc nc c trong phm vi

ngy, Mc tiu ca iu tit tun cng l gim chi ph vn hnh tng trong h

thng.

So vi iu tit ngy v iu tit tun, iu tit nm c nhng c trng hon ton

khc, khng th coi lu lng nc trn sng l t thay i. Hn na, y cn l

nguyn nhn hn ch hiu qu khai thc thy nng ca NMT. V vy nhim v

ca iu tit nm l lm cho dng chy ng u hn nhm khai thc ti a nng

lng ca dng chy.

Hnh 4.4 th hin hiu qu ca bi tan iu tit nm bng h cha (trng hp h

ln v trng hp c th tch hn ch). Nu khng iu tit th v mc l

NMT ch c th vn hnh vi lu lng ti a QTDmax , x ton b lu lng nc

tha, trong khi ma cn ch c th vn hnh vi lu lng rt nh theo dng chy

t nhin ca dng sng. Nu h ln c th vn hnh quanh nm vi lu lng

QS

a)

t365 ngy

Q

max

TDQ QTD(t)

tch nccp nc cp nc

QS

b)

t365 ngy

Q

max

TDQ

QTD(t)

tch nccp nc cp ncx

Hnh 4.4. iu tit nm


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nc cc i (bng lu lng nc trung bnh ca dng sng), ton b lu lng

nc tha tch trong ma l c dng va ht trong ma cn. Khi h c dung tch

hn ch, c mt phn nc tha trong ma l b x trn. Tuy nhin, lng nc

tch ly c y h nng lu lng nc vn hnh trong ma cn ln thm ng

k, cng vi ct nc cng c dng cao. Hiu qu "tn dng nc dng chy" v

hiu qu "dng cao ct nc" ca iu tit nm lm sn lng in nng NMT l

tng ln ng k.

Ngi ta cung thng chia bi ton iu tit nm thnh 2 ma ring bit gii

(ma nc ln v ma nc xung) v chng c nhng c trng c lp tng i.

iu tit nhiu nm, c xt n khi h c dung tch ln (vt trn th tch sanbng hon ton dng chy theo chu k mt nm), trong khi dng chy bin ng

nhiu vi chu k di hn (nhiu nm). Nhiu dng sng c chu k di hn kh r

rt, v d c 6 nm li c mt nm nc cc i, mt nm nc cc tiu (hnh 4.5).

Nu h c dung tch ln, c th tch y h vo cui nhng nm nc to ri dng

dn vo na chu k nc cn. Khi h ln cng c th san bng hon ton dng

chy theo chu k nhiu nm. Hnh 4.5 v ng cong ly tch nhiu nm ca dng

nm

t

1 2 3 4 5 6 7

W m3

-Q0t ng Qtb nm

Vhi san bng dng chy nhiu nm

ng Qtb nhiu nm

QS

Hnh 4.5

Vhi san bng dng chy nm


58/125

chy c chu k 6 nm. Tng t nh chu k mt nm, c th xc nh c dung

tch h cha ln san bng hon ton dng chy nhiu nm theo ng cong ly

tch.

b.Theo tnh cht iu tit, ngi ta chia ra 2 loi : iu tit ngn hn v iu tit

di hn. Trong iu tit ngy v iu tit tun tng ng vi iu tit ngn hn,

iu tit ma, iu nm v iu tit nhiu nm tng ng vi iu tit di hn.

Cch phn loi nh trn khng n thun chia li khong thi gian m ch yu l

do c s khc nhau v tnh cht gia iu tit ngn hn v iu tit di hn.

Vi iu tit di, thc cht v cch thc iu tit l iu chnh li dng chy vn

thay i nhiu tr nn t thay i hn cho ph hp vi nhu cu cung cp in (u

n). V mc tiu iu tit th iu tit di hn nhm tn dng ti a nng lng

dng nc, lm tng sn lng in nng ca nh my thy in ln mc cc i.

Cn vi iu tit ngn hn, ngc li, thc hin iu chnh dng chy vn t thay

i tr thnh thay i nhiu hn, ph hp vi bin ng ca biu cng sut ti.

Mc tiu iu tit l lm gim chi ph vn hnh tng ca h hng xung n mc

cc tiu.

Do c s khc nhau c bn v iu tit di hn v ngn hn nn phng php tnh

ton iu tit cng khc nhau. Trong chng ny ch yu xt phng php iu

tit di hn vi chu k 1 nm.

4.2. tnh ton iu tit di hn dng chy theo phng php

th

I. Bi ton

Xt bi ton iu tit di hn trong chu k mt nm. C th m t nhsau:

- Cho trc biu thy vn ca dng sng. Thc cht l cho chui gi tr lu

lng nc ca nm tnh ton. Ty theo mc y ca s liu, gi tr c

cho ca lu lng nc c th l tr s trung bnh ngy, trung bnh tun, 1/2 thng,

thm ch trung bnh thng.

- Cho trc dung tch hu ch ca h, ct nc cc i Hmax, cc c tnh h

(ng cong c tnh th tch, c tnh mc nc thng lu, h lu ...).


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- Cho tr s lu lng nc ln nht c th lm vic ca cc tua bin QTmax.

Yu cu: xc nh tr s lu lng nc vn hnh ca NMT cc khong thi

gian trong nm, sao cho sn lng in nng tng ca NMT t tr s cc i.

Yu cu trn chnh l hm mc tiu ca iu tit di hn (trong trng hp xt c

lp hiu qu iu tit ca NMT). Cc yu cu khc c th coi l gii hn vn

hnh : cng sut vn hnh ti thiu ca t my (hoc lu lng m bo t nht)

theo nhu cu h thng, gii hn mc nc cao nht, thp nht trong h, lng nc

li trong h cui chu k iu tit ...).

Cng cn ni thm v hm mc tiu trong bi ton iu tit di hn dng chy.

tm cc tr hm mc tiu l sn lng in nng, ni chung, i hi phi m t

hm v s dng nhng phng php ton hc phc tp. l v bi ton cha

nhiu quan h v gii hn phi tuyn. Tng ng vi cc iu kin c th (v cc

iu kin rng buc, phng php gii) ngi ta s dng nhng hm mc tiu gn

ng tng ng khc nhau. Khi p dng phng php th, bi ton thng

c m t hm mc tiu l lm gim lng nc x xung mc thp nht, trong

khi m bo lu lng nc vn hnh ng u nht c th c. L thuyt v cc

tnh ton c th cho thy kt qu in nng nhn c rt gn vi gi tr cc i.

Ngoi ra, nh my cn m bo c rng buc v yu cu ca h thng: pht

cng sut u n gia cc ma.

II. Phng php gii

Ta xt phng php th vi vic s dng cc ng cong ly tch. Bi ton c

thc hin theo cc bc sau:

Bc 1: Da vo s liu thy vn cho v ng cong ly tch nc chy vo h

(t dng sng). Ni chung cc s liu c cho di dng bng s, nn ng

cong ly tch c v gn ng theo ng gp khc v lm trn trn hnh (xem

chng 2).

Bc 2: Da vo tr s dung tch hu ch h cha, v mt ng cong nm di v

cch u ng cong ly tch nc chy vo h, mt on bng Vhi. ng cong

va v c gi l ng cong kim tra. D thy rng ng cong ly tch nc

iu tit qua tua bin ca NMT (ang cn v) ch c th nm gia hai ng cong


60/125

ni trn. l v tnh t gc thi gian n mt thi im bt k th tng lng

nc chy ra khi h ch c th bng hoc nh hn tng lng nc chy vo. Hn

na, khong cch gia 2 ng cong ly tch chnh bng lng nc chnh lch.

Nh vy, khi lng nc chnh lch nh hn hoc bng Vhi th chnh l lng

nc gi li trong h. Lc , ng cong ly tch lu lng nc chy ra khi h

(cng chnh l lu lng nc vn hnh) nm bn trn ng cong kim tra. Nu

th tch nc chnh lch ln hn Vhi c ngha l h phi x trn, khi ng

cong ly tch nc chy ra khi h (bao gm lu lng nc vn hnh v lu lng

nc x) s ct v i xung di ng cong kim tra. Khong cch nm di biu

th th tch nc x.

Bc 3: v ng cong ly tch lu lng nc iu tit ti u t u n cui chu

k iu tit (hnh 4.6).

W

QS

QP

Q'P

Q''P

Vhi

t1 t2 t3 t4 t'5 t5t"5

t'1

t'2 t'3t'4

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

V

Vhi

Hnh 4.6

0

0

0Q0

QP

QT

Q=0

T

T

ng cong kim tra


61/125

thun li cho vic tnh ton, chu k iu tit c chn sao cho thi im u

(t = 0) v thi im cui (t = T) trng vi giai on cui ca ma nc cn (hoc

cui ma l). Trn hnh v chn cc thi im ny tng ng vi thi im

cui ca ma cn. Cch chn ny cho php ta v ngay c phn u ca ng

cong ly tch. l v vo giai on cui ca ma ma nc cn, phng n vn

hnh ti u ca nh my l s dng ht nc ca h, nhm c th tch h ln nht

tch nc ma l sp ti. Lc ny mc nc trong h l thp nht (bng mc nc

cht), th tch nc gi li bng 0, ng cong ly tch lu lng nc vn hnh

(chy ra) phi v trng vi ng cong ly tch nc chy vo h (khong cch

bng 0). Nh vy giai on ny lu lng nc vn hnh cng gi va ng bng

lu lng nc ca dng chy. Qu trnh vn hnh nh trn cng ch c th ko

di n thi im t1, l thi im m lu lng nc trn sng kh ln, ng

bng QTmax (l lu lng nc tng ln nht c th vn hnh qua cc tua bin). Sau

thi im ny lu lng nc trn sng tip tc tng ln, cc tua bin cn vn hnh

vi tr s ti a khng i l QTmax. ng cong ly tch tng ng vi giai on

ny c th v c trng vi ng thng tip tuyn, tip xc vi ng cong ly

tch nc chy vo h ti t1 . nghing ca tip tuyn c th xc nh nh biu

t l xch hnh tia, tng ng vi tr s QTmax bit (xem hnh x). T sau t1 lu

lng nc trn sng ln hn lu lng nc vn hnh nn c mt lng nc tha

c tch tr li trong h. Th tch nc gi li chnh bng khong cch gia hai

ng cong luy tch. T hnh v c th thy c n thi im t2 th h y nc

(ng cong ly tch nc chy ra khi h chm n ng cong kim tra).

Sau thi im t2 lu lng nc trn sng tip tc ln hn lu lng nc vn hnh(vn ang vn hnh ti a bng QTmax), h y nc nn phi x mt lng nc

tha qua p. Th tch nc x tng ng vi din tch nm di ng cong kim

tra (c vch en trn hnh v). Qu trnh x cng ch cn ko di n thi im

t3. l thi im m lu lng nc ca sng gim i, tr li bng QTmax (c

th vn hnh ht c). Phng n vn hnh sau t3 l vn hnh vi ton b lu

lng nc chy vo (bng QS) nhm tip tc gi cho h y nc mt thi gian


62/125

sau ma l. Hn na lu lng lc ny vn kh ln. ng cong ly tch cn v

giai on ny trng vi ng cong kim tra (v h lun y nc). Vic s dng

nc h c bt u t thi im t4 ko di n t5. y l ma nc cn, phng

n iu tit c chn l vn hnh vi tr s lu lng nc khng i, xc nh

theo nghing ca ng thng tip tuyn, tip xc pha trn vi ng cong

kim tra ti t4 v tip xuc pha di vi ng cong ly tch nc chy vo h ti

t5. Trn hnh v ca biu t l xch hnh tia ta xc nh c tr s ny l QP. Sau

thi im t5 h ht nc (ng cong iu tit chm n ng cong kim tra),

ng thi cng l giai on cui ca ma nc cn (ging nh trng thi tai t = 0)

nn phng n iu tit c thc hin lp li nh giai on u.

ng cong ly tch iu tit va v chnh l phng n vn hnh ti u ca

NMT (theo ngha nu). Mi phng n vn hnh khc u hoc l lm cho

lng nc x tng ln, hoc lm cho cng sut pht khng cn ng u. Hy xt

thm v khong thi gian t t4 n t5 . Nu cho vn hnh vi lu lng nc Q'P >

QP (ng cong ly tch iu tit s phi nm cao hn) th t hnh v thy rng n

thi im t'5 h ht nc. Thi im t'5 mi ang gia ma l, do nh my

s phi vn hnh vi cng sut rt nh, tng ng vi lu lng nc ca ma cn.

Ngc li nu cho nh my vn hnh vi Q"P < QP (ng iu tit s nm thp

hn) th cng t hnh v thy rng, h khng lc no ht nc sut trong ma cn.

Hn na n thi im t"5 t h li y nc mat t"5 mi ch l cui ma cn nn

nh my s phi x mt lng nc rt ln vo ma l tip sau. R rng vn hnh

vi lu lng nc QP xc inhl hp l. Cng c th xt tng t cho cc

khong thi gian khc.

Sau khi c ng cong l tch iu tit ti u, ta c th xc nh c mi thng svn hnh khc ca nh my mi khong thi gian trong nm : tr s lu lng

nc vn hnh, th tch nc trong h, mc nc thng lu, mc nc h lu, ct

nc ca nh my t tnh c cng sut vn hnh ca NMT v sn lng

in nng c nm. Hnh 4.7. minh ha kt qu tnh ton mt s i lng nu.


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C th thc hin theo th t sau.

- Theo ng cong iu tit xc nh tr s lu lng nc vn hnh qua tua bin

NMT tng khong thi gian. Kt qu tng ng vi bng sau:

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

Q

T

QTmax QT(t)

QS

t

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

Zhl

T

t

Zhl(t)

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

Zdbt

TZch

Z

t

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

V

Vhi

T t

t1 t2 t4 t5 t'1t3 t'2 t'3 t'4

PT

T t

Hnh 4.7


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Thi gian 0 - t1 t1 - t2 t2-t3 t3-t4 t4-t5 t5-T

QT(t) QS QTmax QTmax QS QP QS

Qx(t) 0 0 Qs- QTmax 0 0 0

- Xc nh bin thin th tch nc trong h (da vo khong cch gia ng

cong ly tch nc chy vo h v ng cong lu lng nc iu tit).

- Bit th tch nc trong h c th tnh c mc nc thng lu theo ng

cong c tnh th tch h.

- Bit tr s lu lng nc vn hnh QT v lu lng nc x Qx xc nh mc

nc h lu theo ng cong c tnh mc nc h lu.

- Tnh ct nc ca NMT:

)()()(TxThlQHQQZQZH +=

- Xc nh cng sut ca NMT:

PT = 9,81HQT .

- Tnh sn lng in nng c nm ca NMT:

ii

t

TDTDttPE

i

= )(

III. Cc trng hp ring

1. Trng hp h c dung tch ln

Trng hp h c dung tch tng i ln, khi tnh ton iu tit ti u lu lng

nc x c th bng 0. Trong trng hp ny vic tnh ton iu tit cn c th

thc hin theo cch n gin hn. Sau khi v ng cong ly tch nc chy vo

h v ng cong kim tra c th xy dng ng cong ly tch iu tit bng

cch v lin tip cc on thng tip tuyn, tip xc pha trn vi ng cong kim

tra v tip xc pha di vi ng cong ly tch nc chy vo h. Phn cn li

ca ng cong c v men theo cc ng cong ni trn. Phng php ny cn

c gi l phng php "si ch cng".

Thng thng vi mi trng hp, ngi ta p dng th u tin phng php si

ch cng. Sau kim tra ring cho ma l bng cch v tia tip tuyn vi ng

cong ly tch nc chy vo h. Nu tip tuyn dc hn ng si ch cng th


65/125

phng php si ch cng tho mn, ly lm kt qu tnh ton. Trong trng hp

ngc li, s dng tia tip tuyn thay cho si ch cng v hiu chnh li ng

cong ly tch iu tit (theo trng hp c x).

2. Trng hp c nhiu nh my thy in xy dng ni tip trn cng mt

dng sng

Khi c nhiu NMT xy dng ni tip trn cng mt dng sng th thc hin iu

tit mt nh my c th c nh hng n cc nh my in cn li. Thng

thng ch vic iu tit cc NMT pha trn c nh hng n cc NMT pha

di. nh hng ngc li khi cc NMT rt gn nhau, mc nc h lu caNMT pha trn b nh hng bi mc nc iu tit pha di.

Tnh ton iu tit trong trng hp ny cn c thc hin teo th t: NMT pha

trn trc, pha di sau. Lu lng nc chy vo h tnh ton cho NMT pha

di c ly bng lu lng dng chy t nhin (khi cha c s iu tit no) tr

i tng th tch cc lng nc b gi li cc h pha trn, tnh trong mt n v

W

t

Hnh 4.7. Phng php si ch cng

QS

QTmax

QTVhi

ng cong kim tra

0


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thi gian v ti cng mt thi im. Trong thc hnh, vic tr ny d dng thc

hin c theo cc ng cong ly tch.

SttQQ =

1

12VQQ

Stt=

213VVQQ

Stt=

T1

T2

Hnh 4.8

W

ng cong kim tra

Qtt1=Qs

Qtt2=Qs-V1

V1

V1

V2

ng cong kim tra

t

t

QT1

QT2

QSW


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Hnh 4.8 minh ha kt qu thc hin tnh ton iu tit cho 2 nh my thy in

kiu p xy dng ni tip trn mt dng sng. iu tit NMT th 2 (pha

di) c gi thit tha mn phng php si ch cng.

C th nhn thy rng, khi thc hin iu tit ng thi cc NMT xy dng ni

tip trn mt dng sng, hiu qu lun lun c tng thm so vi khi ch c mt

nh my. Nh my cng nm pha di th hiu qu "cng thm" cng cao. l

v sau mi iu chnh NMT pha trn dng chy li ng u hn. Lng nc

x NMT pha di gim c, ct nc cng dng cao v ng u nn in

nng thu c ln.

IV. Tnh ton iu tit nhiu nm

Khi dung tch h ln c th tn dng iu tit nhiu nm. Phng php th

gii bi ton iu tit nhiu nm hon ton tng t nh iu tit nm. V h ln

ngi ta thng p dng ban u theo phng php si ch cng. Nu trong ma l

no , theo phng php si ch cng nhn c gi tr QT > QTmax th cng phi

hiu chnh li v c x trn. Trn hnh 4.9 minh ha ng cong iu tit iu tit

nhiu nm khi h c dung tch hn ch, tng ng vi s liu ca hnh 4.5.

nm

t

1 2 3 4 5 6 7

W m3

Vhi san bng dng chy nhiu nm

ng Qtb nhiu nm

QS

Hnh 4.5

ng cong kim tra

Vhi

QTx


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Ngoi cc tnh ton theo c trng thi gian (s dng chui s liu lu lng nc

thng k theo lch thi gian), vi iu tit nhiu nm ngi ta cn thc hin tnh

ton theo c trng xc sut. Mt trong cc bi ton thuc loi ny l xy dng

quan h gia xc sut m bo nc p cho lng nc s dng Wp vi dung tch

hu ch ca h Vhi. Cc i lng thng c tnh trong h n v tng i:

00

;W

W

W

Whip

==

Trong : W0 - l lng nc trung bnh nhiu nm, c ly lm lng c bn

tnh ton th tch nc.

Quan h gia cc i lng trn c xy dng cho dng sng c th, bit trc

cc c trng dng chy nh ng cong tn sut m bo nc (cng tnh trong

h n v tng i), n nh dng Cv, khng i xng Cs . Khi , nu cho

trc 2 trong 3 i lng (, , p) s xc nh c ngay i lng th 3. V d

cho trc th tch hu ch ca h , xc xut m bo nc p, t ng cong quan

h: ),( pf =

c th xc nh c lng nc m bo (tn sut p) l, vi tr s : Wp = W0.

Ngi ta cng xy dng quan h tng hp hn, xt n c s thay i ca h s Cv

( v coi Cs = 2Cv) xy dng ng cong quan h dng:

),,( pCf v = .

H s thng c chn xy dng t 0,2 n 0,9 cn p trong pham vi t 75%

n 97%. ng cong sau khi xy dng c c th dng vo nhiu mc ch tnh

ton khc nhau xt n cc c trng xc sut ca dng chy.

4.3. Biu iu phi h cha

Nh bit (trong chng 2), cc c trng thy vn a ra v s dng cho cc

tnh ton iu tit dng chy u da vo chui cc s liu thng k qu kh. Cc

c trng ny ni chung ch c ngha trung bnh xc xut, bi dng chy c c

tnh ngu nhin. Trong vn hnh NMT thng khng c c cc d bo di hn

tin cy nn c th dn n nhng sai khc ng k, nh hng n hiu qu khai

thc thy nng v tin cy cung cp in cho h thng. x l cc tnh hung


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sai lch ngi ta nghin cu cc quy tc vn hnh h cha theo trng thi hin thc

ca h. Mt trong nhng quy tc vn hnh nh vy c th hin di dng biu

gi l biu iu phi nc ca h cha. Biu gip cho ngi vn hnh

ch ng trong mi tnh hung, x l ng, khc phc c nhng bin ng bt

thng ca dng chy cng nh nhng yu cu t xut v nhu cu nc v nhu

cu nng lng.

Trc honh ca biu iu phi biu th thi gian trong nm (theo lch), trc

tung biu th th tch nc hoc mc nc trong h.

Vi cc ng cong gii hn xy dng c, biu iu phi chia ra thnh cc

vng sau (hnh 4.6):

I- Vng chng x tha (vn hnh vi cng sut ti a);

II- Vng vn hnh bnh thng (pht cng sut nng cao trn mc m bo theo

iu tit ti u);

III- Vng pht theo cng sut m bo;

1

2

3

4

5

Z m

t

Hnh 4.6. Biu iu phi

5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 thng

I

II

III

IV

I


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IV- Vng cung cp in hn ch (gim thp xung di cng sut m bo).

phn chia cc vng trn, cn xy dng cc ng ng mc s dng lu lng

nc (hay cng cng sut pht trung bnh ngay m) ca NMT.

ng gii hn quan trng nht l ng ng mc pht cng sut m bo

(ng 2 trn hnh 4.6). Mc nc di ng ny khng cho php NMT pht

ln trn cng sut m bo tnh ton, nhm duy tr pht cng sut c n nh lu

di (khng b thiu nc). Cng sut m bo tnh ton l cng sut pht thp

nhng c tin cy cao (vi tn xut m bo nc 90%) c xt n kh nng

iu tit ti u dng chy (ca nm nc in hnh tnh ton). Cng sut ny, gi

thit tnh c v tng ng vi lu lng nc m bo Qp. xy dng

ng gii hn ni trn cn da vo s liu dng chy ca cc nm c lng nc

gn vi lng nc ca nm tnh ton (k hiu l Wp). S liu dng chy ca nm

th i, c quy chun bng cch nhn vi t s Wp/Wi (Wi l lng nc ca nm

th i la chn). Na ng cong pha ma cn c xy dng bng cch gii

bi ton iu tit ngc (t cui ma cn) vi gi thit cui ma cn h ht nc

( mc nc cht) v lu lng nc s dng gi khng thay i bng Qp. Na

ng cong tng ng vi ma l cng c thc hin tng t nhng xut pht t

cui ma l, vi mc nc dng bnh thng (ng 1 trn hnh v). Kt qu s

nhn c cc ng cong khc nhau (cho mi nm). V ng baopha trn cc

ng cong ny, ta c ng cong gii hn 2 cn v.

Gp nhng nm c bit t nc (tng ng vi tn sut trn 90%) cn vn hnh

vi lu lng nc hn ch khng phi gim t bin kh nng cung cp in

ca NMT. Thng tnh vi kh nng vn hnh bng 0,8Qp. xy dng ng

cong gii hn trong trng hp ny cn chn ra cc nm t nc c bit trongchui thng k. Quy i lu lng nc mi nm v nm tnh ton v thc hin

tnh ton iu tit ngc t cui ma cn, gi lu lng nc khng i bng

0,8Qp. Kt qu nhn c ng cong 3 nh trn hnh v (l ng baopha trn

ca cc ng cong tnh ton). i khi ngi ta cn xy dng thm ng gii

hn ng vi lu lng bng 0,6Qp p dng cho nhng nm qu t nc. Cch


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xy dng cng tng t. Mc nc h xung di ng cong ny cn gim cng

sut ch cn 60% so vi cng sut m bo (ng 4).

ng cong phn chia ra vng vn hnh chng x tha nm bn trn vng pht

cng sut nng cao. Khi mc nc nm trn ng gii hn ny (trong vng chng

x tha) cn cho nh my vn hnh vi lu lng nc ti a QTmax. y l vng,

theo tnh ton, nu khng vn hnh ti a chc chn phi x tha. ng cong

c xy dng da vo s liu cc nm nc to (tn sut di 10%). S liu c

quy i v nm c tn sut (1-p), ngha l nhn vi t s W(1-p)/Wi. ng gii hn

trong trng hp ny tng ng vi ng baopha dica cc ng cong tnh

ton. ng cong xy dng c thng ch tn ti trong ma cn v u ma l

(ng 4 trn hnh 4.6), bi trong ma l cng sut thng phi pht nng cao n

ti a mi m bo ti u ( lng nc x t nht). Vng pht nng cao v vng

chng x tha trong ma l trng nhau. Hn na, cui ma l cn c th c c

khong thi gian phi x tha bt buc, khng th vn hnh chng x tha.

Biu xy dng c nh trn mi ch l c s cho biu iu phi. Trong thc

t p dng thnh quy trnh vn hnh ngi ta cn xem xt thm li gii ti u

ca bi ton iu tit cc nm (qu kh) hiu chnh cc ng cong, cng nh

hiu chnh theo kinh nghim vn hnh lu nm cho biu iu phi. L do c th

hiu chnh cc ng cong gii hn l v c nhng phn ranh gii khng chc chn

(gia ng bao pha trn v pha di cc ng cong). Ngoi ra xut pht t c

giao trinh thuy dien

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