€ ^ ( N G H I E N C L / U T R A O D O I
HAI CONG SUAT
V A N H U D N G A N H H l / Q N G T R O N G H E T H O N G
•
T I E U T H U V A C U N G C A P D I E N
• • Tran Khac Dien, Pham Hong Minh, Nguyen Anh Son - Vien Do luong Viet Nam
Luu Hong Viet - Dai hoc Bach khoa Ha Noi
Trong thap ky gan day chu de tiet kiem dien nang va chat lUdng dien nang dupe quan tam dac biet bdi nhufng ly do ve van de ton that vdi tai tieu thu phi tuyen cong suat Idn tham gia trong he thong ngay cang nhieu va phuc tap. Che
dp lam viec chuyen mach tan so cao cua cac bp bien ddi cong suat Idn la nguyen nhan true tiep gay ra su khdng tuyen tinh cua cac dang song dien ap, dong dien lam suy giam chat lupng dien nang. MuTc dp anh hudng ngay cang trd nen nghiem trong va cung la mpt thach thufc Idn ddi vdi
cac nha cung cap cung nhu cac nha tieu thu dien nang de lam sao han che/giam thieu tdi da mufc dp anh hudng nay.
Thuc te cho thay da co rat nhieu no lUc dupe thuc hien va yeu cau bat buoc ddi vdi cac tai tieu thu phai tuan thu theo mpt so chuan khi tham gia vao he thong nhu d chau Au ap dung chuan EN61000-3-2, dMy ap dung IEEE 519-1992...
Trong bai viet nay de cap mpt so khai niem xung quanh ve hai cong suat va sir anh hudng cua no, mpt van de luon dupe quan tam hang dau cua cac nha cung cap va tieu thu dien nang ngay nay.
A . K H A I N I E M VE H A l CONG S U A T
Ngay nay vdi sp tham gia ngay cang nhieu cua cac tai phi tuyen1 (pho bien la cac thiet bi chuyen mach tan so cao cong suat IPn) do su phat trien khong ngpng cua cac Png dung dien tP cong suat vdi tan suat ddng ngat cao va gan tdc thdi [1][2], da anh hudng xau tdi chat lupng nguon dien cung cap, dang tin hieu dien ap, dong dien tai vi tri do dem dien nang va hoat dpng chung ciia he thong. Ket qua la dang dien apva dong dien tieu thu khong con du\
tri dupe dang hai co ban hinh sine va co chua nhieu thanh phan ha>
bac cao. Nhuvay khi co spxuat hien cac thanh phan hai dien ap, done dien trong he thong se lam nay sinh hang loat cac van de nhu hieu suat tieu thu, chat luong hoat ddng va sii on dinh cua he thong, va cung co the phat sinh tdn that... 06 la 7 Tai phi tuyen la cac tai khi tham gia vao he thong se tao ra cac ddng dien dan cd dang khde vdi dang cua dien dp, theo quan he phi tuyen.
24 Tir dong hda ngay nay / 12. 2010
nhCfng hien Wong rat pho bien ma :hung ta thuong gap va ban toi khi
</an de tiet kiem nang lupng va on dinh he thong tieu thu dien dang dupe quan tam. Vay khai niem hai cong suat dupe quan niem va hieu nhU the nao? Khi danh gia cac hai cong suat thi cd gi khac biet voi cac thanh phan hai co ban tP lau nay van dang ap dung? IEEE da dUa ra khai niem ve hai: "Ld cac thanh phan
\song dang sine co tan so gap nguyen lan tan so cua song hdi co ban". Dpi khi de nhan manh va dam bao sp ro rang nguoi ta thudng neu day du bac cua song hai vi du, song hai bac 2, bac 3...
Hinh 1: Song sine va cac hai bac 3, 5, va 7
Hinh 2:06 meo cua song sine gay ra bdi cac hai Khi co sU ton tai cua cac thanh phan hai dong va ap thi luong cong suat trao doi trong he thong se trp nen phPc tap hon do phai xet den sp tuong tac cua tat ca cac thanh phan hai cung tham gia vao qua trinh trao doi nang lupng.
B. NGUON GOC CUA HAI CONG S U A T
Hai cong suat la mpt van de khong mong muon sinh ra trong he
^thong tieu thu. Nguon goc cua van :de nay co the dupe dien giai tP
nguyen ly co ban sau: Neu he thong dupe cung cap mpt nguon dien ap hinh sine cho mpt tai tieu thu phi tuyen sinh ra do cac bp bien doi dien tai cong suat Ion lam viec P giai dieu chinh rpng thi trong he thong se xuat hien hay tao ra mpt thanh phan dong dien phi tuyen hay hai dong dien, nhu dugc mo ta trong
I
Hinh 3: Hai ddng dien sinh ra bdi tdi phi tuyen hinh 3.
Cac thanh phan hai dong dien
tuong tac trpc tiep voi cac thanh phan cam, dung trong he thong va mpt each gian tiep sinh ra cac thanh phan hai dien ap bac cao anh hudng tdi nguon cung cap. Hon nUa, sp xuat hien cua hai dong dien vdi tan so cao va spthay doi buoc nhay bien dp Ion se gay ra cac hien tupng sai lech dong bp - differential (symmetri- cal) mode hoac mat doi xPng com- mon (asymmetrical) mode dan toi sp mat on dinh/nhieu loan trong hoat dpng cua he thong. Mpt so yeu to co ban gay ra cac thanh phan song hai co the chi ra nhu sau:
• Tan so chuyen mach cua cac bp chuyen doi nguon
• Toe dp thay doi dong hoac ap trong qua trinh hoat dpng cua thiet bi cong suat
• Trp khang cua nguon cung cap
• Dp dai duong day tP nguon toi tai tieu thu
VPi sp phat trien khong ngpng ve cong nghe phan t d ban dan cdng suat Ion va dng dung rdng rai trong cdng nghiep nhu ngay nay thi hai cdng suat dang la mot nguy co gay tdn that phai ddi mat voi bat ky mdt he thong cung cap hay tieu thu dien nang nao.
CANH HUdNGCUAHAI C O N G SUAT
Da co nhieu nghien cdu va kiem chdng thpc te cho thay rang mdc
anh hudng cua nhieu hai cong suat gay ra tdn that dang ke va anh hudng trpc tiep tdi he thong phu tai va nguon cung cap [3]. Chung co the lam tang mdc dp tdn that, gay nen sp mat can bang va mat ddi xdng trong he thong 3 pha va lam sai lech/nhieu loan hoat ddng cua cac thiet bi cung trong he thong.
Mdt sd hieu dng tieu bieu gay ra bdi song hai cd the ke tdi nhu sau:
• Ddi vdi may bien ap, hai ddng dien lam tang tdn hao ddng va ton hao t d thong lac, con hai dien ap la nguyen nhan gay nen tdn hao sat.
Ndi chung, nhung hieu dng nay gay ra bao hda lam phat sinh nhiet, tang tdn that va giam hieu suat hoat ddng trong he thong. Hieu dng gay bdi hai dien ap va ddng dien deu phu thudc vao tan so, sp tdn hao se gia tang cung vPi sptang cua tan sd.
Do do hai tan so cao se tram trong hon hai tan sd thap khi sinh ra nhiet trong may bien ap. Tdn hao may bien ap bao gdm tdn hao cd tai va tdn hao khong tai. Trong dd, tdn hao cd tai bao gdm tdn hao ' ZR va ton hao lac (stray losses). Tdn hao lac la tdn hao ddng dien xoay (eddy curent losses) do t d thong lac (stray electromagnetic flux) trong cac cudn day, loi sat, vd td, va cac phan cau true khac cua may bien ap. Tdn hao nay se gia tang, t i le voi binh phuong ddng dien tai va binh phuong tan so do hai sinh ra.
• Ddi vdi ddng co, may phat, thanh phan song hai ddng, ap lam tang tdn that nhiet va giam hieu suat hoat ddng gay ra bdi ton that gia tang trong loi sat va anh hudng trpc tiep tdi sp hinh thanh momen trong ddng co va may phat. Hon nPa cd the dan den sp mat on dinh trong hoat ddng do sp dao ddng cua thanh phan momen sinh ra.
Nhung tdn that nay cang Ion neu ddng hoat ddng cang Idn (theo quan he ty le vdi binh phuong bien dp cua ddng hai sinh ra)
• Ddi vdi cac thanh phan tu dien va cam khang, cac thanh phan hai se
Automation Today / 12, 2010
co the gay ra sp cpng hudng trong he thong. Ben canh do cac thanh phan tu se trd thanh cac loi thoat, ngan mach cac thanh phan hai ddng bac cao vi vay lam tang tai hoat dong va giam tuoi tho cua cac tu dien trong he thong.
• Doi vdi cac thiet bi dien td, xd ly tin hieu co hoat dpng nhay cam vdi sp thay doi dang ddng, ap hoac yeu cau nguon cung cap vdi dp on dinh va chinh xac cao rat co the dan tdi sp mat on dinh hoac nhieu loan.
Cac thanh phan hai co the anh hudng trpc tiep hoac qua cac dudng mach khep vong co dat chung neu khong dupe xd ly each ly hay tiep dat tot.
Sp anh hudng cua hai cdng suat co the thay ro la rat da dang va phuc tap, khong chi gay ra ton that, giam hieu suat sd dung voi chinh phu tai (nguon sinh ra hai cong suat) ma con anh hudng tdi cac phu tai xung quanh va he thong nguon cung cap. Vi vay, viec do va danh gia tdn that do hai gay ra la rat kho khan va khong the danh gia day du neu chi can cd vao cac diem do tai cac diem phat sinh hai.
D. CAC CHi SO VA PHUONG P H A P D A N H GIA H A I C O N G S U A T
De danh gia mdc dp anh hudng cua hai cong suat tdi he thong dien ngudi ta sd dung mpt so thong so dac trUng va cung la co sd de cac chuan dinh nghia cac yeu cau mpt each dinh lupng ap dung cho cac he thong dien ngay nay.
Cac chi so ve hai cong suat
Tong do meo nhieu hdi THD - Total Harmonics Distortion la gia tri hieu dung cua thanh phan hai chua trong dang song meo. Thong so dac trung nay dupe ap dung cho ca dien ap va ddng dien.
T H D =
Trong dd Mh la cac thanh phan hai chda trong song meo. Gia tri hieu dung cua song meo se dupe tinh theo gia tri THD nhu sau:
RMS = J_£Af* =-^:Vl + THD:
De dang nhan sp xuat hien/tdn
tai cua hai se lam tang gia tri RMS hay ton that nhiet gay ra ddi vdi he thong co dong dien meo.
Doi vdi dien ap cung da cd nhung qui dinh chuan [4][5] ve mdc dp toi da ap dung cho cac he thong vdi tai tieu thu phi tuyen gay ra nhieu hai nang lupng [IEEE 519- 1992], IEC-61000, EN 50160,...
He so cdng sudt PF - Power Factor:
He so cong suat cho phep danh gia chat lupng cdng suat dpa tren mdc dp meo ve tan so va bien dp cua cac thanh phan dong va ap. Vi vay, he so cdng suat dupe dinh nghia la ti'ch cua he so cong suat meo (TPF) va he so cong suat dich pha (DPF). Trong hau het cac trudng hpp, thanh phan cong suat gay ra bdi cac hai bac cao la nho vi vay:
PF = DPF • TPF = cos(9, -8,) —
\ " )
I P \ Sr' ./;!c o s ( e/ ;- 8/ i)
Trong do, P1, VI, 11 lan lupt la cac gia tri cong suat, dien ap va dong dien tUOng dng vdi thanh phan hai co ban. Ph, Vh, Ih la cac gia tri tuong dng vdi cac thanh phan hai bac h ton tai trong he thong. 5 va 0 tuong dng la pha cua ddng dien va dien ap.
Cac chi so ve dien dp:
- He so dinh CF = - f ^ - He so khong can bang UF=U - He so nhay
\V\
Phuong phap danh gid cong sudt
Trong dieu kien hoat dpng cc sp tham gia cua tai phi tuyen, gia thiet ve nguon dien ap va done dien dang sine khong dupe dam bao. Viec danh gia cdng suat phai dpa tren cac gia tri tdc thdi dc dupe cua dien ap va dong dien.
Cung cd nhieu khai niem ve cong suat da dupe de xuat, va chung khdng hoan toan thong nhat [8]
Budeanu (1927) da dua ra quar niem danh gia va dinh nghia dat tien ve cdng suat cho cac he t h o n g cd ap va dong khong sine Cac dinh nghia cua Budeanu det dupe bieu dien trong mien tan so theo cac thanh phan hai bac cac chda trong cac thanh phan done va ap. Trong cac dinh nghia cu;
Budeanu khdng bieu dien dupe si t u o n g tac cheo gida cac thanh
phan hai bac cao. Chinh vi vf) khdng the hien dupe mpt cadi day du cac thanh phan cong sua hPu d u n g va phan khang nhi quan niem truyen thong. Hon nu;
la rat kho danh gia trpc tiep cha:
lupng dien nang. Cung thdi giar do, Fryze (1930) cung dUa ra dinr nghia cac thanh phan cong suat trong mien thdi gian dpa tren ca;
gia tri hieu dung cua cac thanh phan hai dong va ap. Ve gia tn cac thanh phan cdng suat toar phan va cong suat huu dung di/;
ra t r o n g cac d i n h nghia cu;
Budeanu va Fryze khong co g khac nhau ma chi khac nhau ve each the hien. Budeanu dinh nghia trong mien tan so cdn Fryz;
dinh nghia trong mien thdi gian
TLT dong hoa ngay nay / 12, 2010
Tuy nhien, sir khac nhau the hien d l i n h nghia ve cong suat phan
<hang. Fryze quan diem cdng suat Dhan khang bao g d m toan bd ohan cdng suat khdng tham gia yao cdng suat hOu dung Pw. Trong do Pw thac chat la gia tri trung oinh cua cdng suat tdc thai p(t).
Budeanu thi quan diem cdng suat phan khang chi la tdng cua cac thanh phan cdng suat phan khang d cac tan sd hai ddng va ap tuong dng.
Ca hai tap dinh nghia ve cong suat cua Fryze va Budeanu van gap phai mpt van de khi van dung va tinh toan d che dp qua dp vi cac thanh phan dupe tinh deu dpa tren gia tri hieu dung cua dong va ap.
De khac phuc nhPng nhupc diem tren cua Fryze va Budeanu ly thuyet cdng suat tdc thdi do Akagi vd cac cdng sp de xuat [6][7]
nhanh chong dupe van dung thanh cong trong nhieu dng dung tnang cao chat lupng dien nang
ngay nay (Ipc tich cpc...). Dac biet vPi he thong 3 pha, cdng suat dupe xem xet trong mdt he thong hpp
nhat chP khdng phai sp xep chdng thuan tuy ba pha rieng le nhu quan niem truyen thong.
Khai niem ve cdng suat tdc thdi dupe dinh nghia va ap dung cho he 3 pha. Trong dinh nghia cua Akagi, cac thanh phan cdng suat dupe gpi la cong suat thpc va cdng suat ao, khong sd dung khai niem truyen thong la cdng suat hPu dung hay cdng suat phan khang. Neu so sanh voi cac dinh nghia truyen thong thi quan niem ve cdng suat tuc thoi theo ly thuyet cua Akagi the hien mdt so Uu diem ndi bat:
• Co the ap dung cho he thong 3 pha can bang hoac khdng can bang, dang dien ap va dong sine hoac khdng sine.
• Tinh toan tdc thdi theo gia tri ddng va ap tdc thdi, cho dap Png nhanh.
Viec danh gia cong suat hai tren thpc te la rat phdc tap bdi sp lan truyen cac thanh phan ddng hai va ap, anh hudng tdi tat ca cac thanh phan trong cung he thong cung cap (bao gdm ca cac thanh phan thupc phia nha cung cap va ca cac tai tieu thu khac). Dieu nay co nghia rang khong chi chinh cac phu tai sinh ra hai cdng suat phai chju tdn that ma con anh hudng va gay tdn that vdi cac phu tai va cac thanh phan trong cung he thong.
Chinh vi sp phdc tap nay ma hien nay d mdt so nUdc va khu vpc tren the gidi da dua ra chuan dinh mdc bat budc phai tuan thu voi cac phu tai de kiem soat, han che mdc dp sinh hai dpa tren cac chi so hai nhu THD... Dieu nay da dupe qui djnh cu the trong thong t u 32/20107TT- BCT do bp cdng thuong ban hanh ngay 30 thang 7 nam 2010 [9].
E. KET LUAN
Trong cac he thong tieu thu dien vdi s p t h a m g i a c u a c a c t a i p h i tuyen cdng suat ngay cang tang da lam anh hudng dang ke tdi hoat ddng chung cua he thong.
Hau qua gay ra khdng nhPng lam sai lech/nhieu loan hoat ddng cua cac thanh phan tham gia trong he thong ma con gay ra ton that rat Idn. Ton that gay ra khdng chi vdi chinh phu tai sinh hai ma con anh hudng va gay tdn that vdi ca he t h o n g (gdm cac phu tai xung quanh va he thong cung cap). Van de do lUdng va danh gia mdc dp tieu thu/tdn that gay ra bdi hai cdng suat la rat khd khan va phdc tap. Day la mdt van de can dupe kiem soat va tinh tdi trong van de tieu thu nang lupng cua cac ddi tupng tai phi tuyen. De danh gia mdc dp anh hudng, ngUdi ta ap dung mdt so cac chi so dac trUng ap dung cho cac he thong dong va ap khong sine. Mot trong nhung
no lpc hien nay nham kiem soat va giam thieu hai cdng suat la (bat budc) ap dung dinh mdc chi tieu chat lupng tieu thu ddi vdi cac phu tai ndi ludi, dpa tren cac chi so ve hai, ngoai viec danh gia ve cdng suat tai cac diem do cua phu tai.
Dieu nay se gdp phan tich cpc trong chien lupc thpc hien giam tdn that va tiet kiem nang lupng d Viet Nam chung ta hien nay ./.
TAI LIEU THAM KHAO
[1] E. Acha, V.G.Agelidis, O. Anaya-Lara, T.J.E. Miller, "Power electronic Control in Electrical Power Systems", Newnes, 2002.
[2] Thomas A. Lipo, "Pulse Width
Modulation for Power Converters:
Principles and Practice", Wiley-IEEE, 2003.
[3] Wakileh, George J., "Power System Harmonics", Springer, 2001.
[4] 519-1992 IEEE Recommended
Practices and Requirements for Harmonic Control in Electric Power Systems, IEEE, 1992.
[5] International Standard IEC61000-3-2, IEEE, 2005.
[6] H. Akagi, A. Nabae, and S. Atoh,
"Generalized Theory of the Instantaneous Reactive Power in Three-Phase Circuits", in Proc. IPEC-Tokyo'93 Int. Conf. Power Electronics, Tokyo, 1993, pp. 1375-1386.
[7] H. Akagi, A. Nabae, and S. Atoh,
"Instantaneous Reactive Power Compensator Comprising Switching Devices Without Energy Storage Components", IEEE Trans, on Indus.
Applica., Vol. IA-20, No. 3, pp. 625-630, 1984.
[8] L.H.Viet, "Cdng sudt dien ndng: Khdi niem vd dinh nghia", Tap chi tir dong hoa ngay nay, Sol 11,12/2009.
[9] Bp Cong ThifOng, "Thong tir 32/2010/TT-BCT", 7/2010Q
Automation Today / 12, 2010
