HVDC_Voltage Control.docx - Copy

8/15/2019 HVDC_Voltage Control.docx - Copy

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Transmission and Distribution

EEE-471

Limitations of HVAC Transmission:

1. Reactive Loss and DroFor ac transmission, there are lagging reactive VA loss and leading

reactive VA loss due to L & C of the line and the transmission would be

economical if these two losses balance each other at all points along

the line. This happens when the load is terminated with characteristic

impedance of the line. However, it is not possible to operate the line at

characteristic load conditions all the time. Thus this causes reactive

loss in the line. eactive drop in the line due to L & C is also signi!cant

in case of ac transmission.

!. "tabi#it$ The power transmitted over a line of reactance "#$ per phase is,

P=VsVr

X sinδ

%aimum stead' state power occurs when, δ =90° .

Considering transient condition, δ is not generall' more than 30°

for e(cient stable operation of the s'stem.)tabilit' considerations impose serious limits on the distance of power

transmission over ac lines. For dc transmission problem of stabilit'

doesn$t arise.%. Current Carr$in& Caacit$

*n ac transmission, current carr'ing capacit' is reduced due to

charging current. This charging current is appreciabl' high for

underground cables. This charging current ma' be as high as +A per

-m for -V cable. The length of the cable for which the cable

charging current becomes e/ual to the thermal current limit is 0nown

as critical length and limits the distance of power transmission.4. 'erranti E(ect

The rise of receiving end voltage under leading VA on the line at no

load ma' be serious and limits the distance of power transmission overac lines.

Advanta&es of HVDC Transmission:

+. *n case of dc transmission, onl' two conductors are needed for single

line. 1sing earth return, onl' one conductor is enough and with two

conductors and earth return, the capacit' of the line is doubled. *n case


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of ac transmission, at least three conductors are necessar' and for

double circuit, si conductors would be necessar'.2. High voltage dc line needs less space compared to ac line of the same

voltage and si3e. This reduces space to be maintained.4. The electrical !eld strength at the surface of conductor can be 56

higher on overhead lines and about three times higher on cables incase of ac as compared to dc. Thus dc cables are less epensive than

ac cables.. There is no need to maintain s'nchronism between two ac s'stems

connected together b' a dc lin0. *f transmission is interrupted due to

voltage drop or an' other fault, it is reestablished after clearance of

fault irrespective of s'nchronism.5. 7ower 8ow through dc lines can be easil' controlled via grid control of

the values.9. Terminals & lines can be built in stages. :c line voltage can be

increased graduall' b' stepwise installation of increased number of

conductors and power capacit' of the dc lin0 will also increase

accordingl'.;. Two ac power s'stems having dii? %ono@polar>ii? i@polar>iii? Homo@polar

*n mono@polar line, onl' one conductor is used and earth is used as return

path.


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i@polar lines have two conductors. Bne operating with >ve? polarit' and the

other with D>ve? polarit'. There are two converters of e/ual voltage ratings &

connected in series at each end of the dc line. The =unction of the converters

ma' be grounded at one end or at both ends. *f it is grounded at both ends,

each line can be operated individuall'.

FigE %ono@polar

FigE i@polar

FigE Homo@polar


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+o,er o, in HVDC transmission #ine:

L is the line resistance. er &ci are the resistance of converter & recti!er

respectivel'. The current in the line is given b',

Rcr+ R L+¿ Rci

I d=V ¿cos∝−V oicosβ

¿

*nternal voltage can be controlled b' an' or both of the following methods.

>a? rid Control>b? Tap@change Control

Rectication:

The average value of dc for n@phase is given b',


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V o= 1

2π

n

∫π

2−π

n

π

2+π n

V m sinθdθ

¿V

msin ( π

n)

π

n

For 4@ ∅ , nG4,

V o=

V msin (

π

3)

π 3

V o=3V m

π

√ 32

For 9@ ∅ , nG9,

V o=

V msin (

π

6

)

π

6

∴V o=3V m

π

%- ∅ /rid&e Rectier:


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FigE 4@ ∅ ridge ecti!er

The output voltage for n@phase is,

V o=V

maxsin ( π

n)

π

n

*n this case,

V max=√ 3V m

For nG9,

V o=√ 3V m

sin (

π

6 )π

6

¿3√ 3 Vm

π

0nversion:


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planation of d transmission advantages b' math formulaeE

Pac= E

1 E

2

X sinδ

Pdc= Ed1− Ed2

V d R Ed 2

Ihere, Ed 1− Ed2 can be varied easil' and e


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¿ √ 2

cos ∅

)ince, ∅≤1 , power per conductor in case of dc is more than ac.

+o,er er Circuit:

Let us compare power transmission capabilities of a 4@phase single circuit

line and a bi@polar line.

Pd=2 ρd∧ Pa=3 ρa

Ihere, ρd∧ ρa are power transmitted per conductor of dc & ac lines

respectivel'.

Pd

Pa=

2 ρd

3 ρa=

2V d I d

3V a I acos ∅=

2V d I d

3V d

√ 2 I dcos∅

= 2√ 23cos ∅

= 2.8283cos ∅

)ince, ∅ ≤1 , power transmission capabilit' of bi@polar line is the same as

that of three phase single circuit line.

o "tabi#it$ +rob#em:As we 0now,

Pac= E

1 E

2

X sinδ

Therefore, higher the value of #, lower will be capabilit' to transmit power

and also increase of angle δ which means less stabilit'.

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HVDC_Voltage Control.docx - Copy