International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
Reduce Power Transfer Loss in Transmission Line by Integrating AC &
DC Transmission
Alok Kumar1, Surya Prakash2 ,
Department of Electrical Engineering, CMJ University, Shillong Meghalaya-India¹
Department of Electrical Engineering, SHIATS Deemed University, Allahabad U.P-India²
Abstract— In this paper it is possible to load the
transmission lines very close to their thermal limits by
allowing the conductors to carry usual ac along with dc
super imposed on it. The added dc power flow does not
cause any transient instability. This project gives the
feasibility of converting a double circuit ac line into
composite ac–dc power transmission line to get the
advantages of parallel ac–dc transmission to improve
stability and damping out oscillations. Simulation and
experimental studies are carried out for the
coordinated control as well as independent control of
ac and dc power transmissions. No alterations of
conductors, insulator strings, and towers of the original
line are needed. The present situation demands the
review of traditional power transmission theory and
practice, on the basis of new concepts that allow full
utilization of existing transmission facilities without
decreasing system availability and security. To achieve
this is by simultaneous ac–dc power transmission in
which the conductors are allowed to carry
superimposed dc current along with ac current. Ac and
dc power flow independently, and the added dc power
flow does not cause any transient instability.
Simultaneous ac–dc power transmission was first
proposed through a single circuit ac transmission line.
In these proposals Mono-polar dc transmission with
ground as return path was used. There were certain
limitations due to use of ground as return path.
Moreover, the instantaneous value of each conductor
voltage with respect to ground becomes higher by the
amount of the dc voltage, and more discs are to be
added in each insulator string to withstand this
increased voltage. In this paper, the feasibility study of
conversion of a double circuit ac line to composite ac–
dc line without altering the original line conductors,
tower structures, and insulator strings has been
presented. In this scheme, the dc power flow is point-to
point bipolar transmission system. The novelty of our
proposed scheme is that the power transfer
enhancement is achieved without any alteration in the
existing EHV ac line. The main object is to gain the
advantage of parallel ac–dc transmission and to load
the line close to its thermal limit.
Key Words— Flexible AC transmission system
(FACTS),Extra high voltage (EHV)transmission, MatLab, Simultaneous ac-dc power transmission
ISSN: 2231-5381
I. INTRODUCTION
From In Recent years, environmental, right-of-way, and
cost concerns have delayed the construction of a new
transmission line, while demand of electric power has
shown steady but geographically uneven growth. The
power is often available at locations not close to the
growing load centers but at remote locations. The
wheeling of this available energy through existing long ac
lines to load centers has a certain upper limit due to
stability considerations. Thus, these lines are not loaded to
their thermal limit to keep sufficient margin against
transient instability.
The present situation demands the review of traditional
power transmission theory and practice, on the basis of
new concepts that allow full utilization of existing
transmission facilities without decreasing system
availability and security. To achieve this is by
simultaneous ac–dc power transmission in which the
conductors are allowed to carry superimposed dc current
along with ac current. Ac and dc power flow
independently, and the added dc power flow does not
cause any transient instability. Simultaneous ac–dc power
transmission was first proposed through a single circuit ac
transmission line.
The basic proof justifying the simultaneous ac–dc power
transmission is explained in an IEEE paper “Simultaneous
ac-dc power transmission,” by K. P. Basu and B. H. Khan.
In the above reference, simultaneous ac–dc power
transmission was first proposed through a single circuit ac
transmission line. In these proposals Mono-polar dc
transmission with ground as return path was used. There
were certain limitations due to use of ground as return
path. Moreover, the instantaneous value of each conductor
voltage with respect to ground becomes higher by the
amount of the dc voltage, and more discs are to be added
in each insulator string to withstand this increased voltage.
However, there was no change in the conductor separation
distance, as the line-to-line voltage remains unchanged. In
this paper, the feasibility study of conversion of a double
circuit ac line to composite ac–dc line without altering the
original line conductors, tower structures, and insulator
strings has been presented.
II. PROBLEM DEFINITION
The main object of my paper is to show that by
superimposing DC in AC transmission, the capacity of the
transmission line can be increased by nearly 70 % of that
if only AC is transmitted. In our existing transmission
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
system, long extra high voltage (EHV) ac lines cannot be
loaded to their thermal limits in order to keep sufficient
margin against transient instability. With the scheme
proposed in this project, it is possible to load these lines
very close to their thermal limits. The conductors are
allowed to carry usual ac along with dc superimposed on
it.
This report presents the Power Upgrading of Transmission
line by combining AC and DC transmission. The flexible
ac transmission system (FACTS) concepts, based on
applying state-of-the-art power electronic technology to
existing ac transmission system, improve stability to
achieve power transmission close to its thermal limit.
Another way to achieve the same goal is simultaneous ac–
dc power transmission in which the conductors are
allowed to carry superimposed dc current along with ac
current. Ac and dc power flow independently, and the
added dc power flow does not cause any transient
instability. The authors, H. Rahman and B. H. Khan, have
earlier shown that extra high voltage (EHV) ac line may
be loaded to a very high level by using it for simultaneous
ac–dc power transmission as reported in references [5] and
[6]. The basic proof justifying the simultaneous ac–dc
power transmission is explained in reference [6]. In the
above references, simultaneous ac–dc power transmission
was first proposed through a single circuit ac transmission
line. In these proposals Mono-polar dc transmission with
ground as return path was used. There were certain
limitations due to use of ground as return path. Moreover,
the instantaneous value of each conductor voltage with
respect to ground becomes higher by the amount of the dc
voltage, and more discs are to be added in each insulator
string to withstand this increased voltage. However, there
was no change in the conductor separation distance, as the
line-to-line voltage remains unchanged.
In this paper, the feasibility study of conversion of a
double circuit ac line to composite ac–dc line without
altering the original line conductors, tower structures, and
insulator strings has been presented. In this scheme, the dc
power flow is point-to point bipolar transmission system.
Clerici et al. [7] suggested the conversion of ac line to dc
line for substantial power upgrading of existing ac line.
However, this would require major changes in the tower
structure as well as replacement of ac insulator strings
with high creepage dc insulators. The novelty of our
proposed scheme is that the power transfer enhancement is
achieved without any alteration in the existing EHV ac
line. The main object is to gain the advantage of parallel
ac–dc transmission and to load the line close to its thermal
limit.
III.
inverter bridge at the receiving end. The inverter bridge is
again connected to the neutral of zigzag connected
winding of the receiving end transformer. Star connected
primary windings in place of delta-connected windings for
the transformers may also be used for higher supply
voltage. The single circuit transmission line carriers both 3
–phase ac and dc power. It is to be noted that a part of the
total ac power at the sending end is converted into dc by
the tertiary winding of the transformer connected to
rectified bridge. The same dc power is reconverted to ac at
the received end by the tertiary winding of the receiving
end transformer connected to the inverter bridge. Each
conductor of the line carries one third of the total dc
current along with ac current Ia .The return path of the dc
current is through the ground. Zigzag connected winding
is used at both ends to avoid saturation of transformer due
to dc current flow. A high value of reactor, Xd is used to
reduce harmonics in dc current.
Fig.1 Basic scheme for composite ac–dc transmission.
In the absence of zero sequence and third harmonics or its
multiple harmonic voltages, under normal operating
conditions, the ac current flow will be restricted between
the zigzag connected windings and the three conductors of
the transmission line. Even the presence of these
components of voltages may only be able to produce
negligible current through the ground due to high of Xd.
Assuming the usual constant current control of rectifier
and constant extinction angle control of inverter, the
equivalent circuit of the scheme under normal steady state
operating condition is shown in Fig.2
INTEGRATING AC-DC POWER TRANSMISSION
The circuit diagram in Figure1 shows the basic scheme for
simultaneous ac-dc transmission. The dc power is obtained
through the rectifier bridge and injected to the neutral
point of the zigzag connected secondary of sending end
transformer, and again it is reconverted to ac by the
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
PL = (Ps + Pdr) – (PR + Pdi)
(14)
Ia being the rms ac current per conductor at any point of
the line, the total rms current per conductor becomes:
The dotted line in the figure shows the path of ac return
current only. The ground carries the full dc current Id only
and each conductor of the line carries Id/3 along with the
ac current per phase.
The expressions for ac voltage and current and the power
equations in terms of A,B,C and D parameters of each line
when the resistive drop in transformer winding and in the
line conductors due to dc current are neglected can be
written as:
Sending end voltage:
Vs = AVR + BIR
(1)
Sending end current:
Is = CVR + DIR
(2)
Sending end power:
Ps+ jQS = (- VSV*R)/B* + (D*/B*) Vs₂
(3)
Receiving end power:
PR + jQR = (VS*VR) / B* - (A*/B*)VR₂
(4)
The expressions for dc current and the dc power, when the
ac resistive drop in the line and transformer are neglected,
Dc current:
Id = (Vdrcosα - Vdicosγ)/(Rer + (R/3) – Rci
(5)
Power in inverter:
Pdi = Vdi x Id
(6)
Power in rectifier:
Pdr = Vdr x Id
(7)
Where R is the line resistance per conductor, Rcr and Rci
commutating resistances, α and γ, firing and extinction
angles of rectifier and inverter respectively and Vdr and
Vdi are the maximum dc voltages of rectifier and inverter
side respectively. Values of Vdr and Vdi are 1.35 times line
to line tertiary winding ac voltages of respective sides.
Reactive powers required by the converters are:
Qdi=Pdi tanθI
Qdr = Pdr tanθr
CosθI = (cos γ + cos (γ + μi) ) / 2
Cosθr = (cos α + cos (α + μr) ) / 2
(8)
(9)
(10)
(11)
Where µi and µr are commutation angles of inverter and
rectifier respectively and total active and reactive powers
at the two ends are
Pst = Ps + Pdr and Prt = PR + Pdi
(12)
Qst = Qs + Qdr and Qrt = QR + Qdi
(13)
Total transmission line loss is:
ISSN: 2231-5381
I = sqrt (Ia2 + (Id/3)2) and PL
3I2R
(15)
If the rated conductor current corresponding to its
allowable temperature rise is Ith and
Ia = X * Ith; X being less than unity, the dc current
becomes:
Id = 3 x (sqrt (1-x2) ) Ith
(16)
The total current I in any conductor is asymmetrical but
two natural zero-crossings in each cycle in current wave
are obtained for (Id/3Ia) <1.414. The instantaneous value
of each conductor voltage with respect to ground becomes
the dc voltage Vd with a superimposed sinusoidally
varying ac voltages having rms value Eph and the peak
value being:
Emax = V + 1.414 Ep
Electric field produced by any conductor voltage possesses
a dc component superimposed with sinusoidally varying
ac component. But the instantaneous electric field polarity
changes its sign twice in cycle if (Vd/Eph) <
1.414.Therefore, higher creepage distance requirement for
insulator discs used for HVDC lines are not required.
Each conductor is to be insulated for Emax but the line to
line voltage has no dc component and ELL(max) = 2.45
Eph.Therefore, conductor to conductor separation distance
is determined only by rated ac voltage of the line.
Assuming Vd/Eph = k
Pdc/’Pac
(Vd * Id)/(3 * Eph * Ia * cosθ) = (k * sqrt(1x2)) / (x * cosθ )
(17)
Total power
Pt = Pdc + Pac = (1 + [k * sqrt (1-x2)] / (x * cosθ)) * Pac
(18)
Detailed analysis of short current ac design of protective
scheme, filter and instrumentation network required for
the proposed scheme is beyond the scope of present work,
but preliminary qualitative analysis presented below
suggests that commonly used techniques in HVDC/AC
system may be adopted for this purposes.
IV. EXPERIMENTAL VERIFICATION
The feasibility of the basic scheme of simultaneous ac-dc
transmission was verified in the laboratory. Transformer
having a rating of 2 kVA, 400/230/110V are used at each
end. A supply of 3-phase, 400V, 50Hz are given at the
sending end and a 3-phase, 400 V, 50 Hz,1 HP induction
motor in addition to a 3-phase, 400V, 0.7 KW resistive
load was connected at the receiving end. A 10 A, 110 Vdc
reactor (Xd) was used at each end with the 230V zigzag
connected neutral. Two identical SCR bridges were used
for rectifier and inverter. The dc voltages of rectifier and
inverter bridges were adjusted between 145 V to135 V to
vary dc current between 0 to 3A. The same experiment
was repeated by replacing the rectifier at the sending and
and the inverter at receiving end by 24V battery and a 5A,
25 rheostat respectively, between Xd and ground.
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The power transmission with and without dc component
was found to be satisfactory in all the cases. To check the
saturation of zigzag connected transformer for high value
of Id, ac loads were disconnected and dc current was
increased to 1.2 times the rated current for a short time
with the input transformer kept energized from 400V ac.
But no changes in exciting current and terminal voltage of
transformer were noticed verifying no saturation even with
high value of Id.
Power
Angle
PS (MW)
30°
45°
60°
75°
2311
2379
2391
2349
298
419
505
542
1719
1661
1589
1499
14
35
59
88
288
266
545
219
1991
2057
2066
1999
Pac(MW)
Pdc(MW)
Pac loss
(MW)
Pdc loss
(MW)
PR (MW)
V. AC & INTEGRATING AC – DC MODEL
Power Angle
30°
45°
60°
75°
AC Current(kA)
0.148
0.616
0.09
0.99
DC Current(kA)
5.25
5.07
4080
4.50
AC Power(MW)
292
415
507
563
DC Power(MW)
1688
1628
1549
1157
Total Power(MW)
1977
2039
2051
1718
Figure.3 Diagram for AC transmission system
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Fig.6 Sending and receiving currents
Fig.7 Integrating AC-DC current
VI. RESULT
Figur.4 Diagram for integrating AC & DC transmission
system
TABLE 1 Computed Results
In this paper, it is shown that by injecting DC power in AC
power transmission lines, we can improve the transmission
capacity of the line by 2 to 4 times without altering the
physical equipment. This work can be extended for
analyzing the effect of faults on this type of transmission.
This work is done on double circuit AC transmission lines
but it can be extended to other types of transmission
methods.
VII. DISCUSSION
A simple scheme of simultaneous EHV ac-dc power
transmission through the same transmission line has been
presented. Expressions of active and reactive powers
associated with ac and dc, conductor voltage level and
total power have been obtained for steady state normal
operating condition. The possible applications of the
proposed scheme may be listed as: loading a line close to
its thermal limit, improvement of transient and dynamic
stability and damping of oscillations. In LV and MV
distribution system the proposed scheme may be applied
in a workplace having high ambient temperature or fed
with high frequency supply or with PV solar cells. Only
the basic scheme has been presented with qualitative
assessment for its implementation. Details of practical
adaptation are beyond the scope of the present work.
TABLE 2 Simulation Result
Fig.5 Sending end and receiving end voltages
VIII. CONCLUSION
The feasibility to convert ac transmission line to a
composite ac–dc line has been demonstrated. For the
particular system studied, there is substantial increase in
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the loadability of the line. The line is loaded to its thermal
limit with the superimposed dc current. The dc power flow
does not impose any stability problem. The advantage of
parallel ac–dc transmission is obtained. Dc current
regulator may modulate ac power flow. There is no need
for any modification in the size of conductors, insulator
strings, and towers structure of the original line.
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14. Kimbark I W.’Direct Current Transmission VolI.’Wiley, New York, 1971.
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Authors Biography
Surya
Prakash belongs to Allahabad, Received his
Bachelor of Engineering degree from The Institution of
Engineers (India) in 2003, He obtained his M.Tech. in
Electrical
Engg.(Power
System)
from
KNIT,
Sultanpur.UP-India in 2009. Presently he is working as
Asst. Prof. in Electrical Engg. Dept. SSET, SHIATS
(Formerly Allahabad Agriculture Institute, AllahabadIndia). His field of interest includes power system
operation & control, Artificial Intelligent control e-mail:
sprakashgiri0571@yahoo.com
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International Journal of Engineering Trends and Technology (IJETT) - Volume4 Issue6- June 2013
Alok kumar belongs to Allahabad, He obtained his
M.Tech. in Electrical Engg.(Power System) from SHIATS
Deemed University, Allahabad UP-India in 2012.
Presently he is doing P.hd from CMJ University,
Shillong Meghalaya- India. His field of interest HVDC
Transmission Line, alokkumar1622@rediffmail.com
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