Abstract 661

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UNIVERSITY OF WATERLOO

Term Project ECE 661

Simultaneous AC-DC Power Transmission

Sohail Habib

12/5/2012

This term paper explores the concept of parallel AC and DC transmission

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Abstract:In most power systems around world power demand is increasing due to increase in population and more extreme

weathers, to meet this demand more power needs to be transmitted either on same network or to build new lines . Laying new

lines is expensive and issues like right of way, politics etc makes this idea it very difficult to implement this idea. Transmitting

more power on same lines causes stability issues limiting power transfer as l ines gets longer and the lines cannot be loaded to

its thermal limits, line loadability can be increased by using power electronics based FACTS devices this paper will discuss the

use of power electronics based device to transmit AC and DC current simultaneously on the same l ine the transmission lines will

carry super imposed DC along with AC current. This enhances the power transfer capability and line can be loaded to a very

high value if the conductors are allowed to carry AC current along with superimposed DC current. The added power does not

cause any transient in stability issues.

Introduction:The power systems mostly are AC systems, the HVDC systems are now gaining popularity around the world with

HVDC systems now operating in Europe, Americas and Asia at some places it is being used to interconnect two systems of same

or different frequencies using back to back HVDC converters and at some places it is used to transmit power in EHV lines. This

interest is due to availability of high power electronic converters. Recent trends in power systems involving deregulation and

restructuring are aimed at separating the supply of electrical energy from the service involving transmission from generationsto loads. This can be achived only if the operation of power transmissionis made flexible by introducing fast acting high power

electronics devices. This enables quick and accurate control of power flows, voltage profile and improves stability. FACTS

controllers enables transmission line to carry more power closer to its thermal rating, this can also be archived by the concept

of simultaneous transmission is introduced in the reference [1] and [2]. Before continuing further discussion into simultaneous

AC and DC transmission lets have a brief over view of AC and DC transmission and discuss their advantages and short comings.

AC Transmission:The AC transmission systems are the most common transmission systems, the power flow in AC systems is

determined by Kirchhoffs laws. Even though first commercial electricity generated was DC but the difficulty of transmitting it

over long distance and the difficulty of transforming DC current paved way for AC networks and AC systems were adopted fast

with the development of transformers, synchronous machine and induction motor in the early 20thcentury. Even though in AC

current transformation is easy but the systems are complicated to understand as their size grows due to many variables

involved some short comings of the AC transmission systems are listed below.

1. The power flow in AC transmission is limited due to stability issues, which means that the lines cannot be loaded closeto its thermal limits [3]

2. The lack of fast controls in AC systems the system must be operated at a power level below the maximum loadingcapacity which is itself limited by stability issues, and for keeping systems secure a safety margin is kept for possible

contingencies and in some cases an additional reliability margin is also kept for example WECC(Western Electricity

Coordinating Council)systems are operated a minimum 5% distance away from maximum loadability point when

contingencies are considered.[4]

3. The AC power systems require dynamic reactive power control in order to keep acceptable voltage profiles undervarying loads and transient disturbances.

4. The increase in load is accompanied by higher reactive power consumption in line reactance.5. High surface voltage gradient on conductors has skin effect6. Corona effect

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DC TransmissionEven though DC transmission was an earlier discovery and first electric power transmission was DC developed by

Thomas Edison in late 19th

century, but the technological limitations held the DC transmission back due to lack of availability of

high power DC converters and protection devices. One of the earlier long-distance transmission of electric power was

demonstrated using direct current in 1882 at the Miesbach-Munich Power Transmission, but only 2.5 kW was transmitted. The

work in DC transmission has been done form early 1900s as in 1901 Hewitts mercury-vapor rectifier appeared, the first

commercial HVDC transmission was developed in Sewden in 1954 between the island of Gotland and Swedish mainland. After

the development of semiconductor devices in 1970s the HVDC systems have gained more interest as high power semiconductor

valves became available. Some advantages of DC systems are given below

Less right of way cheaper and simpler towers

1. Low power losses as compared to AC2. Less corona effect3. DC lines dont need reactive power compensation4. Control of power flow using power electronic devices5. Cheaper solution for long distance transmission6. DC systems make asynchronous ties between systems possible.

With all its advantages DC systems also have its drawbacks which are listed below

1. High cost of installation of converter stations at each DC link whereas AC systems needs cheaper transformer stationsat each link

2. Reactive power requirement for both rectification and inversion3. Due to presence of switching devices harmonics are generated which requires filtering4. Costly circuit breakers as braking DC current is difficult5. Complexity of control

Simultaneous AC and DC Transmission:

Advantages of AC and DC transmission can be taken from transmitting AC and DC currents simultaneously on thesame line. Work in this field is still going on and some of the advantages of this transmission scheme are listed below

1. Increased power flow2. Improved transient stability3. Improved dynamic stability4. Provides power systems with damping5. Long EHV lines can be loaded to its thermal limits6. No alterations of conductors, insulator strings and towers of the original line are neededFor simultaneous AC-DC transmission some of the AC power is converted into DC power at the sending end and at the

receiving end this DC power is converted back into AC power. Figure 1 shows circuit for transmitting AC and DC power

simultaneously through a single circuit AC transmission line , the dc current is obtained by the rectifier bridge converter and in

injected into neutral point of the zigzag connected secondary of sending end transformer and is reconverted to AC again by the

inverter bridge at the receiving end. The inverter bridge is again connected to the neutral of zigzag winding of the receiving end

transformer. Each conductor of each line carries one third of the total dc current along with ac current [1-2].

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 and is used as an interfacing device of dc and ac supply. A high value of reactor XD is used to

reduce harmonics in dc current [1-2].

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Figure 1 AC and DC simultaneous power flow through a single circuit AC transmission line [5]

Due to use of ground return path any metallic material may corrode if it comes in its path which limits the use of ground as

return path and this uni-polar DC link cannot be considered for practical application.

To avoid this short coming a double circuit ac line circuit was introduced in reference 6, the circuit is shown in figure 2 and the

equivalent circuit is shown in figure 3. The dotted lines in figure 3 show the path of ac return current only. The second

transmission line carries the return dc current Id and each conductor of the line carries Id /3 along with the ac current per

phase.

Figure 2 AC-DC transmission using a double circuit AC line [6]

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Power Calculations [1, 2, 5, 6, 8]:Assuming constant current control of rectifier and constant extinction angle control of inverter [3, 7], the equivalent

circuit of the scheme under normal steady state operating condition is given in Figure 3. The dotted lines in the figure 3 show

the path of ac return current only. The second transmission line carries the return dc current Id and each conductor of the line

carries Id /3 along with the ac current per phase.

Vdro and Vdio are the no load voltage of rectifier and inverter side dc voltages and are equal to 1.35 times converter ac input

line-to-line voltage. R, L, C are the line parameters per phase of each line. Rcr, Rci are commutating resistances and , are

firing and extinction angles of rectifier and inverter respectively.

Neglecting the resistive drops in the l ine conductors and transformer windings due to dc current, expressions for ac

voltage and current, and for active and reactive powers in terms of A, B, C, D parameters of each line may be written as:

Es= AER+ BIR (1)

Is= CER+ DIR (2)

Ps+ jQs= -EsER */B* + D*ES2/B* (3)

PR+ jQR= Es*ER/B* - A*ER2/B (4)

Neglecting ac resistive drop in the line and transformer, the dc current Id, dc power Pdr and Pdi of each rectifier and inverter

may be expressed as:

Id=[VdroCos - VdioCos +/* Rcr+Req- Rci] (5)

Pdr= VdrId (6)

Pdi= VdiId (7)

Reactive powers required by the converters are:

Qdr= Pdrtanr (8)

Qdi= Pditani (9)

cosr= *cos cos( r)]/2 (10)

cosi= *cos cos( i)]/2 (11)

i and r are commutation angles of inverter and rectifier respectively and total active and reactive powers at the two ends are:

Pst= Ps + Pdrand Prt= PR+ Pdi (12)

Qst= Qs+ Qdrand Qrt= QR+ Qdi (13)

Transmission loss for each line is:

PL = (PS+ Pdr)(PR+ Pdi) (14)

Let Ia being the rms ac current per conductor at any point of the line, the total rms current per conductor becomes:

I = ;

Power loss for each line = PL 3I2R

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The net current I in any conductor is offseted from zero. Now allowing the net current through the conductor equal to

itsthermal limit(Ith):

Ith=

(15)

Let Vphbe per phase rms voltage of original ac line. Let also Vabe the per phase voltage of ac component of simultaneous ac-dc

line with dc voltage Vdsuperimposed on it. As insulators remain unchanged, the peak voltage in both cases should be equal.

Vmax= 2Vph= Vd 2Va (16)

Electric field produced by any conductor possesses a dc component superimpose on it a sinusoidally varying ac component. But

the instantaneous electric field polarity changes its sign twice in a cycle if (Vd/Va) < 2 is insured.

Therefore, higher creepage distance requirement for insulator discs used for HVDC lines are not required. Each conductor is to

be insulated for Vmaxbut the line-to line voltage has no dc component and VLLmax= 6Va. Therefore, conductor to conductor

separation distance of each line is determined only by rated ac voltage of the line. Allowing maximum permissible voltage offset

such that the composite voltage wave just touches zero in each every cycle;

Vd= Vph/2 and Va= Vph/2 (17)

For insulation design point of view:

Let us define factor K1such that K1=dc withstand voltage/rms ac withstand voltage If calculated in straightforward manner for

overhead line

K1=2

The factor K2may be defined as;

K2 = ac insulation level/rated ac voltage

For overhead line K2 2.5

This is because high transient over voltages are possible for ac lines. Similarly for dc side design, a factor K3may be defined as;

K3=dc insulation level/rated dc voltage

For overhead line K3 1.7

The actual ratio of insulation level is (ac/dc):

K = K1(K2Vph/ K3Vd) (18)

Thus converted ac line voltage may be selected a little higher than V a= Vph/2 to have two natural zero crossing in phase voltage

(Va) wave cycle.

The total power transfer through the double circuit line before conversion is;

P/total 3Vph

2Sin1/X (19)

X is the transfer reactance per phase of the double circuit line and 1is the power angle between the voltages at the two ends.

To keep sufficient stability margin, 1is generally kept low for long lines and seldom exceeds 300. With the increasing length of

line, the loadability of the line is decreased. An approximate value of 1may be computed from the loadability curve by

knowing the values of Surge Impedance Loading (SIL) and transfer reactance X of the line.

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P/total= 2.M.SIL (20)

Where M is the multiplying factor and its magnitude decreases with the length of line. The value of M can be obtained from the

loadability curve. The total power transfer through the simultaneous ac-dc line is

Ptotal= Pac+ Pdc= 3Va2Sin2/X + 2VdId (21)

The power angle 2 between the ac voltages at the two ends of the simultaneous ac-dc line may be increased to a high value

due to fast controllability of dc component of power. For a constant value of total power, Pac may be modulated by fast control

of the current controller of dc power converters. Approximate value of ac current per phase per circuit of the double circuit line

may be computed as;

Ia V(Sin/2)/X (22)

The on-line dc current order for rectifier is adjusted as

Id = 3 (23)

Preliminary qualitative analysis suggests that commonly used techniques in HVDC/AC system may be adopted for the purpose

of the design of protective scheme, filter and instrumentation network to be used with the composite line for simultaneous ac-dc power flow. In case of a fault in the transmission system gate signals to all the SCRs are blocked and that to the bypass SCRs

are released to protect rectifier and inverter bridges. CBs are then tripped at both ends to isolate the faulty line. A surge

diverter connected between the zig-zag neutral and the ground protects the converter bridge against any over voltage.

SimulationAC-DC transmission via double circuit depicted in figure 2 was simulated in MATLABs Simulink using blocks of the powersystem

library the Simulink model is shown in figure 5. For the HVDC system 12 pulse converters shown in figure 6 was used rectifier

was controlled in constant current control and constant extinction angle control of inverter.

Control and Protection System:

The control systems of the rectifier and of the inverter use the same Discrete HVDC Controller block from the Discrete ControlBlocks library of the SimPowerSystemsExtras library. The block can operate in either rectifier or inverter mode. At the inverter,

the Gamma Measurement block is used and it is found in the same library. The Master Control system generates the current

reference for both converters and initiates the starting and stopping of the DC power transmission. The protection systems can

be switched on and off. At the rectifier, the DC fault protection detects a fault on the line and takes the necessary action to

clear the fault. The Low AC Voltage Detection subsystem at the rectifier and inverter serves to discriminate between an AC fault

and a DC fault. At the inverter, the Commutation Failure Prevention Control subsystem mitigates commutation failures due to

AC voltage dips. A more detailed description is given in each of these protection blocks [9].

AC-DC Transmission:

To show the difference in power transfer a base case of pure AC transmission is shown in figure 4 where power is

being transferred between two areas connected by a 300KM long line, area one is modeled as a 500KV, 60HZ and 5000 MVA,

while area 2 is modeled as a 345KV, 60HZ and 10,000 MVA.

Figure 7 shows difference between power transfer between a pure AC system and an AC-DC simultaneous transmission system

which shows increment of 65% percent which is a significant gain in the power transmitted, it can also be seen that the

magnitude of the voltage in the AC-DC system is high because of the superimposed DC factor.

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Figure 4

Figure 5

Figure 6 Left Rectifier Right Inverter

phi = 80 deg. 3rd harm.

500kV, 60 Hz

5000 MVA equivalent

345kV, 60 Hz,

10,000 MVA equivalent


Discrete,

Ts= 5e-05 s.

A

B

C

a

b

c

Three-PhaseTransformer

(Two Windings)3

A

B

C

a

b

c


(Two Windings)2

A

B

C

a

b

c


(Two Windings)1

A

B

C

a

b

c


(Two Windings)

0

Multimeter

A

B

C

a

b

c

Line1

A B C

AC filters

60 Hz

600 Mvar

A B C

AC filters

60 Hz

600 Mvar

Open this blockto visualize

recorded signals

Data Acquisition

+

300KM Line1

a

b

c

A

B

C

a

b

c

A

B

C

A

B

C

A

B

C

+

300KM Line2

8

neg

7

pos

6

Cd

5

Bd

4

Ad

3

Cy

2

By

1

Ay

g

A

B

C

+

-

Bridge Y

g

A

B

C

+

-

Bridge D

2

Pd

1

Py

8

neg

7

pos

6

Cd

5

Bd

4

Ad

3

Cy

2

By

1

Ay

g

A

B

C

+

-

Bridge Y

g

A

B

C

+

-

Bridge D

2

Pd

1

Py

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AC System AC-DC System

V

I

P = 2.0 PU P= 3.3 PU

Power Increment = 65%

Figure 7

Rectifier Output Inverter Input

Figure 8 Rectifier and Inverter Output Voltage and Current

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Rectifier Control Signals Inverter Control Signals

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Figure 9

Fault Simulation:

Circuit for fault simulation is shown in figure 9

1. Fault on Phase A on inverter sideThe fault occurs at 0.5 seconds and clears at 0.7 seconds the system response is given in the figures to follow

Figure 10 Top Line 1 voltage and Current Bottom Line 1 Active and Reactive Power

Form figure 10 it can be seen that the system recovers in 0.3 seconds after the fault is cleared at 0.7 second to pre fault level

The reason for this quick recovery is the HVDC system running in parallel with the AC system which gives the system faster

response and provide the system with additional damping


AC-DC Simultanious Transmission System 60HZ

500kV, 60 Hz

5000 MVA equivalent

345kV, 60 Hz,

10,000 MVA equivalent


Read the Model properties for initialisation details

Discrete,

Ts= Tss.

A+

B+

C+

A-

B-

C-

a3

b3

c3

ZigzagPhase-Shifting Transformer3

A+

B+

C+

A-

B-

C-

a3

b3

c3


A+

B+

C+

A-

B-

C-

a3

b3

c3


A+

B+

C+

A-

B-

C-

a3

b3

c3

ZigzagPhase-Shifting Transformer

v+-

Voltage Measurement

Iabc_R

Vabc_R1

Vabc_R

Vabc_R

A

B

C

A

B

C

Three-Phase Fault

Rectifier

Control and Protection

A

B

C

+

-

Rectifier

4

Multimeter

Master Control

Master Control

A

B

C

a

b

c

Line1

Line 4

Line 1

InverterControl and Protection

A

B

C

+

-

Inverter

A B C

AC filters60 Hz

600 Mvar

A B C

AC filters

60 Hz600 Mvar

Fourier

Mag

Phase

DiscreteFourier

Open this block

to visualize

recorded signals

Data Acquisition

i+ -

Current Measurement

IA

B

C

Brecta

b

c

Binv

+

300KV Line2

+

300KV Line1

a

b

c

A

B

C

a

b

c

A

B

C

A

B

C

A

B

C

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Rectifier DC Output Inverter DC Input

Figure 11

The voltage and current profiles are displayed in figure 11 it can be seen that under fault conditions the DC voltage dips and

recovers to pre fault level when the fault is cleared it can be seen in the appendix 1 that voltage on all inverter and rectifier

switch dips and then recovers to its Perrault level. While the currents levels spike.

An EHV line and on occurrence of a fault the transient response of the system for example the voltage profile or the current or

the sudden surge in the reactive power requirement has inherent sluggishness, the system requires a long time to recover. Butby using the simultaneous ac-dc model the transient response is increased and hence the transient stability which can be seen

in figure 13.

The stability is further enhanced because of quicker current control mechanism of HVDC system that is the rectifier and inverter

blocks. The control mechanism consist of a master control that controls and coordinates inverter and rectifier the second level

of control and protection in on inverter and rectifier itself which works on VDCOL control procedures. Whenever the voltage

dips on occurrence of a fault the current is restricted so the fault current is also decreased and the most significant thing is that

it has very small time constant that is it works very quickly, this effect can be seen in figure 12 where it can be seen that the

rectifier goes into current mode and tries to limit the fault current while the inverter goes into alpha max mode in order to have

certain extinction angle to commutate the valves without failing. After the fault has cleared the rectifier returns to minimum

alpha mode to have minimum reactive power requirement and inverter returns to constant current mode to provide load side

with a constant current.

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Rectifier Controls Response Inverter Controls Response

Figure 12

Now the second simulation is for the case when the fault takes a long time to clear so the fault occurs at 2 seconds and it c lears

at 9 seconds it can be seen form figure 13 that the system recovers well to its pre fault level

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Figure 13

Conclusions:Simulations show that the added dc power flows well with the AC power, the stability of system is also enhanced by

added dc power. A weak system becomes unstable when fault is not cleared quickly but here the DC system provides system

with additional damping and control. For the system under consideration it is seen that the maximum power transferred on the

line is increased substantially. The advantages of simultaneous AC-DC transmission are obtained without any change insulator

strings, towers and arresters of the original line as the line to line voltages remains same.

Recommendation and Possible use:1. Both converters require reactive power to operate and this could increase the reactive power demand of the system,

the AC could also be used to provide this additional reactive power or Static Var Compersators(SVC) or any other

FACTS device could be used to provide this additional reactive power

2. The control of DC power flow can be used to tackle stability issues for example If the fault is severe and takes toomuch time to clear the system might become unstable or even crash because of frequency instability as the generator

speed oscillates, this situation can be avoided by temporarily switching off the AC power flow and modulating DC flow

to produce retarding torque to the generators to normal speed.

3. This mode of transmitting power is a relatively new concept and the work in this field is being carried out, thistransmission scheme would be interesting in interfacing renewable electrical power resources such solar can also be a

good prospect by changing the scheme a bit and their DC power can be transferred in parallel with conventional AC

system

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References:

[1] Basu K. P. and Khan B. H., Simultaneous ac-dc Power Transmission,Institution of Engineers (India) Journal-EL, Vol 82,June 2001, pp. 32-35.

[2] Rahman H. and Khan B. H., Enhanced Power Transfer bySimultaneous transmission of AC-DC: A New FACTS conceptIEEE conference on Power Electronics, Machines and Drives, (PEMD 2004). Volume 1, 31 March -2 April 2004, pp. 186-

191.

[3] P. S. Kundur, Power system stability and control, New York: McGrawhill Inc[4] A. Gmez-Expsito, A. J. Conejo and C. A. Caizares, Editors, Electric Energy Systems: Analysis and Operation, CRC

Press, July 2008

[5] K. P. BasuStability Enhancement of Power System by Controlling HVDC Power Flow through the Same ACTransmission Line2009 IEEE Symposium on Industrial Electronics and Applications (ISIEA 2009), October 4-6, 2009,

Kuala Lumpur, Malaysia.

[6] H. Rahman, and B.H. KhanPower Upgrading by Simultaneous ac-dc Power Transfer in a Double Circuit ac Line, IEEEPES conference, Power India (2006).

[7] I.W. Kimbark, Direct Current Transmission, Vol-I, Wiley, New York,[8] K. P. Basu, H. Rahman Feasibility Study of Conversion of Double Circuit ac Transmission Line for Simultaneous ac -dc

Power TransmissionIEEE PEDS 2005

[9] http://www.mathworks.com/help/physmod/powersys/ug/thyristor-based-hvdc-link.html

AppendixAll Rectifier and Inverter Voltage and currents response under fault condition

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0 0.2 0.4 0.6 0.8 1 1.2 1.4

-1000

-500

0

500

1000

Isw1: Inverter/Inverter/Bridge Y

0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500


0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500


0 0.2 0.4 0.6 0.8 1 1.2 1.4

-1000

-500

0500

1000


0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500


0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500


0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500

Isw1: Inverter/Inverter/Bridge D

0 0.2 0.4 0.6 0.8 1 1.2 1.4

-500

0

500


0 0.5 1

-1000

0

1000


0 0.5 1

-1000

0

1000


0 0.5 1

-500

0

500


0 0.5 1

-500

0

500


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0 0.5 1

-1000

0

1000

Isw3: Rectifier/Rectifier/Bridge D

0 0.5 1

-1000

0

1000


0 0.5 1

-1000

0

1000


0 0.5 1

-1000

0

1000


0 0.5 1

-2

0

2

x 105Usw1: Inverter/Inverter/Bridge Y

0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2

x 105Usw1: Inverter/Inverter/Bridge D

0 0.5 1

-2

0

2


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0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


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0 0.5 1

-2

0

2

x 105Usw1: Rectifier/Rectifier/Bridge Y

0 0.5 1

-2

0

2


0 0.5 1

-2

0

2x 10

5Usw3: Rectifier/Rectifier/Bridge Y

0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2

x 105Usw1: Rectifier/Rectifier/Bridge D

0 0.5 1

-2

0

2


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0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


0 0.5 1

-2

0

2


Abstract 661

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