High Voltage Direct Current (HVDC) transmission1. Introduction The growth and expansion of AC power system grid planning is fundamental to the power system construction and solid power grid planning is preliminary to the safety, reliability and economy of power grid operation with the development of smart-grid, the power grid planning techniques should effectively evaluate the economy, risk and reliability of the planning schemes. Thus, it is necessary to develop an intelligent planning system to support power grid planning. With the increasing pressure on environmental protection, energy conserving and persistence develops improves gradually required for society. At the same time, power market-oriented development consistently and provide higher electric energy reliability and quality are required for consumers. It require that the future smart grid must can to provide secure, reliable, clean, high quality power supply, is able to adapt to various of electric power generation, need being able to adapt to highly become market-oriented electric power exchange especially, acting on self’s own being able to adapt to customer especially chooses need, further, improve the ample power grid assets utilization efficiency and beneficial result, provide higher quality service. For this purpose, many countries without exception look upon smart grid as future development direction of power grid. So that the power grid trends to the modern power grid using a new technology in power system design which achieving the reliability of power flows by using power electronics techniques as AC/DC converter transmission line. However to meet the ever growing demand for bulk

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power transmission over long distance, one solution is to build HVDC transmission lines. On the other hand, increasing the capacity of existing transmission lines can achieve the same objective. There are many more AC lines than DC lines in today’s power networks. But, the HVDC transmission is attractive for the transmission of large blocks of power over long distance. The cross-over beyond which dc transmission may become a competitive to alternative ac transmission is around 500Km overhead lines, and 50Km underground cables. But the HVDC became more attractive after the thyristor valves converters are used in a wide area.

This report is discussed the HVDC transmission, types, analysis and equations, consequently, give some examples to illustrate these types. 2. Background 100 years ago, the electric power transmissions are trended to develop and improve transfer capacity and reduce transmission cost. Raising voltage level is the most efficient way to improve transmission power. The electric power transmission at 1000 kV and above AC voltages is known as UHV AC transmission, and the voltages at above 600 kV DC are known as UHV DC transmission[1]. In [2]the authors discussed the new challenges about power system planning and smart grid development in china which concerned to the UHV and some result show that Study of HVDC planning for the receiving-end of the power system, propose the configuration principles for the intelligent dynamic reactive power compensation devices and the planning indices of the HVDC that can improve

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the voltage stability in the multi-indeed HVDC power system. A HVDC can be implemented as direct coupler the back-to-back (B2B) solution or point to point a long distance transmission via DC line. The HVDC links can strengthen the AC interconnections at the same time, in order to avoid possible dynamic problems which exist in such huge interconnections. Due this period time the application of HVDC using in offshore wind farms which, usually have widely dispersed locations in a strong wind area. Furthermore, VSCs have a limited transmission capacity due to limitations on IGBT and capacitors ratings. For these reasons, a multi-terminal HVDC transmission system, which can extract and deliver power from and to several terminals and provide power to more than one terminal, is an attractive method for offshore wind power transmission[3]. Nowadays the HVDC technologies are mature to use and the results get out are good even some research compared the HVDC with HVAC and it found the transmission line length above 400Km it is better to use DC transmission line, because AC transmission line is very expensive[4]. DC systems have three advantages compared to AC: (1) the stability due to synchronization phenomena in AC increases the cost of building long, high capacity transmission lines dramatically. As the transmission length reaches a certain distance, DC transmission becomes economically attractive over AC[5]. In recent years, with the development of power electronics, Voltage source converters (VSC) based on HVDC (VSC-HVDC) transmission link, using self-commutated valves (IGBTs, IGCTs and GTOs), have had an active role in electricity transmission and distribution improvement. Furthermore,

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their significant advantages make this converters suitable for the connection of wind farms to transmission networks[6]: (i) IGBT valve can switch off and on immediately, (ii) there is no commutation failure problem, (iii) no telecommunication required between two stations of HVDC system, (iv) Active and reactive power can be controlled independently, (v) reactive power compensation is not required, (vi) only small filter is required to filter high frequency signal from PWM. After these period the research technology trended to design the HVDC model control to represent the device analysis in difference operation as in the [7].

Consequently, the HVDC transmission line deserves to usage in power system without any constrains.

3. HVDC configurations and Mathematical Models3.1 Configurations of HVDC

The diagram of a basic HVDC system is shown in Fig.1. This is a simple system with two converters C1

and C2, and one DC line. Based on the polarity of DC lines, HVDC systems are classified as:

a. Monopolar linksb. bipolar links c. Homopolar links

The basic configuration of monopolar link is shown in fig.2 below. It is has only one DC line that normally has negative polarity and uses the ground as the current return path. The monopolar system is mainly implemented to reduce the cost of line construction .sometimes instead of ground return, a metallic return maybe used in situations where the earth resistivity is too high or possible to interference with

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MMMMetallic return

-

underground/underwater metallic structures is objectionable. The return ground maybe carry at low voltage[8]. This type is the first stage in the development of bipolar link system.

Fig.1 HVDC basic diagram

Fig.2 Monopolar HVDC linkBipolar systems can eliminate the problems of first type by using two DC lines one positive and the other negative. In Fig.1, the converter has a single bridge. Multibridge converters, comprising several bridges connected in series, can increase the line voltage and reduce harmonics. Multibridge converters can use bipolar and homopolar connections. No need here to add the diagram of bipolar HVDC link, because the illustration above is clear to understand it.

The main equipment in a converter station includes converters, converter transformers, smoothing reactors, AC filters, DC filters, reactive power compensation devices, and circuit breakers. The converter is to transfer energy

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bnbnbVd1 bnbnbVd2

between AC and DC. Converting AC to DC is implemented by rectifiers while converting DC to AC is by inverters. The main element in a converter is the valve. Modern HVDC systems use thyristors as converter valves. Thyristors are usually rated 3–5 kV in voltage and 2.5–3 kA in current[5]. Due to the limited ratings, converters usually consist of one or more converter bridges connected in series or parallel. The DC terminals of a converter connect to DC lines and the AC terminals to AC lines. The converter transformer is a conventional transformer with on-load tap changers. The turns-ratio of the converter transformer can then vary to manage the converter operation. The ‘‘DC side’’ of the converter transformer is usually delta or Y connected with ungrounded neutral, so that the DC line can have an independent voltage reference relative to the AC network. Harmonic voltages and currents arise during converter operation. Harmonics deteriorate the power quality, interfere with wireless communication, and should be filtered out with filters having appropriate parameters. The inductance of the smoothing reactor is very large and can reach 1 H. Its main function is to reduce the harmonic voltages and currents on the DC lines, to prevent commutation failure of inverters, to maintain continuous current under light loading, and to curtail short-circuit current in converters during faults. Converters consume a great amount of reactive power in operation. The reactive power in steady operation can be 50% of the real power transmitted on the DC lines, with much more consumption during system transients. Reactive power compensation near the converters is used to provide the reactive power source for converter operation.

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HVDC is used to convert energy from AC to DC, to transport the energy as DC, and to convert it back from DC to AC. When AC system1 transports energy to AC system2

through DC lines, C1 runs in rectifying mode and C2 in inverting mode. Hence C1 can be seen as source and C2 as load. Given the resistance R of DC line, the line current is:I d=

V d 1−V d2

R (1)

The power transport in the DC line is pure reactive power and can be calculated as below:Pd1=V d1 I d=

V d12 −V d 1V d 2

R∧¿ (2)

Pd2=V d2 I d=V d2

2 −V d1V d 2

R (3)

The rectifier and inverter voltages are given as[9]V d1=n(

3√2V Lr

πcosα−

3 XcrπI d) (4)

V d 2=n(3√2V Li

πcosγ−

3 Xciπ

Id) (5)Where= n is a number of series connected bridge.V Lr ,V Li = line to line AC voltages at the rectifier and inverter bridges, respectively.αAnd γ = rectifier and inverter firing angles, respectively.Pd1 is a power sent out from C1 converter and Pd2 is received power at C2 converter. Note that the DC voltage Vd2 of converter C2 has the opposite direction to the DC current Id. If Vd1 is greater than Vd2, there will be a DC current going through DC line as indicated in (1). The adjustment of DC voltage can control the quantity of power transported on the line. It is important to point out that if the polarity of Vd2 remains unchanged, a higher Vd2 than Vd1 cannot make power flow from C2 to C1. The current in (1) cannot be negative since converters only allow current to flow in one

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direction. To change the power flow direction, the polarities of both converters at the two terminals should be reversed at the same time by the converter control, making C1 operate as an inverter and C2 as a rectifier. The adjustment of DC voltage is achieved through the control of the firing angle of the converter bridge instead of the voltage magnitude of the AC systems. Normal operating ranges of control angles is:

αmin=50 , αmax=(15±3)0 , γmin=150

The adjustment ranges of DC voltage are much greater than AC. HVDC can provide a high power carrying capacity over long distance without any stability constraints, while an AC system would face more difficulties under the same situation. The HVDC control uses electronics to achieve rapid control action. During system transients, fast and large changes of transmission power result in system frequency variation, while generators in the AC system do not take up all the power imbalances. For example, increasing the power transmission will lower the frequency in AC system1 and raise the frequency in AC system2. This converts the rotating kinetic energy in AC system 1 into electric energy and passes it to AC system2. Eventually, the frequency control devices in AC system1 will trigger output increases of generators in the system to restore the frequency. The fast response of HVDC is therefore very important when AC system2 requires emergency power support.

The power system using the HVDC to transport power to the other side does maybe have some assumptions to do prefect transportation as:1. Symmetrical AC system without frequency harmonics.

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2. Constant DC current without fluctuations.3. Convertor transformer id ideal.

The basic operation of the HVDC is depending on the converter equations when is rectifier or inverter according to the send side and receive side of power system such as the value of firing angles which are limited the DC voltage control between +ve and –ve. When α is varied as:Maximum DC voltage when α=00

Rectifier operation when 0<α<900

Inverter operation when 90<α<1800

3.2 Convertors equations Consider the 3 phase full wave bridge in fig.3 below, the equations are given with based on the general assumptions, the AC system (including converting transformer) consists of an ideal voltage source having constant frequency and voltage and a series connected reactance Lc. The instantaneous voltage of the ideal voltage source is:

Fig.3 three phase full wave bridge

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ea=Em cos (ωt+ π3)

eb=Em cos (ωt−π3

) (6) ec=Emcos (ωt−π )

Then the phase to phase voltages are given by:eac¿ea−ec=√3 Emcos (ωt+ π

6)

eba=eb−ea=√3Emcos (ωt−π2

) (7) ecb=¿ ec−eb=√3Em cos (ωt+ 5 π

6)¿

The DC voltage through the HVDC lines is depending on the thyristors firing angle, and the general equation to calculate the average DC voltage (Vd0) converter from the AC voltage as:V d 0=

12π∫0

2π

V d dθ=3 √6πE (8)

Where E is rms voltage. In this case the firing angle is zero. If there is shift in the firing angle the Vd is depending to the α as this function:V d=

62π ∫

−60+α

0+α

eacdθ=¿V d0cosα ¿ (9)

From the above equation, the average DC voltage Vd is less than Vd0 when a is not zero. When a increases from zero to 900, Vd decreases from Vd0 to zero; when a increases from 900 to 1800, Vd decreases from zero to -Vd0. When DC voltage becomes negative, the direction of DC current does not change due to the unidirectional valve characteristics. In this case, the product of DC voltage and current is negative, i.e., the power consumption from the AC system is negative. The real power actually flows from the DC system to the AC system under this operation mode. When a converter provides real power for the AC system, it converts DC energy into AC energy and passes the energy

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into the AC system[8]. If the phase angle difference between AC fundamental frequency voltage and current is the firing angle delayα. From the above analysis the complex power of the AC system is:P+ jQ=3√6

πE I d(cosα+ jsinα ) (10)

Fig.3 above if we added in the end of it, some filter circuits and middle ground DC sources , then use EMTDC/PSCAD software to analysis the operation of the IGBT firing and the output in two side as in fig.4 below. All figures below are showed the output of the converter operated as rectifier and inverter. The firing pulses of thyristor are generated by using PWM model.

Fig.4 example model

Fig.5 AC current

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Fig.6 contains (DC voltage, DC current, AC power, AC reactive power and AC rms (base voltage is 1KV))

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Fig.7 phase to phase AC voltage

Fig.8 , 3 phases AC voltage

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Fig.9 thyristors firing pluses

As we see in the above figures the HVDC is depending totally on the converter and the efficiency also so good when the converters in the two sides are operated well. So that sometimes it used multi-converters to do the converter of power in high accuracy and convenient reliability. When we used HVDC in the power transmission line the power flow equations must included the HVDC equations which, contains the converters, filters and transformer.

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4. Power flow equations including HVDC transmission

We identify a bus as a DC bus or AC bus based on whether it is connected with a converter transformer or not. The bus connecting to the primary side of a converter transformer is called a DC bus. The bus having voltage V t

is a DC bus. Otherwise a bus without connected to converter transformer is pure AC bus. Assume that the number of total buses n and the number of converter nc, the AC buses na=n- nc. The basic principle of building power flow models for AC/DC interconnected systems is as follows. First use the extraction and injection power, P tdc + jQtdc of converter transformers at DC buses to represent converter transformers and the DC system behind them. So that power flow equations are contained two types, AC system equations and DC system equations including the converters equations[5]. These equations are given as General AC node power equations:∆ Pi=P is−V i∑

j∈i

na

V j (Gij cosθij+B ijsin θij )=0

(11)∆Qi=P is−V i∑

j∈ i

na

V j (G ijsin θij−Bijcosθ ij)=0

Note that j in the above equation can be an AC bus as well as a DC bus. For a DC bus, suppose that converter transformer numbered k connects to bus i. The complex power extracted from the bus is:Pidc+ jQidc=V i I i ( cosφk+ jsinφk ) (12)Where=φk is a bus angle. If we considered the converter transformer ratio KTk and inverter ratio K γ , the equ (12) becomes as Pidc+ jQidc=K γKTkV i I dk (cosφk+ jsinφk ) (13)

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Assumed that there are ideal filters on both AC and DC sides, so that harmonic power is zero. And also neglecting the converter power loss. The equ (13) has other expression as Pidc=V dk I dk¿Qidc=V dk I dk tg φk (14)Then the equations of power flow are written as:∆ Pi=P is−V i∑

j∈i

na

V j(Gijcosθ ij+Bijsin θ ij)±V dk I dk¿

¿

(15)∆Qi=P is−V i∑

j∈ i

na

V j(Gijsinθij−Bij cosθij)±V dk I dk tg φk

Where : i=na+k and k=1,2,3… ,nc

The equation (15) is represented the power flow of the HVDC then it solved by using the power flow methods solution such as Newton- Raphson or Gauses- Seidel methods.

5. Example study of HVDCConsider Monopolar HVDC system with two areas

system the area one is sending power to the second area through the HVDC transmission line which, is based on VSC using 12-pluse Bridge. The rectifier control power as a function of phase angle different between the sending end and receiving end voltages. The sending voltage is control by the exciter of the generator and the PWM carries wave frequency is higher than the fundamental frequency. However the inverter in the receive end is control the AC and DC voltage magnitude using the same PWM of sending end side. In this case study it’s concerned to normal and abnormal operation. Abnormal operation is represented by a fault in the receiving end bus bar at 0.2s and clearing at 0.3s. The simulation time is 0.5s. Fig.10

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shows the example study of HVDC from PSCAD library example[10]. The outputs are concerned to show the AC voltage and current and DC voltage and current. This aims to show the rectifier side and inverter side. The outputs in two cases are showed in figures below.

Fig.10 Example study of Monopolar HVDC system.

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Fig.11 Rectifier side AC voltage and DC voltage and current in normal operation.

Fig.12 Inverter side AC voltage and DC voltage and current in normal operation.

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Fig.13 Rectifier waves in abnormal mode.

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Fig.14 Inverter waves in the abnormal mode.As in the figures above the AC and DC voltage of

inverter side is affected by the fault than the rectifier side because the fault in the inverter bus bar side and as soon as the system reached the stability state in small time and the recovery voltage is less or near to zero. This indicates that the HVDC is improving the dynamic operation of power system.

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6. ConclusionsFrom the above topics the HVDC is mature to use in

power system especially in smart power grid, so that the HVDC systems have offer many advantages to the power system transmission lines as:

1. These systems are economical for long distance bulk power transmission by overhead lines.

2. The power per conductor is greater and simpler line construction.

3. Ground return is possible.4. There is no charging current and skin affect.5. There is easy reversibility and controllability of

power flow through a DC link.6. Line losses are smaller.7. No reactive compensation of DC lines is required.8. Low SC current is required on DC line.9. DC cables can be worked at higher voltage gradient.10. Corona loss and radio interference are less as

compared to AC.11.Each conductor can be operated as an independent

circuit.12.The voltage regulation problem is much less for DC,

since only ohm drop is involved. That means steady state stability is no longer a major problem.

13.There is no technical limit to the distance over power transmitted.

The disadvantages of the HVDC are much less as compared with advantages. Furthermore, some of these disadvantages are solved by the new techniques in power electronics. So that the techniques research are effort to completely solve the HVDC problems.

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Considerable research and the development work is under way to provide better performance of the HVDC links to achieve more efficient and economic designs of converters, and future power systems would be more used HVDC transmission. And also the multi-terminal DC (MTDC) systems will play even greater role in the power systems in the future. 7. References

1. Daochun Huang, S.M.I., J.R. Yinbiao Shu, and Y. Hu, Ultra High Voltage Transmission in China: Developments, Current Status and Future Prospects. Proceedings of the IEEE, 2009. Vol. 97, No. 3: p. pp. 555-583.

2. Zhang Ruihua, Du Yumei, and L. Yuhong, New Challenges to Power System Planning and Operation of Smart Grid Development in China. IEEE, 2010. IEEE, 2010 International Conference on Power System Technology: p. (8).

3. Jiebei Zhu and C. Booth, Future Multi-Terminal HVDC Transmission Systems using Voltage Source Converters. UPEC2010 ,31st Aug - 3rd Sept 2010, 2010.

4. Kala Meah and S. Ula, Comparative Evaluation of HVDC and HVAC Transmission Systems. IEEE, 2007: p. (5).

5. Xi-Fan Wang, Yonghua Song , and M. Irving, Modern Power Systems Analysis. BOOK, 2008.

6. Hanif. Livani, M.B., S.L. Yosef. Alinejad, and H. Karimi-Davijani, Improvement of Fault Ride-Through Capability in Wind Farms Using VSC-HVDC. European Journal of Scientific Research, 2009. Vol.28 No.3 (2009): p. (10),pp.328-337.

7. Juan Manuel Mauricio and A.G.e. Exp·osito, Modeling and Control of an HVDC-VSC Transmission System. IEEE, 2006. PES Transmission and Distribution Conference and Exposition Latin America, Venezuela: p. (6),pp.(1-6).

8. KUNDUR, P., power system stability and control. Book, McGraw-Hill,Inc. Edited by:Neal J. Balu and Mark G. Lauby.

9. I J NAGRATH and D.P. KOTHARI, Power System Engineering. book, Copyright 1994,Tata McGraw-Hill publishing Company Limited. , 1994. Third reprint.

10. M. Szechtman, T. Wess, and C.V. Thio, First Benchmark Model for HVDC Control Studies. 1991. Electra, No. 135, April 1991

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