Reduce Power Transfer Loss in Transmission Line by Integrating AC & DC Transmission

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1 Reduce Power Transfer Loss in Transmission Line by Integrating AC & DC Transmission Alok Kumar 1, Surya Prakash 2, 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, Mat- Lab, Simultaneous ac-dc power transmission 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 ISSN: Page 2491

2 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. 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 III. 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 ISSN: Page 2492

3 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 (8) Qdr = Pdr tanθr (9) CosθI = (cos γ + cos (γ + μi) ) / 2 (10) Cosθr = (cos α + cos (α + μr) ) / 2 (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: 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) < 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 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) < 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(1- x2)) / (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. ISSN: Page 2493

4 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) Pac(MW) Pdc(MW) Pac loss (MW) Pdc loss (MW) PR (MW) V. AC & INTEGRATING AC DC MODEL Power Angle AC Current(kA) DC Current(kA) AC Power(MW) DC Power(MW) Total Power(MW) Figure.3 Diagram for AC transmission system ISSN: Page 2494

5 Fig.6 Sending and receiving currents Fig.7 Integrating AC-DC current Figur.4 Diagram for integrating AC & DC transmission system TABLE 1 Computed Results VI. RESULT 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 TABLE 2 Simulation Result Fig.5 Sending end and receiving end voltages 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. 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 ISSN: Page 2495

6 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. References: 1. L. K. Gyugyi, Unified power flow concept for flexible A.C. transmission system, Proc. Inst. Elect. Eng., p. 323, Jul L. K. Gyugyi et al., The unified power flow controller; a new approach to power transmission control, IEEE Trans. Power Del., vol. 10, no. 2, pp , Apr N. G.Hingorani, FACTS flexible A.C. transmission system, in Proc. Inst. Elect. Eng. 5th. Int. Conf. A.C. D.C. Power Transmission, London, U.K., P. S. Kundur, Power System Stability and Control. New York: Mc-Graw-Hill, K. P. Basu and B. H. Khan, Simultaneous ac-dc power transmission, Inst. Eng. (India) J.-EL, vol. 82, pp , Jun H. Rahman and B. H. Khan, Enhanced power transfer by simultaneous transmission of AC-DC: a new FACTS concept, in Proc. Inst. Elect. Eng. Conf. Power Electronics, Machines, Drives, Edinburgh, U.K., Mar. 31 Apr , vol. 1, pp A. Clerici, L. Paris, and P. Danfors, HVDC conversion of HVAC line to provide substantial power upgrading, IEEE Trans. Power Del., vol. 6, no. 1, pp , Jan Stella M., Dash P. K., and Basu K. P. A neurosliding mode controller for STATCOM, Elect. Power Compon. Syst., vol. 32, pp , Feb Szechtman M., Wees T., and C. V. Thio C. V., First benchmark model for HVDC control studies, Electra, no. 135, pp , Apr Sen K. K., SSSC Static Synchronous Series Compensator: Theory, Modelling, and Applications, IEEE Transactions on Power Delivery, Vol. 13, No. 1, January 1998, pp A. Clerici, L. Paris, and P. Danfors, HVDC conversion of HVAC line to provide substantial power upgrading, IEEE Trans. Power Del., vol. 6, no. 1, pp , Jan E. W. Kimbark, Direct Current Transmission. New York: Wiley, 1971, vol. I. 13. H. Rahman H. and Khan B H Stability Improvement of Power System by Simultaneous AC-DC Power Transmission Electric Power System Research Journal, Elsevier, Paper Editorial ID No. EPSRD , Press Article No. EPSR-2560 Digital Object. 14. Kimbark I W. Direct Current Transmission Vol- I. Wiley, New York, Hatziadoniu C. J. and Funk A. T., Development of a Control Scheme for Series- Connected Solid- State Synchronous Voltage Source, IEEE Transactions on Power Delivery, Vol. 11, No. 2, April 1996, pp Kimbark I W. Direct Current Transmission Vol- I. Wiley, New York, Liu Y. H., Zhang R. H., Arrillaga J., and Watson N. R., An Overview of Self-Commutating Converters and Their Application in Transmission and Distribution, 2005 IEEE/PES Transmission and Distribution Conference and Exhibition: Asia and Pacific, Dalian, China, Padiyar K. R., Pai M. A., and Radhakrishna C., Analysis of D.C. link control for system stabilization, in Proc. Inst. Elect. Eng. Conf. Publ. No. 205, London, U.K., PSCAD/EMTDC, User s Guide, Manitoba- HVDC Research Centre. Winnipeg, MB, Canada, Jan [21] Padiyar K.R. HVDC Power Transmission System. Wiley Eastern, New Delhi, 1993). 20. Litzenberger Wayne H., (ed.), An Annotated Bibliography of High-Voltage Direct-Current Transmission and Flexible AC Transmission (FACTS) Devices, Portland, OR, USA: Bonneville Power Administration and Western Area Power Administration, 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 Presently he is working as Asst. Prof. in Electrical Engg. Dept. SSET, SHIATS (Formerly Allahabad Agriculture Institute, Allahabad- India). His field of interest includes power system operation & control, Artificial Intelligent control sprakashgiri0571@yahoo.com ISSN: Page 2496

7 Alok kumar belongs to Allahabad, He obtained his M.Tech. in Electrical Engg.(Power System) from SHIATS Deemed University, Allahabad UP-India in Presently he is doing P.hd from CMJ University, Shillong Meghalaya- India. His field of interest HVDC Transmission Line, ISSN: Page 2497