The development of transmission. HVdc transmission in India. network bottlenecks. Quite a few HVdc transmission projects have been constructed

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1 HVdc transmission in India Skyline courtesy of Wikimedia Commons/Cididity Hat G.D. Kamalapur, V.R. Sheelavant, Sabeena Hyderabad, Ankita Pujar, Saptarshi Bakshi, and Amruta Patil The development of transmission systems closely follows the growing demand on electrical energy. With the increasing size and complexity of transmission networks, the performance of power systems decreases due to problems related to load flow, power oscillations, and voltage quality. Flexible ac transmission systems (FACTS) and high-voltage direct current (HVdc) technologies offer some effective schemes to meet these demands. In recent years, HVdc technology has been considered as one feasible planning alternative in India to increase power grid delivery capability and remove identified Digital Object Identifier 1.119/MPOT Date of publication: 7 January 214 network bottlenecks. Quite a few HVdc transmission projects have been constructed or planned. The interconnection of power systems offers benefits for power transmission, such as pooling of various energy resources, the reduction of reserve capacity in the systems, and increasing the transmission efficiency. However, if the size of the system is too large, dynamic problems can occur that could jeopardize the reliability and availability of the synchronous operation of the interconnected grids. Establishing a desired power condition at the given points are best achieved using power controllers such as FACTS devices and HVdc. HVdc is used to transmit large amounts of power over long distances. The factors to be considered are cost, /14/$ IEEE IEEE POTENTIALS

2 technical performance, and reliability. A FACTS is composed of static equipment used for the ac transmission of electrical energy. FACTS and HVdc technologies offer some effective schemes to meet these demands. FACTS are an effective means of managing power flows. HVdc technologies can also be applied to the power system to improve system reliability. HVdc offers independent frequency and control, lower line cost, power control, voltage control, and stability control. FACTS and HVdc are complementary technologies. An HVdc electric power transmission system uses dc for the bulk transmission of electrical power. Developments in polyphase circuits, the availability of the transformers, the utilization of induction motors, and other ac machines have led to extensive transmission and distribution networks. Advantages of HVdc over HVac Beyond the breakeven distance, HVdc has the ability to transmit large amounts of power with lower capital costs and lower losses than ac. HVdc can carry more power per conductor. The power delivered in an ac system is defined by the root mean square (RMS) of an ac voltage, but RMS is only 7.7% of the peak voltage. The peak voltage of ac determines the actual insulation thickness and conductor spacing. HVdc operates at a constant maximum voltage, with equally sized conductors and insulation to carry more power into an area. For ac used cable transmission, additional current must flow in the cable to charge the cable capacitance, which generates additional losses in the conductors of the cable. There is also a dielectric loss component contributing to power loss. For dc, the cable capacitance is charged only when the cable is first energized or when the voltage is changed; there is no steady-state additional current required. HVdc does not suffer from the skin effect; hence it needs fewer, thinner conductors. Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install. Connecting a remote generating plant to the distribution grid and power transmission and stabilization between unsynchronised ac distribution systems. Stabilizing a predominantly ac power-grid, without increasing prospective short circuit current. Both ac and dc transmission lines can generate coronas, the former case in the form of oscillating particles, in the latter, a constant wind. Due to the space charge formed around the conductors, an HVdc system may have about half the loss per unit length of a HVac system carrying the same amount of power. Tie-line power is easily controlled. Less radio interference, especially in foul weather, for a certain conductor diameter and rms voltage. Reduction of transients and disturbances increases the system stability. This prevents cascading failures from propagating from one part of a wider power transmission grid to another. Synchronous operation is not required in HVdc. The magnitude and direction of power flow through a dc link can be directly commanded and changed as needed to support the ac networks at either end of the dc link. Cables can be worked at a higher voltage gradient, and the line power factor is always unity. The frequency and the intermediate reactive components cause stability problems in the ac line. On the other hand, HVdc transmission does not have the stability problem because of the absence of the frequency. HVdc needs fewer conductors, as there is no need to support multiple phases. This reduces the line cost. Other advantages include simpler line construction, ground as a return path, and each conductor operated as an independent circuit. Distance is not limited by stability and low short-circuits current. Limitations of HVdc The key factors of HVdc are in conversion, switching, control, availability, and maintenance. The scope of application is limited by the following factors: Converters are expensive and require much reactive power. They generate harmonic, hence ac and dc filters are required. The difficulty of breaking dc currents results in the high cost of dc breakers. Multiterminal or network operation is not easy. An inability to use transformers to change the voltage levels. HVdc circuit breakers are difficult to build. Complexity of control. Types of HVdc Monopole, Ground Return Monopole, Metallic Return Fig. 1 Monopole HVDC. Monopole and earth return (Fig. 1) In monopole, one of the terminals of the rectifier is connected to earth ground. The other terminal, at a potential high above or below ground, is connected to a transmission line. If no metallic conductor is installed, current flows in the earth between the earth electrodes at the two stations. One terminal of the converters is connected to earth; the return conductor need not be insulated for the full transmission voltage, which makes it less costly than the HV conductor. Monopole earth return suffers with electrochemical corrosion of long-buried metal objects like pipelines. Bipolar (Fig. 2) In bipolar transmission, a pair of conductors is used, each at a high potential with respect to ground, in opposite polarity. Since these conductors must be insulated for the full voltage, transmission line cost is higher than a monopole with a return conductor. However, there are many advantages to bipolar transmission, which can make it an attractive option. Bipole Capacity Up to Appr. 3, MW Fig. 2 Bipolar HVdc. January/February

3 Back-to-Back Capacity Up to Appr. 1, MW Fig. 3 Back-to-back HVdc. In a homopolar type of link, two conductors having the same polarity can be operated with ground or metallic return. A homopolar link has the advantage of reduced insulation costs, but the disadvantages of earth return outweigh the advantages. Back to back (Fig. 3) Back-to-back HVdc technology enables the interconnection of two asynchronous ac networks. An HVdc system takes electrical power in an ac system and converts it into high-voltage dc using a converter station. It then transmits the dc to a remote system, where it is converted back again to ac by another HVdc converter station. A back-to-back HVdc arrangement is used when two asynchronous ac systems need to be interconnected for bulk power transmission or for ac system stabilization reasons. Back-to-back improves the voltage regulation, system stability, and contribute for effective load flow analysis. HVdc in India In recent years, HVdc transmission has been considered a feasible planning alternative in India to increase power grid delivery and capability and remove identified network bottlenecks. India is one of the few countries that has a large number of HVdc schemes in operation, under commissioning, construction, and planning (Fig. 4). Quite a few HVdc transmission projects have been constructed (Table 1) or planned. There are a couple of HVdc links under active consideration for implementation within three to six- years, along with interlinking with the national grids of neighboring countries, namely Sri Lanka, Bangladesh, Bhutan, and Nepal. Simulation of HVdc back to back An HVdc transmission system consists of three basic parts: 1) a converter station to convert ac to dc, 2) a transmission line, and 3) a second converter station to convert back to ac (Fig. 5). HVdc transmission systems can be configured in many ways on the basis of cost, flexibility, and operational requirements. The simplest one is the back-to-back interconnection, and it has two converters on the same site with no transmission line. This type of connection is used as an intertie between two different ac transmission systems. The monopolar link connects two converter stations by a single conductor line and earth or sea is used as a return path. The multiterminal HVdc transmission systems have more than two converter stations, which could be connected in series or parallel. Among the various components and subsystems existing at an HVdc station, the HVdc transmission line and the converter transformer are the major components that have a significant impact on the total reliability of the HVdc system. Converter transformers are located on either ends of the HVdc transmission line. The transformers used in HVdc have different requirements due to superimposed dc voltage and current. Table 1. HVdc transmission lines existing in India (Reference: Central Electrical Authority of India). a) Bipole line!5 kv Circuit kilometers Chandrapur-Padghe (1999) 1,54 Rihand-Dadri (199) 1,634 Talcher-Kolar (22) 2,738 Padghe North Delhi Sasaram Rihand Vindhyachal East West Barsur Talchar Chandrapur NHVDC Ramagundam Lower Sileru South Bangalore Kolar Fig. 4 HVdc transmission in India. Vishakapatnam North East Long Distance Under Construction Long Distance Implemented Back-to-Back Under Construction Back-to-Back Implemented Balia-Bhiwadi (29) 1,8 Biswanath-Agra (214) 3,6 b) Bipole transmission capacity MW Chandrapur-Padghe (1999) 1,5 Rihand-Dadri (199) 1,5 Talcher-Kolar (22) 2,5 Balia-Bhiwadi (29) 2,5 Biswanath-Agra (214) 4, c) Back-to-back transmission capacity MW Vindhachal (1989) 5 Chandrapur (1999) 1, Gazuwake (29) 1, Sasaram (22) 5 Vizag (199) 5 d) Monopole line Barsur-Lower Sileru (2) 162 circuit kilometer e) Monopole transmission capacity Barsur-Lower Sileru (2) IEEE POTENTIALS

4 C In recent years, HVdc technology has been considered as one feasible planning alternative in India to increase power grid delivery capability and remove identified network bottlenecks. Conn7 Conn8 Conn9 Transformer 2 B A 11 kv, 5 Hz 1, MVA.5 H dc Transmission Line.5 H 11 kv, 5 Hz 1, MVA ac 1 ac 2 Control for Converter Control for Converter Fig. 5 An HVdc back-to-back circuit diagram. In1 In2 Conn8 Conn9 1 Transformer1 Conn7 Conn8 Converter Pi Section Line Out1 Out2 Control for Converter Discrete, s = 5e-5 s Powergui Conn7 In1 In2 Inverter Out1 Out2 Control for Inverter A B C Fig. 6 An HVdc Simulink diagram. January/February

5 Table 2. Simulation parameters with results. Simulation Parameters Rectifier end ac system 1 (SCR = 5) Inverter end ac system 2 (SCR = 3) R = 26.7 X L1 = mh L2 = 98.3 mh R = 2.56 X L1 = mh L2 = mh DC line parameters Rdc =.15 X/km L =.792 mh/km C = 14.4 nf/km Simulation Results Calculated kv Simulated kv Converter output voltage 1,17.2 1,23 7.6% % Variation No-load voltage 1, , % Input voltage to the inverter 912 1, % (kv) Time (s).4 Fig. 7 The converter output voltage..5 Table 3. regulation of HVdc. Distance (km) Load HVdc No-Load Regulation , , , Table 4. regulation of HVac. Distance (km) Load HVac No-Load , , , , , , , , , Regulation Table 5. Fault current and peak overshoot voltage. Distance (km) HVdc Fault Current (ka) , , , Peak Overshoot (MV) (kv) Time (s) Fig. 8 The input to the inverter..4.5 Converter transformers designed for 12 pulse rectifiers have three windings. They are single phase three winding transformers. A bank of three transformers will be used for a 12-pulse converter. Out of the three windings of a converter transformer, one of the windings is connected to the ac network, and the other two are connected to a converter bridge, i.e., one connected in delta and the other in star. As an important component of an HVdc system, the converter transformer is responsible for the stable and reliable power transmission. The back-to-back system was designed as shown in Fig. 6. The HVdc system modeled, using the Simulink, is based on a point-to-point dc transmission system. The dc system is a monopolar, 12-pulse converter using two universal bridges connected in series, rated 1, MW (2 ka, 5 kv) at the inverter. DC interconnection is used to transmit power from a 5 kv, 5, MVA, and 6 Hz network to a 345 kv, 3, MVA, and 5 Hz network. The converters are interconnected through a 3-km transmission line and a.5-h smoothing reactor. The converter transformer is modeled with a three-phase, three-winding transformer. The ac networks, both at the rectifier and inverter end, are modeled as infinite sources separated from their respective commutating buses by system impedances. The simulation parameters, results, and fault analysis are shown in Table 2 5 and Figs Based on a simulation, the breakeven distance of overhead lines of HVdc is found to be 84 km, compared with HVac (Figs. 9 1). Back-to-back technology allows for the two-way flow of electricity, while acting as firewall to isolate disturbances. HVdc transmission does not have the stability problem due to the absence of the frequency. The cost per unit length of an HVdc line is lower than that of an HVac line of the same power capability and comparable reliability, but the cost of the terminal equipment of an HVdc line is much higher than that of an HVac line. Conclusion This article has demonstrated the methodology for modeling an HVdc transmission system rectifier using universally available software Simulink. It focuses on some of the guidelines regarding the areas of applicability of HVdc in India. Particular attention has been paid to back-to-back modeling. With many attractive features, HVdc technology will be more widely considered as a transmission expansion 26 IEEE POTENTIALS

6 option in deregulated energy markets. Future work may be focused on a real-time HVdc analysis in Indian conditions. Read more about it A. L Abbate, G. Migliavacca, U. Hager, C. Rehtanz, S. Ruberg, H. Ferreira, G. Fulli, and A. Purvins, The role of facts and HVDC in the future pan- European transmission system development, in Proc. 9th IET Int. Conf. AC DC Power Transmission, Milan, Italy, Oct , 21, pp A. L Abbate and G. Fulli, Modeling and application of VSC-HVDC in the European transmission system, Int. J. Innov. Energy Syst. Power, vol. 5, no. 1, pp. 8 16, Apr. 21. M. Ramesh and A. J. Laxmi, Stabilty of power transmission capability of HVDC system using facts controllers, in Proc. Int. Conf. Computer Communication Informatics, 212, pp W. Long and S. Nilsson. (27). HVDC transmission: Yesterday and today. IEEE Power Energy Mag. [Online]. 5(2), pp Available: jsp?isnumber= K. Meah and S. Ula, Comparative evaluation of HVDC and HVAC transmission systems, in Proc. Conf. Power Engineering Society General Meeting, June 24 28, 27, pp (kv) A. Kumar, D. Wu, and R. Hartings, Experience from first 8 kv HVDC test installation, in Proc. Int. Conf. Power Systems, Bangalore, India, Dec , 27, pp J. Arrillaga, High voltage direct current transmission, in IEE Power and Energy Series 29, 2nd ed. London, U.K.: The Institution of Electrical Engineers, S. Mukhopadhyay, Towards electricity for all, IEEE Power Energy Mag., vol. 5, no. 5, pp Sept. Oct. 27. V. K. Prasher, D. Kumar, C. Bartzsch, V. Hartmann, and A. Mukherjee, HVDC East-South interconnector II in India: 2 MW, +/-5 kv, in Proc. 7th Int. Conf. AC-DC Power Transmission, Nov. 28 3, 21, pp ,2 1, Distance (km) 9 HVAC HVDC A. Singhal, R. Gera, A. K. Tripathy, T. Adhikari, M. Hanif, K. S. Prakash, and R. L. Das, Design aspects of upgradation from 6 pulse to 12 pulse operation of NHVDC project, in Proc. Int. Conf. Power Electronics, Drives Energy Systems for Industrial Growth, 1996, vol. 2, pp , 1,1 1,2 Fig. 9 The variation of HVac and HVdc with transmission line distance. Regulation Distance (km) 1, HVAC HVDC 1,1 1,2 Fig. 1 The voltage regulation of HVac and HVdc transmission line distance. With many attractive features, HVdc technology will be more widely considered as a transmission expansion option in deregulated energy markets. R. N. Nayak, R. P. Sasmal, Y. K. Sehgal, and S. Mukoo, AC/ DC interactions in multi-in feed HVDC scheme: A case study, in Proc. IEEE Power India Conf., New Delhi, India, June 26, pp A. Tyagi and K. R. Padiyar, Dynamic analysis and simulation of a VSC based back-toback HVDC link, in Proc. 3rd Int. Conf. Power Electronics, Chennai, India, Dec , 26, pp R. D. Begamudre, High Engineering Problems and Solutions, 1st ed. New Delhi, India: New Age International Publishers, 21, pp About the authors G.D. Kamalapur (gdkpur9@ gmail.com) is a professor in the Department of Electrical Engineering at Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. He earned his B.E. (electrical) degree from Karnataka University, Dharwad, and his M.E (control systems) degree from Walchand college of Engineering, Sangli. V.R. Sheelavant (sheel125@ gmail.com) is an assistant professor in the Department of Electrical Engineering at Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. He earned his B.E. (electrical) degree from Karnataka University, Dharwad, and his M.Tech (power systems) degree from the College of Engineering, Pune. Sabeena Hyderabad (sabeena.hyd@ gmail.com) earned a B.E. degree from the Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. Ankita Pujar (ankita.pujar@gmail. com) earned a B.E. degree from the Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. Saptarshi Bakshi (rishialone26@ gmail.com) earned a B.E. degree from the Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. Amruta Patil (29amrutapatil@gmail. com) earned a B.E. degree from the Sri Dharmasthala Manjuntheshwar College of Engineering and Technology, Dharwad. January/February