ISSN 2319-8885 Vol.03,Issue.15 July-2014, Pages:3223-3227 www.semargroup.org, www.ijsetr.com

Design Implementation of 250 kV HVDC Overhead Transmission System 1 2 PHYU WIN WIN AI , THET TIN 1Dept of Electrical , Mandalay Technological University, Mandalay, , E-mail:[email protected]. 2Dept of Electrical Power Engineering, Mandalay Technological University, Mandalay, Myanmar.

Abstract: Most of the high transmission in the world is in the form of (HVAC). Since the development of the , AC power can be generated, transmitted, distributed and used at different and convenient . However, with the proper equipment, AC can be converted to DC electricity. The thyristor or silicon controlled rectifier (SCR) valves make the conversion from AC to DC and thus are the main component of any HVDC converter. Therefore, in this paper, focus is made the thyristor or silicon-controlled rectifier (SCR)s based HVDC system. In this paper, HVDC system design is considered and then the shweli-shwesaryan 110 miles, 250kV HVDC overhead transmission system is implementation designed.

Keywords: CSC, Thyristor, Design Criteria, HVDC, Bipolar.

I. INTRODUCTION A. Converters High voltage DC (HVDC) Transmission system consists of A HVDC system requires an electronic converter for its three basic parts: ability of converting electrical energy from AC-DC or vice  converter station to convert AC to DC versa. There are basically two configuration types of three-  transmission line phase converters possible for this conversion process (Fig. 2):  second converter station to convert back to AC.  Current Source Converter (CSC), and HVDC transmission systems can be configured in many  Voltage Source Converter (VSC). ways on the basis of cost, flexibility, and operational requirements. Alternating current (AC) is the main driving Converter 6-pulse Ld force in the industries and residential areas, but for the long Converter DC line transformer bridges transmission line (more than 400 miles) AC transmission is more expensive than that of direct current (DC). Technically, AC transmission line control is more complicated because of the frequency. By applying interconnections to the CB 1 neighboring systems, power systems have been extended to achieve technical and economical advantages. During their development, power systems become more and more 2 interconnected and heavily loaded. With the increasing size F and complexity of systems and as the result of the 11th,13th liberalization of the electrical markets, needs for innovative HP filters Electrode line applications and technical improvements of the grids will further increase. HVDC plays an important role for these tasks. Fig.1. Principle of HVDC transmission system [1].

II. PRINCIPLE OF HVDC TRANSMISSION SYSTEM Modern HVDC transmission systems can utilize either the A direct-voltage system is a hybrid circuit incorporation traditional Current Source Converter (CSC) or the Voltage AC and DC components. The incoming power is from an Source Converter (VSC) as the basic conversion workhorse. alternating source, which is rectified and filtered before The two converters are actually duals of one another. transmission through the DC system, inversion taking place However, the choice of which option is selected for a at the receiving end in order to provide the usual AC supply particular project is based upon economic and other factors. conditions. The principle of HVDC transmission system is At present VSC are still limited to below 250 MW capacities shown in Fig.1.

Copyright @ 2014 SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved. PHYU WIN WIN AI, THET TIN due to commercial and practical limitations of the electronic .

Fig.2. Converter of the CSC and VSC Types. Fig.4. Bipolar configuration [2]. A. Two-terminal HVDC Links The bipolar link as shown in Fig.4 has two conductors. Two-terminal HVDC links are sub-divided into four Each terminal has two sets of converters of identical ratings, types: in series on the HVDC side. The junction between the two  Monopolar link sets of the converters is grounded at one or both ends.  Bipolar link Normally, both poles operate at equal currents and hence  Homopolar link there is zero ground current flowing under these conditions.  Tripole link From the viewpoint of lighting performance, a bipolar HVDC line is considered to be similar to a double circuit HVAC In the monopolar link arrangement, as shown in Fig.3, there transmission line. is only one conductor, usually negative polarity and the ground or sea is used for the return path. The current flows between the earth electrodes at the two stations.

Fig.3. Monopolar Configuration [2].

Since one terminal of the converters is connected to earth, the return conductor need not be insulated for the full transmission voltage.

International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.15, July-2014, Pages: 3223-3227 Design Implementation of 250 kV HVDC Overhead Transmission System (straight run or corner, river crossing, etc.), material, number Generators of circuits, and geometry. The vertical orientation allows for Converter a more compact right-of-way (ROW), but it requires a taller tower. Surge Arrestors

Converters B. Minimum Clearances Clearances are specified for phase-to-tower, phase-to DC Reactor ground, and phase-to-phase. Phase-to-tower clearance for 500 kV ranges from about 10 to 17 feet, with 13 feet being DC Filter the most common specification. These distances are Surge Arrestors maintained by strings and must take into account possible swaying of the conductors. The typical phase-to- Electrode Line ground clearance is 30 to 40 feet. This clearance is maintained by setting the tower height, controlling the line Ground temperature to limit sag, and controlling vegetation and Electrode DC Line structures in the ROW. Typical phase-to-phase separation is also 30 to 40 feet and is controlled by tower geometry and line motion suppression.

Surge Arrestors C. Insulators Insulator design varies according to tower function. For DC Filter suspension towers (line of conductors is straight), the DC Reactor insulator assembly is called a suspension string. For deviation towers (the conductors change direction), the insulator Converter assembly is called a strain string. For 500-kV lines, the Surge Arrestors insulator strings are built up from individual porcelain disks Converter typically 5.75 inches thick and 10 inches in diameter. The Transformer full string is composed of 18 to 28 disks, providing a long path for stray currents to negotiate to reach ground. At this Shunt Capacitor voltage, two to four insulator strings are commonly used at AC Harmonic Filter each conductor connection point, often in a V pattern to limit Converter Breaker lateral sway.

Receiving AC System D. Lightning Protection Fig.5.DC Transmission system operating in bipolar mode. Since the towers are tall, well-grounded metallic structures, they are an easy target for lightning. To control III. HVDC TRANSMISSION SYSTEM DESIGN the effects of lightning, an extra set of wires is generally CONSIDERATION strung along the extreme top points of the towers. These The altitude range of transmission tower is a rough wires are attached directly to the towers (no insulation), surrogate for weather and terrain. This is important, since providing a path for the lightning directly to and through the nearly all aspects of line design, construction, and towers to the ground straps at the base of the towers. The environmental impacts are linked to weather. The design extra wires are called shield wires and are either steel or wind and ice loading on lines and towers is based on the aluminum-clad steel with a diameter of approximately ½ design load district. This affects insulator specifications as inch. well as tower dimensions, span lengths, tower design, and conductor mechanical strength and wind dampening. E. Selection of Converter Transformer Rating The RMS value of the transformer secondary current A. Tower Specifications (total and not just the fundamental frequency component) The towers support the conductors and provide physical IRMS is given by: and electrical isolation for energized lines. The minimum set T 2 1 2 of specifications for towers are the material of construction, ITRMS  i (t)dt (1) T type or geometry, span between towers, weight, number of 0 circuits, and circuit configuration. The type of tower refers to The alternating line-current wave consists of rectangular basic tower geometry. The options are lattice, pole (or pulses of amplitude Id and width 2π/3 rad. Therefore, monopole), H-frame, guyed-V, or guyed-Y. The span is  2  3 2 1 2 1 2 2 2 commonly expressed in the average number of towers per ITRMS   i (t)dt   Iddt  Id   3 mile. This value ranges from four to six towers per mile. The  2  3 weight of the tower varies substantially with height, duty And hence, International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.15, July-2014, Pages: 3223-3227 PHYU WIN WIN AI, THET TIN 2 ITRMS  Id (2) (6) 3 Where The RMS value of the line-to-neutral transformer Vin(x) = Interference voltage on the telephone line at point x secondary voltage is given by: (in mV/km)  H = Weighting factors which reflect the frequency ELN  Vdo (3) μ 3 6 dependence of the coupling between telephone and HVDC Transformer volt-ampere rating is given by: Three-phase lines rating = 3ELN ITRMS Cμ = “C message“ – weighting factors Iμ(x) = Resulting harmonic current of the ordinal number μ in the HVDC line at point x as the vector sum of the currents (4) caused by the two HVDC stations Ieq = Psophometric weighted equivalent disturbing current Z = Mutual coupling impedance between the telephone and F. Sizing of the Smoothing Reactor HVDC lines While the current and voltage rating of the smoothing reactor can be specified based on the data of the DC circuit, The intensity of interference currents is strongly dependent the inductance is the determining factor in sizing the reactor. on the operating condition of the HVDC. Taking all design aspects above into account, the size of smoothing reactors is often selected in the range of 100 to IV. DESIGN RESULTS FOR 250KV SHWELI- 300 mH for long distance DC links and 30 to 80 mH for SHWESAR YAN HVDC TRANSMISSION SYSTEM back-to-back stations. In an HVDC long-distance The overall schematic connection diagram of proposed transmission system, it seems quite logical that the smoothing Shweli-Shwe Sar Yan 250kV DC Transmission system is reactor will be connected in series with the DC line of the shown in Fig.6 station pole. This is the normal arrangement. Shwe-Sar- Shweli Yan G. Design Criteria for AC Filters 250kVDC T.L The reactive power consumption of an HVDC converter 230/33/11kV Converter 110miles depends on the active power, the transformer reactance and G Converter the control angle. It increases with increasing active power. Station (177.0278 km) Station In addition, a reactive band for the load and voltage range and the permitted voltage step during bank switching must be 11/230kV determined. These factors will determine the size and number 500 MVA of filter and shunt capacitor banks. Harmonic Performance Requirements HVDC converter stations generate Fig.6.Schematic Connection Diagram of Proposed 250kV characteristic and non-characteristic harmonic currents. For a HVDC Overhead Transmission System. twelve-pulse converter, the characteristic harmonics are of the order n = 12k ± 1 (k = 1,2,3 ...). These are the harmonic The line data for Shweli-Shwe Sar Yan 250kVDC components that are generated even during ideal conditions, transmission line is as follows: i.e. ideal smoothing of the direct current, symmetrical AC Voltage - 250kVDC voltages, transformer impedance and firing angles. Current - 2kADC Power - 500MVA H. DC Filter Design Number of Circuit - Single, bipolar Harmonic voltages which occur on the DC side of a Route length - 110 miles converter station cause AC currents which are superimposed on the direct current in the transmission line. These A. Selection of Voltage alternating currents of higher frequencies can create We selected voltage as, interference in neighbouring telephone systems despite Line voltage=230 kV for HVAC and 250 kV for HVDC line limitation by smoothing reactors. DC filter circuits, which are Then, we choose the equivalent spacing (Dm) = 8m for connected in parallel to the station poles, are an effective tool HVAC and 7m for HVDC for combating these problems. The configuration of the DC filters very strongly resembles the filters on the AC side of Current rating, for HVAC is 1568.8866 A and the HVDC station. The interference voltage induced on the its angle is 36.89 degree (lagging). telephone line can be characterized by the following equation: Current rating, for HVDC overhead line is 2 kA(p.f unity).

B. Choice of Conductor (5) (ACSR) conductors are used for high voltage work. The size the conductors selected dependents on the length of the International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.15, July-2014, Pages: 3223-3227 Design Implementation of 250 kV HVDC Overhead Transmission System transmission line, load on the line and voltage of the line. E. Design Results of Sag for Line Conductor and Earth ACSR conductor is selected. For ±250 kVDC, a 954 MCM, Wire 1.196 inches diameter ACSR conductor would be required Table III. Design Results of Sag for Line Conductor and for a single conductor configuration and therefore a twin Earth Wire bundle conductor configuration would be required for 2000 A.

Cross-sectional area= 480 mm2 = 0.744 in2 = 0.0051667 ft2

Approximate Overall diameter, D=1.196 inches =3.03784 cm

Required radius, r =1.1519 cm

DC Resistance = 0.0117 Ω at 50°C F. Converter Design Results Table IV: Design Results of Converter Transformer C. Design Results of HVDC Line Efficiency and Regulation

Sending end HVDC voltage at Shwe Li Vs = 250 kVDC

Receiving end HVDC at Shwe Sar Yan VR = 250 kV - 2000×R= 244.9 kV G. Design Results of Smoothing Reactors and AC Filter Input power, Ps = 400 MW (500MVA×0.8) The smoothing reactors are of air core type and have

2 following main data: Output power, Pr = Ps – PL = 400MW- 2000 ×2.5492 Inductance - 250 mH Rated Voltage - 512 kV DC = 389.8032 MW Rated Current - 2500 A DC TABLE I: Design Results of HVDC Line Efficiency and Regulation The assembly of the selected AC tune filter and high pass damped filter types are shown in Fig.7

D. Insulation of Line Flashover Voltages = 250kV×1.8 = 450 kV. The flashover voltages of HVDC insulator discs are shown in Table II.

Table II. Technical Particulars of HVDC Disc Insulators

Fig.7. Selected AC Filter.

V. CONCLUSIONS This paper presents design consideration and calculation of HVDC overhead transmission line for 250 kV Shweli-Shwe Sar Yan in Myanmar. A high-voltage direct current (HVDC) transmission system uses direct current for the Accordingly the minimum number of discs is to be 7 bulk transmission of electrical power, in contrast with the (7×65=455kV).Thus, taking one numbers for suspension more common alternating current systems. HVDC allows string and two numbers for tension string in excess, the power transmission between unsynchronized AC distribution numbers of insulator discs had been decided as follows: systems, and can increase system stability by preventing Suspension Insulator – 7+1=8 units cascading failures from propagating from one part of a wider Tension Insulator – 18 units (Double string of 9 units) power transmission grid to another. The investment cost for HVDC converter stations are higher than for high voltage AC substations. On the other hand, the costs of transmission

International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.15, July-2014, Pages: 3223-3227 PHYU WIN WIN AI, THET TIN medium (overhead lines and cables), land acquisition or right of way costs are lower in the HVDC system. Moreover, the operation and maintenance costs are lower in the HVDC system. From this paper, the technical knowledge and design consideration and calculation can be contributed to the students, researchers and other engineers.

VI. REFERENCES [1] Arrillaga J., Y.H. Liu, N.R. Watson, 2007, “Flexible Power Transmission”. [2] Dennis A. Woodford, 1998, “HVDC Transmission”. [3] Kala Meah, SadrulUla, 2007, “Comparative Evaluation of HVDC and HVAC Transmission Systems”. [4] Roberto Rudervall, J.P. Charpentier and Raghuveer Sharma, 1998, “High Voltage Direct Current (HVDC) Transmission System”. [5] Hartmut Huang, Markus Uder, Reiner Barthelmess and Joerg Dorn, 2010, “Application of High Power Thyristors in HVDC and FACTS Systems”.

Author Profile: Phyu Win Win Ai received her B.E (Electrical Power) degree from Technological University, in 2009 and now pursuing M.E (Electrical Power) at Mandalay Technological University. Her areas of interest are HVDC overhead bipolar transmission system.

International Journal of Scientific Engineering and Technology Research Volume.03, IssueNo.15, July-2014, Pages: 3223-3227