1 Abstract-- Alternating current (AC) is the main driving force in the industries and residential areas, but for the long 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. DC transmission does not have these limitations, which has led to build long HVDC transmission lines over the last 40 years. HVDC technology made possible to transfer bulk power over long distances. This paper presents a comparative evaluation of HVDC and HVAC transmission systems. Index Terms-- HVDC and HVAC transmission, Transmission cost, Environmental impact. I. INTRODUCTION HE first electric generator was the direct current (DC) generator and hence, the first electric power transmission line was constructed with DC. The basic discoveries of Galvani, Volta, Oersted, Ohm, and Ampere were in the DC field. Thomas A. Edison built the first electric central station in the world in 1882, on the Pearl Street, in the New York, which was the DC current. Despite the initial supremacy of the DC, the alternating current (AC) supplanted the DC for greater uses. This is because of the availability of the transformer, the induction motor, and polyphase circuits in the 1880s and 1890s [1]. The transformer is very simple and easy to change the voltage level for the transmission, distribution and use. The induction motors are the workhorse in the industries and work only with AC. That is why AC has become very useful for the commercial and domestic uses. But for the long transmission, DC is still more favorable than AC because of its economical, technical, and environmental advantages. High voltage DC (HVDC) Transmission system consists of three basic parts: 1) converter station to convert AC to DC 2) transmission line 3) second converter station to convert back to AC. 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 and there is no transmission line. This type of connection is used as an inter tie between two different AC transmission systems. The mono-polar link connects two converter stations * Corresponding Author: Kala Meah is a Ph.D. student in the Electrical and Computer Engineering Department at the University of Wyoming. Tel: 1-307-766-4689; fax: 1-307- 766-2248; e-mail: kala.meah@gmail.com by a single conductor line and earth or sea is used as a returned path. The most common HVDC link is bipolar, where two converter stations are connected by bipolar (±) conductors and each conductor has its own ground return. The multi-terminal HVDC transmission systems have more than two converter stations, which could be connected is series or parallel. II. HVDC VERSUS HVAC TRANSMISSION Alternating current (AC) became very familiar for the industrial and domestic uses, but still for the long transmission lines, AC has some limitations which has led to the use of DC transmission in some projects. The technical detail of HVDC transmission compare to high voltage AC (HVAC) transmission is discussed to verify HVDC transmission for long distances. Current and voltage limits are the two important factors of the high voltage transmission line. The AC resistance of a conductor is higher than its DC resistance because of skin effect, and eventually loss is higher for AC transmission. The switching surges are the serious transient over voltages for the high voltage transmission line, in the case of AC transmission the peak values are two or three times normal crest voltage but for DC transmission it is 1.7 times normal voltage. HVDC transmission has less corona and radio interference than that of HVAC transmission line [2]. The total power loss due to corona is less than 5 MW for a ± 450 kV and 895 kilometers HVDC transmission line [3-4]. The long HVAC overhead lines produce and consume the reactive power, which is a serious problem. If the transmission line has a series inductance L and shunt capacitance C per unit of length and operating voltage V and current I, the reactive power produced by the line is 2 c Q CV ω = and consumers reactive power 2 L Q LI ω = per unit length. If Q C = Q L 1/2 s V L Z I C = = where Z s is surge impedance of the line. The power in the line is 2 n s V P VI Z = = Comparative Evaluation of HVDC and HVAC Transmission Systems Kala Meah * , Student Member, IEEE, and Sadrul Ula, Senior Member, IEEE T 1-4244-1298-6/07/$25.00 ©2007 IEEE. Authorized licensed use limited to: NORTHROP GRUMMAN IT TASC. Downloaded on November 23, 2008 at 14:21 from IEEE Xplore. Restrictions apply.