DC Transmission Lines in Large-Scale Energy Networks

AC systems with a large number of power plants face the challenge of keeping all the generators in sync. The stability of such a system is defined by its technical parameters that ensure the ability of the system to resist perturbations. Of special importance is the need to enable consistently synchronous work of several interconnected energy systems.

Poor resilience of interconnection lines, in case of an unexpected load growth, a short, or breakdown of the system’s generating or transmitting elements, can lead to the disruption of static stability (caused by small perturbations) or dynamic stability (caused by sharp and drastic disturbances). As a result, the generators may go out of sync, which, in turn, may make the energy system’s constituents go out of sync, with dire economic consequences for electricity consumers. Thus, the system’s resilience is the main factor limiting the capacity of long-distance transmission lines.


For these reasons, in the second part of the twentieth century the industry had a renewed interest in DC transmission lines. However, unlike the nineteenth-century’s DC power lines, at this time they had a high capacity, significantly longer length and extra-high voltage.

Design of DC transmission lines

A basic electricity transmission circuit includes a three-phase AC rectifier into high voltage direct current, and an inverting element, which converts direct current into alternating current. The rectifier can generate unipolar (+) direct high voltage on one pole of the line relative to the earthed other pole (unipolar transmission), or bipolar voltage (+ or -) on each pole relative to the earthed middle point of the rectifier (bipolar transmission).

                    On August 14th, 2003, the world’s electrical power industry saw the largest accident in its history, an event which affected the energy generation networks of the east coast of the USA and south of Canada. Because of the power system disintegration that took place, the electricity consumers in eight American states (Ohio, Michigan, New York, Pennsylvania, New Jersey, Vermont, Connecticut, and Massachusetts), as well as in two Canadian provinces (Ontario and Quebec) were devoid of power supply, a total of 61800 MW worth of capacity.   

                    The accident had an impact on six areas of operating control and stopped over 100 generating units at power plants, including 22 reactors at 9 nuclear power plants. 10 airports were closed, with over 700 flights cancelled. 350 thousand people were held up in the New York underground. For many hours, over 50 mln people occupying a territory of 22 thousand square kilometers had no electricity. It took 44 hours to fully restore the energy supply.

                    In the autumn of the same year, the whole Italy suffered a black-out due to a storm damaging two out of four transmission lines that connect the energy systems of Italy and France.

In unipolar transmission, which is often used for laying of submarine cables, the rectifier is connected to the inverting element with one conductor (DC cable). In bipolar transmission, the rectifier is connected to the inverting element with a bipolar DC line. Structurally, this line can be made as both a long-distance overhead line with two polar conductors on supports, or a cable line with two polar DC cables.

During current conversion, a substantial amount of reactive power is consumed (0.5 – 0.6 kVA per 1 kW of active power). Capacitor units required for generating reactive power make the converting substations of DC lines more complicated and expensive.

In the middle of the twentieth century, the power converter of three-phase alternating current was based on mercury tube rectifiers of high capacity. Eventually, the technology of current conversion came to using power semiconductor equipment with twelve-phase electronically-controlled rectifying mode. In the 1960s the industry began applying power thyristors, which were first oil-cooled, and then cooled with deionized water.

                    On the ± 530 kV bipolar DC transmission line with a capacity of 1920 MW, located in Mozambique, where intense military confrontations took place in the 1970s, each pole of the line was installed on separate supports and ran along different routes dozens of kilometers away from each other. This made it possible, in case of support damage, to maintain half of the electricity transmission capacity using the intact pole.
  
                    In 1970 in the USA the Pacific DC transmission line with a capacity of 1400 MW and voltage of ± 400 kV was installed for transmitting electricity from a hydro power plant in the state of Oregon to the energy system of Los Angeles; the length of the line was 1362 km. 

                    From 1973 to 1990 in Canada there were erected three transmission lines about 900 km long, built from the Nelson River hydro power plant near the polar circle to the city of Winnipeg in the south of the country. The capacity of the third DC transmission line was 2000 MW, with a voltage of ± 500 kV. 

                    In 1983 and 1985 in Brazil there were installed two DC power supply circuits of the Itaipu hydro power plant with a capacity of 3150 MW per circuit at a voltage of ± 600 kV. The length of each circuit was about 800 km.

Applications of DC transmission lines

DC transmission lines were widely used for transporting electric energy from powerful hydro or thermal power plants located at a large distance away from the consumers.

Comparison of overhead AC line 800 kV (a) and overhead DC line ±500 kV (b) of the same transmission capacity (dimensions are in metres)

Another application of DC transmission lines was in long-distance intersystem communication. The efficiency of such lines is explained not only by the increase in the  resilience of intersystem connections, but also by low energy losses, lower size of bipolar lines compared to three-phase overhead lines of the same capacity, unlimited length of a transmission line, and the ability to quickly adjust the capacity and transmission direction by converting rectifiers into inverting elements and vice versa.

DC transmission lines turned out to be most applicable for submarine cable lines up to 300 km long with a voltage of 400 kV. Submarine DC cables were quite widespread, especially in Japan and Europe. One of the longest European cable lines is 292 km long; it was laid in 1967 between Italy and the island of Sardinia through the Tyrrhenian sea. In 2005 a 295-km-long DC transmission line was laid between Australia and the island of Tasmania.

In future, powerful DC transmission lines can serve as a means of integrating power supply systems into transcontinental energy networks. Being considered is the possibility of building a powerful multi-substation DC transmission line that would connect the energy systems of Russia, Belorus, Poland, and Germany. It is also possible that a DC connection will be established between Russia and the United States through the Bering Strait.

High-voltage power lines

DC transmission lines are also used as connecting pieces between two AC energy systems that are either asynchronous or of different rated frequency. In addition to that, DC connecting pieces are efficient for various ways of regulating the current and voltage frequency in the energy systems being connected, or if the capacity of these systems is disproportional. The rectifying and inverting convertors of DC connecting pieces are placed at the same substation, and the length of the DC line connecting them is just a few meters.

DC connecting pieces ensure decoupling of adjacent energy systems in terms of frequency, voltage and capacity of a short, while maintaining high controllability of energy transmission in terms of amount and direction to one of the energy systems. In Japan DC connecting pieces are used to connect energy systems operated at AC frequency of 50 and 60 Hz. A powerful DC connecting piece was erected in Russia to connect two large energy supply networks, Russia’s United Power System and the European Network of Transmission System Operators.

Source (Russian language): http://energetika.in.ua

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