direct-current transmission

direct-current transmission

Direct-current transmission

The conveyance of electric power by conductors carrying unidirectional currents. See Direct current

A dc line with two conductors is cheaper to construct and often has lower power losses than a three-phase ac line rated for the same power. Moreover, the same dc line is often considered as equal in reliability of service to a double-circuit three-phase line. The economic advantages are proportional to the line length but are offset by the substantial cost of the converting equipment. However, several other factors influence the selection of dc.

If the ac frequencies at the converting stations are nominally the same but controlled separately, their frequency independence is maintained by the dc link. In other words, the dc system is an asynchronous link. This is the justification of many back-to-back schemes such as the ties between regions in the United States and between European countries.

Although North America operates at 60 Hz and most other parts of the world operate at 50 Hz, on occasion there is a need to interconnect ac systems having different nominal frequencies. The asynchronous nature of dc serves as a frequency changer with the control action of each converter synchronized to its local ac frequency. The Sakuma frequency changer in Japan is a back-to-back scheme, connecting two regions which, for historical reasons, have frequency standards of 50 and 60 Hz.

The electrical shunt capacitance of cables is charged and discharged at the frequency of the voltage. Since the capacitance is proportional to distance, an ac cable longer than a few tens of miles is loaded to its thermal rating by a capacitor charging current, with no power being conveyed to the remote termination. Unless the cable can be sectioned for intermediate compensating measures, dc is obligatory for many cable applications. This is especially the case for submarine links where overhead lines are not an option.

All parts of an ac system function at the same nominal frequency. A system is designed to ensure that the generators do not lose synchronism despite load variations and large fault disturbances. The system is then considered to be dynamically stable. This becomes more of a challenge when the generating stations are geographically dispersed and remote from load centers, as opposed to the relatively tightly knit systems found in Europe. Should an ac connection to another system be contemplated, the combined system is intended to operate in synchronism, although the dynamic stability in either system may deteriorate below acceptable security. In comparison, a dc interconnection maintains the dynamic independence of each system. For example, remote hydroelectric generation at Churchill Falls in Labrador and on rivers entering James Bay prevents Hydro-Quebec from establishing synchronous connections with neighboring power companies for reasons of potential instability. Mutual interties and export contracts for electric power to the New York power pool and New England utilities have been implemented by several dc links.

The automatic control circuits at the converting stations permit the dc power to be accurately set at a value determined by the system control center. Furthermore, the dc power is maintained during dynamic ac frequency disturbances and can be rapidly changed (modulated) in as little as a few milliseconds, on demand. The same control permits the direction of dc power to be reversed equally quickly. This ability to ride through disturbances and to permit precise power scheduling and modulation responsive to dynamic needs has become of increasing value in the operation of power systems. The power flow in an individual ac line cannot be independently controlled to the degree offered by dc transmission.

A typical dc converter station contains conventional ac equipment in its ac switchyard supplemented by equipment specific to the ac-dc conversion. Solid-state converters are connected on the dc side with a center neutral point which is usually connected to a remote ground electrode. Balanced dc currents are circulated on each pole at plus and minus dc voltages with respect to ground. See Converter, Electric power substation