An overview of high voltage direct current systems and applications is presented in this paper. The favorable economics of long-distance bulk-power transmission with HVDC together with its controllability make it an interesting alternative or complement to AC transmission. The higher voltage levels, mature technology, and new converter designs have significantly increased the interest in HVDC transmission and expanded the range of applications

The ABCs of HVDC transmission technologies

Tests on the the modern HVDC system at Square Butte conducted in October 1977 indicated that the HVDC terminal was interacting in an adverse way with an 11.5-Hz torsional mode of an adjacent turbine- generator unit. Subsequent analytical work duplicated the field test observations and was used to develop an understanding of the HVDC-torsional interaction phenomena. As a result of the analytical work, the control system of the HVDC terminal was modified and subsequent tests showed that the changes resulted in stable operation. The paper includes significant field test and analytical results.

Experience with HVDC - Turbine-Generator Torsional Interaction at Square Butte

High voltage direct current (HVDC) technology has characteristics which make it especially attractive in certain transmission applications. The number of HVDC projects committed or under consideration globally has increased in recent years reflecting a renewed interest in this field proven technology. New HVDC converter designs and improvements in conventional HVDC design have contributed to this trend. This paper provides an overview of the rationale for selection of HVDC technology and describes some of the latest technical developments.

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HVDC transmission overview

Utility-interactive photovoltaic, wind-electric and fuel-cell systems are being planned at sufficiently large power levels. The technical and economic feasibility of such systems partially depends on the reliability, cost, and the efficiency of their grid interface. Here, a novel interface, which takes advantage of the system conditions in a utility-scale application is presented. The paper describes the basic principle of operation and characteristics. It is shown that the Total Harmonic Distortion in the line currents meets the 5 percent limit recommended by the IEEE. 519-1992, with one-third the high frequency switch kVA ratings as compared to other topologies. Various options are discussed for utility-scale applications. Simulation results for a 25 kW unit are presented. The new conceptual circuit is validated by a prototype hardware design at the 2.2 kW power level. >

A novel grid interface, optimized for utility-scale applications of photovoltaic, wind-electric, and fuel-cell systems

The paper gives an overview of the operations aspects of parallel operation of HVDC converters, especially in multi-terminal schemes where the converters are located in different locations For UHVDC schemes, plusmn800 kV or even higher the geographic distances between the stations can be very significant Considering the high power rating of UHVDC transmissions, the reliability and availability aspects are considered during all stages of development and design Thus, the consequences of any outage have to be minimized It is important that faulty equipment is quickly and safely isolated from the remaining transmission system In a multi-terminal transmission system the telecommunication is important for coordination of current order and maneuvers in the different stations even if there are back-up controls for not losing the power transmission in case of telecommunication failures The control and protection systems for a multi-terminal UHVDC transmission are conceptually described

Challenges with Multi-Terminal UHVDC Transmissions

Most commercial HVDC plants now operating are equipped with the same type of control system, the so-called individual phase control system. This system is simple and the operating experience is excellent. However, the requirements for future installations will sometimes be difficult to fulfill with the existing control system. This may be the case when the HVDC transmission is connected to a very weak ac network or to a generator station with large variations in frequency.

A Refined HVDC Control System

Subdivision of the total transmission capacity of an HVDC convertor station into smaller parts then one convertor per pole can be done either by series or parallel connection of convertors. Parallel connection which is the subject of this paper, is particularly advantageous when a transmission is built in stages or when the system consists of more than two convertor stations.

Parallel Connection of Convertors for HVDC Transmission

A high voltage direct current transmission includes a rectifier station and an inverter station connected by a DC line. The rectifier station has uncontrolled valves and an arrangement enabling current control to be effective at over-current conditions for controlling the alternating voltage connected to the valves. There are first and second control circuits, the first of which is controlled in dependence on the line voltage and the second in dependence on the line current. A circuit for selecting either the first or the second control circuit is provided between the control circuits and the voltage control arrangement. The valves of the rectifier station may be arranged in two six-pulse groups mutually offset in phase, so reactively connected that they have the commutating reactance partly in common.

Ake Ekstrom

Papers

A Refined HVDC Control System

Parallel Connection of Convertors for HVDC Transmission

Selective voltage and current control for high voltage D.C. transmission systems