10-03-2011, 03:44 PM
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1. ABSTRACT:
In modern power systems network, it is essential to transmit power from one region to another region in order to meet the load demands. This can only possible by having Asynchronous power transmission between two regions operating at different frequency. This Asynchronous power transmission is called HVDC transmission.
Beginning with a brief historical perspective on the development of High Voltage Direct Current (HVDC) transmission systems, this paper presents an overview of the status of HVDC systems in the world today. It then reviews the underlying technology of HVDC systems, and HVDC systems from a design, construction, operation and maintenance points of view. The paper then discusses the recent developments in HVDC technologies. The paper also presents an economic and financial comparison of HVDC system with those of an AC system; and provides a brief review of reference installations of HVDC systems. The paper concludes with a brief set of guidelines for choosing HVDC systems in today’s electricity system development.
In today electricity industry, in view of the liberalization and increased effects to conserve the environment, HVDC solutions have more desirable for the following reasons:
Environmental advantages
Economical (cheapest solution)
Asynchronous interconnections
Power flow control
Added benefits to the transmission (stability, power quality etc.)
2. Basics in HVDC Transmission:
3. The HVDC Technology:
The fundamental process that occurs in an HVDC system is the conversion of electrical current from AC to DC (rectifier) at the transmitting end and from DC to AC (inverter) at the receiving end. There are three ways of achieving conversion:
Natural Commutated Converters.
Capacitor Commutated Converters (CCC)
Forced Commutated Converters.
The components of an HVDC transmission system:
The three main elements of an HVDC system are:
The converter station at the transmission and receiving ends.
The transmission medium.
The electrodes
The converter station: The main components of a converter station are Thyristor valves, VSC valves, Transformers, AC Filters, Capacitor Banks and DC Filters.
Transmission medium: For bulk power transmission over land, the most frequent transmission medium used is the overhead line. This overhead line is normally bipolar, i.e. two conductors with different polarity. HVDC cables are normally used for submarine transmission. The most common types of cables are the solid and the oil-filled ones. The solid type is in many cases the most economic one. Its insulation consists of paper tapes impregnated with high viscosity oil. No length limitation exists for this type and designs are today available for depths of about 1000 m. The self-contained oil-filled cable is completely filled with low viscosity oil and always works under pressure. The maximum length for this cable type seems to be around 60 km.
The development of new power cable technologies has accelerated in recent years and today a new HVDC cable is available for HVDC underground OR submarine power transmission. The new HVDC cable is made of extruded polyethylene, and is used VSC based HVDC systems.
HVDC in the new Electrical Industry:
The question is often asked to when HVDC transmission should be chosen over an AC system. In the past, conventions were that HVDC was chosen when:
Large amounts of power (>500MW) needed to be transmitted over long distance (>500km);
Transmitting power under water;
Interconnecting two AC networks in an asynchronous manner.
HVDC systems remain the best economical and environmentally friendly option for the above conventional applications.
New technologies, such as the VSC based HVDC systems, and the new extruded polyethylene DC cables, have made it possible for HVDC to become economic at lower power levels (up to 200 MW) and over a transmission distance of just 60 km.
HVDC systems enable the bi-directional power flows, which is not possible with AC systems (two parallel systems would be required).
4. Design, construction, operation, Maintenance & Cost structure considerations:
In general, the basic parameters such as power to be transmitted, distance of transmission, voltage levels, temporary and continuous overload, status of the network on the receiving end, environmental requirements etc. are required to initiate a design of an HVDC system.
In terms of construction, it can take from three years for thyristor-based large HVDC systems, to just one year for VSC based HVDC systems to go from contract date to commissioning. The following table shows the experience for the different HVDC technologies.
To the extent that the term operation denotes the continual activities tat are aimed at keeping the system availability at designed levels, modern HVDC links can be operated remotely, in view of the semiconductor and microprocessor based control systems included. There are some existing installations in operation completely unmanned. Moreover, modern HVDC systems are designed to operate unmanned. This feature is particularly important in situations or countries where skilled people are few, and these people can operate several HVDC links from one central location.
Maintenance:
Maintenance of HVDC systems is comparable to these of those of high voltage AC systems. The high voltage equipment in converter stations is comparable to the corresponding equipment in AC substations, and maintenance can be executed in the same way. Maintenance will focus on: AC and DC filters, smoothing reactors, wall bushings, valve- cooling equipment, thyristor valves. In all the above, adequate training and support is provided by the installation, commissioning and initial operation period.
Cost structure: The cost of an HVDC system depends on many factors, such as power capacity to be transmitted, type of transmission medium, environmental conditions and other safety, regulatory requirements etc.
