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Electrical transmission lines are very widely used to transmit high frequency signals over long or short distances with minimum power loss. One familiar example is the down lead from a TV or radio aerial to the receiver.
Short Description
Transmission Lines is a conductor or conductors designed to carry electricity or an electrical signal over large distances with minimum losses and distortion.
In many electric circuits, the length of the wires connecting the components can for the most part be ignored. That is, the voltage on the wire at a given time can be assumed to be the same at all points. However, when the voltage changes in a time interval comparable to the time it takes for the signal to travel down the wire, the length becomes important and the wire must be treated as a transmission line. Stated another way, the length of the wire is important when the signal includes frequency components with corresponding wavelengths comparable to or less than the length of the wire.
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution.
Transmission Constraints and their Effects on Operations and Reliability
As the transmission system has expanded over the years, surplus capacity available on transmission lines always seems to be consumed as the system grows or as transmission users find more economical ways of meeting system demands. Expansion leads to more usage that leads to more expansion. Transmission congestion results when a particular electricity transmission path cannot accommodate increased power flow.
When line faults occur, protective transmission line components remove the line from service to protect terminal equipment from serious damage. Once the faulted line is removed from service, other transmission lines in the system experience increased loads as they compensate for loss of the faulted line. Overloading can then occur on these transmission lines, which might exceed thermal operating constraints. If not controlled promptly, additional transmission line faults may occur.
Endpoint-to-endpoint long-haul bulk power transmission without intermediate taps, for example, in remote areas.
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Increasing the capacity of an existing power grid in situations where additional wires are difficult or expensive to install.
Allowing power transmission between unsynchronized AC distribution systems.
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Reducing the profile of wiring and pylons for a given power transmission capacity, as HVDC can carry more power per conductor of a given size.
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Stabilizing a predominantly AC power grid without increasing the maximum prospective short-circuit current.
Reducing corona losses (due to higher voltage peaks) compared to HVAC transmission lines of similar power.
TRANSMISSION LINES AND CABLES:
For bulk power transmission over land, overhead transmission lines are most frequently used. These lines most often employ a bipolar configuration using two conductors with opposite polarity.
CONSTRUCTION:
Construction of HVDC transmission line systems typically takes from 3 years for large, thyristor-based systems, to just 1 year for VSC-based systems, from contract date to commissioning. Modern HVDC links with microprocessor-based control systems can be operated remotely, and some existing installations in operation are completely unmanned. Since such systems are designed to operate this way, a few skilled people can operate several HVDC links from one central location.
MAINTENANCE:
Maintenance of HVDC systems is comparable to that of HVAC systems. The highvoltage equipment in converter stations is comparable to the corresponding equipment in AC substations, and maintenance can be executed in much the same way.
One week per year of normal routine maintenance is recommended. Newer systems may go for 2 years before requiring maintenance. Bipolar systems can continue to operate at near normal levels during maintenance, while one pole continues to operate at elevated load while the other is stopped for maintenance
COST :
Comparing the costs of a thyristor-based HVDC system to an HVAC system, the investment costs for HVDC converter stations are higher than those for HVAC substations, but the costs of transmission lines and land acquisition are lower for HVDC. Furthermore, the operation and maintenance costs are lower in the HVDC case. Initial loss levels are higher in the HVDC system, but they do not vary with distance. In contrast, loss levels increase with distance in a HVAC system.