what are the problems with communication over DC transmission lines?

Answer1

What is High Voltage Direct Current (HVDC)?

High Voltage Direct Current, or "HVDC" power lines have the ability to transmit large amounts of power over long distances with lower capital costs and much lower electrical losses than typical AC power lines. 


What are the Advantages of High Voltage Direct Current Transmission Lines over the Standard AC Power Lines?


High Voltage Direct Current power lines have several distinct advantages over the typical AC power lines.  

High Voltage Direct Current, or "HVDC" power lines have the ability to transmit large amounts of power over long distances with lower capital costs and much lower electrical losses than typical AC power lines. 

Depending on the voltage level and construction details, losses for HVDC are about 3% per 1000 km. 

High-voltage direct current transmission allows use of energy sources remote from load centers.

In a number of applications High Voltage Direct Current, HVDC is more effective and efficient than AC transmission lines. 

Examples where HVDC is more effective, and efficient, than AC power lines, include the following:

*  Undersea cables, where high capacitance causes additional AC losses.

*  Endpoint-to-endpoint long-haul bulk power transmission without intermediate 'taps.'  

*  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. 

*  Stabilizing a predominantly AC power-grid, without increasing maximum prospective short circuit current. 

*  Reducing line cost since HVDC transmission requires fewer conductors (i.e. 2 conductors; one is positive another is negative) 

*  Long undersea cables have a high capacitance. While this has minimal effect for DC transmission, the current required to charge and discharge the capacitance of the cable causes additional I2R power losses when the cable is carrying AC. In addition, AC power is lost to dielectric losses.

*  High Voltage Direct Current transmission lines can carry more power per conductor, because for a given power rating the constant voltage in a DC line is lower than the peak voltage in an AC line. 

*  Increased stability of power systems - because High Voltage Direct Current transmission lines allow power transmission between unsynchronized AC distribution systems, it can help increase system stability, by preventing cascading failures from propagating from one part of a wider power transmission grid to another. Changes in load that would cause portions of an AC network to become unsynchronized and separate would not similarly effect a DC link, and the power flow through the DC link would tend to stabilize the AC network. The magnitude and direction of power flow through a DC link can be directly commanded, and changed as needed to support the AC networks at either end of the DC link. This has caused many power system operators to contemplate wider use of HVDC technology for its stability benefits alone.


Disadvantages of High Voltage Direct Current Transmission Lines

The required static inverters are expensive and have limited overload capacity. At smaller transmission distances the losses in the static inverters may be bigger than in an AC transmission line. The cost of the inverters may not be offset by reductions in line construction cost and lower line loss.

In contrast to AC systems, realizing multi-terminal systems is complex, as is expanding existing schemes to multi-terminal systems. Controlling power flow in a multi-terminal DC system requires good communication between all the terminals; power flow must be actively regulated by the control system instead of by the inherent properties of the transmission line.



Answered by: siva85     On: 27-Apr-2011


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