Tuesday, December 6, 2011
1:20 PM | Posted by
Muslim |
TRANSMISSION LINES
Raising Voltage to Reduce Losses
The cost due to losses decreases dramatically when the current is lowered. The power losses in conductors are calculated by the formula I2R. If the current (I) is doubled, the power losses quadruple for the same amount of conductor resistance (R)! Again, it is much more cost effective to transport large quantities of electrical power over long distances using high-voltage transmission lines because the current is less and the losses are much less.
Bundled Conductors
Bundling conductors significantly increases the power transfer capability of the line. The extra relatively small cost when building a transmission line to add bundled conductors is easily justified since bundling the conductors actually doubles, triples, quadruples, and so on the power transfer capability of the line. For example, assume that a right of way for a particular new transmission line has been secured. Designing transmission lines to have multiple conductors per phase significantly increases the power transport capability of that line for a minimal extra overall cost.
Why use high-voltage transmission lines? The best answer to that question is that high-voltage transmission lines transport power over long distances much more efficiently than lower-voltage distribution lines for two main reasons. First, high-voltage transmission lines take advantage of the power equation, that is, power is equal to the voltage times current.
Therefore, increasing the voltage allows one to decrease the current for the same amount of power. Second, since transport losses are a function of the square of the current flowing in the conductors, increasing the voltage to lower the current drastically reduces transportation losses. Plus, reducing the current allows one to use smaller conductor sizes.
Above Figure shows a three-phase 500 kV transmission line with two conductors
per phase. The two-conductors-per-phase option is called bundling.
Power companies bundle multiple conductors—double, triple, or more to increase the power transport capability of a power line. The type of insulation used in this line is referred to as V-string insulation. V-string insulation, compared to I-string insulation, provides stability in wind conditions. This line also has two static wires on the very top to shield itself from lightning. The static wires in this case do not have insulators; instead, they are directly connected to the metal towers so that lightning strikes are immediately grounded to earth. Hopefully, this shielding will keep the main power conductors from experiencing a direct lightning strike.
per phase. The two-conductors-per-phase option is called bundling.
Power companies bundle multiple conductors—double, triple, or more to increase the power transport capability of a power line. The type of insulation used in this line is referred to as V-string insulation. V-string insulation, compared to I-string insulation, provides stability in wind conditions. This line also has two static wires on the very top to shield itself from lightning. The static wires in this case do not have insulators; instead, they are directly connected to the metal towers so that lightning strikes are immediately grounded to earth. Hopefully, this shielding will keep the main power conductors from experiencing a direct lightning strike.
Raising Voltage to Reduce Current
Raising the voltage to reduce current reduces conductor size and increases insulation requirements. Let us look at the power equation again:
Raising the voltage to reduce current reduces conductor size and increases insulation requirements. Let us look at the power equation again:
Power = Voltage × Current
VoltageIn × CurrentIn = VoltageOut × CurrentOut
VoltageIn × CurrentIn = VoltageOut × CurrentOut
From the power equation above, raising the voltage means that the current can be reduced for the same amount of power. The purpose of step-up transformers at power plants, for example, is to increase the voltage to lower the current for power transport over long distances. Then at the receiving end of the transmission line, step-down transformers are used to reduce the voltage for easier distribution.
For example, the amount of current needed to transport 100 MW of power at 230 kV is half the amount of current needed to transport 100 MW of power at 115 kV. In other words, doubling the voltage cuts the required current in half.
For example, the amount of current needed to transport 100 MW of power at 230 kV is half the amount of current needed to transport 100 MW of power at 115 kV. In other words, doubling the voltage cuts the required current in half.
The higher-voltage transmission lines require larger structures with
longer insulator strings in order to have greater air gaps and needed insulation. However, it is usually much cheaper to build larger structures and wider right of ways for high-voltage transmission lines than it is to pay the continuous cost of high losses associated with lower-voltage power lines. Also, to transport a given amount of power from point “a” to point “b,” a higher-voltage line can require much less right of way land than multiple lower-voltage lines that are side by side.
longer insulator strings in order to have greater air gaps and needed insulation. However, it is usually much cheaper to build larger structures and wider right of ways for high-voltage transmission lines than it is to pay the continuous cost of high losses associated with lower-voltage power lines. Also, to transport a given amount of power from point “a” to point “b,” a higher-voltage line can require much less right of way land than multiple lower-voltage lines that are side by side.
The cost due to losses decreases dramatically when the current is lowered. The power losses in conductors are calculated by the formula I2R. If the current (I) is doubled, the power losses quadruple for the same amount of conductor resistance (R)! Again, it is much more cost effective to transport large quantities of electrical power over long distances using high-voltage transmission lines because the current is less and the losses are much less.
Bundled Conductors
Bundling conductors significantly increases the power transfer capability of the line. The extra relatively small cost when building a transmission line to add bundled conductors is easily justified since bundling the conductors actually doubles, triples, quadruples, and so on the power transfer capability of the line. For example, assume that a right of way for a particular new transmission line has been secured. Designing transmission lines to have multiple conductors per phase significantly increases the power transport capability of that line for a minimal extra overall cost.
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