TRANSMISSION LINES Flashcards
when HVDC transmission is used?
1) long distances OHL(>400 miles)
2) long underwater cable power transfer
3) providing a asynchronous means of joining different ps
transmission lines characterized by what?
series resistance and inductance ,shunt capacitance. these values determines the power carrying capacity of TL and the voltage drop across it at full load
formula for dc resistance?
Rdc=P(density)*L/A
which is higher in conductor AC resistance or DC resistance?
AC resistance is higher than DC resistance due to skin effect
series inductance of transmission lines consists how many components?
two components : internal and external inductances,which are due to the magnetic flux inside and outside the conductors.
how will be the inductance if there is greater spacing between the phases of TL?
inductance will be greater
how will be the inductance if there is greater the radius of the conductors in TL?
lower the inductance of line
why two or more conductors are bundled together?
1) to reduce corona loss
2) alternate to using heavy and inflexible conductors of large radii
how will be the capacitance if there is greater spacing between the phases of TL?
lower the capacitance
how will be the capacitance if there is greater the radius of the conductors in TL?
capacitance will be higher
does bundling increase capacitance?
yes
hove OHL lines below 50 miles can be modeled?
modeled as series reactance and series resistance
maximum power flow occurs at which angle?
angle 90 degree
what controls power flow through the line?
The angle δ controls the power flow through the line. Hence, it is possible to control power flow by placing a phase-shifting transformer
relationship between maximum power handling capability of TL and series reactance?
The maximum power handling capability of a transmission line is inversely proportional to its series reactance (some very long lines include series capacitors to reduce the total series reactance
The angle δ in a transmission line should typically at what?
The angle δ in a transmission line should typically be ≤ 30o to ensure that the power flow in the transmission line is well below the static stability limit
what should be included in medium lines(50-150miles)?
the shunt admittance must be included in calculations. However, the total admittance is usually modeled (π model) as two capacitors of equal values (each corresponding to a half of total admittance) placed at the sending and receiving ends.
how capacitance and series impedances are modeled in long transmission lines(>150 miles)?
For long lines, both the shunt capacitance and the series impedance must be treated as distributed quantities. The voltages and currents on the line are found by solving differential equations of the line.
•However, it is possible to model a long transmission line as a π model with a modified series impedance Z’ and a modified shunt admittance Y’
why shunt reactors are used?
Shunt reactors are used to compensate the line shunt capacitance under light load or no load to regulate voltage
why series capacitors are used in TL?
Series capacitors are often used to compensate the line inductive reactance in order to transfer more power
power flow through TL depends on what?
Therefore, the power flow through a transmission line depends on the angle between the input and output voltages.
•Maximum power flow occurs when δ = 90o.
