TRANSMISSION LINE LOADABILITY
INTRODUCTION
Loadability of a transmission line is defined as the optimum power transfer capability of a
transmission line under a specified set of operating criteria. The loadability of short
transmission lines is limited by the thermal rating of the conductors, medium line voltages
regulation and long lines by stability consideration which is lower than the thermal rating.
Compensation can be used to increase loadability of long lines toward their thermal limit.
OBJECTIVES
1. To understand the use of series compensation for increasing transmission line
loadability.
2. To compare effect of compensation at receiving end, sending end and at both ends.
THEORY
There are some physical properties associated to the transmission system that limit power
transfer in spite of the capability of the generator or therequirement of the load.Transmission
systems are designed to operate according to specific voltagelevels.
Depending on the characteristic of the transferred power, the voltage at thetransmission line
ends, for instance, can be either below or above certain limits,modifying the system capacity to
transfer power. Actions are frequently taken to recover the assigned voltagelevels, allowing the
system to attend to the power demand at adequate operating condition.
The physical parameters of transmission lines, which depend upon the linelength and voltage
level, strongly restrain power transfer. As stated before,the loadability of short transmission
lines is limited by the thermal rating of the conductors. This is the magnitude of the current,
continuingover time and increasingly heating the conductor that limits the loading.As the
conductor heats up, the resistance of theconductor increases with temperature, it stretches,
and the line sags (can be irreversible).
Series and shuntcompensations have been traditionally used to modify the natural parameters
oftransmission lines. Compensation generally describes the intentional insertion of reactive
power devices (inductive or capacitive) into a power network to achieve a desired effect.
Series capacitors are sometimes used in long lines to increase the loadability. Capacitor banks
are installed in series with each phase conductor at selected points along the line. They reduce
, the net series impedance of the line in series with the capacitor bank thereby reducing line
voltage drops and increasing the steady state stability limit.
A disadvantage of series capacitor banks is that automatic protective devices have to be
installed to bypass high currents during faults and to re-instate the capacitor banks after the
fault has passed. They can also excite low frequency oscillations (sub-synchronous resonance)
which may damage turbine-generator shafts. However, there are techniques to counteract this
effect such as use of static filters.
The characteristic of power transfer (P-V characteristic) relates the voltage at the receiving-end
bus bar to the active power reaching it, for a given sending-end voltage, power factor and
impedance of transference. It is affected by changes either in the sending-end voltage
magnitude or in the impedance of transference between sending and receiving ends, or even in
the transfer power factor.
The graph below depicts a P-V characteristic where curves 1, 2 and 3 depict reactance X 1, X2 and
X3 respectively where X1> X2> X3.The line VSM shows the point with maximum power transfer.
INTRODUCTION
Loadability of a transmission line is defined as the optimum power transfer capability of a
transmission line under a specified set of operating criteria. The loadability of short
transmission lines is limited by the thermal rating of the conductors, medium line voltages
regulation and long lines by stability consideration which is lower than the thermal rating.
Compensation can be used to increase loadability of long lines toward their thermal limit.
OBJECTIVES
1. To understand the use of series compensation for increasing transmission line
loadability.
2. To compare effect of compensation at receiving end, sending end and at both ends.
THEORY
There are some physical properties associated to the transmission system that limit power
transfer in spite of the capability of the generator or therequirement of the load.Transmission
systems are designed to operate according to specific voltagelevels.
Depending on the characteristic of the transferred power, the voltage at thetransmission line
ends, for instance, can be either below or above certain limits,modifying the system capacity to
transfer power. Actions are frequently taken to recover the assigned voltagelevels, allowing the
system to attend to the power demand at adequate operating condition.
The physical parameters of transmission lines, which depend upon the linelength and voltage
level, strongly restrain power transfer. As stated before,the loadability of short transmission
lines is limited by the thermal rating of the conductors. This is the magnitude of the current,
continuingover time and increasingly heating the conductor that limits the loading.As the
conductor heats up, the resistance of theconductor increases with temperature, it stretches,
and the line sags (can be irreversible).
Series and shuntcompensations have been traditionally used to modify the natural parameters
oftransmission lines. Compensation generally describes the intentional insertion of reactive
power devices (inductive or capacitive) into a power network to achieve a desired effect.
Series capacitors are sometimes used in long lines to increase the loadability. Capacitor banks
are installed in series with each phase conductor at selected points along the line. They reduce
, the net series impedance of the line in series with the capacitor bank thereby reducing line
voltage drops and increasing the steady state stability limit.
A disadvantage of series capacitor banks is that automatic protective devices have to be
installed to bypass high currents during faults and to re-instate the capacitor banks after the
fault has passed. They can also excite low frequency oscillations (sub-synchronous resonance)
which may damage turbine-generator shafts. However, there are techniques to counteract this
effect such as use of static filters.
The characteristic of power transfer (P-V characteristic) relates the voltage at the receiving-end
bus bar to the active power reaching it, for a given sending-end voltage, power factor and
impedance of transference. It is affected by changes either in the sending-end voltage
magnitude or in the impedance of transference between sending and receiving ends, or even in
the transfer power factor.
The graph below depicts a P-V characteristic where curves 1, 2 and 3 depict reactance X 1, X2 and
X3 respectively where X1> X2> X3.The line VSM shows the point with maximum power transfer.
