Power Transfer Capability of Transmission Lines

Power Transfer Capability of Transmission Lines

AU : Dec.-13

The main constraints in the capability of power transfer for transmission line are thermal limit, voltage drop limit and the stability limit. When the current flows through the transmission line conductors, due to its resistance there is loss which generates heat increasing the temperature. This gives the thermal limit. Thus it is required to keep the temperature of overhead line conductors within a safe limit. This avoids excessive line sag between the towers. It also helps in preventing irreversible stretching maintaining the proper ground clearace. Due to this there is limit on maximum safe current carried by overhead line.

In addition to current carried by line conductors, the other factors such as geometry and size of conductors, spacing between towers influence the temperature rise. The ambient temperature, wind velocity are some of the influencing operating factors.

The underground cables are more sensitive towards the thermal limit as there is limited possibility for heat transfer. But there is no question of sag in case of cables. If the cables are overheated then the insulation will start deteriorating and may get completely damaged in the near future.

The stability limit is another consideration in the power transfer capability of transmission lines. We have already seen the equation for power at receiving end given by,

P_R = | V_S | . | V_R | / X_L sin δ

For short transmission lines, the limit for transmission capability as per above equation is given by δ = 90°. But normally 8 is limited to 30 to 60° for maintaining the synchronism in the line.

In case of short transmission lines, the power transfer capability is set by the thermal limit instead by stability limit. But it is exactly opposite in case of long lines. This can be seen from Fig. 2.20.1.

For a long transmission line

Equating the above equation with V_S = V_R + B I_R

With the increase in line length, β l increases and P_R decreases.

The voltage drop in the line is the main consideration in case of medium transmission lines for maximum power transfer.

There is always rating for line in terms of current beyond which a line is not be operated for longer duration. The transmission lines are always designed for particular voltage level depending on string insulators and clearances. Sometimes the rating of line is given in terms of power which is multiplication of current and voltage. For extra high voltage lines (EHVAC), the MVA rating is almost equal to its MW rating.

Review Question

1. Write a short note on loadability limits based on thermal loading.