1. Course: M.Tech
Subject : Flexible AC Transmission Systems
Topic : Loading Capability Limits of Transmission Lines
By A. Raja
• The loading capacity or thermal rating of short power
transmission lines and distribution networks is often based on
the maximum permissible temperature of the conductors.
• In the operation of transmission lines, the conductor
temperature should be maintained within a safety limit.
3. Loading Capability Limits:
The basic limitations on loading capability of transmission
1. Thermal Limit
2. Voltage Limit
3. Dielectric Limit
4. Stability Limit
4. 1. Thermal Limit:
• Thermal capability of an overhead line is a function of the
ambient temperature, wind conditions, conductor size & its
cross section area and ground clearance etc.
• The main limiting factor in a transmission system is the
• As the thermal limit exceeds the transmission line due to
the heat generated by the line current, sags and may be the
line contact with the trees etc.
5. Thermal Limit: Cont…
•If the system is operated close to its thermal limit, i.e. it is
being used to its maximum capacity. But the thermal limit is
hardly achieved since voltage or stability limits restrict the
capacity of a transmission system.
•Thermal limits establish the maximum amount of electrical
current that a transmission line or electrical facility can
conduct over a specified time period before it sustains
permanent damage by over heating or before it violates public
6. 2. Voltage Limit:
• The transmission line voltages must be maintained within the
range of acceptable minimum and maximum limits.
• For Example, minimum voltage limits can establish the
maximum amounts of electric power that can be transferred
with out causing damage to the electric system or customer
• Voltage limits normally requires that the voltage level within
a transmission system be maintained within a specified
interval, for instance plus/minus 5% of the nominal voltage.
7. 3. Dielectric Limit:
• An insulation point of view, many lines are designed very
• For a given normal voltage rating, it is often possible to
increase normal operation by 10% voltage i.e. 500kv – 550kv
or even higher voltages.
• Care is needed to ensure that the dynamic and transient over
voltages are within limits.
•The FACTs technology could be used to ensure acceptable
over voltage and power flow conditions.
8. 4. Stability Limit:
•The interconnected power system shall remain stable upon loss
of any one single element without system cascading that could
result in the successive loss of additional elements.
•There are a number of stability issues that limit the
i. Transient Limit
ii. Thermal Limit
iii. Steady State Limit
iv. Frequency Collapse
9. Stability Limit: cont…
v. Voltage Collapse
vi. Sub Synchronous Response etc.
•Conventional power systems do not have fast and flexible
control over the transmission networks. Their stability limits are
usually less than their thermal limits. This means that the power
transfer capabilities of existing power networks are not fully
•But the solution to the above problem is FACTs. FACTs has
emerged to implement fast and flexible control over transmission