Loading Capability Limits of Transmission Lines
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This document discusses the four main loading capability limits of transmission lines: thermal, voltage, dielectric, and stability limits. The thermal limit depends on ambient temperature, wind conditions, conductor size and is usually the main limiting factor. Voltage limits require the transmission voltage to be maintained within a specified range, like plus/minus 5% of nominal. The dielectric limit concerns insulation and allows for some increase in normal operating voltage. Stability limits involve ensuring the power system remains stable after the loss of a single element to prevent cascading outages. FACTS technology can help utilize more of the thermal limits and improve stability.
Loading Capability Limits of Transmission Lines
- 1. Course: M.Tech Subject : Flexible AC Transmission Systems Topic : Loading Capability Limits of Transmission Lines By A. Raja
- 2. Introduction: • 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 lines are: 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 thermal limit. • 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 safety requirements.
- 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 facilities • 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 conservatively. • 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 transmission capability. These include: 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 utilized. •But the solution to the above problem is FACTs. FACTs has emerged to implement fast and flexible control over transmission network.
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