Transmission line low voltages and overload situations

Q: I want to know just what the surge impedance loading (SIL) is but its relevance towards the improvement of stability and reliability of a power network especially an already existing one with various degrees of low voltages and overload situations?

A: The surge impedance loading will provide you with an easy way of determining if your transmission line is operated as a net reactor (above SIL, so external sources of reactive power are required) or as a net capacitor (below SIL, so external "sinks" for the excess reactive power are required).

Since you mentioned "overload", it seems clear that you are above SIL. Overload is a function of the rated current of the conductor and it should (typically) be above SIL for overhead lines. Cables are usually operated below SIL, so long EHV cables might have reactors connected at one or both ends of the cable.

And since you mentioned low voltages, probably you don't have enough controlled reactive power sources to hold voltage. This is usually done by the synchronous generators operating in automatic voltage regulator (AVR) mode, complemented by shunt capacitors or on-load tap changers in the transformers.

By relevance to stability and reliability, I guess you mean capability of network to be able to take required power to the load. Load is intimately related to the voltage at the load end. Therefore surge impedance load represents load that can be carried by the network in an ideal manner (resistance less line assumed), without any drop in reactances of network. That accounts for reliability as well as stability (voltage/ load stability). Simultaneously existing low voltage at far end and high voltage near source end are special conditions which need to be analyzed on a case to case basis.

Of all limiting factors that normally set a ceiling on how much power can be carried by a particular transmission line, three major line loading limitations are considered:

(1) Thermal limitation
(2) line-voltage-drop limitation
(3) steady-state-stability limitation

In contrast with the line voltage drop limitation, the steady state stability limitation has been discussed quite extensively in the technical literature.

However, one important point is rarely made or given proper emphasis; that is, the stability limitation should take the complete system into account, not just the line alone. This has been a common oversight which, for the lower voltage lines generally considered in the past, has not led to significant misinterpretations concerning line loadability

At higher voltage classes such as 765 kV and above, the typical levels of equivalent system reactance at the sending and receiving end of a line become a significant factor which cannot be ignored in determining line loadability as limited by stability considerations, so surge impedance loading plays a fundamental role in reliability and stability.