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# Transmission line absorb or produce reactive power

A transmission line can either absorb or produce reactive power based on whether the loading is greater or less than the surge impedance loading of the line. What is the surge impedance load?

#1

09-17-2013 12:53 AM

Top #2

The surge impedance loading (SIL) of a line is the power load at which the net reactive power is zero. So, if your transmission line wants to "absorb" reactive power, the SIL is the amount of reactive power you would have to produce to balance it out to zero. You can calculate it by dividing the square of the line-to-line voltage by the line's characteristic impedance.

09-17-2013 03:26 AM

Top #3

how a transmission line can produce reactive power?

09-17-2013 05:32 AM

Top #4

I strongly recommend a textbook like Elements of Power System Analysis, by W. D. Stevenson, Jr.

Really? Still asking about SIL of a transmission line? It is all there, in any good textbook. Go learn something, today.

Or, if you prefer, search one of the dozens of similar "debates" about reactive power in AC power systems that have already popped up in LinkedIn. Just be careful, you have to distinguish the correct posts from those with utter gibberish.

By the way, Mr. Russell has already provided all the answer you need to know about SIL.

Really? Still asking about SIL of a transmission line? It is all there, in any good textbook. Go learn something, today.

Or, if you prefer, search one of the dozens of similar "debates" about reactive power in AC power systems that have already popped up in LinkedIn. Just be careful, you have to distinguish the correct posts from those with utter gibberish.

By the way, Mr. Russell has already provided all the answer you need to know about SIL.

09-17-2013 07:54 AM

Top #5

SIL=sq root of L/C,if I remember correct

09-17-2013 10:32 AM

Top #6

Transmission lines can be considered as, a small inductance in series and a small capacitance to earth, - a very large number of this combinations, in series. Whatever voltage drop occurs due to inductance gets compensated by capacitance. If this compensation is exact, you have surge impedance loading and no voltage drop occurs for an infinite length or, a finite length terminated by impedance of this value (SIL load). (Loss-less line assumed!). Impedance of this line can be proved to be sqrt (L/C). If capacitive compensation is more than required, which may happen on an unloaded EHV line, then you have voltage rise at the other end, the ferranti effect. Although given in many books, it continues to remain an interesting discussion always..

09-17-2013 01:25 PM

Top #7

The capacitive reactive power associated with a transmission line icreases directly as the square of the voltage and is proportional to line capacitance and lenght.

Capacitance has two effects:

1 Ferranti effect

2 rise in the voltage resulting from capacitive current of the line flowing through the source impedances at the terminations of the line.

SIL is Surge Impedance Loading and is calculated as (KV x KV) / Zs their units are megawatts.

where Zs is the surge impedance....be aware...one thing is the surge impedance and other very different is the surge impedance loading.

I highly recommend to read "Transmission line Reference Book, 345 KV and Above second edition" by Electric power research institute. Check at page 22.

Also u can "Practical concepts in capability and performance of transmission lines" by H.P. ST. Clair

and

"analytical development of loadability characteristics for EHV and UHV Transmission lines" by R.D. Dunlop IEEE Transactions Vol PAS-98

Regards

Capacitance has two effects:

1 Ferranti effect

2 rise in the voltage resulting from capacitive current of the line flowing through the source impedances at the terminations of the line.

SIL is Surge Impedance Loading and is calculated as (KV x KV) / Zs their units are megawatts.

where Zs is the surge impedance....be aware...one thing is the surge impedance and other very different is the surge impedance loading.

I highly recommend to read "Transmission line Reference Book, 345 KV and Above second edition" by Electric power research institute. Check at page 22.

Also u can "Practical concepts in capability and performance of transmission lines" by H.P. ST. Clair

and

"analytical development of loadability characteristics for EHV and UHV Transmission lines" by R.D. Dunlop IEEE Transactions Vol PAS-98

Regards

09-17-2013 03:32 PM

Top #8

Thanks all for your feedback. However the importance of my question was not just to knowing what the SIL is but its relevance towards the improvement of stability and reliability of a power network especially an already existing one with various degree of low voltages and overload situations?

09-17-2013 06:13 PM

Top #9

The SIL, per se, has nothing to do with system performance, stability and reliability.

The SIL 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.

The SIL 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.

09-17-2013 08:44 PM

Top #10

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 SIL 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.

09-17-2013 11:07 PM

Top #11

Mr. Ekow;

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 SIL plays a fundamental role in reliability and stability.

Regards

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 SIL plays a fundamental role in reliability and stability.

Regards

09-18-2013 01:42 AM

Top #12

Dear Enrique can you kindly provide me with literature material on the points you discussed. Would most appreciate. Thanks