How can protection schemes protect 500kV lines with 65% S.C.

[QBplanner]

Hi:
I am not a protection guy. I know if the line series compensations go beyond 50% the distance relay may mis-operate due to the possible negative impedance (Or out of the cycle).
Differential relay is preferred. However, our 500kV system uses microwave not fiber. So one of my friends told me differential relay can not be applied by micro wave. Is it true?

I read some books it said phase comparison relay can also be applied .For microwave communication, can phase relay be applied.
Another question is that, our P&C guys said if the series cap can be located at exactly the mid point of the line. then they may make 65% series compensation work based on distance relay. But due to the Geo. limits, mid point location will be highly unlikely the case. therefore, we've been told the compensation level can not go beyond 50%.
I don't know exactly why the location becomes a very important factor?
I knew back to 1960-1970s US installed a lot of series compensation more than 70%. Don't know exactly how they apply the protection scheme and which type of communication they used at that time.

I know we can take the risks by relaying on series cap short out right after the fault by MOVs. But I would like to learn if we don' not relay on MOVs what should be the right thing to do
Hope any experts who did 500kV line protection with more than 60% series compensation provide some directions.
Thanks!

 

[davidbeach]

I know of lines compensated to nearly 100%. Protection is traveling wave relays. Line differential can work over digital microwave. 500kV is its own animal, and is handled differently than most anything at lower voltages, although some 345kV and even a few highly critical 230kV lines get treated like 500kV lines. Someday, maybe, I'll know a whole lot more about 500kV.

 

[QBplanner]

100% compensation will definitely push the system into resonance zone. I doubt very much about it.you may have to run A frequency scan to see the frequency domain first. Maybe your system is highly damped system I guess it may be below 230kV system.
Daviabeach, could you be more specific of the system with 100% compensation I will be more than interested in knowing how people make it work and how the system look like.

 

[davidbeach]

If I remember, I can provide line impedance on Monday. Beyond that this isn't an appropriate forum (too public) for detailed transmission information.

 

[QBplanner]

Davidbeach:
That is fine not need to provide me the information. Since I seldom see such a system just personal curiosity. I guess u are in NORTH.AMERCIA
Many Thanks!

 

[davidbeach]

Line in question has an inductive reactance of 0.1 pu on an arbitrary base. Series capacitors total 0.8011 pu on the same base, or about 80% of inductance. Portions of the line are compensated over 100%. Caps are shorted out of the line during faults significantly raising the line impedance.

 

[SMB1]

yes, I have seen lines compensated to nearly 100% in Mexico.

but this was not the plan. the line was 50% compensated and then one substation built in the midle of the line.

 

[QBplanner]

SMB1: I won't consider that a 100% compensation case unless the line opens then the system may be in deep trouble with any small triggers such as any chances creating harmonics sources. However, even in North America, people starting ignoring the problems by pushing the projects. Good luck