Voltage rise on transmission lines

[Marke]

Adding capacitance to a transmission line will cause the line voltage to rise.
For a given capacitance and a given transmission line, will adding significant inductance in series with the capacitor reduce the voltage rise? i.e. series resonant between 3rd and fourth harmonics.
Best regards,
Mark

Mark Empson
Advanced Motor Control Ltd

 

[collies99]

Since it is the capacitive current flowing through the inductive elements that produces the voltage rise, adding series reactance will increase the voltage rise and add more operational losses.

Too much capacitive charging current on 230kv lines and higher are normally mitigated by shunt reactors during lightly loaded conditions, which may be switched out during periods of heavy loading and also when voltage levels are low.

 

[cranky108]

To reduce the voltage in the case of too much capactance, you need to increase the load flowing throughing the line.

 

[crshears]

Where I work we routinely switch one of a multiple of 500 kV circuits between points A & B out of service, meaning completely off potential, during light load periods. Two beneficial effects: [a] gets rid of all the MX being generated by that lightly loaded line, and typically loads up the remaining circuits and pushes them closer to their surge impedance limit so they consume more of their own MX production.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[QBplanner]

switching off lines is an operational solution to limit the surplus Var but can only apply to the network which has more than 2 lines in parallel. other wise it may cause system separation.

 

[crshears]

Generally true, but not always...

If there are only two circuits tying together portions of a system, fair enough; but there are situations where the loss of the remaining 500 kV circuit simply 'downloads' the transfer onto the 230 kV network below it. If in this instance the system operator is desperate enough to get rid of all those unwanted VARs they will, with discretion, configure the balance of the system for this recognized contingency and live with the risk.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[fx007]

Where the load is too small, adding reactance can help mitigate reactive power and overvoltage at the receiving end. Otherwise, keeping generator voltages lower (for a small island system), can linearly reduce kVAr.
Adding more load is real solution/or alternatively disconnecting it.

Example of island operation, (from my head, not calculated):
120kV transmission: Generator 20kV -> 18KV, plus tap changer on 120kV transformer can further turn it down; goal reaching 120-5%kV at the delivery transformer.
Once you reach this point, you capacitive power will linearly be lower than what you would expect at full 120kV sending (and maybe 130kV at the receiving end).

 

[QBplanner]

i don't know which part of the network world you are operting but parallel 500kV with lower voltage network is not a good idea especially may potentially O/L 230kV lines when loss of the 500kV lines and may cause upstream generation units O.O.S.

 

[crshears]

Ontario, Canada; many of the network's 230 kV and 500 kV circuits normally operate in parallel. Even some 115 kV ckts run in parallel as well as both of the foregoing, although generally the 115 is run radially. Real-time network analysis tools and more than one type of Special Protection System are employed to respect pre-contingency posture.



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[QBplanner]

Thanks CR! good to know Ontario transmisison network is operating like this. The fact is whenever there is sth happening, without properly planned network , the SPS will mess up first. remember what happened in the First Energy. As far as the real time network analysis tools,they cannot foresee anything.
I don't know who in Ontario after the deregulation still seriously do the transmisison planning work. Sorry to ask but Do your company still have transmission planning department or within IESO? I heard that Hydro one only hires outsiders to do the T planning work. Best Wishes with the parallel operations.

 

[crshears]

I'd have to disagree with you about the real time contingency analysis tools; whatever else may have been sub-optimal, the ones we use are very, very good. As to transmission planning, company has its own...but whether or not 'only outsiders' are hired I have no idea; not at all my department. As to First Energy, I worked the first and second overnights immediately following that event; unbelievably educational!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[QBplanner]

good to know you have a good real time contingency analysis tool. However,it still cannot help you. All the system black outs were cause by more than one contingencies and when that happens, these tools are not helpful at all because it has to be predetermined. Good to know you were in the shift when the 2002 black out happened. That must be stressful. Ontario parallel network is only a minor problem comparing to the big loop problem between canada and US. best wishes!

 

[crshears]

"Cannot help" is too absolute...

Maybe it is better to say the tools aren't perfect, but they do calculate single, double, and all manner of other contingencies, and as such don't leave much ground uncovered.

That being said, the Blackout of 2003 was 'the perfect storm,' and as such was one of those scenarios that defies the ability of any predictive analysis tool since that concatenation of circumstances was 'beyond the normal scope of coverage' [one of those phrases that says a lot less than it seems to, similarly to 'the system functioned as designed,' one of my personal favorites].

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]