DC Offset on HV Transmission Lines

[scottf]

Does anyone have any information on DC on AC HV transmission lines. I.e. DC current that is either induced or injected on transmission lines under steady state conditions.

I know that EHV lines can have some DC currents induced by the DC magnetic field between line and ground. What magnitudes are these currents generally?

I also know that some DC can be injected from generation stations or other industrial loads, but that this would generally not pass through the power transformer up to the transmission line. True?

Please note that I am not concerned with "DC Offset" as related to transients and time constants.

 

[jbartos]

Suggestions:
Look into:
1. Geostorms
2. AC-DC, DC-DC, DC-AC Converters
3. Common mode currents
4. Transients
5. Oscillations

 

[electricpete]

The only significant steady-state dc CURRENT that I have heard of on the hv lines is for geostorms as jbartos mentioned. More commonly known as geomagnetically induced current. Causes difference in ground potential between points several hundred miles apart which will affect tranmission lines connected between those points with grounded-wye hv-side transformers (very common) on each end. Big event in 1989 caused damage to a generator stepup in New Jersey and many problems in Canadian system. 11 year cycle didn't cause many problems in 2000... will peak again around 2001. Try searching geomagnetically induced current or g.i.c.

I don't know of many practical examples but dc VOLTAGE can of course be trapped on the capacitance of an isolated section of transmission line or bus with no connected grounded wye power transformers. You may have heard of high voltage magnetic potential transformers which cook because in some configurations they may be the only discharge path for this voltage on an otherwise isolated bus/line in some configurations.

 

[scottf]

Pete-

Thanks for your comments. I am familiar with the Geo-storms and the trapped DC charge on line capacitances.

I have heard of some utilities that say they have a constant DC current componant on their EHV lines. In particular, Hydro Quebec estimates as much as 50A Dc on some of their 765 kV lines.

I am trying to determine if their is any effect of possible DC currents on metering accuracy CTs operating at low currents. I am concerned if the potential DC offset of the B-H curve would cause accuracy problems and to determine this I need to know some relative magnitudes.

I don't think DC currents of this magnetude would exist on transmission lines, but I have heard many people say it is so...hence the question.

 

[electricpete]

Scott - I can only add that hydro-quebec was the Canadian utility that got whacked by the g.i.c storm in 1989. I can only guess that perhaps they are more susceptible to routine g.i.c. current for some reason. The only factor for susceptibility that I remember is long lines (long distance between grounded-wye transformers) since the induced voltage increases with distance. I believe that 765kv is typically only used for long transmission distances.

 

[jbartos]

Suggestion: The biggest DC offsets for CTs to negotiate are coming form lightnings. The CTs are supposed to return to their normal accuracy after the lightning DC offsets. Else, they would have to be replaced after each lightning DC offset. The continuous small DC offset in the EHV transmission lines is nowhere near to the DC offsets caused by lightnings.
See:
A.R. Van C. Warrington "Protective Relays Their Theory and Practice," Volume II, 3rd Edition, John Wiley & Sons, 1977,
section 7.2.4 Effects of d.c. Offset in Primary Current, and Figure 7.11 Effect of transient d.c. component upon c.t. flux.
The DC offset can cause about 18 times steady state flux peak during the DC transient component upon c.t. flux for a 100% offset wave since the DC component of the offset current wave is X/R times symmetrical a.c. component for a 100% offset wave.

 

[scottf]

Jbartos...thanks for your comments. I understand your point about having CT cores "magnitized" after a transient. This is really not my concern here, but more from a point of steady-state DC offsets.

For CT cores that have high residual flux due to DC during transients or immediate current reduction, etc...the CT cores tend to "de-magnetize" themselves, with the time period depending on what the primary current levels return to once they are re-energized. My company has done a fairly detailed study on this.