Utilities that are furthest North with long transmission lines would be most concerned I would have thought.
My limited understanding is that transformers (often generator step up transformers) are subject to a relatively high DC off set current that causes overheating. For a small amount of overheating the generator/circuit load can be reduced. Particularly vulnerable transformers can have a resistor added to the earth connection to reduce the DC current flow at times of high solar activity.
If you google geomagnetically induced currents you should get more info.
The currents generally would follow the lines of the arrora, or East to West (I supose they could go West to East). So North to South lines are not as affected.
There was talk at one time about using capacitors instead of resistors. But I don't think that was ever done.
Also in areas of low impedance soils this is not as much of a concern.
I would think series capacitors can be used in transmission lines to reduce DC flows.
This is a really interesting question. Does anybody know of a utility that actually implements GIC protection schemes like the ones in the attached .pdf? Where I work, I have never seen a customer specification for a GSU transformer requiring anything to protect against GICs.
I guess I don't completely understand why a fuseable disconnect is inadequate here to protect the transformer.
My only guess is that the transmission lines are not the only source of induced current in the system that would cause an overload. The transformers themselves, due to the presence of their windings, also induce current. That fact, in and of itself, would render the transformers vulnerable even in the presence of disconnects.
This looks like the idea EPRI floted years ago, but to my knoledge I don't know of any takers.
The problem with leaving the neutral open or high impedance grounded is it reduces the ability to flow ground fault currents, and thus the ability to detect a ground fault on the line. And because ground faults can transition into phase to phase faults we do want to clear them fast.
The transmission lines are not the only inducable conductor. In most of these cases the earth also acts like an inducable conductor, which is why the DC potential in the earth causes current in some lines.
Also note there can also be current flow in shield wires, pipelines, etc, although they go un-noticed because there are no seen effects.
Putting disconnects and other junk between the transformer neutral and ground is not a popular concept with utilities. They like their grounded wye transmission systems and they like to keep things simple.
I believe Hydro Quebec or Hydro Ontario had some serious issues back in a large event about 1990 or so. I seem to recall they did install some protection in the transformer neutral to ground path.
It seems like geomagnetic storm damage to transformer assets is not a very big deal to utlities--at least as it pertains to equipment protection schemes. These HILF(high impact low frequency) events only justify (finacially) having the necessary operations response and grid management initatives rather than the implementation of expensive and complex GIC protection schemes. Perhaps in Canada and in higher/lower global latitudes this is more widely pursued by utilities. Any comments?
Next week IEEE is discussing this topic at their Fall meeting at Nashville Tenesee.Some of the topics presented are
Effect of GIC on Power Transformers", by Dr. Ramsis Girgis (ABB Inc.)
"Simulating GIC flow in power systems, and the evaluation of potential impact of VAR swings on voltage stability and harmonics on system protection", by Dr. Emanuel Bernabeu (Dominion)
"GIC monitoring and examples of actual GIC signatures", by Dr. David Fugate (Electric Research & Management, Inc.)
"An overview of the NERC GMD TF activities, conclusions, and recommendations", by Mr. Frank Koza (PJM Interconnection)
"Processes / procedures presently used by utilities in North America to mitigate the effects of GIC", by representatives of North America utilities
You can have all these presentations from IEEE site after 15th ie
I work for a very northern utility with long transmission lines. We implement a scheme similar to that shown in the paper posted by 86ed, with a capacitor and surge arrestor across in the neutral.
We recently installed a new transformer onto our line that is most likely to be affected by GIC which has a solidly-earthed neutral. I also have a box of special CT's and relays sitting at my desk waiting to be installed on this and several other soon-to-be-purchased transformers. These will measure the DC component of the neutral current, and provide an alarm for our system operator to take action if the DC current starts to become high. It looks like we'll miss out the present solar storm, which would have been a good test to see how it all works.
Adding to my misery - it's cloudy and snowing out, so no Northern Lights show!
Specail CTs and relays?? Do you mind sharing the wiring schematics/settings of how these will be implemented? Just curious and want to learn about these types of protection schemes.
I worked in a utlity that is pretty far south so I haven't had the opportunity to see something like this before.
Some years back,in India we had a similar problem. In a 500 kV HV DC line (two lines with +500 kV and -500KV DC) when one line is down,still the other line used to transmit power with ground as return for 1000 A. Normally this used to work.But in one particular line end,there was high earth resistivity.There the return DC current entered in to the neutral of nearby transformers due to high earth resistivity,moved through transmission lines and returned back to ground through the neutral of a far away transformers. This created core satuartion in both transformers with loud humming.To prevent current entry in to transformers, neutral DC filters were used consisting of a capacitor with a bypass switch to short capacitor during the line fault.
I don't have the schematics in front of me, but I don't recall there being anything unusual in how they're connected - just a CT into a transducer which gives a 4-20mA output to SCADA.
My understanding is that the problem is on N-S lines and that it is solved by grounding one end of the line neutral going through a capacitor. This will stop the DC which will stop the saturation of the transformer. The DC current flow is induced when the magnetic field shifts due to the solar storms.
At least that is what I have seen in Canada (Prince George to points 300km North).
