Answers to Your Geomagnetic Disturbances Questions Part III
On January 15, 2013, I presented a webcast on Energy Central titled, “Geomagnetic Disturbances and their Impacts on Power Transformers”. You can view the presentation here.
The presentation generated many questions from the audience that I did not have time to address. This blog post addresses a few of those questions. Stay tuned for more blog posts with more questions and answers.
Question: Are distribution transformers/equipment susceptible?
Answer: Distribution transformers/equipment are less susceptible to Geomagnetically Induced Currents (GIC), because of the shorter line lengths and the fact that GIC flows tend to accumulate more significantly on high-voltage lines.
Question: Are lines in any particular direction more susceptible to GIC?
Answer: The spatial geometry of the Geomagnetic Disturbances (GMD) and transmission lines are critical to determining the magnitude of GIC. However, the direction of the GMD can change during a storm. In general, analytical estimates of worst-case GIC flow are calculated by varying the orientation of the storm such that maximum coupling is achieved between ground potentials and the affected transmission lines.
Question: Are the risks increased if the transmission lines are carrying power from variable or intermittent sources?
Answer: Transformer vulnerability is a function of loading history (i.e. what is the remaining insulation life). So intermittent or variable loading would generally reduce transformer's risk of GIC. In other words, if you minimize the loss of insulation life, you can increase the transformer’s ability to withstand GIC-induced heating.
Question: How important is the age of transformers to their risk? What percentage of transformers in the U.S. are vulnerable at the 69 kV + level
Answer: Age plus thermal capacity, remaining insulation life and expected GIC magnitude/duration at each location are vulnerability factors. Age is not a direct indicator, since insulation life is reduced by loading history plus poor maintenance condition (e.g. excess insulation moisture levels). So a newer, heavily loaded or poorly maintained transformer may have less remaining insulation life than an older transformer.
As such, transformers must be individually assessed in regards to the expected high GIC levels. As a result, it is impossible to determine how many transformer locations are of concern.
Question: What is your estimate of the percentage of the transformers in use now are at risk? Are larger transformers 10+ MVA at greater risk
Answer: Without a detailed analysis of many transformers, it is difficult to give an accurate figure of how many transformers are at risk. Clearly, the older, high-voltage, single-phase and large MVA capacity transformers near the end of their useful life are most likely to suffer as a result of thermal impacts related to GIC. Transformers matching this description should be evaluated to determine if these critical assets require additional protection or replacement.
Question: Can you discuss shunt reactor susceptibility to GIC?
Answer: Shunt reactors are not susceptible to GIC heating, even at extreme dc levels. This is because reactors are designed with a high-core reluctance via the use of air core gaps. The high reluctance means that GIC-caused saturation has only a minimal effect on transformer's stray flux.
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