number of winding turns) generally determine the core steel volume requirements of a transformer.
Therefore, designing for unsaturated operation with the high level of GIC present would require a
core of excessive size. The ability to even assess existing transformer vulnerability is a difficult under-
taking and can only be confidently achieved in extensive case-by-case investigations. Each transformer
design (even from the same manufacturer) can contain numerous subtle design variations. These
variations complicate the calculation of how and at what density the stray flux can impinge on internal
structures in the transformer. However, the experience from contemporary space weather events is
revealing and potentially paints an ominous outcome for historically large storms that are yet to occur
on today’s infrastructure. As a case in point, during a September 2004 Electric Power Research Industry
workshop on transformer damage due to GIC, Eskom, the power utility that operates the power grid in
South Africa (geomagnetic latitudes 278 to 348 ), reported damage and loss of 15 large, high-voltage
transformers (400 kV operating voltage) due to the geomagnetic storms of late October 2003. This
damage occurred at peak disturbance levels of less than 100 nT=min in the region (Kappenman, 2005).
16.4 An Overview of Power System Reliability and Related
Space Weather Climatology
The maintenance of the functional integrity of the bulk electric systems (i.e., power systems reliability) at
all times is a very high priority for the planning and operation of power systems worldwide. Power
systems are too large and critical in their operation to easily perform physical tests of their reliability
performance for various contingencies. The ability of power systems to meet these requirements is
commonly measured by deterministic study methods to test the system’s ability to withstand probable
disturbances through computer simulations. Traditionally, the design criterion consists of multiple
outage and disturbance contingencies typical of what may be created from relatively localized terrestrial
weather impacts. These stress tests are then applied against the network model under critical load or
system transfer conditions to define important system design and operating constraints in the network.
GIC050100150200External tank temp Top oilTemperature (°C)TimeGIC and tank temperature
5/10/92GIC (A)− 30− 20− 100102030405060704:094:194:294:394:494:595:095:195:295:395:49FIGURE 16.6 Transformer hot spot heating due to stray flux can be a concern in operation of a transformer with
GIC present. This transformer experienced stray flux heating that could be monitored with a thermocouple mounted
on the tank exterior surface. This storm demonstrated that the GIC and resulting half-cycle saturation produced a
rapid heating in the tank hot spot. Notice also that transformer top-oil temperature did not show any significant
change, indicating that the hot spot was relatively localized. (Courtesy Phil Gattens.)