GIC and Substation Coordinates for Line Taps
To properly perform geomagnetic induced current (GIC) analysis, substation geographic coordinates (longitude, latitude) must be provided for all substations that contain terminals of transmission lines for which a GIC-induced equivalent DC voltage is applied. Generally this includes all lines greater than the minimum length and minimum voltage specified on the GIC Analysis Form. The need for coordinates applies regardless of whether the substation contains one or more grounded transformers. (However, substations without grounded transformers do not need accurate grounding resistances, as they contain no path to ground for GIC.)
Consider the simple 7-bus, 4-substation example below.
Substation 4 is a tap between substations 2 and 3 and contains no high-side grounded transformers.
- Geographic coordinates must be provided for sub 4. If they are left blank, Simulator cannot calculate GIC-induced voltage on the lines connecting sub 4 (fields in bold orange text in the diagram). However, if the lines connecting sub 4 are compensated with series capacitors, and if there are no connections to ground along the line, then it is not necessary to provide coordinates for sub 4. The line would appear an open circuit to GIC and it would not be necessary to model a GIC DC input voltage.
- The geographic coordinates for sub 4 need not be exact, as the E-field dot products are such that the induced voltage on the 2 lines that connect sub 4 should sum to the same value, regardless of the sub 4 coordinates. However, error may be introduced by numeric precision if the coordinates are not close to the correct location. It is suggested that substations like sub 4 be located within about 100 km or 60 miles of their correct locations.
- Because there is no ground path in sub 4, the assumed grounding resistance does not impact GIC calculations.
The files in this example are provided here. For an example of error introduced by numeric precision, change the coordinates of substation 4 to lat=40, long=-90, which moves it about 760 km from its original location. The sum of the induced voltages changes from 1555.6 to 1509.2 (about 3% error).
March 26, 2015