Knowledge Base

GIC Modeling Data Requirements

Some general guides for substation and transformer inputs needed for GMD analysis may be found in the following links, with some specifics as they pertain to PowerWorld Simulator:

  1. A spreadsheet template (updated 2/14/2017) which describes the input data for substations and transformers needed to perform GIC analysis. (These are inputs that do not occur in FERC 715 load flow models.)
  2. A presentation that explains some inputs and calculations for assumed coil resistance (when user does not specify), effective transformer GIC, and K-Factor

A brief explanation of the spreadsheet follows. (Much of this information is also contained as “comments” in select cells.)

  1. The substation inputs are shown on the “Substations” sheet. Buses must be assigned to substations (especially transformer buses in the GIC study footprint). PowerWorld can provide bus-substation assignments to Simulator GIC customers for FERC715 cases.
  2. Key substation inputs are grounding resistance (Ohms), Latitude, and Longitude. (Western hemisphere longitudes have a negative sign.) PowerWorld can provide latitude and longitude for FERC715 cases to Simulator GIC customers.
  3. Substation grounding resistance is the resistance in ohms between the substation neutral and earth ground (zero-potential reference). An actual “fall of potential” test is the best way to determine this resistance. It is very important to specify this value for all substations with grounded transformers. There is not a reliable method for estimating it.
  4. Transformer inputs are shown on 2 sheets. Either may be used depending on the purpose and the level of detail of available data:
    • “Transformers Simple” includes a simple format for the most essential data fields. This is sufficient if data is provided to PowerWorld or an outside consultant for use in software. (It should be roughly compatible with other GIC software input requirements, though the specific column headings might need to be changed.)
    • “Transformers Complete” has additional detail and would be preferred if the user intends to paste the data directly into PowerWorld Simulator. The most essential fields are highlighted yellow. Additional inputs that may be known are highlighted green.
  5. Essential transformer fields:
    •  Coil Resistance for high and medium voltage windings. This is DC resistance in ohms, per phase. (Values for ungrounded delta windings are generally not needed as these are open circuits to GIC.)
    • Configuration for high and medium windings. (The tertiary is normally assumed to be delta connected and open circuit to GIC.)
    • Is Autotransformer: yes or no
    • Core Type
  6. Additional transformer fields (shown only on the “Transformers Complete” sheet)
    • GIC Blocked for Transformer Neutral: Is a capacitive blocking device in series with the transformer neutral? (yes or no)
    • Coil Resistance and Configuration for tertiary windings (normally assumed to be delta connected and not needed)
    • GIC Model Type (K-Factor: default or user-specified 2 segment piecewise linear)
    • GIC Model segment slopes and break point
  7. Many transformer fields have only certain allowable values. These are defined on the “Transformer Options” sheet. Some points of clarification:
    • Manually Enter Coil Resistance: Use “Yes” in PowerWorld Simulator if the value is to be specified. Otherwise the coil resistance will be derived from the transformer’s per-unit impedance in the load flow model.
    • GIC Model Type: “Default” means the K-factor will be derived from nominal voltage and core type. “Piecewise Linear” means the K-Factor will be the user-specified values in the “GIC Model First Segment Slope”,  “GIC Model Param Break Point”, and “GIC Model Second Segment Slope” fields. The values relate effective per-unit effective GIC flow to per-unit GIC reactive power losses.
  8. Switched Shunts and Line Shunts can also provide paths to ground for GIC. Resistance data are entered on the “Switched Shunts” and “Line Shunts” sheets, respectively. Zero values are treated as infinite resistance. (This is also the default in Simulator, if no data is provided.)
  9. Transmission Line resistance may also be provided on the sheet “Transmission Lines”. If none is provided, Simulator calculates DC resistance from AC per-unit resistance in the power flow model. (Skin effect is assumed negligible at 50 or 60 Hz.)

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February 14, 2014