You've hit on a common and frustrating issue. Here's a breakdown of why the discrepancy occurs and what you can do about it

Why the Discrepancy Exists

The difference between the SKM-calculated load-side short circuit values and the utility-provided values comes down to a few key factors:1. Different Calculation Methods and Assumptions



SKM's Conservative Approach

SKM is designed to be conservative. It typically uses worst-case scenarios and makes assumptions that maximize the short-circuit current. This is important for safety and proper equipment rating. The idea is to ensure that anything that happens in reality is something that your overcurrent protection can handle.

Utility's Operational Focus

The utility's load-side fault current number is a projection of the fault current they expect to see at that point in the system. This is a snapshot in time. They are using the number for system planning and protection coordination purposes. The utility may use different calculation methods, consider system impedance further upstream, or include assumptions about load diversity that SKM would not.2.

System Impedance Variations



Source Impedance Fluctuation

The utility's system impedance (especially at the source) is not static. It can vary depending on generation dispatch, transmission line availability, and other factors. The utility might be providing you with a value that is typical but not necessarily the absolute worst-case.

Upstream Network Complexity

The utility's network upstream of your service entrance is complex and constantly changing. Accurately modeling this in SKM is virtually impossible.3.

X/R Ratio and DC Offset



Impact of X/R Ratio

The X/R ratio is crucial. A higher X/R ratio leads to a greater DC offset component in the fault current, which can significantly increase the initial peak fault current. SKM will account for this.

Utility Assumptions

The utility may use simplified methods that don't fully capture the DC offset, leading to lower calculated values.4.

Transformer Model Variations SKM uses a transformer model that attempts to be as detailed and accurate as possible. This is intended to provide an accurate representation of the transformer impedance and the resulting short circuit current. Utilities may use a more simplified model or make assumptions that result in lower values.5. Fault Type Assumptions

The fault type used for calculation can have an impact. SKM can perform various types of fault studies (e.g., three-phase, line-to-ground). Utilities may provide short-circuit values based on a specific fault type.

What You Should Do

Here's a strategy for addressing this issue:1. Understand the Utility's Number



Ask Questions

Don't just accept the number. Ask the utility: "What type of fault does this represent (3-phase, line-to-ground)?" "Is this the maximum fault current, or a typical/design value?" "What assumptions did you make about system loading and source impedance?" "What method was used to calculate the value, and what is the basis for that method?" "Can you provide a range of possible fault currents?"

Consider Their Perspective

The utility is concerned with grid stability and coordination. Their number might reflect a design value that allows them to maintain protection schemes.2.

Validate Your Model



Double-Check Input Data

Ensure you've entered the utility's data (transformer impedance, voltage, kVA, etc.) accurately. A simple typo can cause significant differences.

Transformer Impedance Tolerance

Transformers have impedance tolerances (typically +/- 10% on the nameplate impedance). Experiment within reason with adjusting the transformer impedance within the tolerance range in your SKM model. You might get closer to the utility's value.

Crucially, don't go outside the nameplate tolerance just to match the utility's number! Model Complexity

Don't overcomplicate. Unless you have detailed knowledge, avoid overly complex models of the utility grid.3.

Document and Justify the Difference



Explain the Methodology

In your report, clearly explain that you used SKM, a conservative and industry-standard software for short-circuit calculations.

Acknowledge the Utility Data

State that you received short-circuit data from the utility.

Compare and Contrast

Present both the utility's value and the SKM-calculated value. Explain the likely reasons for the difference (as outlined above).

Focus on Conservative Results

Emphasize that you are using the SKM result for equipment selection and protection coordination because it represents a more conservative (worst-case) scenario.

Sample Wording

"The utility provided a load-side fault current value of 900A at the transformer secondary. The SKM model, using a conservative short-circuit calculation methodology, yields a fault current of 3200A. This difference is likely due to [reasons from above]. For the purposes of this study, we are using the SKM-calculated value of 3200A to ensure a conservative approach to equipment rating and overcurrent protection coordination."4.

The "Hybrid" Approach (Use with Caution)



Don't Force a Match

Resist the urge to arbitrarily manipulate the SKM model to exactly match the utility's number. This undermines the integrity of your study.

Consider Adjustment (With Justification)

If, after careful investigation and consultation with the utility, you believe their value is a more accurate representation of the actual short-circuit current, you could consider the following:

Adjust Source Impedance

Adjust the utility source impedance in the SKM model within a reasonable range to bring the calculated load-side fault current closer to the utility value. This is a complex task and requires a solid understanding of system impedance. Document your reasoning extensively.

Transformer Impedance Adjustment (Within Tolerance)

As mentioned, adjust the transformer impedance within its tolerance range only.

Explain the Adjustment

Explicitly state in your report that you adjusted the model to reflect the utility's data, the reasons for the adjustment, and the impact on the results.5.

Presentation is Key



Be Transparent

Don't hide the discrepancy. Address it head-on.

Confidence

Project confidence in your understanding of the issue and your rationale for the chosen approach.

Education

Use this as an opportunity to educate your client about the complexities of short-circuit analysis.

Regarding Your Specific Questions



Real-Life Losses vs. Theoretical Calculations

"Real-life" losses are partially captured in the transformer impedance. The bigger issue is the difference in assumptions and calculation methods.

Another Value to Request

Ask for a range of possible fault currents, not just a single value. Also, ask about the source impedance they used for their calculation (even if it's just a typical value).

Tolerance on Z%

Use the nameplate tolerance, only.

Removing Utility XF

While some do this, keeping the transformer is preferable. It provides valuable information about the system configuration.

In summary: The most important thing is to understand why* the numbers differ, document your assumptions and calculations, and clearly explain the discrepancy in your report. Focus on the conservative approach and the safety implications of using the higher (SKM-calculated) fault current. Good luck! Flag for review