Welcome to the forum! This is a great question and definitely gets into the nuances of fault analysis. Let's break down the potential reasons for your observed sequence current behavior.

Understanding the Ideal Single-Phase-to-Ground Fault Scenario

You're correct. In a textbook single-phase-to-ground fault with zero fault impedance (a bolted fault at the point of fault), the ideal scenario is: I0 = I1 = I2 in magnitude All three sequence currents are in phase (0-degree phase angle relative to each other) This is because the positive, negative, and zero sequence networks are connected in series.

Why Your Results Deviate: Potential Explanations

Here's a breakdown of why your measured sequence currents might show a smaller zero-sequence current:1. Fault Impedance (Rf)

This is the most likely culprit. Any fault impedance will alter the sequence current distribution. The presence of Rf affects the zero sequence current.

Impact

A significant Rf will reduce the magnitude of all fault currents, but it will reduce the zero sequence current to a greater degree. This is because the zero-sequence network often has higher impedance than the positive and negative sequence networks.

Why

Think of it as adding resistance to the zero-sequence path more than to the positive and negative sequence paths.2.

Transformer Grounding and Zero-Sequence Impedance



Transformer Winding Configuration

The winding configuration of the transformer is critical. The zero sequence currents will pass through the grounded windings. The transformer winding configuration might influence the magnitude of zero sequence impedance seen by the fault.

Grounding Type

The method of grounding the transformer's neutral point (solidly grounded, resistance grounded, ungrounded) will significantly impact the zero-sequence impedance. A high-resistance ground will dramatically limit I0.

Zero-Sequence Impedance (Z0)

The transformer's Z0 can be significantly different from its positive (Z1) and negative (Z2) sequence impedances. A higher Z0 will restrict I0.3.

Remote Fault Location

You mentioned measuring the currents at a distance from the fault.

Line Impedance (ZL)

While ideally the sequence networks are in series, line impedance adds impedance to each sequence network separately. The positive and negative sequence line impedances are equal, but the zero sequence line impedance can be significantly higher. This will affect the ratio of sequence currents.

Impact

The line impedance, particularly the zero-sequence line impedance, will cause a voltage drop between the fault location and your measurement point.4.

Partial Fault Involving Multiple Phases (Less Likely, But Possible)



Two Phases to Ground with Asymmetrical Impedances

It is unlikely that you will have two phases with identical impedances, but if the impedance from one phase to ground is lower, it could result in a signature that deviates from the textbook.

Impact

Complex to analyze without more details.5.

Measurement Error/Instrument Transformer Issues



CT Saturation

Current transformers (CTs) can saturate during high fault currents, especially if the CTs are not sized appropriately. This will distort the waveform and affect your symmetrical component calculations. This is especially true if the CTs are of an older vintage.

PT Accuracy

Potential Transformer (PT) accuracy might influence the voltage calculations that contribute to the symmetrical component calculations.

Calibration

Make sure that your equipment is properly calibrated.

Impact

Garbage in, garbage out. If there is a measurement error, it can significantly impact your results.

Troubleshooting Steps and Recommendations1. Review Transformer Data

Critically examine the transformer's nameplate data and any available impedance information, especially Z0. Understand how the transformer's neutral is grounded.
2.

Model the System

Create a positive, negative, and zero sequence impedance diagram, extending from your measurement point back to the source and the fault location. Include line impedances and transformer impedances. This will help you theoretically calculate the expected sequence currents for different fault impedances. You can use software like ETAP, SKM, or even a spreadsheet for simpler models.3.

Experiment with Fault Impedance in Simulations

Once you have a model, simulate the single-phase-to-ground fault, varying the fault impedance (Rf) and observe how it affects the magnitude of I0, I1, and I2. This should give you a much better understanding of what Rf value would produce the currents you're seeing.4.

Consider the Measurement Setup

Verify the accuracy of your current and voltage measurements. Check for CT saturation. Compare the fault current levels to the CT ratings and burden. Ensure proper phasing of your CT connections.5.

Analyze the Waveforms Themselves

Even without uploading images, look closely at the shape* of the fault current waveforms for each phase. Are there any unusual distortions or asymmetries? This might give you clues about the fault type or a measurement issue.6.

Check for High-Resistance Grounding

Determine how the transformer neutral is grounded (solidly grounded, resistance grounded). If it is resistance grounded, that will inherently limit the zero sequence current.

In summary, the most likely explanation for I0 being smaller than I1 and I2 is the presence of fault impedance, possibly coupled with the influence of zero-sequence impedance in the transformer or grounding resistance. Building a model and experimenting with different parameters is the best way to confirm this. Let me know if you can provide more details about the system and transformer configuration! Flag for review