You've accurately diagnosed the problem: circulating currents in the delta secondary, even at no load, are causing unbalanced line-to-ground voltages which, in turn, are tripping your sensitive ground fault relays. Delta systems are notorious for this, especially when you're dealing with slightly mismatched transformer windings or capacitance to ground.Here's a breakdown of possible causes and solutions

Understanding the Problem Deeper:
Capacitive Coupling to Ground

This is usually the biggest culprit. Every conductor (including transformer windings) has a small capacitance to ground. In a delta system, these capacitances form a network. Any imbalance in these capacitances, especially if the transformer core isn't perfectly grounded, leads to voltage differences between each phase and ground.

Transformer Imperfections

Minor differences in winding impedance (reactance and resistance) between phases, core asymmetries, and even manufacturing tolerances can contribute to unbalanced circulating currents.

Harmonics

Even if the source voltage appears clean, harmonics (especially 3rd and its multiples) can circulate strongly in a delta, causing voltage distortions.

Relay Sensitivity

Some ground fault relays are designed to be very sensitive to even small imbalances, aiming to detect high-resistance faults. This is good for safety but can lead to nuisance tripping in your scenario.

Potential Solutions (Prioritized)


Grounding Resistors (Most Recommended)

This is the most robust and common solution for delta systems prone to this problem. You effectively create a "corner-grounded" or "high-resistance grounded" system without actually grounding one of the corners. Here's how:

Zig-Zag Grounding Transformer

This is the ideal solution. A zig-zag transformer has a specialized winding configuration that provides a low-impedance path for zero-sequence (ground fault) currents while presenting a high impedance to normal balanced currents. It essentially creates a neutral point. When a ground fault occurs, the zig-zag transformer allows current to flow, which the ground fault relay can then detect.

Advantages

Superior fault current control, effective for high-resistance faults.

Disadvantages

Higher initial cost, physical space requirements.

Neutral Grounding Resistor (NGR)

If a zig-zag transformer is too expensive or space-constrained, you can use a single resistor connected between an artificial neutral (created by a wye-delta transformer) and ground, or simply across one of the phases to ground. The artificial neutral can be created by three equal value impedance or resistor voltage divider connected in Wye from each phase.

Advantages

More affordable than zig-zag transformer, limits ground fault current to a known value.

Disadvantages

Still requires some sort of neutral creation, might not detect very high-resistance faults as effectively as a zig-zag transformer.

Sizing the Resistor

The resistor value is chosen to limit the ground fault current to a safe and manageable level. You'll need to coordinate the resistor value with your ground fault relay settings.2.

Relay Adjustments (Carefully Considered)

Before changing relays entirely, try these (with extreme caution and understanding):

Increase Trip Threshold

The Littelfuse EL3100 might have adjustable settings for the trip current and time delay. Carefully increase the trip current threshold just enough to avoid nuisance tripping at no load. Do not disable the ground fault protection entirely! You are trading off some sensitivity for reliability. Document this adjustment thoroughly.

Increase Time Delay

A short time delay (e.g., 0.1-0.2 seconds) might allow the unbalanced voltages to settle down after energization before the relay trips. Again, be careful; you are slowing down the response to a real fault.

Important Warning

Altering relay settings must be done by qualified personnel with a thorough understanding of ground fault protection principles and the system's safety requirements. Improper settings can compromise safety.3.

Relay Replacement



Solid-State Differential Relay

These relays compare the currents entering and leaving the transformer. If there's a significant difference (indicating a fault), they trip the breaker. They are more sophisticated but less prone to nuisance tripping from capacitive imbalances. However, they can be more expensive.

Ground Fault Relay with Adjustable Filters

Some relays have built-in filters to attenuate harmonic frequencies or other noise that might be contributing to the imbalance.4.

Investigate and Reduce Capacitive Coupling



Shielded Cables

If you have long cable runs on the secondary side, consider using shielded cables. The shield should be properly grounded at one end to drain away stray currents.

Physical Separation

Keep secondary conductors physically separated from grounded structures as much as possible.

Transformer Core Grounding

Verify that the transformer core is properly grounded to the station ground. A poor or intermittent core ground can exacerbate capacitive imbalances.5.

Your Coworker's Suggestion (Delta-Wye Transformer)

This is not recommended as a primary solution. While it could theoretically work, it's essentially a workaround and doesn't address the underlying problem of unbalanced currents in the delta. It also adds another transformer to the system, increasing cost and complexity. The small delta-wye transformer would essentially be acting as a voltage transformer with the ground fault relays on the secondary. This would work, but is not ideal.

Steps to Take


Thoroughly Inspect

Visually inspect the transformers, wiring, and grounding connections. Look for loose connections, damaged insulation, or anything that could be contributing to leakage currents.2.

Measure Capacitance to Ground

Use a capacitance meter to measure the capacitance to ground of each phase on the secondary side. Significant differences between phases can indicate a problem.3.

Analyze Harmonics

Use a power quality analyzer to measure the harmonic content of the voltage and current on the secondary side. High harmonic levels can be a contributing factor.4.

Consult a Power Systems Engineer

Before making any significant changes to the system, consult with a qualified power systems engineer. They can perform a detailed analysis of your system and recommend the best solution.5.

Implement a Grounding Strategy

A well-designed grounding system is crucial for safety and reliability in any electrical system. The zig-zag grounding transformer is usually the preferred method, but a neutral grounding resistor may be acceptable for some applications.

Key Considerations



Safety

Ground fault protection is essential for safety. Never compromise safety by disabling or improperly adjusting ground fault relays.

Coordination

If you have multiple levels of ground fault protection, ensure that the relays are properly coordinated to avoid cascading trips.

Documentation: Thoroughly document any changes made to the system, including relay settings, grounding configurations, and test results.By systematically investigating the problem and implementing the appropriate solutions, you should be able to eliminate the nuisance tripping and ensure reliable ground fault protection for your transformers. Start with investigating the capacitive coupling and then consult with a power systems engineer about implementing a grounding resistor strategy. Flag for review