You've hit on a very real and often overlooked issue with active harmonic filters (AHFs) on Wye-Wye utility services, particularly when combined with older 6-pulse VFDs. Your suspicion is highly plausible: the harmonic currents generated by the VFDs and actively injected by the AHFs are likely polluting the utility's Wye-Wye distribution system, causing nuisance tripping or other power quality issues for other customers.Here's a breakdown of why this is happening and what to consider

Why Wye-Wye Transformers are Problematic with AHFs and 6-Pulse VFDs:
Triplen Harmonics (Especially 3rd)

6-pulse VFDs are notorious for generating significant 3rd, 9th, 15th, and other triplen harmonics (multiples of 3). These currents are zero-sequence currents. They don't cancel out in a balanced 3-phase system; instead, they add up in the neutral conductor.

Wye-Wye Transformer Path for Triplens

Unlike Delta-Wye transformers, Wye-Wye transformers do not block zero-sequence currents. The triplen harmonic currents generated by the VFDs (and enhanced by the AHFs trying to correct the distortion) have a direct path to flow through the Wye-Wye transformer and back into the utility's neutral.

Potential for High Neutral Currents and Voltages

This circulation of triplen harmonics can cause: Excessive neutral currents on both the customer and utility side. Elevated neutral-to-ground voltages, which can damage sensitive electronic equipment. Voltage distortion on the utility's distribution system, affecting other customers. Resonance, if the system impedance matches the harmonic frequency.

AHF Exacerbates the Problem

AHFs work by injecting current waveforms into the system to cancel the existing harmonic currents. However, if those injected currents simply circulate back into the utility grid through the Wye-Wye transformer, you're not solving the problem; you're just moving it elsewhere. You are essentially using the AHF as an active current source back into the utility.

Why Your THD Readings are Low (Voltage) and Moderate (Current)



Low VTHD

The utility source impedance can be relatively low, especially with a 1500kVA transformer. So, while the current distortion might be significant, the voltage distortion (VTHD) is limited due to the utility's ability to absorb the harmonic currents without a large voltage drop.

I THD at 20%

This is a moderate level of current distortion, indicating the VFDs are producing harmonics. This is not an ideal situation, and can be improved. The 20% THD-I is the reason the site invested in the AHF units.

What to Consider / Investigate


Confirm the Utility Transformer Configuration

Double-check the nameplate data or contact the utility directly to absolutely confirm that it's a Wye-Wye transformer. Get the exact model number. This is your starting point.2.

AHF Configuration and Settings

If you can get access to the AHF's setup software, check the settings. Are they even configured correctly for the system? Are they programmed with the right compensation strategy? Are they injecting at the proper level to even matter?3.

Neutral Current Measurement

Measure the neutral current before and after enabling even one AHF. You should expect to see a dramatic increase in neutral current when the AHF is operating. This is a key indicator that the harmonics are circulating back into the utility grid.4.

Utility Coordination

Before doing anything with the AHFs, contact the utility. Explain the situation and that you are investigating harmonic issues at your site and the AHF may be injecting noise back onto the utility service. They may have records of previous issues or specific requirements for harmonic mitigation. They might even offer to perform their own power quality analysis on your system. They will most likely NOT want you to turn these back on unless you have a resolution to the harmonic noise being injected.5.

Alternative Harmonic Mitigation Strategies

If the Wye-Wye transformer is confirmed, consider these alternatives to AHFs:

Passive Harmonic Filters

Install properly sized passive harmonic filters at each VFD. These are typically tuned to specific harmonic frequencies (e.g., 5th, 7th). They provide a low-impedance path for the harmonic currents, preventing them from propagating back into the system. Passive filters are often a more robust solution for Wye-Wye systems. However, system impedance and possible resonance conditions need to be carefully considered.

12-Pulse VFDs

Replace the 6-pulse VFDs with 12-pulse VFDs or better (e.g., active front-end (AFE) VFDs or VFDs with multi-pulse rectifiers). 12-pulse VFDs significantly reduce the 5th and 7th harmonics, and AFE VFDs virtually eliminate harmonics altogether. This is the most effective (but also most expensive) solution.

Line Reactors/DC Chokes

Since you already have 3% line reactors, ensure they are properly sized and installed. While they offer some harmonic reduction, they are not as effective as dedicated filters or 12-pulse VFDs. Consider adding DC chokes on the DC bus of the VFDs, as this can reduce harmonic currents further.6.

System Impedance Scan Have a power systems company conduct a full system impedance scan. Use this report to properly tune the AHF's to avoid issues with the utility system.

Has Anyone Successfully Used AHFs on Wye-Wye Systems?

Yes, but it's much more challenging and requires careful design and coordination. Success depends on: Low Harmonic Penetration

The amount of harmonic current injected must be very small compared to the utility's available fault current. Small VFDs or a very stiff utility grid are better.

Careful AHF Tuning

The AHF must be tuned very precisely to avoid over-compensation or resonance. It may require advanced control algorithms and real-time monitoring.

System-Wide Analysis

A detailed power system study (including harmonic analysis and impedance scanning) is crucial to identify potential resonance points and ensure the AHF doesn't exacerbate existing problems.*

Utility Collaboration

Ongoing communication and coordination with the utility are essential to monitor the impact of the AHF on the grid and make adjustments as needed.

In your case, given the existing Wye-Wye transformer, the older 6-pulse VFDs, and the inoperable AHFs, I strongly recommend exploring passive filters or 12-pulse/AFE VFDs as a more reliable and less problematic solution. The active filters may not be the best method here.Don't simply try to "fix" the AHFs without addressing the underlying system incompatibility. You could end up causing more problems than you solve, and potentially damaging equipment or disrupting the utility's service.

Start with confirming the transformer type and contacting the utility. That's the most important first step. Flag for review