Okay, let's break down your situation and the calculations to see if the 0.2 ohms you're measuring makes sense.

Understanding the Setup PV Inverter

You're measuring between the AC output phases (A, B, C) of a PV inverter downstream of contactors and fuses. This means you're isolating the inverter output circuitry itself.

MV Transformer

These AC phases connect directly to the primary windings of a 3630 kVA, 690V/MV (Y-Delta) transformer without a primary disconnect. This is crucial. You're measuring the impedance through the transformer windings and back through the other inverter phases.

Measurement

You're measuring 0.2 ohms between the phases.

Analyzing Your Math (Based on your Picture)

It's critical that I can see the image to verify the math. However, based on your description, I can give you a breakdown of what needs to be done and probable assumptions.1. Base Impedance Calculation (Assuming Correct)

You're trying to relate the transformer's nameplate data (kVA and voltage) to an equivalent impedance. The core concept is: `Z_base = V_base^2 / S_base` where `V_base` is the primary voltage (690V), and `S_base` is the transformer kVA (3630 kVA).2.

Per-Unit (pu) Impedance (Assuming Correct)

You're likely calculating the transformer's per-unit impedance (Zpu). This is usually a value provided by the transformer manufacturer on its nameplate or datasheets. It's essential for calculations. You must get the Zpu from the transformer datasheet. Typical values for this size transformer might be in the range of 0.04 to 0.10 pu, but it varies. Do not assume a Zpu.3.

Actual Impedance Calculation (Critical Point)

You're calculating the actual impedance of the transformer referred to the primary side (which is the 690V side). `Z_actual = Z_pu Z_base`4.

Phase-to-Phase Impedance vs. Measured Impedance

This is where the interpretation is important. Your meter reads 0.2 ohms between phases. This is NOT the same as the calculated impedance to neutral. Given the Y-Delta configuration, you are reading a winding-to-winding impedance. The actual transformer winding impedance between the Y primary coils are likely less than this value.

Is 0.2 Ohms Reasonable? It's Possible, but Needs Confirmation

Without knowing the exact Zpu of your transformer, it's difficult to say definitively. It is a low impediance reading, so a check of calculations and procedure will be important. But you also have to remember this is a winding to winding reading and therefore, reasonable.

Possible Concerns and Things to Check1. Transformer Datasheet

The most important thing is to get the transformer's nameplate data, especially the Zpu value. Also note the vector group. This is critical.2.

Metering Accuracy

Ensure your meter is accurate at low impedance measurements. Many meters aren't very precise in this range. Try a different meter if possible. Also ensure your test leads are properly compensated, shorted, and tared to remove lead resistance from the equation.3.

Connections

Double-check all connections are tight and clean (at the inverter output, fuses, contactors, and transformer terminals). Loose or corroded connections can affect impedance measurements.4.

Short Circuit/Ground Faults

While less likely if everything is functioning, a ground fault on the primary side (even a developing one) could lower the impedance. However, a direct short circuit is much more common with these conditions, and will result in voltage depression.5.

Transformer Health

Consider a transformer insulation resistance test (Megger test) to check for winding insulation degradation. This is a standard maintenance practice and can reveal potential problems.

Steps to Take


Get Transformer Data

Find the transformer's datasheet. Focus on the kVA, primary voltage, and, most importantly, the Zpu (per-unit impedance) and vector group.2.

Re-Check Measurements

Ensure your meter is accurate, and re-take the impedance measurements between phases.3.

Verify Calculations

With the correct Zpu, repeat the calculations to find the expected impedance.4.

Compare

Compare the calculated impedance to your measured impedance.5.

Insulation Resistance Test

Conduct a Megger test on the transformer primary windings.6.

Consult Experts

If you're still unsure, consult with a qualified electrical engineer experienced with PV systems and transformer testing.

In summary, 0.2 ohms could be reasonable, but it depends on the transformer's Zpu and the other factors mentioned above. You need to do a thorough investigation to be certain. Flag for review