What does the testing of photovoltaic transformers involve?

Oliver Watts

Field: Inspection and testing

China

1. Specificities and Testing Requirements of Photovoltaic Transformers

As a new energy systems technician, I recognize photovoltaic transformers' unique design and application traits: Inverter - output AC carries abundant 5th/7th - order odd harmonics, with PCC harmonic current distortion reaching 1.8% (higher voltage distortion under low load), causing winding overheating and accelerated insulation aging. Photovoltaic systems use TN - S grounding, requiring reliable N - phase output from the secondary side to avoid short circuits. Environmentally, they must withstand 60°C desert heat, coastal salt spray, and industrial EMI.

These specifics dictate testing uniqueness: Beyond conventional DC resistance, voltage ratio, insulation, and withstand voltage tests, add harmonic detection (Fluke F435 for THD), temperature rise monitoring (infrared imagers), grounding system checks (four - terminal method for ≤0.1Ω contact resistance), and short - circuit impedance testing. The core aim is ensuring safe operation in power electronic environments while preventing harmonic, thermal, and grounding - related risks.

2. Conventional Testing Items and Tool Selection for Photovoltaic Transformers
2.1 DC Resistance Test

This key test identifies inter - turn short circuits or loose connections in windings. The four - terminal method is used to eliminate line resistance interference, with procedures including power-off discharge, winding cleaning, temperature measurement, current selection (1A/10A), and temperature correction. The ZSCZ - 8900 DC resistance tester (accuracy: 0.2%±2μΩ, resolution: 0.1μΩ) meets high - precision requirements. Measured values must be compared with standards/historical data; significant deviations may indicate faults - as seen in a case where poor winding contact was detected via DC resistance testing and later repaired.

2.2 Voltage Ratio Test

This verifies whether winding turns ratios align with design specifications to ensure stable voltage output under load. The dual - voltmeter method calculates ratios by measuring primary/secondary voltages under no - load conditions, while the voltage ratio bridge method offers higher precision. For instance, a voltage imbalance in the low - voltage output of an 800V/400V transformer, caused by a high - voltage side open circuit, was identified through voltage ratio testing.

2.3 Insulation Performance Test

Insulation Resistance Test: Using an MI - 2094H megohmmeter, measure insulation resistance between windings and between windings and the core (required ≥300MΩ).
Withstand Voltage Test: Apply 2× rated voltage for 60 minutes to check for breakdown. Ensure power is off, disconnected from live equipment, and surfaces are clean before testing.

2.4 Short - Circuit Impedance Test

The volt - ampere method evaluates short - circuit tolerance: one side is shorted, and a test voltage is applied to the other side to drive rated current through windings, measured by a CS - 8 impedance tester. A change >±2% from the factory value may indicate winding deformation. Note: Test current should be controlled at 0.5% - 1% of rated current to avoid waveform distortion.

2.5 Temperature Rise Test

After full - load operation, measure temperatures of windings, core, and casing using thermometers or infrared thermometers. Temperature rises should be ≤60K for oil - immersed transformers and ≤75K for dry - type transformers. A dry - type transformer operating in a 60°C environment that maintained a temperature rise within 65K extended its service life effectively.

2.6 Grounding System Test

The four - terminal method measures grounding continuity to avoid misjudgments from the two - terminal method. Common faults include rusted connections or plastic washer misuse, requiring regular inspection. Four - terminal ground resistance testers ensure measurements meet the 0.1Ω standard.

2.7 Harmonic Detection

A unique test for photovoltaic systems, using Fluke F435 at the PCC to detect harmonics up to the 50th order (focusing on 5th/7th orders). Results must comply with GB/T 14549 - 93, providing data for equipment optimization.

3. On - site Testing Procedures and Safety Specifications for Photovoltaic Transformers
3.1 Pre - testing Preparation

Develop detailed plans specifying project information, testing items, and equipment lists (including high - precision power analyzers, power quality testers, infrared thermal imagers, etc.). Check equipment integrity and power voltage (220V±10%), and monitor environmental conditions - such as irradiance ≥700W/m², irradiance variation <2% in the preceding 5 minutes, no strong winds or clouds - to ensure test accuracy.

3.2 Electrical Connection Inspection

Use a phase volt - ampere meter to verify inverter output polarity matches the transformer's primary corresponding terminal, preventing circulating current losses. Inspect cable connections for tightness. For oil - immersed transformers, check oil level and color; for dry - type transformers, verify cooling fans operate normally.

3.3 Insulation Resistance Test

With power off, use a megohmmeter to test high/low - voltage windings and grounding, recording 1 - minute stable values. A sudden resistance drop indicates insulation issues. Detailed test reports must be compiled post - testing.

3.4 AC Withstand Voltage Test

Connect the withstand voltage device's output to test points, set parameters to 2× rated voltage, gradually increase voltage while monitoring for breakdown, and maintain for 60 minutes before reducing voltage.

3.5 Load Test

Measure output voltage, current, and power under full - load operation to calculate efficiency and voltage regulation rate, while monitoring temperature rise. Increase load current gradually and record parameter changes for analysis.

3.6 Short - Circuit Impedance Test

Apply voltage to the high - voltage side with the low - voltage side shorted (using wires with sufficient cross - section). Control test current at 0.5% - 1% of rated value and correct results for temperature (75°C for oil - immersed, 120°C for dry - type) to avoid misjudging winding deformation.

3.7 Harmonic Detection

Use a power quality analyzer at the PCC to monitor odd - order harmonic content and calculate THD, ensuring compliance with national standards for safe operation in harmonic environments.

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Solar Transformer

Inspection and testing

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Oliver Watts

China

Expertise area

Inspection and testing

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