What Are the Main Tests Conducted Before the Delivery of an Oil-Immersed Transformer?

Oil-immersed transformers are critical components in modern power systems. Before shipment, they must pass a comprehensive series of tests to verify compliance with international and national standards such as IEC 60076 and GB/T 1094. These tests ensure optimal electrical performance, insulation reliability, and operational safety during long-term service.

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Electrical Performance Tests

Voltage Measurement: Both offline and online voltage ratio tests confirm accurate voltage conversion under rated conditions. This prevents damage from overvoltage or undervoltage and ensures consistent performance.

DC Resistance Test: Measures the direct current resistance of windings to detect short circuits, open circuits, or poor connections. Deviation from previous test data must not exceed 2%, as per technical specifications.

Insulation Performance Tests

Insulation Resistance Test: Evaluates insulation between windings, bushings, and the transformer core. The insulation resistance must meet minimum standard values, and the polarization index or absorption ratio should be no less than 1.3.

Dielectric Loss (tanδ) Test: Measures the loss factor of insulation. The dielectric dissipation factor must not exceed 130% of the factory value, or must comply with the manufacturer's limits, reflecting insulation aging and quality.

Power Frequency Withstand Voltage Test: Applies a power frequency voltage (usually for 1 minute) to ensure insulation strength under rated voltage without any breakdown or flashover.

Induced Overvoltage Withstand Test: Simulates overvoltage operating scenarios by applying higher-than-rated voltage, assessing inter-turn and inter-layer insulation integrity in the windings.

Fault Detection and Withstand Tests

Partial Discharge (PD) Test: High-voltage pre-stress is applied to detect internal insulation imperfections like air pockets or contamination. The discharge level should not exceed 10 picoCoulombs (pC), helping to prevent long-term degradation.

Impulse Voltage Test: Simulates lightning or switching surges using 1.2/50 μs impulse waveforms. The waveform and amplitude must match product-specific technical data to confirm resilience against extreme overvoltage.

Operational Performance Verification

No-Load Test: Measures core magnetization loss and current under rated voltage. No-load loss must meet design standards, and no-load current is typically between 1%–5% of rated current.

Load Test: Evaluates load loss and impedance voltage under full load. The measured impedance deviation should be within ±10% of the design value.

Temperature Rise Test: The transformer is operated under rated current, and temperature rise is measured at key locations such as windings, oil surface, and core. The winding temperature rise must not exceed 65 K, validating heat dissipation effectiveness.

Oil Quality Testing

Dissolved Gas Analysis (DGA): Uses gas chromatography to detect methane, acetylene, CO, and other gases dissolved in the insulating oil. Based on IEC 60599 or DL/T 722, this test reveals possible internal faults such as overheating or insulation failure.

Insulating Oil Tests: A complete oil evaluation includes dielectric strength, dissipation factor, moisture level, gas content, and acidity. These parameters, based on GB 2536, determine whether the insulating oil meets performance and chemical stability requirements.

Pre-delivery testing of oil-immersed transformers is a crucial quality control step to guarantee safety, reliability, and efficiency in power applications. Each test, from partial discharge to insulating oil analysis, ensures compliance with stringent standards and provides confidence in the transformer's long-term performance.