When a transformer is newly installed or undergoes major maintenance, a transformer impulse test is critical before formal commissioning. This test verifies insulation strength, mechanical robustness, and relay protection performance. This guide explains the types of impact tests in power systems, the specific purposes of transformer impulse closing tests, and the standard procedure.

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1. General Types of Shock/Impact Tests in Power Systems

Impact tests in power systems fall into three main categories. Each serves a different purpose:

Test Type	Description	Application
Pulse test (sine wave)	Uses a sine wave vibration testing machine	Aerospace, defense, communications; detects early faults and structural strength
Impact spectrum test	Controls impact force magnitude and duration on an impact test bench	Verifies product adaptability to shock; tests packaging durability during shipping/handling
Testing machine method	Specialized equipment (e.g., crushed stone impact tester)	Automotive material and coating resistance to gravel impact

Temperature Classifications

Impact tests are also divided by temperature:

• Normal temperature impact test – Conducted at 23°C ± 5°C.
• Low temperature impact test – Sample stored in a low‑temperature medium for a set time, then removed for immediate testing.

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2. Why Perform a Transformer Impulse Test in Power Grids?

When a transformer is first commissioned, engineers follow handover test standards (insulation, protection, secondary circuits). However, before formal operation, they perform a no‑load full‑voltage closing impulse test – usually 5 consecutive closings. This transformer impulse test serves two critical purposes:

2.1 Verify Insulation Strength Against Overvoltage

When you open a no‑load transformer, operating overvoltage can occur. The magnitude depends on the neutral grounding method:

Neutral Grounding	Maximum Overvoltage (phase voltage multiple)
Ungrounded or via arc suppression coil	4 – 4.5×
Directly grounded	3×

Therefore, the impulse test checks whether the transformer’s insulation can withstand full voltage or these operating overvoltages.

2.2 Assess Mechanical Strength & Relay Protection

When you close a no‑load transformer, an excitation inrush current flows. This current can reach 6 to 8 times the rated current. Such a large current generates significant electromagnetic forces.

• Mechanical strength assessment – The winding and core must resist these forces without deformation.
• Relay protection evaluation – The inrush current can cause differential protection to maloperate. The impulse test confirms that protection settings are correct and won’t trip falsely during inrush.

✅ Passing the impulse test means the transformer is ready for live grid operation.

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3. Standard Procedure for Transformer Impulse Closing Test

Newly installed transformers typically require five no‑load closings. Here is the standard process:

• Number of impacts: 5 times
• Duration after first impact: Run for at least 10 minutes before the next closing.
• Interval between subsequent impacts: Wait at least 5 minutes before each next closure.
• Total test time: Approximately 1 hour for a full impulse test.

Why five times? Because the closing angle varies each time, the excitation inrush current also varies – sometimes large, sometimes small. Multiple impacts thoroughly test the transformer‘s insulation, mechanical strength, and differential protection under different inrush conditions.

Once the impulse test is completed successfully, the substation can be put into formal operation.

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4. Key Takeaways for Engineers and Operators

Aspect	Key Point
Purpose	Verify insulation strength against overvoltage and mechanical strength against inrush current
Test count	5 no‑load closings
Time per test	~1 hour total
Critical risk	Operating overvoltage (up to 4.5× phase voltage) and inrush current (6–8× rated)
Success indicator	No insulation breakdown, no protection maloperation, no mechanical damage