1250kVA Dry-Type Transformer Testing Case | DC Resistance, Insulation Resistance & AC Withstand Voltage Test

In May 2026, HKV Test engineers participated in the commissioning test of a 1250kVA dry-type transformer installed in an underground distribution room of a commercial building in Wuhan, China.

Before the transformer could be energized, a complete series of electrical tests was required to verify its operating condition and compliance with commissioning requirements.

The testing included:

• DC winding resistance measurement
• Insulation resistance testing
• AC withstand voltage testing

These tests were performed to evaluate transformer integrity and ensure reliable operation before being placed into service.

The customer required a complete commissioning test within a limited maintenance schedule.

The primary objectives were:

• Verify winding resistance consistency
• Confirm insulation performance
• Ensure dielectric strength met operating requirements
• Complete all testing before energizing the transformer

Accurate and efficient testing was essential to avoid delays in project delivery.

During this project, HKV Test engineers used the following equipment:

20A 10A DC Resistance Tester for Transformer Testing

The HZB1202 DC resistance tester provides fast and highly accurate winding resistance measurements for power transformers.

Main advantages include:

• High-current output
• Automatic measurement
• Stable and repeatable results
• Large LCD display

Digital Insulation Resistance Tester | High Voltage Megohmmeter

The insulation resistance tester evaluates transformer insulation quality and automatically calculates the Polarization Index (PI) and Absorption Ratio (DAR), helping engineers assess insulation condition efficiently.

AC Withstand Voltage Test Equipment Oil Immersed High Voltage Test Transformer Manufacturer

The HZD1601 AC Withstand Voltage Test Equipment was used to perform the dielectric withstand test after the insulation resistance measurement. This test verifies that the transformer insulation can safely withstand the specified test voltage without flashover or breakdown before being placed into service.

The transformer winding resistance was measured on both the high-voltage and low-voltage windings using the HZD1601 AC Withstand Voltage Test Equipment.

The measured phase values were highly consistent, indicating balanced winding resistance and good winding integrity.

Using the HZ3035 Digital Insulation Resistance Tester, insulation resistance was measured between:

• High voltage to low voltage and ground
• Low voltage to high voltage and ground
• High voltage to low voltage

The measured absorption ratios exceeded 2.2, indicating excellent insulation condition.

The transformer successfully passed the dielectric withstand test.

Test voltage:

• HV to LV & Ground: 24 kV / 1 minute
• LV to HV & Ground: 1 kV / 1 minute

No insulation breakdown or abnormal discharge was observed during testing.

The transformer successfully passed all commissioning tests.

Key results included:

✅ Stable winding resistance values

✅ Excellent insulation resistance performance

✅ Absorption Ratio greater than 2.2

✅ Successful AC withstand voltage test

These results confirmed that the transformer met commissioning requirements and was ready for normal operation.

Comprehensive transformer testing before commissioning helps engineers identify hidden defects that cannot be detected through visual inspection alone.

The combination of:

• DC resistance testing
• Insulation resistance testing
• AC withstand voltage testing

provides a complete assessment of transformer electrical health and significantly improves long-term operational reliability.

Using HKV Test equipment enabled the customer to:

• Complete commissioning on schedule
• Improve testing efficiency
• Obtain accurate digital test records
• Verify transformer readiness before energization
• Reduce future maintenance risks

• DC resistance testing verifies winding integrity.
• Insulation resistance testing evaluates insulation health.
• AC withstand voltage testing confirms dielectric strength.
• Comprehensive commissioning testing improves transformer reliability.
• Professional testing equipment supports safe power system operation.

Wuhan Hongzhi Gaocetest Electrical Technology Co., Ltd. (HKV Test) is a trusted manufacturer of high-voltage electrical testing equipment with over 16 years of manufacturing experience. Our solutions are widely used in substations, power plants, utilities, industrial facilities, EPC projects, and electrical maintenance applications worldwide.

Our main products include:

• Transformer Test Equipment
• Cable Fault Locator Systems
• Insulation Resistance Testers
• AC/DC Hipot Test Equipment
• Relay Protection Test Systems
• SF6 Gas Test Equipment
• High Voltage Test Instruments

Website: https://www.hkvtest.com

It verifies winding continuity, detects loose connections, and identifies potential winding defects.

It evaluates the condition of transformer insulation between windings and ground.

An absorption ratio above 1.3 is generally considered acceptable, while values above 2.0 indicate very good insulation condition (requirements may vary depending on applicable standards and equipment specifications).

It confirms that transformer insulation can safely withstand operating voltage without electrical breakdown.

Before initial energization, after major maintenance, or following transformer replacement.

👉 Transformer DC Resistance Tester

👉 Digital Insulation Resistance Tester (Digital Insulation Resistance Tester | High Voltage Megohmmeter)

👉 AC Withstand Voltage Test Equipment (AC Withstand Voltage Test Equipment Oil Immersed High Voltage Test Transformer Manufacturer)

• How to Measure Transformer Winding Resistance
• What Is Insulation Resistance Testing?
• Transformer Commissioning Test Checklist
• AC Withstand Voltage Test Explained