2025-06-18
Insulation resistance testing (performed using an ohmmeter) is crucial for assessing the condition of transformers, but there is no universal "pass/fail" standard value. This is because the values can be affected by various factors.
1、Absolute Minimum "Rule of Thumb":
Often cited as: IR (in MΩ) ≥ (Rated Voltage (V) / 1000) + 1 (for windings rated ≤ 1kV).
Example: A 480V transformer: (480 / 1000) + 1 = 1.48 MΩ minimum (corrected).
Note: This is an absolute minimum sanity check. Healthy transformers typically measure orders of magnitude higher (hundreds or thousands of MΩ). Values near this minimum indicate serious problems.
2、Comparison is Key (Most Important Principle):
Trending: Compare current readings to previous tests (corrected to the same temperature) on the same transformer. A significant drop (e.g., > 25-30%) is a major red flag, even if the absolute value seems high.
Similar Units: Compare readings to identical or similar transformers under the same conditions (temp, humidity, test voltage).
3、Polarization Index (PI) & Dielectric Absorption Ratio (DAR):
These ratios assess insulation quality and dryness better than a single spot reading.
PI = (IR @ 10 minutes) / (IR @ 1 minute)
DAR = (IR @ 60 seconds) / (IR @ 30 seconds) (Faster alternative)
4、Major Standards Referencing IR Testing:
IEEE Std 62: *IEEE Guide for Diagnostic Field Testing of Electric Power Apparatus - Part 1: Oil Filled Power Transformers, Regulators, and Reactors.* (Provides methodology, interpretation guidance).
IEEE Std 43: IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery (Principles & PI widely used for transformers).
IEC 60505: Evaluation and qualification of electrical insulation systems (Includes diagnostic tests like PI).
IEC 60076: Power Transformers (Series covering various aspects).
NETA MTS-2019: Maintenance Testing Specifications for Electrical Power Equipment and Systems (Provides specific test voltages and acceptance criteria tables - widely used in North America).
1.Winding-to-Winding & Winding-to-Ground (Oil-Filled): ≥ 1000 MΩ (For units ≤ 10 MVA) / ≥ 2000 MΩ (For units > 10 MVA). (Note: These are typical minimums for new/good condition; trending is still vital).
2.Bushings: Compare to manufacturer data or similar units. Often > 10,000 MΩ when new/clean/dry.
3.Dry-Type Transformers: Values are generally lower than oil-filled. Consult manufacturer data. Typical new minimums might be in the range of 100 MΩ to 1000 MΩ+ depending on voltage/size. Trending is critical.
1.Safety First: Follow LOTO procedures. Windings must be completely isolated and discharged before/after testing.
2.Temperature Correction: MANDATORY for valid comparison. Use formulas/tables from IEEE 43 or the megohmmeter manual.
3.Manufacturer Data: Always consult the transformer's test report or manual for factory values and specific recommendations.
4.Surface Leakage: Clean bushings and terminals. Use the guard terminal on the megohmmeter if surface leakage is suspected to bypass it.
5.Interpretation: Low IR or poor PI/DAR indicates moisture, contamination (dust, oil carbon), or physical damage. Requires further investigation (e.g., oil tests, power factor testing, visual inspection).
