Very Low Frequency Withstand Test
The Very Low Frequency Withstand Test (VLF) is an electrical test method used to verify the insulation withstand capability of capacitive electrical equipment. It applies an AC high voltage at an extremely low frequency (typically 0.01Hz to 0.1Hz, with 0.1Hz being the most common) to simulate the electric field stress during operation, thereby determining whether the insulation has defects.
Working Principle and Core Advantages
Traditional power frequency (50Hz/60Hz) withstand tests on large capacitive equipment such as cables and large generators generate significant capacitive charging currents, resulting in bulky, heavy, and expensive test transformers. By reducing the frequency to 1/500 of the power frequency, the VLF test significantly reduces the capacitive current of the test object and the capacity requirements of the test power supply.
Core Advantages:
Portable Equipment: The volume and weight of the required equipment are approximately 1/500 of traditional power frequency equipment, greatly facilitating on-site mobile testing.
Non-destructive: Compared to DC withstand tests, the voltage polarity of VLF AC withstand tests changes periodically, posing a lower risk of insulation damage and effectively avoiding the "false pass" phenomenon that may occur in DC testing.
High Defect Detection Rate: AC voltage is more likely to expose hidden defects such as mechanical damage and water tree aging within the insulation.
Main Application Scenarios
The VLF withstand test is mainly applicable to various capacitive electrical equipment and is widely used in acceptance tests for new equipment and preventive tests for equipment in service:
Power Cables: Withstand tests for plastic insulated cables such as 10kV and 35kV cross-linked polyethylene (XLPE) cables.
Large Generators: Insulation withstand tests for stator windings of steam turbines and hydro generators, particularly effective in detecting insulation defects at generator ends.
High-voltage Capacitors: AC withstand tests for shunt capacitors and coupling capacitors.
Key Parameters and Test Criteria
Test Voltage: Determined based on equipment type and rated voltage. For example, for 10kV cables, the peak test voltage is typically set to
calculated in combination with the equivalent coefficient.
Test Duration: Set according to relevant regulations, such as acceptance tests for new cables typically requiring a withstand duration of 60 minutes.
Acceptance Criteria: Within the specified test voltage and duration, if the test object shows no destructive discharge (such as breakdown or flashover), no abnormal sounds, smoke, or odor, and the leakage current remains stable, the insulation is deemed acceptable.
Test Precautions
Phase-by-phase Testing: When testing equipment such as generators, tests should be conducted phase by phase, with the tested phase energized and the non-tested phases short-circuited and reliably grounded.
Abnormality Handling: During voltage increase or withstand testing, if output waveform distortion, abnormal current increase, voltage instability, or odor or abnormal sounds from the test object are observed, stop the test immediately and investigate the cause. If caused by surface contamination or air humidity, clean and dry before retesting.
Scope Limitations: Although VLF equipment has significant advantages in testing cables at 35kV and below, for higher voltage levels (such as 110kV and above) or large transformers with high equivalence requirements, variable frequency series resonance withstand testing remains the more mainstream and irreplaceable choice.
