Very Low Frequency (VLF) Withstand Voltage Test
The Very Low Frequency (VLF) withstand voltage test is an efficient alternative to the power frequency withstand voltage test, primarily referring to AC high voltage tests with output frequencies of 0.1Hz, 0.05Hz, or 0.02Hz. Since the required power capacity of the test power supply is theoretically only 1/500 of that for power frequency tests, the equipment size is significantly reduced, making it particularly suitable for insulation withstand voltage testing of capacitive equipment such as cross-linked polyethylene (XLPE) insulated power cables up to 35kV, large high-voltage rotating machines, and power capacitors.
Core Working Principle
The core of the VLF withstand voltage test lies in significantly reducing the charging current of capacitive test objects by lowering the test frequency, thereby reducing the demand on the test power supply capacity. The working process typically involves: converting 50Hz AC voltage to DC through rectification and filtering, then inverting it to 1kHz AC voltage, and finally modulating it through a VLF sine wave oscillator to output a 0.1Hz high-voltage sine wave.
Compared to traditional DC withstand voltage tests, VLF AC withstand voltage better matches the actual operating conditions of cables. AC voltage can periodically change the direction of the electric field, making it easier to expose defects such as mechanical damage and water tree aging inside the insulation, avoiding insulation damage or "false acceptance" phenomena that may occur with DC withstand voltage tests.
Main Features and Advantages
Lightweight Equipment: Utilizing digital frequency conversion and microcomputer control technology, the fully electronic design reduces its size and weight to only 1/500 of traditional power frequency equipment, greatly facilitating on-site mobile testing operations.
Non-Destructive to Insulation: It is a non-destructive AC withstand voltage test that does not introduce harmful polarization charges into the dielectric, causing far less damage to cable insulation than DC withstand voltage tests.
High Automation: Voltage boosting, reduction, measurement, and protection are fully automated, equipped with a large-screen LCD display that shows the output waveform in real-time and automatically stores and prints test reports.
Comprehensive Safety Protection: Built-in multiple protection mechanisms such as overvoltage, overcurrent, flashover, and sudden voltage/current changes, with extremely short action times (millisecond level), ensuring the safety of test equipment and personnel.
Test Standards and Specifications
In China, VLF withstand voltage tests on cables are mainly carried out in accordance with GB 50150-2016 "Standard for Acceptance Tests of Electrical Equipment in Electrical Installations" and DL/T 596-2021 "Regulations for Preventive Tests of Power Equipment":
Acceptance Tests: The test voltage is typically 3U₀ (RMS value) with a duration of 60 minutes. For example, for a cable with U₀/U=8.7/10kV, the test voltage is 26.1kV.
Preventive Tests: Standards are relatively diverse. For instance, DL/T 596-2021 specifies a test voltage of 2.5U₀ for 15 to 60 minutes; some local regulations may specify 2.1U₀ for 5 minutes, or 3U₀ for 20 minutes.
Acceptance Criteria: During the entire specified withstand voltage process, if the test object does not experience destructive discharge (no breakdown, no flashover), it is deemed qualified. Before and after the test, insulation resistance should be measured using a megohmmeter and compared for analysis.
Main Application Areas
Power Cables: Acceptance and preventive tests for insulated power cables such as 10kV and 35kV cross-linked polyethylene (XLPE), to identify hidden hazards like water trees and local damage.
Large Generators: Insulation withstand voltage testing of stator windings in turbine and hydro generators, replacing bulky high-capacity power frequency test transformers.
High-Voltage Capacitors: AC withstand voltage tests for shunt capacitors and coupling capacitors to evaluate insulation performance.
Operation and Safety Precautions
Equipment Selection: Appropriate equipment should be selected based on the voltage level and length (capacitance) of the cable under test. For example, at 0.1Hz frequency, the standard load capacity is 1.1μF, suitable for cables up to 10kV; if testing 35kV or longer cables, the 0.05Hz or 0.02Hz settings should be selected to enhance load capacity.
Safety Regulations: Before testing, the equipment under test must be fully discharged, and a warning area should be demarcated; operators must wear insulating gloves, use insulating mats, and ensure reliable grounding of the instrument.
Standard Operation: Strictly follow standard procedures for voltage boosting and stabilization, continuously monitor the high-voltage circuit and abnormal sounds. After the test, use a discharge rod to fully ground and discharge, and only after confirming no residual voltage can wiring be disconnected.
