In the insulation test system of power equipment, the external applied voltage test mainly assesses the main insulation, including the insulation between the windings and the ground, as well as the insulation between windings. However, there are limitations in evaluating the longitudinal insulation such as inter-turn, inter-layer, inter-section, and inter-phase insulation of the windings. The three times frequency induction withstand voltage test device is a dedicated test equipment that can output a 150Hz frequency multiplier power supply, solving the technical problem of difficultly applying sufficient test voltage under the power frequency condition. It is an important test equipment for inspecting the insulation quality of transformers and transformers-type equipment.
Standard Basis and Technical Positioning
The design and use of the three times frequency induction withstand voltage test device need to comply with relevant national and industry standards. The insulation level of power transformers and the test methods related standards clearly specify the technical indicators of induction voltage tests; the general technical conditions for high-voltage test devices specify the technical conditions of the three times frequency test power supply device; the electrical equipment preventive test regulations determine the execution cycle and operation methods of induction withstand voltage tests for various power equipment.
This device can be used for induction withstand voltage tests of power transformers, electromagnetic voltage transformers, and current transformers, and can also be used to conduct inter-turn insulation detection of generator stator windings. For graded insulation voltage transformers, the insulation grades of the high-voltage windings at the beginning and end are inconsistent, and external power frequency withstand voltage tests cannot be conducted. The three times frequency induction withstand voltage is an effective way to assess the insulation performance of such equipment.
Working Principle: Solving the Problem of Power Frequency Conditions
Under power frequency conditions, the transformer core is close to magnetic saturation under the rated voltage. If the 2-3 times rated voltage is directly applied for withstand voltage tests, the core will enter a deep saturation state, the excitation current will rise significantly, causing the equipment to overheat, and in severe cases, the equipment will be damaged, making the test work unable to be carried out normally.
Based on the principle of induced electromotive force, increasing the power supply frequency can obtain a higher induced voltage without increasing the magnetic flux density. The three times frequency device relies on the nonlinear saturation characteristic of ferromagnetic materials, and through the star or open delta connection of the three-phase power frequency power supply, it outputs a 150Hz voltage. The core of the tested equipment will not be saturated, and the three times frequency and voltage multiplication insulation assessment can be successfully completed.
Key Points of Test Procedures
Test Voltage and Time
The test voltage is generally taken as two to three times the rated voltage of the tested equipment and the larger working phase voltage. The duration of withstand voltage and the test frequency are related. When using a 150Hz test frequency, the withstand voltage duration is 40 seconds; when the test frequency is no more than twice the rated frequency, the withstand voltage duration is 60 seconds.
Test Wiring
For current transformer tests, all secondary windings need to be short-circuited and grounded. The three times frequency power supply output is connected to any one of the secondary windings to complete excitation. For voltage transformer tests, the primary winding of the fully insulated transformer is grounded at any one end; for graded insulation transformers, the grounding end must be reliably grounded, and all secondary windings and the equipment shell should also be properly grounded. For generator tests, a three times frequency voltage is applied to the stator winding in a single phase, and the current changes during the voltage application process should be observed to ensure the stability of the current.
Voltage Rise Operation
The voltage rise operation must start from zero and prohibit impulse closing. Before the voltage reaches 75% of the test voltage, rapid voltage rise can be performed; after exceeding this voltage, the voltage is smoothly increased at a rate of approximately 2% of the test voltage per second until the target voltage is reached; when the withstand voltage test is over, the voltage drop operation should be immediately carried out.
Result Judgment Criteria
For the equipment test to be qualified, multiple conditions need to be met simultaneously: there is no breakdown sound or abnormal current during the entire voltage application process; there is no significant change in the DC resistance values before and after the test; the excitation characteristic curve has no significant deviation; if local discharge detection is conducted simultaneously, the local discharge quantity needs to be controlled within the allowable limit.
If the amplitude of the DC resistance change and the deviation of the excitation characteristic exceed the allowable range, it is likely that the inter-turn insulation has been damaged, and further verification should be carried out.
Post time: Sep-10-2026