Its core function is to convert the input power frequency (50Hz) into a 150Hz three-frequency power supply, mainly used for induced withstand voltage tests on transformers, voltage transformers, and other equipment.
Here is a detailed introduction to the three-frequency generator:
1. Main Uses
Induced withstand voltage test: This is its most core function. According to national standards (such as GB/T 16927, DL/T 848.4, etc.), it conducts longitudinal insulation (turn-to-turn, layer-to-layer, section-to-section) and main insulation withstand voltage tests on power transformers and voltage transformers (especially series-excited voltage transformers of 35kV-220kV levels).
Solving magnetic saturation problem: If a voltage of 2 or 3 times the power frequency (50Hz) is applied to the transformer for withstand voltage testing, the iron core will quickly saturate due to excessive magnetic flux density, causing a sharp increase in excitation current, making it impossible to reach the test voltage and potentially damaging the equipment. By increasing the frequency to 150Hz (three times the frequency), according to the formula
Φ remains within the normal range, thereby avoiding iron core saturation.
Other applications: It can also be used as a short-term 150Hz power supply, or for induced tests on windings of motors and small transformers.
2. Working Principle
The three-frequency generator usually utilizes the saturation characteristics of the magnetic circuit to generate the third harmonic:
Input: Connect to a three-phase 380V, 50Hz sinusoidal power supply.
Harmonic extraction: The iron core inside the equipment is designed to saturate under specific conditions. When a sinusoidal voltage is applied to the saturated iron core coil, the excitation current becomes distorted, containing rich odd harmonics, with the third harmonic (150Hz) component being the most significant.
Output: Through specific wiring methods (such as open delta connection), the fundamental wave and other harmonics are filtered out, extracting a 150Hz voltage with higher amplitude as the output power supply.
Voltage boosting: The output 150Hz low voltage usually needs to be boosted by a dedicated three-frequency test transformer to reach the high test voltage required by the tested equipment.
3. Main Technical Parameters (Typical Values)
Specifications vary among different manufacturers and models; common parameters are as follows:
Input voltage: Three-phase 380V ±10%, 50Hz.
Output voltage: 0-300V, 0-400V, 0-500V or higher (referring to the generator body output, which can reach tens to hundreds of kilovolts after the test transformer).
Output frequency: 150Hz ±5%.
Rated capacity: From small 3kVA to large 100kVA or even higher, depending on the capacity of the tested object.
Waveform distortion: Usually required to be ≤5%.
Operating time:
No-load operation: Generally allowed for a longer time (e.g., ≤5 minutes).
Load operation: Due to significant heating, usually limited to within 40-60 seconds (depending on the test standard; induced withstand voltage test time is usually 40 seconds).
4. Equipment Structure Forms
Monobloc type: Integrates the three-frequency generation part and control operation part into one box, suitable for occasions with small capacity and high portability requirements.
Split type: The three-frequency generator, control box (console), and step-up transformer are designed separately. Suitable for large-capacity tests, easy to transport and assemble on site.
5. Typical Wiring and Operation Procedure
Wiring:
Connect the three-phase 380V power supply to the input terminals (A, B, C) of the generator.
Connect the output terminals (a, x) of the generator to the low-voltage side of the step-up transformer or directly to the tested object (depending on the specific wiring scheme).
Must be reliably grounded.
Check: Confirm the wiring is correct and the voltage regulator is reset to zero.
Power on: Close the power switch.
Voltage increase: Slowly adjust the voltage regulator, observe the voltmeter and frequency meter, until the required 150Hz test voltage is reached.
Timing: Start timing after reaching the specified voltage (usually 40 seconds).
Voltage reduction and power off: After the time is up, quickly and uniformly reduce the voltage to zero, then cut off the power supply.
6. Application Scenarios
Factory tests of power equipment manufacturers.
Equipment handover tests and preventive tests of power supply companies and substations.
Insulation performance research in electric power research institutes.
Summary
The three-frequency generator is an indispensable equipment in high-voltage testing of power systems. It cleverly utilizes the relationship between frequency and magnetic flux to solve the iron core saturation problem during high-multiple induced withstand voltage tests on high-voltage equipment, and is an important means to ensure the insulation quality of transformers and instrument transformers.
