Three-Frequency Power Generator Maintenance
According to national test standards, the induced test voltage for power transformers and voltage transformers is approximately 2-3 times the maximum working phase voltage. As is well known, when a transformer operates at rated frequency and rated voltage, the iron core is close to saturation. If a test voltage higher than the rated voltage is applied across the winding of the transformer under test using a power frequency source, the no-load excitation current will increase sharply to an unacceptable level. From the relationship of induced electromotive force, it can be seen that to apply a test voltage higher than the rated voltage without saturating the iron core, the method of increasing the power supply frequency must be adopted, requiring a frequency multiplier power generator. It is simple to operate, reliable in performance, and can well meet the needs of induced withstand voltage tests for transformers and instrument transformers.
Variable frequency generator sets are bulky, inconvenient to transport, and not suitable for on-site use. When the capacity is small, the waveform is also not good. Three single-phase transformers connected in an open delta can provide a 150Hz power supply, but the waveform and capacity will vary with the load, and resonance with the test object is very likely to occur, causing resonant overvoltage that can damage the test object.
Basic Principles
This product is designed to meet the requirements of the three-frequency induced withstand voltage test and partial discharge test in the "Regulations for Preventive Tests of Electrical Equipment".
The induced withstand voltage test for transformers and instrument transformers is an important test to verify whether the product meets national standards. The longitudinal insulation induction test of interlayer, inter-turn, segment, and phase-to-phase insulation of windings is an important item to assess whether the winding meets the requirements. To increase the test voltage of the winding without causing excessive saturation of the transformer or instrument transformer iron core, which would lead to excessive excitation current, the test frequency must be increased, typically to 100Hz-400Hz.
There are several methods to obtain a 100Hz-400Hz power supply, among which the more applicable ones are: ① a combination of three variable frequency generators (100Hz-400Hz); ② a combination of three single-phase transformers (150Hz). The most convenient is the second method.
Combination of three single-phase transformers: The primary winding is connected in star, and the secondary winding is connected in open delta. When the iron core is not saturated, the main magnetic flux it produces must be a flat-topped wave. Since the flat-topped main magnetic flux contains a significant component of third harmonic magnetic energy Φ3, and the frequency of the third harmonic magnetic flux is f3=3f1, the main magnetic flux induces a third harmonic phase electromotive force.
Since the primary is star-connected and the secondary is open delta-connected, the iron core operates in a saturated state, and the no-load current I is a peaked wave. In addition to the fundamental component, it also contains higher harmonics. Among the higher harmonics, the amplitude of I is relatively large. Due to the open secondary, the three-phase fundamental components cancel each other out, leaving only the third harmonic output.
The three-frequency power generator utilizes the saturation characteristics of the magnetic circuit to extract the third harmonic voltage, which has the largest component among the harmonics, as the power supply for the generator. It is used for frequency multiplication and voltage multiplication tests on inductive coil-type electrical products such as transformers and instrument transformers, for inter-turn, inter-segment, and interlayer tests, to assess the insulation strength and withstand voltage level of the coils.
Operating Environment
1. Ambient temperature: -25°C to 55°C
2. Altitude: not exceeding 4000 meters
3. Used for AC induced withstand voltage tests of voltage transformers, motors, and transformers, to assess the main and auxiliary insulation strength, and can also be used as a 150Hz power supply for short-term operation.
Technical Parameters
1. Working power supply: three-phase AC 380V ±10% 50Hz
2. Rated capacity: 3KVA-30KVA (optional)
3. Input voltage: 380V 50Hz sine wave
4. Output voltage: 250V-1000V 150Hz, waveform distortion ≤5%
5. Output capacity: 3kVA, 5kVA, 10kVA, 20kVA, 50kVA
6. No-load operation time: ≤5 minutes
7. Load operation time: 40-60s
This product is designed according to the People's Republic of China electric power industry standard DL/T848.4—2004 and the "Regulations for Preventive Tests of Electrical Equipment" issued by the former Ministry of Water Resources and Electric Power in January 1985, to meet the requirements of the power system for frequency multiplier induced withstand voltage test equipment for high-voltage voltage transformers. It is widely used for AC withstand voltage tests of cascade voltage transformers at 35-220kV levels in power systems, to assess the main and auxiliary insulation strength of the transformers. It can also be used for induction tests on windings of motors and small transformers, and can serve as a 150Hz power supply for short-term operation.
