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, at rated frequency and rated voltage, the iron core of a transformer is close to saturation. If a test voltage higher than the rated voltage is applied across the windings of the transformer under test using power frequency, 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 frequency can be adopted, and a frequency multiplier generator must be used. 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 change with the load, and it is very easy to resonate with the test object, causing resonant overvoltage and damaging the test object.
Basic Principle
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 of 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 test whether the windings meet the requirements. In order to increase the test voltage of the windings without over-saturating the iron core of the transformer or instrument transformer, which would cause excessive excitation current, the test frequency should be increased, generally to 100Hz-400Hz.
There are various methods to obtain a 100Hz-400Hz power supply, among which the more applicable ones are: ① Combination of three variable frequency generator sets (100Hz-400Hz); ② Combination of three single-phase transformers (150Hz), and the most convenient is the second method.
Combination of three single-phase transformers: The primary side is connected in star, and the secondary side 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 large component of third harmonic magnetic energy Φ3, and the third harmonic magnetic flux frequency f3=3f1, the main magnetic flux induces a third harmonic phase electromotive force.
Since the primary side is star-connected and the secondary side 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 wave, 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 source of the generator. It is used for frequency multiplication and voltage multiplication tests on inter-turn, inter-segment, and interlayer insulation of inductive coil-type electrical products such as transformers and instrument transformers, to assess the insulation strength and withstand voltage level of the coils.
Operating Environment
1. Ambient temperature: -25℃ - 55℃
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 longitudinal insulation strength, and can also be used as a short-time 150Hz power supply.
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 series-excited voltage transformers at 35～220kV levels in power systems to assess the main and longitudinal insulation strength of the transformers. It can also be used for induction tests on windings of motors and small transformers, and can also be used as a short-time 150Hz power supply.
