The complete set of variable frequency series resonance test equipment mainly consists of a variable frequency controller, excitation transformer, high-voltage reactor, high-voltage voltage divider, etc; Variable frequency controllers are divided into two categories: desktop controllers with 20KW and above, and portable box controllers with 20KW and below. They consist of a controller and a filter.
The main function of a variable frequency controller is to convert a fixed amplitude and frequency 380V or 200V AC power frequency sine wave into an adjustable amplitude and frequency sine wave. For the entire equipment, the excitation transformer is used to increase the output voltage of the variable frequency power supply to the appropriate test voltage. The reactor L is an important component of the resonant circuit. When the power frequency is equal to 1/(2 π√ LCX), it resonates in series with the test CX.
Insulation tolerances for large capacity electrical equipment with variable frequency series resonance, such as large generator sets, power transformers, power capacitors, GIS, power cables, etc. It is equivalent to power frequency withstand voltage within a certain frequency range, providing the possibility of using the inductance of the frequency conversion test device and the capacitance of the sample in series to generate resonant voltage for AC withstand voltage testing.
Characteristics of frequency conversion series resonance withstand voltage test device
When the power frequency (f), inductance (L), and measured capacitance (C) satisfy the following equation, the circuit is in series resonance: F=1/2 π√ LC, loop current I=ULX/R, which is the excitation voltage for the output voltage and test circuit quality factor. The voltage detection device UCX=I/ω Cx ratio is Q=UCX/ULX=(ω L)/R. Due to the small resistance R of the test circuit, the test circuit has a large quality factor. In general, under normal circumstances, the output voltage can reach 50 or more, which is 50 times the excitation voltage. Therefore, using a lower capacity test transformer can obtain a higher test voltage, which solves the problem of AC withstand voltage test for test transformer capacity and cannot meet the test requirements. The relationship between capacitance and inductance is ω L=1/ω C, because capacitance is inherent to the test sample, and the adjustable inductance being tested is very expensive. Therefore, the solution to this problem is to change the resonant frequency of the power frequency circuit, adjust the frequency of the circuit at the initial voltage, and observe the maximum change in UC. When the frequency increases or decreases, the resonant voltage will decrease. In addition, due to the resonant state of the test circuit, the circuit itself has good filtering effect, and the waveform of high-order harmonic components is greatly reduced in both devices to output a good sine waveform. When the sample discharges or breaks down, that is, the equivalent capacitance in the circuit is short circuited, the resonance state is destroyed, the voltage drops significantly, the recovery voltage slowly rises, and the transient voltage does not occur on the sample. Due to the limitation of reactance, the short-circuit current of the power supply decreases, thereby limiting the degree of damage to the equipment in the test.