How to prevent overvoltage generated by the frequency conversion series resonance test device

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The frequency conversion series resonance test device is used to evaluate the insulation strength of electrical equipment. It has a decisive significance in determining whether the electrical equipment can be put into operation and is also an important means to ensure the insulation level of the equipment and avoid insulation accidents. Because the frequency conversion series resonance test device can fully reflect the actual operation of electrical equipment under AC voltage, it can truly and effectively detect insulation defects.  

The frequency conversion series resonance adopts the principle of resonance, which generates a certain frequency of voltage in the system circuit and resonates with the capacitors and reactance in the circuit. After the resonant voltage is generated at both ends of the capacitor (test sample), the voltage at both ends of the capacitor is boosted to reach the test voltage.  

Based on the above principles and actual experimental conditions on site, the overvoltage in the general frequency conversion series resonance test device occurs in two situations: one is during the process of finding the resonance point and generating resonance voltage in the instrument; One is when the boost reaches the test voltage.  

When finding the resonance point through series resonance test and boosting it to the test voltage, if there are phenomena such as unqualified withstand voltage of the test sample or no significant changes in the on-site environment, the test will not generate overvoltage protection or other faults. However, due to the fact that the voltage of the power grid is not constant and the input voltage of the power supply fluctuates, the high-voltage output also has a certain degree of fluctuation, which may cause overvoltage protection due to voltage peaks. If there is a fluctuation in the power supply voltage, the instrument's overvoltage protection can be adjusted to increase the overvoltage protection setting. We generally require the overvoltage protection to be set to 1.1-1.2 times the voltage protection, and setting it to 1.2 times is basically not a problem.

The above is a relatively simple problem, but overvoltage caused by voltage fluctuations is difficult to occur when set better than voltage protection. The overvoltage of a general variable frequency series resonance test device will occur during the scanning phase of the instrument, which is the process of finding the resonance point. Those who have used the variable frequency series resonance test device know that during the process of finding the resonance point, the voltage and frequency of the device exhibit a parabolic linear relationship. The system defaults to finding the highest voltage, which is the vertex of the parabola, as the resonance point. Due to the theory of resonance principle, low voltage can be resonated up to 80 times (generally not exceeding 30 times due to factors such as quality factor). The voltage required for the frequency conversion series resonance test device to sweep the frequency is generally 20-50V, and the voltage after excitation transformation is generally several hundred volts. Through the above principles, we have found that if the test voltage of the sample we need is lower than the voltage at the resonance point of the system, the system may experience overvoltage protection when automatically searching for the resonance point. At this time, the entire variable frequency series resonance test device cannot withstand voltage in a timely manner, and the test cannot be completed.  

The solution to this problem is relatively complicated. The variable frequency series resonance test device cannot generate the test voltage, but the resonance voltage is higher than the test voltage. We know that the system defaults to the vertex of the parabola as the resonance voltage, which means that during the process of the parabola rising to the vertex or falling, there will be a point that is consistent with the test voltage point. We only need to manually test the frequency conversion series resonance by manually finding the frequency point that is consistent with the voltage used, and then withstand the voltage to perfectly solve the overvoltage problem during the process of finding the resonance point.