I. What is the significance of measuring the dielectric loss tangent value tg ?
The dielectric loss tangent value, also known as the dielectric loss factor or simply dielectric loss, is a highly sensitive test item. It can detect overall moisture, deterioration, and local defects in the insulation of power equipment, whether through or not through, especially for small-volume tested devices. For example, the normal tg value of a transformer bushing is 0.5%, but after moisture, it becomes 3.5%, a seven-fold difference; however, when measured by insulation resistance, the difference before and after moisture is not significant.
Due to its high sensitivity in reflecting the above defects, the measurement of dielectric loss factor is widely used in electrical manufacturing and in acceptance and preventive tests of power equipment. The regulations for dielectric loss testing of electrical equipment such as transformers, generators, and circuit breakers have been specified.
II. What is the current status and technical difficulties of domestic dielectric loss testers?
The technology of dielectric loss testers is developing rapidly. The QS1 Schering bridge, previously widely used in power systems, is being replaced by intelligent dielectric loss testers. The new generation of dielectric loss testers has built-in voltage boosting equipment and standard capacitors, and features simple operation, accurate data, and convenient reading of test results. Although the technology of dielectric loss testing is developing rapidly, there is still a significant gap compared with international levels in many aspects, mainly in the following areas:
(1) Anti-interference capability
Since dielectric loss testing is a highly sensitive project, the test data is easily affected by external electric field interference. Currently, the anti-interference methods adopted by dielectric loss testers mainly include: phase reversal method, phase shifting method, and frequency conversion method. Although these methods can solve the interference problem to a certain extent, when the external interference is strong, large deviations can still occur.
(2) Accuracy issues in reverse connection method
Many power equipment on site are grounded, so the reverse connection method must be used for testing. However, in reverse connection, there are many factors affecting the test data, often resulting in large deviations, especially when the tested object has a small capacitance (such as a bushing) and the high-voltage wire is dragged on the ground during testing (some dielectric loss testers' high-voltage wires can be used on the ground, but the leakage current to the ground is large), which seriously affects the accuracy of the test.
III. What is the "fully automatic anti-interference source" and what are its characteristics compared with other anti-interference methods?
The so-called "fully automatic anti-interference source" means that the instrument has a built-in detection device that can detect the amplitude and phase of external interference signals, transmit the relevant information to the CPU, and the CPU outputs commands to the "anti-interference source control device". This device generates an "anti-interference signal" with the same amplitude but opposite phase as the interference signal, which cancels out the interference signal when superimposed, achieving the purpose of anti-interference. Since the "anti-interference source" is automatically generated during the entire test process without user intervention, we call it the "fully automatic anti-interference source".
IV. The traditional anti-interference methods mainly include phase reversal method, phase shifting method, and frequency conversion method. How do they work?
1. Phase reversal method
The instrument's working power is tested twice with reversed phases, and the two test results are analyzed and processed to achieve anti-interference. This method has a certain effect when the external interference is weak.
2. Phase shifting method
The idea originates from the "phase reversal method", but instead of reversing the working power phase, it shifts the phase to be the same as the interference signal phase to reduce the interference effect. Practice shows that in the case of strong interference, the data still has large deviations.
3. Frequency conversion method
This is a method developed in recent years. Its basic principle is that the frequency of the working power is not 50Hz, i.e., different from the power frequency. In this way, the sampled signal is the superposition of two signals of different frequencies (test current and interference current). By filtering the signal through analog and digital filters to attenuate the power frequency signal, the purpose of anti-interference is achieved. Practice shows that this method has better anti-interference capability than the "phase reversal method" and "phase shifting method", but in some specific occasions, due to interference, the data still has deviations, and even negative values may appear. In addition, due to its own principle characteristics, there are several contradictions:
(1) Frequency selection issue: The closer the frequency is to the power frequency, the weaker the anti-interference capability, but the better the equivalence; the farther the frequency is from the power frequency, the stronger the anti-interference capability, but the worse the equivalence.
(2) To enhance equivalence, some instruments use "dual frequency conversion", i.e., two frequencies can be selected for testing, such as 40Hz and 60Hz. But the problem is: what if the test results at the two frequencies are inconsistent? Can a simple average be equivalent to the power frequency?
(3) Analog filters all have phase shift issues. Fixed phase shifts can be compensated by the computer, but when the phase shift characteristics change due to temperature and other conditions, it will seriously affect the test results of the dielectric loss value.
