1. 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, for a bushing of a transformer, the normal tg value is 0.5%, but after moisture, the tg value becomes 3.5%, a difference of 7 times. However, when using insulation resistance measurement, the values before and after moisture are not significantly different.
Due to the high sensitivity of measuring the dielectric loss factor in reflecting the above defects, it has been widely used in electrical manufacturing and in handover 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.
2. 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, which was 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, compared with the international level, there is still a large gap in many aspects, mainly in the following aspects:
(1) Anti-interference capability
Since dielectric loss testing is a highly sensitive project, the test data is also 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 different frequency method. Although these methods can solve the interference problem to a certain extent, when the external interference is strong, there will still be large deviations.
(2) Accuracy issue of the reverse connection method
Many power equipment in the field are grounded, so the reverse connection method must be used for testing. However, when reversed, 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 when 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 will seriously affect the accuracy of the test.
3. 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 the external interference signal, transmit the relevant information to the CPU, and the CPU outputs instructions to the "anti-interference source control device". This device will generate an "anti-interference signal" inside the instrument with the same amplitude but opposite phase as the interference signal, which cancels out the interference signal to achieve 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".
4. The traditional anti-interference methods mainly include the phase reversal method, phase shifting method, and different frequency method. How do they work?
1. Phase reversal method
The working power supply of the instrument is reversed twice for testing, and the two test results are analyzed and processed to achieve the purpose of 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 supply, it shifts the phase to be the same as the interference signal to reduce the impact of interference. Practice shows that in the case of strong interference, the data still has large deviations.
3. Different frequency method
This is a method developed in recent years. Its basic principle is that the frequency of the working power supply 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 the anti-interference capability of this method is better 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", which allows testing at two frequencies, such as 40Hz and 60Hz. But the problem is: what if the test results at the two frequencies are inconsistent? Can a simple average processing 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.
