The dielectric loss tangent test, commonly known as the tan δ test, is a highly sensitive method used to evaluate the insulation condition of electrical equipment. This test can detect overall moisture, aging, and even small defects within the insulation system. In the pre-regulation standards, this test was widely implemented in most pilot projects for various types of electrical equipment. It remains one of the most effective techniques for assessing the health of insulation systems.
**First, the principle of testing**
The dielectric loss tangent (tan δ) is an important parameter that reflects the energy loss in an insulating material. From a practical standpoint, it is desirable for the tan δ value to be as low as possible. When insulation becomes damp or aged, the resistive current through the material increases, which leads to a higher tan δ value. This increase can indicate the presence of internal defects.
However, if the defect is localized rather than widespread, the tan δ test may not be very sensitive. For example, in motor and cable insulation tests under the "pre-regulation," where the bulk of the insulation is large and the defective area is small, detecting local faults using the overall tan δ measurement can be challenging. Therefore, in such cases, this test may not be performed.
The tan δ test for electrical equipment is based on the equivalent circuit model of a capacitor and a resistor connected in parallel. By using an AC bridge, the tan δ and capacitance Cx of the test object can be determined by comparing the known values in the bridge arms. The balance condition of the bridge allows for accurate calculation of these parameters.
**Second, test instruments and wiring methods**
In China, two main types of bridges are commonly used: the Xilin Bridge and the Unbalanced Bridge. Recently, digital automatic dielectric loss measuring instruments have also been introduced. This section will focus primarily on the Xilin Bridge and briefly explain the Unbalanced Bridge.
**1. Working principle of Xilin Bridge**
The QS1 type Xilin Bridge is a specialized instrument used to measure tan δ and Cx for electrical equipment insulation. It is an AC bridge that offers high sensitivity and accuracy. The bridge operates at 10kV and supports three wiring configurations: positive, reverse, and diagonal. Positive and reverse wiring are typically used in practice.
In the positive wiring configuration, the sample’s two ends are not grounded. In contrast, the reverse wiring is used when the device being tested is already grounded and cannot be disconnected. In this case, R3 and Z4 are at a high potential, so an insulating rod is used with R3 and C4 to ensure safety. Testers should stand on an insulating mat during this process.
In the bridge diagram, CN represents a loss-free standard capacitor, while Zx is the test object composed of Cx and Rx in parallel. R4 is a non-inductive fixed resistor, C4 is a variable capacitor, and R3 is a decimal resistor box. By adjusting R3 and C4, the bridge can be balanced, resulting in no current flow through the galvanometer.
When the bridge is balanced, the voltage ratios between the arms are equal to their impedance ratios. This relationship allows for the calculation of Cx and tan δ. A typical bridge frequency is 50Hz, with w = 2π × 50 = 314 rad/s.
If R4 is set to 3184 Ω (equal to 10ⴠ/ π), then tan δ = 10ⶠ× C4, meaning that the value of C4 (in microfarads) directly corresponds to the tan δ value in percentage. With CN = 50 pF and R4 = 3184 Ω, Cx = 159,200 / R3 (in picofarads).
For samples with capacitance greater than 3000 pF, a 100Ω shunt resistor is added to the bridge arm, divided into 98.8Ω and 1.2Ω. The 1.2Ω resistor serves as a trimming resistor. Using this configuration, Cx and tan δ can be calculated accordingly.
**2. Judgment and analysis of test results**
(1) **Evaluating the tan δ value**: According to the pre-regulation, the measured tan δ must not exceed specified limits. If it does, further investigation is necessary, and decomposition testing may be required.
(2) **Comparing test values**: Comparing the current tan δ reading with previous measurements for the same equipment or similar devices helps identify abnormal changes. A significant increase should be taken seriously.
(3) **Analyzing the tan δ vs. voltage curve**: For good insulation, the tan δ value should remain relatively constant with varying voltage. However, for deteriorated insulation, the tan δ value tends to rise as voltage increases.
(4) **Considering temperature effects**: Temperature significantly affects insulation performance. When comparing results, it is essential to do so under the same temperature conditions to ensure accuracy.
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