First, the principle of testing
    The dielectric loss tangent tan δ of the insulation can be used to reflect the dielectric loss. From the viewpoint of dielectric loss, it is required that the tan δ value is as small as possible.
    When the insulation is wet and aged, the active current I R through the resistor will increase, and the measured tan δ can reflect the distribution defect of the entire insulation. If the defect is concentrated, measuring tan δ at this time is not very sensitive. In the "pre-regulation" for the insulation of motors, cables, etc., because the bulk of the insulation is large, and the volume of the defective part is small, it is difficult to find the local defects by measuring the tan δ of the whole, so this test is not performed.
    The tan δ test for electrical equipment is based on the parallel equivalent circuit of the capacitor and the resistor, and the tan δ and capacitance C of the test object are obtained by using the known capacitance and resistance in the bridge arm by the principle of AC bridge balance. x .
Second, test instruments and wiring methods
    At present, two types of bridges, Xilin Bridge and Unbalanced Bridge, are commonly used in China. In recent years, digital automatic dielectric loss measuring instruments have appeared. The following focuses on the Xilin Bridge and a brief introduction to the unbalanced bridge.
    1. The working principle of Xilin Bridge
    The QS1 type Xilin bridge currently used is a special instrument for measuring tan δ and capacitance Cx of electrical equipment insulation. It is a balanced AC bridge with the advantage of being sensitive and accurate. The bridge operates at 10kV and has three types of wiring: positive, reverse and diagonal (see Figure 2-10). Generally, both positive and negative wiring methods are used.
    The positive wiring is used when the two ends of the sample are not grounded. The reverse wiring is used when the ground device has a fixed ground and cannot be opened. At this time, R3 and Z4 are at a high potential. To ensure safety, an insulating rod is mounted on R3 and C4. To ensure the safety of the tester, the tester should stand on the insulating mat.
    In Figure 2-10, CN is a loss-free standard capacitor; Zx is the test object (Zx is composed of Cx and Rx in parallel); R 4 is a non-inductive fixed resistor; C 4 is a variable capacitance box; and R 3 is a decimal resistance box. . Adjust R 3 and C 4 to bridge balance, galvanometer G has no current, and the following relationship exists.
U CA =U CB ;U AD =U BD (equal size, same phase); U AB =0
    And there are: U CA /U AD =U CB /U BD
    When the bridge is balanced, the ratio of the voltages of the bridge arms should be the ratio of the impedances of the bridge arms, that is,
Z X /Z 3 =Z N /Z 4 ;Z X Z 4 =Z N Z 3
    The virtual and real parts on both sides of the equal sign are equal, and you can get
C x =(R 4 /R 3 )C N
Tanδ=wC 4 R 4
The frequency of a typical bridge is 50Hz, w=2Ï€50=314(rad/s)
Take the resistance R 4 = 3184 (Ω) = 10 4 / π (Ω), then
Tanδ=10 6 C 4 =C 4 (μF)
C 4 often uses a micro-level capacitor, so the C 4 (μF) value read from the bridge is the tan δ value (%) of the test object, and the capacitance C N = 50pF, R 4 = 3184Ω, so C x =159 200/ R 3 (pF).
When the sample capacitance is greater than 3000pF, a 100Ω shunt resistor should be added to the bridge arm, divided into 98.8Ω and 1.2Ω, and 1.2Ω is the trimming resistor (see Figure 2-11). At this time, Cx and tan δ can be obtained by the following formula.
    Where n is the shunt resistor whose value is determined by the position of the tap.
         P——fine-tune the line resistance, the value is between 0~1.2Ω.
2. Judgment and analysis of test results
    (1) Judging of tan δ value. According to the requirements of the "pre-regulation", the specified value should not be exceeded.
    If there is any excess, the cause should be ascertained and the test product should be subjected to a decomposition test if necessary.
    (2) Comparison of test values. The measured tan δ value is compared with the previous measured data, with the same equipment, and with other equipment of the same type; when the difference is significantly increased, it should be taken seriously.
    (3) Test the tan δ versus voltage curve. For good insulation, tan δ does not change with voltage; for poor insulation, tans will rise with voltage.
    (4) The effects of temperature should be fully considered. When comparing, it should be done on the same temperature.
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