The dielectric loss tangent test is a highly sensitive method used to detect overall moisture, aging, and small defects in the insulation of electrical equipment. This test was included in nearly all pilot projects for electrical equipment under the pre-regulation, making it an effective way to assess insulation condition.
**First, the principle of testing**
The dielectric loss tangent (tan δ) reflects the amount of energy lost as heat in an insulating material. Ideally, this value should be as low as possible. When insulation becomes wet or aged, the resistive current increases, which can be detected through the tan δ measurement. However, if the defect is localized, the tan δ test may not be very sensitive. In cases where the bulk of the insulation is large but the defective area is small, such as in motors or cables, measuring the overall tan δ might not reveal local issues. Therefore, this test is sometimes omitted in such scenarios.
The tan δ test for electrical equipment is based on a parallel equivalent circuit of a capacitor and a resistor. Using the AC bridge balance principle, the tan δ and capacitance (Cx) of the test object are determined by comparing known values in the bridge arms.
**Second, test instruments and wiring methods**
In China, two types of bridges are commonly used: the Xilin Bridge and the Unbalanced Bridge. Recently, digital automatic dielectric loss measuring instruments have also emerged. The focus here is on the Xilin Bridge, with a brief overview of the Unbalanced Bridge.
**1. Working principle of the Xilin Bridge**
The QS1 type Xilin Bridge is widely used to measure tan δ and Cx for electrical equipment insulation. It's a balanced AC bridge known for its sensitivity and accuracy. The bridge operates at 10kV and has three wiring configurations: positive, reverse, and diagonal. Typically, both positive and reverse wiring are used.
Positive wiring is applied when neither end of the sample is grounded. Reverse wiring is used when one end is fixed to ground and cannot be disconnected. In this case, R3 and Z4 are at high potential, so an insulating rod is used with R3 and C4. The operator should stand on an insulating mat for safety.
In the diagram, CN is a loss-free standard capacitor, Zx represents the test object (composed of Cx and Rx in parallel), R4 is a non-inductive resistor, C4 is a variable capacitor, and R3 is a decimal resistance box. By adjusting R3 and C4 until the bridge is balanced (no current through the galvanometer), the following relationships hold:
Uca = Ucb; Uad = Ubd (same magnitude and phase); Uab = 0
And:
Uca / Uad = Ucb / Ubd
When balanced, the impedance ratio of the bridge arms equals the voltage ratio:
Zx / Z3 = Zn / Z4 → Zx * Z4 = Zn * Z3
Equating real and imaginary parts gives:
Cx = (R4 / R3) * Cn
Tanδ = ωC4R4
With a typical frequency of 50Hz, ω = 2π × 50 = 314 rad/s.
If R4 = 3184 Ω (which is 10^4 / π Ω), then:
Tanδ = 10^6 * C4 = C4 (in μF)
Since C4 is usually in microfarads, the value read from the bridge directly corresponds to the tan δ percentage. With Cn = 50pF and R4 = 3184Ω, Cx = 159,200 / R3 (in pF).
For samples with capacitance greater than 3000pF, a 100Ω shunt resistor is added to the bridge arm, divided into 98.8Ω and 1.2Ω. The 1.2Ω is a trimming resistor. At this point, Cx and tan δ are calculated using adjusted formulas.
**2. Judgment and analysis of test results**
(1) Evaluate the tan δ value according to the pre-regulation standards. If the value exceeds the limit, further investigation is needed, and additional tests may be required.
(2) Compare the measured tan δ with previous data, similar equipment, and other devices of the same type. A significant increase should raise concern.
(3) Plot the tan δ versus voltage curve. For good insulation, tan δ remains stable with voltage. For poor insulation, tan δ increases as voltage rises.
(4) Consider temperature effects. Comparisons should be made at the same temperature to ensure accurate results.
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