**How the Dielectric Loss Tester Works**
The DCJS-S automatic anti-interference dielectric loss tester is a high-precision instrument designed for the automatic measurement of dielectric loss tangent (tan δ) and capacitance in various high-voltage power equipment found in power plants, substations, and industrial facilities. This device is essential for evaluating the insulation quality of electrical components, ensuring safe and efficient operation of power systems.
**Working Principle**
When an alternating voltage is applied to a dielectric material, part of the electrical energy is consumed and converted into heat, which is referred to as dielectric loss. This loss is closely related to the phase difference between the voltage and current in the dielectric. The angle between the voltage and current, known as the dielectric loss angle (δ), determines the amount of energy lost during the process. The tangent of this angle, tan δ, is a key parameter used to assess the dielectric loss.
The measurement circuit of the tester consists of two main parts: a standard loop (Cn) and a test loop (Cx). The standard loop includes an internal high-stability capacitor and a measurement circuit, while the test loop includes the sample under test and its associated measurement system. A sampling resistor with a preamplifier and an A/D converter is used to capture the current signals from both loops. These signals are then analyzed using digital real-time acquisition techniques and vector operations to determine the capacitance and tan δ value of the sample.
To ensure accurate measurements even in the presence of external interference, the instrument incorporates advanced anti-interference features that help eliminate noise and maintain precision.
**Instrument Structure**
The DCJS-S tester is composed of several key components:
- **Measuring Circuit**: Performs Fourier transforms, complex arithmetic, range switching, and variable frequency power supply control.
- **Control Panel**: Includes a printer, keyboard, display, and communication relay for user interaction and data output.
- **Variable Frequency Power Supply**: Uses SPWM switching technology to generate a stable high-power sine wave output.
- **Step-up Transformer**: Boosts the output voltage to the required level for testing, with a maximum reactive power of 2KVA for one minute.
- **Standard Capacitor (Cn)**: Acts as a reference for measurement accuracy.
- **Cn Current Detection**: Measures the current through the standard capacitor, ranging from 10μA to 1A.
- **Cx Positive Wiring Current Detection**: Used for positive wiring measurements, also covering a range of 10μA to 1A.
- **Cx Reverse Wiring Current Detection**: Designed for reverse wiring measurements, with the same current range.
- **Reverse Wiring Digitally Isolated Communication**: Transmits the reverse wiring current signal to the low-voltage side using a precision MPPM digital modem, with an isolation voltage of 20KV.
**Working Principle (Continued)**
Once the measurement is initiated, the set voltage is sent to the variable frequency power supply, which uses a PID algorithm to adjust the output to the desired level. The measuring circuit then fine-tunes the low-voltage output to ensure accurate high-voltage delivery. Based on the selected wiring configuration (positive or reverse), the measuring circuit automatically selects the appropriate input range and performs vector calculations using Fourier transform to filter out interference and calculate the tan δ value.
After multiple measurements, the system averages the results to provide a more reliable reading. At the end of the test, the step-down command is issued, and the power supply gradually reduces the voltage to zero.
**Measurement Methods and Principles**
There are two primary methods for measuring dielectric loss: positive wiring and reverse wiring. The choice depends on whether the tested object is grounded or not.
In the **positive wiring method**, the sample is ungrounded, and the full current is measured through the sampling resistor. In the **reverse wiring method**, the sample is grounded, and the current flows directly into the machine from the high-voltage end.
Both methods rely on comparing the standard current (from the internal capacitor) with the sample current to determine the tan δ value. This involves decomposing the current into horizontal and vertical components and calculating their ratio.
**Dielectric Loss Calculation Formula**
Under a constant electric field, the dielectric loss can be expressed as:
$$ W = \frac{U^2}{R} = \frac{(Ed)^2 S}{\rho d} = \sigma E^2 Sd $$
Where:
- $ W $ is the dielectric loss,
- $ U $ is the voltage,
- $ R $ is the resistance,
- $ E $ is the electric field strength,
- $ d $ is the thickness,
- $ S $ is the surface area,
- $ \sigma $ is the conductivity,
- $ \rho $ is the resistivity.
The dielectric loss rate per unit volume is defined as:
$$ \omega = \sigma E^2 $$
In an alternating electric field, the electric displacement $ D $ and electric field $ E $ become complex vectors, and the dielectric constant becomes a complex number. The imaginary part represents the energy loss in the dielectric.
From a circuit perspective, the current density in the dielectric is given by:
$$ J = \frac{dD}{dt} = \frac{d(\varepsilon E)}{dt} = J_\tau + iJ_e $$
Where:
- $ J_\tau $ is the active current density (in phase with $ E $),
- $ J_e $ is the reactive current density (90° ahead of $ E $).
The loss tangent is defined as:
$$ \tan \delta = \frac{J_\tau}{J_e} = \frac{\varepsilon''}{\varepsilon'} $$
Here, $ \delta $ is the loss angle, and $ \tan \delta $ reflects the ratio of active to reactive current, indicating the energy loss per unit volume in the dielectric.
**Importance of Dielectric Loss**
Dielectric loss is proportional to the applied voltage, frequency, capacitance, and tan δ. However, it is not always the best indicator of insulation quality because it depends on factors like voltage, size, and shape of the material. Therefore, tan δ is preferred as it provides a more consistent and independent measure of the dielectric's performance.
In engineering applications, tan δ is widely used to evaluate the condition of insulating materials in high-voltage equipment. It is a dimensionless quantity that reflects the internal losses of the material and is crucial for assessing the long-term reliability of electrical systems.
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