The principle of partial discharge detectors is a question many users want to understand, along with detailed operating methods. We can explain it from different perspectives with examples to make the principle easier to grasp. For more information about partial discharge detectors, please follow KeGang Power.
1. What is Partial Discharge?
Before understanding the detector principle, it is first necessary to clarify what partial discharge is.
Partial discharge refers to a localized breakdown and discharge phenomenon that occurs in the insulation system of high-voltage electrical equipment (such as transformers, cables, GIS, etc.) due to uneven electric field distribution, bubbles or impurities inside the insulation. However, it does not form a discharge channel penetrating the entire insulation.
It can be compared to a dam: most parts are solid, but there is a small crack or ant hole causing water seepage. Although the water does not collapse the dam instantly, the continuous seepage will gradually erode the dam body and eventually lead to a dam failure.
Similarly, although partial discharge has small energy and an extremely short duration, its long-term existence will continuously corrode insulation materials, which may eventually cause breakdown of the entire insulation system, resulting in serious equipment damage and power outages. Therefore, partial discharge detection is an important means to evaluate the insulation status of electrical equipment and provide fault early warning.
2. Basic Principle of Partial Discharge Detector
The core principle of a partial discharge detector is to capture various physical and chemical phenomena generated during partial discharge and convert them into measurable and analyzable electrical signals.
Partial discharge mainly produces the following detectable phenomena:
Electric pulse: A steep current pulse is generated at the moment of discharge.
Electromagnetic wave: The rapidly changing current radiates electromagnetic waves to the surroundings, covering a wide frequency range (from tens of Hz to several GHz).
Ultrasound: The discharge produces a tiny explosion instantaneously, generating mechanical vibration in the insulating medium, namely ultrasound.
Light: Discharge produces optical radiation in some transparent media (such as SF₆ gas).
Chemical change: Discharge produces new chemicals such as ozone and nitrogen oxides, and may decompose insulating oil to generate characteristic gases.
According to different detection objects, partial discharge detectors are mainly divided into the following categories:
3. Detailed Explanation of Main Detection Methods and Principles
1. Electrical Detection Method (The most classic and direct method)
This is the standard method recommended by the International Electrotechnical Commission, mainly used for off-line or laboratory testing.
Detection principle: Based on the high-frequency current pulse generated by partial discharge. When partial discharge occurs at both ends of the test object, an instantaneous charge change is formed. This change generates a pulse voltage on the detection impedance through the coupling capacitor.
Core components:
Coupling capacitor: Provides a low-impedance path for the pulse current.
Detection impedance: Converts the pulse current into a measurable voltage signal.
Amplifier: Amplifies weak pulse signals.
Measurement and display system: Processes, analyzes and displays signals.
Typical circuits: Direct method and balanced method.
Advantages: High sensitivity, capable of quantitatively measuring discharge quantity (unit: picocoulomb, pC), serving as the benchmark for calibrating other methods.Disadvantages: Susceptible to on-site electromagnetic interference, usually requiring power-off detection and electrical connection with the tested equipment.
2. High Frequency Current Transformer (HFCT) / Rogowski Coil Method
This is the most common form of electrical detection method in on-site applications, belonging to non-intrusive detection.
Detection principle: A clamp-type high-frequency current transformer (HFCT) is clamped on the grounding wire of the tested equipment. When the partial discharge pulse current flows through the grounding wire, a voltage signal is induced in the HFCT according to the principle of electromagnetic induction.
Advantages:
Easy installation, no need to disconnect the line, supporting on-line detection.
Relatively strong anti-interference ability.
Capable of measuring the amplitude and frequency of discharge.
Application: Widely used for grounding wire detection of cables, transformers, GIS and other equipment.
3. Ultra-High Frequency (UHF) Method
Mainly used for the detection of Gas Insulated Switchgear (GIS) and transformers.
Detection principle: Partial discharge excites electromagnetic waves with a frequency as high as 300MHz ~ 3GHz. UHF sensors (antennas) installed inside the equipment cavity or at the basin insulator can receive these electromagnetic wave signals.
Advantages:
High detection frequency, effectively avoiding low and medium frequency power harmonic interference on site, with extremely strong anti-interference ability.
Very high sensitivity.
Capable of realizing discharge location.
Disadvantages:
Unable to directly calibrate discharge quantity (pC), usually expressing signal intensity in dBm.
Large signal attenuation during propagation.
4. Ultrasonic Detection Method
A completely non-electrical detection method with high safety.
Detection principle: Sonic and ultrasonic signals (usually 20kHz ~ 300kHz) generated by partial discharge propagate to the equipment shell through the insulating medium. The ultrasonic sensor attached closely to the equipment shell can detect these mechanical vibration signals and convert them into electrical signals.
Advantages:
Completely immune to electrical interference.
Capable of realizing precise positioning. The discharge point can be accurately located by measuring the time difference of signal arrival through multiple sensors.
Disadvantages:
Large signal attenuation in insulating oil and solid media, complex propagation path, resulting in relatively low sensitivity.
Susceptible to environmental mechanical vibration noise.
5. Transient Earth Voltage (TEV) Method
Mainly used for live detection of medium-voltage switch cabinets.
Detection principle: When partial discharge occurs inside the switch cabinet, the generated electromagnetic waves propagate along the metal cabinet wall and generate a transient earth voltage on the cabinet surface. This voltage can be measured by a capacitive coupling sensor attached to the cabinet door.
Advantages: Simple and fast operation, suitable for general survey and screening of switch cabinets.Disadvantages: Measurement results are greatly affected by sensor position and cabinet surface condition, only serving as a reference for qualitative or relative comparison, and cannot accurately measure discharge quantity.
4. Working Process of Modern Partial Discharge Detector
Modern advanced partial discharge detectors usually adopt multi-technology integration, and can use multiple sensors for detection simultaneously to improve the accuracy and reliability of detection.
The basic working process is as follows:
Signal sensing: Capture raw signals through HFCT, UHF, ultrasonic, TEV and other sensors.
Signal conditioning: Amplify and filter signals, remove noise, and extract effective features.
Data acquisition and processing: Convert analog signals into digital signals, and conduct in-depth analysis using digital signal processing technologies (such as wavelet analysis, FFT, pattern recognition).
Analysis and diagnosis:
Display patterns: Show PRPD patterns, waveforms, frequency spectra of discharge, etc.
Identify discharge type: Automatically or manually identify discharge types (such as internal discharge, surface discharge, corona discharge, etc.) according to pattern features (such as mushroom cloud, triangular hump, etc.).
Location and evaluation: Locate the discharge point combined with multi-sensor information, evaluate the severity of discharge, and give diagnosis conclusions and maintenance suggestions.
Summary
In short, the principle of a partial discharge detector is to capture traces of discharge. Through electrical, acoustic, optical, chemical and other approaches, it converts invisible insulation defects into analyzable data, thus realizing intelligent diagnosis and early warning of the insulation status of high-voltage electrical equipment.