What is the partial discharge (PD) in a switch cabinet?
Inside the switch cabinet, due to defects in the insulating materials, aging, or installation issues, the electric field becomes highly concentrated in certain areas. When the electric field strength at this location exceeds the tolerance limit of the material, a local, non-throughout breakdown discharge will occur, but it does not form a complete short circuit of the entire insulation channel. This is known as partial discharge.
Harm: Although the PD is weak, it will continuously erode the insulating material, causing thermal and chemical corrosion effects. Eventually, it may lead to insulation breakdown, causing power outages and even equipment explosions.
Characteristic: The PD signal is extremely weak (typically at the pC level) and is often overwhelmed by strong electromagnetic noise.
The core principle and mainstream technologies of the partial discharge detection device for switch cabinets
Due to the fact that switch cabinets are in a strong electromagnetic interference environment and have a closed structure, special technologies are required to capture the PD signals. The mainstream technologies include the following:
Transient Earth Voltage (TEV) Method
Principle: When a partial discharge occurs, the rapid transfer of charges generates high-frequency electromagnetic waves. Some of these waves will propagate along the metal enclosure and “leak” out at the joints and gaps of the cabinet, thereby creating a transient earth voltage on the surface of the cabinet.
Detection method: A capacitive coupling sensor (similar to a metal patch) is attached to the metal casing of the switch cabinet to measure this weak voltage pulse signal.
Feature:
Advantages: Non-invasive, simple to operate, capable of live detection, high cost-effectiveness, and it is currently the most widely used method.
Disadvantages: It is prone to interference from external noises (such as mobile phones, walkie-talkies), unable to precisely locate the power supply, and the signal strength is greatly affected by the structure of the cabinet.
2. Ultrasonic (AE) Method
Principle: During the PD process, a transient explosion effect is generated, which excites ultrasonic wave signals (typically in the 20kHz – 200kHz range) in the air or solid insulating materials.
Detection method: Utilize an ultrasonic sensor (probe) to contact the surface of the cabinet or through air coupling, receive the sound wave signal, and convert it into an electrical signal for analysis.
Feature:
Advantages: Highly sensitive to air discharges (such as suspended potential discharges); Strong anti-electromagnetic interference capability; Capable of positioning through the time difference of sound wave propagation.
Disadvantages: The signal decays rapidly in the medium, the propagation path is complex, and its sensitivity to discharges in solids or oils is relatively low.
3. Ultra High Frequency (UHF) Method
Principle: The electromagnetic waves generated by PD have a very wide frequency spectrum, reaching several GHz. The gaps and observation windows of the switch cabinet can act as “antennas”, allowing ultra-high frequency (300 MHz – 3 GHz) electromagnetic waves to leak out.
Detection method: Utilize a specially designed UHF antenna sensor, which receives these high-frequency electromagnetic signals through the gaps on the cabinet or through a dedicated interface.
Feature:
Advantages: Extremely high sensitivity; Extremely strong resistance to low-frequency electromagnetic interference (power frequency noise is not within this frequency range); Capable of achieving precise time difference positioning.
Disadvantages: High cost; the cabinet needs to have a gap for leakage signals or pre-installed built-in sensors; the detection effect is greatly influenced by the cabinet structure.
4. High-frequency Current Transformer (HFCT) Method
Principle: The PD current pulses will flow back to the ground through the grounding wire or the cable shielding layer.
Detection method: Place the HFCT clamp sensor on the grounding wire or cable of the switch cabinet to detect the high-frequency current signals generated by the partial discharge.
Feature:
Advantages: High detection sensitivity, good signal-to-noise ratio, and capable of quantitatively measuring the discharge amount (in relative terms).
Disadvantages: It requires being close to the grounding wire. In indoor environments with many switch cabinets, the operation may sometimes be inconvenient.
Post time: Jun-16-2026