How to detect partial discharge in switchgear
What is switchgear partial discharge?
Partial discharge (PD) in switchgear refers to localized electrical discharges that occur within the insulation system or internal components of the switchgear under high-voltage stress. These discharges do not immediately cause complete insulation breakdown, but they release localized high-energy pulses that progressively degrade insulation materials. Over time, this degradation may lead to insulation failure and serious electrical faults. Therefore, effective monitoring and detection of partial discharge in switchgear are critically important for ensuring operational safety and system reliability.
Within switchgear, PD typically occurs at points of insulation defects, impurities, voids, or inhomogeneities within the insulation structure. The phenomenon results in localized, transient electrical discharges that produce energy in the form of electromagnetic waves, acoustic signals, and transient voltages.
How to detect partial discharge in switchgear?
There are three common partial discharge methods for switchgears in the following:
Ultrasonic (AA/AE) Detection
Before partial discharge occurs, the electric field stress, dielectric stress, and particle forces around the defect site remain in a relatively balanced state. Partial discharge involves a rapid release or migration of electrical charges, disturbing this equilibrium and causing transient mechanical and particle forces to oscillate rapidly. These oscillations induce vibration in the surrounding insulation medium, generating acoustic signals. By using piezoelectric transducers, these ultrasonic signals can be detected and measured.
Since ultrasonic waves propagate through air and along the switchgear enclosure, the ultrasonic sensors must be firmly attached to the cabinet surface for effective monitoring.
TEV (Transient Earth Voltage) Detection
During partial discharge, due to the skin effect, high-frequency discharge signals travel along the internal metallic surfaces of the switchgear enclosure. At points where the metal is interrupted or at insulation joints, the electromagnetic waves transfer to the outer surface.
As the rising edge of these electromagnetic waves reaches the metallic surface, a transient voltage to earth (TEV) is produced, typically in the frequency range of 3 MHz to 100 MHz. The amplitude of the TEV signal depends on the discharge magnitude, the propagation path, and attenuation characteristics of the internal structure.
Capacitive TEV sensors are used to detect these signals. By analyzing the amplitude and waveform of both the TEV and ultrasonic signals, partial discharge activity can be effectively monitored and evaluated.
UHF (Ultra High Frequency) Detection
The insulation strength of power equipment is extremely high, withstanding high breakdown field intensities. When partial discharge occurs within these insulators, the breakdown process happens rapidly and generates pulse currents with rise times less than 1 ns, simultaneously exciting ultra high frequency (UHF) electromagnetic waves, often reaching frequencies in the GHz range.
Using UHF sensors, these high-frequency electromagnetic signals produced by PD activity can be captured and analyzed.
In practical applications, a combination of multiple monitoring methods is often employed to improve the accuracy, sensitivity, and reliability of partial discharge detection. Through the integrated use of these detection techniques, early identification of PD activity within power equipment is possible, enabling timely preventive measures to be taken and ensuring the safe and reliable operation of power systems.
