The key to effectively reducing partial discharge of transformers is to understand the language of partial discharge signals, namely the partial discharge frequency range. Partial discharge signals will present different characteristics in different frequency ranges. Only by understanding these partial discharge frequency ranges can we accurately determine the location of transformer partial discharge and take effective measures. This article will introduce how to reduce transformer partial discharge in combination with the partial discharge frequency range.
What is partial discharge frequency range ?
“Partial discharge frequency range is very wide, mainly including Ultra High Frequency (UHF), Very High Frequency (VHF) / High Frequency (HF), Ultrasonic (AE), Transient Earth Voltage (TEV), and Narrowband / Power Frequency Harmonics.
UHF frequency range is 300 MHz – 3 GHz. These high-frequency signals are propagated in the form of electromagnetic waves and generally occur in GIS gas insulated switchgear, large transformer bushing areas, cable terminals and other equipment. In order to monitor and reduce this type of partial discharge, UHF sensors are often installed inside the equipment or on dedicated interfaces to monitor high-frequency electromagnetic waves.
VHF frequency range is 3 MHz – 300 MHz, in the form of guided waves in transformer windings, bushings and other equipment. In order to reduce this type of partial discharge, HFCT is usually installed on the ground wire or bushing lead to detect high-frequency transient current.
The frequency range of Ultrasonic/AE is 20 kHz – 300 kHz. Ultrasonic sensors are usually installed in the equipment or transformer tank to detect sound waves, so as to detect partial discharge early.
The frequency range of TEV partial discharge is 3 MHz – 100 MHz, which usually occurs inside the metal casing of the switch cabinet or bus duct. To monitor this type of partial discharge, it is necessary to install TEV sensors on the surface of the metal casing of the switch cabinet or bus duct to monitor voltage pulses.
Narrowband / Power Frequency Harmonics) The frequency direction is 50/60 Hz. The harmonic components of the power frequency current and voltage signals are often analyzed to indirectly infer whether there is partial discharge. “
How to reduce partial discharge in transformer based on the partial discharge frequency range?
“The frequency range characteristics of partial discharge (PD) signals are crucial for the detection, diagnosis and determination of effective reduction measures for transformer partial discharge.
UHF signals are mainly propagated inside the transformer (especially inside the oil tank and bushing) and decay quickly. So how can we reduce transformer partial discharge? During the transformer manufacturing stage, partial discharge caused by internal bubbles can be eliminated by injecting transformer oil under extreme vacuum. Secondly, the purity of transformer oil should be strictly controlled to eliminate sharp corners and burrs on transformer windings and components. At the same time, keep the transformer insulation material dry. During the operation of the transformer, the discharge inside the transformer can be monitored by installing a combination of DGA and UHF.
VHF signals can propagate in the transformer windings and bushing leads, or they can be transmitted through the ground path. In this case, internal discharges, surface discharges or winding discharges can be detected by installing HFCT and TEV at the transformer winding leads, bushing connections, and ground connections.
Ultrasonic signals are mechanical vibration waves generated by PD, which propagate through the wall of the transformer tank. This local discharge frequency range can be located by the multi-sensor time difference method to locate the local discharge generation area.
TEV signals may usually occur in transformer control boxes, lead boxes, etc. When there are insulation defects inside these metal casings, pulses can be induced on the outside. Therefore, TEV sensors can be installed to monitor local discharges. “
